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Arc segmentation and landscape evolution in the Bhutan Himalaya: Insights from integrated thermochronometry, cosmogenic radionuclide and geomorphometry studies
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  • Matthew Peter Wood,
  • Phuntsho Pelgay,
  • Mike Sandiford,
  • Barry Kohn,
  • Alexandru Codilean,
  • Guangwei Li
Matthew Peter Wood

Corresponding Author:matthewpeterwood@gmail.com

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Phuntsho Pelgay
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Mike Sandiford
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Barry Kohn
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Alexandru Codilean
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Guangwei Li
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The principal structural elements of the Himalayan arc can be traced more or less continuously for nearly 2500 km. It is therefore understandable that along-strike variations in structure and denudation have not received the same attention as equivalent arc-normal trends. However, it is now clear that arc segmentation can be controlled by lateral variations in the geometry of the Main Himalayan Thrust (MHT). The Bhutan Himalaya has a distinctive physiography and hosts nominal modern seismicity despite experiencing long-term strain accommodation comparable to the wider arc. This enigmatic section of the orogen presents an opportunity to test the case for local arc segmentation through applied tectonic geomorphology. By integrating low temperature thermochronology, cosmogenic radionuclide methods and quantitative geomorphometry, this study documents the spatial and temporal variability of denudation to infer partitioning of deformation across crustal structures. Laboratory methods include apatite fission track (AFT), and (U-Th)/He dating of zircon from in situ bedrock, synorogenic sediments and modern detrital samples. Additionally, 10Be concentrations from detrital quartz samples adds to a nation-wide compilation of previously published data. Results suggest prominent along- and across-strike variation in deformation within Bhutan. High normalised channel steepness and hillslope characterise a prominent east-west trending zone of elevated millennial-scale erosion rates. Here termed the Naka Zone, this geomorphic region is coincident with the estimated rupture extent of an early 18th Century great earthquake and terminates to the east in the vicinity of the Kuru Chu reentrant in the Main Central Thrust (MCT). Greater Himalaya basement rocks west of the Sakteng Klippe show a phase of rapid, monotonic cooling, the timing of which is largely latitude-dependent, consistent with exhumation above a mid-crustal ramp on the MHT ~100 km from the front followed by horizontal translation above the AFT partial annealing zone. This framework explains the decoupling of AFT ages in sampled catchments from millennial-scale erosion rates. Small catchments that straddle the MCT show bimodal distributions in single grain AFT ages, suggestive of activity on the MCT during the late Miocene. Further, central ages show a marked decrease towards Arunachel Pradesh, suggesting that in far eastern Bhutan the mid-crustal ramp extends towards the foreland, possibly invoking a lateral ramp. Synorogenic detrital thermochronometers are unreset and thus provide information on source area bedrock cooling and provenance. ZHe and AFT age distributions in the Siwaliks are bimodal. Comparisons with large modern drainage systems links a young age peak (Mio-Pliocene) to the Greater Himalaya and a dominant older age peak (Mid Miocene) to the Lesser Himalaya and points to persistent elevated topography in the range front east of Kuru Chu.