Frank et al.
Progress in Earth and Planetary Science(2024) 11:34
https://doi.org/10.1186/s40645-024-00641-y

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessDeformation mechanisms and fluid
conditions of mélange shear zones associatedwith seamount subduction
Madison Frank1* , Kohtaro Ujiie1, Ginta Motohashi1 and Takayoshi Nagaya2

Abstract
Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocatedwith slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamountsubduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Islandpreserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange
deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone,
siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone
and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The
basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S–C fabrics in both mélanges show
top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstonedominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructuralobservations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creepof quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange
was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possiblepressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association
with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.
Keywords Deformation mechanisms, Fluid conditions, Microstructure, Mélange shear zone, Seamount subduction*Correspondence:
Madison Frankmfrank@geol.tsukuba.ac.jp
1
Graduate School of Science and Technology, University of Tsukuba,
1‑1‑1 Tennodai, Tsukuba, Ibaraki 305‑8572, Japan2
Department of Environmental Science, Tokyo Gakugei University, 4‑1‑1
Nukui Kitamachi, Koganei, Tokyo 184‑8501, Japan© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

1 Introduction
Seamounts derived from hot spot volcanism increase
seafloor roughness (e.g., Heuret et al. 2012; Nishimura
2014; Wang and Bilek 2014; Tilley et al. 2021; Gase et al.
2023), and seamount subduction promotes lithologic heterogeneity by introducing volcaniclastic and calcareous
sediments into the subduction interface (e.g., Harris et al.
2013; Barnes et al. 2020). Subducting seamounts also
influence tremor and slow slip events in several margins,
including New Zealand (e.g., Todd et al. 2018), Japan(e.g., Nishikawa et al. 2019; Tonegawa et al. 2020), andCosta Rica (e.g., Norabuena et al. 2004; Outerbridge et al.
2010). Compositionally heterogeneous materials associated with seamount subduction are thought to affect thefrictional and rheological properties of subduction shear
zones (Barnes et al. 2020; Vannucchi et al. 2022). Whilethe deformation processes and mechanisms preserved insandstone-mudstone mélange shear zones have been wellstudied (e.g., Fisher and Byrne 1987; Ujiie 2002; Fagereng2011), the behavior of seamount-related mélange shear
zones remains poorly understood. Vannucchi et al.
(2022) studied a mélange formed in association with seamount subduction and showed that hydrothermal alteration prior to subduction strengthened the volcaniclastic
mélange matrix. They propose the failure of weak basalt
lenses in a strong volcaniclastic matrix as the mechanism
for tremor. In contrast, Phillips et al. (2020a) suggestthat tremor and slow slip are characterized by velocityweakening altered basalt embedded in a weaker velocitystrengthening argillaceous matrix. Seamount-derived
sediments have been suggested to accommodate megathrust shear during seamount subduction (Bonnet et al.
2020; Leah et al. 2022; Barbero et al. 2023), but the deformation processes and mechanisms of megathrust shear
are still unknown, particularly in cases where seamounts
have subducted deeper into the subduction zone where
temperature-dependent viscous deformation may befavored (e.g., Fagereng and Beall 2021; Ando et al. 2023).
In this paper, we aim to define the lithology, structural characteristics, fluid conditions, and deformation
mechanisms preserved in an exhumed accretionary complex associated with seamount subduction on AmamiOshima Island, Ryukyu Arc. We describe an outcrop of
the Chichibu accretionary complex that contains twomélange shear zones: one composed of limestone mixed
with seamount-derived basalt, and one dominated byterrigenous sediments. From field and microstructural observations and Raman spectra of carbonaceous
materials, we determined the deformation styles andmechanisms recorded in each mélange shear zone andexamined temporal changes in fluid pressure and stressstate during seamount subduction to sub-greenschist
facies metamorphic conditions. We also try to relate thePage 2 of 17

deformation in the mélange shear zones to tremor associated with seamount subduction.

2 Geological setting
Amami-Oshima Island is located in the central Ryukyu
Arc, which lies between the Tokara Strait to the northand the Kerama Gap to the south (Fig. 1a). Basement
rocks of the island consist of the Eocene Wano Formation and Shimanto accretionary complex in the east andthe Chichibu accretionary complex in the west, separated
by the Butsuzo Tectonic Line (Osozawa 1984; Takeuchi
1993; Osozawa et al. 2009) (Fig. 1b). The Shimanto andChichibu accretionary complexes in Amami-OshimaIsland are the southwestern extension of the southernmost part of the Shimanto Belt and the Chichibu Beltin the Outer Zone of Southwest Japan (Taira et al. 2016;
Wallis et al. 2020).
The Chichibu accretionary complex on AmamiOshima Island is a mélange characterized by lenses ofsiliceous mudstone, sandstone, chert, limestone, andbasalt in a mudstone matrix (Osozawa 1984). Limestone
contains fusulinid, coral, and conodont fossils of Carboniferous to Permian ages, while interbedded limestone
and chert contain Triassic radiolarians (Osozawa 1984).
Basaltic rocks include pillow lava, breccia, and hyaloclastite (Takeuchi 1993). Geochemical analysis indicatesthat the basaltic rocks were derived from oceanic island
basalt (Motohashi et al. 2023b). The lithological assemblage constituting the mélange, the ages of limestone andchert, and the geochemical characteristics of basalt in
the Chichibu accretionary complex on Amami-OshimaIsland are well correlated with those in the southernmost
part of the Chichibu Belt in the Outer Zone of Southwest
Japan, which are interpreted to represent the accretion ofa seamount chain along the subduction zone (Matsuoka
1992). The ages of the terrigenous sandstone and mudstone in the Chichibu accretionary complex on AmamiOshima Island remain unknown, but the correlations of
the Chichibu accretionary complex between AmamiOshima and the Outer Zone of Southwest Japan suggest a Late Jurassic to Early Cretaceous accretionary age(Takeuchi 1993).
The Shimanto accretionary complex on AmamiOshima Island consists of mudstone and sandstoneintercalated with basalt, hemipelagic mudstone, andsilicic tuffs (Osozawa 1984; Osozawa et al. 2009). The
accretionary age was Early to Middle Cretaceous basedon radiolarians and ammonites in terrigenous sediments (Ishikawa and Yamakuchi 1965; Osozawa 1984;
Matsumoto et al. 1966). At the easternmost edge of theisland, Eocene Wano Formation composed of sandstone
and conglomerate containing basalt, chert, and marl is
exposed (Osozawa 1984).

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

The Chichibu accretionary complex was exhumed
above the Shimanto accretionary complex by the Butsuzo Tectonic Line. Osozawa et al. (2009) identified
outcrop-scale out-of-sequence thrust faults in the Chichibu accretionary complex. Otherwise, no widespreadexhumation-related structures have been identified in the
Chichibu accretionary complex.
Several intrusive rocks have been identified on AmamiOshima Island. Small granitic bodies of Eocene age haveintruded the Shimanto accretionary complex (Shibata
and Nozawa 1966). Middle Cretaceous camptonite andspessartite lamprophyres with chemical characteristics
representative of ocean island basalt have been identified
in both the Chichibu and Shimanto accretionary complexes (Kanisawa et al. 1983; Osozawa 1984). Recently,
middle Miocene alkaline basalt representative of forearc
alkaline magmatism was reported from Amami-OshimaIsland (Motohashi et al. 2023a).

3 Methods
Thirty-nine samples were collected from the various
lithologies of the Chichibu accretionary complex for
petrological, microstructural, and Raman spectroscopic
analyses. Forty-six thin sections cut parallel to lineationand perpendicular to foliation were made from the collected samples.

Page 3 of 17

3.1 SEM and EDS analysis

Field emission scanning electron microscopy (SEM) andenergy-dispersive X-ray spectroscopy (EDS) analyses wereconducted at the University of Tsukuba using a JEOL
JSM-IT300HR scanning electron microscope to investigate microstructures and mineral composition. Thin sections were coated with carbon prior to analysis to prevent
charging. EDS mapping was conducted under a high vacuum using an accelerating voltage of 15 kV, a working distance of 10 mm, and a beam current of 60–70 μA.
3.2 EBSD analysis

Electron backscatter diffraction (EBSD) data were collected from a representative sample of a micritic limestonelens to investigate the grain size, intragrain distortion, andcrystallographic preferred orientations (CPO) of the calcite. The thin section was polished with diamond pastes ofprogressively finer sizes (0.25 μm at minimum) and finallywith colloidal silica. EBSD analysis was conducted at the
University of Tokyo using a JEOL JSM-6510LV scanningelectron microscope and Oxford Aztec software. The
operating conditions were as follows: 10 kV accelerating
voltage and 24–25 mm working distance. Data were collected in a low vacuum (10 Pa) at × 40 magnification.
EBSD mapping was carried out using a step size of
1 μm over 516,683 pixels. Automatic indexing of calcite (a = b = 4.97 Å, c = 16.96 Å, and β = 120°; Paquette
and Reeder 1990) and chlorite (a = 5.24 Å, b = 9.07 Å,

Fig. 1 Tectonic setting and geological map of Amami-Oshima Island. Modified from Motohashi et al. (2023a, 2023b). a Distribution of the Shimanto
and Chichibu accretionary complexes in southwest Japan and the Ryukyu Arc. Amami-Oshima Island (red box) is located within the central Ryukyu
Arc. b Geological map of Amami-Oshima Island (modified from Takeuchi 1993). The field area, Asanzaki, is highlighted in red

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

c = 14.29 Å, and β = 97.0°; Eggleston and Bailey 1967)
grains was performed using up to 12 Kikuchi bands, a
Hough resolution of 60, and mean angular deviation(MAD) values less than 2.0°. Quantitative analyses of
EBSD data, including map analyses (e.g., grain reconstruction, grain size, and misorientation), CPO pole
figures, and orientation distribution functions (e.g.,
M-index), were carried out using the MTEX toolbox for
MATLAB (Hielscher and Schaeben 2008; Mainprice et al.
2014). Grain boundaries were constructed using a misorientation threshold criterion of 10 degrees, and grainsof isolated indexed points with ≤ 3 pixels were excluded.
Grain orientation spread (GOS) of calcite was defined as
the intragranular misorientation for a given pixel withineach of the calcite grains relative to the mean orientation of the parent calcite grain. CPO patterns of calcite
were calculated using a dataset of the mean orientationsof each calcite grain constructed based on EBSD indexing
to reduce any influence by large grains. In this study, no
significant change in calcite CPO patterns due to data filtering conditions using MAD was observed. CPO fabricstrength was assessed using the M-index (Skemer et al.
2005). All other methods of the EBSD analyses followed
Nagaya et al. (2017).
3.3 Raman spectroscopic analysis

Three mudstone samples were collected with ~ 50 m
spacing to determine the peak temperature from Raman
spectroscopic analysis of carbonaceous material. Samples were analyzed at Nagoya University using a Nicolet
Almega XR (Thermo Scientific, Yokohama, Japan) with a
532 nm Nd-YAG laser passed through a confocal microscope (Olympus, BX51: Olympus, Tokyo, Japan) with a
100 × objective (Olympus Mplan-BD 100X, NA = 0.90).
To avoid influence from mechanical polishing, carbon
grains embedded within transparent minerals were targeted in standard petrographic thin sections and irradiated for 30 s with a 1–3 mW laser. Forty carbon grains
were measured per sample.
Raman spectra peaks were fitted using the methodology described by Kouketsu et al. (2014). Spectral peak
patterns were manually analyzed initially to identify
the best fit of G-, D1-, D2-, D3-, and D4-bands following the flowchart in Kouketsu et al. (2014). After the initial manual peak fitting, the spectra were automaticallydecomposed using PeakFit v4.12 (Systat Software Inc.).
We corrected the Raman spectra by subtracting a linear
baseline in the range of 1000–1800 ­cm−1, and then the
spectra were decomposed with a Gaussian–Lorentzian
Sum Voigt function. In all cases, the D4-band was fixedat 1245 ­cm−1, and peak width and shape were allowed tovary. In measurements where the D1-band peak shape
was asymmetric (i.e., Fitting F of Kouketsu et al. 2014),

Page 4 of 17

the G-band was fixed at 1593 ­cm−1 with a Lorentz function. Temperatures were calculated using the full width at
half maximum of the D1-band (FWHM-D1):

T (◦ C) = −2.15 × (FWHM - D1) + 478.

(1)

Values exceeding 2 × the standard deviation (σ) wereidentified as outliers and excluded prior to calculation of
the mean temperature.

4 Results
4.1 Field observations

The study area is located in Asanzaki, in the northwestern part of Amami-Oshima Island (Fig. 1b), where theoutcrop is most laterally accessible. Here, two types ofmélanges are recognized in the Chichibu accretionary
complex: basalt–limestone mélange (BLM) and mudstone-dominated mélange (MDM) (Fig. 2a). The BLM
is distributed in the northwest and intruded by basaltic
sills along the westernmost margin. The MDM is distributed in the southeast and cut by a series of minor
NW-dipping thrust faults. The shear direction of thethrust faults, determined by quartz slickenfiber steps,
is top-to-SE or ESE reverse shear with one exception
showing top-to-S shear (Fig. 2b). No thrust faults areobserved within the BLM. Foliations in the BLM and
MDM are parallel; both dip moderately WNW (Fig. 3a).
The BLM overlies the MDM, and the contact between
the BLM and MDM is parallel to foliations in the BLM
and MDM (Fig. 3b). The characteristics of the BLM and
MDM are described below.
4.1.1 Basalt–limestone mélange (BLM)

The BLM is characterized by lenses of micritic limestone
and basalt in a green-gray matrix (Fig. 3c). The minimum
thickness of the BLM, limited by outcrop exposure, is
18 m. Lenses in the BLM are subangular to subrounded,with a long axis of 0.05–20 m. The proportion of limestone to basalt in the matrix varies across the outcrop,from sections of limestone with almost no macroscopic
basalt lenses observable to zones with an approximately
4:1 limestone/basalt lens ratio. Basaltic pillow lava andbreccia and interbedded calcareous mudstone/greenstone exist locally in the BLM (Fig. 2a). Bedding ininterbedded calcareous mudstone/greenstone is approximately parallel to the foliation of the BLM but locallyfolded. The matrix of the BLM is well foliated and characterized by anastomosing polished surfaces with slickenlines showing scattered trends (Fig. 3d, f ). Limestone andbasalt lenses are surrounded by matrix foliation (Fig. 3e)and locally elongated parallel to the foliation. In places,
asymmetric lenses and the matrix foliations show S–C
fabric consistent with top-to-SE shear sense (Fig. 3c). S
surfaces, defined by the alignment of matrix foliations

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

and long axes of basalt and limestone lenses, are cut by C
surfaces at less than 45°.
Within the BLM, calcite extension and shear veinsare rarely observed. Calcite shear veins near-parallel tofoliation were identified in outcrops where the matrix
is intensely foliated but cannot be traced for more thana few centimeters along their length. Slickenfibers andslickensteps on calcite shear veins indicate top-to-SE or-ESE shear (Fig. 3f ). Within individual basalt and micritic
limestone lenses, extension veins of < 10 cm in length areobserved at various angles to foliation. Extension veins athigh angles to the foliation sometimes cross-cut multiple
lenses and are displaced by shear surfaces.
Basaltic sills intruding into the BLM have sharp boundaries and display chilled margins. Calcite veins are rarely
observed near basaltic sills. Foliations within the sills
are subparallel to those in the BLM and the MDM. The

Page 5 of 17

geochemical features of the basaltic sills and timing of
intrusion suggest that the basaltic sills originated frompetit-spot volcanism in the oceanic plate, while othernonintrusive basaltic rocks show geochemical featuresconsistent with hot spot volcanism (Motohashi et al.
2023b).
4.1.2 Mudstone‑dominated mélange (MDM)

The MDM is characterized by lenses of sandstone, siliceous mudstone, and basalt in a mudstone matrix(Fig. 4a). The length of the long axis of the lenses ranges
from 0.02 to 20 m. The minimum thickness of the MDM
is 31 m or thinner, as thrust faults may contribute to repetition of the MDM (Fig. 2a). Matrix foliations are anastomosing and wrap phacoids, showing a scaly fabric.
Slickenlines on the polished foliations show a scatteredtrend (Fig. 4c). Asymmetric lenses and scaly foliations

Fig. 2 Geological map of Asanzaki. a Geological map showing two types of mélanges. b Stereonet showing poles to thrust faults in the MDM(black dots) and quartz slickenfibers (hollow diamonds) on the thrust faults

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 6 of 17

Fig. 3 Basalt–limestone mélange (BLM) at Asanzaki. a Stereonet showing poles to foliation and average foliation in BLM (green) and MDM (blue). b
Contact between the BLM and MDM. c BLM: limestone (L) and OIB basalt lenses in a greenish matrix showing S–C fabric consistent with top-to-SE
shear. d Polished surface with slickenlines (red lines) in the BLM matrix. White patches on the polished surface are calcite. e BLM showing that matrix
foliation surrounds gray limestone lenses (L). f Stereonet showing average foliation of the BLM (green circle), slickenfibers from calcite shear veins(open box), and slickenlines on polished surfaces (open green circles)

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 7 of 17

constitute S–C fabric with top-to-SE shear sense (Fig. 4a).
S surfaces defined by the alignment of lenses and matrix
foliations are transected or deflected by C surfaces at lessthan 45°.
Both quartz-filled extension and shear veins areobserved in the MDM. The density of quartz veins varies throughout the MDM with no clear correlation to thepresence of thrust faults or the distribution of basaltic
or sedimentary lenses. The extension veins at moderate
to high angles to the foliation are generally restricted tolenses of sandstone and siliceous mudstone with spacingof 1–2 cm (Fig. 4b). Some extension veins at high angles
to the foliation penetrate the mudstone matrix and are
displaced by shear surfaces. The extension veins parallelto the foliation or the S-surface are laterally continuous
for less than 1 m (Fig. 4b, d) and locally underwent sigmoidal deformation associated with top-to-SE viscousshear (Fig. 4f ). Shear veins, defined by the presence ofslickenfibers (Fig. 4e), develop parallel to the C-surface(Fig. 4b). Shear sense, determined from slickenfibers andsteps, indicates top-to-SE thrust movement (Fig. 4d).

Coarse-grained calcite is restricted to the margins of
micritic limestone lenses and extension veins. Extension
veins that crosscut both lenses and matrix at moderate
to high angles to foliation are displaced by shear surfaces(Fig. 6a). Calcite veins generally consist of blocky, coarsegrained calcite > 40 μm in size (Figs. 5a, 6c). In contrast to
microcrystalline calcite, e-twins are widespread in coarse
calcite grains and display bulging boundaries (Fig. 6c).
Lamellae morphology and width of calcite twins wereassessed using optical microscopy to establish the dominant twin type and associated deformation temperatures(Burkhard 1993). Calcite twin morphology is defined byrepeated straight twins of 1–10 μm width, corresponding
to type II calcite twins formed at temperatures between
150 and 300 °C (Burkhard 1993). We also considered thecalcite twin density paleopiezometer where differential
stress (σd) is calculated from calcite twin density (NL), thenumber of twins per mm, using the following equation(Rybacki et al. 2013):√
σd = (19.5 ± 9.8) × NL
(2)

4.2 Mineral assemblage, microstructures, and temperature
conditions
4.2.1 Basalt–limestone mélange (BLM)

Twin density was measured in 76 grains from 7 samples
across the BLM. The average NL ranged from 24 to 200,
yielding differential stresses between 97 and 276 MPa(Fig. 6d).

Limestone lenses are composed of microcrystalline calcite and radiolarian tests replaced by calcite (Fig. 5a).
EBSD mapping of a limestone lens gives an average grain
size of 7.8 μm (Fig. 5b) for microcrystalline calcite. Since
the proportion of unindexed grains (Fig. 5d), representative of the smallest grain size is high, the average grain
size is expected to be overestimated. Microcrystalline
calcite has small misorientation angles of mostly < 1–2°
(Fig. 5c). The limestone lenses show no chemical zoning. The crystallographic fabric is random (Fig. 5e), with
an M-index (Skemer et al. 2005) of 0.01. Albite grains areoften observed within limestone lenses.
Basalt lenses are aphyric and composed of fine-grained(< 15 μm) albite with microcrystalline calcite and Fe–Ti
oxide minerals (Fig. 5f ). The BLM matrix is composed ofchlorite with minor illite. Foliations, defined by the alignment of chlorite with lesser illite, wrap limestone andbasalt lenses to form an interconnected network or S–C
fabric showing top-to-SE shear sense (Fig. 5f ). Fe-Ti oxideminerals and illite are concentrated in dark seams within
the chloritic matrix (Fig. 6b). Pressure shadows aroundcalcite or albite grains or chlorite strain shadows aroundbasalt or limestone lenses have not been observed. The
chilled margins of intrusive basalt sills contain glassy
groundmass that is locally altered to chlorite (Motohashi
et al. 2023b), but otherwise there is no mineralogicalchange in the BLM with respect to sill proximity.

4.2.2 Mudstone‑dominated mélange (MDM)

The mudstone matrix of the MDM is primarily composedof quartz, albite, illite, and chlorite with lesser calcite and
Fe–Ti oxide minerals (Fig. 7a). Illite is the dominant phyllosilicate in the matrix. Albite grains range in size from15 to 60 μm and display rare twins. Quartz grain size ishighly variable; quartz in the mudstone matrix ranges
from 15 to 80 μm, while grains in quartz veins can havea long axis of up to 1.5 mm. Quartz grains in the matrix
and in veins commonly display undulose extinction, andbulging grain boundaries are present in quartz veins(Fig. 7b). Under the microscope, the quartz-filled foliation-parallel extension veins have a crack-seal texture
defined by stylolitized phyllosilicate solid inclusion bands
aligned nearly parallel to the vein margins (Fig. 7c). Spacing of solid inclusion bands is variable, but commonly
ranges from 0.1 to 0.6 mm. Quartz grains in crack-sealveins are generally blocky (Fig. 7d), but elongated grains
with the long axis perpendicular to the vein margin are
also observed. In some cases, quartz shear and extension
veins are associated with veins of blocky, coarse-grained
calcite. Microscopically, S–C fabric is characterized by
S surfaces defined by the alignment of lenses and dark
seams composed of illite and titanomagnetite with lesser
chlorite that is truncated or deflected by the C surfaces (Fig. 7e). Pressure shadows of quartz and illite areFrank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 8 of 17

Fig. 4 Mudstone-dominated mélange (MDM) at Asanzaki. a Dark, foliated mudstone with siliceous mudstone and sandstone lenses showing
S–C fabric consistent with top-to-SE shear. b Quartz extension and shear veins in the MDM. Extension veins at high angles to foliation (greenline) are confined to a sandstone lens. The extension veins (orange arrow) are oriented parallel to S-surfaces. Extension veins and foliation are cutby the shear vein (blue arrow) parallel to the C-surface. c Stereonet showing average foliation of the MDM (blue circle) and slickenlines (open blue
circles) on polished surfaces. d Stereonet showing average foliation of the MDM (blue circle), average shear vein (black dashed circle), and average
foliation-parallel extension vein (orange circle). Slickenfibers from quartz shear veins are shown as hollow boxes, poles to foliation-parallel extension
veins are shown in orange dots, and poles to shear veins are shown in black dots. e Quartz slickenfibers with steps (yellow arrow) on shear vein
showing reverse, top-to-SE shear. f Viscously deformed foliation-parallel extension veins

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 9 of 17

Fig. 5 Micritic limestone lenses in the BLM. Panels b–e were generated using MTEX software (Hielscher and Schaeben 2008; Mainprice et al.
2014). a Microcrystalline calcite (Cc) with radiolarian tests showing partial recrystallization to calcite. b Histogram of microcrystalline calcite grain
size calculated from the diameter of reconstructed EBSD grains. c Histogram of grain orientation spread (GOS) of calcite, showing a low degreeof intragranular misorientation. d GOS map of calcite on EBSD band contrast image of the micritic limestone. A given pixel within each of the calcite
grains is colored by the angular degree of intragranular misorientation relative to the mean orientation of the parent grain. Most white grains
correspond to chlorite and albite grains, identified by EBSD indexing and optical microscopy. Unindexed pixels are black. e CPO pole figuresof microcrystalline calcite grains. CPO is plotted as equal-area, upper hemisphere projections. The crystal directions shown here were selected
to examine the possibility of deformation under low-temperature calcite slip systems (De Bresser and Spiers 1997). The M-index (m) indicatesthat the fabric intensity is very weak (Skemer et al. 2005). Grains are plotted with the X-direction parallel to lineation and the Z-direction normal
to foliation. N represents the number of measured points. f Micritic limestone (Cc) and basalt lenses in green chloritic matrix with S–C fabricshowing top-to-SE shear sense. Dark seams contain Fe and Ti oxide minerals

observed adjacent to quartz and albite grains oriented
parallel to the S surfaces (Fig. 7f ).
Siliceous mudstone lenses contain minor amounts ofphyllosilicates compared to the mudstone matrix. Sandstone lenses are composed of quartz and albite with alarger grain size (generally > 40 μm) and a relatively minor

proportion of matrix dominated by illite. Similar to basalt
lenses in the BLM, basalt lenses in the MDM are composed of fine-grained albite with abundant Fe–Ti oxideminerals and chlorite.
Raman spectra of carbonaceous material in the matrix
of MDM show prominent peaks of the D1- and D2-bands

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 10 of 17

Fig. 6 Photomicrographs of coarse-grained calcite in the BLM. Panels a and b are plane-polarized light, panel c is cross-polarized light. a
Coarse-grained calcite (Cc) is restricted to the margins of micritic limestone (Cc) lenses and extension veins. The extension vein at high angle
to foliation (green arrows) is displaced by dark seams. b Close-up of the chlorite (Chl) matrix along the edge of a micritic limestone (Cc) lens.
Dark seams are composed of iron (Fe), titanium (Ti), and illite (Il). c Thin section (cross-polarized light) with calcite showing type II (Burkhard 1993)
twins. Bulging grain boundaries (BLG) can be observed at the contact between coarse calcite grains. d Average differential stress calculated
from the e-twin density (NL) paleopiezometer (Rybacki et al. 2013) of coarse-grained calcite. Individual data points represent the twin densityof individual calcite grains(Fig. 8a). The peak deformation temperatures, estimatedfrom the full width at half maximum of the D1-band, are
281 ± 11 °C, 286 ± 12 °C, and 289 ± 12 °C (Fig. 8b, Supp.
Info S1). The calculated uncertainties are smaller than theuncertainty of ± 30 °C inherent in the calibration (Kouketsu et al. 2014). The estimated temperatures are consistent with the occurrence of bulging recrystallization
in MDM quartz veins at temperatures ≥ 280 °C (Stippet al. 2002) and the development of type II calcite twins
between 150 and 300 °C (Burkhard 1993) in BLM calcite
veins.

5 Discussion
5.1 Deformation mechanisms of the mélanges

At macroscale, the scaly fabric of the MDM matrix isdefined by anastomosing polished surfaces with slickenlines (Fig. 4a), indicating that deformation of thematrix was accommodated by frictional sliding of illite.
However, under the microscope, pressure shadows areobserved around quartz and albite grains in the matrix(Fig. 7f ), and dark seams composed of illite with Ti andFe oxide minerals likely represent zones of insoluble residue (Fig. 7a, f ), consistent with deformation by pressure
solution. Furthermore, stylolitized solid inclusion bands
within crack-seal quartz veins (Fig. 7c) also indicate

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 11 of 17

Fig. 7 Photomicrographs of the MDM. Panels a, c, e, and f are plane-polarized light, and panels b and d are cross-polarized light. a The mineral
assemblage of the mudstone matrix consists of quartz (Qtz), albite (Ab), and rare calcite (Cc) grains in a matrix of illite (Il) with lesser chlorite (Chl).
Dark seams contain titanium (Ti), iron (Fe) oxide minerals, and carbonaceous material (Cm). b Quartz grains from a foliation-parallel extension vein
display bulging grain boundaries (BLG) and undulose extinction. c Photomicrograph (plane-polarized light) of a crack-seal extension vein parallel
to foliation. Stylolitized phyllosilicate inclusion bands (yellow arrows) subparallel to the vein margin (blue line) indicate that the vein opened
at a high angle to the vein margin. d Photomicrograph (cross-polarized light) of the crack-seal extension vein in panel c showing blocky quartz
grain morphology. e S–C fabric showing top-to-SE shear sense. f Pressure shadow composed of quartz (Qtz) and illite (Il) around an albite (Ab) grainin the mudstone matrix

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

deformation by pressure solution. Quartz grains in the
MDM exhibit undulose extinction and bulging grainboundaries (Fig. 7b), consistent with deformation by dislocation creep. The deformation and microstructural features of the MDM are therefore interpreted to represent
the product of combined frictional sliding of illite andpressure solution creep of quartz (e.g., Bos and Spiers
2002), in conjunction with quartz dislocation creep.
In the BLM, polished surfaces with slickenlines (Fig. 3d)
indicate deformation of the chloritic matrix by frictionalsliding. At the microscopic scale, dark seams composedof Ti and Fe oxide minerals and illite (Fig. 6b) likely represent insoluble material associated with pressure solution. However, pressure shadows around limestone andbasalt lenses and stylolites in calcite are not recognized.
EDS mapping of limestone lenses shows no chemicalzoning. The e-twinning and bulging grain boundaries incoarse-grained calcite in calcite extension veins (Fig. 6c)
are indicative of intragranular deformation by dislocation
climb and the onset of dynamic recrystallization (Burkhard 1993; Passchier and Trouw 2005; Lacombe et al.
2021). In contrast, the paucity of twins, limited intragranular deformation, and random CPO fabric (Fig. 5c–e)suggest that dislocation creep is not a dominant deformation mechanism for microcrystalline calcite in limestone

Page 12 of 17

lenses. Rather, the small size of the microcrystalline
calcite could facilitate grain size-dependent diffusion
mechanisms (Schmid et al. 1977; Walker et al. 1990; Houet al. 2022; Ujiie et al. 2022). The deformation and microstructural features of the BLM are therefore interpreted
to represent frictional sliding of chlorite and dislocation
creep of coarse-grained calcite, with possible pressure
solution creep in the BLM matrix and diffusion creep offine-grained calcite in limestone lenses.
5.2 Origin of the mélanges

The BLM is characterized by the mixing of micritic limestone and basalt and does not contain terrigenous materials. Geochemical analyses indicated that basaltic lenses
in the BLM were classified as ocean island basalt (OIB)(Motohashi et al. 2023b). The chloritic matrix in the
BLM may be formed by alteration of basaltic glass on theseafloor or during subduction, as reported from basaltbearing mélanges in exhumed accretionary complexes(Ujiie et al. 2007; Kameda et al. 2011; Phillips et al. 2020b;
Leah et al. 2022). Motohashi et al. (2023b) and Osozawa
et al. (2009) suggested that mixing of the OIB-derived
basalt and micritic limestone above the seamount likely
occurred prior to subduction during submarine landslides along the seamount flank. One possible location

Fig. 8 Raman spectroscopic analysis of carbonaceous materials in the mudstone matrix of the MDM. a Representative Raman spectraof carbonaceous material (upper) and modeled spectra with decomposed peaks (lower). b Histograms showing the frequency distributionof temperature calculated using FWHM-D1 (Eq. 1) from 40 spot measurements of carbonaceous material. All data are shown in the histograms,
but outliers greater than 2σ were excluded prior to calculation of the mean temperature. Dashed red lines and text indicate mean temperatureand gray shaded regions show 2σ

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

for such mixing is at the foot of a steeply dipping fault
scarp formed by normal faulting associated with oceanic
plate bending in the outer rise region (e.g., Kobayashi
et al. 1987; Taira et al. 1989). We also interpret the BLM
as the result of a submarine landslide at the seamountmargin (Fig. 9a).
OIB-derived basalt lenses are present in the MDM(Motohashi et al. 2023b), while terrigenous materials
are absent from the BLM. The BLM lies above the MDM
without a fault plane (Fig. 3b). The BLM overlying the
MDM by isoclinal folding is unlikely due to the absenceof the opposite stratigraphic/structural relationship (i.e.,the MDM overlying the BLM) and the absence of a hingezone (Fig. 2a). We therefore interpret that, while some
OIB-derived basalt was incorporated into the trenchfill terrigenous sediments as lenses, there was no mixing between the trench-fill terrigenous sediments andthe BLM during the juxtaposition of the BLM over the
trench-fill terrigenous sediments by a submarine landslide (Fig. 9a).
Based on the absence of an asymmetric shear fabric andthe presence of overlapping planar cleavages in the lensesof oceanic material, Osozawa et al. (2009) interpreted themixing of terrigenous and oceanic materials to be due
to debris flows from an older accreted oceanic material
into trench-fill terrigenous sediments. However, both the
BLM and the MDM preserve an asymmetric S–C fabricshowing SE-directed shear consistent with megathrust
shear (Figs. 3c, 4a, b). In addition, the foliations overprint
both the BLM and the MDM (Fig. 3a). Thus, we suggestthat the BLM was juxtaposed above the MDM by submarine landslide and then underwent subduction-related
megathrust shear together to form a mélange shear zone.
The mineral assemblage of basaltic rocks lacks epidote
and actinolite, quartz grains show bulging recrystallization that becomes dominant at ~ 280 °C (Stipp et al.
2002) and undulose extinction (Fig. 7b), and Raman spectra of carbonaceous material give a peak temperatureof ~ 281–289 °C (Fig. 8) suggesting that the mélange shear
zone underwent sub-greenschist facies metamorphism(Fig. 9b).
5.3 Temporal changes in differential stress and fluidpressure

The extension veins in the sandstone and siliceous mudstone lenses generally develop at high angles to thefoliation (Fig. 4b). Some extension veins penetrate bothcompetent lenses and mudstone matrix, indicating thattensile fracturing persisted as the block-in-matrix fabric developed. Extension veins in boudinaged lenses andmatrix foliation are commonly observed in subduction
mélanges composed of sandstone lenses in mudstonematrix and are thought to be formed by layer-parallel

Page 13 of 17

extension under subvertical maximum principal stress(σ1) during subduction (Fisher and Byrne 1987; Ujiie
2002). Similarly, the extension veins at high angles tothe foliation in the MDM are thought to form duringsubduction.
Extension veins parallel to foliation suggest rotation ofσ1 from sub-perpendicular to parallel to foliation. Solid
inclusion bands subparallel to vein margins indicate
repeated fracturing and silica precipitation. These features suggest repeated generation of near-lithostatic fluidpressures. Sigmoidal deformation of foliation-parallelveins with top-to-SE shear sense indicates that viscousshear by pressure solution creep occurred after tensile
failure under near-lithostatic fluid pressures (Fig. 4f ).
The stylolitized solid inclusion bands also suggest that
deformation by pressure solution creep during which
σ1 rotated to perpendicular to the S surface followed
repeated tensile failure under near-lithostatic fluid pressure (Fig. 7c). Quartz-filled shear veins parallel to the C
surfaces truncate sigmoidal fabrics, likely representing
dilational shear at lower differential stress and higher
fluid pressure than viscous shear by pressure solution
creep (Fig. 9c).
In contrast to the MDM, quartz veins are not observedat the outcrop scale in the BLM, in part due to the low
silica content of the basaltic and limestone rocks. At the
microscopic scale, the calcite-filled extension veins at highangles to the foliation are displaced along shear surfaces
in the matrix (Fig. 6a), while calcite-filled extension andshear veins at low angles to the foliation are rarely observed(Fig. 9c). Fewer veins in the BLM relative to the MDM could
represent lower fluid pressures and/or higher strength of
the BLM relative to the MDM. For example, the tensile
strength of limestone (T0 = 5–25 MPa; Winkler 1997) andbasalt (T0 = 10–30 MPa; Winkler 1997) is greater than thatof pelitic rock (T0 = 1 MPa; Lockner 1995). Another possibility for fewer veins in the BLM is a decrease in calcite solubility (Weyl 1959) at temperatures of ~ 281–289 °C under
sub-greenschist facies metamorphic conditions.
Calcite twins formed by dislocation creep in the BLM
record σd of 97–276 MPa (Fig. 6d), comparable to σd of
30–255 MPa from calcite twinning in chloritic Gwana
mélange (Leah et al. 2022). Dislocation creep of quartz
and combined pressure solution creep with frictionalsliding of phyllosilicates in the MDM are favored at lower
fluid pressure and higher σd. In contrast, considering
T0 = 1 MPa (Lockner 1995), foliation-parallel extension
veins and shear veins in the MDM can develop at σd = 4
and 4 < σd < 5.66 MPa, respectively (Secor 1965). Therefore, the mélange shear zone, at least the MDM, records
temporal changes in σd, and fluid pressure.
Foliation-parallel veins suggest σ1 parallel to the
mélange shear zone. Such a stress field is likely downdip

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

Page 14 of 17

Fig. 9 Schematics showing the development of the BLM and MDM as a result of seamount subduction. a A seamount composed of OIB basalt(dark green) capped by micritic limestone (light blue) entering a subduction zone. The landward edge of the seamount may be displaced
by normal faults accommodating plate flexure at the outer rise region. A submarine landslide at the steeply dipping normal fault scarps
results in mixing of micritic limestone and OIB basalt and deposition above terrigenous trench-fill sediments. b The basalt–limestone deposit
and terrigenous trench-fill sediments are subducted to sub-greenschist facies metamorphic conditions and form megathrust shear zones downdip
of a subduction seamount, resulting in the formation of the BLM and MDM. c Deformation features in the BLM and MDM showing an S–C fabric.
The BLM is characterized by OIB basalt (dark green) and micritic limestone (light blue) lenses wrapped by the chloritic matrix (light green). The
MDM is represented by sandstone and siliceous mudstone lenses (yellow) surrounded by the illitic matrix (blue) and records progressive quartz vein
formationof the subducting seamount, where tectonic compression
is enhanced, rather than updip, where extension in thewake of the subducting seamount is expected (Sun et al.
2020; Bangs et al. 2023; Gase et al. 2023). The clusteredquartz veins in the subducting rocks are thought to represent a geological fingerprint of the tremor (Fagereng
et al. 2011; Ujiie et al. 2018, 2024; Giuntoli et al. 2022).
The brittle deformation under near-lithostatic fluid pressure recorded in the quartz-filled extension and shear
veins may also represent tremorgenic deformation in themegathrust-related shear zones downdip of a subducting seamount, as observed in the northern Hikurangi
subduction zone (Todd and Schwartz 2016; Todd et al.
2018; Barnes et al. 2020). In addition to tremor, slow slip
events are observed downdip of a subducted seamount(Nishimura 2014; Collot et al. 2017; Barker et al. 2018;
Todd et al. 2018). Viscous shear in the MDM is accommodated by combined pressure solution creep withfrictional sliding of illite in conjunction with quartz dislocation creep. Stress greater than 100 MPa is requiredfor pressure solution creep in metasedimentary rocks toaccommodate slow slip strain rates (Condit and French2022). This suggests that slow slip accommodated by viscous shear is unrealistic in the MDM. Leah et al. (2022)show that frictional sliding of chlorite at elevated fluidpressure may accommodate slow slip. This may be comparable to the viscous shear in the BLM that was accommodated by frictional sliding of chlorite. However,evidence for elevated fluid pressure is generally lacking in
the BLM.

6 Conclusions
We examined the lithology, deformation mechanisms,
and fluid conditions of the mélange shear zones formedin association with the subduction of a seamount. The
mélange shear zones are composed of basalt–limestonemélange (BLM) originating from the mixing of limestone
and basalt at the foot of a seamount and mudstone-dominated mélange (MDM) derived from the terrigenous
sediments. The juxtaposition of the BLM over the MDM

Frank et al. Progress in Earth and Planetary Science(2024) 11:34

occurred by submarine landslide from the seamount totrench-fill terrigenous sediments. The mineral assemblage of basaltic rocks, quartz grains with bulging grainboundaries and undulose extinction, and a peak temperature of ~ 281–289 °C determined by Raman spectraof carbonaceous material suggest that the mélange shear
zones were deformed under sub-greenschist facies metamorphism. The deformation mechanisms of the BLM
are frictional sliding of chlorite and dislocation creep ofcoarse-grained calcite, possibly accompanied by diffusion creep of very fine-grained calcite, whereas the MDM
was deformed under the operation of dislocation creepof quartz and combined quartz pressure solution andfrictional sliding of illite. The MDM records temporalchanges in differential stress and fluid pressure downdip
of a subducting seamount; quartz-filled shear and extension veins formed during periods of very low differential
stress and near-lithostatic fluid pressure, while dislocation creep and pressure solution creep combined withfrictional sliding of illite operated under higher differential stress and lower fluid pressure. The brittle deformation under very low differential stress and near-lithostatic
fluid pressure may represent tremor downdip of a subducting seamount. However, viscous shear in the MDM
and BLM under higher differential stress and lower fluidpressure is unlikely to accommodate slow slip events.
Abbreviationsσ	Standard deviation
σ1	Maximum principle stressσd	Differential stress
BLM	Basalt–limestone mélange
CPO	Crystallographic preferred orientation
EBSD	Electron backscatter diffraction
EDS	Energy-dispersive X-ray spectroscopy
FWHM-D1	Full width at half maximum of the D1-band
GOS	Grain orientation spreadMAD	Mean angular deviation
MDM	Mudstone-dominated mélange
NL	Number of twins per mm
OIB	Ocean island basalt
SEM	Scanning electron microscopy
SSE	Slow slip event
T0	Tensile strength

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​024-​00641-y.
Additional file 1.
Acknowledgements
We would like to thank the Amami Gunto National Park Management Office(Amami Wildlife Conservation Center) of the Ministry of the Environment for
permission to sample at Asanzaki, Y. Kouketsu for their analytical support ofRaman analysis, T. Sano for review of geochemical interpretations, S. Wallis
for providing the EBSD system at the University of Tokyo, Y. Yamasaki for fieldassistance, and N. Nishiyama for both field assistance and critical discussion of
our results. Constructive comments from T. Byrne, P. Vannucchi, A. Fagereng,and an anonymous reviewer significantly improved the manuscript.

Page 15 of 17

Author contributions
M.F. and K.U. conceptualized the study, the methodology was determined by
M.F., K.U., and T.N. K.U. obtained funding and supervised the project. M.F., K.U.,
and G.M. contributed to fieldwork and, in collaboration with T.N., data acquisition and investigation. M.F. wrote the manuscript and all authors contributed
to the review and editing process and visualization.
Funding
This project was supported by funding from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants JP16H06476, 20K21050, 20KK0078, and21H05203 to KU.
Availability of data and materials
Results of the Raman spectra analysis are available in Supplementary Information. The datasets supporting the conclusions of this article including structural measurements, Raman spectra, and EBSD data, are available in the OpenScience Framework repository, https://​osf.​io/​ydbha/. Code for R Project for
Statistical Computing (R Core Team 2021) v4.1.0 with RStudio v2022.07.1+554(used for calculation of Raman temperature and plot generation) and MATLAB
R2022a with the MTEX (Mainprice et al. 2014) v5.8.2 toolbox (EBSD analysis) isavailable from the GitHub repository, https://​github.​com/​madis​onfra​nk/​Asanz​
aki_​Defor​matio​nMech​anisms.

Declarations
Competing interests
The authors declare that they have no competing interests.
Received: 27 November 2023 Accepted: 10 June 2024

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