Wedge tectonics in South China: constraints from new seismic data.

Collisional orogens form when tectonic forces amalgamte fragments of Earth's continental lithosphere. The sutures between individual fragments, or terranes, are potential sites of weakness that facilitate subsequent continental breakup. Therefore, the lithospheric architecture of collisional orogens provides key information for evaluating the long-term evolution of the continental interior: for example, the South China Block (SCB), where the tectonic history is severely obscured by extensive surface deformation, magmatism, and metamorphism. Using new passive-source seismic models, we show a contrasting seismic architecture across the SCB, with three prominent crustal dipping structures across the Jiangnan Orogen. Combined with constraints from multi-disciplinary regional geophysical datasets, these pronounced dipping patterns are interpreted as relict wedge-like lithospheric deformation zones initiated in the fossil collisions that assembled the Yangtze Block and the SCB. The overall trend of these tectonic wedges implies successive crustal growth along paleo-continental margins and is indicative of northward subduction and docking of accretional terranes. In contrast, no such dipping structures are preserved in the Cathaysia Block, indicating a weak and reorganized lithosphere. The variations in the deformation responses across the SCB reflect the long-term modifications of the lithosphere caused by prolonged collision and extension events throughout the tectonic history of the SCB. Our results demonstrate the critical roles that suture zones played in the successive growth and evolution of the continental lithosphere.

Direct structural evidence of Indian continental subduction beneath Myanmar.

Indian continental subduction can explain Cenozoic crustal deformation, magmatic activity and uplift of the Tibetan Plateau following the India-Asia collision. In the western Himalayan syntaxis and central Himalaya, subduction or underthrusting of the Indian Plate beneath the Eurasian Plate is well known from seismological studies. However, because information on the deep structure of the eastern Himalayan syntaxis is lacking, the nature of the Indian subduction slab beneath Myanmar and the related tectonic regime remain unclear. Here, we use receiver function common conversion point imaging from a densely spaced seismic array to detect direct structural evidence of present-day Indian continental subduction beneath Asia. The entire subducting Indian crust has an average crustal thickness of ~30 km, dips at an angle of ~19°, and extends to a depth of 100 km under central Myanmar. These results reveal a unique continental subduction regime as a result of Indian-Eurasian continental collision and lateral extrusion.