Geoscience

Tectonic Remnants of Continental Evolution in Lithospheric Mantle Discovered

Prof. Zheng Tianyu and his colleagues from the Institute of Geology and Geophysics, CAS presented a new approach to distinguish between the multiple waves generated by the crustal structure and the velocity discontinuities in the lithospheric mantle by synthetic test of common conversion point (CCP) stacking images. The velocity discontinuities in the lithospheric mantle were constructed for 6 profiles spanning different tectonic units in the North China Craton (NCC). The homogeneity of the lithospheric mantle beneath profile NCISP-4 and the northern part of profile NCISP-7 is a structural characteristic of the western NCC. The local high-velocity volumes in the lithospheric mantle beneath the eastern NCC (profiles NCISP-2, NCISP-3 and NCISP-6) are interpreted to be the remnants of lithospheric delamination or subduction slab occurred only within local areas. The imaging results beneath profile NCISP-5 and the southern part of profile NCISP-7 clearly displayed intermittent and juxtaposed velocity interfaces which reveal a flat subduction channel resulting from the continent-continent collision between the NCC and the Yangtze craton. The projections of the top of velocity interfaces onto the surface are close to the Shangdan suture to the north and to the Mianlue suture to the south. This seismological research of the lithospheric mantle structure in the NCC indicates that the remnants of continental subduction in the lithospheric mantle since the Mesozoic could be observed, but the slab relict beneath the Paleoproterozoic amalgamation in the NCC might have disappeared off the topmost mantle. It is difficult to identify the previously relict architectures in the fertile juvenile lithospheric mantle in the reactivating eastern NCC. This research was recently published in Journal of Geophysical Research (2012, 117, B03308, doi:10.1029/2011JB008873).

Implications of Tectonic Movement in Altyn Tagh Fault

The Altyn Tagh left-lateral strike-slip fault is the geological boundary between the Tibetan Plateau and the Tarim Basin in northwest China. It is one of the major region of stress consumption and transmission and geomorphological deformation in northern Tibetan Plateau. The tectonic movements in this region recorded that the stress of India-Eurasia collision had transferred to the northern Tibetan Plateau. In order to constraint on evolution of the Altyn Tagh fault in northern Tibetan Plateau, substantial geologic research in field were done by some geologists in the Institute of Earth Environment, CAS. Dr. Chang Hong and his colleagues discover that the sedimentary facies changed from lacustrine to fluvial-diluvial in Pliocene on the basement about magnetostratigraphy, magne tic rock and deposition facies in the Suerkuli intermontane basin between north and south branches of the Altyn Tagh. Although climatic change can influence sedimentary environment, Geomorphlogical location of the section, unconformity between Pliocene and Pleistocene, sedimentation rate variation and previous data about growth in the northern Tibetan Plateau by others suggest that tectonic movement is the main cause to this deposition facies change. Geological evidents reveal that Altyn Tagh uplifted and deformed in late Pliocene. This tectonic movement resulted in the modern geomorphogical Altyn Tagh and expansion of the Tibetan Plateau to the northeastword. This result is coincided with that the northern Tibetan Plateau had rapid grown in late Pliocene which was constraint by atmospheric modelling on eolian in eastern Asian. Recently, this result was published in the Journal Asian Earth Sciences.