Channel flow and the Himalayan-Tibetan orogen: a critical review

The movement of a low-viscosity crustal layer in response to topographic loading provides a potential mechanism for (1) eastward flow of the Asian lower crust causing the peripheral growth of the Tibetan Plateau and (2) southward flow of the Indian middle crust to be extruded along the Himalayan topographic front. Thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface. Isotopic constraints on the timing of fault movement, anatexis and thermobarometric evolution of the exhumed garnetto sillimanite-grade metasedimentary rocks support mid-crustal channel flow during the Early to Mid-Miocene. Exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface, defined by the South Tibetan Detachment System, and strain softening along the base, bounded by the Main Central Thrust. Neotectonic extrusion, bounded by brittle Quaternary faults south of the Main Central Thrust, is positively correlated with the spatial distribution of precipitation across a north–south transect, suggesting climate–tectonic linkage over a million-year time scale. A proposed orogen-wide eastward increase in extrusion rate over 20 Ma reflects current precipitation patterns but climate–tectonic linkage over this time scale remains equivocal. Although our understanding of the creation and subduction of oceanic lithosphere has advanced rapidly over the past few decades, the processes that control mountain building within the continents remain highly contentious. Recent developments in quantitative modelling of lithospheric deformation, coupled with an improved understanding of the mechanical behaviour of crustal materials, have provided a context within which the interactions between surface erosion and deep crustal deformation can now be explored. Knowledge of the strength of lithospheric materials is essential for understanding their behaviour. The effective viscosity of the lithosphere varies widely, and is determined largely by temperature, composition and, most importantly, the distribution of melt; the viscosity of partially melted protoliths between liquidus and solidus temperatures varies by about 14 orders of magnitude (Cruden 1990). Because (1) temperature generally increases monotonically with depth, thus decreasing the viscosity of a homogeneous body, and (2) viscosity increases across an isothermal boundary from a quartz-dominated fusible lithology to a more refractory one where olivine dominates (i.e. the Moho), the lower crust can form a layer of low viscosity, relative to the bounding lithologies above and below. If viscosities are sufficiently low within this layer, the material within it may flow in response to lateral variations in lithostatic load. Thus lower crustal flow provides a possible means by which lateral pressure gradients equilibrate and so moderate topography and variations in crustal thickness. The same process can occur in the middle crust if highly fusible lithologies, such as pelitic metasedimentary units, predominate. Crustal flow was first modelled by Bird (1991) in terms of laboratory flow laws. It was proposed initially as a means of modelling the response to extensional tectonics of the Basin and Range province, where a channel of 10–15 km thickness was inferred to flow as a result of a low viscosity of 10 –10 Pa s (Kruse et al. 1991). Burov & Diament (1995) argued that a wide range of crustal thicknesses could be explained by a ‘jelly sandwich’ in which a weak lower crust is sandwiched between a strong brittle–elastic upper crust and an elastic–ductile lithospheric mantle. More recently, a weak lower crust has been proposed to account for the uplift and topographic variations of convergent regimes, as exemplified by the Tibetan Plateau (Royden et al. 1997). This paper reviews the evolution of ideas that has led some geoscientists to believe that mechanical weakening in lower or middle crust explains diverse phenomena observed in many orogenic belts, including the Andes (Gerbault & Martinod 2005), the Appalachian orogenic belt (Merschat et al. 2005), the Canadian cordillera (Williams & Jiang 2005) and the Himalayan–Tibet orogen (Grujic et al. 2002). Specifically, it examines the quantitative evidence for the hypothesis that flow of a lowviscosity channel is linked to orography and surface precipitation in collisional orogens such as the Himalaya (Beaumont et al. 2001). Modelling the mechanical behaviour of the Tibetan Plateau The Himalayan arc, and the Tibetan Plateau that lies to the north (Fig. 1), is Earth’s type example of continuing collision tectonics. The tectonic regime that exists today is the result of a collision between a northward moving Indian plate and the Eurasian plate at about 50 Ma when convergent velocities decelerated from 150 to 50 mm a 1 (Patrait & Achache 1984). Following the initial collision, India has continued to migrate northward by about 2000 km. To explain the uplift of a wide plateau and the observation that significant thrusting across much of the plateau surface is largely absent, Zhao & Morgan (1985) were the first to invoke a weak lower crust beneath Tibet (6 3 10 Pa s). They suggested that Tibet was elevated by hydraulic pressure as the subducted Indian plate was intruded into the weak lower crust of Tibet. However, this is not strictly the first application of a channel-flow mechanism to the uplift of the Tibetan orogen, as Zhao & Morgan required no lateral movement of the weakened layer in response to topographic loading. Many of the orogen-scale features observed in the Tibetan orogen, such as the diffuse zones of seismicity and the width and height of the plateau, can be explained by assuming that the lithosphere behaves as a continuous medium akin to a thin viscous sheet that homogeneously thickens during the collision of two continental plates (England & McKenzie 1982). Homogeneous thickening of the lithosphere has thermal consequences, one of which is the postulated convective removal of the thickened keel of the lithosphere with consequent isostatic uplift followed by east–west spreading (England & Houseman 1989). Recent tectonic behaviour of the Tibetan Plateau is characterized by east–west extension across north–south-trending graben (Molnar & Tapponnier 1975; Armijo et al. 1986), and seismicity over much of the high plateau is characterized by normal faulting focal mechanisms (Chen & Molnar 1983). Other mechanisms have since sought to explain the crustal extension in southern Tibet without recourse to homogeneous thickening of the Tibetan lithosphere; for example, by invoking basal drag from underthrusting Indian lithosphere beneath southern Tibet (McCaffrey & Nabalek 1998). Although homogeneous thickening accounts for many of the first-order features of the uplift of a wide plateau at a continental collision zone, it implies that crustal thickening by thrusting is not significant on the scale of the Tibetan Plateau. An alternative treatment of crustal thickening assumes that the crust behaved as a rigid–plastic layer deformed by the motion of two rigid plates according to critical Coulomb wedge theory (Davis et al. 1983; Dahlen 1984). In this model, as formulated by Willett et al. (1993), thickening is restricted to the crust and the processes involved are treated as essentially brittle, at least in the early stages. In contrast to a thin viscous sheet, where thickening is homogeneous throughout the lithosphere, the lithospheric mantle of the Indian plate continues to be subducted beneath the Tibetan Plateau and so is not involved in thickening. Willett et al. noted that such rigid tectonics would be modified in time by viscous flow induced in the lower crust. The mechanical behaviour of viscous wedges has been more fully discussed by Medvedev (2002). Many geophysical studies have considered the implications of variations in mechanical strength with depth in the Tibetan lithosphere, as diverse lines of evidence suggest that depthdependent mechanical behaviour needs to be taken into account to develop more realistic models, particularly during isostatic readjustment following thickening of the lithosphere. Shortwavelength Bouguer anomalies and topographic variations observed on the Tibetan Plateau require compensation within the crust, indicative of a rheologically layered plate (Jin et al. 1994), and analysis of digital topography across recent graben from central and southern Tibet suggested a ductile, viscous (c. 10 Pa s) lower crust (Masek et al. 1994). Such geodetic and geophysical data were invoked by a study by Royden et al. (1997) of surface deformation in eastern Tibet, which found little surface evidence for deformation over the past 4 Ma despite abundant evidence for crustal shortening; those workers deduced that upper crustal deformation had been decoupled from the motion of the underlying mantle by a weakened lower crust. They suggested that flow in the lower crust was induced by lithospheric thickening beneath the central plateau causing its peripheral extension. This model allowed differential shortening and thickening of the lower crust around the margins of the plateau without associated upper crustal deformation. Such an analysis seeks to explain the outward growth of the Tibetan Plateau, rather than provide a mechanism for initial lithospheric thickening. The approach was further developed by Clark & Royden (2000), who modelled the topography of the eastern margin of the Tibetan Plateau in terms of the Poiseuille flow (whereby channel boundaries are assumed to be static) of a Newtonian fluid through a 15 km thick channel within the lower crust. They demonstrated that steep, abrupt margins, such as observed across the southern Himalaya, could result from a fluid of viscosity 10 Pa s whereas low-gradient margins, such as Fig. 1. Geological sketch map of southern Tibet and the Himalaya. ITSZ, Indus– Tsangpo suture zone; STDS, South Tibetan Detachment System; MCT, Main Central Thrust; MBT, Main Boundary Thrust. Dashed line marks hinge of North Himalayan antiform (NHA). N. HARRIS 512