Mesozoic Tectonic Evolution Of The Southeast China Block

The Mesozoic basins of Southeast China attract little attention due to their small in scale, very different in depositional style and complicated in formation. Lack of studies of these Mesozoic basins in large scale makes the geology history of South China especially the lithospheric structure and geodynamic process still under hot debate. This study thus focuses on a series of Mesozoic basins located in the center section of South China block, and conducted sedimentology and structural deformation based on geological survey, analysis on the drilling and seismic data, combined with petrology, geochronology and geochemistry studies on the mafic rocks developed in the study area, the following conclusions has been achieved:(1) The Mesozoic terrestrial deposition of the Mesozoic basins in the center South China has very different time-space distribution pattern. The late Triassic and early Jurassic strata occurred in the whole study area sporadically. However, the middle and late Jurassic mainly distributed at the center part and middle-east part of the study area respectively. The lower Cretaceous strata have a symmetric distribution pattern with center region almost missing the early Cretaceous strata with minor deposition of late lower Cretaceous strata while the early lower Cretaceous occurred in east and west part of the study area. The upper Cretaceous strata mainly distributed at the inland part of the study area.(2) Based on depositional record and structure features, the Mesozoic basins of center South China block have experienced two major stages:Late Triassic to Early Jurassic post-orogenic depression and fault subsidence basin since middle Jurassic. The two different stage of basins also suggesting two main depositional stages:the source area of late Triassic and early Jurassic deposition were located at the north area of the study area, and with relatively stable depositional style, mainly fluvial-lacustrine facies. Varies depositional facies occurred since middle Jurassic, and the source regions may constrained by the shape of the basins. (3) Three mafic magmatic eruption stages can be identified with our new40Ar/39Ar age and geochemistry features combined with previous studies:stage1(195-160Ma) basalts; stage2(160-110Ma) basalts; stage3(110-64Ma) basalts. Each stage basalt has different distribute style and time-space variation of geochemistry regularity which is a reflection of the secular changes of the magmatic source:the early period (195-160Ma) source region can be the asthenospheric mantle with negligible to minor lithosphere contamination; the middle period (160-110Ma) source region could be the asthenosphere mantle with enhanced asthenosphere-lithosphere interaction; the late period (110-64Ma) have two different source regions, the coastal source region involved partial melting of newly formed SCLM at the base of lithosphere that was metasomatised by slab-induced melts and fluid or from SCLM within normal subduction zone, the inland source region may be the mantle of asthenosphere-origin with a small contribution from the SCLM.(4) Based on basin analysis, tectonic studies combined with dataset of mafic rocks of center section of South China block, the subduction of paleo-pacific plate is the major factor of evolution of Mesozoic South China block. And the Mesozoic geological features of South China can be interpret as a results of geodynamics processes related to the break-up, foundering and retreating of an early Mesozoic flat-subducted oceanic slab.