Coupling of Rock Uplift and River Incision in the Namche Barwa–Gyala Peri Massif, Tibet

Coupling of rock uplift and river incision in the Namche Barwa–Gyala Peri massif, Tibet Noah J. Finnegan†* Bernard Hallet David R. Montgomery Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, P.O. Box 351310, Seattle, Washington 98195, USA Peter K. Zeitler Department of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, Pennsylvania 18015, USA John O. Stone Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, P.O. Box 351310, Seattle, Washington 98195, USA Alison M. Anders Department of Geology, University of Illinois at Urbana-Champaign, 245 Natural History Building, 1301 West Green Street, Urbana, Illinois 61801, USA Liu Yuping Chengdu Institute of Geology and Mineral Resources, 2 Beisanduan, Yihuanlu, Chengdu 610081, Sichuan Province, China ABSTRACT of local erosion, rock uplift, thermal weak- rates of tectonic uplift is also documented in ening of the lithosphere, and deformation: numerical models (Whipple and Tucker, 1999; Geodynamic modeling demonstrates the (1) Whereas along the rest of the Himalayan Anders, 2005). Such work has led to wide strong potential for erosion to infl uence the front, high relief and high rock uplift rates acknowledgment of the idea that rates of rock pattern and style of deformation in active are essentially continuous, the high relief and uplift with respect to the geoid and rates of sur- mountain belts, but fi eld studies yield con- rapid exhumation in the syntaxis is restricted face erosion should be driven toward a dynamic fl icting views on the importance of erosion to a “bull’s-eye” pattern exactly where the balance in actively uplifting ranges. Coupling in infl uencing orogenesis. Here we compare largest river in the Himalaya, the Yarlung between rock uplift and surface erosion, in turn, patterns in river power, inferred excess fl u- Tsangpo–Brahmaputra, has the most energy has important implications for geodynamics. vial-transport capacity, topographic relief, per unit area available to erode its channel This is because patterns in topography and cli- precipitation, and mineral-cooling ages to and transport sediment. (2) The location mate, to the extent that they reveal erosion rate assess the coupling between surface erosion of rapid incision on the Yarlung Tsangpo– patterns, can provide constraints on spatial gra- and rock uplift within the vicinity of the Brahmaputra has been pinned for at least dients in rock uplift rates, which commonly lack Namche Barwa–Gyala Peri massif, an active 1 m.y., and without compensatory uplift of clear surface expression in heavily dissected antiformal structure within the eastern the Namche Barwa–Gyala Peri massif dur- and hard to access terrain (e.g., Seeber and Gor- Himalayan syntaxis. Our rich and dense data ing this time the river would have eroded nitz, 1983; Finlayson et al., 2002; Wobus et al., set reveals a tight spatial correspondence headward rapidly, incising deeply into Tibet. 2003; Kirby et al., 2003; Wobus et al., 2006). of fl uvial incision potential, high relief, and Additionally, recent models demonstrate that young cooling ages. The spatial coincidence is Keywords: river incision, rock uplift, climate, the width and height of active mountain ranges most easily explained by a sustained balance tectonics and landscape evolution, eastern Hima- may be controlled by rates of erosion (Whipple between rock uplift and denudation driven layan syntaxis, Namche Barwa. and Meade, 2004; Stolar et al., 2006; Roe et by river incision over at least the last ~1 m.y. al., 2006; Willett et al., 2006). Finally, coupled The Yarlung Tsangpo–Brahmaputra River INTRODUCTION surface and thermo-mechanical models show is the largest and most powerful river in the that focused erosion, even on the comparatively Himalaya, and two lines of evidence point The potential for spatial patterns in erosion to local scale of a large river valley system, can to its active role in the dynamic interaction infl uence the location of deformation in active have dramatic consequences for the deforma- mountain belts is well established in numerical tion of crustal lithosphere, leading to localized models (e.g., Willett, 1999; Beaumont et al., feedbacks between erosion, deformation, and †E-mail: [email protected] *Present address: Department of Earth and At- 2001; Koons et al., 2002). Additionally, the ten- rock uplift (Koons et al., 2002). mospheric Sciences, Cornell University, Snee Hall, dency for rivers to grow steeper, convey more The latter scenario, dubbed a tectonic aneu- Ithaca, New York 14853-1504, USA water, and become more erosive with higher rysm, arises from the dynamic interactions of GSA Bulletin; January/February 2008; v. 120; 1/2; p. 142–155; doi: 10.1130/B26224.1; 12 fi gures; Data Repository item 2008002. 142 For permission to copy, contact [email protected] © 2007 Geological Society of America Namche Barwa Uplift Incision localized erosion, topographic stresses, rock The second and more general goal of this by a spectacular arcuate defl ection around the uplift, thermal weakening of the lithosphere, and paper is to provide additional insight into how indenting Indian plate corner of geologic units, deformation (Koons et al., 2002). The Nanga sustained high erosion rates are expressed topo- structural fabric, topography, and global posi- Parbat–Haramoosh massif in Pakistan exhibits graphically and geomorphologically in active tioning system (GPS)–derived plate velocity the interconnected geomorphic, geophysical, mountain belts. Despite considerable attention, vectors (Tapponnier et al., 1982; Royden et al., petrologic, and geochemical evidence that ini- there is still little consensus on which topo- 1997; Hallet and Molnar, 2001; Zhang et al., tially inspired the aneurysm model (Zeitler et graphic metrics are likely to refl ect adjustment 2004; Sol et al., 2007). al., 1993, 2001a; Meltzer et al., 1998; Park and to long-term rates of rock uplift. The Namche Embedded within the syntaxis is an active Mackie, 2000): a large, powerful, and rapidly Barwa–Gyala Peri massif and its surround- antiformal metamorphic structure, the Namche incising river, the Indus, cuts a deep gorge adja- ings are ideally suited as a natural laboratory Barwa–Gyala Peri massif, which is composed cent to an isolated, high-relief massif marked by to elucidate the coupling between erosion and of Precambrian Himalayan basement that has extremely rapid cooling, and an upwardly bowed rock uplift in active mountain belts, because recently been undergoing active deformation and Moho. Nanga Parbat’s eastern counterpart, the the study area is characterized by large and rapid unroofi ng, partially coeval with an anatec- Namche Barwa–Gyala Peri massif, punctuates now well-constrained gradients in exhuma- tic episode under way for the past 10 m.y. (Burg the eastern terminus of the Himalaya Arc. The tion, precipitation, fl uvial power, fl uvial-sedi- et al., 1997; Booth et al., 2004; Malloy, 2004; major east-fl owing orogen-parallel river, the ment transport capacity, and topographic relief. Zeitler et al., 2006). Field mapping and thermo- Yarlung Tsangpo–Brahmaputra, wraps around Thus, the likelihood of discerning a signal in chronometry indicate that the northern tip of the the Himalayan arc here, cutting a spectacular both exhumation and topography that is above syntaxis is cross-cut by a major north-dipping gorge into the easternmost high Himalaya. Field inherent noise levels is arguably higher here crustal-scale shear zone and fault, the Nam-La observations, analysis of coarse-scale digital- than elsewhere in the Himalaya. thrust zone (Ding et al., 2001) (Fig. 1A). The elevation data, and mineral-cooling age data Finally, because the extent to which patterns Nam-La thrust bounds to the south an anti- suggest that the superposition of the 5-km-deep in precipitation alone control patterns in exhu- formal crustal pop-up (Burg et al., 1997) that Yarlung Tsangpo gorge and the rapidly cooled mation remains vigorously debated (Reiners et contains the two highest peaks for several hun- and deeply incised Namche Barwa–Gyala Peri al., 2003; Burbank et al., 2003; Wobus et al., dreds of kilometers along the Himalaya: the massif is consistent with local coupling between 2003; Thiede et al., 2005), our third goal is to 7782 m Namche Barwa and the 7294 m Gyala erosion and crustal deformation in this region examine patterns in precipitation independently Peri. Cooling ages drop to the north across this (Burg et al., 1998; Zeitler et al., 2001b; Koons of patterns in river incision potential to assess inferred structure, indicating that the Namche et al., 2002). However, until recently, neither the their spatial relationship to apparent exhuma- Barwa–Gyala Peri antiform has been recently geomorphology nor the thermal history of this tion rate gradients. This is possible within the and rapidly exhumed relative to the surrounding region has been characterized with suffi cient study area because, in contrast to the majority of terrain (Burg et al., 1997; Malloy, 2004; Zeitler detail to confi rm such coupling with any satis- other Himalayan rivers with little source area on at al., 2006). The antiform itself is complexly faction, and hence to begin to address the unique the Tibetan Plateau, the Yarlung Tsangpo–Brah- folded but shows fabrics and fold axes consis- geodynamics of this region. maputra River conveys a huge volume of water tent with N-S to NW-SE compression (Burg et The fi rst of the three goals

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