Lithospheric Stretching Styles During Continental Breakup: Evidence From the Hyper‐Extended Xisha Trough

Abstract

The northwestern margin of the South China Sea experienced complex continental breakup, yet the specific lithospheric stretching modes and degree of crustal thinning in the Xisha area remain highly debated. Specifically, determining whether the Xisha trough is underlain by relatively thick or hyper‐extended continental crust is crucial for understanding the region's deep thermal structure and passive margin geodynamics. To address this, we employed a 2D multi‐observable modeling approach (LitMod2D). By jointly fitting free‐air and Bouguer gravity anomalies, geoid height, topography, and surface heat flow, while incorporating seismic reflection and refraction constraints, we constructed a robust lithospheric thermo‐chemical structure model for the region. Our results reveal significant lateral heterogeneity in the depth of the lithosphere‐asthenosphere boundary (LAB). The LAB beneath the Xisha block is comparatively deep and laterally uniform, occurring at depths of ∼90 km. In contrast, the LAB beneath the surrounding troughs and oceanic basins is significantly shallower, rising to depths of 50–70 km. Modeling results indicate that the Xisha trough is underlain by hyper‐extended continental crust (∼4–6 km thick), consistent with regional high heat flow anomalies and observed tectonic patterns. This indicates that thermo‐tectonic processes play a controlling role in lithospheric stretching and evolution. Synthesis shows that lithospheric extension in this region is consistent with predictions of the depth‐dependent stretching mechanism. Stretching strain is mainly concentrated in the upper crust and uppermost mantle, whereas the rheologically weaker middle and lower crust undergoes plastic flow, acting as a slip layer that buffers overall lithospheric deformation.