Geodetically Constraining Indian Plate motion and Implications for Plate Boundary Deformation

Edwin (Trey) Apel, Roland Bürgmann, and Paramesh Banerjee (Wadia Institute of Himalayan Geology)

Introduction

Traditionally tectonic Indian plate motion has been estimated using closed plate circuit models and summing motions across mid-ocean ridges. More recently Paul et al., (2001), using limited GPS velocity vectors, calculated a pole of rotation that suggested motion slower than that of the rates suggested by sea-floor spreading. Socquet et al., (2006) estimate an India-Eurasia geodetic pole with rates $\sim $5 mm/yr slower than Paul et al., (2001). We present new data spanning a larger and more significant portion of the "stable" Indian plate than previous studies. Our preliminary India-Eurasian pole estimates are consistent with Socquet et al., (2006). In addition to the above mentioned sites, we also include published data from numerous sources. However, many of the sites in this region are on or near plate boundaries (eg. along the Himalayan front or above the Sumatra subduction zone) making it difficult to use them for plate pole parameter determination. We use a block modeling approach to incorporate both rigid block rotation and near-boundary elastic strain accumulation effects in a formal inversion of the GPS velocities. Considered models include scenarios with and without independent microplates and a number of different plate boundary locations and locking depths.

GPS Velocities

The GPS velocities used in our inversion come from a solution of 164 global stations, including some unpublished campaign style sites from central and northwestern India. These data were processed using GAMIT/GLOBK and processed by Paramesh Banerjee from the Wadia Institute of Himalayan Geology. Processing details can be found in (Banerjee and Bürgmann, 2002). In addition to our own analysis, we integrate GPS-station velocities from published work along the Himalayans, throughout China, and southeast Asia (Bettinelli et al., 2006; Bock et al., 2003;Calais et al., 2003; Shen et al., 2005; Socquet et al., 2006; Zhang et al., 2004). We integrated these velocities into the reference frame of our own solutions by estimating translation and/or rotation parameters that minimized the differences in horizontal velocities for common sites. Our combined solution contains $\sim $1800 global velocities.

Plates

We define our plates as rigid blocks on a spherical earth bounded by dislocations in an elastic halfspace and invert for poles and rates of rotation that minimize the misfit to the GPS velocities using an extension of the block modeling code by Meade and Hager (2005). The segments that bound the blocks represent uniformly slipping elastic dislocations locked to some specified depth. Because our inversion combines rigid block rotation with elastic strain accumulation effects, the parameterization of the block boundary geometry is critical. Geometry of the block boundaries is based heavily on seismicity and adopted from prior analyses (eg. Socquet et al., 2006, Reilinger et al., 2006, Meade, IN PRESS) or adjusted as indicated by the geodetic data. We invert the horizontal GPS velocities for poles of rotation constrained by the prescribed block geometry defined above. Systematic patterns in the residual velocities (observed minus predicted) are used as an indicator of where and how the model matches the observed surface velocities. Misfit statistics are used to formally evaluate the statistical significance of the plate kinematic scenarios we test.

Results and Discussion

Our preliminary results constrain a pole of rotation for India with respect to Eurasia to lie at 29.04$^{\circ }$$\pm$0.7$^{\circ }$ N 16.74$^{\circ }$$\pm$1.6$^{\circ }$ E with a counterclockwise angular velocity of 0.42$^{\circ }$$\pm$0.01$^{\circ }$ Myr$^{-1}$. This pole confirms that the NUVEL-1A rate is 20% too fast and the India-Eurasia convergence rates computed along the Himalaya front range from 34 mm/yr to 45 mm/yr between 72$^{\circ }$ N and 96$^{\circ }$ N longitude. Residual velocities from our inversion are shown in figure 18.1 and show near zero motion within uncertainties. The geographic distribution of stations within the Indian plate allows us to generate the most robust realization of Indian plate motion to date.

Figure 18.1: Residual velocities on the Indian plate from our robust inversion. Dashed lines show small circle pole traces from the NUVEL-1A IND-EUR pole of rotation, dotted lines are pole traces from this study. Circles show locations of GPS sites along the Himalayan range front however, velocity vectors at these locations have not been shown for the sake of brevity.
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References

Banerjee, P., and R. Bürgmann, Convergence across the northwest Himalaya from GPS measurements, Geophysical Research Letters, 10.1029/2002GL015184, 2002. Bettinelli, P. J.Avouac, M. Flouzat, F. Jouanne, L. Bollinger, P. Willis, G. Chitrakar, Plate motion of India and interseismic strain in the Nepal Himalaya from GPS and DORIS measurements Journal of Geodynamics, 10.1007/s00190-006-0030-3, 2006. Bock, Y., L. Prawirodirdjo, J. F. Genrich, C. W. Stevens, R. McCaffrey, C. Subarya, S. S. O. Puntodewo, and E. Calais, Crustal motion in Indonesia from Global Positioning System measurements, Journal of Geophysical Research, 10.1029/2001JB000324, 2003. Calais, E., M. Vergnolle, V. San'kov, A. Lukhnev, A. Miroshnitchenko, S. Amarjargal, and J. Deverchere, GPS measurements of crustal deformation in the Baikal-Mongolia area (1994-2002): Implications for current kinematics of Asia, Journal of Geophysical Research, 108(B10), 2003. Meade, Brendan, Mechanics of the India-Asia Collision Zone, Geophysical Research Letters, IN PRESS. Meade, B. J., and B. H. Hager, Block models of crustal motion in southern California constrained by GPS measurements, Journal of Geophysical Research, 110, B03403, 10.1029/2004JB003209, 2005. Paul, J., Bürgmann, R., Gaur, V. K., Bilham, R. Larson, K. M., Ananda, M. B., Jade, S., Mukal, M., Anupama, T. S.. Satyal, G., Kumar, D., The motion and active deformation of India, Geophysical Research Letters 28(4), 2001. Shen, Z.-K., J. Lü, M. Wang, and R. Bürgmann, Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau, Journal of Geophysical Research, 10.1029/2004JB003421, 2005. Socquet, A., Vigny, C., Chamot-Rooke, N., Simons, W., Rangin, C. and Ambrosius, B., India and Sunda plates motion and deformation along their boundary in Myanmar determined by GPS, Journal of Geophysical Research, 111, B05406, 10.1029/2005JB003877, 2006. Zhang, P. Z., Z. Shen, M. Wang, W. J. Gan, R. Bürgmann, and P. Molnar, Continuous deformation of the Tibetan plateau from global positioning system data, Geology, 32(9), p. 809, 2004.

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