3D Crustal Architecture and Along-Strike Variation in the Mid-Northern South China Sea Rifted Margin

Rifted margins often show apparent along-strike variations at the continent-ocean transition (COT) region, typically occurring gradually over hundreds-to-thousands of km between the "magma-rich" and "magma-poor" endmembers. In contrast, comparatively smaller-scale lateral variabilities of rifting structure and faulting style in hyper-thinned continental crust are not yet well described. The South China Sea (SCS) was recently defined as the archetype of a new "intermediate-type" margin with neither excess magmatism nor mantle exhumation. However, along-strike crustal variations along this new type of margin are still little explored. Based on previously unpublished 2D grid of seismic reflection lines and overlapping 3D seismic volumes, we have investigated the crustal structure, faulting style, and the lower crustal reflectivity along similar to 400 km of the mid-northern SCS margin. Our new maps of crystalline-crust thickness and top basement show that the SCS extension developed in two segments with contrasting tectonic styles separated by a narrow boundary. The eastern domain, resembling a wide-rift model, displays low-angle detachment faults and high-velocity lower crust mainly associated with syn-rift magmatism indicating weak crustal rheology. The neighboring western domain, resembling a narrow rift model, displays steep crustal-scale faults indicating comparatively strong rheology. The change from wide to narrow rift-or weak to strong rheology-occurs abruptly, which appears at odds with the current conventional wisdom of mostly gradual transitions over a long distance between rift models. We speculate that the inherited heterogeneity in mantle composition and 3D melt focusing effect may drive this abrupt along-strike variability in the mid-northern SCS.