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

Many seismic and magnetotelluric experiments within Tibet provide proxies for lithospheric temperature and lithology, and hence rheology. Most data have been collected between c. 888E and 958E 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 Beneath northern Tibet the upper mantle has not been replaced by subducting Indian and Asian lithospheres, and is warmer than to the south.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. ‘Only fools, or unusually insightful individuals not yet recognized to be ahead of their time, would doubt that the warmth of the lower crust in regions of extension makes decoupling of upper crust and upper mantle more likely there than in other settings. Thus, the real challenge to understanding how such decoupling or coupling occurs will require study of regions of intracontinental crustal shortening’ (Molnar 2000). Lateral strain variations, and vertical strain and strength profiles in Tibet Tibet forms the largest and highest plateau on Earth today. Two extreme and opposed views of the mechanism(s) responsible for regional uplift and accommodation of shortening are that: (1) discrete tectonic blocks, internally relatively undeformed, are being expelled eastward between lithospheric strike-slip faults (e.g. Tapponnier et al. 1982, 2001); and (2) deformation is essentially continuous, with diffuse deformation of the crust and upper mantle over broad areas (e.g. Dewey & Burke 1973; England & Houseman 1986; England & Molnar 1997; Shen et al. 2001), in addition to the convergence taken up at the plateau margins. With the increasing availability of global-positioning data from Tibet and adjacent regions, both in number of sites (now exceeding 500: Zhang et al. 2004) and length of their time series (up to nine years: Chen et al. 2004), the extreme model of coherent blocks separated by narrow fault zones along which most deformation is localized seems a less plausible description of the active deformation of Tibet. At the Earth’s surface there is a continuously varying surface strain field across Tibet when observed at scales of 100 km (Wang et al. 2001; Zhang et al. 2004). The major active strike-slip faults such as the Kunlun and Jiali faults (Fig. 1) have motions c. 10 mm a and separate internally deforming blocks in which about two-thirds of the total 25 mm a eastward extrusion of Tibet is accommodated by smaller normal faults and conjugate strike-slip faults (Taylor et al. 2003; Chen et al. 2004; Zhang et al. 2004). In contrast, vertical strength and strain profiles in Tibetan lithosphere cannot be directly measured, and so remain less certain and more contentious. From: LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications, 268, 39–70. 0305-8719/06/$15.00 # The Geological Society of London 2006. Opposing views are that the entire lithosphere deforms homogeneously (‘vertical coherent deformation’) (e.g. England & Houseman 1986; England & Molnar 1997; Flesch et al. 2005), or that deformation is dominated by a more rapid ductile flow in the middle and/or lower crust above a stronger upper mantle (‘channel flow’) (e.g. Zhao & Morgan 1987; Bird 1991; Shen et al. 2001; Beaumont et al. 2004). The depth extent of such channelized flow – middle crust, lower crust, or both – depends critically on the actual strength profile. These opposing views have recently been conflated with a new controversy about vertical strength profiles for the continental lithosphere. Laboratory experiments have long been used to infer a combined frictional plus ductile behaviour of the lithosphere, with a quartz-dominated strength maximum in the uppermiddle crust, and an olivine-dominated strength maximum at the top of the upper mantle (Brace & Kohlstedt 1980). It is also widely held that the greater part of lithospheric strength is in the upper Fig. 1. Location map of main geophysical experiments referred to in this paper. Heavy black lines and upper-case italicized names are generalized location of seismic experiments; heavy dotted lines and lower-case italicized names are magnetotelluric profiles; thin lines are major sutures and faults; thin dotted line is the 3000 m contour. Faults: KKF, Karakax fault; ATF, Altyn Tagh fault; QF, Qaidam Border fault; KF, Kunlun fault; JRS, Jinsha River suture; BNS, Banggong–Nujiang suture; JF, Jiali fault; IYS, Indus–Yarlung suture; MCT, Main Central thrust; MBT, Main Boundary thrust. Himalayan syntaxes: Np, Nanga Parbat; Nb, Namche Barwar. Representative geothermal areas: Ybj, Yangbajain graben in Yadong-Gulu rift; Pu, Puga. Representative north Himalayan gneiss domes: Kd, Kangmar dome; Tmc, Tso Morari complex. Profile names (and selected references): seismic, INDEPTH, IN-1 (Zhao et al. 1993; Makovsky et al. 1996), IN-2 (Nelson et al. 1996; Kind et al. 1996; Alsdorf et al. 1998b; Makovsky & Klemperer 1999), IN-3 (Zhao et al. 2001; Ross et al. 2004; Huang et al. 2000; Rapine et al. 2003; Tilmann et al. 2003); Sino-French, SF-1 (Hirn et al. 1984a), SF-2 (Zhang & Klemperer 2005), SF-3 (Hirn et al. 1995; Galve et al. 2002b), SF-4 (Herquel et al. 1995; Vergne et al. 2003), SF-5 (Wittlinger et al. 1998), SF-6 (Galve et al. 2002a; Vergne et al. 2003), SF-7 (Wittlinger et al. 2004); PASSCAL (McNamara et al. 1994, 1995, 1997; Rodgers & Schwartz 1998; Sherrington et al. 2004); WT, West Tibet (Kong et al. 1996); TB, Tarim basin (Gao et al. 2000; Kao et al. 2001); NP, Nanga Parbat (Meltzer et al. 2001); HIMPROBE (Rai et al. 2006); TQ, Tarim-Qaidam (Zhao et al. 2006); HIMNT, Himalaya– Nepal–Tibet experiment (Schulte-Pelkum et al. 2005); NE INDIA (Mitra et al. 2005). Near-vertical reflection profiles are IN-1, IN-2 and locally along IN-3, TB and HIMPROBE; controlled-source wide-angle profiles are SF-1, SF-2, SF-3, SF-6, IN-1, IN-2, IN-3, WT and TQ; teleseismic recording experiments (typically for tomography, receiverfunction, shear-wave-splitting, and waveform inversions) are PASSCAL, IN-2, IN-3, SF-3, SF-4, SF-5, SF-6, SF-7, TB, NP, HIMPROBE, HIMNT and NE INDIA. Magnetotelluric profiles: INDEPTH, in-100, in-200 (Chen et al. 1996), in-500 (Wei et al. 2001), in-600 (Unsworth et al. 2004), in-700 (Spratt et al. 2005), in-800 (Unsworth et al. 2005); himprobe (Gokarn et al. 2002); nf, Nepal–French (Lemonnier et al. 1999); wt, West Tibet (Kong et al. 1996). S. L. KLEMPERER 40