Following cessation of rifting, the continental margin basins of the East and South China sea entered a post-rift stage, during which basement subsidence has traditionally been attributed to lithospheric cooling and thermal contraction. However, quantitative analyses from multiple sub-basins indicate that rapid or accelerated subsidence persisted into the post-rift period and locally exceeds predictions of classical thermal subsidence models. This is anomalous subsidence compared to simple rift models. In this study, we conduct a comparative analysis of post-rift subsidence histories in several representative basins in the East and South China seas. Regional porosity-depth relationships were established based on drilling data or generic models, and backstripping analyses were performed to reconstruct tectonic subsidence histories after accounting for sediment loading. Thermal subsidence and stretching factors were further calculated and compared with theoretical extensional subsidence models. Our results show that pronounced anomalous subsidence has developed since ~5.3 Ma in the Xihu Sag of the East China Sea, since ~5 Ma in the Yinggehai-Song Hong Basin, since ~2–5.3 Ma in the Qiongdongnan Basin, and since ~18 Ma in the Zhu III Depression and Baiyun Sag of the Pearl River Mouth Basin. Although these accelerated subsidence events all occurred during the post-rift stage, they exhibit marked differences in timing and persistence between basins. The mismatch between observed subsidence and model-predictions suggests that post-rift subsidence cannot be explained solely by lithospheric cooling, but is likely influenced by additional processes such as deep-seated mantle up or downwelling, tectonic reactivation, or mid and lower crustal flow driven by sedimentary loading. These findings highlight the stage-dependent and diachronous nature of post-rift anomalous subsidence in the East and South China sea marginal basins and provide new quantitative constraints on post-rift basin dynamics and sedimentary responses.

How to cite: Zhao, X., Li, C.-F., and Clift, P.: A Comparative Study of Anomalous Post-rift Subsidence in the East and South China seas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13723, https://doi.org/10.5194/egusphere-egu26-13723, 2026.

Tectonic structures associated with continental rifting depend on numerous factors: the nature and mechanical behaviour of the stretched lithosphere, geological inheritance, thermal conditions, and geodynamic forces. The South China Sea exhibits a complex geodynamic history, marked by pre-existing structures (granitoids, etc.). Oceanic accretion in the South China Sea began in the east around 32 Ma and propagated southwestward around 22 Ma, accompanied by a change in the extension direction. Spreading stopped around 16 Ma. The rifting phase lasted longer in the west, leading to the development of a wide rift, accompanied by core complexes and exhumed mantle.

Previous numerical modelling studies show that the formation of a wide rift requires a ductile lower crust and high temperatures at the base of the crust. Structural and thermal inheritance promotes distributed deformation. However, in 3D, an additional mechanism is required to slow down oceanic propagation in order to allow the formation of a wide rift. One possibility is the action of compressive stresses, which, in the case of the South China Sea, may be linked to the topography of the Indochinese block resisting rift propagation.

Here we explore another hypothesis for slowing an oceanic propagator: the transition from N–S extension to an N–S strike-slip system. The opening kinematics of the South China Sea remains debated, between (1) a pull-apart model linked to left-lateral motion along the Red River Fault associated with extrusion of the Indochinese block, and (2) a continental-rifting model induced by subduction of the proto–South China Sea. Modelling rift propagation toward a major transform fault allows us to assess how different kinematic scenarios influence the opening of the South China Sea, the formation of crustal structures, and topography. End-member models fail to reproduce a wide rift, whereas intermediate conditions better account for the slowing of rift propagation, the width of the rift, and the oblique localization of deformation in the southwest basin.

How to cite: Watremez, L., Le Pourhiet, L., Pubellier, M., Delescluse, M., Chamot-Rooke, N., Jourdon, A., and Zhou, F.: Testing the Pull-Apart vs. Subduction-Driven Rifting Debate with 3D Geodynamic Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21741, https://doi.org/10.5194/egusphere-egu26-21741, 2026.

The South China Sea (SCS) formed in a forearc and post-orogenic environment related to the Mesozoic subduction of the Paleo-Pacific plate. It started rifting at >65 Ma and ended at 32 Ma in the eastern part and 23–19 Ma in the southwestern part, respectively. The driving mechanisms for the rifting of the SCS mainly include two models. One is the pull-apart basin model, which attributes the driving force to the collision between the Indian and Eurasian plates, inducing block extrusion along the Red River Fault System (RRFS) with sinistral strike-slip motion, while the other is the rifted basin model, which emphasizes the pull from the southward subduction of the Proto South China Sea (PSCS) plate, leading to dextral motion of the RRFS. In this study, we interpret a ~300 km-long seismic line with velocity structure by combining multi-channel seismic (MCS) and wide-angle seismic (WAS) data in the Qiongdongnan Basin (QDNB), which is located in the northwestern SCS and close to the RRFS. We identify a hyper-thinned continental crust with southward (oceanward)-dipping detachments cutting through the crust and sometimes offsetting the Moho vertically up to 8 km. This contrasts with most observations in the SCS that indicate northward (continent-ward) vergence of extensional crustal structures. Based on the thermo-mechanical model of Zhou et al. (2025), we interpret these structures as the products of reactivated orogenic inherited crustal structures. Combining other seismic observations in the northwestern SCS, there is an east to west transition in crustal vergence from northward to symmetric and then to southward. Furthermore, considering the location of our seismic line between the RRFS and the QDNB, we also propose that this reflects the influence of  strike-slip motion along the RRFS, leading to preferential activity of southward-dipping crustal structures under the effect of a horsetail structure. This indicates a dextral activity of the southern RRFS during the SCS rifting, in agreement with the PSCS hypothesis.

How to cite: Zhou, F., Delescluse, M., Pubellier, M., Le Pourhiet, L., and Watremez, L.: Transition from continental- to ocean-verging crustal-scale normal faults in the northwestern South China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9922, https://doi.org/10.5194/egusphere-egu26-9922, 2026.

The Manila–Taiwan–Ryukyu subduction system provides a natural laboratory for investigating the transition from active oceanic subduction to mature arc–continent collision at a complex plate junction. While dense geophysical observations across Taiwan are commonly interpreted in terms of arc–continent collision and crustal-scale orogenic processes, the contributions of underlying mantle circulation and interacting slabs remains poorly quantified. This deficiency is critical because subduction-driven mantle flow can influence regional stress and deformation over distances of up to ~600 km, the ~500-km-long Taiwan orogen—particularly where neighboring slabs interact through slab edges, slab gaps, and potential tearing or detachment. Here, we investigate these processes using three-dimensional finite-element geodynamic models ASPECT. Starting from an simplified double-subduction configuration, we isolate the first-order signatures of slab–slab interactions from the complexity of regional tectonics. Systematic sensitivity tests varying inter-trench distance and convergence geometry are conducted to quantify their effects on mantle flow and regional stress–strain patterns. To connect model dynamics to seismological observables, we further predict seismic anisotropy by tracking the development of crystal preferred orientation within the modeled mantle flow. Model prediction of stress, strain, and seismic anisotropy are compared with earthquake focal mechanisms, island-wide GNSS-derived strain rates and SKS splitting observations. These comparisons constrain the extent to which double-subduction–driven mantle flow contributes to geophysical observables, and they identify which observables are most sensitive to specific subduction parameters and slab–slab interaction geometries.

How to cite: Hu, W.-L. and Tan, E.: Geodynamic Modeling of Slab–Slab Interactions in the Manila–Taiwan–Ryukyu Subduction System , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2911, https://doi.org/10.5194/egusphere-egu26-2911, 2026.

Southeast Asia records a prolonged and complex tectonic history from the Permian to the Cenozoic, marked by the accretion of continental fragments, subduction-related magmatism, and repeated reorganization of plate boundaries. Within this framework, the Palawan Continental Block (PCB) contains one of the most complete records of Mesozoic–Cenozoic continental margin evolution in the western Philippines and records key evidence of pre-Cenozoic tectonic events. However, the genetic relationships among its internal tectonic blocks remain unresolved. This study utilizes zircon U–Pb geochronology and Hf isotopic data from Paleozoic to Cenozoic strata extending from northern Palawan to Mindoro Island to constrain the timing, provenance, and tectonic affinity of rocks forming the PCB.

Igneous and detrital zircons from the Mindoro Metamorphics define a coherent Middle to Late Permian age population (270–254 Ma) with uniformly low Th/U ratios (<0.1), indicating a major Permian tectonothermal event related to the Indosinian orogeny. Jurassic strata from northwest Panay yield distinct unimodal age peaks at ~252 Ma (Early Triassic) and ~174 Ma (Middle Jurassic). The Middle Jurassic population overlaps with unimodal Early to Middle Jurassic age peaks (189–182 Ma) from strata on Busuanga Island, recording the development of Yanshanian arc magmatism along the South China continental margin. The recognition of both Permian-Triassic Indosinian (ca. 255-202 Ma) and Jurassic-Cretaceous Yanshanian (ca. 200-100 Ma) tectono-magmatic signatures, characteristic of the South China margin, establishes a direct temporal and genetic link between the tectonic evolution of the PCB and that of southern China. Moreover, the comparable detrital zircon age spectra and Hf isotopic signatures of the Permian-Eocene strata from Palawan and Mindoro indicate derivation from a common continental source, supporting the interpretation of the PCB as a single continental fragment rather than a collage of discrete accreted terranes.

How to cite: Labis, F. A., Mouthereau, F., Pubellier, M., Le Pourhiet, L., Bulois, C., Larvet, T., Tabilog, A. E., Balanial, N., Valera, G. T., and Payot, B.: From Indosinian to Yanshanian tectonic events recorded in the Palawan Continental Block, Philippines: New constraints from zircon U-Pb-Hf isotopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19323, https://doi.org/10.5194/egusphere-egu26-19323, 2026.

Formation of mantle plumes is often linked to the large low-shear-velocity provinces (LLSVPs) along the core-mantle boundary (CMB) evidenced by their strong spatial associations. However, a small portion of mantle plumes lie far from the major Pacific and African LLSVPs, and their formation cannot be explained by the classical models. Geophysical observations reveal that a few mantle plumes locate nearby the high seismic velocity anamolies (e.g., slab graveyard) along the CMB, indicating the influence of the subduction slabs on plume generation. In this study, we propose that the heterogenous accumulation of subduction slabs along the CMB may facilitate plume generation.

To test this hypothesis, we established a thermo-mechanically coupled numerical model. The model incorporated variations in slab distribution along the CMB to simulate slab graveyard heterogeneity and tracked the distribution of chemical components. The key results are: (1) In the models with heterogenous distribution of subduction slabs along the CMB, mantle plumes are formed induced by the lateral sliding of the heavy subduction slabs. This process is driven by gravity-induced migration and convergence of low-viscosity, high-buoyancy thermo-chemical material in the thermal boundary layer, which generates local thermal-buoyancy anomalies. (2) In the models with homogeneous distribution of subduction slabs or no subduction slabs, mantle plumes failed to form. (3) The initiation rate of mantle plumes correlates positively with slab accumulation height variations and thermal boundary layer thickness; conversely, a higher proportion of dense chemical components in the slab graveyard suppresses plume initiation.

Our modeling results may provide new insights on mantle plume formation away from the LLSVPs along the CMB, which could explain the observed mantle plumes that locate far from the LLSVPs in the present day.

How to cite: Liao, J., Xu, H., Li, Y., and Fan, Z.: Mantle plume formation away from the LLSVPs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10677, https://doi.org/10.5194/egusphere-egu26-10677, 2026.

The role of magmatism in modulating continental breakup remains a topic of debate¹. We present new, high-resolution 3D seismic reflection data from the South China Sea that reveals voluminous lower crustal magmatism occurred ~7–10 Myr before breakup along a >1,000 km long, NE–SW trending belt offset 100 km landwards of the eventual continental rupture. Through integration with numerical geodynamic models of continental extension, we show that a thermal anomaly associated with such lower crustal magmatic intrusion facilitate continental breakup. Specifically, our models show focused magma intrusion weakens the crust, promoting strain localization and migration that can lead to continental rupture 10’s–100’s km away from the site of initial magmatism.

How to cite: Deng, H., Chen, H., Bodur, Ö., Thybo, H., Magee, C., Bai, Z., Rey, P., and Keir, D.: Continental breakup facilitated by lower crustal magmatism, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3631, https://doi.org/10.5194/egusphere-egu26-3631, 2026.

Most magmatism on Earth is linked to passive mantle upwelling at mid-ocean ridges, dehydration of subducting plates, or mantle plume, additionally, subduction zones are proposed to induce melting of hydrated mantle, thereby driving magmatism (Yang & Faccenda, 2020). Geophysical observations show that some global hotspot associated mantle plumes do not directly penetrate the mantle transition zone (MTZ) but stall beneath it, nevertheless, significant low velocity anomalies and volcanism persist in the upper mantle and lithosphere (Hua et al., 2022; Tang et al., 2014). This phenomenon indicates that deep stalled mantle plumes can trigger shallow magmatism, yet the underlying dynamic processes and mechanisms remain unclear.

To clarify the nature of such spatially discontinuous plume-related magmatism, we developed a thermodynamic-geodynamic coupled model to systematically explore its core dynamic processes and mechanisms (Gerya & Yuen, 2003). Results demonstrate that after ascending to the region beneath the MTZ, the mantle plume is trapped by the phase transition barrier at the 660 km depth boundary. Its sustained heating preferentially melts the hydrated mantle within the MTZ, weakening rock strength and forming a melt-enriched layer. Subsequent disturbances from the subducting plate ultimately drive the melt to breach the boundary barrier and ascend to the base of the lithosphere. The model confirms that hydrated mantle in the MTZ is the direct source of shallow ascending melt, which remains uncontaminated or only minimally contaminated by mantle plume material. This study further quantifies the regulatory effects of mantle plume temperature, water content of the MTZ hydrated mantle, and phase transition parameters at the 660 km boundary on melt generation, enrichment, and ascent.

Our model results are highly consistent with observed shallow low-velocity anomalies associated with global stagnant mantle plumes, providing a plausible explanation for magmatism in these regions. This research deepens our understanding of shallow volcanism, and provides a new dynamic perspective for interpreting the discontinuous distribution of upper mantle low-velocity anomalies and inferring the spatiotemporal characteristics of intraplate volcanism.

Reference

Gerya, T. V., & Yuen, D. A. (2003). Rayleigh–Taylor instabilities from hydration and melting propel “cold plumes” at subduction zones. Earth and Planetary Science Letters, 212(1-2), 47-62.

Hua, Y., Zhao, D., & Xu, Y.-G. (2022). Azimuthal anisotropy tomography of the Southeast Asia subduction system.Journal of Geophysical Research: Solid Earth, 127, e2021JB022854.

Tang, Y., Obayashi, M., Niu, F. et al.(2014). Changbaishan volcanism in northeast China linked to subduction-induced mantle upwelling. Nature Geoscience, 7, 470-475.

Yang, J., & Faccenda, M. (2020). Intraplate volcanism originating from upwelling hydrous mantle transition zone. Nature, 579, 88-91.

How to cite: Kou, J. and Liao, J.: Intraplate magmatism driven by secondary plumes in the upper mantle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22538, https://doi.org/10.5194/egusphere-egu26-22538, 2026.

Mud diapirs are widely observed offshore southwestern Taiwan and are generally interpreted to originate from the overpressured Gutingkeng Formation, which experienced rapid sedimentation. Fluid enrichment within this formation reduces its bulk density relative to the surrounding strata, allowing buoyancy forces to drive upward migration and diapir formation. However, recent gravity analyses challenge this classical diapirism model by indicating a positive density contrast associated with the observed diapirs. This apparent contradiction raises the question of whether buoyant diapirism can coexist with a positive density anomaly.

In this study, we use numerical simulations incorporating visco–elasto–plastic rheology to investigate the formation mechanisms of mud diapirs under varying physical conditions. The models explore the effects of viscosity, elastic moduli, and density contrasts between diapiric material and the overlying sedimentary layers. Our results demonstrate that diapiric structures with a positive density contrast can be successfully reproduced. We further show that diapirism is systematically accompanied by the development of sedimentary basins filled with unconsolidated sediments, which introduce a strong negative density contrast relative to surrounding rocks. Gravity forward modeling indicates that a sedimentary basin with a thickness of approximately 500 m is sufficient to generate a gravity anomaly of ~5 mGal, consistent with observed data. These results suggest that the presence of positive-density diapirs does not preclude buoyancy-driven ascent and can be reconciled through the combined effects of diapirism and syn-deformational sedimentation.

How to cite: Lee, F.-Y., Tan, E., and Le Pourhiet, L.: Probing into the Diapirism in Southwest Taiwan by Numerical Simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21221, https://doi.org/10.5194/egusphere-egu26-21221, 2026.

In Southeast Asia, the contact between oceanic plates of Pacific affinity (Caroline and West Philippine seas) and the Eurasia and Australia plates of continental nature is rarely marked by high mountain ranges. Instead, it is characterised by fold-and-thrust belts involving volcanic arcs and slices made of continent-derived sediments. At every location, the tectonic style results from the oblique docking of the oceanic plates against the continental margins.

In this compressional setting, we identify two discrete systems developing the one after the other. The first system is marked by arcuate frontal thrusts bounded by oblique strike-slip lateral ramps, that are ubiquitous and vary in size from a few tens of meters to a few tens of kilometers. The regional convergence obliquity leads to a migration of the plates contact with consecutive tectonic periods of 1 to 2 Myrs, controlling the rapid triggering of subduction jumps that progressively change the location of the plates boundary over time. The second system corresponds to the onset of shear partitioning marked by the formation of a new subduction zone along which long, subparallel strike-slip faults form. Therefore, these new structures intersect previous ones, and the resulting sliver plate is affected by a margin-parallel stretching regime accommodating the progression of the docking and velocity variations during the convergence.

Thus, our study describes the evolution from one system to the other in Taiwan, Northern Philippines, Southern Philippines, Eastern Indonesia and Papua New Guinea. It also highlights important shifts that are necessary to discriminate small, Recent-to-Actual displacements imposed by GPS data (from 0Ma to 2Ma) from those deduced from longer-term motions documented along the main faults. In the frontal units of the Taiwan foothills or in the Luzon sedimentary wedge, recent tectonic slices typically disappear as we go backward in time by just a few Ma. In Southern Philippines, compression began in the latest Miocene–earliest Pliocene times with flat-and-ramp system, before being progressively replaced by the N–S Philippine Fault throughout the entire archipelago. In Eastern Indonesia and Papua New Guinea, the Sorong Fault also crosscuts Mid-Miocene docking structures marked by the flat-and-ramps features. Any regional reconstruction requires to unravel the two systems and date them carefully.

How to cite: Bulois, C., Pubellier, M., Chamot-Rooke, N., Mouthereau, F., Delescluse, M., Labis, F.-A., Bufféral, S., and Le Pourhiet, L.: Tracking the subduction-collision transition in the Taiwan-Philippine-New Guinea regions: a simple structural scheme to assist kinematic reconstructions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14104, https://doi.org/10.5194/egusphere-egu26-14104, 2026.

The Tanintharyi Shelf in the Andaman Sea, a key yet understudied segment of the Indo-Eurasian plate collision-related back-arc basin, hosts a Cenozoic carbonate platform with substantial hydrocarbon potential. This study integrates high-resolution 2D/3D seismic reflection data, well logs, regional stratigraphy, and global tectono-climatic records to systematically decipher the origin, morphological evolution of this platform, with a focus on tectono-climatic coupling mechanisms. Our findings reveal that the platform did not develop as a typical continental shelf-marginal rimmed system but on a fault-bounded restricted basement high as isolated patch reef. The evolution of the platform was governed by a trinity of coupled factors: (1) the rift tectonics provide basement highs as substrate; (2) the eustatic sea-level fluctuations drove aggradation-backstepping cycles and subaerial exposure; (3) and the Neogene Tibetan plateau uplift intensified Asian monsoons, increasing siliciclastic input through the Irrawaddy-Salween river systems, ultimately drowning the platform. This study establishes a predictive tectonostratigraphic framework for fault-bounded carbonate platforms in back-arc rift settings, linking far-field tectonics (Tibetan uplift), regional structural dynamics, and local sedimentary processes.

How to cite: Luan, X.: Tectonic–climatic controls on the growth and drowning of carbonate platforms: evidence from the Tanintharyi Shelf, Andaman Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22558, https://doi.org/10.5194/egusphere-egu26-22558, 2026.

Izu-Bonin-Mariana (IBM) subduction zone, which situated in the western Pacific and eastern Japan, is a major segment of the Pacific subduction system. Based on the geochemical investigations of ophiolites (Ishizuka et al., 2011), IBM is generally considered initiated approximately 52–51 Ma through a gravitational collapse across the transform fault. And then the subsequential spontaneous subduction of IBM began, which corroborated by numerical simulations (Maunder et al., 2020; Ritter et al., 2024). However, recent studies indicate that the IBM has involved horizontal convergence in its subduction initiation (Li et al., 2022). Two-dimensional geodynamic models have verified that the early-stage IBM subduction pattern dominated by horizontal compression is consistent with geochemical observation (Liu et al., 2024), whereas the source of such horizontal forces remains unclear.

Geological reconstructions reveal that the initiation of IBM was synchronous with the subduction of Izanagi-Pacific Ridge. Based on that, we hypothesize that the subduction of the mid-ocean ridge played a important role in the initiation of this new subduction. To verify this idea, we use the 3D thermomechanical coupled numerical code I3VIS to construct a subduction model incorporating both the Izanagi-Pacific Ridge and the transform fault where IBM subduction initiated. Model results demonstrate that when the Izanagi-Pacific Ridge caused subduction obstruction, stress redistributed laterally, thereby inducing horizontal compression along the transform fault. The new subduction was first triggered locally, and then gradually expanded across the entire transform fault, ultimately forming the full-scale initiation of the new subduction zone.

This model confirms that the subduction obstruction of the mid-ocean ridge can redistribute local stress to lateral weak structures, thereby triggering the transition or expansion of the new subduction zone, which as a potential process for the initiation of the IBM subduction. This proposed model validates a new mechanics of subduction initiation driven by indirect factors, and provides novel insights into subduction dynamics.

Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J., 2011. The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth Planet. Sci. Lett. 306, 229–240.

Maunder, B., Prytulak, J., Goes, S., Reagan, M., 2020. Rapid subduction initiation and magmatism in the western pacific driven by internal vertical forces. Nat. Commun. 11, 1874.

Ritter, S., Balázs, A., Ribeiro, J., Gerya, T., 2024. Magmatic fingerprints of subduction initiation and mature subduction: numerical modelling and observations from the izu-bonin-mariana system. Front. Earth Sci. 12, 1286468.

Li, H.-Y., Li, X., Ryan, J.G., Zhang, C., Xu, Y.-G., 2022. Boron isotopes in boninites document rapid changes in slab inputs during subduction initiation. Nat. Commun. 13, 993.

Liu, Liang, Li, H.-Y., Liu, Lijun, Ryan, J.G., Morgan, J.P., Ren, K.-X., Xu, Y.-G., 2024. Horizontally forced initiation of the izu-bonin-mariana subduction zone. Commun. Earth Environ. 5, 91.

How to cite: Wang, G. and Liao, J.: Mid-Ocean Ridge obstruction Cause A New Subduction zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10965, https://doi.org/10.5194/egusphere-egu26-10965, 2026.