THE TERTIARY HISTORY OF THE TIBET PLATEAU: THE DOGAI CORING FOLD-THRUST BELT OF THE QIANGTANG TERRANE

The central Tibet Plateau comprises from north to south the Songpan-Ganzi−Hoh Xil terrane, the Qiangtang block, and the northern Lhasa block; they have accreted successively to the Eurasian Plate during the Mesozoic and have finally been involved into the India-Asia collision since ca. 55 Ma. While the Miocene−Recent history of the reverse-sinistral growth of the northern Tibet Plateau is relatively well known, the early growth history of the Plateau is still a matter of controversy. The remote Dogai Coring thrust-fold belt (DCFTB) is an ideal area to address the early collision, as it lies west of the broad zone of distributed dextral shear that is imposed by the material flow around the eastern Himalayan syntaxes. Here, we address this early evolution with structural, paleomagnetic, and geo-thermochronologic techniques.

The DCFTB within the northern Qiangtang block comprises coarse-grained sandstones to conglomeratic red beds that conformably overlie green shales and sandstones. This sequence overthrusted Jurassic limestone in the south and is covered by Cenozoic basalts in the north. The age of this red-green sequence is unclear: the youngest detrital U-Pb zircon age cluster is at ca. 130 Ma; lithologic correlation to other red bed sequences, in particular to the Fenghuoshan area to the northeast, suggests a Late Cretaceous to early Tertiary age. Based on map interpretation, evaluation of structural data, paleomagnetic results, and apatite fission-track and Ar-Ar ages, we interpret the DCFTB as a Cenozoic sinistral transpressional structure that formed over a decollement in the Mesozoic cover; the dextral strike-slip faults and the anticlockwise rotation result from book-shelf rotation of the blocks between the thrust splays. Overly, the DCFTB belt records a history quite similar to the Miocene−Recent one of the northern Tibet Plateau. Even in its initial stage, the India-Asia deformation seems to have affected a broad region in front of the Indian intender, and deformation appears to have encompassed both N-S and E-W lengthening.