Stress heterogeneities in exhumed high-pressure rocks shed light on deep subduction interface transient coupling

Domain: earth_science
Published: 2025-10-14
Editorial outcome: major_revision

Abstract
Stress distribution and creep mechanisms along modern subduction interfaces remain poorly understood, due to the lack of high-resolution constraints on the rheology of the heterogeneous plate interface and associated fluid distribution in the so-called transition zone (40–60 km depth), where deep slow slip events (SSEs) occur. Here, we document the presence of dislocation-based creep in a blueschist facies metachert block exhumed from the North Qilian paleo-subduction interface, NE Tibet. This block exhibits foliation-parallel micro-shear zones inferred to have formed under relatively fast strain rates of 10−9 s−1 and shear stresses exceeding 100 MPa. These values are paradoxical, as they are significantly higher than those estimated for the surrounding, weaker calcschist matrix, raising questions about their physical origin. We propose that the observed micro-shearing in the block, formed over a duration on the order of decades, sheds light on the local coupling induced by transient block interactions during large-scale SSEs along the deep subduction channel.

Introduction
Along the subduction interface, the transition zone between the seismogenic zone (hosting regular earthquakes of up to M > 9) and the creeping zone (steady deformation accommodated by ductile shearing) embraces a depth range between about 30–40 km updip to 60–80 km downdip and is characterized by several instrumentally-constrained kinematic and physical conditions1,2. Kinematic observations include deep slow slip events3,4, which are sometimes seen to correlate with fluctuations of Vp/Vs, suggesting changes in pore fluid pressure or rock properties5,6. Another notable feature, particularly in fast convergence zones, is the occurrence of deep aftershocks (2010 Maule earthquakes7) and continuous deep interface seismicity (2014 Iquique earthquake8) in the transition zone, well below the locked seismogenic zone. This deep seismicity—localized within the plate interface zone—may be linked to the seismogenic zone or be separated by a “soft barrier” from the latter3,9. Stress drops in this region are around 1 MPa10, lower than those typically observed in shallower aftershocks along the seismogenic plate interface11,12. In contrast, the overall stress in the transition zone is higher by an order of magnitude (20–40 MPa), as recovered from lithosphere-scale rheological and force analysis3,9,13, bridging the downdip end of seismic coupling with the peak stresses to the much lower stresses in the continuously creeping zone below 60 km depth.

While these instrumental observations provide valuable insights into certain properties of the deep transition zone, the physical mechanisms governing the plate interface in this region are not as well understood or analyzed as their equivalent in the shallower depth range of the seismogenic zone14,15. A range of mechanisms has been invoked: the importance of pressure solution creep, as a form of viscous deformation at low temperatures (<350 °C), has been recognized for decades16–18. On the other hand, recent studies have brought to light the significant potential of dislocation creep in accommodating the transient oscillations of higher-temperature SSEs occurring at the base of the seismogenic zone, without switching to seismic slip19–21. Furthermore, the widespread presence of free fluids and highlighted cyclic pore fluid pressure swings in the transition zones22,23 play an important role in the transition of aseismic-seismic behavior and the occurrence of tremor signals24,25. Based on the lithological heterogeneity of the subduction interface, numerical studies highlight stress accumulation formed by rigid block interaction hosted by the weak matrix, which could strongly alter the rheological properties of the interface26–28. Therefore, identifying the transient phenomena recorded in exhumed rocks from the transition zone of the subduction interface—where field and fabric observations are far more limited compared to the seismogenic zone—and determining their associated creep mechanisms are essential for advancing our understanding of the rheology of the transition zone.

Rheological constraints on exhumed fossil subduction channels from this depth domain can be derived from several observations, including the hydraulic state and its variations22,29, deformation fabrics diagnostic of the underlying physical deformation mechanisms17,18, the range of lithologies involved20, and the role of recrystallized grain size paleopiezometry30–33. However, the heterogeneity of subduction mélanger rocks poses challenges for the accurate application of paleopiezometry developed for monomineralic rocks34. Metachert blocks locally preserved in paleo-subduction channels, characterized by high quartz purity comparable to quartzite, could represent an ideal target for applying quartz paleopiezometry at outcrop scales in order to estimate shear stress and strain rate distribution across the block-in-matrix setting. Additionally, local stress heterogeneity at this temperature range26,35 limits the extrapolation of small-scale paleopiezometric data to the entire subduction channel.

Here, we investigate the North Qilian high-pressure/low-temperature (HP/LT) mélange (NE Tibet) that represents the depth range of the former transition zone along the plate interface during Early Paleozoic fast subduction of Proto-Tethyan Ocean. In this mélange, metachert blocks are embedded in a calcschist matrix and contain foliation-parallel ultramylonitic micro-shear zones, which are viewed as recording dislocation-based creep below the base of the seismogenic zone and associated with anomalously high stress and strain rates. This finding provides key insight into coupling heterogeneities in the deep SSEs region as well as transient block interactions along the subduction interface.

Results
Geological and structural setting
The WNW–ESE-striking North Qilian HP/LT mélange belt (NQMB) represents the exhumed Early Paleozoic paleosubduction interface and extends over 50 km along the northeastern margin of the Qinghai-Tibet Plateau (Fig. 1a). It resulted from progressive northward subduction of the Paleo-Qilian Ocean (a branch of the Proto-Tethyan Ocean) beneath the Alxa block, along with fast plate convergence velocities during its closure, demonstrated by the subduction erosion-controlled active continental margin36. The NQMB, exposed as a tectonic slice between shallower greenschist facies slivers, comprises two distinct geological units, representing segments of different depths along the subduction interface (Fig. 1a, b): the eclogite facies unit and the overlying blueschist facies unit (Fig. 1c). The blueschist facies unit represents a coherent tectonic slice composed mainly of foliated glaucophane-bearing micaschists, calcschists, metacherts, and minor epidote blueschists. Previous data, including Lu-Hf isochron ages, suggest that the blueschist facies unit underwent solely blueschist facies metamorphism (450–550 °C and 1.2–1.6 GPa) at 467–462 Ma37, with subsequent cooling ages spanning 460 to 440 Ma38.

Along the Qingshui Gorge profile, we focus on a ~500-meter-wide metachert block within the host calcschist matrix of the blueschist facies unit. It can be divided into two structurally distinct portions: a micro-sheared metachert (MSMC) and a coarse-grained metachert (CMC), based on integrated fine-scale field mapping, systematic sampling at 50 m intervals, and oriented thin-section observations (Fig. 1b, c; Supplementary Table S1). Pronounced ductile shearing, localized in the MSMC portion with an outcrop thickness of ~300 m, is indicated by outcrop-scale mylonitic foliation, stretching lineation (Fig. 1d, g, h), and pervasively developed foliation-parallel micro-shear zones (Supplementary Table S1). The portion of CMC (~200 m thick) is characterized by top-to-the-SSW thrust-related folds, enclosed along the foliation within the MSMC portion (Fig. 1e), with no evidence of micro-shearing observed (Supplementary Table S1). The overall fabric orientations of the block and matrix are consistent, with the foliation dipping moderately to the NNE (14/47N) and the gently east-northeast–plunging stretching lineation defined by elongated quartz (Fig. 1a), which also correspond well to the top-to-the-south shearing sense observed in the blueschist unit (Supplementary Fig. S1). At the flank of the thrust fold, the graphite-bearing metachert foliation is embedded and lies parallel to the coarse-grained metachert (Fig. 1f). Furthermore, quartz-filled sheared veins within the centimeter-layered blueschist facies calcschist are ductilely elongated, exhibiting boudinage and/or pinch-and-swell geometry along the foliation in the northern margin of the metachert block (Fig. 1i). The development of brittle fault/fracturing zones is partially preserved within the calcschist matrix (Supplementary Fig. S1). Extensive field observations indicate that metachert blocks contain far fewer veins compared to the surrounding calcschist matrix (Fig. 1d–h; Supplementary Fig. S1). Pseudotachylyte has been observed in neither the matrix nor the metachert blocks.

Deformation of metachert block versus calcschist matrix
Electron backscatter diffraction (EBSD) and microstructural analysis reveal and quantify the deformation styles of quartz in the calcschist matrix and the metachert block. In the calcschist matrix, quartz sheared veins range in thickness from a few millimeters to a centimeter and host large quartz grains along with dispersed phengite platelets, which are loosely aligned with the main foliation (Fig. 2a, b). The surrounding matrix consists of near-equigranular calcite–quartz aggregates, where quartz grains are commonly in contact and form a load-bearing framework (Supplementary Fig. S2). Recrystallized quartz grains appear weakly lobate along the edges of relict quartz grains with undulose extinction (Fig. 2c). Viscous foliations in CMC are defined by coarse-grained quartz ribbons interspersed with sparsely distributed, recrystallized quartz grains (Fig. 2e–g), along with 1 vol.% elongated mica identified through rock surface scans (Fig. 2e; Supplementary Fig. S3). The microstructures of the oriented MSMC samples are marked by penetrative foliation-parallel micro-shear zones (2–6 vol.%), ranging from 50 to 400 μm in thickness (Fig. 3; Supplementary Figs. S4–5 and Table S1). These micro-shear zones commonly cross-cut coarse-grained ribbons composed of undulatory quartz, fragmented glaucophane, and elongated phengite (Fig. 3a–c). In addition, the boundaries between the quartz ribbons and micro-shear zones are commonly decorated by aligned mica aggregates (Fig. 3b, c), as further corroborated by TIMA analysis showing that the MSMC sample (E-52) contains an appreciable amount of mica (8 vol.%, Supplementary Fig. S6). Within the micro-shear zones, quartz grains appear as fine, equant, strain-free recrystallized grains mantling larger, undulatory relict grains with higher internal lattice distortions (Fig. 3d, e, Supplementary Figs. S4–5).

The analyzed sites, including quartz sheared veins, micro-shear zones, and coarse-grained ribbons, demonstrate consistent patterns in their crystallographic preferred orientations (CPO), misorientation angle distributions (MAD), and crystallographic vorticity axis (CVA) (Figs. 2–3; Supplementary Figs. S3–5). Additional details are available in the Supporting Information. According to the GOS-based separation method39, the root mean square (RMS) grain sizes of recrystallized quartz and subgrain populations were determined for both the matrix and the block: quartz sheared veins (100–112 μm and 98–105 μm; Fig. 2d), coarse-grained ribbons (16–34 μm and 17–29 μm; Fig. 2h), and micro-shear zones (7–10 μm and 6–12 μm; Fig. 3h).

Assessment of P-T Conditions for micro-shearing
Graphite occurs as flattened flakes and/or dispersed aggregates (~50–400 μm in size), aligned with the foliation, as identified by Raman spectroscopy in the coarse-grain ribbons of the CMC (G, D1, and D2 bands; Supplementary Table S2). The Raman Spectra of Carbonaceous Material (RSCM) geothermometry40 yielded peak temperatures at the analyzed sites (including sample C-61; Fig. 1e, f) ranging from 446 to 525 °C, with a mean temperature (RMS) of 480 °C and a standard deviation of 22 °C (Supplementary Table S2).

Electron probe micro-analyses of white mica from block and matrix samples reveal homogeneous chemical compositions, with no significant variations among micro-shear zones, coarse-grained ribbons, and quartz sheared veins (see detailed mineral chemistry in Supplementary information). White micas are phengites (solid solution between celadonite and muscovite) with silica per formula unit (p.f.u) values ranging from 3.42 to 3.46 in MSMC sample (E-52) (Fig. 4a, Supplementary Data 1). Phase equilibrium modeling using the Perple_X software41 for sample E-52 was conducted based on synkinematic petrological relationships and bulk-rock compositions calculated from TIMA estimates (Supplementary Fig. S6). By combining RSCM geothermometry, the RMS silica p.f.u. value in phengite, and the mineral assemblage of sample E52 observed through TIMA scanning, the average equilibrium P-T conditions of micro-shearing were constrained to 490 ± 10 °C and 15.0 ± 1 kbar (Fig. 4b). These P-T conditions are regarded as the synkinematic peak conditions during micro-shearing, supported by the absence of microstructures indicative of retrograde metamorphic reactions (e.g., consistent CPO of quartz, uniform silica content across phengite, and flat profiles of sodium content and XMg value across glaucophane; Supplementary Fig. S7 and Supplementary Data 1–2). Additionally, phengite 40Ar–39Ar dating on calcschist matrix yields two uniform plateau ages of 467 ± 3 Ma (Supplementary Fig. S8 and Table S5).

Discussion
In the Qingshui Gorge mélange, the mechanisms and kinematic framework of viscous creep events are quantitatively extracted from the obtained quartz fabrics. No significant divergence in the active slip systems of various quartz grain populations was observed among the calcschist matrix, CMC, and MSMC (Figs. 2–3, Supplementary Figs. S3–5), all of which coincided with the shift from the rhomb<a> slip to the prism<a> slip42,43. This, combined with the strong CPO and intragranular distortion patterns of quartz aggregates, suggests that dislocation creep driven by subgrain rotation recrystallization (SGR) dominated deformation in both the calcschist matrix and the metachert block. The similar grain sizes and CPO patterns between recrystallized quartz grains and their subgrains imply negligible overprinting on these textures during late-stage exhumation. Furthermore, the near-horizontal vorticity axis and top-to-the-south shear sense, inferred from the CVA y-axis maximum and the counterclockwise rotation of asymmetric c-axis girdles, correspond well to the regional subduction kinematic framework (Fig. 2 Legend).

Outcrop-scale quartz piezometry conducted on the studied metachert block and quartz-rich sheared veins within the calcschist matrix provides insight for evaluating stress distribution during peak burial metamorphism. Based on the “sliding resolution” piezometer of Cross et al.39 and measured RMS recrystallized quartz grain sizes, shear stress estimates range from 23 to 25 MPa for quartz sheared veins, 57 to 75 MPa for coarse-grained ribbons, and 126 to 143 MPa for micro-shear zones (Supplementary Table S4). The shear stress values of quartz sheared veins are considered the upper bound for the strength of calcschist matrix, as the boudinage and/or pinch-and-swell geometry of the quartz veins suggests they are more competent than the host calcschist44. The quartz load-bearing framework observed throughout the calcschist thin-sections (Supplementary Figs. S2 and S7) suggests that the rock’s strength is mainly governed by quartz, thereby implying a relatively homogeneous strain distribution at the whole-rock scale45.

Although these shear stress estimates, derived from micro-shear zones, exceed previously reported paleopiezometry-based values from exhumed subduction rocks18,30,32,33,46,47 (~10–100 MPa or less), they remain valid due to the observed dominance of dislocation creep. The presence of intragranular Dauphiné twins, which do not cross grain boundaries (yellow twin boundaries in Figs. 2–3, Supplementary Figs. S3–5), indicates differential stresses of ~50–145 MPa at ~500 °C, based on the study of Minor et al.48.

According to estimated deformation P-T conditions (490 °C and 15 kbar), the spread in flow stress translates to an approximately 600-fold difference in strain rate. Quartz sheared veins from the host calcschist exhibit rates on the order of 10−12 s−1, while estimates for block-hosted micro-shear zones locally exceed 10−9 s−1 (Fig. 5a, Table S4). The rheological results obtained through the application of various piezometers and flow laws are broadly consistent, as illustrated in Supplementary Fig. S9. On the other hand, given the closure temperatures of muscovite49 (425 °C ± 70 °C) being slightly lower than the constrained deformation temperatures, the 40Ar–39Ar plateau ages of 467 Ma are thought to represent the initial cooling age of the dislocation creep event. Our estimated P-T conditions and timing are both compatible with the reported blueschist facies metamorphism in the NQMB37. Consequently, the studied calcschist matrix and the adjacent metachert block are regarded as part of the North Qilian paleo-subduction channel and are here interpreted to have coevally recorded a single deformation event in the Lower Ordovician. Under the lithostatic gradient of the overriding continental plate, the viscous deformation is inferred to have occurred at a depth of 50-60 km in this paleo-subduction channel, which falls at the base of the modern deep SSEs zone, typically situated between 40 and 60 km1.

Among viscous creep mechanisms, pressure solution has been reported as active along the shallow subduction interface17,18, particularly favored in areas with small grain sizes50. The application of the dislocation creep-based flow law in this study may be influenced by the unaccounted role of pressure solution creep, which is likely leading to grain size re-equilibration and further misestimation. Even so, the deformation mechanism map (Fig. 5a) shows that our obtained values for quartz veins (matrix) and metachert (block) fall within the dislocation creep field (observed dominance of dislocation creep; Figs. 2–3, Supplementary Figs. S3–5), suggesting that pressure solution creep has little to no impact on our rheological estimates. Furthermore, recent numerical and microphysical models predict the prevalence of dislocation creep within the high-temperature SSEs zone, highlighting its potential to accommodate transient perturbations through accelerated creep without switching to seismic slip19–21. Here, the deformation style identified in the metachert block and calcschist matrix provides compelling geological evidence of dislocation-based creep within the SSEs zone, bridging the gap between model predictions and field observations.

Within the metachert block, a positive correlation between phyllosilicate content and creep rate exists, i.e., micro-shear zone formation and strain rate increase in the mica-rich MSMC (8 vol.%) while micro-shearing is absent in the mica-poor CMC (1 vol.%). The role of mica in facilitating strain localization is commonly demonstrated by deformation experiments51 and natural observations52. The addition of a mere 10 vol.% of mica content can significantly alter the rheological behavior of quartz, and increase the strain rate by an order of magnitude relative to pure quartzite. Furthermore, there is no evidence of pre-existing foliation-parallel brittle fractures within the metachert that could have been exploited by micro-shearing53. This textural variation is therefore interpreted as the product of strain localization driven by variations in phyllosilicate content.

Quartz micro-shearing, as a snapshot of transient deformation conditions, has been evidenced by both experimental results54 and geological observations31,55,56, including for other HP/LT paleo-accretionary systems worldwide57. The overprinting of coarse-grained ribbons by micro-shear zones, along with their consistent CPO, can therefore be interpreted as two stages of a deformation event: coarse-grained ribbons represent steady-state creep, which subsequently transitioned to transient higher stress and strain rate events that are recorded in micro-shear zones. We emphasize that the micro-sheared samples (collected across the ~300-meter-wide MSMC) contain penetrative, foliation-parallel micro-shear zones (~2–6 vol.%, based on optical scanning of oriented X-Z thin sections; Fig. 3; Supplementary Fig. S4–5 and Table S1). Rheological envelope calculations (Fig. 5b) show that the pore fluid pressure factor (λ) required for stable plastic behavior is lower within the micro-sheared metachert block (λ < 0.86) compared to the vein-bearing calcschist matrix (λ > 0.97), implying that micro-shearing is activated within the block under physical conditions (i.e., at lower fluid content and higher differential stresses) distinct from those in the matrix. This variability in pore fluid pressure (between the block and the matrix) is independently confirmed by (i) the greater outcrop-scale abundance of quartz veins in the matrix (Fig. 1d–i; Supplementary Fig. S1) and (ii) a higher amount of hydrated minerals (e.g., epidote and mica; Figs. 2–3; Supplementary Table S1) of the matrix. Elevated pore fluid pressure (λ > 0.97) within the calcschist matrix is regarded as a key factor promoting unstable slip and subsequent cataclasis. However, direct evidence of cataclasis is rarely preserved in nature, as it is prone to being overprinted by deformation-assisted recrystallization during multiple seismic cycles, ultimately erasing the original brittle markers14,58,59 (e.g., Fig. S1e, f). In this context, although the occurrence of former SSEs within the calcschist matrix cannot be directly demonstrated, they remain the most likely mechanism of stress release in this segment of the subduction interface—an area known to host such events2–5.

Deep SSEs occur beneath the seismogenic zone, manifesting as transiently elevated slip velocities that exceed the long-term plate motion rate with durations ranging from days to years60. Modern subduction interfaces are characterized by relatively low bulk strength level3,9,10,13 (1–40 MPa) in the transition zone compared to estimates from paleopiezometry18,30,32,33,46,47, as well as heterogeneous plate coupling along strike, influenced by variations in the effective strength of locked patches61,62. From our geological perspective, these discrepancies likely arise from sheared, heterogeneous lithologies at small-scale26,28, such as the presence of “strong” metachert/basaltic blocks within a “weak” metasedimentary matrix20,63,64. Here, the power-law equation for dislocation creep indicates that shear stress and strain rates in the stronger, metachert block are significantly higher than those in the calcschist matrix, unexpectedly underscoring their contrasting rheological behavior. The presence of various metachert blocks exposed within the studied blueschist facies mélange (Fig. 1a–c) suggest that these relatively strong blocks could have mechanically interacted together during deep-seated shearing along the subduction interface, leading to the amplification of localized stress19,28. This transient configuration is closely linked to their local interconnected geometric arrangement, which creates a force-chain-like phenomenon, as previously suggested by numerical studies19,27. Meanwhile, the integration of stress over the block surface28,35 and/or the indentation of relatively strong blocks within a weaker matrix could lead to higher internal stresses65 within blocks than in the matrix, possibly also playing a role in strengthening this phenomenon. Alternatively, the strength-inversion model66 suggests that comparably high stress concentration and associated brittle failure may also arise from a potentially local configuration, in which mechanically weak blocks are embedded within a stronger, viscous matrix—without requiring direct mechanical coupling between blocks.

Numerical modeling studies19,28 have demonstrated that a critical threshold of approximately 50–55% block content is required to maintain Voigt-type iso-strain-rate behavior, which in turn leads to the mechanical protection of the matrix through a well-connected network of strong blocks. This highlights the broader role of lithological heterogeneity, as also evidenced by geophysical observations67 showing that subduction zone mechanics are strongly influenced by the structural complexity of the downgoing plate (e.g., asperities, seamounts, fracture zones, mélanges, and olistostromes). At the North Qilian paleo-subduction interface, our field observations (Fig. 1a) indicate that the block content in the mélange is heterogeneous and does not systematically reach the critical threshold (~50%), implying that such localized force-chain phenomena65 could not have persisted over prolonged durations. Instead, transient and localized stress transfer along force chains may have occurred only episodically, during temporary enrichment of the downgoing crust in strong blocks, as illustrated in the inset sketch of Fig. 6. While during the rest of the time, the block content was too low to generate interaction or exert a first-order control on matrix rheology, as supported by previous modeling studies19,28. Alternatively, it cannot be excluded that other events, such as asperity subduction62, could also have occasionally contributed to the stress transfer recorded in the metachert blocks. In any case, our observations suggest that stress increase associated with the activation of micro-shear zones was not sustained over geological timescales of millions of years but rather occurred over a shorter duration (i.e., within a timeframe of ~5 to 40 years; see calculations in Method), likely on the long-term SSEs timescales68,69 (5–15 years). Shear strain rates (~10−8 s−1–10−4 s−1; see Method)14 of SSEs below the seismogenic zone can be estimated from geodetic slip rate70 (300–1500 mm/yr) and defined slip thickness of SSEs (10-4–50 m)71, with the lower limit almost reaching the constrained strain rate of micro-shearing in the blocks (10−9 s−1). Linking the exhumed geological evidence to present-day geophysical observations of SSEs is intrinsically hampered by (i) a pronounced mismatch in spatial and temporal scales; (ii) the scarcity of unequivocal rock fabrics and/or structures attributable to single events; and (iii) the prolonged history of subduction, accretion, and exhumation, which complicates the interpretation of the rock record14,72. Nevertheless, constrained analogies with modern subduction channels remain an indispensable—though indirect—framework for deciphering how transient SSEs may be archived in the deep geological record20. Here, extrapolating from observations in modern subduction analogs, such as slip transient would be spatially embraced by the large-scale (several to hundreds of kilometers), multi-fault network59,71,73,74 of the weaker matrix (Supplementary Fig. S1), which is expected to host the SSEs. The aforementioned associations in space and strain rate imply that the stress perturbation observed within the block may be related to, or even triggered by, large-scale SSEs occurring in the surrounding faulted matrix network. Notably, large megathrust ruptures are also known to be followed by viscoelastic relaxation along the subduction interface that can persist for several decades (such as for the 1960 Great Chile Earthquake75), in line with the duration estimated from observed microstructures. Such post-seismic stress relaxation processes could also have contributed to further block interaction and the subsequent generation of the micro-shear zones observed within the metachert blocks.

In summary (Fig. 6), the studied micro-shear zones formed during transient block interactions in a paleo-subduction channel exhumed from below the downdip end of the seismogenic zone. Block interaction processes aseismically generated dislocation-based creep while recording anomalously high stress and strain rates within the block compared to the adjacent, more fluid-rich, weaker calcschist matrix. These findings offer unprecedented high-resolution insights into the mechanical feedback between hydraulic state, stress heterogeneities, and mixed lithologies along the deep SSEs region in the transition zone of the subduction interface.

Methods
Electron backscatter diffraction (EBSD)
Thin sections (X–Z plane) were prepared by polishing with the Buehler MasterPrep 0.05-μm alumina polishing suspension and the Buehler EcoMet300-AutoMet300 grinder–polisher, followed by coating with 7.5 nm layers of carbon to reduce charging. Quartz crystallographic orientation mapping was also performed using the FEI Quanta 450 FE-SEM equipped with an Oxford Symmetry S3 EBSD detector in high vacuum mode at the Institute of Geology, Chinese Academy of Geological Sciences (CAGS), Beijing. The orientation of the thin sections in the analysis chamber was set with the stretching lineation parallel to the SEM X-axis and a tilt angle of 70°. Operating conditions for electron backscatter analysis were: accelerating voltage of 20 kV, beam current of 110–120 μA, step size of 1.5 μm for quartz sheared veins, 0.75–1.0 μm for coarse ribbons, and 0.18–0.9 μm for micro-shear zones. The image collection was completed using the Aztec Crystal software, and data were processed with the open-source toolbox76 for MATLAB (https://mtex-toolbox.github.io/; v. 5.8.1). Grains were defined as areas of a single phase with a misorientation of less than 10°, and the crystallographic vorticity axis (CVA) patterns were determined after77.

TESCAN integrated mineral analyzer (TIMA)
Mineral identification and mica content measurement of coarse-grained metachert (CMC) and micro-sheared metachert (MSMC) thin sections were conducted using the TIMA mineralogy system at the Institute of Geology, CAGS, which comprises a Tescan Mira Schottky field emission scanning electron microscope equipped with four silicon drift energy dispersive (EDS) detectors arranged at 90° intervals around the chamber. The analyses were performed with an accelerating voltage of 25 kV, a beam current of 8 nA, a working distance of 15 mm, and a spot size of 90 nm. The classification of mineral phases and the calculation of their volume ratios were performed by matching the obtained spectroscopic data with built-in mineral definition files.

Electron probe micro-analyzer (EPMA)
Mineral chemical compositions of the calcschist matrix and the metachert block were acquired on the JEOL JXA-iHP200F field emission electron probe micro-analyzer at the Institute of Geology, CAGS. An accelerating voltage of 15 kV, a probe current of 20 nA, and a beam diameter of 5 μm were used for acquisitions. Ten oxide compositions were measured with the following standards: SiO2, TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, CaO, Na2O, and K2O. The lines and spectrometer crystals used for the major elements are: Si (Kα, TAP), Ti (Kα, PETJ), Al (Kα, TAP), Cr (Kα, LIFL), Fe (Kα, LIFH), Mn (Kα, LIFH), Mg (Kα, TAPL), Ca (Kα, PETJ), Na (Kα, TAPL), and K (Kα, PETJ). Count times were set to 10 s for the peak and 5 s for the background for each element. Utilizing natural minerals for standardization and performing ZAF corrections yielded an estimated precision of ±2% for major elements.

Raman spectroscopy
Raman spectra of graphite on CMC thin sections were obtained using a Horiba LabRAM HR Evolution spectrometer, coupled with an Olympus BX41 microscope at the Institute of Geology, CAGS. A 532 nm air-cooled frequency-doubled Nd:YAG laser was used, delivering 20 mW on the sample with a ×50 objective lens. The scattered light was received by an 1800 gr/mm optical grating through a confocal 100 μm pinhole. After calibration with a silicon standard, we use an acquisition time of 10 s and 2 scans were accumulated within the Raman shift range of 1000 cm-1 to 3500 cm-1. The raw spectral data were fitted using the Voigt function in Fityk 1.3.1 software78. Three peaks, corresponding to the G (1580 cm⁻¹), D1 (1350 cm⁻¹), and D2 (1620 cm⁻¹) bands were identified. Based on the determined peak parameters (Table S2), the quantitative parameter of the graphitization degree (R2 ratio) and related temperature are estimated following40.

Phase equilibria modeling
The major element composition of the MSMC sample E52 was calculated based on the volume fractions of classified mineral phases and their corresponding chemical formulas, which in weight percent is: SiO2 (90.48), Al2O3 (4.13), FeO (1.56), MnO (0.07), MgO (1.31), CaO (0.05), and Na2O (0.69). The phase equilibria modeling of sample E52 was conducted using Perple_X version 6.8.141 and the internal thermodynamic database79 in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) system. Ferric iron has been neglected in sake of simplicity because analyzed minerals exhibit low ferric iron contents. The a–x relations are provided in the Supporting Information. Mineral abbreviations are given following80. The isopleths of silica content per formula unit in phengite are also constrained in the pseudosection. The comparison between EPMA analyses from the MSMC sample (E52-B site) and modeled mineral compositions (for the best-fit area shown in Fig. 4b), presented in Table S3, reveals minor yet acceptable deviations.

Flow law and slip rate estimation
The EBSD-derived RMS recrystallized grain sizes of quartz (dRMS) were converted into differential stress (σ) using the “sliding resolution” piezometer39, expressed as: dRMS = 104.22 × σ−1.59. The estimated differential stress values were multiplied by 2/√3 to obtain the equivalent shear stress under plane stress conditions46, which was confirmed by the finite strain estimation (Supplementary Fig. S11). The “dislocation creep” flow law from Hirth et al.81 and the derivation details of the deformation mechanism map are included in the Supporting Information.

Assuming the North Qilian paleo-subduction channel underwent down-dip simple shear deformation, the shear strain rate (γ̇) can be expressed in terms of the shear strain (γ) accumulated across an idealized shear width (w), resulting from a finite slip displacement (d) parallel to the subduction interface over a given shearing duration (t). Thus, shear strain (γ) is defined as γ = d/w and the shear strain rate (γ̇) is given by γ̇ = γ/t82. Strain estimates for the micro-sheared metachert range from 0.71 to 2.69, as constrained by syn-kinematic microstructures shown in Supplementary Fig. S10 and the calculation method detailed in the Supporting Information. With constrained strain rates of 10−9 s−1, the process of micro-shearing is estimated to last 5–40 years. Using the relationships described above, the shear strain rate (γ̇) along the subduction interface can be expressed in terms of slip rate (VS = d/t) and idealized shear width (w) as: γ̇ = VS/w.

40Ar–39Ar dating
Syn-kinematic phengite grains were manually selected under a binocular microscope from two crushed blueschist facies calcschist samples (B61 and B62). The phengite grains were ultrasonically cleaned with deionized water and methanol, weighed, placed into aluminum packets, and loaded into quartz vials for irradiation. Irradiation was conducted for 24 hours in the B4 position of the nuclear reactor at the Chinese Institute of Atomic Energy, Beijing, with a neutron flux density of ~2.65 × 1013 n cm−2 s−1. After irradiation, 40Ar–39Ar standard step analyses were performed using a Helix MC mass spectrometer at the Institute of Geology, CAGS. Correction factors for interfering isotopes were calculated during irradiation using irradiated K2SO4 and CaF2. Their values were: (36Ar/37Ar)Ca = 0.0002398, (40Ar/39Ar)K = 0.004782, and (39Ar/37Ar)Ca = 0.000806. All isotopic data are provided in Table S5.

Data availability
The data generated in this study are provided in this published article and its Supplementary Information files.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors argue for the occurrence of deformation at very high shear stresses within an exhumed subduction zone interface melange, at a depth range where stress/strength within the interface is usually considered low. The authors base their argument on structural and micro-structural observations of micro-shear zones. The stress state and strength of the subduction zone interface below the seismogenic zone is an area of ongoing research with implications for understanding the diverse slip styles (slow slip events, tremor, earthquakes) inferred to occur there globally. The authors’ arguments for the high stress state of these micro-shear zones are robust, and will be of interest to a wide range of researchers studying subduction zone mechanics and seismology. I am particularly convinced by how the range of processes throughout the melange are reconciled, such as the contrast in mica content and presence of veins/fluids between the blocks and matrix. I have only one main comment below, regarding the authors’ conclusions about the observed deformation being transient. The main conclusion of the paper regarding localised high stresses is robust and important, even without the point about transience, and I suggest the paper is published after minor revisions addressing this concern.\nMain comment:\nI am not convinced that there is evidence described in the paper that the micro-shear zones record transient deformation on the time-scale of years. The authors make this hypothesis and therefore relate deformation to slow slip events which occur on similar time-scales (L24, L300, L400). The 1-2 year time-scale is derived in L398 by assuming a strain of ‘at least 0.2 … for quartz dynamic recrystallisation to occur’, such that the years time-scale is a minimum. I do not see an explanation for why the micro-shear zones could not have been deforming on longer geological time-periods. If there is a micro-structural or field constraint on the maximum strain supporting this inference, then the authors should detail this. The formation and breaking of force chains modelled in papers cited by the authors (e.g., Beall et al. 2019) tends to occur at almost an order of magnitude larger strains than described here. The occurrence of slow slip also requires the build-up and release of elastic strain, which is difficult to explain by viscous processes without a sudden loss of strength usually attributed to rapid weakening through fracturing or fluid release/ingress. There is no explanation of rapid weakening in the paper.\nMinor comments:\nL51 – ‘higher-temperature SSEs’ – It is unclear what this terminology refers to. If you mean SSEs occurring at the base of the seismogenic zone, rather than SSEs occurring up-dip of the seismogenic zone near the trench, then please clarify.\nL75 – ‘by 26,35’ – This is a minor stylistic comment that you could include the cited authors names if allowed by the journal as this currently reads awkwardly. This occurs through the paper.", "reviewer_2": "Wu et al\nStress heterogeneities in exhumed high-pressure rocks shed light on deep subduction interface transient coupling\nReview for Nature Communications.\nGeneral Comments\nThis paper presents paleopiezometric data from metasedimentary rocks in the North Quilian paleosubduction zone in Tibet. The stress values they obtain from a tectonic block of metachert are significantly higher than those in the surrounding calc-schist matrix, and the authors suggest that this represents transient coupling during slow slip events along the subduction zone interface. The techniques they have applied to determine paleostresses are cutting edge, and their observations, measurements, and calculations all appear to be correct. The interpretation is more speculative; it is interesting and provocative, but it may need some more detailed explanation, and possibly rethinking (see below). Overall, I recommend publication after minor revisions.\nMy concern with the interpretation is as follows. The authors show that the metachert block has experienced substantially higher stresses and strain rates than the surrounding calc-schist matrix. They suggest that the metachert blocks were relatively strong, and interacted to form force-chains, amplifying the stresses in the blocks for short periods of time (1-2 years). They suggest that this resulted in transient strain-rate events that could correlate with geodetically observed slow slip events in modern subduction zones. My concern is, that if the blocks interacted in this way, the structure of the subduction channel would approximate what is known as the Voigt iso-strain-rate condition, where the blocks protect the matrix from the high stresses, so that the strain-rate in the blocks and matrix are approximately the same. This explains the high stresses in the blocks, but it doesn’t predict high strain rates in the blocks (300 times the strain-rate in the matrix, according to the authors’ calculations). If the strain-rate in the blocks is so high, they can hardly be described as ‘rigid’ as the authors do in this paper. I think the authors need to explain their mechanical model more carefully, or possibly revise it.\nDetailed comments\nLine 29. “decoupling zone”. This is common terminology among seismologists, but in the context of this paper, it is rather misleading. The zone of ductile shearing below the seismogenic zone involves coupling across the plate interface, in the sense that stress and deformation are transferred across it. The deformation reflects distributed shear, and the lack of seismicity means that there is no evidence of loss of continuity (i.e., slip) across the interface. The authors use “coupling” to refer to this viscous deformation elsewhere in the paper (e.g., line 25). I suggest the authors avoid using this term, as it is ambiguous and misleading.\nLines 112, 647. “thrust folds”. I’m not sure what the authors mean by this. Many folds are related, either directly or indirectly, to thrusting; for example fault propagation folds formed ahead of propagating thrusts, and ramp anticlines formed by thrusting over a ramp. But a fold is not a fault, so I think the term “thrust fold” is a contradiction in terms.\nLine 214. Not clear what “these” refers to. May be some text went missing here? The authors appear to be referring to Dauphiné twins.\nLines 220. The difference is stress is about a factor of 4, so with a stress exponent of 4, that gives a difference of strain rate of ~300. That’s large, but not 3 orders of magnitude, as suggested in the text.\nLine 289. “Rigid” means the material is so strong that it doesn’t deform. Given that these blocks show very high strain rates, this word does not seem appropriate. Maybe you should say “relatively strong”. But I don’t see how you explain the very high strain rates in the blocks if you propose that they are strong enough to form force-chains. That would mean that the strain-rates should be equal to, or less than, those in the matrix.\nFigure 2 (i). There is some discrepancy about grain sizes here. The histograms suggest modes around 10-12 µm for the recrystallized grains and subgrains in B61, which conflict with the values of 55 – 75 µm shown on the plot. The images shown in (a) and (b) suggest that B61 is quite coarse-grained, so it seems the problem is with the histograms. This needs sorting out.\nLines 660-661. The quartz veins are recrystallized by SGR; this is not a deformation mechanism. The deformation mechanism is dislocation creep.\nLine 664. Please show or explain what the reference frame is for the CVA plots.\nLine 671. The rectangle is in part (e), not part (a).\nFigure 3 (d). The resolution in the figure is quite low, and the letters A-A’ and B-B’ are quite illegible in the review copy of the manuscript. Similarly, the plot at the top right of part (d) is completely illegible. In the source file of the figure, these features are visible (if you zoom in a lot). The authors and the production staff will need to work together to make sure the figures are legible in the published version. I would suggest (a) reducing the amount of information in the figure; for example little use is made of the misorientation plot and the MAD plot in the manuscript, and multiple histograms for each sample are not really necessary, and (b) break the figure up into multiple figures so that the individual parts are larger. Much the same is true of Figure 2 and several of the others: they are too “busy” and have too much information in them, not all of which is necessary.\nJohn Platt, March 24, 2025.", "reviewer_3": "This paper is about paleopiezometry in quartz-bearing rocks in an exhumed mélange in China, which is used to document stress heterogeneities in subducted blocks at the plate interface. The paper integrates EBSD mapping of quartz with pseudosection modelling and mica Ar/Ar dating. I have read the paper with interest, I think it is well written, the methodology used up to date, and the quality of the data is good. However, I think the authors make strong assumptions that are never discussed in the manuscript, and there are methodological errors related to the pseudosection modelling and Ar/Ar data. For this reason, I think they need more data to document their model, and to restructure the paper to better address the limitations of their approach. I think a new pseudosection in the MnNCKFMASTHO system is needed, using the chemistry of all the minerals that are available. I also think that they cannot prove the model they propose, about stress heterogeneities along the subduction interface based on this data alone.\nFirst and foremost, the authors use EBSD data from (1) quartz veins in calcschists, (2) metacherts, and (3) micro shear zones in metacherts, and use paleopiezometry and flow laws to estimate differential stress and strain rates from quartz from these rocks. There are two strong assumptions here that are never discussed. First, by analyzing the differential stress recorded by quartz veins in calcschists, it is assumed that those veins registered the same differential stress of the surrounding carbonate-rich matrix, but it is very likely that there was strain partitioning between quartz and calcite, which makes it very difficult to compare this rock with the quartz-rich metacherts. It is like comparing apples and oranges. Second, these microstructures are not necessarily coeval. Micro shear bands may register the latest strain increments in a shear zone, as documented in SC fabrics in the South Armorican shear zone (Berthé et al., 1979; Bukovska et al., 2016).\nSecond, another strong limitation is that all these rocks were deformed at the same time and that, as such, they register high stresses in the metachert blocks and low stresses in the calcschist matrix, but this assumption is too strong, since the authors could be looking at tectonic slices that were accreted subsequentially from the plate interface and that contain – and this may occur even if the Ar/Ar mica ages from calcschists and metacherts are undistinguishable (Figure S5), because the associated uncertainty of +- 3 Ma does not allow to tell for sure that deformation occurred at the same time (for example, the blocks may have been accreted 1 or 2 Ma after one another). Rapid and subsequent accretion of slices in subduction zones may be expected. There are examples of this from the Sanbagawa belt of Japan for instance (Special volume about Sanbagawa and Ryoke belts on Elements, 2024).\nAnother strong limitation of this approach is that the Ar/Ar date does not necessarily represent the peak of contact metamorphism, but may represent a cooling age when the blocks were being exhumed together in the subduction channel above the plate interface. In this case, the age is the same because they are dating retrograde metamorphism. This would be consistent with the observation that the closure temperature of mica in the Ar/Ar system (e.g. Harrison et al., 2009) is lower than the peak metamorphism recorded by RSCM thermometry, which is nearly 500 °C (Figure 4).\nTo better document if deformation occurred at the same time, the authors needs to rely on other dates of peak metamorphism (e.g. Lu/Hf garnet dating, monazite dating) or even to check if the sedimentary age of the different protoliths is similar or very different with U/Pb zircon dating. If the protoliths have very different ages, the authors are likely looking at different blocks affected by retrograde metamorphism above the plate interface.\nFor the same reason, I have concerns that pseudosection modelling and RSCM thermometry is providing the peak metamorphic temperature. There might be a mismatch between RSCM T, which records the peak metamorphic temperature reached by a sample (which may occur during retrograde metamorphism) and metamorphic assemblage, which may keep the record of the entire PTt path, including the pressure peak. Therefore, it is incorrect to use the best fit between mica isopleths and RSCM temperatures as in Figure b. The measured mica isopleths are stable in most of the pseudosection calculated in Figure 4b, and according to Fig. 4a, there are different generations of white mica. So, why not using these generations to reconstruct a PT path appropriately? I am also surprised by the fact that according to the modelling there is free garnet in this sample (Figure 4b), but I am very surprised that garnet compositions are not shown, because garnet isopleths are very reliable to estimate the PT conditions of metamorphism.\nMoreover, when building a pseudosection, it is good practice to provide the mineral chemistry of all minerals in the rock and discuss it to check if it fits or not with the observed assemblage. This should be done with glaucophane, chlorite, and biotite, which are expected to be present based on modelling.\nI have also noted that the authors modelled the system in the NCKFMASH system and do not model nor provide information about Mn, Ti, and Fe2+/Fe3+ (O). This is tricky, because the presence of Mn could lower the stability of minerals significantly (stabilizing for instance spessartine garnet at lower T) and Fe2+/Fe3+ affects the stability of amphibole, garnet, chlorite, and muscovite, and mica isopleths. So why not measuring and using Mn and O for the modelling?\nI think there is not enough discussion about the limitations of paleopiezometry and flow laws, which have a large associated uncertainty related to (1) separating recrystallized grain size and relic grains (Cross et al., 2017), (2) estimating radii from sections without knowing the 3D shape of grains, and (3) the uncertainty associated with power laws. In general, it is good practice to try to use different relationships as done by Stipp (2002a, b) in the Tonale Fault. It is also necessary to check if the rocks involved where deformed at the same fluid conditions. For example, if we are in fluid-saturated conditions or not, as this strongly affects the recrystallization of quartz (Stipp et al., 2006, 2010; Menegon et al., 2011; Austrheim papers).\nRegarding the calcschists, I am also concerned by the fugacity of CO2 and H2O in the fluid, which may affect calcite and quartz behavior (Pennacchioni paper: Why Calcite can be stronger than quartz?). Crucially, there are no microphotographs showing the microstructure of the calcschists. Indeed, the mismatch between paleostress measurements with this approach and differential stresses measured in modern subduction zones might indicate that there is an intrinsic error with the method that it is not addressed.\nFinally, the author only discuss one model proposed for the formation of SSE at the subduction interface, the interaction between strong blocks in a weak matrix, which is only one of the models proposed. Crucially, they do not provide evidence of interaction between blocks in their rocks, so why do they focus on this model only when there could be other explanations related to e.g. fluid activity/fluid overpressure, strain partitioning, accretion of different blocks with different microfabrics and so on.\nAnother minor -but important- observation: I do not think it is necessary to stress that recrystallization is by SGR if you have EBSD data, as when you have those small micro-shear bands, you need to do a very detailed microstructural work to prove this. For example, to me, you have both BLG and SGR (Stipp et al., 2002a, b), but it is still dislocation creep, so it is not important.\nFor the reasons stated above, I do not think the data provided in the paper can be easily tied to the subduction interface or be demonstrated that the quartz microfabrics on which EBSD was performed were coeval and all representative of deformation at the plate interface, as most of this may have also been occurred in the exhumation channel above the plate interface and over a range of PT conditions.\nBut the quartz EBSD data shown is of excellent quality and I think the authors can use it to address issues pertaining to quartz deformation, such as the origin and propagation of micro shear bands or SC fabrics in a deforming rock (e.g. papers by Trepmann et al., Hentschel et al., Finch et al. etc.; Ceccato et al., Kjoll et al., etc.)."}, "author_response": "Point-by-point response to reviewers’ comments for\nNature Communications Manuscript: NCOMMS-25-11434\nManuscript Title: Stress heterogeneities in exhumed high-pressure rocks shed light\non deep subduction interface transient coupling\nAuthors: Yawei Wu, Samuel Angiboust, Jianxin Zhang, Bo Zhang, Xiaohong Mao,\nOnno Oncken\nDear Reviewers,\nWe sincerely thank you for your thoughtful evaluation of our manuscript. Your\nconstructive comments and suggestions have substantially improved this work. Please\nfind below our point-by-point responses, shown in blue. Corresponding revisions to the\nmanuscript are indicated in red where appropriate to address your inquiries.\nReviewer #1 (Remarks to the Author):\nThe authors argue for the occurrence of deformation at very high shear stresses within\nan exhumed subduction zone interface melange, at a depth range where stress/strength\nwithin the interface is usually considered low. The authors base their argument on\nstructural and micro-structural observations of micro-shear zones. The stress state and\nstrength of the subduction zone interface below the seismogenic zone is an area of\nongoing research with implications for understanding the diverse slip styles (slow slip\nevents, tremor, earthquakes) inferred to occur there globally. The authors’ arguments\nfor the high stress state of these micro-shear zones are robust, and will be of interest to\na wide range of researchers studying subduction zone mechanics and seismology. I am\nparticularly convinced by how the range of processes throughout the melange are\nreconciled, such as the contrast in mica content and presence of veins/fluids between\nthe blocks and matrix. I have only one main comment below, regarding the authors’\nconclusions about the observed deformation being transient. The main conclusion of\nthe paper regarding localised high stresses is robust and important, even without the\npoint about transience, and I suggest the paper is published after minor revisions\naddressing this concern.\nMain comment:\nI am not convinced that there is evidence described in the paper that the micro-shear\nzones record transient deformation on the time-scale of years. The authors make this\nhypothesis and therefore relate deformation to slow slip events which occur on similar\ntime-scales (L24, L300, L400). The 1-2 year time-scale is derived in L398 by assuming\na strain of ‘at least 0.2 … for quartz dynamic recrystallisation to occur’, such that the\nyears time-scale is a minimum. I do not see an explanation for why the micro-shear\nzones could not have been deforming on longer geological time-periods. If there is a\nmicro-structural or field constraint on the maximum strain supporting this inference,\nthen the authors should detail this. The formation and breaking of force chains modelled\nin papers cited by the authors (e.g., Beall et al. 2019) tends to occur at almost an order\nof magnitude larger strains than described here.\nResponse: As Reviewer #1 insightfully pointed out, determining the credible strain\nrange of micro-sheared metachert from its microstructural evidence would be pivotal\nfor constraining the timescale of micro-shearing. Although the metachert is generally\nhomogeneous in mineral composition, a few representative strain domains were\nnevertheless identified in the X–Z thin section of the micro-sheared metachert sample\nC13, including the syn-kinematic microstructures: (a) a σ-type porphyroclast composed\nof micro-sheared aggregates of quartz and phengite (Fig. R1a); and (b) a σ-type\nporphyroclast consisting of an elongated magnetite crystal rimmed by recrystallized\nquartz (Fig. R1c).\nThe selection of appropriate mineral strain markers is critical for finite strain analysis.\nGiven its high modal abundance and internal uniformity, quartz is the most reliable\nindicator of whole-rock strain in micro-sheared metachert (e.g., Ramsay and Huber,\n1983; Robin and Cruden, 1994; Bailey and Eyster, 2003). In the X–Z thin section of the\nmicro-sheared metachert sample C13, a σ-type porphyroclast—comprising micro-\nsheared aggregates of quartz and phengite—has been analyzed using the Fry method\n(Fry, 1979) to determine the whole-rock strain ellipse (Fig. R1a–b), where (18.3°)\n𝜃𝜃\nrepresents the angle between the long axis of the strain ellipse and the y-axis.\nWe assume that the original siliceous rock was homogeneous (with a strain ellipse axial\nratio of 1) and was subjected solely to simple shear strain ( ). Under simple shear,\nthe\n𝛾𝛾\ndeformation gradient tensor F is expressed as (Fig. R1e):\n1 𝛾𝛾\n𝐅𝐅 =  \nThe corresponding Green–Lagrangian strai0n ten1sor E is calculated by:\n1 𝑇𝑇 1 1 𝛾𝛾 1 0 1 0 𝛾𝛾\n𝐄𝐄 = (𝐅𝐅 𝐅𝐅−𝐈𝐈) = ( 2− ) =  2\nThe principal dir2ections corresp2ond𝛾𝛾 to 1th+e 𝛾𝛾eigenve0cto1rs of 2the𝛾𝛾 sym𝛾𝛾metric Green–\nLagrangian strain tensor E. The angle ( ) between the long axis of deformed strain\n𝜑𝜑\nellipse and the y-axis can be calculated using the following equation:\n2𝐸𝐸𝑥𝑥𝑥𝑥\n2𝛾𝛾 −2\ntan 2𝜑𝜑 = = 2 =\n𝐸𝐸𝑥𝑥𝑥𝑥 −𝐸𝐸𝑥𝑥𝑥𝑥 0−𝛾𝛾 𝛾𝛾\nThe relationship between and , where is the angle between the long axis of the\n𝜑𝜑 𝜃𝜃 𝜃𝜃\nstrain ellipse and the x-axis, is given by:\n𝜑𝜑 = 90°− 𝜃𝜃\nApplying the trigonometric identity:\ntan 2𝜑𝜑 = tan(180°− 2𝜃𝜃) = −tan 2𝜃𝜃\nThus, the simple shear strain can be expressed as:\n(𝛾𝛾)\n2\ntan 2𝜃𝜃 =\nThis relation is also derived and presented in Ra𝛾𝛾msay and Huber (1983), Equation 2.5.\nBy substituting the Fry method obtained angle into the derived equation,\n𝜃𝜃 = 18.3°\nthe estimated simple shear strain ( ) is 2.69. Since this strain estimate is based on the\n𝛾𝛾\nassumption that deformation was fully accommodated by 100% simple shear—as\nopposed to natural general shear, which typically involves a combination of simple and\npure shear—the calculated shear strain should be considered the maximum possible\nbulk strain for the rock.\nOn the other hand, σ-type porphyroclasts of magnetite crystals rimmed by recrystallized\nquartz are also observed in the thin section of sample C13 (Fig. R1c). It is well\nestablished that magnetite belongs to the cubic crystal system, indicating that the aspect\nratio of magnetite crystals prior to deformation was originally equal to 1. In Fig. R1d,\nthe best-fit ellipse of the deformed magnetite crystal was determined using MATLAB’s\nImage Processing Toolbox, yielding a long-to-short axis ratio (R ) of 2.93. The\nX-Z\nequation used to calculate the elongation strain ( ) of this magnetite crystal relative to\n𝑒𝑒\nits original (undeformed) state is derived as follows (Fig. R1d–e):\nThe magnetite crystal was assumed to be originally equant (i.e., assumed ,\n𝑎𝑎 = 𝑏𝑏 = 1\nthus ), and deformed into an ellipse with a measured axial ratio of\n′ ′\n𝑎𝑎/𝑏𝑏 = 1 𝑎𝑎 /𝑏𝑏 =\n. We further assume that the deformation occurred under area-conserving\n2.93\nconditions (plane strain; Fig. R5). As described by equation 3-27 of Ramsay (1967), the\nchange in area due to deformation is expressed as:\n1/2\nΔ = (𝜆𝜆1𝜆𝜆2) −1 = 0\nwhere represents the area change, and and are the quadratic elongation of the\nΔ 𝜆𝜆1 𝜆𝜆2\nlong and short axes of the deformed ellipse, respectively. The lengths of the deformed\nlong and short axes are given by and , where\n′ ′\n𝑎𝑎 = 1+𝑒𝑒1 = √𝜆𝜆1 𝑏𝑏 = 1+𝑒𝑒2 = √𝜆𝜆2\nand are the elongation strains in the respective directions.\n𝑒𝑒1 𝑒𝑒2\nIt can be easily derived from the above equations that , implying ,\n′ ′ ′ ′\n𝑎𝑎 ⋅𝑏𝑏 = 1 𝑏𝑏 = 1/ 𝑎𝑎\nand = 2.93.\n′ ′ ′2\n𝑎𝑎 /𝑏𝑏 = 𝑎𝑎\nSolving for , we obtain: . Therefore, the elongation strain ( ) is\n′ ′\n𝑎𝑎 𝑎𝑎 = √2.93 = 1.71 𝑒𝑒\ncalculated as:\n′\n𝑎𝑎 −𝑎𝑎 1.71−1\n𝑒𝑒 = = = 0.71\nGiven that magnetite is rheologically stronger than quartz at high temperatures (Till and\n𝑎𝑎 1\nRybacki, 2020), this estimated elongation strain ( ) should serve as a lower-\n𝑒𝑒 = 0.71\nbound estimate of the bulk strain experienced by the micro-sheared metachert.\nOverall, the plausible range of strain values for the micro-sheared metachert (Sample\nC-13) is estimated to lie between 0.71 and 2.69. This constrained range of strain values\naligns with the shear strain of 1.75 required for force chain formation in the modeling\nstudy by Beall et al. (2019).\nRegarding the timescale of micro-shearing, as noted in Lines 337 and 446, given the\nconstrained strain rates on the order of 10−9 s−1, the revised strain estimates (0.71–2.69)\ncorrespond to a micro-shearing duration of approximately 5 to 40 years. Hence, we\npropose that the micro-shearing was not sustained over geological timescales of\nmillions of years, but rather occurred over a shorter duration, likely comparable to the\ntimescales of long-term slow slip events (e.g., ~5 years for Rousset et al., 2019; ~15\nyears for Shelly, 2017). These timescales are also broadly consistent with the duration\nof post-seismic relaxation processes documented following megathrust ruptures, such\nas the 1960 Great Chile Earthquake (Wang et al., 2007).\nThe discussion on the timescale of micro-shearing has been updated accordingly in the\nmanuscript and its methods section (Lines 337 and 446), and the strain estimation\nprocess—including the relevant measurement methods, figure (Fig. S10), and\nequations—has been provided in the Supplementary Information.\nFig. R1: Strain estimation for the oriented X–Z plane of MSMC sample C13 at sites B and C.\n(a) Micro-sheared quartz and phengite aggregate showing selected grain centroids (red dots), used\nas input for the Fry method. (b) Fry plot of sample C13-B with a fitted strain ellipse (red) generated\nusing the software EllipseFit v 3.5.2. (c) σ-type porphyroclast defined by an elongate magnetite\n(Mag) crystal mantled by recrystallized quartz grains was observed in sample C13-C. (d) Schematic\nof the elongate magnetite crystal (white area) and its best-fit ellipse derived using MATLAB’s\nImage Processing Toolbox. (e) Idealized simple shear model (modified after Fossen, 2010), illustrating the\ndeformation of a block containing an initial reference circle into an ellipse. Simple shear strain,\n, where is the angle between the originally perpendicular line and deformed block line.\n𝛾𝛾 =\nThe is the angle between the long axis of the deformed ellipse and the vertical axis. The angle\ntan 𝜓𝜓 𝜓𝜓\nbetween the long axis of the deformed ellipse and the horizontal axis is related to shear strain ( )\n𝜑𝜑\nby the equation: (Ramsay & Huber, 1983).\n𝜃𝜃 𝛾𝛾\ntan2𝜃𝜃 =2⁄𝛾𝛾\nThe occurrence of slow slip also requires the build-up and release of elastic strain,\nwhich is difficult to explain by viscous processes without a sudden loss of strength\nusually attributed to rapid weakening through fracturing or fluid release/ingress. There\nis no explanation of rapid weakening in the paper.\nResponse: We appreciate Reviewer #1’s kind comments regarding the potential rapid\nweakening mechanisms associated with slow slip events occurring along the deep\nsubduction channel. However, we would like to clarify that in this study, we do not\ndirectly link the micro-shearing observed in the block to slow slip events (SSEs)\nthemselves; rather, we suggest that the micro-shearing may be related to, or even\ntriggered by, large-scale SSEs (or post-seismic stress relaxation) occurring in the\nsurrounding faulted matrix network. The reasons are as follows: (1) there is no direct\nevidence of SSEs within the studied block, such as fracturing and/or cataclasis; (2)\ninstrumental observations of SSEs have identified rupture lengths ranging from several\nkilometers to hundreds of kilometers, with rupture thicknesses of a few meters to\nseveral tens of meters (e.g., Chalumeau et al., 2024; Huang et al., 2025), which are\nfar larger than the micro-shearing we observed.\nConversely, we suggest that SSEs, likely accompanied by cataclasis, occurred along\nslipping planes within the calcschist matrix, potentially facilitated by rapid weakening,\neven though direct evidence for cataclasis is scarce. The trace of cataclasis was probably\nerased due to repeated shearing associated with multiple seismic cycles, which led to\nthe formation of a recrystallized, foliated matrix that has largely overprinted earlier\nmarkers of brittle events associated with rapid weakening (see Kirkpatrick and Rowe,\n2013; Oncken et al., 2022; and Angiboust et al., 2024 for further discussion on this\nmatter). Such velocity-weakening behavior can lead to a loss of cohesion within the\nmaterial, ultimately resulting in brittle failure and the release of accumulated elastic\nstrain (Marone, 1998). High pore fluid pressures in the calcschist matrix could have\nplayed a key role in maintaining the system in a critical state (λ > 0.97, Fig. 5b), close\nto rupture. Nearly lithostatic pore fluid pressures are also supported by the presence of\nnumerous vein sets in the calcschist. We stress that the main point of the paper is not\nthe report of potential SSE in the matrix but the finding of stress-related micro-shear\nzones within the relatively strong metachert blocks. We now better explain in the\nmanuscript that the concept of having SSEs in the matrix is a hypothesis, which beyond\nthe fact that it is totally supported by geophysical data, cannot be directly demonstrated\nhere from the field record (Lines 288 and 347).\nMinor comments:\nL51 – ‘higher-temperature SSEs’ – It is unclear what this terminology refers to. If you\nmean SSEs occurring at the base of the seismogenic zone, rather than SSEs occurring\nup-dip of the seismogenic zone near the trench, then please clarify.\nResponse: We indeed refer to SSEs that occur at the base of the seismogenic zone,\nrather than those located up-dip near the trench. To clarify this point, we have added\nthe phrase “occurring at the base of the seismogenic zone,” after “higher-temperature\nSSEs” as suggested (Line 53).\nL75 – ‘by 26,35’ – This is a minor stylistic comment that you could include the cited\nauthors names if allowed by the journal as this currently reads awkwardly. This occurs\nthrough the paper.\nResponse: We have revised the stylistic issue accordingly by either deleting phrases\nsuch as “has been shown by” or incorporating the cited authors' names where\nappropriate, as suggested (Lines 76, 212, and 227).\nReviewer #2 (Remarks to the Author):\nWu et al\nStress heterogeneities in exhumed high-pressure rocks shed light on deep subduction\ninterface transient coupling\nReview for Nature Communications.\nJohn Platt, March 24, 2025.\nGeneral Comments\nThis paper presents paleopiezometric data from metasedimentary rocks in the North\nQuilian paleosubduction zone in Tibet. The stress values they obtain from a tectonic\nblock of metachert are significantly higher than those in the surrounding calc-schist\nmatrix, and the authors suggest that this represents transient coupling during slow slip\nevents along the subduction zone interface. The techniques they have applied to\ndetermine paleostresses are cutting edge, and their observations, measurements, and\ncalculations all appear to be correct. The interpretation is more speculative; it is\ninteresting and provocative, but it may need some more detailed explanation, and\npossibly rethinking (see below). Overall, I recommend publication after minor revisions.\nMy concern with the interpretation is as follows. The authors show that the metachert\nblock has experienced substantially higher stresses and strain rates than the surrounding\ncalc-schist matrix. They suggest that the metachert blocks were relatively strong, and\ninteracted to form force-chains, amplifying the stresses in the blocks for short periods\nof time (1-2 years). They suggest that this resulted in transient strain-rate events that\ncould correlate with geodetically observed slow slip events in modern subduction zones.\nMy concern is, that if the blocks interacted in this way, the structure of the subduction\nchannel would approximate what is known as the Voigt iso-strain-rate condition, where\nthe blocks protect the matrix from the high stresses, so that the strain-rate in the blocks\nand matrix are approximately the same. This explains the high stresses in the blocks,\nbut it doesn’t predict high strain rates in the blocks (300 times the strain-rate in the\nmatrix, according to the authors’ calculations). If the strain-rate in the blocks is so high,\nthey can hardly be described as ‘rigid’ as the authors do in this paper. I think the authors\nneed to explain their mechanical model more carefully, or possibly revise it.\nResponse: Mixing models for estimating the elastic properties of single-phase\npolycrystalline materials have been developed based on the assumption of uniform\nstrain rate (iso-strain or Voigt bound) or uniform stress (iso-stress or Reuss bound)\n(Ioannidi et al., 2021). However, in the significantly more complex natural context of\nsubduction channel systems, spatial heterogeneity, localized stress concentrations, and\nthe dynamic evolution of phase connectivity can lead to mechanical behaviors that\ndeviate substantially from these idealized bounds. The force chain model of block\ninteractions, as interpreted in the manuscript, suggests that failure of an individual force\nchain does not necessarily lead to system-wide stress release (Beall et al., 2019). Instead,\nother load-bearing chains remain active and continue to constrain the matrix, thereby\npreventing a uniform increase in matrix strain rate. As a result, the overall strain\ndistribution diverges from that predicted by a Voigt-type iso-strain-rate model.\nPrevious mechanical studies (Beall et al., 2019; Ioannidi et al., 2021) have shown that\none needs more than 50% of rigid elements to enable the formation of Voigt-type\ninteraction. At the North Qilian paleo-subduction interface, our field observations (Fig.\n1a) indicate that the block content in the mélange is heterogeneous and does not\nsystematically reach the critical threshold (~50%). This suggests that such localized\nforce-chain phenomena could not have persisted over prolonged durations, and that the\nbulk of the matrix has never been ‘protected’ by the Voigt-type model of hard blocks.\nWe thus postulate that the low-stress, low-strength portion of the deep subduction\ninterface matrix that was largely not affected by localized force chain dynamics (the\ngreen strain rate line shown in Fig. 6), which is only visible in metachert, stronger\nblocks. The related discussion has been updated in the manuscript (Line 317). The\ncaption of figure 6 (inset) has been also updated in order to explain that the sketch\ndepicts the timing of the block interaction and micro-shear zone formation, making\nclear that it does not correspond to a steady-state mechanical regime, but rather a\npunctual event occurring from time to time when a block-rich segment of the\ndowngoing crust subducts. In the paper, we now also leave the door open for other\nprocesses, like asperity subduction (e.g., seamount), that may also occasionally locally\nimpact interface coupling and leave an imprint in the rock record–even though this is\nnot our preferred option.\nDetailed comments\nLine 29. “decoupling zone”. This is common terminology among seismologists, but in\nthe context of this paper, it is rather misleading. The zone of ductile shearing below the\nseismogenic zone involves coupling across the plate interface, in the sense that stress and\ndeformation are transferred across it. The deformation reflects distributed shear, and the\nlack of seismicity means that there is no evidence of loss of continuity (i.e., slip) across\nthe interface. The authors use “coupling” to refer to this viscous deformation elsewhere\nin the paper (e.g., line 25). I suggest the authors avoid using this term, as it is ambiguous\nand misleading.\nResponse: Thank you for the meticulous comment. We agree that the term ‘decoupling\nzone’, though commonly used in seismology, may be misleading in the context of our\nstudy, as it can imply a lack of mechanical interaction across the deep subduction\ninterface. In this study, the deep portion of the subduction interface we describe is\ncharacterized by viscous deformation, accompanied by stress transfer and mechanical\ncoupling that, although reduced compared to the shallower seismogenic zone, still\npersists. To avoid this ambiguity and to more accurately reflect the distributed, ductile\nshearing behavior observed in this region, we have replaced ‘decoupling zone’ with\n‘creeping zone’ in Line 29.\nLines 112, 647. “thrust folds”. I’m not sure what the authors mean by this. Many folds\nare related, either directly or indirectly, to thrusting; for example fault propagation folds\nformed ahead of propagating thrusts, and ramp anticlines formed by thrusting over a\nramp. But a fold is not a fault, so I think the term “thrust fold” is a contradiction in\nterms.\nResponse: As you rightly pointed out, the term “thrust folds” is not appropriate in this\ncontext. We have replaced “thrust folds” with “thrust-related folds” to more accurately\ndescribe folds that formed synkinematically with thrusting (Lines 114, 726, and Figure\n1e).\nLine 214. Not clear what “these” refers to. May be some text went missing here? The\nauthors appear to be referring to Dauphiné twins.\nResponse: For clarification, we have added “estimates, derived from micro-shear zones”\nafter “these shear stresses” (Line 221). These stress values remain consistent with the\nstress range in which Dauphiné twins form in experimental studies (e.g., Minor et al.,\n2018), further supporting the reliability of our stress estimates.\nLines 220. The difference is stress is about a factor of 4, so with a stress exponent of 4,\nthat gives a difference of strain rate of ~300. That’s large, but not 3 orders of magnitude,\nas suggested in the text.\nResponse: We have replaced ‘strain rates spanning at least three orders of magnitude’\nwith ‘a strain rate difference of approximately 600-fold’, as suggested (Line 229). The\nchange in strain rate difference (from ~300 to ~600) stems from reprocessing of the\nEBSD data, updated grain size distributions, and a revised estimate of the flow stress\nfor sample B-61 (see details below).\nLine 289. “Rigid” means the material is so strong that it doesn’t deform. Given that\nthese blocks show very high strain rates, this word does not seem appropriate. Maybe\nyou should say “relatively strong”. But I don’t see how you explain the very high\nstrain rates in the blocks if you propose that they are strong enough to form force-chains.\nThat would mean that the strain-rates should be equal to, or less than, those in the matrix.\nResponse: Indeed, we have replaced “rigid” with “relatively strong” throughout the\nmanuscript to better reflect the mechanical properties of the blocks (Lines 309 and 314).\nFigure 2 (i). There is some discrepancy about grain sizes here. The histograms suggest\nmodes around 10-12 µm for the recrystallized grains and subgrains in B61, which\nconflict with the values of 55 – 75 µm shown on the plot. The images shown in (a) and\n(b) suggest that B61 is quite coarse-grained, so it seems the problem is with the\nhistograms. This needs sorting out.\nResponse: We appreciate your helpful comments on the grain size mismatch between\nthe grain size histograms, their RMS value, and the coarse-grained microstructure\nobserved in sample B61. Upon closer examination of the EBSD data, we found that this\ndiscrepancy results from pseudo-phase identification during EBSD mapping.\nSpecifically, under certain orientations, accessory minerals (e.g., phengite) may\nproduce Kikuchi patterns that partially overlap with those of quartz, leading to pseudo-\nsymmetric misindexing, where these phases are incorrectly identified as fine-grained\nquartz in the raw EBSD data. This phenomenon of pseudo-symmetric mis-indexing is\nalso reported in the Oxford Instruments technical note (Addressing pseudo-symmetric\nmis-indexing in the EBSD analysis of gamma-TiAl - Nanoanalysis - Oxford\nInstruments) as well as the EBSD study of natural deformed rock (Dempsey et al., 2011).\nThis leads to a misleading shift toward smaller grain sizes for the recrystallized grains\nand subgrains in the histograms.\nTo address this, we refined the post-processing workflow for sample B-61 in\nAZtecCrystal by first removing grains smaller than 4 pixels, followed by manually\ninspecting and batch-removing misindexed, spurious small ‘quartz’ grains located\nwithin phengite domains. For instance, the comparison between the previous version of\nthe EBSD map, the reprocessed EBSD map, and the corresponding micrograph of the\nzoomed-in area in B61-A clearly shows that the misindexed ‘quartz grains’ originating\nfrom the phengite domains have been effectively eliminated (Fig. R2). Following this\npost-processing workflow, the EBSD datasets of B61-A and B61-B were reprocessed.\nTheir EBSD-derived maps, CPO patterns, and grain size histograms have been updated\nin Fig. 2 of the manuscript, as well as in Fig. S3 and Table S5 of the Supplementary\nInformation.\nThe EBSD-derived maps and CPO patterns remain consistent before and after data\nreprocessing. On the other hand, the RMS values of recrystallized grain sizes for\nsamples B61-A and B61-B increased systematically from 55–76 μm to 100–112 μm, as\nexpected after the elimination of misindexed small grains, with peak modes typically\ncentered around 100 μm, in good agreement with the grain size features observed in\noriented photomicrographs. This adjustment in grain size affected the piezometric\nestimation, resulting in a slight reduction in the inferred differential stress for sample\nB61—from 36–40 MPa to 23–25 MPa. These updated grain sizes and stress values were\nalso incorporated into the estimation of the flow law and pore fluid pressure factor (λ)\nthroughout the revised manuscript.\nFig. R2: Comparison of misindexed grains before and after data reprocessing in M2M maps\nof quartz shear veins within the calcschist matrix (B61-A). (a) Photomicrograph of the analyzed\nsite (B61-A) showing the quartz shear vein with foliated epidote (Ep); the red dashed box marks the\nregion magnified in b. (b) The zoomed-in area of B61-A, which shows the phengite (Phe) platelets\ndistributed within the quartz aggregate. (c) The previous version of the M2M map (enlarged area\nfrom b) prior to reprocessing, in which misindexed pixels—represented as small ‘quartz grains’\nlocated within phengite domains—are highlighted by white arrows. (d) Reprocessed M2M map\nillustrating the effective elimination of misindexed ‘quartz grains’ (red arrows in phengite domains).\nLines 660-661. The quartz veins are recrystallized by SGR; this is not a deformation\nmechanism. The deformation mechanism is dislocation creep.\nResponse: We replaced ‘subgrain rotation recrystallization (SGR)’ by ‘dislocation\ncreep’ as suggested (Line 224).\nLine 664. Please show or explain what the reference frame is for the CVA plots.\nResponse: The spatial reference framework used in the CVA plot is depicted in the\nlegend of Fig. 2 (bottom right). A corresponding caption has been added in Line 757 to\nclarify this, stating: “the spatial relationship between the subhorizontal CVA (red line\nterminated with black triangles at both ends) and the geometry of the North Qilian\npaleosubduction channel.”\nLine 671. The rectangle is in part (e), not part (a).\nResponse: We confirm that the EBSD-related plots in Fig. 2d correspond to sample\nB61-A (quartz shear veins within a calcschist matrix). Therefore, the rectangle\nindicating the sample location is correctly shown in part (a), not part (e). To avoid\nconfusion, we have revised the caption by replacing ‘box in part a’ with ‘location\nmarked by the rectangle in part a’ (Line 745).\nFigure 3 (d). The resolution in the figure is quite low, and the letters A-A’ and B-B’ are\nquite illegible in the review copy of the manuscript. Similarly, the plot at the top right\nof part (d) is completely illegible. In the source file of the figure, these features are\nvisible (if you zoom in a lot). The authors and the production staff will need to work\ntogether to make sure the figures are legible in the published version. I would suggest\n(a) reducing the amount of information in the figure; for example little use is made of\nthe misorientation plot and the MAD plot in the manuscript, and multiple histograms\nfor each sample are not really necessary, and (b) break the figure up into multiple figures\nso that the individual parts are larger. Much the same is true of Figure 2 and several of\nthe others: they are too “busy” and have too much information in them, not all of which\nis necessary.\nResponse: Following the reviewer’s constructive comments, we have revised and\nupdated Figure 3 and Figure S3 to improve overall clarity and readability. Specifically,\nthe original Figure 3e (GOS map) has been relocated to Supplementary Figure S3 to\nreduce the amount of information presented in the main figure and enhance visual\nclarity. In addition, the misorientation traverses (A–A′ and B–B′), which were\npreviously shown in the upper right corner of Figure 3d, have been enlarged for\nimproved legibility and are now presented as a standalone panel (Figure 3e). All\nassociated figure panels, captions, and in-text references have been updated accordingly\nin both the manuscript and Supplementary Information.\nReviewer #3 (Remarks to the Author):\nThis paper is about paleopiezometry in quartz-bearing rocks in an exhumed mélange in\nChina, which is used to document stress heterogeneities in subducted blocks at the plate\ninterface. The paper integrates EBSD mapping of quartz with pseudosection modelling\nand mica Ar/Ar dating. I have read the paper with interest, I think it is well written, the\nmethodology used up to date, and the quality of the data is good. However, I think the\nauthors make strong assumptions that are never discussed in the manuscript, and there\nare methodological errors related to the pseudosection modelling and Ar/Ar data. For\nthis reason, I think they need more data to document their model, and to restructure the\npaper to better address the limitations of their approach. I think a new pseudosection in\nthe MnNCKFMASTHO system is needed, using the chemistry of all the minerals that\nare available. I also think that they cannot prove the model they propose, about stress\nheterogeneities along the subduction interface based on this data alone.\nResponse: Please find our detailed responses below.\nFirst and foremost, the authors use EBSD data from (1) quartz veins in calcschists,\n(2) metacherts, and (3) micro shear zones in metacherts, and use paleopiezometry and\nflow laws to estimate differential stress and strain rates from quartz from these rocks.\nThere are two strong assumptions here that are never discussed. First, by analyzing the\ndifferential stress recorded by quartz veins in calcschists, it is assumed that those veins\nregistered the same differential stress of the surrounding carbonate-rich matrix, but it is\nvery likely that there was strain partitioning between quartz and calcite, which makes\nit very difficult to compare this rock with the quartz-rich metacherts. It is like\ncomparing apples and oranges.\nResponse: Thanks for this remark. We have completed our paper with thin section\nimages of the calcschist matrix (Fig. R3). In Fig. R3, the aggregate of near-equigranular\ncalcite and quartz grains form a load-bearing framework with a relatively high quartz\ncontent, and where quartz grains are commonly in contact with each other.\nExperimental studies have indicated that the strength of calcite–quartz aggregates\nincreases with increasing quartz grain fraction, reaching a peak at ~20 vol.% quartz\n(Rybacki et al., 2003). In the samples studied here, the quartz content of the aggregate\napproaches this threshold. Beyond this critical fraction, the bulk rheology of the\naggregates is largely controlled by the mechanical properties of the load-bearing\nframework (Handy, 1990), namely quartz. Hence, the microstructures observed suggest\nthat the strength of the calcschist matrix is mainly controlled by quartz, which would\nresult in a relatively homogeneous strain distribution without significant strain\npartitioning.\nIn addition, σ-type quartz porphyroclasts within the matrix of the calcite–quartz\naggregate are observed under the microscope (Fig. R3d), along with outcrop-scale\nboudinage structures and/or pinch-and-swell geometries of the quartz veins. These\nfeatures unambiguously demonstrate that the quartz veins remained mechanically\nstronger than the surrounding matrix during shearing.\nGiven the inapplicability of piezometers to polymineralic aggregates, and the\ncomparably coarse quartz grain sizes (several hundred micrometers) between the\ncalcite–quartz aggregates and the quartz shear vein and/or quartz-bearing domain, we\nselected the synkinematic quartz shear vein and/or the quartz-bearing domain for EBSD\nanalysis (B61-A and B61-B). In summary, these microstructural characteristics validate\nthe application of piezometers to the quartz shear vein and/or the quartz domain as\nreliable proxies for the upper bound of whole-rock strength.\nFig. R3: Oriented photomicrograph (X–Z plane) of the calcschist matrix in the NQMB. (a)\nQuartz (Qz) shear vein and the surrounding aggregate of calcite (Cal) and quartz (Qz), with the\nEBSD analysis site (B61-A) indicated. (b) Foliation composed of elongated calcite and quartz, with\nthe EBSD analysis site (B61-B) located in the quartz domain, marked by a rectangle in the upper\nright. (c) Aggregate of calcite and quartz with a relatively high quartz content, where quartz grains\nare in contact and form a load-bearing framework. (d) σ-type quartz porphyroclast within the matrix\nof calcite and quartz, indicating that quartz behaved mechanically stronger than the calcite–quartz\nmatrix. (e) Aggregate of near-equigranular quartz and calcite grains, with a fairly homogeneous\nspatial distribution.\nFurther clarification of this issue has been incorporated into the manuscript (Lines 136\nand 218), and thin section images (Fig. R3) have been added to the Supplementary\nInformation as Fig. S2.\nSecond, these microstructures are not necessarily coeval. Micro shear bands may\nregister the latest strain increments in a shear zone, as documented in SC fabrics in the\nSouth Armorican shear zone (Berthé et al., 1979; Bukovska et al., 2016).\nResponse: As noted, we examined the key studies on S–C fabrics in the South\nArmorican shear zone (Berthé et al., 1979; Bukovská et al., 2016), along with related\nliterature. These studies demonstrate that S- and C/C′-fabrics formed under distinct\nmetamorphic conditions—temperatures exceeding 550 °C for the S-fabric, and 300–\n350 °C at 100–400 MPa for the C/C′ shear bands. They also suggest that microcracks\nmay act as precursors to micro-shear bands (see also Fusseis and Handy, 2008).\nFollowing the initiation of early brittle microcracks under high differential stress (up to\ncreep, accompanied by a marked decrease in differential stress to 17–26 MPa in the\nSouth Armorican shear zone.\nIn contrast, our data indicate that quartz veins in calcschists and coarse-grained\nmetacherts, as well as micro-shear zones in metacherts, exhibit consistent kinematics,\nas shown by CPO and CVA plots (Figs. 2–3). While we are not certain that the micro-\nshear zones represent C-type shear bands, if they do, their formation appears to reflect\nkinematic continuity rather than the overprinting of an earlier planar fabric by a later\ncleavage, as observed in S–C fabrics from the South Armorican shear zone (e.g.,\nBukovská et al., 2016). Furthermore, syn-kinematic minerals in quartz veins within\ncalcschists, coarse-grained metacherts, and micro-shear zones consistently formed at\npeak metamorphic P–T conditions, without evidence of retrograde overprinting (see\ndetails below). No precursor microfractures are observed in the metacherts, and our\nsamples lack both macro- and microstructural evidence for polyphase ductile\ndeformation or retrograde overprinting. In addition, the high differential stress (109–\nmuch lower stress conditions (~10 MPa) reported for late-stage C′ fabrics by Bukovská\net al. (2016).\nTaken together, these observations favor an interpretation in which the microstructures\nreflect a single deformation event rather than a sequence of overprinting fabrics. The\nobserved heterogeneities in stress and strain between the micro-sheared metachert and\ncoarse-grained metachert are best explained by the influence of phyllosilicates (mica)\non deformation and strain localization, in line with both natural and experimental\nstudies (Tullis and Wenk, 1994; Holyoke and Tullis, 2006; Tokle et al., 2023;\nGottardi et al., 2024).\nSecond, another strong limitation is that all these rocks were deformed at the same time\nand that, as such, they register high stresses in the metachert blocks and low stresses in\nthe calcschist matrix, but this assumption is too strong, since the authors could be\nlooking at tectonic slices that were accreted subsequentially from the plate interface\nand that contain – and this may occur even if the Ar/Ar mica ages from calcschists and\nmetacherts are undistinguishable (Figure S5), because the associated uncertainty of +-\nexample, the blocks may have been accreted 1 or 2 Ma after one another). Rapid and\nsubsequent accretion of slices in subduction zones may be expected. There are\nexamples of this from the Sanbagawa belt of Japan for instance (Special volume about\nSanbagawa and Ryoke belts on Elements, 2024).\nResponse: We agree that the uncertainty inherent to Ar–Ar mica dating (±3 Ma) limits\nthe ability to conclusively demonstrate synchronous deformation of block and matrix.\nHowever, we still interpret the blueschist-facies unit—characterized by a homogeneous\nmetamorphic grade within the blueschist facies and composed of these lithologies—as\na snapshot of the transition zone (part of subduction interface) at a specific time (ca.\nrock types record the same P–T conditions without evidence of retrograde overprinting\n(Fig. R5, see discussion below) and (2) they exhibit comparable deformation patterns,\nsuggesting kinematic continuity (Fig. 2–3 and S3–5).\nIn contrast, the eclogite-facies unit represents a deeper portion of the subduction\ninterface (Fig. 1b) and was juxtaposed with the blueschist-facies unit during late-stage\nexhumation (as shown by the observation of localized greenschist-facies mylonites\nalong the main contact between the two units). The blueschist-facies unit (1.5\nGPa/490 °C; Cheng et al., 2016; this study) and the eclogite-facies unit (2.4 GPa/510 °C;\nZhang et al., 2007) represent upper and lower portions of the subduction interface,\nrespectively, having formed at different depths and possibly at different times. The\ngeological configuration in Qilian here indeed recalls what can be seen in Sanbagawa\nBelt for instance where the eclogite-facies Iratsu unit and the blueschist- to greenschist-\nfacies Shirataki unit formed at different depths along the subduction interface (~55–80\nkm and ~30–45 km, respectively) and were subsequently juxtaposed during exhumation\n(Endo et al., 2024). To conclude, what matters for our study is to demonstrate the\nrelative coherence of the blueschist facies unit, which is supported by the ages and the\nfield structures. This is now clearly stated at Lines 99 and 101 in the manuscript.\nAnother strong limitation of this approach is that the Ar/Ar date does not necessarily\nrepresent the peak of contact metamorphism, but may represent a cooling age when the\nblocks were being exhumed together in the subduction channel above the plate interface.\nIn this case, the age is the same because they are dating retrograde metamorphism. This\nwould be consistent with the observation that the closure temperature of mica in the\nAr/Ar system (e.g. Harrison et al., 2009) is lower than the peak metamorphism recorded\nby RSCM thermometry, which is nearly 500 °C (Figure 4).\nTo better document if deformation occurred at the same time, the authors needs to rely\non other dates of peak metamorphism (e.g. Lu/Hf garnet dating, monazite dating) or\neven to check if the sedimentary age of the different protoliths is similar or very\ndifferent with U/Pb zircon dating. If the protoliths have very different ages, the authors\nare likely looking at different blocks affected by retrograde metamorphism above the\nplate interface.\nResponse: Indeed, the Ar–Ar age may reflect cooling age rather than the timing of peak\nmetamorphism, considering that the closure temperature of mica of the Ar–Ar system\n(425 ℃ ± 70 ℃; Harrison et al., 2009) is slightly lower than the peak temperature\nrecorded by RSCM thermometry (~490 °C; Figure 4). Relevant published data also\nprovide insights into this interpretation. In the Qingshui Gorge blueschist unit, Lu–Hf\nisochron dating of garnet, glaucophane, and whole rock yielded a peak metamorphic\nage of 467 ± 2 Ma (Fig. R4a; Cheng et al., 2016), which is indistinguishable from our\nAr–Ar mica age of 467 ± 3 Ma. This consistency suggests that the blueschist-facies unit\nlikely experienced rapid cooling following peak metamorphism. Unfortunately, neither\nprevious studies nor our work yielded zircon U–Pb metamorphic ages, likely due to the\nabsence of metamorphic zircon growth. Therefore, based on the available\ngeochronological constraints, we infer that the peak metamorphic age of the blueschist-\nfacies unit is ca. 467 Ma, as shown in Fig. 6.\nAt present, the available constraints on protolith ages come from our unpublished U-\nPb dating results from the Qingshui Gorge, involving two samples related to this\nstudy—a coarse-grained metachert (C13) and the calcschist matrix (B61). Both samples\nyield comparable minimum age clusters around 513–512 Ma (Fig. R4b–c), suggesting\na shared maximum sedimentation age and, i.e., a common protolith age. This\ninterpretation is further supported by a maximum sedimentation age of ~511 Ma\nobtained from the blueschist-facies unit of the Qingshui Gorge by Cheng et al. (2016),\nbased on zircon U–Pb analyses. It is worth noting that the presence of a block-in-matrix\nstructure does not necessarily imply a juxtaposition of rocks with contrasting P–T–t\npaths. In the Italian Alps, numerous recent works have demonstrated that units\nexhibiting block-in-matrix mixing (that were originally formed as olistostrome basins)\nstill have strikingly consistent metamorphic trajectories through the eclogite facies (e.g.,\nMonviso: Angiboust et al., 2012; Aosta valley: Tartarotti et al., 2017). Therefore,\nalthough the protolith ages of the blueschist facies unit remain partially constrained,\nprevious studies and our work together demonstrate that this unit consistently records\na well-defined peak metamorphic stage, which provides more robust evidence for\ncontemporaneous deformation. Same modification of the draft as for the previous\ncomment (Lines 101 and 103).\n[REDACTED]\nFig. R4: (a) Lu–Hf isochron plot for garnet fraction, bulk-rock powder, and glaucophane\nseparate of the blueschist (Cheng et al., 2016). Error bars are significantly smaller than the size\nof the symbols. (b–c) Zircon U-Pb age KDE and histogram plots for the coarse-grained\nmetachert (C13) and calcschist matrix (B61). The zircon U–Pb isotopic dating was performed\nusing an AnalytikJena PQMS Elite ICP-MS instrument, attached to a RESOlution 193 nm laser\nablation (LA) system at Beijing Createch Testing Technology Co., Ltd. Analytical conditions were\nas follows: a beam diameter of 24 μm, a repetition rate of 6 Hz, and an energy density of 6 J/cm2.\nFor the same reason, I have concerns that pseudosection modelling and RSCM\nthermometry is providing the peak metamorphic temperature. There might be a\nmismatch between RSCM T, which records the peak metamorphic temperature reached\nby a sample (which may occur during retrograde metamorphism) and metamorphic\nassemblage, which may keep the record of the entire PTt path, including the pressure\npeak. Therefore, it is incorrect to use the best fit between mica isopleths and RSCM\ntemperatures as in Figure b.\nResponse: See the previous answer on the fact that peak burial temperature and peak\nburial age are the same in our samples. This interpretation is further supported by the\ndata presented in Table S3, which shows comparable chemical compositions between\ncore and rim mica analyses, thereby indicating a lack of significant retrograde overprint.\nIn order to better link mineral chemistry with microtextures, we have performed new\nscanning electron microscope (SEM) analysis of peak burial minerals (phengite and\nglaucophane), both within matrix and micro-shear zones, and all the compositional profiles show a\nstrikingly flat pattern across the studied crystals (see Figure R5). We decided to undertake\nSEM analyses because the interaction volume is much smaller (~10 µm3) than for the\nEPMA (~100 µm3), enabling a higher-resolution detection of potential retrogressive\nimprints on the crystal rims. Even though the values of absolute concentration may not\nbe as accurate as those of the EPMA, the sensitivity of the instrument enables the\nidentification of fine compositional changes, if any. The absence of systematic zoning\ntrends (in particular towards crystal rims) confirms that retrogression did not affect the\nstudied crystals in their respective microstructural position. We can therefore safely\ndiscard this possibility and confirm that the mechanical processes proposed in our study\ndid occur at peak burial conditions. The corresponding addition has been incorporated\ninto the manuscript at Line 188, and Figure R5 has been included as Supplementary\nFig. S7.\nFig. R5: SEM transect analyses of phengite and glaucophane conducted within the EBSD-\nanalyzed sites of the MS, CMC, and MSMC samples. (a–d) Photomicrographs showing the\nlocations of SEM transect analyses, highlighted by rectangles. (e–m) Backscattered electron (BSE)\nimages illustrating compositionally homogeneous phengite and glaucophane grains, with the\npositions of SEM line transects marked. Panels (e–g) are rotated 90° counterclockwise. (n) Si p.f.u.\nprofiles across selected phengite grains, displaying consistently flat compositional trends. (o) Na\np.f.u. profile across a glaucophane grain (I–I′), showing nearly uniform composition from core to\nrim. Analytical conditions for SEM analyses were: 15 kV, 10 nA, 2 μm beam size, 30s counting\ntime per point, and internal standard calibration. The SEM used was a JEOL IT800HL equipped\nwith an Oxford Instruments EDX detector X-Max 50 operated at ENS de Lyon.\nThe measured mica isopleths are stable in most of the pseudosection calculated in\nFigure 4b, and according to Fig. 4a, there are different generations of white mica. So,\nwhy not using these generations to reconstruct a PT path appropriately?\nResponse: We kindly disagree with this statement. The mica compositions are\nremarkably clustered in the 45-60% celadonite joint (Fig.4a is strongly zoomed in,\nwhich gives a visual impression of dispersion). Our new transect analyses (Fig. R5)\nconfirm that the crystals are unzoned. We added the EPMA points on the small triangle\non the bottom left of Fig.4a to better show the homogeneity of compositions.\nI am also surprised by the fact that according to the modelling there is free garnet in\nthis sample (Figure 4b), but I am very surprised that garnet compositions are not shown,\nbecause garnet isopleths are very reliable to estimate the PT conditions of\nmetamorphism.\nResponse: Garnet exhibits a very homogeneous, Mn-rich composition, with\nrepresentative data presented in Table R1 and also complemented by Supplementary\nTable S4. Note that the modal proportions of garnet are quite low—0.02% based on\nTIMA mapping and 0.25% in the best-fit area of our pseudosection modeling—and thus\nexert little impact on the phase relationships. Because of the strong fractionation effect\nof garnet (also visible in Table R1), we cannot use the modeled garnet composition for\nconstraining the peak burial conditions accurately (see for instance Konrad-Schmolke\net al., 2008). Using garnet for constraining peak conditions thus appears a very risky\noperation.\nBecause of the strong fractionation effect of garnet (also visible in Table R1), we cannot\nuse the modeled garnet composition for constraining the peak burial conditions accurately\n(see for instance Konrad-Schmolke et al., 2008). Using garnet for constraining peak\nconditions thus appears a very risky operation.\nMoreover, when building a pseudosection, it is good practice to provide the mineral\nchemistry of all minerals in the rock and discuss it to check if it fits or not with the\nobserved assemblage. This should be done with glaucophane, chlorite, and biotite,\nwhich are expected to be present based on modelling.\nResponse: It is well known that achieving a perfect fit between modeled minerals and\nanalyzed phases is extremely difficult, as the solution models are often simplified and\nnever perfectly replicate actual phase compositions. For instance, the measured X of\nMg\nphengite (0.67) and Na/(Na+Ca) ratio of glaucophane (0.97) closely match the modeled\nvalues of 0.64 and 0.97, respectively (see data in Table R1). Glaucophane X value is\nMg\nslightly different (0.60 modeled versus 0.72 analyzed), same for garnet (X : 0.12\nMg\nmodeled vs. 0.17 analyzed). Again, there is not a single study on such a complex system\nwhere a perfect match is achieved between modeled and measured mineral\ncompositions. A model is never perfect, and the ‘best model’ is the one that most\naccurately matches the observed natural system. We believe that we did our best and\ncannot converge towards a model more perfectly describing the natural system at stake.\nSuch considerations might be relevant for a study focused solely on petrology and phase\nrelationships, but we would like to kindly remind that the core topic of our work is the\ndocumentation and mechanical interpretation of the micro-shear zones. Whether the\nmicro-shear zone formed at 1 GPa or 1.5 GPa has little to no implication for our\nproposed geodynamic and mechanical model. The temperature, which exerts a greater\neffect on rheological calculations, is much better constrained thanks to RSCM estimates.\nIn order to avoid an unnecessary imbalance between pseudosection modelling and\nEBSD/microstructures and keep our draft sharp and concise, we prefer to avoid such\ntechnical considerations.\nTable R1 is now provided as Table S4 in the Supporting Information. A sentence related\nto the accuracy of the match between the analyzed and modeled phases is now provided\nat Line 429.\nE52-B Modeled\nMinerals\nPhe Grt Gln Phe Grt Gln\nSiO2 51.22 36.13 57.11 — — —\nTiO2 0.11 0.42 0.00 — — —\nAl2O3 25.73 19.92 8.12 — — —\nFeO 3.05 2.39 10.18 — — —\nMnO 0.35 36.08 0.00 — — —\nMgO 3.45 0.28 14.71 — — —\nCaO 0.00 4.65 0.39 — — —\nNa2O 0.57 0.00 7.44 — — —\nK2O 11.00 0.00 0.00 — — —\nCations p.f.u\nSi 3.44 2.97 7.88 3.44 3.00 7.98\nTi 0.01 0.03 0.00 0.00 0.00 0.00\nAl 2.04 1.93 1.32 2.12 2.00 1.95\nFe2+ 0.17 0.16 1.17 0.16 1.30 1.24\nFe3+ 0.00 0.00 0.00 — — —\nMn 0.02 2.51 0.00 0.00 1.27 0.00\nMg 0.35 0.03 3.03 0.28 0.20 1.83\nCa 0.00 0.41 0.06 0.00 0.23 0.06\nNa 0.07 0.00 1.99 0.02 0.00 1.93\nK 0.94 0.00 0.00 0.98 0.00 0.00\nCompositional parameter\nXMg 0.67 0.17 0.72 0.64 0.12 0.60\nNa/(Na+Ca) — 0.00 0.97 — 0.00 0.97\nTable R1: Representative chemical compositions of the MSMC sample (E52-B site), along with\npseudosection-derived mineral compositions (best-fit area shown in Fig. 4b) from the\nQingshui Gorge mélange zone. Phe–Phengite, Grt–Garnet, and Gln–glaucophane.\nI have also noted that the authors modelled the system in the NCKFMASH system and\ndo not model nor provide information about Mn, Ti, and Fe2+/Fe3+ (O). This is tricky,\nbecause the presence of Mn could lower the stability of minerals significantly\n(stabilizing for instance spessartine garnet at lower T) and Fe2+/Fe3+ affects the stability\nof amphibole, garnet, chlorite, and muscovite, and mica isopleths. So why not\nmeasuring and using Mn and O for the modelling?\nResponse: The point regarding Mn is totally correct: its presence strongly controls the\nstability of garnet towards lower temperatures. In fact, MnO was already included as a\ncomponent in the grid shown in Fig. 4b; however, we inadvertently omitted it from the\nfigure caption. This oversight has been corrected in the revised version. Thank you for\nraising this matter. The amount of MnO is extremely low, 0.07 wt.% (and was estimated\nin the same way as for the other elements). Because of the strong fractionation effect of\ngarnet (see Konrad-Schmolke et al., 2008, for instance), we cannot use the modeled\ngarnet composition for constraining the peak burial conditions, as also illustrated in\nTable R1.\nAs for the Fe2+/Fe3+ ratio, we state in the manuscript (Line 425) that phengite, chlorite,\nand glaucophane in the vicinity of the micro-shear zone have no ferric iron in their\nstructural formula (stoichiometric calculations report all iron as ferrous, and in\nparticular for glaucophane; see Table R1). Given that glaucophane has an Fe3+ end-\nmember (riebeckite), if oxygen fugacity had been high, this would have resulted in a\nsignificant riebeckite component, which is not supported by the analyzed glaucophane\ncompositions (Table R1). In addition, the presence of magnetite but the absence of\nhematite indicates a moderate oxygen fugacity, below the hematite-magnetite (HM)\nbuffer. Hence, we are somewhere between the iron-wüstite and HM buffer but\ndetermining accurately the amount of O in the system is extremely challenging and\n2\nrisky. Nevertheless, we have run the model with various Fe2+/Fe3+ ratios and this\ninvariably leads to the stabilization of aegirine, which is an artefact that is well known\nby the metamorphic petrologist community. Since aegirine has not been observed in the\nnumerous thin sections that have been studied, we conclude that adding O or Fe3+ in\n2\nour calculations brings an unnecessary level of complexity. We thus prefer to keep\nworking in a simplified, Fe3+-absent system that more accurately reproduces our natural\nsystem.\nI think there is not enough discussion about the limitations of paleopiezometry and flow\nlaws, which have a large associated uncertainty related to (1) separating recrystallized\ngrain size and relic grains (Cross et al., 2017), (2) estimating radii from sections without\nknowing the 3D shape of grains, and (3) the uncertainty associated with power laws. In\ngeneral, it is good practice to try to use different relationships as done by Stipp (2002a,\nb) in the Tonale Fault.\nResponse: These concerns are addressed below one by one:\n(1) Separating recrystallized grain and relic grains\nThe details of separating recrystallized and relic quartz grains follow Cross et al. (2017).\nSpecifically, we applied the Grain Orientation Spread (GOS) threshold method to\nEBSD data to distinguish strain-free recrystallized grains (small grain sizes, low GOS\nvalues) from internally deformed relic grains (larger sizes, high GOS values). Only\nfully identified grains—those not intersected by the edges of the mapping area—were\nincluded in the grain size analysis to ensure reliable measurements. In our strongly\ndeformed sample, the maximum intragranular misorientation of quartz (~15°) is well\nabove the angular resolution limit of conventional EBSD (nominally ~0.5°), effectively\neliminating the possibility of misclassification due to instrumental limitations.\nAlthough 2D imaging may introduce cutting effects—where fewer than 25% of large\ngrains are mistakenly identified as small due to sectioning—the measured proportion\nof small relic grains in 2D remains statistically representative of their actual proportion\nin 3D. Comparisons between 2D and 3D mylonitic microstructure datasets (Berger et\nal., 2011) further support this conclusion.\n(2) 3D grain shape\nThe metachert outcrop commonly displays both mylonitic foliation and stretching\nlineation (Fig. 1), suggesting that it should be classified as an LS-tectonite—typically\nassociated with strain conditions approximating plane strain (e.g., K ≈ 1; Flinn, 1962;\nRamsay and Huber, 1987). Finite strain estimations based on Fry analyses of oriented\nthin sections from MSMC sample E51 (X–Z and Y–Z planes) yield axial ratios of R\nX-Z\n= 1.75 and R = 1.32, respectively (Fig. R5a–d). These values further support a near-\nY-Z\nplane strain geometry, consistent with L=S tectonites, as shown by the Flinn diagram\n(K = 1.03) and Hsü diagram (ν= –0.02; Hsü, 1966; see Fig. R5e–f). In plane strain, the\nprincipal strain axes follow the relationship X > Y = 1 > Z, with extension along X, no\nstrain along Y, and shortening along Z (Fossen, 2010). Given this 3D strain geometry,\nmeasuring grain size in the X–Z plane would minimize the influence of area distortion\ncaused by extension and shortening along the principal axes (e.g., Platt et al., 2018;\nTulley et al., 2020; Ujiie et al., 2022). This approach was adopted in our study, as all\nEBSD mappings were performed on X–Z plane thin sections, cut perpendicular to the\nfoliation and parallel to the stretching lineation. Moreover, the primary focus of the\npalaeopiezometric analysis—small, strain-free recrystallized quartz grains within the\nmicro-shear zones—is typically equant in shape, rendering their size measurements\nlargely insensitive to 2D thin-section orientation.\nFig. R5: Three-dimensional strain ellipsoid shape data from the MSMC sample E51. (a)\nsynkinematic quartz band in the X-Z thin section from sample E51, with selected grain centroids\n(red dots) marked for Fry method analysis. (b) Fry plot of the X-Z planes with red fitted strain\nellipses. (c) Deformed quartz aggregate in the Y-Z thin section of sample E51, showing selected\ngrain centroids (red dots) used for Fry method analysis. (d) Fry plot on the Y-Z planes with blue\nfitted strain ellipses. (e) distribution of MSMC sample (red star) on Flinn diagram (K = 1.03). (f)\nsample position (red star) on the Hsü diagram (ν = -0.02).\n(3) Multiple flow laws and piezometers\nTo constrain the uncertainty inherent in flow laws, multiple dislocation-based flow laws\nfor quartz are plotted in Fig. R6, including: (a) Hirth et al. (2001); (b) the experimental\ndata of Luan and Paterson (1992), with parameters re-evaluated by Fukuda and Shimizu\n(2017); (c) Fukuda et al. (2018); and (d) Tokle et al. (2019).\nFig. R6: Deformation mechanism maps were constructed based on the following flow laws:\n(a) Hirth et al. (2001); (b) Luan and Paterson (1992) with parameters redetermined by Fukuda\n& Shimizu (2017); (c) Fukuda et al. (2018); (d) Tokle et al. (2019) for dislocation creep, and\nDen Brok et al. (1998) for diffusion creep. The boundary between dislocation creep and pressure\nsolution creep fields shown as a red solid line. Blue dashed line (RG-S&T): recrystallized grain size\npiezometer of Stipp and Tullis (2003). White dash-dot-dash line (CG-C): piezometer for coarser-\ngrained quartz, derived from 1 μm step-size EBSD mapping by Cross et al. (2017). Black solid line\n(SR-C): \"sliding resolution\" piezometer of Cross et al. (2017) as adopted in this study. The semi-\ntransparent white bands indicate the uncertainty bounds associated with the above-mentioned\npiezometers.\nAs shown in Fig. R6, strain rate variations among the different flow laws applied to our\ndataset remain within one order of magnitude and consistently fall within the\ndislocation creep regime. Three different piezometers are also included in the\ndeformation mechanism maps (Fig. R6): the recrystallized grain size piezometer of\nStipp and Tullis (2003); the piezometer for coarser-grained quartz derived from 1 μm\nstep-size EBSD mapping by Cross et al. (2017); and the ‘sliding resolution’ piezometer\nproposed by Cross et al. (2017). The differential stresses calculated using these\npiezometers show minimal variation, with discrepancies of less than 10 MPa.\nFigs. R5 and R6 are provided as Supplementary Figs. S11 and S9, respectively. The\ncorresponding explanatory text has been incorporated into the main manuscript (Lines\n232 and 438).\nIt is also necessary to check if the rocks involved where deformed at the same fluid\nconditions. For example, if we are in fluid-saturated conditions or not, as this strongly\naffects the recrystallization of quartz (Stipp et al., 2006, 2010; Menegon et al., 2011;\nAustrheim papers).\nResponse: The hydrolytic weakening effect in flow strength has been observed for\ndecades in experiments on quartzites (e.g., Hirth and Tullis, 1992), while exerting little\nto no influence on the quartz piezometer relationship (Stipp et al., 2006). Moreover,\nfurther studies have shown that this weakening effect is primarily controlled by the\nwater fugacity (highly related to confining pressure) rather than by water content alone\n(Tullis et al., 1979; Tullis and Yund, 1989; Post et al., 1996). Accordingly, the water\nfugacity term ( ) has been incorporated into the power-law constitutive equation for\n𝑓𝑓𝐻𝐻2𝑂𝑂\ndislocation creep and subsequently extrapolated to crustal deformation conditions\n(Kohlstedt et al., 1995; Hirth et al., 2001). In this study, water fugacity is derived under\nconsistent P–T conditions of rocks (see above) using the equation of state proposed by\nPitzer and Sterner (1994), which provides a thermodynamically robust formulation\nvalid from near-ambient to upper mantle pressures and is directly fitted to experimental\ndata. This approach enables accurate and continuous estimation of water fugacity across\nthe P–T range relevant to both laboratory experiments and natural deformation\nconditions, thereby allowing for the reliable incorporation of hydrolytic weakening\neffect into quartz flow law calculations.\nCase studies of natural rocks have revealed deformation contrasts related to variations\nin water and/or melt content, which are typically driven by the following factors: (1)\nvariations in melt or fluid release associated with metamorphic facies, governed by\ncontrasts in bulk-rock composition and P–T conditions (Menegon et al., 2011;\nGiuntoli et al., 2022; Bukała et al., 2024); (2) infiltration of external fluids along brittle\nfractures or shear zones (Austrheim, 1987; Jamtveit et al., 2018). In the studied metachert\nblock, both the whole-rock composition (high quartz content) and the peak metamorphic\nP–T conditions (see above) are notably homogeneous. No evidence of syn-metamorphic\nveining with high-pressure minerals, brittle fracturing, or associated bands of hydrous\nassemblages has been identified inside the metachert block. Based on these evidences,\nwe suggest that there was no significant variation in fluid content (pressure and\ncomposition) within the metachert block.\nA discrepancy in pore fluid pressure factor (λ) between the calcschist matrix (λ > 0.97)\nand the metachert block (λ < 0.86) is concluded from our study (Fig. 5b). Nevertheless,\nthe consistent activation of quartz slip systems—from rhomb <a> slip to prism <a>\nslip—together with subgrain rotation recrystallization (SGR)-dominated dislocation\ncreep observed in both block and the matrix, suggests that relatively minor difference\nin pore fluid pressure (as reflected by a ~0.1 difference in λ) did not exert a significant\ninfluence on the dislocation creep mechanism under consistent P–T conditions. Hence,\nwe suggest that our piezometric and flow law estimations for both the block and the\nmatrix remain robust. This is now more explicitly explained in the Flow laws derivation\nsection of the Supplementary Texts.\nRegarding the calcschists, I am also concerned by the fugacity of CO2 and H2O in the\nfluid, which may affect calcite and quartz behavior (Pennacchioni paper: Why Calcite\ncan be stronger than quartz?). Crucially, there are no microphotographs showing the\nmicrostructure of the calcschists\nResponse: Fair enough. According to the flow law plots of calcite and quartz presented\nby Mancktelow and Pennacchioni (2010), calcite is rheologically stronger than quartz\nonly under the following conditions:\n(1) the presence of abundant fluid, such that water significantly reduces the rheological\nstrength of quartz while having little effect on calcite;\n(2) deformation occurs at high to moderate temperatures (greater than 550 °C);\n(3) the calcite grains are coarse-grained, with grain sizes on the order of millimeters;\n(4) the differential stress is sufficiently low (less than 10 MPa).\nIn the setting of this study (the transition zone), high water fugacity may exist in the\ncalcschist; however, conditions (2)–(4) are demonstrably not satisfied. Moreover, as we\nmentioned above, both the outcrop (boudinage structures and/or pinch-and-swell\ngeometries of the quartz veins; Fig 1i) and microstructure (σ-type quartz porphyroclast\nwithin the matrix of the calcite–quartz aggregate; Fig. R3d) indicated that quartz\nremained mechanically stronger than calcite during shearing.\nAdditionally, the corresponding microphotographs depicting the microstructure of the\ncalcschists are presented in Fig. R3. Nevertheless, we now include additional material\nin the supplementary files (Fig. S2 and S7) in order to better show what the calcschist\nmatrix looks like.\nIndeed, the mismatch between paleostress measurements with this approach and\ndifferential stresses measured in modern subduction zones might indicate that there is\nan intrinsic error with the method that it is not addressed.\nResponse: In the block-in-matrix subduction channel setting, numerical modeling\n(Kenkmann and Dresen, 1998; Kenkmann, 2000; Ioannidi et al., 2021) has shown that\nthe integration of stress over the surfaces of relatively rigid blocks can lead to elevated\ninternal stresses within the blocks. Yamato et al. (2019) have also shown numerically\nthat indentation of rigid blocks in a weaker matrix can lead to transiently extremely\nhigh stress within touching blocks. In addition, large megathrust ruptures are also\nknown to be followed by viscoelastic relaxation along the subduction interface that can\npersist for several decades (such as for the 1960 Great Chile Earthquake, Wang et al.,\n2007), in line with the duration estimated from observed microstructures. Such post-\nseismic stress relaxation processes could also have contributed to the generation of the\nmicro-shear zones observed within the metachert blocks. Notably, the interpretation of\nthe force-chain-like phenomenon formed by block interactions in this study is also\nconsidered to contribute to the pronounced amplification of internal stress within the\nblocks (Beall et al., 2019). These stress amplification effects provide a plausible\nexplanation for the mismatch between stress values estimated from microscale\npalaeopiezometry (tens of MPa; Behr and Platt, 2013; Platt et al., 2018; Tulley et al.,\n2020; Ujiie et al., 2022) and those inferred from large-scale geophysical observations\n(≤ 1 MPa; Folesky et al., 2024). This concern has been clarified in the manuscript at\nLines 76, 221, 298, 304, 317, and 347.\nFinally, the author only discuss one model proposed for the formation of SSE at the\nsubduction interface, the interaction between strong blocks in a weak matrix, which is\nonly one of the models proposed. Crucially, they do not provide evidence of interaction\nbetween blocks in their rocks, so why do they focus on this model only when there\ncould be other explanations related to e.g. fluid activity/fluid overpressure, strain\npartitioning, accretion of different blocks with different microfabrics and so on.\nResponse: We respectfully disagree with this statement. We do not claim to propose a\nnew model for the formation of SSEs. Rather, the objective of our study is to document\nthe mechanical effects of block interaction within the region where SSEs are inferred\nto occur. It is clear that we cannot document the interaction itself because this block-\nblock interaction occurred 467 Ma ago, and the former contact zones have been\nreworked right after block interaction and later upon exhumation and tectonic\namalgamation during the closure of the oceanic realm. Alternative options, such as\nasperity subduction, are now offered in the paper to acknowledge that several\nphenomena can lead to the same rock imprint (Lines 320 and 332). We also evoke the\npossibility of having post-seismic stress relaxation processes for triggering block\ndeformation, in addition to deep SSEs (Lines 347 and 350).\nAnother minor -but important- observation: I do not think it is necessary to stress that\nrecrystallization is by SGR if you have EBSD data, as when you have those small\nmicro-shear bands, you need to do a very detailed microstructural work to prove this.\nFor example, to me, you have both BLG and SGR (Stipp et al., 2002a, b), but it is still\ndislocation creep, so it is not important.\nResponse: Thank you for the kind reminder. As noted above, this issue was also raised\nby Reviewer #2 and has been resolved accordingly. While SGR appears to be the\ndominant recrystallization mechanism in the micro-shear zones—evidenced by the\nshape of newly formed grains and the transition from rhomb <a> slip to prism <a>\nslip (Fig. 3 and Fig. S4–5)—we also acknowledge the presence of bulging at the margins of\nlarge grains (BLG). In light of this, we have removed the emphasis on ‘recrystallization\nby SGR’ and now refer more broadly to ‘recrystallization by dislocation creep’ in the\nmanuscript (Lines 224, 251, and 737).\nFor the reasons stated above, I do not think the data provided in the paper can be easily\ntied to the subduction interface or be demonstrated that the quartz microfabrics on\nwhich EBSD was performed were coeval and all representative of deformation at the\nplate interface, as most of this may have also been occurred in the exhumation channel\nabove the plate interface and over a range of PT conditions.\nResponse: As already discussed above, we prove that we have excellent control over\nthe place and the timing at which the studied quartz microfabrics formed. It is neither\nrelated to exhumation nor other processes, as already discussed in the discussion section\nof the draft. Nevertheless, we have added additional elements in order to clarify these\npoints in the manuscript.\nBut the quartz EBSD data shown is of excellent quality and I think the authors can use\nit to address issues pertaining to quartz deformation, such as the origin and propagation\nof micro shear bands or SC fabrics in a deforming rock (e.g. papers by Trepmann et al.,\nHentschel et al., Finch et al. etc.; Ceccato et al., Kjoll et al., etc.).\nResponse: Thank you for the constructive comments. Following established EBSD\nanalytical methods (e.g., Kjøll et al., 2015; Ceccato et al., 2017; Trepmann et al., 2017;\nHentschel et al., 2022), we obtained crystallographic orientations of quartz grains from\ndifferent deformation domains (micro-shear zones and coarse-grained ribbons). These\nare presented as IPF-X maps with oriented grain shapes, and pole figures color-coded\naccording to the IPF-X directions of individual data points.\nWithin the micro-shear zone at the E52-A site, the <c> axis of the host quartz grain is\npreferentially oriented subparallel to the Z-axis of the sample's kinematic framework,\naccompanied by intense subgrain rotation recrystallization (see bottom left of E52-A in\nFig. R7). These types of host grains, referred to as XZ-type by Ceccato et al. (2017),\nare interpreted as having crystallographic orientations that favor early strain hardening,\nSGR recrystallization, and the development of micro-shear zones. However, the\ncomparison between micro-shear zones and coarse-grained ribbons reveals a consistent\npattern in the IPF-X maps and pole figures (E52-B, C71-A, C31-A, and C13-A in Fig.\nR7), suggesting rather random crystallographic orientations of quartz. Hence, we infer\nthat the formation of micro-shear zones was, to some extent, facilitated by the quartz\ninitial crystallographic orientations with c-axes perpendicular to the Y-axis, as\nsuggested by Ceccato et al. (2017). Fig. R7 has been included in the Supplementary\nInformation as Fig. S5, with the corresponding description provided in the EBSD\nanalysis section of the Supplementary Texts and the Manuscript (Line 158).\nFig. R7: IPF-X maps with overlaid crystal shapes of trigonal quartz, and corresponding IPF-\nX color-coded pole figures, from micro-shear zone sites in the micro-sheared metachert metachert."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for addressing my comments. I am satisfied with the responses and changes, and recommend publication.", "reviewer_2": "This is a revised version of the manuscript by Wu et al. I reviewed the original version earlier this year, and recommended publication after minor revisions.\nI have looked through the revised version, focusing on the parts of the text that have been revised. I am satisfied that the authors have considered my earlier comments and suggestions carefully, and that they have made appropriate revisions. I have a few corrections of an editorial nature, noted below, but otherwise I am happy to recommend publication.\nLine 224. I suggest rewriting the last sentence in this paragraph as follows, for clarity:\n“The presence of intragranular Dauphiné twins, which do not cross grain boundaries (yellow twin boundaries in Fig. 2–3, Supplementary Figs. S3–5), indicate differential stresses ….”\nLine 229. I suggest “translates to an approximately 600-fold difference in strain rate.”\nLine 751. I think the rectangle locating part h of Figure 2 is located in part e, not a.\nJohn Platt, June 3, 2025.", "reviewer_3": "Dear authors,\nI apologize for my late review. I have read with interest the replies to my previous concerns and the revised manuscript and I think many answers are appropriate and satisfactory. The authors did an impressive job in digging through the literature, improving the paper, and providing additional supplementary material that further strengthen the manuscript. I only have minor concerns related to some of the answer and still some more serious concerns regarding the metamorphic assemblage. For this reason, I think minor revisions are still needed before the manuscript can be published.\nFor the rest, my comments were meant to be mostly though-provoking. I do believe the authors really have excellent EBSD data, Ar/Ar data, and P-T data, but my problem is always: how to correlate a geochronological data that has only +-3 Ma uncertainty (very low in geology) with geophysical data that gathers information about ETS in real time? Same for a rock fabric that is maybe instantaneous in the geological time, but it is the sum of thousands of deformation events.\nI do not think the quartz crystal shown in Fig. 3d can be considered a sigma-porphyroclast (or at least it is really difficult to see), but the first part of the response in the rebuttal letter was already satisfactory even without invoking sigma-porphyroclasts. Anyway, I suggest changing the caption or removing this figure.\nThe response about Mn and Fe2+/Fe3+ is generally satisfactory, but the author state to have calculate stoichiometrically the Fe3+ in chlorite and phengite. Unfortunately, it is not possible to calculate Fe2+/Fe3+ stoichiometrically for minerals that contain more than one partially filled site such as biotite, white mica, chlorite... (see for example Forshaw & Pattison, 2021. Contributions to Mineralogy and Petrology 176:63). Anyway, this can be done for glaucophane and if the authors do not find Fe3+ in glaucophane, and do not have Fe3+-bearing oxides in their samples, then the assumption of all iron to be divalent is correct. Is this the case?\nThe profile shown for glaucophane (Figure R5n) is not satisfactory, because it only shows the Na content (also according to line 188), whereas Na-amphibole is commonly zoned in Mg, Fe, and Mn. It is necessary to show at least a consistent XMg across the mineral to document that it formed under a single event. In the supplementary, I can only find EPMA chemical data of phengite, and only representative analyses of garnet and glaucophane. ALL chemical analyses should be provided and accessible.\nI still do not think the interaction of blocks in the mélange is a suitable explanation for your structures, because you always see them isolated in many mélanges. Their interaction, as proposed by the models cited in your manuscript, leads to clogging, so in their finite strain state you should find them coupled. Other models (e.g. Vannucchi et al., 2022. Nature communications 13, 2311) consider clogging to be physically not possible and argue e.g. for weak blocks in a strong matrix.\nI think with your data you could critically consider all these limitations and try to evaluate these models or propose alternatives that fits your data and observation better. Alternatively, why discussing things you have little data about (you have much microstructural data but little field documentation of the blocks, the mélange fabrics and their interaction).\nI still do not think all your fabrics to be coeval, because it is hard to tie Ar/Ar ages in mica to quartz microfabrics. One may argue that mica does not recrystallize in shear bands and you may be dating fragments of relic micas disrupted by the shear bands. In any case, I accept the fact that you assume your fabrics to be coeval (as you state in your reply), but I think this should be clearer in the text for the readers. This is crucial, because a slight change in the T of deformation would change the estimation from paleopiezometry and pseudosection modelling also has an intrinsic P-T error of at least +- 1 kbar and +-50 °C, likewise RSCM thermometry.\nIn general, leaving more space in the text to discuss the limitations of bridging geological data (with its uncertainties) to geophysical data (currently observed SSEs) would make the manuscript more appealing even to non-geologists.\nSome minor comments below:\n201-202. This is not correct. SGR is not always happening at 420-500 °C. This temperature range derives from the works of Stipp et al. (2002a, b), and these authors stated in multiple works that this temperature range is valid only for the Tonale Fault (this is also reported by Law, 2014). I know that SGR is kinda active around these temperatures but there are conditions at which it might not be, for example at dry conditions (e.g. Menegon et al., 2011). In your case this is irrelevant: you have a pseudosections and other T estimates, so I suggest removing this statement."}, "author_response": "Point-by-point response to reviewers’ comments for\nNature Communications Manuscript: NCOMMS-25-11434A\nManuscript Title: Stress heterogeneities in exhumed high-pressure rocks shed light\non deep subduction interface transient coupling\nAuthors: Yawei Wu, Samuel Angiboust, Jianxin Zhang, Bo Zhang, Xiaohong Mao,\nOnno Oncken\nDear Reviewers,\nWe are grateful for your continued and thoughtful evaluation of our revised manuscript,\nwhich has contributed to improving its clarity and robustness. Please find below our\npoint-by-point responses, shown in blue. Corresponding revisions to the manuscript are\nindicated in red where appropriate to address your inquiries.\nReviewer #1 (Remarks to the Author):\nThank you for addressing my comments. I am satisfied with the responses and changes,\nand recommend publication.\nResponse: We sincerely thank you for your positive feedback and recommendation for\npublication.\nReviewer #2 (Remarks to the Author):\nThis is a revised version of the manuscript by Wu et al. I reviewed the original version\nearlier this year, and recommended publication after minor revisions.\nI have looked through the revised version, focusing on the parts of the text that have\nbeen revised. I am satisfied that the authors have considered my earlier comments and\nsuggestions carefully, and that they have made appropriate revisions. I have a few\ncorrections of an editorial nature, noted below, but otherwise I am happy to recommend\npublication.\nJohn Platt, June 3, 2025.\nResponse: Thanks for your supportive evaluation of our revised manuscript and the\npublication recommendation. The editorial suggestions have been addressed as detailed\nbelow.\nLine 224. I suggest rewriting the last sentence in this paragraph as follows, for clarity:\n“The presence of intragranular Dauphiné twins, which do not cross grain boundaries\n(yellow twin boundaries in Fig. 2–3, Supplementary Figs. S3–5), indicate differential\nstresses ….”\nResponse: We appreciate your careful comment and have revised the last sentence of\nthe paragraph in Line 223 following your suggestion to improve clarity.\nLine 229. I suggest “translates to an approximately 600-fold difference in strain rate.”\nResponse: We replaced “translates to a strain rate difference of approximately 600-fold”\nby “translates to an approximately 600-fold difference in strain rate” as suggested (Line\n228).\nLine 751. I think the rectangle locating part h of Figure 2 is located in part e, not a.\nResponse: Thank you for pointing this out. The error has been corrected, and the text\nnow correctly refers to the “rectangle in part e” in Line 767.\nReviewer #3 (Remarks to the Author):\nDear authors,\nI apologize for my late review. I have read with interest the replies to my previous\nconcerns and the revised manuscript and I think many answers are appropriate and\nsatisfactory. The authors did an impressive job in digging through the literature,\nimproving the paper, and providing additional supplementary material that further\nstrengthen the manuscript. I only have minor concerns related to some of the answer\nand still some more serious concerns regarding the metamorphic assemblage. For this\nreason, I think minor revisions are still needed before the manuscript can be published.\nFor the rest, my comments were meant to be mostly though-provoking. I do believe the\nauthors really have excellent EBSD data, Ar/Ar data, and P-T data, but my problem is\nalways: how to correlate a geochronological data that has only +-3 Ma uncertainty\n(very low in geology) with geophysical data that gathers information about ETS in real time?\nSame for a rock fabric that is maybe instantaneous in the geological time, but it is the\nsum of thousands of deformation events.\nResponse: Thanks for your kind and thoughtful feedback on our previous revision. As\nearlier detailed, we utilize an integrated approach combining EBSD-based\nmicrostructural analysis, metamorphic petrology, and geochronology to clearly\ndemonstrate that the studied blueschist-facies rocks—including metachert blocks and\nthe surrounding calcschist matrix—were part of, and underwent synchronous\ndeformation and metamorphism within, the transition zone of the Early Paleozoic North\nQilian paleo-subduction channel. Within this broader framework, the initial cooling\nand/or peak metamorphic age of 467 Ma—obtained from our Ar–Ar dating and the Lu–\nHf dating of Cheng et al., (2016), with an uncertainty of ±2–3 Ma—does not\ncompromise the interpretation that these rocks belonged to, and recorded related\ndeformation and metamorphism within, this transition zone. In other words, our spatial\nand temporal constraints are sufficiently robust to support the aforementioned\ninterpretation, even though precise absolute timing—down to the scale of years—is not\nattainable due to the limitations of current geochronological techniques. More\nimportantly, while absolute dating at year-scale resolution is not feasible, the relative\nduration of micro-shearing events can be indirectly constrained by integrating finite\nstrain estimates with estimated strain rates, yielding timescales on the order of\napproximately 5 to 40 years (see Figure S10 and the ‘Finite Strain Estimation’ section\nin the Supplementary Information). Together, these findings enable us to reconstruct a\nplausible scenario in which the micro-shearing we observe occurred as a transient event\n(lasting several decades) within the ~470–464 Ma timeframe (based on uncertainties of\ngeochronological data), at a depth of ~55 km in the transition zone of the North Qilian\npaleo-subduction channel.\nOn the other hand, we would like to clarify that we do not propose a direct one-to-one\ncorrelation between the decades-long micro-shearing observed in the block and real-\ntime slow slip events (SSEs), for the following reasons: (1) there is no direct evidence\nof SSEs within the studied block, such as fracturing and/or cataclasis; and (2)\ninstrumentally detected SSEs typically exhibit rupture lengths ranging from several\nkilometers to hundreds of kilometers, with rupture thicknesses of a few meters to\nseveral tens of meters (e.g., Chalumeau et al., 2024; Huang et al., 2025), which are\nsignificantly larger in scale than the micro-shearing within the blocks we observe.\nRather, we highlight several commonalities in spatial distribution, duration, and strain\nrate between the observed micro-shearing recorded in paleo-subduction channels and\nmodern SSEs observed by geophysical instruments. Given these parallels, we suggest\nthat the micro-shearing may be related to, or even triggered by, large-scale SSEs (or\npost-seismic stress relaxation) occurring in the surrounding faulted matrix network.\nAdmittedly, this analogy inevitably introduces inherent uncertainties, including the fact\nthat slow earthquake phenomena arise from a combination of structures and\nmechanisms rather than any single controlling factor (Kirkpatrick et al., 2021), and that\nthe preservation and overprinting of these structures complicate their interpretation in\nthe geological record (Platt et al., 2024). With this caution in mind, we have elaborated\non these limitations in the manuscript (Lines 346–354) to ensure scientific rigor.\nAs for the rock fabric, we acknowledge that the observed rock fabric—particularly that\nassociated with long-term steady-state creep—may indeed represent the cumulation of\nmultiple (perhaps even thousands of) deformation stages. Nevertheless, our estimated\ndecades-long duration of micro-shearing constrains the deformation to an exceptionally\nwell-defined relative timescale (transient in geological time). Even if it’s possible\nindividual stages remain analytically unresolved, they are nonetheless confined within\nthe broader geochronological framework established above (~ 467 Ma). Hence, the rock\nfabric formed over a certain short time interval should still provide meaningful average\nconstraints over the whole event, including active slip systems, crystallographic\nkinematic frameworks, and flow law estimations. Numerous case studies also\ndemonstrate that well-preserved and carefully evaluated rock fabrics and structures\ncould serve as natural snapshots to elucidate the geological mechanisms of slow slip\nevents in subduction channels (e.g., Behr and Platt, 2013; Fagereng et al., 2014;\nAngiboust et al., 2015; Tulley et al., 2020), and even regular earthquakes (e.g.,\nCampbell and Menegon, 2019; Papa et al., 2020).\nI do not think the quartz crystal shown in Fig. 3d can be considered a sigma-porphyroclast (or at least it is really difficult to see), but the first part of the response in\nthe rebuttal letter was already satisfactory even without invoking sigma-porphyroclasts.\nAnyway, I suggest changing the caption or removing this figure.\nResponse: Thank you for the meticulous comment. We concur that the quartz domain\nshown in Figure R3d cannot be legibly identified as a σ-type porphyroclast.\nAccordingly, we have removed the kinematic indicator from the upper right corner and\nupdated the caption to: “The quartz domain in the center is hosted within a matrix of\ncalcite and quartz.” as suggested (Fig. S2d of the Supplementary Information).\nThe response about Mn and Fe2+/Fe3+ is generally satisfactory, but the author state to\nhave calculate stoichiometrically the Fe3+ in chlorite and phengite. Unfortunately, it is\nnot possible to calculate Fe2+/Fe3+ stoichiometrically for minerals that contain more\nthan one partially filled site such as biotite, white mica, chlorite... (see for example\nForshaw & Pattison, 2021. Contributions to Mineralogy and Petrology 176:63). Anyway, this can be done for glaucophane and if the authors do not find Fe3+ in\nglaucophane, and do not have Fe3+-bearing oxides in their samples, then the\nassumption of all iron to be divalent is correct. Is this the case?\nResponse: Thanks for the insightful comments regarding the stoichiometric calculation\nof Fe2+/Fe3+ of chlorite and phengite. As noted by Forshaw and Pattison (2021), stoichiometric calculations of Fe3+ content are not valid for minerals with more than\none partially occupied crystallographic site, such as biotite, white mica, chlorite, and\nstaurolite (Schumacher, 1991). This limitation arises from the structural complexity of\nthese minerals, where multiple partially occupied sites and common vacancies render\ncharge-balance-based Fe3+ estimates inherently underdetermined and thus unreliable.\nTherefore, Fe3+ content in these minerals cannot be accurately resolved from EPMA\nanalysis alone. In this way, the Fe3+ content of phengite is replaced with ‘—’ instead of\n‘0.00’ in Table S5 of the Supplementary Information, to reflect that it cannot be reliably\ncalculated. Nevertheless, stoichiometric calculations for glaucophane remain valid\n(Schumacher, 2007; Hawthorne et al., 2012), indicating that all iron is present as Fe2+\n(Table S4). Combined with the absence of Fe3+-bearing minerals (e.g., aegirine) in the\nsystem, we have therefore modeled the system as Fe3+-free, which would more\naccurately reflect the natural assemblage.\nThe profile shown for glaucophane (Figure R5n) is not satisfactory, because it only\nshows the Na content (also according to line 188), whereas Na-amphibole is commonly\nzoned in Mg, Fe, and Mn. It is necessary to show at least a consistent XMg across the\nmineral to document that it formed under a single event. In the supplementary, I can\nonly find EPMA chemical data of phengite, and only representative analyses of garnet\nand glaucophane. ALL chemical analyses should be provided and accessible.\nResponse: Thank you for this helpful reminder. We agree that including an X profile\nMg\nof glaucophane would strengthen the interpretation and better demonstrate its formation\nduring the peak metamorphic stage. Accordingly, we have updated Figure S7o and its\ncaption in the Supplementary Information. The revised figure now presents strikingly\nflat profiles of both Na p.f.u. and X from core to rim, thereby reinforcing the\nMg\nrobustness of our interpretation. In addition, we have included the full set of SEM\nanalytical data for phengite and glaucophane in Table S4 in the Supplementary\nInformation to ensure transparency and data accessibility. Corresponding revisions\nhave also been made to the manuscript (Lines 188–189), and all references to the\nupdated Supplementary Figure number have been corrected throughout the manuscript.\nI still do not think the interaction of blocks in the mélange is a suitable explanation for\nyour structures, because you always see them isolated in many mélanges. Their\ninteraction, as proposed by the models cited in your manuscript, leads to clogging, so\nin their finite strain state you should find them coupled. Other models (e.g.\nVannucchi et al., 2022. Nature communications 13, 2311) consider clogging to be physically not possible and argue e.g. for weak blocks in a strong matrix.\nI think with your data you could critically consider all these limitations and try to evaluate these models or propose\nalternatives that fits your data and observation better. Alternatively, why discussing\nthings you have little data about (you have much microstructural data but little field\ndocumentation of the blocks, the mélange fabrics and their interaction).\nResponse: We appreciate the reviewer’s valuable and critical perspective. While our\nstudy primarily focuses on the microstructural analysis of individual blocks, we would\nlike to clarify that it is also informed by systematic field observations of both blocks\nand matrix in the Qingshui Gorge mélange (See ‘Geological and structural setting’\nsection of the Manuscript). These observations constrain their spatial distribution,\ngeometries, and fabric orientations, as shown in Figure 1 and Table S1, where the blocks\nare commonly preserved as isolated, yet closely spaced bodies. In fact, the force chain\nmodel we adopt (Beall et al., 2019a; Ioannidi et al., 2021) does not require that block\ninteraction leads to sustained jamming or clogging. Instead, their numerical modelling\nsuggests that force chains composed of randomly distributed clasts after finite shear\nstrains of about 1, but would subsequently reorganize in response to shear strains of as\nlittle as 0.1 (Beall et al., 2019b). This mechanical instability implies that even if blocks\nwere transiently coupled with each other at the deep subduction interface during certain\nstages of geological history, such block interaction configurations would be inherently\nshort-lived (transient in geological time), with failure leading to stress redistribution\ninto the broader matrix and eventual block decoupling. We therefore consider this\nmechanism as the plausible explanation for the present-day field observation of isolated,\nyet closely spaced blocks in the paleo-subduction mélange.\nAlternatively, the strength-inversion model proposed by Vannucchi et al. (2022)\nprovides a contrasting view. Based on integrated field observations and experimental\ndata, this model suggests that blocks in subduction mélanges may undergo\nhydrothermal alteration and become mechanically weaker than the surrounding matrix.\nIn such configurations, localized tremor or brittle failure can arise from mechanically\nweakened blocks embedded within a relatively stronger, creeping matrix—thereby\ncircumventing the need for sustained block interaction or clogging. Although the\nmetachert blocks studied here consist predominantly of quartz, which is relatively inert\nduring metamorphism and hydrothermal alteration compared to other rock-forming\nminerals (Bucher et al., 2002), their rheological strength is nonetheless highly sensitive\nto water fugacity (e.g., Austrheim, 1987; Mancktelow and Pennacchioni, 2005;\nMenegon et al., 2011; Giuntoli et al., 2022; Bukała et al., 2024). This, coupled with the\ninherent mechanical heterogeneity of subduction mélanges, leaves the door open to the\npossibility that the blocks may have been locally weaker than the surrounding matrix.\nIn any case, both the force chain model and the weak-blocks-in-strong-matrix model\nallow for transient mechanical coupling and stress transfer within the block-in-matrix\nsetting. Therefore, we have incorporated the strength-inversion model into the\ndiscussion of manuscript (Lines 316–320), alongside mechanisms such as asperity\nsubduction (e.g., asperities and seamounts; Moreno et al., 2014) and post-seismic stress\nrelaxation (e.g., Wang et al., 2007), as potential drivers for the formation of micro-shear\nzones within the blocks.\nI still do not think all your fabrics to be coeval, because it is hard to tie Ar/Ar ages in\nmica to quartz microfabrics. One may argue that mica does not recrystallize in shear\nbands and you may be dating fragments of relic micas disrupted by the shear bands. In\nany case, I accept the fact that you assume your fabrics to be coeval (as you state in\nyour reply), but I think this should be clearer in the text for the readers. This is crucial,\nbecause a slight change in the T of deformation would change the estimation from\npaleopiezometry and pseudosection modelling also has an intrinsic P-T error of at least\n+- 1 kbar and +-50 °C, likewise RSCM thermometry.\nResponse: Indeed, the dataset presented in our previous revision demonstrates a well-\nconstrained relationship among quartz CPOs, syn-kinematic mineral compositions, and\nboth metamorphic and protolith ages in the metachert block and calcschist matrix. On\nthe basis of this integration, we infer that the quartz microfabrics in both lithologies\nformed during a single deformation event, rather than reflecting multiple overprinting\nepisodes. However, due to the absence of currently developed, datable isotopic systems\nin quartz at measurable concentrations, it remains challenging to directly constrain the\nabsolute timing of deformation from quartz fabrics alone. In light of this limitation, we\nacknowledge that our interpretation—that quartz fabrics in both the block and matrix\nare coeval—carries a degree of uncertainty. Nonetheless, this uncertainty has been\nminimized to an acceptable level through the integration of structural, metamorphic,\nand geochronological data. To improve clarity, we have revised the manuscript (Line\n239) to explicitly state this interpretation and make transparent the underlying\nassumptions.\nIn general, leaving more space in the text to discuss the limitations of bridging\ngeological data (with its uncertainties) to geophysical data (currently observed SSEs)\nwould make the manuscript more appealing even to non-geologists.\nResponse: We appreciate this constructive suggestion. To broaden the manuscript’s\nappeal to a multidisciplinary readership, we have added two concise sentences (Lines\n346–353) that summarise both the key limitations and the broader scientific relevance\nof linking geological records to present-day SSE observations.\nSome minor comments below:\n201-202. This is not correct. SGR is not always happening at 420-500 °C. This\ntemperature range derives from the works of Stipp et al. (2002a, b), and these authors\nstated in multiple works that this temperature range is valid only for the Tonale Fault\n(this is also reported by Law, 2014). I know that SGR is kinda active around these\ntemperatures but there are conditions at which it might not be, for example at dry\nconditions (e.g. Menegon et al., 2011). In your case this is irrelevant: you have a\npseudosections and other T estimates, so I suggest removing this statement.\nResponse: As you rightly pointed out, the temperature range commonly associated with\nsubgrain rotation (SGR) recrystallization (420–500 °C; Stipp et al., 2002a, b) is not\nuniversally applicable. This range was derived from their studies in the Tonale Fault\nZone, and its general applicability is limited, as also indicated by Law (2014), which\nnoted that SGR activity is sensitive to factors such as water content, strain rate, grain\nsize, and deformation geometry. Moreover, Menegon et al. (2011) demonstrated that\nSGR can be effectively suppressed under dry lower-crustal conditions, even at\ntemperatures exceeding 500 °C. Given that our study already includes robust\ntemperature constraints from pseudosection modelling and RSCM geothermometry, we\nagree that this general statement is redundant and have removed it from the revised\nmanuscript (Line 202), as suggested."}, {"round": 3, "reviewer_comments": {"reviewer_3": "Thank you for addressing my comments. I am satisfied with the replies and recommend acceptance of the manuscript in its present form."}, "author_response": ""}]

Decision letter