# Rifting a post-orogenic continental crust: Insights from the South China Sea

Fan Zhou, Laetitia Le Pourhiet, Manuel Pubellier, Matthias Delescluse

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Fan Zhou, Laetitia Le Pourhiet, Manuel Pubellier, Matthias Delescluse. Rifting a post-orogenic continental crust: Insights from the South China Sea. Earth and Planetary Science Letters, 2025, 671, 10.1016/j.epsl.2025.119679. hal-05318168

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# Rifting a post-orogenic continental crust: Insights from the South China Sea

Fan Zhou<sup>a,* ID</sup>, Laetitia Le Pourhiet<sup>b,c ID</sup>, Manuel Pubellier<sup>a ID</sup>, Matthias Delescluse<sup>a ID</sup>

## ARTICLE INFO

Editor: Dr C Carolina Lithgow-Bertelloni

Keywords:

Continental rifting

South China Sea

Post-orogenic process

Inheritance

Hyper-extension

Wide rift

## ABSTRACT

The South China Sea (SCS) is an oceanic marginal basin that transects continental lithosphere and comprises two sub-basins. Despite a nearly constant stretching rate along strike in both space and time, continental breakup in the Southwest Subbasin is delayed by 9 to 13 Myr relative to the East Subbasin. During this delay, a wide rift formed and accommodated fast extension at 4 cm/yr. This rift features synchronous detachments and basins, a non-planar Moho, and two crustal necking zones spaced 500 kilometers apart. These characteristics are not predicted by conventional wide-rift nor fast-rifting models. We test the hypothesis that structural inheritance, in particular preserved crustal nappe stacks formed in a forearc setting during post-orogenic processes linked to Pacific subduction, may explain these features. Using 2D numerical models, we show these inherited structures promote distributed deformation and systematically delay continental breakup. Variations in lower crustal thickness, initial dip angle, and mantle temperature control crust-mantle coupling and boudinage, producing three distinct Moho textures and two necking zones. One zone accommodates final breakup; the other fails at different maturity stages or develops into a secondary spreading center, defining four lithospheric architectures. Our results match the observed margin tectonic architecture only under conditions of a warm mantle, a thick viscous lower crust, and shallow-dipping upper-crustal structures.

## 1. Introduction

The South China Sea (SCS) is one of the marginal basins in the western Pacific (Taylor & Hayes, 1983; Lester et al., 2014). Its originality as a marginal basin relies on the fact that it did not form by arc splitting in a back-arc position but by fast-rifting of a continental margin. The SCS's oceanic spreading is well recorded by magnetic anomalies (Taylor & Hayes, 1983; Briais et al., 1993; Li et al., 2014) that have been calibrated by drilling (Sun et al., 2018). As the continental breakup propagated by pulses from East to West, the oceanic basin forms a series of three imbricated V-shaped oceanic propagators reported in Fig. 1A (Sibuet et al., 2016). V-shaped oceanic propagators marks periods in which propagation of continental breakup in map view is as slow or slower than plate tectonic velocities (Le Pourhiet et al., 2018). The first one constitutes the edge of today East Sub-Basin (ESB). It initiates at 32 Ma with rates of spreading varying between 6 and 2 cm/yr and a well-accepted averaged value of 40 km/Myr marked in blue in Fig. 1A. The second pulse of propagation of continental breakup, marked in yellow in Fig. 1A, occurred at 23 Ma with a 15° obliquity to the first one and similar rate of spreading (Li et al., 2014; Sibuet et al., 2016). The

last pulse occurred at 19 Ma and corresponds to the continental margin marked in magenta in Fig. 1A. These two last pulses correspond to the opening of the Southwest Sub-Basin (SWSB). Oceanic spreading ceased everywhere at 15 Ma.

Le Pourhiet et al. (2018) recalculated the pole of rotation for the first spreading period (32-23 Ma) and showed that it was located far away to the East, discarding a mode of scissor opening. As a result, the rate of spreading in the ESB must be equal to the extension rate imposed to continental lithosphere before the continental breakup of the SWSB in the area that extends from the western tip of the ESB to the East to the Red River Fault to the west; i.e. 40 km/Myr.

This continental rifting rate is, by all means, a very fast rate for continental extension. Active continental rifts indeed exhibit extension rates ranging from 0.2 to 20 km/Myr (Tetreault and Buiter, 2018). At such fast rates, only two end-member styles of rifted margins are expected to form from a horizontally layered, initially undeformed continental lithosphere: (1) narrow margins if the lower crust is strong, or (2) wide asymmetric margins resulting from fast rift migration (Tetreault and Buiter, 2018).

Instead, the continental margins of the SWSB (Fig. 1B) show widely

<footnote><sup>a</sup> Laboratoire de Géologie, CNRS UMR 8538, Département de Géosciences, École Normale Supérieure, PSL University, 75005 Paris, France</footnote>

<footnote><sup>b</sup> Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre Paris, ISTE P UMR 7193, F-75005 Paris, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France, France</footnote>

<footnote>* Corresponding author.
E-mail address: zhou@geologie.ens.fr (F. Zhou).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Digital Elevation Model of the area around the South China Sea with the outline of the sedimentary basins and geophysical observations with interpretation. (A) Digital Elevation Model of the area around the South China Sea with the outline of the sedimentary basins (modified from Sautter & Pubellier, 2022). Solid green lines are some selected and simplified faults zones, where shortening or/and exhumation took place before the rifting. (B) Multi-channel results with interpretation (C) of the black line in (A) (Chang et al., 2022), TWT-two way travel time in second. Basin names: DG: Dangerous Grounds, PRM: Pearl River Mouth, PK: Phu Khan. ESB: East Sub-Basin of the SCS; SWSB: Southwest Sub-Basin of the SCS. The break-up and spreading time of the ESB and the SWSB is from Briais et al. (1993). The green and red arrows mark the positions of two crustal neckings of the SWSB.</figcaption></figure>
spread half grabens that are all the same age. An arrangement that is compatible with neither of these end-member fast-rifting scenarios nor with typical wide continental rift scenario. Moreover, the parametric analysis of Tetreault and Buiter (2018) predicts continental rifting to last only 3-5 Myr before continental breakup, while we just mentioned above that this phase lasted between 9 and 13 Myr in the SWSB.

In section 2, we detail how the half-grabens associated with the period of fast continental rifting are rooted in low-angle detachment faults. These faults reactivate former orogenic and post-orogenic structures (Fig. 1A), which predominantly dip toward continental China (Franke et al., 2014; Liang et al., 2019; Ye et al., 2020; Deng et al., 2021; Sautter & Pubellier, 2022; Li et al., 2025). In section 3 we review in more details the existing models of wide rifts and show that they explain only partly these observations and justify the design of new simulations to understand the structure of the SWSB. After presenting the simulation results in terms of sensitivity of Moho geometry as a function of mantle potential temperature and the geometric nature inherited from the orogenic crust, we argue on what may have effectively been the initial conditions in the region and discuss the applicability of our simulations to other wide rifts in the world.

## 2. Tectonic context of the South China Sea (SCS)

Prior to the period of fast continental rifting, the margin of the South China Block underwent a long-lived history of subduction and collisions of crustal blocks, from Permian to Early Cretaceous (Faure et al., 2007; Charvet et al., 2013; Cochelin et al., 2022). During that time period, a large amount of granitic rocks intruded the crust. This magmatic activity stopped by the middle of the Cretaceous during the Yanshanian orogenic phase that corresponds to the docking (a soft collision able to stop the subduction) of a Gondwanian-derived micro-block referred to as the

Luconia Block (Fyhn et al., 2010; Clements et al., 2012; Nanni et al., 2017). This Cretaceous shortening is marked by anticlines (Chan et al., 2010; Yan et al., 2014) and the main sutures from Paleozoic (blue) to Cretaceous (green) are thrusts which dip toward the continent (Fig. 1A, Morley et al., 2011; Franke et al., 2014; Zhou et al., 2018; Clerc et al., 2018; Sautter & Pubellier, 2022). An idealized cross-section of the thickened crust at this stage named “orogeny” is provided in Fig. 2A panel 1.

From the Paleocene (~60 Ma, Fig. 2A, panel 2) to the onset of the fast-rifting period (~35 Ma, Fig. 2A, panel 3), the Yanshanian orogeny underwent gravitational collapse. During that period, there is clear evidence that many of the Cretaceous thrusts were reactivated as low-angle normal faults (Wei et al., 2015; Liang et al., 2019; Ye et al., 2020), and deep seismic data image large bulges interpreted as lower crustal sheath folds that are typical post orogenic extensional structures in the lower crust (Zhang et al., 2020).

During this post-orogenic collapse stage, the future continental margins of the SCS were located between the South China Block and the passive margin of an oceanic basin named the Proto-South China Sea. Today’s remnants of that basin is the tertiary suture (yellow) shown in Fig. 1A, whose dip indicates that the SCS was in the lower plate of the subduction zone. The continental rifting rate at the continental margin of the lower plate of a subduction zone accelerates and might lead to continental breakup as the subducting oceanic slab reaches the 410 km mantle discontinuity (Larvet et al., 2022). The fast extension that led to continental breakup in the South China Sea was most likely driven by slab pull forces generated by the southward subduction of the Proto-South China Sea oceanic domain (Larvet et al., 2023; Chang and Pubellier, 2024).

Our contribution focuses on the fast-rifting period, which lasted from 32 Ma to 23–19 Ma, and affected the wide margins of the SWSB, while

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Concept figure and model design configuration. (A) Concept figure showing the development of inherited dipping structure of continental crust. (B) Model domain with the design of four reference models. A 5 km thickness of sediments, 35 km thickness of continent and 360 km thickness of mantle is set from top to bottom, the domain of model is 400 km in depth and 800 km in distance. (C) The yield strength and temperature envelope used in this study. Compositions of different layers refer to the color codes. Stratified design refers to the continental crust with both weak and strong units. BDT, brittle-ductile transition. LAB, lithosphere-asthenosphere boundary. S, the spacing between the strong and weak units in the upper crust.</figcaption></figure>
the ESB was already an oceanic basin accommodating divergence by
spreading (Franke et al., 2011; Sibuet et al., 2016; Taylor, 2025).

The cross-sections in Fig. 1B and 1C are located across the last pulse of continental break-up at 19 Ma. The profiles show that the acceleration of rifting is marked by the preservation of a regional erosional surface (unconformity), which predates the structures related to the fast-rifting stage that we aim to better understand through modelling. It also displays a change of organization in subsidence at 24-23 Ma, i.e. the second pulse of propagation that marks the start of continental breakup in the SWSB.

Fig. 1C clearly shows two wavelengths: one crustal, displaying a
series of horsts and grabens that were active simultaneously; and one
lithospheric, with two centres of crustal necking; one in the north, which
shows hyper-thinned continental crust beneath the sediments, and one
in the south, which served as a locus for continental breakup of the
SWSB (Franke et al., 2014, Sibuet et al., 2016).

Geophysical results show the width of SWSB margin can be >800 km (Cameselle et al., 2020). The continental crust thickness varies from ~7 km to ~17 km, with two minimum where the crust is <10 km in thickness (stretching factor β>3): one is located at the OCT (ocean-continent transition), the other one located some 500 km North towards the South China Block (Fig. 1B, Savva et al., 2014; Chang et al., 2022; Pubellier et al., 2022; Zhang et al., 2025). In between, the Moho is not flat, and a series of synchronous basins and detachments are visible. The convergence to the north tend to predominate. Seismic refraction indicates large lateral variation of velocity in the upper crust with repeating low velocity bodies (LVBs) in the upper crust (Pichot et al., 2014).

Abundant geological observations, mostly from seismic data, record
the opening of the SCS, faults distribution and reactivation (Sun et al.,
2014; Franke et al., 2014; Sun et al., 2018, Yan et al., 2014; Wei et al.,
2015; Nanni et al., 2017; Ye et al., 2020; Deng et al., 2021). The ob-
servations in the SWSB show that the margin of the SCS features a series
of synchronous basins and detachments, hyper-extended continental
crust, non-flat Moho surface and asymmetric wide margin with narrow
OCT (Ye et al., 2020; Deng et al., 2021; Franke et al., 2014; Liang et al.,
2019; Chang et al., 2022; Legeay et al., 2024).

3. Modelling

### 3.1. Modes of wide continental rifting

Buck (1991) identified three continental rifting modes: core com-
plex, wide rift, and narrow rift based on 1D simulations to build 2D
conceptual models. In these 1D simulations, wide continental rifts can
only form by rift migration as they result from strengthening of the
lithosphere with thinning. This is inconsistent with the simultaneous age
of the grabens and basins in the SCS (Franke et al., 2014; Pichot et al.,
2014; Liang et al., 2019; Chang et al., 2022; Pubellier et al., 2022;
Legeay et al., 2024). Meanwhile the classification of Buck (1991) has
been widely tested, partly confirmed and largely improved using 2D and
3D thermo-mechanical simulations (Brune et al., 2014; Brune et al.,
2017; Le Pourhiet et al., 2018; Qing et al., 2024), especially the wide rift
mode and the core complex mode in the continents.

Regarding the wide rift mode, these simulations show how the
rheology of the lithosphere, and in particular the capacity of the lower
crust to flow, influences the distribution and activity in space and time of
individual grabens at the passive margin scale. While the original wide
rift mode of Buck (1991) would now be referred as rift migration mode
(Brune et al., 2014; Pérez-Gussinyé et al., 2020) that are characterized
by in-sequence grabens and some Moho topography, other types of wide
margins emerge from modelling. Using a well mechanically decoupled
crust and mantle lithosphere, and a white noise as inheritance indeed
permits to form wide margins in 2D too, with several grabens being
active at the same time (Brune et al., 2017), yet in these simulations the
Moho remains flat. In 3D, the presence of a slow oceanic propagator (Le
Pourhiet et al., 2018) or that of an inherited mechanically strong

lithospheric blocks (Qing et al., 2024) both enforce kinematically the
width of the deformation zone and also permits to explain the coeval
formation of grabens observed in the SWSB (Pichot et al., 2014; Chang
et al., 2022; Pubellier et al., 2022).

3.2. Factors favoring the formation of low angle detachments

While some metamorphic core complexes correspond to the classi-
fication of Buck (1991), i.e. exhumation of weak lower crustal material,
others display large asymmetric detachments (Labrousse et al., 2016).
The first ones correspond to viscous gneiss domes exhumed in a more or
less diapiric mode named either spreading domes (Huet et al., 2011a) or
double domes (Rey et al., 2011). They typically form in association with
a flat Moho as the decoupling is very strong, and the lithospheric mantle
is warm and weak (Tirel et al., 2008). The others require the existence of
a long-lived asymmetric detachment faults. This asymmetric kinematics
require weak fault (Lavier et al., 1999; Lecomte et al., 2011) and the
presence of a very strong viscosity contrast. The strong viscosity contrast
may be introduced implicitly by a Winkler basement (perfect fluid) for
the bottom boundary condition (Lavier et al., 1999). Yet in many other
models, it has been introduced explicitly by adding a layer of very low
viscosity inside the model setup. Brun et al. (1994) obtained detachment
faults thanks to the contrast between sand and silicone putty in their
analogue models. Huet et al. (2011a) considered a post-orogenic lower
crust with a felsic tectonic unit underthrusted underneath a more mafic
one. Mezri et al. (2015) found that limiting the retromorphose of rocks
to the ductile detachment introduces a sufficient mechanical contrast to
localize strain over a detachment fault. At margin scale, Duretz et al.
(2016) demonstrated that a mechanically stratified lithosphere
enhanced boudinage favoring the formation of detachment in the
inter-boudins compared to homogeneous models without viscosity
contrast. However, the large-scale asymmetry in these models often
arises from boundary conditions (Huet et al., 2011a; Li et al., 2025), flow
in the lower crust (Brun et al., 1994) or the asthenospheric mantle
(Lavier et al., 1999), or initial dipping heterogeneities (Le Pourhiet et al.,
2004; Huet et al., 2011b; Balázs et al., 2018).

3.3. Modelling post-orogenic processes

All the initial dipping and some horizontal heterogeneities
mentioned in the above paragraph were introduced to mimic inheri-
tance from former orogeneses. Dipping layer was introduced either to
mimic the effect of a suture zone (Balázs et al., 2018) or that of a former
crustal nappe-stack (Le Pourhiet et al., 2004; Huet et al., 2011b).
Similarly, the use of a horizontal yet compositionally less viscous layer
in the lower crust than in the upper crust also leads to the formation of
detachments that exhume high grade rocks (Huet et al., 2011a; Le
Pourhiet et al., 2012). The idea behind these last three contributions is
motivated by the fact that thickening the crust in simple shear leads to
the formation of a compositionally weak post-orogenic lower crust, i.e. a
kind of rheological polarity reversal. In all these contributions, the goal
was to account for post-orogenic setting without modelling explicitly the
orogenic phases. Another possibility would be to use so-called "accor-
dion" approach that consists of reverting the polarity of the boundary
conditions to model the orogenic phase. This approach has mainly been
used to study the effect of rifting on orogenesis (Buiters & Pfiffner, 2003;
Jammes & Huismans, 2012; Jourdon et al., 2019) but has only recently
been applied in the opposite direction and, to our knowledge, solely to
model the opening of the South China Sea (Li et al., 2025). The study of
Li et al. (2025) was specifically designed to reproduce the exact kine-
matics inferred from the tectonic setting described in Part 2. However, it
starts after the subduction and only simulates the effect of continental
lithosphere thickening in pure shear. They show how lateral variations
in crustal thickness at the end of the collision introduce lateral variation
of GPE (gravitational potential energy) that controls the localization of
the deformation during rifting in a similar manner to what was shown in
the Gulf of Corinth with more archaic models (Le Pourhiet et al., 2003). Their intended simplified pure shear shortening simulations did not permit the production of an initial stage involving crustal-scale nappe-stacking. Here, we complement this study by adhering to the parameterized inherited structures approach, which allows for diversification of idealized crustal structures to specifically address the role of crustal-scale rheological polarity reversals introduced by simple shear deformation in post-orogenic crust.

### 3.4. Numerical Setup

Simulations use the open-source pTatin2D code which solves the conservation of momentum, energy and mass using Boussinesq approximation using the finite element method (FEM) for a visco-plastic rheology (May et al., 2015). The mesh is arbitrary Lagrangian-Eulerian, the domain can deform with time at a velocity that is not that of the solution. Material point method is used to integrate the displacement. The code also incorporates erosion-sedimentation processes and thermodynamics-based density prediction. Details, references and governing equations with notation are found in supplementary material. We use 576 by 64 Q2P1 elements for 800 km by 400 km initial model domain. Mesh is refined only in the vertical direction with 30 elements representing 40 km at the top, 20 elements representing 80 km in between 40 and 120 km and only 14 elements between 120 km and 400 km depth.

Our set up reflects the former fore-arc (Hall & Morley, 2004) and collisional (Sautter & Pubellier, 2022) environment from the Early Paleozoic suture to the Cretaceous one (Ye et al., 2020) (Fig. 2A). The LVBs (low-velocity bodies) in the upper crust (Pichot et al., 2014) also serve as evidence for us to explore a dipping layered design consisting of weak and strong units of the upper crust (UC), as well as varying thicknesses of the strong lower crust (LC). The domain is initially 800 km wide and 400 km deep. On both sides of the models, 150 km wide blocks refer to the South China block and Dangerous Ground on both sides of the SWSB. In between, the structure of continental crust is declined in three horizontal layered designs and a parametric dipping layered design.

All designs have 35 km total crustal thickness ($H_c$) and an extra 5 km horizontal layer of sediments set atop (Liang et al., 2019). Initial Moho depth is 40 km. The horizontal layered design includes weak crust, strong crust and stratified crust (with both strong and weak units in the crust) (Fig. 2B). The dipping-layered design features a stack of strong and weak units of the upper crust dipping at a constant angle $\theta$ overlying a horizontal layer of strong lower crust of thickness $H$. Dipping units correspond to crustal-scale nappes, comparable to those of the internal zones of the Alps. They involve different lithologies that were either

detached along a flat décollement at the Moho ($H=0$) or within the middle crust ($H>0$), assuming a mafic lower crust of thickness $H$ that remained coupled to the mantle during orogenesis. $H$ varies in the range of 0/5/10/15 km and modifies the coupling between the crust and the mantle. $\theta$ represents the dip of ramp structures at the end of the collapse and varies in the range of 15°/30°/45°/60°. It primarily imprints a wavelength in the crustal deformation as the spacing between 2 adjacent dipping layers (strong and weak units) is $S = (H_c - H)/\sin\theta$.

The change in strength is measured by the surface area of the yield strength envelope represented for “strong” and “weak” crust in Fig. 2C. Frictional parameters are kept constant across crustal units, in accordance with Byerlee’s law (1978). Consequently, the change in strength, for a given temperature, arises solely from the variation in dislocation creep parameters. This together with the dip of the tectonic units leads to lateral variation in the depth of the crustal brittle-ductile transition (BDT) illustrated by the blue line in Fig. 2B.

The weak crustal units follow a quartz-based flow law (Table 1) and are assumed to have a pelitic composition, which has been used to construct the tabulated arrays that contains its density and melting portion as a function of pressure and temperature. If melting would to occur in the crust (it does not in our simulations), the melt residue would have a gneissic composition (Table 1). In contrast, the strong crustal units are assigned the composition of a gneiss and deform according to a diorite flow law (Table 1). The composition, density values and melting point used for all units are fully detailed in the supplementary material of Larvet et al. (2022).

Aforementioned observations constrain the extension rate and the crustal strain in our models, we set a 2 cm/yr horizontal velocity on each side with opposite horizontal direction and run models typically for 20 Myr but only the period prior to continental break-up is important.

Other boundary conditions include a free surface on the top and an inflow velocity $V_y (>0)$ that ensure that the free surface stays at 0 on the lateral boundary where no strain occurs. The temperature increases from 0 °C at the surface to a parameter $T_{LAB} = 1295°C/1340°C/1385 °C$ at 120 km depth, a depth beneath which non-adiabatic initial temperature gradient is zero. $T_{LAB}$ is therefore more a mantle potential temperature than the temperature of the LAB. The 1295°C isotherm in the models varies effectively between 108 km to 120 km. This last parameter may affect the strength of the upper mantle, the coupling between the crust and the mantle and the buoyancy of the asthenosphere during necking.

<caption>**Table 1**</caption>

Rheological parameters used in this study. Dislocation creep flow laws are based on the wet quartz from Ranalli & Murphy (1987) for sediments and weak crustal units (C.U.), on the anorthite law of Rybacki & Dresen (2000) for the strong crustal units and the lower crust and on the dry olivine of Brace & Kohlstedt (1980) for the mantle.

<table><thead><tr><th></th><th>Sediments</th><th>Weak C.U.</th><th>Strong C.U. & Lower Crust</th><th>Mantle</th></tr></thead><tbody><tr><td></td><td></td><td></td><td>Friction</td><td></td></tr><tr><td>C<sub>0</sub> - C<sub>∞</sub> (MPa)</td><td>2-1</td><td>20-1</td><td>20-1</td><td>20-10</td></tr><tr><td>φ<sub>0</sub> - φ<sub>∞</sub> (°)</td><td>30-10</td><td>30-10</td><td>30-10</td><td>10-5</td></tr><tr><td>ε<sub>min</sub> - ε<sub>max</sub></td><td>0-1</td><td>0-1</td><td>0-1</td><td>0-1</td></tr><tr><td></td><td></td><td>Dislocation creep flow laws</td><td></td><td></td></tr><tr><td>n</td><td>2.4</td><td>2.4</td><td>3.00</td><td>3.50</td></tr><tr><td>A (MPa<sup>n</sup>s<sup>-1</sup>)</td><td>6.8 × 10<sup>-6</sup></td><td>6.8 × 10<sup>-6</sup></td><td>13.4</td><td>2400</td></tr><tr><td>Q (kJmol<sup>-1</sup>)</td><td>156</td><td>156</td><td>345</td><td>540</td></tr><tr><td>V (cm<sup>3</sup>mol<sup>-1</sup>)</td><td>0</td><td>0</td><td>38</td><td>18</td></tr><tr><td></td><td colspan="2">Melt & Density according to Larvet et al. (2022)</td><td></td><td></td></tr><tr><td>Rocks Init. Res.</td><td>Metapelite-Gneiss</td><td>Metapelite-Gneiss</td><td>Gneiss-Gneiss</td><td>Lherzolite-Harzburgite</td></tr><tr><td>η<sub>melt</sub> (Pa.s)</td><td>10<sup>16</sup></td><td>10<sup>16</sup></td><td>X</td><td>10<sup>10</sup></td></tr><tr><td>Max melt fraction</td><td>10%</td><td>10%</td><td>X</td><td>1%</td></tr><tr><td>ρ<sub>melt</sub> (kgm<sup>-3</sup>)</td><td>2700</td><td>2700</td><td>X</td><td>2900</td></tr></tbody></table>
# 4. Results

## 4.1. Reference models

We ran first 9 models with the horizontal layered design of the continent, consisting of only weak crust, strong crust, and a combination of weak and strong crust (stratified design) under different thermal states ($T_{LAB}=1295$ °C/1340 °C/1385 °C). We then ran 48 models with the dipping layered design of the continent, varying the dipping angle ($\theta=15°/30°/45°/60°$), and the thickness of strong lower crust ($H=0/5/10/15$ km) under the same thermal states mentioned above. The total 57 simulations are available as supplementary material.

Four reference models (model R1/R2/R3/R4) with different initial design of continental crust structure (Fig. 3) are picked out to show different signatures of continental rifting. Model R1 and R2 (Fig. 3A, B) with only weak and strong crust design results in almost symmetric margin, while other two models (Fig. 3C, D) feature asymmetric margin. All reference models are under the same thermal state ($T_{LAB}=1340$ °C). The models are described below:

(1) Weak crust design. Continent with only weak crustal units in model R1 (Fig. 3A) leads to diapiric and symmetric exhumation of the lower crust within synchronous spreading domes, which are outlined by the viscous strain (blue) in the lower crust.

(2) Strong crust design. Model R2 (Fig. 3B) features only strong crustal units of continent, it produces a series of synchronous symmetrical basins and wavy Moho as the signature of crustal boudinage, ductile strain is limited in the LC (blue viscous strain) while upper crust (UC) localizes all brittle strain (black brittle frictional strain). This model displays the wavy Moho signature of the margin of the SWSB but does not display the two necking zones.

(3) Stratified design with horizontal layered continental crust. Model R3 (Fig. 3C) employs horizontal layered structure of continental crust consisting both weak and strong crustal units similar to Duretz et al. (2016). This design leads to lithospheric boudinage with two crustal asymmetric necking zones. One fails before continental break-up leaving behind an hyper-thinned continental crust and the other accommodate break-up and become a spreading center (see movie of model R3 in the supplementary materials). This simulation produces the two necking zones but not the wavy Moho signature.

(4) Stratified design with dipping layered continental crust. Model R4 (Fig. 3D) uses dipping layered structure of continent with 60° angle and 10 km thickness of strong LC. Left, exhumed mantle is produced as the signature of lithospheric boudinage, and a series of synchronous detachments (black plastic strain) also occur as the consequence of inherited structure.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Results of reference models. $T_{LAB}$, $\theta$ and $H$ refers to the initial temperature of LAB, dipping angle between weak and strong units of UC and the thickness of LC. None of the models produce rift migration. (A) Weak homogeneous crust results in upper crustal boudins separated by spreading domes over a flat Moho. (B) Strong homogeneous crust leads to crustal scale symmetric boudinage with wavy Moho. (C) Horizontal layered crust leads to lithospheric scale boudinage accommodated by large scale detachment. (D) Dipping crustal units result in lithospheric-scale boudinage and upper crustal brittle-ductile asymmetric detachments. The syn-rift sediments age refers to the age of newly deposited sediments since the rifting, from ~35 Ma as dated in the SCS case. Stratified design refers to the continental crust with both weak and strong units. White line in (A-D) all refer to the isotherm contour of 1340 °C. Cumulative brittle frictional strain and viscous strain refer to the brittle and viscous detachments in continental crust, mantle viscosity is also shown above.</figcaption></figure>
The different results between these four reference models highlight how variations in the initial structure of continental crust leads to different signatures at crustal and lithospheric scales, including spreading domes and boudinage, accompanied (or not) by the formation of synchronous basins and detachments. However, only R4 seems close to the observations on the margin of the SWSB justifying the parametric approach that follows.

## 4.2. Moho textures

We select six models (C11, C21, and C31 with a horizontal layered design, while C12, C22, and C32 have a dipping layered design of the continental crustal structure) in Fig. 4 representing three distinct Moho textures, we will use to classify our results: flat Moho, hummocky Moho, and wavy Moho.

Models C11 and C12 (Fig. 4A, B) both exhibit a flat Moho: they display no characteristic wavelength within a distance of 300 km. In

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Moho textures within dipping layered models. T<sub>LAB</sub>, θ and H refers to the initial temperature of LAB, dipping angle between weak and strong units of UC and the thickness of LC. None of the models produce rift migration and all feature detachments. (A-B) Model C11 and C12 displaying a flat Moho without boudinage. (C-D) Model C21 and C22 displaying a hummocky Moho, small teeth of lower crust indent the upper part of the mantle causing strain localization. (E-F) Model C31 and C32 displaying a wavy Moho formed by crustal boudinage. The syn-rift sediments age refers to the age of new generated sediments since the rifting of continental crust, from ~35 Ma in nature. Cumulative brittle frictional strain and viscous strain refer to the brittle and viscous detachments in continental crust, mantle viscosity is also shown above.</figcaption></figure>
model C11, horizontal viscous flow (Fig. 4A) occurs in the lower crust along the bottommost interface between the strong and weak crustal units of the upper crust, indicating predominant horizontal deformation. In contrast, model C12 features dipping viscous flow in the weak crustal units with detachments (Fig. 4B) above them.

Models C21 and C22 (Fig. 4C, D) both show a hummocky Moho displaying small notches along the Moho surface every 10's to 100 km. These notches are due to localized mantle deformation, either by necking (weak lower crust, i.e. C21) or because the inherited detachments transmit strain localization into the mantle (no lower crust for C22).

Model C31 and C32 (Fig. 4E and 4F) exhibits a wavy Moho with smooth variation of depth over wavelength of 10's to 100 km. Model C31 displays typical symmetric crustal boudinage with conjugate faults (plastic strain in black). Model C32 (Fig. 4F) displays typical asymmetric boudinage with detachment faults reusing inherited dipping structure without rooting into the mantle.

### 4.3. Modes of continental break-up

Fig. 5 reveals four distinct rifting styles observed in our simulations.

(1) The single spreading center style corresponds to a mode of rifting by lithospheric necking (single neck) rather than lithospheric boudinage (multiple neck). It is exemplified in Fig. 5A with a simulation that features a wide rift with flat Moho despite the multitude of basins forming synchronously on top of asymmetric detachments that cut across the whole crust.

The three other modes display two necks as the signature of lithospheric boudinage and are distinguished by the timing at which the second neck becomes tectonically inactive.

(2) Single spreading center with hyper-thinned crust is exemplified in Fig. 5B with a simulation that features a wide rift with flat Moho, a series of basins forming synchronously on top of asymmetric detachments that root in the thick lower crust and most importantly participate to the reduction of the thickness of the crust to less than 10 km thick at the second neck.

(3) Single spreading center with exhumed mantle is exemplified in Fig. 5C with a simulation that features a wide rift with wavy Moho, a series of synchronous basins forming on top of asymmetric detachments that root on the Moho, deforming it. One of

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Lithospheric architecture for dipping crustal units. T<sub>LAB</sub>, θ and H refers to the initial temperature of LAB, dipping angle between weak and strong units of UC and the thickness of LC. None of the models produce rift migration and all feature detachments. (A) Model L1 without lithospheric scale boudinage resulting single spreading center without secondary crustal neck. (B) Model L2 with slow lithospheric scale boudinage resulting in hyperextended continental crust neck and spreading center. (C) Model L3 with lithospheric scale boudinage resulting in exhumed mantle neck and spreading center. (D) Model L4 with fast lithospheric scale boudinage resulting in two active spreading centers. The syn-rift sediments age refers to the age of new generated sediments since the rifting of continental crust, from ~35 Ma in nature. White line in (A-D) refers to the isotherm contour of 1295 °C, 1340 °C, 1385 °C and 1295 °C, respectively. Cumulative brittle frictional strain and viscous strain refer to the brittle and viscous detachments in continental crust, mantle viscosity is also shown above.</figcaption></figure>
these detachments actually exhumes the mantle beneath the sediments at the location of the second neck.

(4) Double spreading centers is exemplified in Fig. 5D. The two synchronous active necking zones that develop in response to lithospheric boudinage are not abandoned and lead to the formation of double spreading centers. These simulations correspond to situations that are not observed in nature.

All four rifting styles feature the formation of synchronous basins, asymmetric detachments, and wide rift. The detachment results in asymmetric necks and partly in asymmetric thinning in the vicinity of the necks. The latter three styles, characterized by the presence of two synchronous necking zones, reflect the competition between the growth rate of lithospheric boudinage and the activity of asymmetric crustal detachments which recouple the mantle and crustal deformation during the final necking. The exhumation of the lower crust leading preferentially to hyperextension of the continent while direct coupling to the mantle might favor its exhumation. Most important message is that in our models, wide rifts and continental hyperextension both originate from the inherited structure of the continent rather than from rift migration (Brune et al., 2014).

# 5. Discussion

## 5.1. Crustal and lithospheric boudinage/hyperextension: Necking a Post-orogenic Crust

Using the aforementioned classifications, the Fig. 6 explore more systematically the influence of the thermal state ($T_{LAB}$), the dipping angle ($\theta$), and the thickness of the flat lying lower crust ($H$) on the break-up time (blue color shading and numbers), the Moho textures identified by colored circles and the continental break-up modes identified by colored squares. Our simulations indicate that the geometry of the Moho is primarily controlled by the initial dip of the crustal layering and the

thickness of the strong lower crust. The transition between a flat and a wavy Moho reflects the ability of crustal-scale detachments to deform the Moho surface. When the lower crust is thick and strong, it acts as a viscous buttress that resists the downward propagation of detachments into the mantle, particularly at low dip angles. In this case, deformation is distributed throughout the entire crust, producing a wavy Moho. When deformation is instead restricted to the upper crust, a flat Moho tends to form. Temperature plays a secondary role: higher Moho temperatures reduce the viscosity of the lower crust, weakening the buttress and promoting greater deformation of the Moho.

Continental break-up is delayed either when the lower crust is thick or when the initial layering has a high dip angle. In both cases, deformation is more distributed. A thick lower crust accommodates the strain across a broader area, while a high-angle layering activates more inherited weak zones, lowering the strain rate along each and further distributing deformation. These conditions prolong the synchronous activity of two necking zones and result in a dominant mantle signature, which may manifest as hyper-thinned crust, exhumed mantle, or the development of two spreading centers.

The character of this mantle signature is strongly influenced by the mantle potential temperature. When the mantle is relatively cold, necking proceeds passively, and the crust may break up along two centers. As the mantle temperature increases, buoyancy forces at the base of the lithosphere become strong enough to promote active diapiric upwelling, leading to a focused, single break-up center (Huismans et al., 2001).

Whether this single break-up results in hyper-thinned crust or in mantle exhumation depends on the initial dip angle. High dips combined with a thin lower crust tend to favor rolling-hinge-style detachment systems (e.g. Lavier et al., 1999), similar to those observed in oceanic mega millions. Shallower detachments, in contrast, typically root above the Moho and are unable to exhume mantle material.

To summarize, our models reveal that:

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Synthetic matrix with the lithospheric architecture (squares), Moho texture (circles) and timing of continental break-up (colormap) for all the dipping layered simulations (detailed in supplementary material). Exhumed mantle (orange squares) only occurs for large initial dip angle, wavy Moho (dark green circles) occurs only at low dip angle in presence of thick strong lower crust. Absence of strong lower crust is most favorable for hummocky Moho. Well-developed lithospheric boudinage leading to double spreading center is the signature of delayed continental breakup (light blue color), high dipping angle (60°) and thick LC (15 km) also delays the break-up of continental crust. The break-up time of continent refers to the duration of extension until the break-up of continent happens.</figcaption></figure>
1. Moho geometry depends on the dip angle and lower crustal thickness, with a flat or wavy Moho reflecting the degree to which detachments can deform the Moho, moderated by thermal effects on viscosity.

2. Delayed continental break-up can result from either a thick lower crust or a high initial dip, both of which promote distributed crustal deformation and lead to a dominant mantle signature.

3. The mode of continental break-up, i.e. double spreading centers versus hyper-thinning or exhumation, is governed by mantle potential temperature, which controls the transition from passive to active necking.

4. The transition between hyper-thinned crust and exhumed mantle is driven by the initial dip angle, with high dips favoring rolling hinge detachment and mantle exhumation.

## 5.2. Implications for the rifting and spreading of the SWSB

In this study, we do not calibrate models in the sense of adjusting parameters to fit specific datasets or prior interpretations. Instead, we perform a parametric exploration that generates a range of mechanically self-consistent outputs that we have synthetized in Fig. 6. To select a preferred model for comparison with observations from the SWSB, we

use a set of validation criteria derived from its main margin features. These include the development of a wide rift with asymmetric margins, a wavy Moho, continental hyperextension, multiple necking zones, inland hyper-thinned crust, and synchronous basins. In the SWSB, the distance between the inland hyper-thinned crust and the ocean-continent transition (OCT) is ~500 km (Fig. 7C), while the Moho undulations have a characteristic wavelength of ~100 km. We quantified these features across all simulations of our parametric study and found that, among all possible configurations, the model featuring $T_{LAB}=1385$ °C, $\theta=30^\circ$ and $H=15$ km is the one that best satisfied the geometrical constraints. It produces a wavy Moho with ~100 km wavelength, a narrow OCT and a neck of hyper-thinned crust located >400 km from the OCT, along with a series of synchronous syn-rift detachments and basins. These features result from the inherited structure, which includes both strong and weak units in the upper crust (UC). If we assume that the weak units in the UC (Fig. 7B) are less mafic and therefore lower velocity than the strong units and the lower crust, we can use our models and the multi-channel seismic line (Chang et al., 2022; Fig. 1B and 1C) to reinterpret the origin of LVBs (low velocity bodies) shown in Vp structure (Pichot et al., 2014; Fig. 7C) as the signature of the existence of former thrusts putting in contact units of different lithologies. The hyper-thinned crust in model SCS originates from the relic of the abandoned left necking zone

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Comparison between preferred model SCS and observations in the SWSB. (A) Result of model SCS. (B) Zoomed area of red rectangle in (A). (C) Final velocity model showing LVBs (low velocity bodies) in the UC (upper crust) and high velocity lower crust (Pichot et al., 2014). Red solid circles with number 1-49 refer to the codes and positions of OBS, circled 1 to 6, A and B indicate the LVBs in the UC, the velocity contours are drawn every 0.25 km/s. (D) Conceptual model in this study showing the coupled lithospheric-crustal boudinage can lead to the formation of wide rifts, like the SCS. The white line in (A) refers to the temperature contour of 1385 °C. The syn-rift sediments age refers to the age of new generated sediments since the rifting of continent, from ~35 Ma in nature. Cumulative brittle frictional strain and viscous strain refer to the brittle and viscous detachments in continental crust, mantle viscosity is also shown above.</figcaption></figure>
after the competition with the right one (see the supplementary for the evolution video of model SCS). Following this reinterpretation, the asymmetric necking style can now be attributed to the inherited structure of continental crust, which leads to the continent-ward dipping of upper crust. The existence of dipping structure also permits to explain the ~100 km wavelength of the non-flat Moho observed in SWSB (Pichot et al., 2014; Zhang et al., 2025) featuring boudin-like crustal structure. Note, that our model with strong crust and no dipping structure do also results in wavy Moho (Fig. 4E), but the wavelength which is solely controlled by the thickness of the crust is only of 55 km, one order of magnitude smaller than observations. Moreover, these models do not result in double-necking zone. Therefore, strengthening the case for the existence of pre-existing dipping structures. Brune et al. (2017) employed a single layer continental crust consisting of only weak quartz model is consistent with their seismic line located on the margin of the ESB. This model failed to explain the boudin-like structure of continental crust with wavy Moho, wide rift with asymmetric margin, and synchronous syn-rift basins/detachments observed in the west where the timing is different. Our models, accounting for the inheritance from post-orogenic processes, provide a novel explanation for the formation of wide rifts with a strong lower crust, like the margins of the SWSB (Fig. 7D), rather than wide rift mode with rift migration which would require a weak lower crust (Buck, 1991).

Furthermore, the opening of the SWSB can be further connected with continental hyperextension through lithospheric boudinage. Except for model SCS, the other 17 cases shown in Fig. 6 (see yellow squares) also exhibit hyper-thinned continental crust and wide rift with asymmetric margin, even under varying thermal states (see supplementary for final results of models). This indicates that when structural and compositional inheritance are considered, the occurrence of hyper-thinned continental crust coupled with asymmetric margins can be common.

Our models, however, did not consider lateral variations in crustal thickness, such as those introduced by Le Pourhiet et al. (2003) or Li et al. (2025). We acknowledge that these types of variations might well compete with the lithospheric-scale boudinage that dominates the long-wavelength deformation in our study. Besides, the spacing (S) in Fig. 2B between adjacent strong and weak crustal units in the upper crust remains constant in each model with a dipping layered design of the continental crust. This results in an almost constant wavelength of the Moho surface in models with a hummocky or wavy Moho. Varying spacing may lead to different scenarios, such as variations in the Moho wavelength and different degrees of continental crustal thinning, which can be explored in future work.

## 6. Conclusions

Our simulations, based on the geological setting of the South China Sea, reproduce three distinct Moho textures: flat, hummocky, and wavy; and four types of lithospheric architecture: (1) single spreading center style, (2) single spreading center with hyper-thinned crust, (3) single spreading center with exhumed mantle, and (4) double spreading centers. These styles arise as responses to crustal and lithospheric boudinage, respectively.

We show that all models incorporating structural inheritance allow:

1. The distribution of initial subsidence into synchronous asymmetric basins;

2. A prolonged period (≥10 Myr) of fast rifting.

Our comprehensive parametric study reveals that only a limited range of initial conditions can produce both the observed tectonic architecture, characterized by: multiple active basins, a wavy Moho and two necking zones, as well as the extended (13 Myr) phase of rapid continental rifting observed in the Southwest Subbasin.

Specifically, only models with a thick lower crust or a large initial dip delay the continental breakup by 13 Myr after the onset of rifting,

matching the natural record (up to 19 Ma). However, models with a large dip fail to reproduce the observed Moho texture in the SCS and can therefore be excluded.

By accounting for the Moho waviness, we further infer that mantle temperature must have been high (~1385 °C), and that the tectonic units in the upper crust had relatively shallow initial dips at the onset of fast rifting.

## Open research

Original data of all models in this study can be found (https://zenodo.org/records/15983402), simulations use open source code pTatin2d (https://bitbucket.org/ptatin/ptatin2d/).

## CRediT authorship contribution statement

**Fan Zhou:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation. **Laetitia Le Pourhiet:** Writing – review & editing, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Manuel Pubellier:** Writing – review & editing, Validation, Supervision, Investigation, Formal analysis, Conceptualization. **Matthias Delescluse:** Writing – review & editing, Validation, Supervision, Investigation, Formal analysis, Conceptualization.

## Declaration of competing interest

We declare that we have no financial interests or personal relationships with other people or organizations that could have appeared to influence our work reported in this paper.

## Acknowledgments

Sentences in this article are polished with ChatGPT, figures are made using Paraview5.11.1, Adobe Illustrator 2020 and Matlab2024. The simulations were performed on the MeSU platform at Sorbonne Université. Fan Zhou is supported by the China Scholarship Council (CSC) and other authors by ANR-22-CE49-0015 Collisea. We thank Jie Chen for providing good suggestions on improving the quality of figures. We also thank Sung-Ping Chang for providing high-resolution multi-channel seismic results with interpretation.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2025.119679.

## Data availability

I have shared the link to my data, see Open Research

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