RiseV8

AbstractThe N-S trending rift zones commonly exist in the southern Tibet due to the E-W extension of the plateau. In order to understand the structure of these rifts, a temporary seismic array across the Yadong-Gulu rift is deployed by Peking University. We use receiver function method to analyze the data collected, and produce an image of crustal interfaces using pre-stack Kirchhoff migration. The result shows large structural variation across the rift zone. We found that the upper-mid crustal low-velocity zone is only profound inside the rift zone. The result also indicates a thicker crust in the rift zone. We interpret this observation using mantle magmatic underplating model, which also agrees with geochemistry and other seimsic observations. If the magmatic underplating is a common mechanism in the southern Tibet rift zones, the deepest Moho in Earth can be found beneath those rifts.

Introduction

Tibetan Plateau is created by the Indian-Eurasian continental collision, which started around 50 Myr ago (Patriat 1984). The 2000-km north-south convergence between the two continents uplifts the highest plateau with an average elevation of 4 km. The collision thickens the plateau crust to around 70 km (e.g. Nabelek 2009), and induces the clockwise crustal flow around the Eastern Himalayan Syntaxis (Gan 2007). The eastwards migration of lithospheric material may relate to the development of the widespread north-south trending rifts in the southern Tibet (e.g. Yin 2000, Zhang 2013).

Several seismic surveys (INDEPTH I - IV, HICLIMB, etc.) have been conducted on the Tibetan Plateau to explore the tectonic process of the collision. Researchers published many observations and interpretations based on these surveys, e.g. the underthrusting of the Indian plate crust (Zhao 1993), the existence of partial melt in the plateau’s mid-lower crust (Brown 1996), the possible downwelling or subduction of the Indian lithosphere (e.g. Tilmann 2003, Li 2008, Nabelek 2009), the detailed mantle flow pattern from the seismic anisotropy observations (e.g. Hirn 1995, Huang 2000, Fu 2008).

Most of these studies focus on the N-S lithospheric shortening and the plateau uplift mechanism, yet some studies highlight the N-S trending, regularly spaced rift zones, which are probably created by the E-W extension of Tibetan plateau (e.g. Molnar 1978, Armijo 1986, Yin 2000, Kapp 2004, Kapp 2008). The orientations of these rifts are N-S in general, with a systematical clockwise rotation from NW-SE to NE-SW from western Tibet to eastern Tibet (Kapp 2004). The spacing between these rifts decreases from 191 km south of Indus-Yalu suture to 101 km in central Tibet (Yin 2000). These rifts started to develop in Southern Tibet from 18 Myr to 13 Myr (Williams 2001), which is believed to be the time when the stress setting in Tibet Plateau transit from compression to extension (Kapp 2005).

Although the N-S heterogeneity dominates the Tibet Plateau lithospheric structure, the E-W heterogeneity is also significant across the rifts (e.g. Zhang 2005, Zhang 2013). Unfortunately, most of the seismic surveys in the southern and central Tibet are inside the N-S trending rift zones, mostly due to the transportation limitation. As a result, the seismic observations are biased towards the rift structure instead of the average plateau structure, which may effect our interpretations if the lithospheric structures inside and out of the rift zones are significantly different.

Yadong-Gulu Rift (YGR) is one of the longest N-S trending rifts in the plateau. It extends from the high Himalaya, across Yalu-Zangbu suture, and ends to east of Namucuo lake around 31\({}^{\circ}\)N. West of the rift’s central-northern section locates the Nyainqentanglha Massif as the footwall of the extensional normal fault (Figure \ref{fig:topo}). The central part of the rift (Yangbajain portion) initiated normal faulting around 8 Ma ago and the northern part (Gulu portion) started at around 5 Ma (Harrison 1995, Stockli 2002). The thermochronological results suggest that the Nyainqentanglha Massif has been uplifted approximately 12-17km since then (Harrison 1995, Kapp 2005). The total horizontal extension accumulated on this rift system depends on the dipping angle of the normal faults, which is still under debate. For a 15 km uplift, the horizontal extension may range from 9 km on a high-angle (60\({}^{\circ}\)) normal fault to 21 km on a possible low-angle (35\({}^{\circ}\)) detachment fault (Cogan 1998).