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Implications of Shortening in the Himalayan Fold-Thrust Belt for Uplift of the Tibetan Plateau

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Implications of Shortening in the Himalayan Fold-Thrust Belt for Uplift of the Tibetan Plateau TECTONICS, VOL. 21, NO. 6, 1062, doi:10.1029/2001TC001322, 2002 Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau Peter G. DeCelles, Delores M. Robinson, and George Zandt Department of Geosciences, University of Arizona, Tucson, Arizona, USA Received 20 August 2001; revised 13 September 2002; accepted 18 September 2002; published 31 December 2002. [1] Recent research in the Himalayan fold-thrust belt Plateau. The need for Tibetan crust to stretch laterally provides two new sets of observations that are crucial as the Greater Indian lower crust was inserted may to understanding the evolution of the Himalayan- explain the widespread east-west extension in the Tibetan orogenic system. First, U-Pb zircon ages and southern half of the Plateau. Our reconstruction of the Sm-Nd isotopic studies demonstrate that (1) Greater Himalayan fold-thrust belt suggests that Indian cratonic Himalayan medium- to high-grade metasedimentary lower crust, of presumed mafic composition and high rocks are much younger than true Indian cratonic strength, should extend approximately halfway across basement; and (2) these rocks were tectonically the Tibetan Plateau, to the Banggong suture. From mobilized and consolidated with the northern margin there northward, we predict that the Tibetan Plateau is of Gondwana during early Paleozoic orogenic activity. underlain by more felsic, and therefore weaker, lower These observations require that Greater Himalayan crust of Greater Himalayan affinity. Two slab break-off rocks be treated as supracrustal material in restorations events are predicted by the model: the first involved of the Himalayan fold-thrust belt, rather than as Indian Neotethyan oceanic lithosphere that foundered 45– cratonic basement. In turn, this implies the existence 35 Ma, and the second consisted of Greater Indian of Greater Himalayan lower crust that is not exposed lithosphere (most likely composed of Greater anywhere in the fold-thrust belt. Second, a regional Himalayan material) that delaminated and foundered compilation of shortening estimates along the 20–10 Ma. Asthenospheric upwelling associated Himalayan arc from Pakistan to Sikkim reveals that with the break-off events may explain patterns of (1) total minimum shortening in the fold-thrust belt is Cenozoic volcanism on the Tibetan Plateau. Although up to 670 km; (2) total shortening is greatest in the model predicts a northward migrating topographic western Nepal and northern India, near the apex of the front due solely to insertion of Greater Indian lower Himalayan salient; and (3) the amount of Himalayan crust, the actual uplift history of the Plateau was shortening is equal to the present width of the Tibetan complicated by early-middle Tertiary shortening of Plateau measured in an arc-normal direction north of Tibetan crust. INDEX TERMS: 8102 Tectonophysics: the Indus-Yalu suture zone. This information suggests Continental contractional orogenic belts; 8110 Tectonophysics: that a slab of Greater Indian lower crust (composed of Continental tectonics—general (0905); 8120 Tectonophysics: both Indian cratonic lower crust and Greater Dynamics of lithosphere and mantle—general; 9320 Information Himalayan lower crust) with a north-south length of Related to Geographic Region: Asia; 9604 Information Related to 700 km may have been inserted beneath the Tibetan Geologic Time: Cenozoic; KEYWORDS: collisional orogenic belts, crust during the Cenozoic orogeny. We present a Tibetan Plateau, Himalaya, lithosphere dynamics, delamination, modified version of the crustal underthrusting model thrust belts. Citation: DeCelles, P. G., D. M. Robinson, and G. Zandt, Implications of shortening in the Himalayan fold-thrust belt for Himalayan-Tibetan orogenesis that integrates for uplift of the Tibetan Plateau, Tectonics, 21(6), 1062, surface geological data, recent results of mantle doi:10.1029/2001TC001322, 2002. tomographic studies, and broadband seismic studies of the crust and upper mantle beneath the Tibetan Plateau. Previous studies have shown that incremental Mesozoic and early Cenozoic shortening had probably 1. Introduction thickened Tibetan crust to 45–55 km before the [2] The origin of high-elevation orogenic plateaus is a onset of the main Cenozoic orogenic event. Thus, the topic of considerable interest, given the potential climato- insertion of a slab of Greater Indian lower crust with logical, geochemical, and environmental side effects of maximum thickness of 20 km (tapering northward) plateau growth and maintenance. Explanations of orogenic could explain the Cenozoic uplift of the Tibetan plateaus have been best refined for the case of the Tibetan Plateau, which has been studied intensively since the mid- 1970’s [e.g., Molnar and Tapponnier, 1975]. The Hima- Copyright 2002 by the American Geophysical Union. layan fold-thrust belt, which forms the southern rim of the 0278-7407/02/2001TC001322$12.00 Plateau (Figure 1), is obviously a result of shortening of 12 - 1 12 - 2 DECELLES ET AL.: HIMALAYAN-TIBETAN OROGENY Figure 1. Regional tectonic map of the Tibetan Plateau, showing major suture zones, terranes, fault systems, Cenozoic magmatic centers, and the Gangdese magmatic arc [after Yin and Harrison, 2000; Hacker et al., 2000; Tapponnier et al., 2001]. Elevations after Fielding et al. [1994]. Indian rocks to the south of the suture zone that marks the butions of Greater Indian crust to thickening of the Plateau. paleosubduction zone between the Indian and Eurasian Greater India is regarded as the landmass of the Indian plates [Gansser, 1964]. However, no consensus exists on subcontinent before the onset of the Indo-Eurasian collision the timing and mechanism(s) of formation of the Tibetan [Veevers et al., 1975]. The north-south length of Greater Plateau and the nature of its relationship to the Himalaya Indian lower crust available to thicken the Tibetan Plateau [e.g., Dewey et al., 1988; Harrison et al., 1992; Molnar et should be equivalent to the amount of supracrustal shortening al., 1993; Matte et al., 1997; Yin and Harrison, 2000; in the Himalayan fold-thrust belt [Klootwijk et al., 1985]. Tapponnier et al., 2001]. Moreover, the kinematic history of the fold-thrust belt should [3] Our corporate understanding of the development of the provide a gauge of the timing of addition of Greater Indian Tibetan Plateau has developed to its current state largely crustal material to the Tibetan Plateau. These simple concepts unaided by a thorough consideration of the potential contri- have been difficult to exploit because estimates of shortening DECELLES ET AL.: HIMALAYAN-TIBETAN OROGENY 12 - 3 based on geological data from the Himalayan fold-thrust belt contributed to growth of the Plateau, are the main subjects of are sparse and vary by a factor of roughly two. Instead, the this paper. principal constraints on amounts of Himalayan shortening have come from paleomagnetic data, which are inherently 2. Models for Tibetan Plateau Uplift subject to large uncertainties and provide only the coarsest kinematic information [Molnar and Tapponnier, 1975; Ach- [5] Models for the origin of the Tibetan Plateau begin with ache et al., 1984; Patriat and Achache, 1984; Klootwijk et the following boundary conditions: (1) approximately 2500 al., 1985; Besse et al., 1984; Besse and Courtillot, 1988, km of post-collision convergence has taken place between 1991; Patzeltetal., 1996]. In addition, the high-grade India and Eurasia [Molnar and Tapponnier, 1975; Patriat metamorphic rocks of the Greater Himalayan sequence that and Achache, 1984; Besse and Courtillot, 1988, 1991; crop out in the medial part of the fold-thrust belt historically Patzelt et al., 1996]; (2) the Plateau is underlain by con- have presented a puzzle in terms of how to restore cross tinental crust that is approximately twice as thick as normal sections through the Himalaya: Should these rocks be treated crust [Hirn, 1988; Owens and Zandt, 1997; Zhu, 1998; Kind as Indian cratonic basement [e.g., Gansser, 1964; Mattauer, et al., 2002]; and (3) seismic phase velocities in the upper 1986; Srivastava and Mitra, 1994; Hauck et al., 1998], or mantle (at 100–300 km depth) are generally fast compared to should they be treated as an exotic tectonostratigraphic adjacent regions, and Pn wave velocity and Sn propagation terrane that was structurally elevated prior to the Cenozoic are relatively slow and inefficient, respectively, beneath the orogenic event [e.g., Parrish and Hodges, 1996; DeCelles et northern half of the plateau but relatively fast and efficient al., 2000]? Adding to the dilemma is the uncertainty of the beneath the southern half [Ni and Barazangi, 1984; Molnar, structure in the mantle and lithosphere beneath the Tibetan 1988; Holt and Wallace, 1990; McNamara et al., 1997; Plateau. However, recent progress on the Greater Himalayan Owens and Zandt, 1997; Tapponnier et al., 2001]. issue, as well as recent deep crustal seismic reflection profil- [6] Five general categories of models for the Tibetan ing [Nelson et al., 1996; Hauck et al., 1998] and broadband Plateau have been proposed: (1) crustal thickening by pure seismic experiments across the Tibetan Plateau [Owens and shear during the Mesozoic and Cenozoic [Murphy et al., Zandt, 1997; Kosarev et al., 1999; Kind et al., 2002], and 1997] or entirely during the Cenozoic [England and House- mantle tomographic studies [Grand et al., 1997; Van der Voo man, 1988; Molnar et al., 1993]; (2) crustal underthrusting, et al., 1999] provide an opportunity
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