Mechanisms of continental collision

Like all other major zones of continental deformation (e.g. Sections 7.6, 8.6, 10.2.5), the evolution of colli-sional orogens is governed by the balance among regional and local forces, the strength and rheology of the continental lithosphere, and by processes that change these parameters over time. To determine how interactions among these factors control the development of the Himalayan-Tibetan orogen, geoscientists have developed physical and analogue models of continental collision. This section provides a discussion of the main results and different approaches used in this field of study.

1 Precollisional history. The strength and rheology of the continental lithosphere at the start of continental collision is governed by the pre-collisional history of the two colliding plates. In the case of the Himalayan-Tibetan orogen, millions of years of subduction, arc magmatism, terrane accretion, and crustal thickening along the southern margin of Eurasia (Section 10.4.2) weakened the lithosphere. During the India-Eurasia collision, the many suture zones, thick flysch sequences, and other weak zones that characterized Eurasia allowed deformation to extend deep into the interior of the continent (Yin & Harrison, 2000; Tapponnier et al., 2001).

Unlike Eurasia, the relatively cool and deeply rooted Precambrian shield of India resulted in a relatively strong plate that resisted shortening during collision. The generally high mechanical strength and high elastic thickness of the Indian lithosphere led to its underthrusting beneath southern Tibet (Section 10.4.3). An exception to its generally high strength is the sediment that was deposited on the passive continental margin of northern India from the Early Proterozoic to Paleocene. During collision, these weak sequences failed and were scraped off the downgoing plate, forming the Himalayan fold and thrust belt.

2 Continental underthrusting. The underthrusting, or subduction, of continental lithosphere beneath another continental plate is one of the most important mechanisms that accommodates convergence in zones of continental collision. The rheology of the two plates and the degree of mechanical coupling between them control shortening and the evolution of stresses within the overriding plate. In the Himalayan-Tibetan orogen, the underthrusting of Indian continental lithosphere drives intra-plate shortening at the leading edge of the Indian plate and in Tibet, and, possibly, also farther north into Asia. The resultant shortening has generated crust that is up to 70-80 km thick (Section 10.4.5) and has contributed to the uplift and growth of the Tibetan Plateau. Like its counterpart in the central Andes (Section 10.2.4), the plateau is associated with high crustal temperatures and widespread intra-crustal melting that have weakened the crust sufficiently to allow it to flow. This process has decoupled the Tibetan crust from the underlying convergent motions and has altered the dynamics of the orogen. Although geophysical observations show that Indian lithosphere is underthrust to at least a point beneath central Tibet, interpretations differ on how this process is accommodated (Dewey et al., 1989; Yin & Harrison, 2000; Johnson, 2002). The main problem is that the underthrusting requires the removal or displacement of Asian lithosphere from under Tibet (Section 10.4.5). Several mechanisms may alleviate this problem, including the downturning of Indian mantle lithosphere beneath the Bangong-Nujiang suture (Figs 10.21, Plate 9.4(bottom) (between pp. 244 and 245), the convective removal or delamination of the lithospheric mantle beneath Tibet (England & Houseman, 1988; Molnar et al., 1993), the southward subduction of Asian mantle (Willett & Beaumont, 1994), and the removal of Asian mantle by strike-slip faulting during the lateral escape of Tibet (Section 10.4.3).

Although the role of these various processes remains uncertain, it seems likely that a combination of mechanisms accommodates shortening beneath Tibet.

3 Indentation, lateral escape, and gravitational collapse. A comparison between the total amount of convergence between India and Eurasia since they collided and estimates of the total amount of shortening accommodated by fold-thrust belts in the orogen has yielded a shortening deficit ranging anywhere from 500 km to over 1200 km (Dewey et al., 1989; Johnson, 2002). This deficit has led to numerous attempts to explain how the convergence not accounted for by folding and thrusting has been accommodated. A leading hypothesis involves the indentation of India into Asia and the lateral escape of eastern Tibet (Section 10.4.3). Indentation is the process by which a rigid block presses into and deforms a softer block during convergence. The theory of indentation originally was developed by mechanical engineers to predict the configuration of lines of maximum shear stress, or slip lines, in deforming plastic materials. In geologic applications, the slip lines correspond to dextral and sinistral strike-slip faults whose pattern is controlled by the shape of the indenter and by lateral constraints placed on the plastic medium (Tapponnier & Molnar, 1976; Tapponnier et al., 1982).

In one pioneering application, Tapponnier et al. (1982) explored the effects of indentation as a rigid 50-mm-wide block (India) penetrates into a softer block (Asia) made of laminated plasticine. Figure 10.22 shows two evolutionary sequences where the plasticine is either bilaterally confined at the two edges parallel to the motion of the indenter (Fig. 10.22a-c) or unilaterally confined at only one of these edges (Fig. 10.22d-f). The bilaterally confined case produces a symmetric pattern of slip lines ahead of a "dead triangle" that rapidly welds to the indenter. The penetration proceeds by the creation of numerous, short-lived, dextral and sinistral faults near the triangle's apex. The unilateral

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(g) 70° E 80° E 100° E 120° E 140° E 150° E 60° N 50° N

Strike-slip fault Motions with respect to Siberia

„t*** Normal fault ^ Direction of extension

Thrust fault Oceanic crust in SE Asia -cf*^ Subuduction zone

Indentation Tectonics Indentation Tectonic Triangular

Figure 10.22 (a-f) Indentation experiments on Plasticine and (g) schematic map illustrating extrusion tectonics in eastern Asia (redrawn from Tapponnier et al., 1982, with permission from the Geological Society of America). F, major fault. Numbers associated with arrows in (g) are extrusion phases: I ~ 50-20 Ma; 2 ~ 20-0 Ma.

Figure 10.22 (a-f) Indentation experiments on Plasticine and (g) schematic map illustrating extrusion tectonics in eastern Asia (redrawn from Tapponnier et al., 1982, with permission from the Geological Society of America). F, major fault. Numbers associated with arrows in (g) are extrusion phases: I ~ 50-20 Ma; 2 ~ 20-0 Ma.

case generates an asymmetric pattern where faults that allow displacement towards the free edge predominate, such as F1. The block translated sideways rotates about 25° clockwise and is followed by the extrusion of a second block along another sinistral fault, F2, which allows a continued rotation of the first block by up to about 40°. Pull-apart basins (Section 8.2) develop along the sinistral faults because of their irregular geometry. As these movements progress, a gap grows between the indenter and extruded plasticine. Tapponnier et al. (1982) suggested that these results explain the dominance of sinistral offsets in China (Fig. 10.22g). The pull-apart structures may be analogous to the extensional regimes in Shansi, Mongolia, and Baikal. The Altyn Tagh Fault may correlate with the major dislocation F2, and the Red River Fault with F1. The comparison also suggests that indentation causes the curvature of fault systems located east of Tibet. Finally, lateral extrusion between and to the southeast of the Altyn Tagh and Red River faults results in extension that resembles patterns observed in the South China Sea and the Gulf of Thailand (Fig. 10.22g).

Since its development in the late 1970s and early 1980s, the indentation model of continental collision has evolved considerably. Although the model of Tapponnier et al.

(1982) explains the general pattern and distribution of strike-slip faulting in eastern Tibet and southeast Asia, it has been less successful at explaining other aspects of the deformation. One problem is that it predicts lateral displacements of hundreds to a thousand kilometers on the large strike-slip faults within Tibet. However, estimates of the magnitude of slip on major strike-slip faults, so far, have failed to confirm the extremely large magnitudes of displacement. The Altyn Tagh Fault, for example, may only have 200 km of left-lateral slip and the Xianshuihe Fault about 50 km (Yin & Harrison, 2000). These observations suggest that while lateral escape is occurring, it may occur on a smaller scale than originally predicted.

Another problem with the application shown in Fig. 10.22 is that it does not predict, or take into account, the effects of variations in crustal thickness during deformation. In addition, the region of east-west extension and normal faulting in Tibet has no analogue in the model. One possible explanation for the extension is that it results from gravitational buoyancy forces associated with the great thickness and high elevations of the plateau. In this view, an excess in gravitational potential energy enhanced by the presence of a buoyant crustal root and the possible removal of mantle lithosphere by convective erosion or delamination (see also Section 10.2.5) drives the gravitational collapse of the overthickened crust (Dewey, 1988; England & Houseman, 1989). Lateral gradients in gravitational potential energy may help the plateau spread out and move laterally toward the eastern lowlands where it interacts with other lithospheric elements. Whereas other origins for this extension also have been proposed, quantitative considerations of these forces suggest that the evolution of the plateau depends as much upon buoyancy forces and local boundary conditions as it does on indentation or stresses arising at the edges of the Indian and Eurasian plates (Royden, 1996; Liu & Yang, 2003). The gravitational collapse of over-thickened continental crust also explains the evolution of orogens after convergence stops where, in many areas, it has been linked to the formation of extensional metamorphic core complexes (Section 7.3).

To account for these effects, investigators have simulated the deformation of Asia using a viscous sheet that deforms in response to both the edge forces arising from continental collision and the internal forces generated by differences in crustal thickness (England & Houseman, 1989; Robl & Stuwe, 2005a). Rather than modeling displacements on individual faults, these continuum models simulate deformation as a zone of distributed flow between two colliding plates. Most predict that a zone of shortening and thickening crust grows in front of and, with the appropriate boundary conditions, to the side of an advancing indenter. The results suggest that the zone of deformation directly related to the India-Eurasia collision is much smaller than that predicted by the plasticine models and that other regions of deformation in Southeast Asia are unrelated to the local and tectonic forces arising from the collision. Instead, deformation in these latter regions may result from regional tectonic stress fields related to the plate boundaries located south and east of Asia.

Continuum models of indentation, in general, have been successful at explaining the asymmetry of deformation in Asia, including the lateral escape of eastern Tibet. They also are well suited for examining the effects of variations in lithospheric strength and rheology on the style of deformation observed in India and Asia. Robl & Stuwe (2005a, 2005b), for example, explored the effects of variations in the shape, convergence angle, and rheology of a continental indenter on both lateral and vertical strain patterns in Asia during lateral escape. These authors investigated the sensitivity of a deforming viscous sheet to indentation involving combinations of these parameters. An especially interesting aspect of their application is the investigation of how buoyancy forces arising from crustal thickening are balanced by edge forces from indentation.

In the experiments of Robl & Stuwe (2005a) Asia is modeled as a viscous sheet consisting of a regular square mesh with 3200 triangles (Fig. 10.23a). The eastern, western, and northern boundaries of the mesh are rigid and cannot move. These constraints simulate the effects of the Tarim Basin to the north and Pamir to the west. At the southern boundary an indenter of width (D/2) and length (ffl) moves northward into the mesh with a velocity scaled to be 50 mm a-1. This motion results in deformation and thickening that is distributed between the indenter and the foreland to the north (Fig. 10.23b). Two important variables include the viscosity contrast (n) between the indenter and the foreland and the angle (a) between the indenter front and the direction of indentation. All materials are described using a power law rheology with exponent (n), which describes how strain rates are related to stress (Section 2.10.3).

The effect of indenter shape on the distribution of deformation is best illustrated in simulations where the indenter is strong. Viscosity contrasts of n = 1000 and n = 100 simulate this condition. The results show that for an indentation angle of a = 45° and a strong, viscous indenter, deformation localizes along the interface between the colliding blocks and the horizontal velocity field is highly asymmetric. Crustal thickening is at a maximum north of the western tip and slightly less in front of the northeastern edge (Fig. 10.24a,b). A band of eastward-moving material develops on the northeast side of the indenter (Fig. 10.25a). These asymmetries contrast with the symmetric patterns that surround rectangular indenters with high viscosities (Figs 10.23b, 10.25b). For a low viscosity indenter (n = 2 or 3), the indenter angle plays only a minor role. In these latter cases, the indenter accommodates most of the shortening and thickening, with the pattern becoming progressively more symmetric and delocalized through time (Fig. 10.24c,d). Figure 10.25c and d show that the horizontal velocity field for a rectangular indenter is similar to that with an indenter angle of a = 45°. These results indicate that, for the given set of boundary

10 km

Crustal thickness

40 50 60 70 80

Figure 10.23 Geometry and boundary conditions of a finite element model of indention (image provided by J. Robl and modified from Robl & Stuwe, 2005a, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union).

(a) Regular mesh consisting of3200 triangles. Shaded region is the indenter and light region is the foreland.

(b) Typical model result scaled for a length scale of D = 5000km and an indentation velocity of 50mm a~'. Gray scale indicates crustal thickening distributed between indenter and foreland.

8 x 103 km

Figure 10.23 Geometry and boundary conditions of a finite element model of indention (image provided by J. Robl and modified from Robl & Stuwe, 2005a, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union).

(a) Regular mesh consisting of3200 triangles. Shaded region is the indenter and light region is the foreland.

(b) Typical model result scaled for a length scale of D = 5000km and an indentation velocity of 50mm a~'. Gray scale indicates crustal thickening distributed between indenter and foreland.

Figure 10.24 Finite element model showing the influence of viscosity contrast on the evolution of oblique indenters (image provided by J. Robl and modified from Robl & Stuwe, 2005a, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). In both model runs, the geometry was identical, and n = 3 and a = 45°. Bold dotted line is the outline of the indenter. (a,b) Viscosity contrast r = 1000. (c), (d) Viscosity contrast r\ = 2. (a) and (c) show finite element mesh after 40 Myr, (b) and (d) show corresponding diagrams contoured for crustal thickness.

x 10 km

x 10 km

Figure 10.24 Finite element model showing the influence of viscosity contrast on the evolution of oblique indenters (image provided by J. Robl and modified from Robl & Stuwe, 2005a, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). In both model runs, the geometry was identical, and n = 3 and a = 45°. Bold dotted line is the outline of the indenter. (a,b) Viscosity contrast r = 1000. (c), (d) Viscosity contrast r\ = 2. (a) and (c) show finite element mesh after 40 Myr, (b) and (d) show corresponding diagrams contoured for crustal thickness.

x 10 km

x 10 km conditions, lateral escape of the crust increases with indenter angle for relatively strong indenter rheologies and simulates the patterns of displacement observed in eastern Tibet.

In situations where Asian lithosphere is especially viscous and strong, lateral escape results mainly from horizontal compression as blocks move out of the way of the rigid indenter. In these cases, buoyancy forces arising from crustal thickening contribute little to the horizontal velocity field because the thickening tends to be either highly localized or inhibited by the high strength of the material. As the strength of Asia decreases, the magnitude and distribution of crustal thickening increase and gravitational buoyancy forces become increasingly important. The numerical simulations of Robl & Stuwe (2005a) and others (Liu & Yang, 2003) suggest that buoyancy forces developing in weak thick crust such as that in Tibet enhance the rate of lateral escape.

Distance x 104 km

Figure 10.25 Results from finite element modeling showing the instantaneous lateral displacement field during indentation for two different indentor rheologies and two indentor angles (a) (image provided by J. Robl and modified from Robl & Stuwe, 2005a, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). (a) and (b) show a viscosity contrast between indentor and foreland of r = 100; (c) and (d) show a contrast of r = 3. Indentor angles of a = 0° and a = 45° are represented. Grayscale bar shows horizontal velocity. Contour interval is 1 mm a~'. Eastward moving area is largest for the oblique indentor shown in (a).

Distance x 104 km

Figure 10.25 Results from finite element modeling showing the instantaneous lateral displacement field during indentation for two different indentor rheologies and two indentor angles (a) (image provided by J. Robl and modified from Robl & Stuwe, 2005a, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). (a) and (b) show a viscosity contrast between indentor and foreland of r = 100; (c) and (d) show a contrast of r = 3. Indentor angles of a = 0° and a = 45° are represented. Grayscale bar shows horizontal velocity. Contour interval is 1 mm a~'. Eastward moving area is largest for the oblique indentor shown in (a).

A three-dimensional viscoelastic model developed by Liu & Yang (2003) illustrates how various possible driving forces and a rheological structure involving both vertical and lateral variations influence deformation patterns in Tibet and its surrounding regions. This model, like most others, involves a rigid Indian plate that collides with a deformable Eurasian continent at a constant velocity relative to Eurasia. The two plates are coupled across a fault zone that simulates the Main Boundary Thrust (Fig. 10.26a). On the eastern and southeastern sides of the model, boundary conditions are assigned to simulate the lateral escape of the crust. On the west, the effects of a spring or roller simulate the lateral resisting force of a rigid block in Pamir. On the northern side of the model boundary conditions approximate the resistance to motion by the rigid Tarim Basin. The top surface approximates the real topography and the bottom lies at 70 km depth. A vertical topographic load is included by calculating the weight of rock columns in each surface grid of the finite element model. An isostatic restoring force is applied

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