Interactions between syn-rift magmatism and tectonic extension at intermediate rifted margins

Intermediate rifted margins exhibit neither seaward dipping reflectors nor exhumed mantle at the continent-ocean transition (COT). Instead, they transition into normal-thickness, magmatic Penrose-type oceanic crust, and thus diverge from the classic magma-rich and magma-poor end-member models. However, several intermediate margins, such as the South China Sea (SCS), display detachment faulting similar to magma-poor margins and magmatic underplating typical of magma-rich ones. How tectonics and magmatism interact in these intermediate environments is poorly understood. Here we use 2D numerical models to demonstrate that the elevated initial geotherm inherited from prior plate subduction in the SCS explains several key observations: an early phase of wide rifting, subsequent localization onto core complexes with substantial footwall magmatic intrusions, and eventual formation of normal igneous oceanic crust at break-up. Thermal weakening caused by syn-rift footwall magmatic intrusions facilitates lower crustal ductile flow, promoting the development of rolling-hinge type detachment faults and exhumation of core complexes. These structures are associated with accelerated tectonic subsidence, which is later moderated by detachment-related doming, as observed in the SCS. Normal-thickness oceanic crust occurs after break-up, even under ultra-slow extension rates used in our simulations, highlighting the importance of inheritance in determining margin architecture, the spatio-temporal distribution of syn-rift magmatism, and the nature of the COT. This behavior contrasts sharply with magma-poor margins, where a cooler lithosphere and similar ultra-slow extension produce no syn-rift magmatism, leading instead to crustal embrittlement, mantle serpentinization and exhumation at the COT.

💡 Research Summary

The paper addresses a long‑standing paradox in the classification of rifted continental margins. Classical end‑member models separate margins into magma‑rich (characterized by seaward‑dipping reflectors, thick magmatic crust) and magma‑poor (thin crust, mantle serpentinization, exhumed mantle at the continent‑ocean transition, COT). However, several margins—most notably the South China Sea (SCS)—exhibit a hybrid suite of features: wide‑rift detachment faulting typical of magma‑poor settings, extensive foot‑wall magmatic intrusions, and ultimately normal‑thickness Penrose‑type oceanic crust. The mechanisms that allow these apparently contradictory processes to coexist have not been resolved.

Using two‑dimensional thermo‑mechanical numerical experiments, the authors test the hypothesis that an elevated initial geotherm, inherited from a previous subduction episode, controls the evolution of such intermediate margins. The model domain represents a 150‑km‑thick continental lithosphere with a pre‑existing high‑temperature gradient. Various extension rates are imposed, with a focus on ultra‑slow rates (≈0.5 cm yr⁻¹) comparable to those inferred for the SCS. Magma is introduced as a low‑viscosity, buoyant fluid that can intrude into the foot‑wall during extension.

Key findings are as follows:

1. Early Wide‑Rift Phase – The hot geotherm reduces lithospheric viscosity, allowing a broad region to undergo simultaneous extension. This reproduces the observed early, wide‑rift geometry of the SCS.

2. Thermal Weakening by Foot‑Wall Intrusions – As extension proceeds, magmatic intrusions accumulate in the foot‑wall. The heat supplied by these intrusions further weakens the surrounding rocks, creating a positive feedback loop: weaker lithosphere permits more extension, which in turn drives additional magmatic intrusion.

3. Development of Rolling‑Hinge Detachment Faults and Core Complexes – The thermally weakened foot‑wall promotes lower‑crustal ductile flow and the nucleation of large‑scale, rolling‑hinge style detachment faults. These faults exhumate core complexes, matching the SCS’s observed detachment‑fault architecture.

4. Accelerated Subsidence Followed by Detachment‑Related Doming – The rapid formation of detachment faults generates a phase of pronounced tectonic subsidence. Subsequent uplift of the detached foot‑wall creates domal topography that moderates further subsidence, a pattern documented in seismic and gravity data from the SCS.

5. Normal‑Thickness Oceanic Crust at Break‑up – Even under ultra‑slow extension, the continued magmatic supply produces a normal‑thickness (~7 km) Penrose‑type oceanic crust after break‑up. This demonstrates that a hot inherited geotherm can sustain magmatism independent of extension rate.

6. Contrast with Magma‑Poor Margins – When the same ultra‑slow extension is applied to a cooler lithosphere (representative of magma‑poor margins), no syn‑rift magmatism occurs. Instead, the lithosphere becomes brittle, the mantle serpentinizes, and exhumed mantle is observed at the COT, consistent with classic magma‑poor behavior.

The authors argue that “thermal inheritance” is the primary control on whether a rifted margin follows a magma‑rich, magma‑poor, or intermediate evolutionary pathway. The feedback between magmatic intrusion, thermal weakening, and fault development explains the coexistence of detachment faulting and substantial magmatic underplating in the SCS. This insight reshapes our understanding of margin architecture, the spatial‑temporal distribution of syn‑rift magmatism, and the nature of the continent‑ocean transition. It also suggests that other intermediate margins worldwide may be governed by similar inherited thermal structures, with implications for crustal thickness predictions, hydrocarbon prospectivity, and seismic hazard assessments.