STRATIGRAPHIC AND GEOCHRONOLOGIC EVIDENCE FOR LATE TRIASSIC INITIAL SUBDUCTION OF THE BANGONG-NUJIANG (TETHYS) OCEAN BENEATH QIANGTANG

The Bangong-Nujiang suture zone (BNSZ) in central Tibet is the boundary separating the Qiangtang block to north and Lhasa block to the south. The suture zone bears critical information of the birth and demise of the Bangong-Nujiang (Tethys) Ocean (BNO) and the rise of the Tibetan plateau. By far, controversies are still present concerning the Wilson-cycle evolution of the BNO. Herein, sedimentological and geochronological studies along a north-south traverse across the BNSZ near Gaize, Tibet, provide evidence for a Late Triassic–Jurassic accretionary wedge accreted to the south margin of Qiangtang. This wedge, preserved as the Mugagangri Group (MG), records evidence for the northward subduction of the BNO beneath Qiangtang. The MG strata comprise two coarser intervals (lower olistostromes and upper conglomerates) intercalated within sandy turbidites, which are consistent with timing and forearc stratigraphy during subduction initiation, predicted by geodynamic modeling. Following the model, the northward subduction of the BNO beneath Qiangtang and subsequent arc-magmatism are inferred to have begun, respectively, at ~220Ma and ~210Ma, with respect to depositional ages constrained by youngest detrital-zircon ages. The initiation of arc-magmatism is also supported by provenance transition, reflected in sandstone detrital modes and age patterns of detrital zircons. Moreover, the ages are consistent with the occurrences of exhumed, high-pressure metamorphic rocks along the BNSZ in the Amdo and Dongcuo areas, both of which were interpreted to have been subducted along with the BNO beneath Qiangtang and to have attained their peak metamorphism by ~190Ma.

Previously, evidence for an incipient arc in Qiangtang, induced by the northward subduction of the BNO, was lacking, but the timing of Late Triassic BNO subduction and related volcanism is coincident with an important Late Triassic magmatic event in central Qiangtang that probably represents the “missing” arc. Other Qiangtang events, such as exhumation of the Qiangtang metamorphic belt as a source area, development of the Late Triassic Nadigangri deposits and bimodal volcanism, are more easily explained in the tectonic context of early northward subduction of the BNO beneath Qiangtang, beginning at about 220Ma.