Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

K. J. Zhang ¹

Affiliations
1 Department of Earth Sciences, Nanjing University, Nanjing 210093, CN

Abstract

The Songpan-Ganzi terrane of central China has long been believed to be floored by oceanic crust. However, several recent deep seismic profiles across the terrane reveal that it could be underlain by Precambrian continental crust. In general, the upper crust of the terrane with a P velocity of less than 6.35 km/s is about 29-32 km thick and could consist of Mesozoic and Paleozoic sedimentary rocks and Precambrian basement. Within the terrane there exist two suites of Permo-Triassic sedimentary rocks in tectonic contact with each other. These suites were deposited in two distinct settings: a flysch association and a carbonate-coarse grained clastics association formed in a stable platform environment. The Songpan-Ganzi terrane is also characterized by consistently northeasterly verging folds and thrusts and by subsequent strike slip movements, with the main thrusts and detachments occurring between the Permo-Triassic flysch association and the continental crust and between the upper and lower crusts. The detachments are highlighted by steady low velocity zones. A new tectonic model is presented for the Mesozoic evolution of the Songpan-Ganzi terrane. The Permo-Triassic flysch present in the terrane is proposed to represent allochthonous flakes that were thrust over the lower para-autochthonous Sinian-Triassic continental sedimentary rocks of the terrane. Concurrently, the Precambrian basement could have detached from the lower crust and could have been transported northeastwards. The deformation mainly resulted from the mid-Mesozoic suturing between the Songpan-Ganzi and Qiangtang terranes and represents a product of multiphase collision between the Tethyan terranes through the Mesozoic.

Keywords

Flysch, Crustal Structure, Continental Basement, Continental Subduction, Tectonic Evolution, Songpan-Ganzi Terrane, Northeastern Tibet.

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K. J. Zhang ¹, Q. F. Wang ², H. N. Lu ², B. G. Zhang ², B. D. Xia ¹, G. M. Wang ³

Affiliations
1 Department of Earth Sciences, State Key Laboratory for Mineral Deposit Research, Nanjing University, Nanjing 21 0093, CN
2 Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, CN
3 Department of Resources, Petroleum University, Dongying, Shandong 257061, CN

Abstract

The magnitude of Cenozoic crustal shortening within the Tibetan plateau is a question of key importance in the models for its development. Earlier investigators proposed that the Cenozoic shortening within the plateau is generally less than 40%. Here, we present two exactly constrained structural sections, which show that the late Cenozoic crustal shortening within southern Qiangtang is upto 57-67%. This indicates that the internal crustal shortening within the plateau can play a vital role in the doubling of the crust of the Tibetan plateau.

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