Is the Songpan-Ganzi Terrane (Central China) Really Underlain by Oceanic Crust?

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.