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Climatic forcing of erosion, landscape, and tectonics in the Bhutan Himalayas

¹ Department of Earth Sciences, Dalhousie University, Halifax B3H 4J1, Canada
² Université de Lille I, UMR 8110, 59655 Villeneuve d'Ascq Cedex, France
³ Institute for Crustal Studies, University of California–Santa Barbara, Santa Barbara, California 93106, USA
⁴ Université de Rennes 1, Géosciences Rennes, 35042 Rennes Cedex, France
⁵ Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA
⁶ Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0–1.8 mm/yr) than inside of it in the east (0.55–0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region's recent tectonic evolution.

Key Words: tectonics • climate • erosion • paleolandscape • Himalaya • apatite fission-track

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