LATE MESOZOIC RETRO-ARC SHORTENING AND BACK-ARC EXTENSION IN SOUTH CHINA REVEALED BY SEISMIC REFLECTION PROFILING

South China’s eastern margin evolved into an Andean-type convergent margin associated with the subduction of the Paleo-Pacific plate during the Late Mesozoic. It provides a natural laboratory to unravel the long-distance deformation, sedimentation, and magmatism in retro-arc and back-arc tectonic settings. We reveal parts of the crustal architecture of South China via SinoProbe deep seismic reflection profiles. From west to east, the crustal architecture is characterized by (1) flat-lying reflections in the westernmost South China (Sichuan Basin), consistent with weak or undeformed feature of a depocentre; (2) extensively folded and truncated reflections in the upper crust of central South China, which correspond to thin- and thick-skinned fold-and-thrust belts associated with northwestward thrust imbrication and shortening; (3) the underlying mid-crustal flat-ramp-flat décollement, which forms a weak, viscous layer accommodating deformation decoupling between the extensively deformed upper crust and the nearly undeformed middle crust; (4) two seismically distinct terranes separated by a low-angle set of crust-penetrating reflections in eastern South China, with the latter documenting the Cretaceous extensional reactivation of an ancient fault or suture zone; and (5) widespread seismic transparency truncated by high-reflective, subhorizontal bands of layered reflections in the coastal area of the easternmost South China, with the latter representing mantle-derived mafic sills and documenting magmatic underplating in a back-arc extensional setting. The above-stated seismic observations, coupled with surface tectono-magmatic records, provide evidence for a broad (>1000-km-wide) retro-arc shortening system with its eastern part destructed by intense back-arc extensional deformation and magmatism. We interpret the large-scale retro-arc shortening and back-arc extension as resulting from low-angle slab subduction and subsequent rollback along the Paleo-Pacific subduction zone in the Late Mesozoic. The magmatic underplating associated with slab rollback appears to have added large volumes of mantle-derived material to the crust, compensating for some crustal thinning and contributing to the generation of the broadly flat Moho across eastern South China during the extension.