GPS Measurements from the Ladakh Himalaya, India: Preliminary Tests of Plate-Like Or Continuous Deformation in Tibet

GPS measurements from the Ladakh Himalaya, India: Preliminary tests of plate-like or continuous deformation in Tibet Sridevi Jade CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore 5650037, India B.C. Bhatt Indian Institute of Astrophysics, Bangalore 560034, India Z. Yang Department of Surveying Engineering, Chang’an University, Xi’an 710054, People’s Republic of China R. Bendick† Department of Geological Sciences, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boul- der, Colorado 80309-0399, USA and COMET, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK V. K . Gaur CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore 5650037, India and Indian Institute of Astrophysics, Bangalore 560034, India P. Molnar Department of Geological Sciences, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA M.B. Anand Dilip Kumar CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore 5650037, India ABSTRACT argue for plate-like behavior of the Tibetan large-scale continental tectonics have prevailed. Plateau. Convergence between Ladakh and In one, continental tectonics results from the rela- Observations of relative motion in a geo- the Indian subcontinent at 18.8 ± 3 mm/yr at tive movements of a small number of essentially detic network in Ladakh, India, and across 224° ± 17° (1σ) differs little from estimates of rigid plates of mantle lithosphere, with deep southern Tibet indicate slow shear on the convergence across the central segment of the structure blurred by deformation of the overlying Karakorum fault, rapid east-west extension Himalaya. Finally, lengthening of the baseline thick crust (e.g., Tapponnier et al., 2001); in the across the whole of southern Tibet, and con- between Leh, Ladakh, and Lhasa (in south- other, continental lithosphere undergoes con- stant arc-normal convergence between India eastern Tibet) at 17.8 ± 1 mm/yr or between tinuous deformation (e.g., England and Jackson, and southern Tibet along the Himalayan arc. Leh and Bayi (farther to the southeast) at 18 1989; Molnar, 1988). Asia has provided both the Measurements of ten campaign-style and ± 3 mm/yr, is consistent with an extrapolation source of data for developing these views and the six permanent sites with global positioning of rates of east-west extension of the Tibetan testing ground exploited by supporters of both system (GPS) precise geodesy provide these Plateau based both on shorter GPS baselines views. Among the geological interpretations that bounds on the style and rates of the large-scale (e.g., Lhasa-Simikot) and on diverging slip separate proponents of each view, perhaps none deformation in the Tibet-Himalaya region. vectors of earthquakes in the Himalaya. We is more controversial than the role of strike-slip Divergence between sites at Leh, Ladakh, interpret these results to indicate that Tibet faults. Proponents of plate-like deformation India, and Shiquanhe, western Tibet, as well behaves more like a fl uid than like a plate. argue that slip on major strike-slip faults, at least as slow relative motion among sites within in Asia, occurs rapidly and that these faults con- the Ladakh network, limit right-lateral slip Keywords: geodesy, geodynamics, Tibetan trol the deformation fi eld, such as “the growth of parallel to the Karakorum fault to only 3.4 ± Plateau, Himalaya, Karakorum, continuum the Tibetan highlands” (Tapponnier et al., 2001, 5 mm/yr. This low rate concurs with a recent mechanics. p. 1671). Proponents of continuous deformation, estimate of 3–4 mm/yr for Late Holocene however, consider slip rates on the strike-slip time, but disagrees with the much higher INTRODUCTION faults to differ little from those on normal and rate of 30–35 mm/yr that has been used to thrust faults, and that all faults constitute passive Following the recognition of plate tectonics in markers of discontinuities in the strain fi eld that †Corresponding author e-mail: [email protected]. largely oceanic domains, two extreme views of translate and rotate with a deforming continuum. GSA Bulletin; November/December 2004; v. 116; no. 11/12; p. 1385–1391; doi: 10.1130/B25357.1; 4 fi gures; 2 tables. For permission to copy, contact [email protected] © 2004 Geological Society of America 1385 JADE et al. Of course, where two extremes prevail, show that deformation should be localized at the along strike do not alone preclude a single pole there is always a middle ground; at some level edges of strong inclusions within a deforming for Tibet-India relative velocity, with additional the deformation fi eld seen at the surface of all continuum (e.g., England and Houseman, 1985; constraint on kinematics applied by the geom- continental regions can be described in terms of Neil and Houseman, 1997). etry of the Himalayan arc (Avouac and Tappon- relative movements of blocks of crust. The dis- The other dominant strike-slip fault in the nier, 1993). parity is primarily a function of spatial scale. At Tibetan Plateau is the Karakorum fault, which Concurrent with the underthrusting of India one extreme, blocks may be so numerous (and crosses the western end of Tibet with a south- beneath the Himalaya, southern Tibet deforms therefore small) that their relative movements westerly strike. Prominent linear valleys, clearly by active normal faulting (e.g., Armijo et al., are best treated as resistant inclusions within a recognizable on satellite imagery, follow the 1986; Molnar and Tapponnier, 1978; Ni and continuous medium; at the other, blocks may be fault trace (e.g., Liu, 1993; Molnar and Tap- York, 1978), so that the ESE-WNW dimen- so few (and therefore large) that a description ponnier, 1978). Fresh scarps mark offsets of sion of the region extends. Although a rate of in terms of rigid blocks (plates) provides more alluvial and debris-fl ow fans that are easily seen extension was initially inferred from the varia- insight into the deformation fi eld than con- on the ground and from high-resolution satellite tion in slip vectors of earthquakes along the tinuous deformation. A logical boundary on the imagery (e.g., Brown et al., 2002; Liu, 1993; Himalaya and the rate of convergence across sizes of blocks might be 100 km, for to remain Liu et al., 1992). Peive et al. (1964) inferred the Himalaya (Molnar and Chen, 1982, 1983; strong, smaller ones would almost surely not hundreds of kilometers of right-lateral slip on Molnar and Lyon-Caen, 1989), corroboration include mantle lithosphere. Conversely, blocks this fault, and Peltzer and Tapponnier (1988) both from estimates of rates of slip on normal substantially larger than 100 km should include suggested 650 km of such slip, though Searle faults (Armijo et al., 1986) and from GPS mea- upper mantle. Thus, in the context of testing (1996) has argued for only 120 km. Although surements (Bilham et al., 1997; Jouanne et al., whether continental tectonics is plate-like or the high terrain (~5000 m) both east and west 1999; Larson et al., 1999; Wang et al., 2001) has fl uid-like, let us lump large-block models in of the fault requires Cenozoic crustal thicken- been limited to a fraction of the relevant zone. with the former and small-block models with ing, active faulting is sparse, if present at all, GPS measurements from both the western and the latter. The mechanical properties of crustal on the west side of the fault. Thus, in one view, eastern extremes of the Himalaya test whether blocks may also depend on the time over which this fault would be the boundary of plate-like extension persists throughout the arc and place a deformation is observed. We make the assump- Tibetan lithosphere, and in the other it would tight constraint on the total rate. tion that geodetic velocities are indicative of the mark a shear zone within a deformable region Motivated by these questions concerning the regional velocity over much longer times than if deformation has ceased in parts of the region. rate and direction of movement of the western the time span of observation. Inferred slip rates range from 30 to 35 mm/yr end of the Himalaya with respect to western Perhaps the best testing ground for plate-like (Avouac and Tapponnier, 1993; Liu, 1993; Liu Tibet east of the Karakorum fault, to India, versus fl uid-like behavior is Tibet itself. Major et al., 1992; Tapponnier et al., 2001), to 11 ± and to eastern Tibet, we installed a network of strike-slip faults surround much of the region: 4 mm/yr (Banerjee and Bürgmann, 2002), to GPS control points in Ladakh, India, which lies the Altyn Tagh fault in the north and the Kara- ≤9 mm/yr (Molnar and Lyon-Caen, 1989), to north of the Himalaya and near the western end korum fault in the west of Tibet. The area is ≤8 mm/yr (Searle et al., 1998), and fi nally to of the Tibetan Plateau. A second small array of large enough that if it behaved as a single plate only 3–4 mm/yr (Brown et al., 2002). GPS measurements near the eastern syntaxis (e.g., Avouac and Tapponnier, 1993), slip on the GPS measurements from the western and east- of the Himalaya, east of Lhasa, extends direct 1000-km-scale strike-slip faults should be much ern ends of the Himalaya also allow us to address geodetic observations across the entire extent more rapid that on the network of faults that other aspects of Asian tectonics relevant to the of the southern Tibetan Plateau (Fig. 1) when surrounds blocks of ~100–300 km in dimension plate versus fl uid controversy. For instance, we combined with observations reported in the within Tibet.

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