Oblique convergence between India and Eurasia

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B5, /2001JB000636, 2002 Oblique convergence between India and Eurasia Muhammad A. Soofi and Scott D. King Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, USA Received 23 May 2001; revised 31 December 2001; accepted 4 January 2002; published 30 May [1] Oblique convergence is the general rule along convergent boundaries. Such convergence produces asymmetric deformation (topography) along the collision boundary. To quantify this asymmetry, we develop a method that allows us to calculate the asymmetry in topography with respect to a reference line drawn perpendicular to the collision boundary. The asymmetry in topography is presented as a number, or series of numbers, called here the asymmetry index. A positive asymmetry index indicates that there is more area with topography greater or equal to certain value to the east (right) of the reference line than to the west (left). The asymmetry index is negative for the opposite topography distribution. We use oblique convergent boundary conditions with a thin viscous sheet model to investigate the observed topography along the India-Eurasia collision boundary. The goal of our study is to reproduce the observed asymmetry in the topography (i.e., the high topography in the Tibetan Plateau is shifted to the east). We consider a laterally varying velocity (both magnitude and angle, which we call the angle of obliquity) along this boundary and use a homogeneous lithosphere for Eurasia. With boundary conditions alone we are unable to account for the degree of asymmetry present in the observed topography. The asymmetry in the calculated topography is enhanced by considering a rheologically stronger anomalous zone representing the Tarim basin. The use of a constant velocity (both magnitude and angle) boundary condition, as in earlier studies, produces topographic asymmetry in the opposite direction to the observed topography. This indicates that in the study of obliquely converging boundaries it is important to consider the variation of velocity (magnitude and angle) in the collisional boundary. INDEX TERMS: 8110 Tectonophysics: Continental tectonics general (0905); 8159 Evolution of the Earth: Rheology crust and lithosphere; 8120 Tectonophysics: Dynamics of lithosphere and mantle general; 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: Convergent boundary, India-Asia collision, viscous flow, oblique convergence 1. Introduction Copyright 2002 by the American Geophysical Union /02/2001JB000636$09.00 [2] Reviews of global plate motion [e.g., DeMets et al., 1994] reveal that the relative motion between two plates at a convergent boundary almost always contains an oblique component of motion. Oblique convergence gives rise to both compressional and shear (transcurrent) components of strain [Lu and Malavieille, 1994; Haq and Davis, 1997]. Consequently, a variety of structures can be produced along a collisional boundary depending on the ratio of compressional to shearing motion. Because oblique convergence is the rule, rather than the exception, oblique boundary conditions should be investigated along with rheology and heterogeneities in the lithosphere when studying convergent boundaries [Lu and Malavieille, 1994; Sobouti and Arkani-Hamed, 1996; Hu et al., 1997; Haq and Davis, 1997]. [3] The mechanics of oblique convergence have been studied using analytical techniques [e.g., Beck, 1986, 1991; McCaffrey, 1992; Platt, 1993; Ellis et al., 1995], numerical models [e.g., Braun, 1993; Hu et al., 1997] and mechanical models [e.g., Pinet and Cobbold, 1992; Lu and Malavieille, 1994]. Fitch [1972] suggested that in the case of high-oblique-angle convergence, complete decoupling occurs, decomposing the velocity vector into strikeparallel and strike-normal components. The strike-parallel component causes lateral displacement of a sliver of the overriding plate. This type of decoupling was later termed Sunda-style tectonics by Beck [1983]; however, McCaffrey [1992] has shown that complete decoupling is an end-member case and is not common in nature. Generally, the oblique convergence partitions into strikeslip and dip-slip components where the dip-slip vector makes an angle less than the obliquity of the velocity vector, measured from the trench-normal component, thus producing partial decoupling [McCaffrey, 1992]. [4] Numerous studies have investigated the effect of the collision of India with Eurasia [e.g., Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1976; England and McKenzie, 1982; England and Houseman, 1986; Peltzer and Tapponnier, 1988; Jolivet et al., 1990; Neil and Houseman, 1997]. These studies have shown that the amount of topographic uplift along the India-Eurasia plate boundary and the stress distribution in the adjacent Eurasian plate, as recorded by earthquake focal mechanisms, can be explained by the collision; however, the asymmetric pattern of the topography (i.e., a wide, parallel to convergence, zone of uplift along the eastern boundary and a narrow zone of uplift along the western boundary) remains the subject of debate [e.g., England and Houseman, 1985; Peltzer and Tapponnier, 1988; Houseman and England, 1996]. [5] A stress pattern consistent with crustal thickening was observed in the plane stress numerical experiment of Vilotte et al. [1982]. In addition, they found that the pattern of stress correlated well with the distribution of normal and thrust faults in the region. In a separate plane stress numerical experiment, where gravitational effects and boundary forces were considered, England and McKenzie [1982] showed that most of the topography in Asia north of India can be accounted for by the collision of India with Eurasia. In a further modification of the thin viscous sheet formulation, Houseman and England [1986] introduced a moving indenter boundary condition, showing that to obtain the present topography of the Tibetan Plateau requires km of crustal shortening. This is close to the estimated crustal shortening obtained from paleomagnetic data [Patzelt et al., 1996]. Le Pichon et al. ETG 3-1

2 ETG 3-2 SOOFI AND KING: OBLIQUE CONVERGENCE 50N 60ûE 80ûE 100ûE 120ûE 50N EURASIA 40N 30N A TARIM BASIN N 30N INDIA B 20N 60E 80E 100E 120E 20N Figure 1. Observed topography along the India-Eurasia collision boundary and neighboring regions from ETOPO5. Topography is in meters. The asymmetry in the topography of the Himalayas is apparent by tracing the 4000-m contour. This contour line is much closer to the AB plate boundary to the west than it is to the east. [1992] attempted a reconstruction of the India-Eurasia collision over the last 45 Myr and concluded that the total volume of crustal shortening is more than the crustal volume needed to explain the uplifted Himalaya and Tibetan Plateau. They suggested that a combination of lateral extrusion and eclogitization of the continental crust is necessary to reconcile the crustal shortening with the observed deformation and topography. A reconciliation between the lateral extrusion and crustal thickening models can be reached by considering that the lithosphere deforms first by thickening and then, as the buoyancy force increases, lateral extrusion by strike-slip deformation becomes important [England and Houseman, 1986]. Neil and Houseman [1997] showed that a relatively small change in the strength of the lithosphere, specifically a stronger Tarim basin, produces an asymmetric topography distribution that is more consistent with the observed Tibetan Plateau. [6] While the amount and general distribution of crustal thickening can be reconciled with the predicted crustal shortening from the collision, the shape of the Himalaya and Tibetan Plateau (Figure 1) is not well predicted by the thin viscous sheet models. Attempts to produce this asymmetry include consideration of the Tarim basin as a stronger region [England and Houseman, 1985; Houseman and England, 1996; Neil and Houseman, 1997] and the use of a lithostatic boundary condition on the eastern boundary of Eurasia, to allow the extrusion of Eurasia to the east, and oblique convergence between India and Eurasia [Houseman and England, 1993, 1996]. The results, based on constant velocity (magnitude and angle) across the convergent boundary, produce topographic asymmetry opposite in sense of the observed topography (Figure 1). [7] Houseman and England [1996] investigated the collision boundary between India and Eurasia showing that for normal convergence, use of a lithostatic eastern boundary for Eurasia produces the correct sense of asymmetry in the calculated topographic (crustal thickness) distribution. Adding a stronger Tarim basin further enhances the topographic asymmetry; however, by adding an oblique but constant convergence boundary condition (Figure 2c), the topographic asymmetry decreases. The model that best matches the observed topographic (crustal thickness) distribution includes a lithostatic eastern Eurasia boundary, a strong Tarim basin, and constant oblique convergence. [8] Our study differs from previous work in that we use an oblique and variable velocity boundary condition along the length of the indenter. This boundary condition not only has a velocity that is oblique to the boundary, but the obliquity angle and magnitude of the velocity boundary condition vary along the length of the boundary (Figure 2d). The variable velocity and angle of obliquity used in this study are obtained from the Le Pichon et al. [1992] review of motion of India since collision with Eurasia. Figure 2. (a) Boundary conditions and mesh size used in this study. These remain unchanged for all the models. (b) Boundary condition for the case of normal convergence. (c) Boundary condition for convergence with laterally constant velocity (including both magnitude and angle). (d) Boundary condition for convergence with laterally varying velocity (including both magnitude and angle).

3 SOOFI AND KING: OBLIQUE CONVERGENCE ETG 3-3 Table 1. Parameters in the Calculation of Elevation Parameter Value h w 2.5 km h b 5.0 km L 100 km r w 1.03 Mg/m 3 r b 2.8 Mg/m 3 r c 2.8 Mg/m 3 r m 3.33 Mg/m 3 [9] Our objective is to understand how much of the asymmetry in the topography along the India-Eurasia collision boundary is due to the oblique convergence. Our numerical investigation indicates that variable velocity (both magnitude and angle) produces the correct sense of asymmetry in the calculated topography. This is then enhanced by inclusion of the Tarim basin as a stronger zone. 2. Model [10] The assumptions in the thin viscous sheet (TVS) model have been discussed by England and McKenzie [1982], Houseman and England [1986], and Houseman and England [1996]. In the TVS approximation the lithosphere is treated as a continuous medium (i.e., role of faulting in lithospheric deformation is neglected) overlying an inviscid fluid (i.e., the asthenosphere is much weaker than the lithosphere). This implies that the top and bottom surfaces of the lithosphere are traction free. Further, the TVS approximation assumes that the lithosphere is isotropic and incompressible and that gradients of crustal thickness are small, so that horizontal gradients of vertical stresses can be neglected. We further assume that the lithosphere deforms by power law creep with no rheological layering and the crust always maintains Airy isostatic equilibrium. [11] We use the finite element program Basil [Houseman and England, 1986] which is capable of performing both plane stress (i.e., the thin viscous sheet formulation discussed above) and plane strain analyses. The program calculates velocity and stress distributions, crustal thicknesses, strain rates, and rotation. [12] In the TVS model, lithospheric deformation is a function of two parameters, the Argand number, a ratio of the buoyancy force to the plate boundary force, given by glr c 1 r c r m A r ¼ 1=n ; ð1þ B Uo L and n, the exponent in the stress-strain relationship, comes from t ij ¼ B _E ð1 n 1Þ _e ij ; ð2þ where g is gravity, L is the thickness of the lithosphere, r c is the crustal density, r m is the mantle density, U o is the imposed collision velocity, B is the vertical average of compositional and thermal influences on the lithospheric rheology, t ij and _e ij are the vertically averaged components of the deviatoric stress and strain rate tensor, Table 2. Rotation Poles for Postcollision India-Eurasia Relative Motion Age, Ma North Rotation Pole East Angular Velocity w, deg/myr Table 3. Normalized Linear Velocity Components Along Line AB in Figure 1 Latitude, deg respectively, and _E is the second invariant of the strain rate tensor. The strength of the lithosphere varies inversely with the Argand number. As the Argand number increases, the lithosphere is less able to support topographic loads and topography relaxes more quickly. An increase in n allows a more rapid decrease in the effective viscosity with increasing strain rate. [13] The effect of n and A r on the style of lithospheric deformation has been investigated by England and McKenzie [1982] and Houseman and England [1986]. As A r increases, the length scale of deformation increases, and the crustal thickness decreases because of the decreased ability of the lithosphere to support topography; however, A r has little influence on the length scale of deformation in a transcurrent environment [Sonder et al., 1986]. From equation (1), as n increases A r increases if all other parameters are fixed. From collision studies [e.g., Houseman and England, 1986; Sobouti and Arkani-Hamed, 1996] it has been shown that 3 n 5 and 1 A r 3 are reasonable values for studying lithospheric deformation. [14] To obtain the topography from our results, we assume isostatic equilibrium between a column of continental lithosphere and a reference column at the mid-ocean ridge. The elevation (e) (i.e., topography) of the continental crust is then given by [England and Houseman, 1986] e ¼ S r m r a Longitude, deg 1 r c r m Normal Component Tangential Component þ L 1 r m h w r a 1 r w r a Obliquity From Perpendicular to Line AB, deg + cw a a Positive angles clockwise. h b 1 r b ; r a ð3þ where h w is depth of ocean at the mid-ocean ridge, h b is thickness of the oceanic crust, L is thickness of the lithosphere, S is thickness of the continental crust, r m is density of the mantle, r b is density of the oceanic crust, r a is density of the asthenosphere, r c is density of the continental crust, and r w is the density of water. We assume 35 km as the precollision thickness of the Eurasian crust. The top surface of the crust is assumed to be at sea level, and the elevation corresponding to a given crustal thickness is then calculated from equation (3) using the values in Table 1. [15] For the relative motion of India with Eurasia we use three different rotation poles from three different time periods (Table 2). For each rotation pole the linear velocity is calculated at six different locations. The velocities obtained for each time period are then averaged over time, normalized and resolved into normal and tangential components with respect to the straight line drawn between the points 35 latitude, 73 longitude and 28 latitude, 96 longitude (i.e., points A and B, respectively, on Figure 1). These components are listed in Table 3. A least squares fit is then made to the normalized velocity components to determine the variation in the normal and tangential components of the linear velocities. The correlation coefficients for these fits are 0.99 and 0.96, respectively. This velocity profile is then use as the boundary condition in the finite element model. [16] The finite element models are run for the total convergence of km corresponding to 40 Myr of convergence at a rate of

4 ETG 3-4 SOOFI AND KING: OBLIQUE CONVERGENCE Normal Constant Velocity Variable Velocity (a) (b) (c) Figure 3. Calculated topography, in kilometers, for (a) normal convergence (b) constant oblique convergence, and (c) variable oblique convergence. The mesh and boundary conditions used are as given in Figure 2. The stress-strain exponent (n) is 3, and the Argand number (A r ) is 1. See text for the definition of n and A r. 50 km/myr. The horizontal dimension of the model, D, is 5000 km (Figure 2d), and elements are used. The boundary conditions for (1) variable velocity (including both magnitude and velocity), (2) constant velocity (including both magnitude and velocity) and (3) normal (nonoblique) convergence are shown in Figure 2. The indentation velocity is tapered on both the sides by a cos 2 function. In the initial experiments the velocity boundary conditions do not vary as a function of time. Experiments with time-varying velocity boundary conditions produce topographic distributions that are qualitatively similar to those shown below. 3. Results [17] The contours of topography for a model with normal convergence and A r = 3 and n = 1 are shown in Figure 3a. As Figure 5. Calculated asymmetry indices for the case of constant but oblique velocity corresponding to the boundary condition shown in Figure 3b. The asymmetry indices are shown corresponding to (a) 4-km and (b) 2-km topographic contours. Constant Velocity Variable Velocity RL (a) y x Figure 4. Profile coverage and location of the reference line used to determine the asymmetry index. All calculation use n = 3 and A r = 1. (a) Constant velocity (both magnitude and angle) and (b) variable velocity (both magnitude and angle). Profiles are drawn to cover the 2-km topographic contour. RL (b) y x expected, the deformation and resulting topography are symmetric about a line through the center of indentation. As the goal of this work is to explore the effect of oblique convergence on the asymmetry of topography along the India-Eurasia collision boundary, the normal convergence will not be discussed further. The resulting topography from calculations with a constant velocity (Figure 3b) and a variable velocity (Figure 3c) with the same parameters is shown for comparison. [18] To compare the results of the constant velocity boundary condition (i.e., Figure 3b) with the variable velocity boundary condition (i.e., Figure 3c), we define a parameter which we call the asymmetry index (AI). The asymmetry index is defined as the volume of a profile on one side of the reference line (RL) that equals or exceeds a minimum topography minus the volume of the profile on the other side of the reference line (RL) that equals or exceeds the same minimum topography. In both the calculations and the data we use 2-km elevation as the minimum topography. In Figure 4 we choose our coordinate systems such that x is parallel to RL and y is perpendicular to RL with y = 0 at RL. Thus a positive asymmetry index indicates a greater volume above 2-km elevation on the positive y side of RL than on the negative y side of RL. The asymmetry index is negative for the opposite distribution of topography about RL. The orientation of RL is obtained geometrically by calculating a line perpendicular to the endpoints of the boundary condition, and the location of RL along the boundary is chosen so

5 SOOFI AND KING: OBLIQUE CONVERGENCE ETG 3-5 as the cutoff for the asymmetry index, this case is nearly symmetric for all distances from the indenter. With the 4-km cutoff the asymmetry index profiles quantify what is apparent visually in the crustal thickness contour plot (Figure 3b), the region of high elevation is skewed to the left (west) of the center of the indenter. In addition, the profile shows that the skewed topography extends 700 km perpendicular from the indenter. [20] For the case of variable velocity (Figure 3c) the calculated asymmetry index profiles are shown in Figure 6. In this model the higher elevations are skewed to the right (east) of the reference line in contrast to the previous model. The extent of the anomalous topography is 700 km from the indenter, identical to the previous model. When considering the elevation within the 2-km contour, there is a transition between right (east) skewed and left (west) skewed topography at 400 km; however, the degree of asymmetry is smaller. [21] The relatively small difference in the boundary conditions, between a variable oblique indenter and a uniform oblique indenter where the angle of obliquity is the average value of the variable case, has a pronounced effect on the distribution of topography within the 4-km elevation contour. The asymmetry profile of the uniform oblique boundary condition is opposite to the distribution observed with the variable oblique boundary condition. [22] To decide whether either of the models discussed above better explain the asymmetry observed in the topography along the Figure 6. Calculated asymmetry indices for the case of variable oblique velocity corresponding to the boundary condition shown in Figure 3c. The asymmetry indices are shown corresponding to (a) 4-km and (b) 2-km topographic contours. that there is an equal volume of anomalous topography on either side of the line. The asymmetry index can be written mathematically as AIðyÞ ¼ Z yp RL topoðx; yþdx Z RL y n topoðx; yþdx; ð4þ where RL is the reference line (in this case, y = 0), y n is the coordinate where the profile crosses the 2-km contour in the negative y direction, and y p is the coordinate where the profile crosses the 2-km contour in the positive y direction. To obtain the asymmetry index, uniformly spaced, parallel profiles with a horizontal spacing of 0.02D, or 100 km in the x direction, are drawn across the region enclosed by the 2-km contour line (Figure 4). The integral in equation (4) is divided by a characteristic volume, which is one half the length of the indenter times the space between the profiles (100 km) times 4 km elevation. With this scaling, the asymmetry index ranges between ±1. For the case where the topography on the positive y side of RL was 4 and 0 km on the negative y side of RL, the asymmetry index would be 1. [19] The asymmetry index obtained for the constant velocity boundary condition (Figure 3b) is shown in Figure 5. The first observation that can be made is that when using the 2-km contour Figure 7. Calculated asymmetry indices for the topography along the India-Eurasia collision boundary using the ETOPO5 data set. (a) Asymmetry indices for the topography 4 km. (b) Asymmetry indices for the topography 2 km.

6 ETG 3-6 SOOFI AND KING: OBLIQUE CONVERGENCE the magnitude of the asymmetry index for the 2-km contour. Comparing the asymmetry index profiles for the observed India- Asia topography (Figure 7) and the strong Tarim basin model (Figure 8), the 2-km profiles have the same transition from leftskewed (west) to right-skewed (east) and back to left-skewed (west) asymmetry. This is most clear in the 2-km profile of the model. The 4-km profile from the model is not as clear; however, 4-km elevation is close to the maximum elevation attained anywhere in the model. In the 2-km profile of the model the transition between left-skewed (west) and right-skewed (east) topography occurs closer to the indenter than in the observed topography. Figure 8. Calculated asymmetry indices for the model with an embedded anomalous zone 10 times stronger than the surrounding material. The surrounding material has the rheology n = 3 and A r =1. (a) Asymmetry indices for the topography 4 km. (b) Asymmetry indices for the topography 2 km. India-Eurasia collision boundary, we also calculate the asymmetry index for the observed topography using the ETOPO5 data set regridded to 15 min. The distribution of the asymmetry indices for the 4-km and 2-km contours is shown in Figure 7. In the India- Asia topography, there are two obvious feature that are not reproduced by either model. First, there is a shift in both the 2-km and 4-km profiles from left (west) skewed to right (east skewed) asymmetry. This occurs at 500 km from the indenter in both cases. Second, the topography extended to distances >1500 km from the indenter. [23] Because the asymmetry indices obtained from our variable oblique boundary condition model (Figure 3c) are still not consistent with the topographic data, we investigate the effect of a stronger anomalous zone such as the Tarim basin [England and Houseman, 1985] on the development of the asymmetric topography. We use the same rheology for this case (i.e., n =3,A r = 1) as before. The anomalous zone is considered to be 10 times stronger than the surrounding material, and the boundary condition is identical to that used for Figure 3c (i.e., variable velocity). The calculated topographic asymmetry indices for the 4-km and 2-km contours are shown in Figure 8. [24] One noticeable difference between the topographic asymmetry obtained from the model with a strong Tarim basin (Figure 8) and that without an anomalous zone (Figure 6) is the difference in 4. Discussion [25] In our study of the collision of India with Eurasia the convergence rate and the obliquity angle remain constant for the whole period of convergence. Our boundary condition represents the average of the convergence rate and obliquity angle for the period of convergence from 0 to 45 Ma. This average is obtained from the kinematic history of India-Eurasia collision summarized by Le Pichon et al. [1992]. According to Le Pichon et al. [1992], convergence velocity between India and Eurasia since 49 Ma falls in the range km/myr, and our convergence velocities vary from 46 to 61 km/myr, consistent with this range. Also, in this study the obliquity angle (measured from the perpendicular to the line between points A and B in Figure 1) varies from 14.7 in the east (location B, Figure 1) to 32.8 in the west (location A, Figure 1). This distribution follows the same trend as that reported by Le Pichon et al. [1992] (i.e., 20 in the east and 30 in the west); however, according to them the obliquity angle has decreased considerably due to 20 clockwise rotation of the motion vectors at 7 Ma. [26] In our estimation of the linear velocities, geographical location of the boundary between India and Eurasia remains unchanged over the last 45 Myr. We have only considered the change in the location of the pole of rotation, as indicated by Le Pichon et al. [1992]. In the real world the geographical location of a plate boundary changes continuously. Thus our approach in estimating the average linear velocities along the India-Eurasia collision boundary over the last 49 Myr is a simplification of the collisional history along this boundary. Still, this approach represents an improvement over the constant oblique velocities or normal velocities that have been used in previous studies. [27] In the case of constant velocity (Figure 6) the topography has negative asymmetry indices, whereas topography along the India-Eurasia collision boundary has positive asymmetry indices near the indenter. This has led investigators to consider a lithostatic eastern boundary condition. For the case of variable velocity, however, the asymmetry indices near the indenter (Figure 7) are consistent with the observed topography. [28] The reason for the pronounced difference in the topographic distribution with boundary conditions can be explained by looking at the normal and tangential components of the velocity vector with respect to the boundary. When a constant obliquity and velocity are applied across the collision boundary, the normal and tangential components of velocity remain constant across the boundary. The normal component of velocity remains constant along the boundary, and it does not contribute to the asymmetric distribution of deformation along the collision boundary (as in Figure 4a). Thus the asymmetry is due entirely to the tangential component of velocity. Along the India-Eurasia collision boundary, where the average obliquity is 22 counterclockwise from the normal to the boundary (Table 3), the tangential component of the velocity remains constant, directed from east to west. This is the applied boundary condition in Figure 3b. More deforming material is piled to the west than to the east, producing an asymmetric distribution of topography that is opposite to the observed topography near the indenter.

7 SOOFI AND KING: OBLIQUE CONVERGENCE ETG 3-7 [29] In the variable velocity boundary condition case the convergence velocity decreases from east to west, while the obliquity increases is the same direction. This produces normal component of convergence that decreases from east to west, resulting in a greater accumulation of deforming material to the east than to the west. The asymmetry so obtained is consistent with the observed topography. The decreasing linear velocity from east to west makes the tangential component of velocity less effective in producing the asymmetry. [30] A comparison of Figures 5 and 6 indicates that use of variable velocity produces the correct sense of asymmetry in the topography near the indenter; however, through boundary conditions alone, we are not able to produce the transition in asymmetry observed in the actual topography. There is a pronounced transition in the skewness of the topographic asymmetry in the India- Eurasian collision (Figure 7) that we have been unable to reproduce with changes along the collisional boundary alone. The model that most closely resembles the observation has a mechanically strong zone representing the Tarim basin (Figure 8). This is consistent with the findings of England and Houseman [1985] and Houseman and England [1996]. [31] We did not find any improvement in the topography by considering the boundary between Eurasia and Pacific plates as a lithostatic boundary. In addition, we do not incorporate the effect of lateral movement along the strike-slip faults present near and farther north of the India-Eurasia collision boundary. The presence of right-lateral strike-slip faults north of this collision boundary could also contribute to the asymmetry in the observed topography of southern Eurasia. Peltzer and Tapponnier [1988] have used Landsat images and geological maps to decipher the geological history of eastern Eurasia. They suggested that right-lateral movement on the order of 1000 km along the Karakorum-Zangbo strikeslip fault moved central Tibet to the east with respect to India. If true, such movement would give rise to significant asymmetry in the topographic distribution. [32] There are several other assumptions in the thin viscous sheet model that could be important in the India-Asia collision problem. First, in our formulation the indenter (India) does not deform. This seems justified by the observation that the anomalous topography associated with the collision boundary is primarily on the Eurasian plate. Second, we have not included any effect from the underthrusting plate. It is possible that shear traction from the underthrusting Indian plate is responsible for deformation north of the collision boundary. Especially if the underthrusting part of the Indian plate is continental crust which should be strongly coupled to the Eurasian plate because of its relative buoyancy. Finally, the thin viscous sheet approximation assumes a simple relation between topography and crustal thickness (equation (3)). If lithospheric thinning has occurred in part of the region [England and Houseman, 1989; Neil and Houseman, 1997], then the relationship between topography and crustal thickness is not as simple as we have assumed. 5. Conclusions [33] In this study we have investigated the factors responsible for the topographic asymmetry along the collision boundary between India and Eurasia. In the process, we have developed a technique to quantify the asymmetry present in the topographic data set and in the results of our models. The technique is applicable to a digital data set once the orientation of the reference line is defined. The estimated parameter, which we call the asymmetry index (AI), is positive when more area with topography equal or greater than a specified topography is to the east (right) of the reference line and is negative when more area with such topography is to the west (left) of the reference line. The technique enables us to quantify the asymmetry in the topography as a function of distance from the collision boundary. [34] In this study we observe that the laterally varying obliquity angle and convergence rate between India and Eurasia are responsible for some of the asymmetry in the topography along this collision boundary. This boundary condition more realistically captures the tectonic history along the India-Asia collision boundary, although it is insufficient to explain the asymmetric distribution of elevation with respect to the collisional boundary. The topographic asymmetry is significantly enhanced when a heterogeneous block (i.e., Tarim basin) is considered reinforcing the conclusion that the presence of nonhomogeneous blocks (e.g., Tarim and Qaidam basins) is most likely responsible for the majority of the asymmetry in the India-Eurasia collision zone. [35] Acknowledgments. We wish to thank G. A. Houseman, L. J. Sonder, and an anonymous reviewer for constructive comments on this and an earlier version of this manuscript. M. Soofi was supported by a David Ross Fellowship at Purdue University. References Beck, M. E., On the mechanism of tectonic transport in zones of oblique subduction, Tectonophysics, 93, 1 11, Beck, M. E., Model for late Mesozoic Early Tertiary tectonics of coastal California and western Mexico and speculations on the origin of the San Andreas fault, Tectonics, 5, 49 64, Beck, M. E., Coastwise transport reconsidered: Lateral displacement in oblique subduction zone, and tectonic consequences, Phys. 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