Crustal flow in Tibet: geophysical evidence for the physical state of Tibetan lithosphere, and inferred patterns of active flow

Abstract

Many seismic and magnetotelluric experiments within Tibet provide proxies for lithospheric temperature and lithology, and hence rheology. Most data have been collected between c. 88°E and 95°E in a corridor around the Lhasa-Golmud highway, but newer experiments in western Tibet, and inversions of seismic data utilizing wave-paths transiting the Tibetan Plateau, support a substantial uniformity of properties broadly parallel to the principal Cenozoic and Mesozoic sutures, and perpendicular to the modern NNE convergence direction. These data require unusually weak zones in the crust at different depths throughout Tibet at the present day. In southern Tibet these weak zones are in the upper crust of the Tethyan Himalaya, the middle crust in the southern Lhasa terrane, and the middle and lower crust in the northern Lhasa terrane. In northern Tibet, north of the Banggong-Nujiang suture, the middle and probably the lower crust of both the Qiangtang and Songpan-Ganzi terranes are unusually weak. The Indian uppermost mantle is cold and seismogenic beneath the Tethyan Himalaya and the southernmost Lhasa terrane, but is probably overlain by a northward thickening zone of Asian mantle beneath the northern Lhasa terrane. Beneath northern Tibet the upper mantle has not been replaced by subducting Indian and Asian lithospheres, and is warmer than to the south. These inferred vertical strength profiles all have minima in the crust, thereby permitting, though not actually requiring, some form of channelized flow at the present day. Using the simplest parameterization of channel-flow models, I infer that a Poiseuille-type flow (flow between stationary boundaries) parallel to India-Asia convergence is occurring throughout much of southern Tibet, and a combination of Couette (top-driven, between moving boundaries) and Poiseuille lithospheric flow, perpendicular to lithospheric shortening, is active in northern Tibet. Explicit channel-flow models that successfully replicate much of the large-scale geophysical behaviour of Tibet need refinement and additional model complexity to capture the full details of the temporal and spatial variation of the India-Asia collision.

Seismic signature of the collision between the east Tibetan escape flow and the Sichuan Basin

“…Table 1 The steepest topography gradient around the Tibetan plateau margin is commonly accepted to result from the strong interaction between the eastern Tibetan escape flow with the resistive Sichuan Basin. From our passive source seismic experiment, we contend that the deep process (or processes) that have occurred (and/or are occurring) beneath eastern Tibet may also contribute to the steep rise in topography at LMS, in addition to middle/lower crust channel flow and tectonic escape (Royden, 1996;Klemperer, 2006;Zhang et al, 2009). The convergence of the eastward escape flow from eastern Tibet with the Sichuan Basin should produce a downward undulation (or detachment according to Houseman et al 1981) of the lithosphere beneath both the western margin of the Sichuan Basin and the eastern margin of eastern Tibet (Zhang et al, 2009).…”

“…Subsurface compensation may have occurred in a variety of different ways, such as by crustal thickening (Allegrè, 1984), denudation (Meng et al, 2006), slip partitioning (Chen et al, 1994;Tapponnier et al, 2001), subduction of the Indian mantle lithosphere (Kosarev et al, 1999;Kumar et al, 2006;Li et al, 2008a), lithospheric detachment (Houseman et al, 1981, Molnar, 1988, subduction of the Asian lithosphere (Willett and Beaumont, 1994;Kind et al, 2002) and eastward escape (Royden et al, 1997Clark and Royden, 2000;Klemperer, 2006). GPS displacement vectors (Gan et al, 2007) and SKS anisotropy measurements (Wang et al, 2008) indicate that the Tibetan crust (and possibly also the lithosphere and asthenosphere) is escaping eastwards, and that the main portion of the flow is being redirected towards the south east after it encounters the Sichuan Basin ( Figure 1).…”

“…thickening or subduction) remains poorly understood. The powerful interaction between eastern Tibet and the Sichuan Basin is characterized by the Longmen Shan (LMS), which has the steepest topography gradient in Tibet (Royden et al, 1997;Clark and Royden, 2000;Klemperer, 2006). The LMS fault system, which marks the border of the Songpan-Ganzi block of eastern Tibet and the Sichuan block of the Yangtze craton, is dominated by dextral strike-slip and has a significant thrust component (Burchfiel et al, 1995;.…”

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

“…Geophysical studies have revealed similar geophysical anomalies in the mid-crust of the Lhasa terrane (block) and the Himalaya range (Nelson et al, 1996;Unsworth et al, 2005), and a channel flow model has been proposed to explain this phenomenon (Beaumont et al, 2001(Beaumont et al, , 2004Clark and Royden, 2000;Hodges, 2006;Hodges et al, 2001;Klemperer, 2006;Medvedev and Beaumont, 2006;Nelson et al, 1996). Based on a study of the Kuday dykes emplaced on the south side of the Indus-Yalu suture (IYS), King et al (2007) proposed southward ductile flow of Asian crust beneath Southern Tibet.…”

Extensional Seismogenic Stress and Tectonic Movement on the Central Region of the Tibetan Plateau

“…The characteristics and mechanism of extensional motions at the high altitudes of the Tibetan plateau have been analyzed [25][26][27][28][29]. The activities of normal faulting earthquakes concentrated at high altitudes were investigated based on distribution characteristics of stress field, GPS data, as well as geothermal data in this paper.…”