Coeval doming and stretching of the eastern end of the India-Asia collision zone - Namche Barwa Syntaxis, Tibet

The eastern part of the India-Asia collisional zone is marked by a N-plunging, non-cylindrical fold, the Namche Barwa Syntaxis (NBS) with a topographic relief of >7 km. The NBS exposes rapidly exhuming Indian- and parts of the overlying Asian crust. Structural, paleomagnetic and GPS studies reveal clockwise rotation of the crust and mantle around a steep axis located close to the mutual borders of India, China and Myanmar (e.g. Sol et al. 2007; Liebke et al. 2011). Teleseismic tomography beneath the NBS reveals 170 km thick Indian lithosphere that is not connected with a slab anomaly to the N beneath the Tibetan Plateau, but that appears laterally continuous with north and E-dipping slab anomalies along strike of the Himalayan chain, respectively, to the W and E of the NBS (Zhang et al. 2012). A closer look at available structural, geochronological, and petrological data reveals Late Oligocene, N-directed subduction of Indian continental lithosphere followed by two stages of pronounced exhumation in Late Oligocene to Miocene time and unroofing in Pliocene time. This history is intimately related to the mantle structure imaged beneath the NBS. Timing of subduction is not constraint in the NBS, but Booth et al. (2009) suggest Palaeocene to Eocene ages for subduction-related plutonism along strike of the India/Asia suture zone. High-pressure (HP) conditions in granulites in the western NBS at 25-24 Ma preceded decompression beginning no later than 18 Ma (Su et al. 2012). These granulites are separated from granulites without HP assemblages by a moderately W-dipping mylonitic fault, indicating that this fault was a thrust (Xu et al. 2012) which accommodated S to E directed exhumation of the HP granulites in its hangingwall. This first stage of exhumation coincided with peak temperatures at ~10 Ma (Booth et al. 2009). The granulites and their intervening thrust fault are folded by the NBS (Geng et al. 2006), indicating that this first stage of exhumation preceded doming of the NBS. The NBS is bounded to the W and E by steep sinistral and dextral transpressional faults, respectively. Based on map-scale relations, we link these faults to the South Tibetan Detachment (STD) which in the NBS area is sited along the Indus-Yarlung Suture Zone and juxtaposes the Late Oligocene granulites in its footwall with Eocene or older amphibolite-facies rocks in its hangingwall. The STD cuts mylonitic faults in its footwall, including the aforementioned thrust separating the HP and non-HP granulites. Taken together, the STD is interpreted as a system of stretching faults that attenuated the crust in a N-S direction during doming of the NBS. The age of extensional exhumation is poorly constrained, but certainly pre-dated rapid exhumation and cooling (>5 mm/a, >50° C/ Ma) of the NBS in Pliocene time (4-2 Ma) related to ductile-brittle faulting at ≤300° C of the northern part of the NBS and northerly adjacent Asian units (Stewart et al. 2008). We attribute Miocene doming and extensional exhumation of the NBS to tearing of the Indian slab. The lateral propagation of this tear allowed Asian asthenosphere to escape clockwise around the front of the subducting Indian plate.