SUBDUCTION POLARITY AND HISTORY OF THE MESO-TETHYAN BANGONG-NUJIANG OCEAN BETWEEN THE QIANGTANG AND LHASA BLOCKS

The amalgamation history of the Qiangtang and Lhasa blocks is key to understanding the tectonic evolution of the Tibetan Plateau prior to the India-Asia collision. Recent paleomagnetic studies suggest a 4500-km-wide ocean, the Bangong-Nujiang Ocean of the Meso-Tethys realm, existed between the two blocks before it closed in the Early Cretaceous. Three models have been proposed to explain the surface geology along the Bangong-Nujiang suture zone: (1) northward subduction of the Bangong-Nujiang under Qiangtang; (2) simultaneous north- and south-ward subduction of the Bangong-Nujiang under Qiangtang and Lhasa, respectively; (3) northward, low-angle subduction of the Neo-Tethyan oceanic lithosphere from the southern margin of the Lhasa terrane. These hypotheses predict distinct locations and geometries of subducted slabs in the mantle, and are therefore testable via seismic tomography models. We map the midslab surfaces using several global seismic tomography models to interpret the slabs in 3D. We first recognize the Neo-Tethyan slabs, which form a province of plate-like, south-dipping, semi-continuous high-velocity anomalies that can be traced from 1600-km depth under southern India to the Indo-Asian collision zone in the lithosphere. Structurally lower than the Neo-Tethyan slabs are two plate-shaped high-velocity anomalies that are interpreted as the remnant slabs of the Bangong-Nujiang Ocean. The northern slab dips steeply to the north, and lies at ca. 1100-1800 km depth under the Qiangtang block. The bottom of the slab is slightly overturned and merges into the “slab graveyard” in the lowermost mantle. The southern slab dips moderately to the south and is located at ca. 1100~2000 km depths under the southern Tibet and northernmost India. It may extend deeper and further to the south, but the resolution is too low to interpret confidently. This study provides evidence, independent from surface geology or paleomagnetism, to the second tectonic model in which the Bangong Nujiang Ocean underwent doubly vergent subduction in the late Mesozoic. Slab geometry also indicates a relatively stable trench to the north, and a northward-retreating trench to the south of the Bangong-Nujiang Ocean. Moreover, this study may help to constrain the longitudinal motion of plates in the lower mantle reference frame.