Cretaceous lithospheric extension and surface responses in South China

Lithospheric extension at passive margins is commonly classified into three end-member scenarios: pure shear, simple shear, and depth-dependent deformation. However, how lithospheric extension evolves in an intraplate setting remains enigmatic due to the lack of reliable constraints on the deep lithospheric architecture. Here we integrated seismic reflection profiles and surface geology across the ~800-km-wide Cretaceous intraplate extensional system of South China to illustrate depth-dependent kinematic decoupling of extension in a mechanically stratified lithosphere. The extension was initially distributed in magma-poor conditions as expressed by normal faulting in the upper crust and lower-crustal flow toward the rift axis. Necking of the crust and Moho uplift led to mantle shear-zone formation, lower-crustal flow toward the rift flanks, and deep mantle flow. We demonstrate that the extensional modes vary with decreasing mantle strength from magma-poor to magma-rich domains, as reflected in decreasing crust-mantle decoupling with increased Moho temperatures and the replacement of a two-layer (brittle vs ductile) mantle by a fully ductile mantle. These findings reveal a first-order lithospheric configuration of intraplate depth-dependent extension driven by far-field stresses attributable to slab retreat. Extension-related strain fields across the lithosphere are uniform ~NW-SE, indicating vertically coherent deformation. Stress transmission across this coherent system might occur as follows: (1) basal shear stresses at the lithospheric base may enhance the simple shearing at the crust-mantle Moho interface, promoting ductile stretching in the lower crust, and (2) mantle shear zones localized strain by acting as strain-transfer structures between the lower crust and the lithospheric mantle. We establish the crustal and lithospheric mantle deformation fields accompanying the retreating subduction. Our data compilation suggests a tectonic coupling between slab rollback, mantle flow, and lithospheric extension in South China. Rollback-induced mantle flow likely drove lithospheric extension in South China by imposing shear forces at the lithosphere base.