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Asymmetric Exhumation of the Mount Everest Region: Implications for the Tectono-Topographic Evolution of the Himalaya

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Asymmetric Exhumation of the Mount Everest Region: Implications for the Tectono-Topographic Evolution of the Himalaya Asymmetric exhumation of the Mount Everest region: Implications for the tectono-topographic evolution of the Himalaya B. Carrapa1, X. Robert2,3, P.G. DeCelles1, D.A. Orme1,4, S.N. Thomson1, and L.M. Schoenbohm5 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 2Institut des Sciences de la Terre, Université Joseph Fourier, CNRS, F-38041 Grenoble Cedex 9, France 3Université Grenoble Alpes, CNRS, IRD, IFSTTAR, ISTerre, F-38000 Grenoble Cedex 9, France 4Department of Geological Sciences, Stanford University, Stanford, California 94305, USA 5Department of Earth Sciences, University of Toronto, Toronto, Ontario M5S 3B1, Canada ABSTRACT (e.g., Harris and Massey, 1994), mostly between The tectonic and topographic history of the Himalaya-Tibet orogenic system remains ca. 22 and 12 Ma (e.g., Cottle et al., 2015, and controversial, with several competing models that predict different exhumation histories. references therein). Here, we present new low-temperature thermochronological data from the Mount Everest region, which, combined with thermal-kinematic landscape evolution modeling, indicate CLIMATE ACROSS MOUNT EVEREST asymmetric exhumation of Mount Everest consistent with a scenario in which the southern The Himalaya constitutes an orographic edge of the Tibetan Plateau was located >100 km farther south during the mid-Miocene. barrier to northward movement of southerly Northward plateau retreat was caused by erosional incision during the Pliocene. Our results air masses; the result is a sharp gradient in suggest that the South Tibetan Detachment was a localized structure and that no coupling precipitation, with rainfall of 1–3 m/yr south between precipitation and erosion is required for Miocene exhumation of Greater Himalayan of the divide and <0.5 m north of the divide Sequence rocks on Mount Everest. (Bookhagen and Burbank, 2006). Mount Ever- est is flanked on the south and east by the steep INTRODUCTION and apatite (U-Th)/He (AHe) thermochronologi- Khumbu and Kangshung Glaciers, and to the Mount Everest is the culmination of the High cal data, which, when combined with existing north by the less-steep East Rongbuk and West Himalayan topographic crest, which forms the data, can test these models. We couple this with Rongbuk Glaciers (Fig. 2A). The Khumbu and boundary between the arid Tibetan Plateau and numerical modeling to constrain the north-to- Kangshung Glacier headwalls truncate the West the more rugged and humid southern side of the south tectono-topographic and exhumation his- Rongbuk and East Rongbuk Glacier headwalls, Himalaya (Fig. 1). Although much work has been tory across the highest part of the Himalaya. respectively (Fig. 2A), indicating the importance done on the middle to late Cenozoic tectonic his- of headward glacial erosion south of the divide tory of Greater Himalaya Sequence (GHS) mid- GEOLOGICAL SETTING OF MOUNT (Scherler et al., 2011). Advances of the Rongbuk crustal rocks in the Mount Everest region (e.g., EVEREST and Khumbu Glaciers were broadly synchronous Murphy and Harrison, 1999; Cottle et al., 2015), The Himalayan thrust belt is composed of and correspond to times of Holocene strength- the timing and magnitude of exhumation of these upper-crustal rocks that have been thrust south- ened monsoon precipitation (Owen et al., 2009). rocks remain largely unresolved. Also, the devel- ward since early Cenozoic time. The northern- opment of Himalayan topography is uncertain: most major thrust fault in the range is the Main THERMOCHRONOLOGICAL RESULTS Does the modern topography represent the cul- Central Thrust (MCT), which places amphibo- FROM MOUNT EVEREST mination of southward growth of Tibet, or north- lite-grade GHS rocks on top of Lesser Himala- We collected samples from the Rongbuk and ward erosional retreat of the plateau? yan low-grade metasedimentary rocks (Fig. 1). Gyachung Chhu Rivers draining the northern The exhumation history of GHS rocks with The most significant structure in the Mount flank of Mount Everest and neighboring sum- respect to the South Tibetan Detachment System Everest region is the northward-dipping STDS, mits (Fig. 2) and 12 samples of GHS gneiss (STDS) is important for testing tectonic models which separates rocks of the Tethyan Himala- and leucogranite in the footwall of the STDS for the Himalaya. The STDS has been inter- yan sequence (THS) above from the GHS below along the eastern wall of Rongbuk Gorge (Fig. preted as a gravity-driven normal fault (Burg et (Figs. 1B and 1C; Burchfiel et al., 1992; Burg 2) for AFT, AHe, and white mica 40Ar/39Ar ther- al., 1984; Pêcher, 1991), the northern boundary et al., 1984). The THS consists of Paleozoic– mochronology (see the GSA Data Repository1). of an extruded wedge of GHS rocks (Burchfiel et Mesozoic sedimentary and low-grade metasedi- The detrital data set provides the first catchment- al., 1992), the top of a midcrustal ductile channel mentary rocks that were incorporated into the wide exhumation record for the Mount Everest (Beaumont et al., 2004; Jamieson et al., 2004), Himalayan orogenic wedge during Eocene region. Seven of the bedrock samples produced and a passive roof thrust above a southward- and Oligocene time (Aikman et al., 2008) and AFT ages, and one sample was analyzed for verging wedge of GHS rocks (Yin, 2006). These buried GHS protoliths to depths sufficient for AHe thermochronology (Table DR1 in the GSA models make different predictions for the exhu- Barrovian metamorphism. The GHS was sub- Data Repository). The combination of dated mation history of GHS rocks. sequently exhumed and thrust southward on top minerals constrains the cooling history of these Thermochronological data are sparse and of Lesser Himalayan rocks along the MCT dur- mostly available for the southern side of the ing early Miocene time (e.g., Hodges, 2000). 1 GSA Data Repository item 2016198, analyti- High Himalaya (e.g., Thiede and Ehlers, 2013, Cenozoic leucogranites are common in GHS cal information and data tables, is available online at and references therein). Here, we present new rocks and formed by decompression anatexis www.geosociety.org/pubs/ft2016.htm, or on request white mica 40Ar/39Ar, apatite fission-track (AFT), of metapelites and gneisses during STDS slip from [email protected]. GEOLOGY, August 2016; v. 44; no. 8; p. 1–4 | Data Repository item 2016198 | doi:10.1130/G37756.1 | Published online XX Month 2016 GEOLOGY© 2016 Geological | Volume Society 44 | ofNumber America. 8 For| www.gsapubs.org permission to copy, contact [email protected]. 1 A A India-Asia collision zone Figure 1. A: Simplified digital elevation model Great Counter Thrust 29°N of Asia and geological map of central Hima- Tibet North Himalayan domes laya and southern Tibet, modified from Yin (2006); inset shows broader geographic con- Tethyan Himalaya India text. B: Schematic north-south cross section of central Himalayan thrust belt, modified from study area TIBET Murphy (2007). C: Topographic profile across Shisha Pangma Mount Everest with locations of samples from Mt. Everest this study, Streule et al. (2012), and Sakai et STDS Makalu 28° al. (2005). MCT—Main Central Thrust; MBT— Main Boundary Thrust; STDS—South Tibetan Detachment System; Him—Himalaya; Fm.— Lesser HimalayaNEPAL Greater Himalaya Formation; GHS—Greater Himalaya Sequence. 5 km 4.1 INDIA rocks through the ~350–60 °C temperature win- 3.2 dow (Reiners and Brandon, 2006). 2.3 MCT 27° A’ AFT ages of bedrock samples range between 1.4 0 100km MBT samples by Streule et al. (2012) 15.6 ± 2.8 Ma and 12.7 ± 1.5 Ma and are within 85° 86° 87° 88° 89°E error of each other, with a mean age of 14.8 ± India-Asia 3.2 Ma (Fig. 2C; Table DR1), indicating rapid B A' collision zoneGreat Counter A cooling during the middle Miocene. For some Thrust Main Central Thrust Xigaze S Tethyan Himalaya Forearc Kailas Fm. N samples, these ages may represent a maximum Gangdese Arc 2C Liuqiu Fm. due to low track densities/uranium concentra- Lesser Him. Sequence subduction complex Asian plate tions. The few AHe ages are between ca. 16 and Indian plate ~25 km Greater Himalayan Lhasa terrane 0 ~100 km Sequence ca. 3 Ma and show no correlations with grain size or eU (Table DR2; cf. the Data Repository). STDS C Mt. Everest The Rongbuk River detrital sample shows a dis- S N Apatite Fission-Track ages (Ma) 40 39 STDS tribution of white mica Ar/ Ar ages with a ca. hangin gwall 0-2 2-4 16 Ma peak, and AFT ages characterized by a STDS footwall 8 ) Estimated eroded material 4-6 single population at ca. 15 Ma; AFT ages from 6 6-8 8-10 10-12 the Gyachung Chhu River show a detrital popu- this study 4 GHS 12-14 40 39 Elevation (m 14-16 lation at ca. 14 Ma (Fig. 2B). The Ar/ Ar and 2 16-18 18-20 AFT cooling ages of bedrock and sand samples 20-30 0 indicate that the STDS footwall north of Mount 50 0 Figure 2. A: Shaded digi- 86°45'0"E 87°0'0"E tal elevation model of A B Mount Everest region. N Gyachung Chhu River 8,848 m Rongbuk and Gyachung (Everest 7-9-14 2PK ) Chhu River watersheds 28°15'0" are indicated in green and STDS 4,400 m 15.1 ± 1.2 red, respectively; princi- Rongbuk pal glaciers are indicated 13.8 ± 1 by blue fields; truncated headwalls are indicated Rongbuk River Kernel density function watershed by small white arrows; 0 10 20 30 40 50 60 70 80 90 100 trace of South Tibetan Rongbuk River Ages (Ma) Detachment System Gyachung Chhu River, AFT (n=48) Rongbuk River sand, AFT (n=104) (STDS) is after Murphy Rongbuk River sand, 40Ar/39Ar (n=112) and Harrison (1999) and GHS rocks, S of Everest, AFT (Streule et al., 2012) 40 39 AFT ages from GHS rocks, N of Everest (this study) Searle (2003).
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