Lower crustal extrusion in the distal continental margin of the South China Sea

Ductile deformation dominates the evolution of hot hyperextended distal margins before continental breakup. However, how ductile deformation interacts with magmatism remains understudied, which prevents a clear understanding of distal margin evolution. This study uses high-resolution 3-D seismic reflection data integrated with borehole-constrained stratigraphy in the distal margin of the northern South China Sea to show the structure and evolution of a system of gneiss domes (D1−D6) and associated detachment fault. D1, D3, D4, D5, and D6 developed in the synrift stage (40−33 Ma), while D2 developed in the postrift stage (33−23 Ma). These domes were likely driven by deep magma intrusion and ductile deformation in response to Rayleigh-Taylor instability. Dome evolution comprised three stages: (1) magma intrusion and partial melting drove the initial doming, (2) increased magmatism and melt fraction led to dome amplification, and (3) the residual high geothermal condition during the thermal cooling resulted in localized dome growth. Domes impinged the draping detachment fault and the supradetachment basin, and small depocenters were developed in response to lower crustal doming and detachment faulting. The coexistence of simple- and pure-shear deformation during ductile stretching led to a complex 3-D evolution of the Liwan detachment fault. This study provides new insights into the spatial and temporal evolution of lower crustal extrusion during hyperextension of ductile crust that shaped distal margin architecture.