Orogen-parallel extension and exhumation enhanced by denudation ...

Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal Micah J. Jessup* Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA

Dennis L. Newell Hydrology, Geochemistry and Geology Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

John M. Cottle Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK

Aaron L. Berger James A. Spotila Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA

ABSTRACT Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal flow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW–striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100–300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogenparallel extension that was enhanced by, or potentially coupled with, denudation in the transHimalayan Arun River gorge. This model provides important insights into the mechanisms by which orogen-parallel extension, which has dominated the southern margin of the Tibetan Plateau since the middle Miocene, has exhumed trans-Himalayan antiformal structures. Keywords: exhumation, mid-crustal flow, extension, denudation, Himalaya, Tibetan Plateau.

INTRODUCTION In the context of the Himalayan orogen, channel flow (Beaumont et al., 2001, 2004) and tectonic aneurism models (Zeitler et al., 2001) described the coupling between the flow of midcrustal rocks, exhumation, and denudation along the orogenic front. Tectonic aneurism models are applied to the eastern (Namche Barwa) and western (Nanga Parbat) syntaxes, where exhumation rates of mid-crustal rocks are extreme and metamorphic massifs coincide with major river systems (Fig. 1). In these models, focused fluvial incision of deep gorges is instrumental in exhumation of warm and weak mid-crustal rocks, driven by the local stress field to extrude upward as a bivergent wedge (Butler and Prior, 1988; Zeitler et al., 1993, 2001; Burg et al., 1998; Schneider et al., 2001; Koons et al., 2002). Channel flow models propose southward extrusion of a lowviscosity mid-crustal channel beneath the Tibetan Plateau (Nelson et al., 1996), driven by horizon*E-mail: [email protected].

tal gradients in lithostatic pressure between the Tibetan Plateau and lowlands of the Indian plate and coupled with focused denudation along the range front (Grujic et al., 1996, 2002; Beaumont et al., 2001, 2004; Searle et al., 2006). The spatial coincidence between focused precipitation and exhumation (Thiede et al., 2004, 2005; Bookhagen et al., 2005b, 2005a, 2006; Bookhagen and Burbank, 2006) as well as elevated erosion index and young metamorphic massifs along the Himalaya (Finlayson et al., 2002; Montgomery and Stolar, 2006) have been used as evidence for a fine-scale coupling between mid-crustal flow and denudation. The trans-Himalayan Arun River flows along the western limb of the Ama Drime Massif and through the Yo Ri gorge, before it enters the main Arun River gorge, where it passes into the eroded core of the Ama Drime Massif and continues into the foothills of Nepal (Wager, 1937; Brookfield, 1998) (Figs. 1 and 21). Mont1

Figure 2 is a separate insert.

gomery and Stolar (2006) proposed that exhumation of the eroded core of the antiformal structure might be partially accommodated by the youngest generation of faulting; however, without detailed structural data, they were unable to make the final link between the geomorphic features and specific structures and/or faults. Our field-based structural data and (U-Th)/He apatite ages provide a new context in which to view the evolution of this region of the Himalaya. PREVIOUS INTERPRETATIONS OF THE AMA DRIME MASSIF Wager (1937, p. 246) used the relationship between the drainage network and gorges to demonstrate that the Ama Drime Massif was a “localized region of special uplift.” Subsequent investigations of the Ama Drime Massif focused on the eastern (Burchfiel et al., 1992) and western margins (Lombardo et al., 1998; Lombardo and Rolfo, 2000; Visonà and Lombardo, 2002) of the range and resulted in two different interpretations. Burchfiel et al. (1992) documented

© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY, 2008 Geology, JulyJuly 2008; v. 36; no. 7; p. 587–590; doi: 10.1130/G24722A.1; 2 figures; 1 insert; Data Repository item 2008138.

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Figure 1. Simplified interpretive block diagram of the Everest region, Tibet-Nepal. Location of Main Central thrust is based on Hubbard (1989). Location of South Tibetan detachment (STD) system is constrained by field-based data and interpretation of Landsat 7 images. Location of North Himalayan gneiss domes is approximate and based on Landsat 7 image interpretation. Inset map shows location of Namche Barwa, Ama Drime, Gurla Mandhata, and Nanga Parbat. ADD—Ama Drime detachment; NRD—Nyönno Ri detachment; MBT— Main Boundary thrust.

that on the eastern limb the pervasive foliation within the core of the Ama Drime Massif is deflected into a north-south–striking shear zone and brittle fault that dips 45°E and contains a mylonite zone in the footwall rocks with a welldeveloped S-C fabric that records top-down-tothe-east sense of shear (inset A, Fig. 2). Triangular facets, 1 km high, define the fault trace, fault scarps offset Quaternary deposits, and the fault offsets the South Tibetan detachment system by 20 km of right-lateral separation (Burchfiel et al., 1992) (Fig. 1 and Fig. 2, inset B). This same fault system appears to be related to the western margin of the Xainza-Dinggye rift located to the east of the Ama Drime Massif (Zhang and Guo, 2007) (Figs. 1 and 2). The 40Ar/39Ar muscovite, biotite, and K-feldspar ages from the hanging wall (ca. 19–13 Ma) are older than muscovite and biotite ages from the footwall of this fault system (ca. 13–10 Ma) (Hodges et al., 1994; Zhang and Guo, 2007). The western margin of the Ama Drime Massif is defined by an ~100–300-m-thick shear zone and fault system in leucogranite, calc-silicate, quartzite, and marble. Hanging-wall rocks are composed of migmatitic orthogneiss of the Greater Himalayan Series, whereas footwall rocks include granulite facies, migmatitic Ama Drime orthogneiss with mafic lenses cored by fresh eclogite pods (Lombardo et al., 1998; Lombardo and Rolfo, 2000; Groppo et al., 2007)

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(Fig. 2). This shear zone was originally interpreted as the Main Central thrust zone (Lombardo et al., 1998). The U(-Th-)Pb ages suggest that the ductile fabric within the shear zone developed ca. 12–13 Ma (Liu et al., 2007). The Sm/Nd model ages of leucogranites jump from an average of 2.0 Ga in the hanging wall to 2.9 Ga in the footwall of the western limb of the Ama Drime Massif (Visonà et al., 2000). Visonà and Lombardo (2002) suggested that the model ages are similar to paragneiss from the base of the Greater Himalayan Series (2.0 Ga) and orthogneiss of the Lesser Himalayan sequence (2.5 Ga) in Nepal (Robinson et al., 2001). Subsequent investigations have either projected the Main Central thrust zone along the western side (Borghi et al., 2003) of the Ama Drime Massif, or around the entire Ama Drime Massif (Lombardo and Rolfo, 2000; Groppo et al., 2007; Liu et al., 2007). STRUCTURAL EVOLUTION AND EXHUMATION OF THE AMA DRIME MASSIF New structural data from the western limb (between locations D and C, Fig. 2) demonstrate that the main shear zone fabric generally strikes NNE-SSW and dips 4°–66°W with an approximate downdip stretching lineation (stereonet in Fig. 2). In some areas (location A, Fig. 2), rocks within the shear zone record a complex deformation history that culminated in the development of

the prominent NNE-SSW–striking shear fabric. The earliest phase of deformation is recorded by a stretching lineation, defined by elongate quartz on quartz-rich surfaces [L1 average; trend (310°) and plunge (10°)]. L1 is folded by a set of tight to open folds [F2 representative; trend (250°) and plunge (20°)] with an axial surface [S2 representative; strike (090°) and dip (46°S)]. The pervasive mylonitic shear fabric [S3 average; strike (168°) and dip (19°W)] and stretching lineation [L3 average; trend (281°) and plunge (18°)] variably overprint the earlier history. S3 is broadly warped by a final set of transport parallel folds. In the structurally lower part of the shear zone, feldspar augen are ductilely deformed (Fig. 2, insets A and C). With decreasing structural depths, the feldspars begin to behave as semirigid clasts and shear bands develop along the margins that record top-down-to-the-west sense of shear (Fig. 2, inset C). These are overprinted by foliation-parallel mylonite to ultramylonite zones that record deformation that occurred at lower temperatures. Brittle faults [strike (201°) and dip (45°NW)], filled with fault gouge, are oriented subparallel to the shear fabric (Fig. 2, inset D). Structural mapping along two east-west drainages across the Ama Drime Massif defines the central section of an elongate north-plunging structural dome (Figs. 1 and 2). Mafic lenses are parallel to the main foliation, frequently boudinaged, and contain cores of

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DISCUSSIONS AND CONCLUSIONS The major range-bounding faults and shear zones of the Ama Drime Massif strike approximately NNE-SSW, dip away from the core of the range, and record opposite sense of shear. We term the western limb the Ama Drime detachment and the eastern limb the Nyönno Ri detachment, and emphasize that additional structural data are needed to establish how these two structures merge toward the north (Figs. 1–3). These detachments offset the South Tibetan detachment system and overprint most evidence for early deformation, except for at least one location along the western limb and some locations in the core of the Ama Drime Massif. Within the Ama Drime detachment, pos2 GSA Data Repository item 2008138, discussion of results, description of analytical methods, and Table DR1, is available online at www.geosociety. org/pubs/ft2008.htm, or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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sibly due to strain partitioning, metasedimentary rocks record early polyphase folding that predates the main shear zone fabric. The possibility of an early fault history is also supported by Sm/Nd model ages that suggest that the core of the Ama Drime Massif might be part of the Lesser Himalayan sequence. However, we recognize that multiple interpretations for these data exist and are hesitant to rely on this interpretation without more extensive structural and isotopic data. These results imply that reactivation and/or offsetting of preexisting structural weaknesses or unconformities might have played a significant role in accommodating the transition from crustal thickening to orogen-parallel extension. This interpretation differs from the Gurla Mandhata core complex, where the Gurla MandhataHumla extensional shear zone offsets the South Tibetan detachment system and Main Central thrust zone (Murphy et al., 2002; Murphy and Copeland, 2005; Murphy, 2007) (Fig. 1). Consistency in kinematics between high- to low-grade deformation within the Ama Drime detachment and Nyönno Ri detachment suggest that ductile mid-crustal flow was dominated by orogen-parallel extension that prevailed as the rocks were exhumed through the brittleductile transition. The 40Ar/39Ar ages from the Nyönno Ri detachment suggest that exhumation of the hanging wall, presumably related to movement on the South Tibetan detachment system, ended by ca. 13 Ma, at which point the locus of exhumation migrated into the footwall block that accommodated exhumation later than 13 Ma. The early stages of this transition from south-directed to orogen-parallel mid-crustal flow potentially involved movement on the South Tibetan detachment system, Ama Drime detachment, and Nyönno Ri detachment. New (U-Th)/He apatite ages define a minimum exhumation rate of ~1 mm/yr between 1.5 and 3.0 Ma and confirm that the Ama Drime Massif is a locus of rapid, recent exhumation set within a more slowly eroding zone. Quaternary deposits that are offset by apparent normal sense displacement along the Nyönno Ri detachment brittle fault suggest that orogen-parallel extension remains active today. Crustal extension in core complex–like scenarios associated with upwelling warm midcrust flow is predicted to result in the most rapid exhumation rates of any of the major domeforming mechanisms (Whittington, 2004). Far from the potential effects of focused denudation in the interior of the Tibetan Plateau, other rift systems such as the Yadong-Gulu and Lunggar are bounded by low-angle detachment faults with mylonite zones and Miocene granites in the footwall (Kapp et al., 2008). Among others, the Xainza-Dinggye rift extends southward to the margin of the Tibetan Plateau, where it coincides with the Ama Drime Massif. Our results suggest that exhumation of the Ama Drime Massif is

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pristine eclogite. The U(-Th-)Pb geochronology demonstrates that granulite facies metamorphism (750 °C and 8–10 kbar), injection of leucogranite dikes, and migmatization occurred ca. 12 Ma, suggesting that rocks in the core of the Ama Drime Massif were capable of ductile flow at that time (Cottle, 2007). The eastern limb of the range, as exposed near Demon’s Lake (Figs. 1 and 2), is defined by a 300-m-wide, NNE-SSW–striking shear zone within leucogranite and orthogneiss. Shear bands that formed around semirigid feldspar augen (Fig. 2, inset C) as well as S-C fabrics in mylonite and ultramylonite zones that developed subparallel to the shear zone fabric [representative strike (011°) and dip (42°E) approximate downdip stretching lineation] all record topdown-to-the-east sense of shear (Fig. 2, inset A). Fault scarps are subparallel to 1-km-tall triangular facets and offset recent lacustrine, fluvial, and colluvial deposits (Fig. 2, inset B). The (U-Th)/He apatite ages from the two transects across the Ama Drime Massif range from 1.44 to 4.79 Ma (Fig. 3), excluding two ages from the eastern limb that appear to be too old based on regional 40Ar/39Ar data. An age versus elevation plot for five samples from the western limb yields a minimum exhumation rate of ~1 mm/yr (R2 = 0.95) between 1.5 and 3.0 Ma (Fig. 3). The (U-Th)/He apatite ages (4.16–4.79 Ma) from the three highest samples suggest that exhumation prior to ca. 2–3 Ma might have been slower (dashed line, Fig. 3). One (U-Th)/He apatite age (3.29 Ma) from the western limb, which reproduced poorly, is too old to fit this trend (see the GSA Data Repository2). We speculate that this age is anomalously old, due to parent nuclide zonation or U- and Th-bearing microinclusions, but more data are required to test this hypothesis.

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Figure 3. Age versus elevation plot for (U-Th)/He apatite ages from Ama Drime Massif. Linear regression through five of six samples from west side of range yields exhumation rate of ~1 mm/yr (R2 = 0.95). Dashed line connects most robust age from east ridge area with west side data. X-axis error bars are 2σ standard deviation; Y-axis error bars are ±100 m.

related to a combination of focused denudation and orogen-scale tectonic instabilities. We propose that the Ama Drime Massif records orogenparallel extension that has dominated the southern margin of the Tibetan Plateau since the middle Miocene and that exhumation on the southern end of the range is enhanced by focused denudation in the trans-Himalayan Arun River gorge. ACKNOWLEDGMENTS We thank R. Parrish, M. Searle, R. Law, J. Fletcher, and K. Karlstrom for insightful discussions and Sonam Wangdu for helping us traverse the Ama Drime Massif. Reviews from B. Bookhagen, J. Lee, M. Murphy, P. Kapp, D. Grujic, and two anonymous reviewers helped strengthen previous versions of this manuscript. The program Stereonet 6.3.2 was used to plot the stereonet in Figure 2. Expeditions were funded by American Alpine Club, Sigma Xi, 2010 Fellowship from the College of Science, Virginia Tech to Jessup, a National Science Foundation Integrative Graduate Education and Research Traineeship (DGE-9972810), and University of New Mexico research grants to Newell and a New Zealand Tertiary Education Commission Doctoral scholarship to Cottle. REFERENCES CITED Beaumont, C., Jamieson, R.A., Nguyen, M.H., and Lee, B., 2001, Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation: Nature, v. 414, p. 738–742, doi: 10.1038/414738a. Beaumont, C., Jamieson, R.A., Nguyen, M.H., and Medvedev, S., 2004, Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan-Tibetan orogen: Journal of Geophysical Research, v. 109, B06406, doi: 10.1029/2003JB002809. Bookhagen, B., and Burbank, D.W., 2006, Topography, relief, and TRMM-derived rainfall variations along the Himalaya: Geophysical Research Letters, v. 33, p. 5, doi: 10.1029/ 2006GL026037. Bookhagen, B., Thiede, R.C., and Strecker, M.R., 2005a, Abnormal monsoon years and their control on erosion and sediment flux in the high, and northwest Himalaya: Earth and Planetary Science Letters, v. 231, p. 131–146, doi: 10.1016/j.epsl.2004.11.014.

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