Rapid exhumation of the eastern Himalayan syntaxis since the late Miocene

Abstract

The Himalayan syntaxes are exceptionally dynamic landscapes characterized by high-relief topography and some of the most rapid and focused crustal exhumation on Earth. In the eastern Himalayan syntaxis, it has been hypothesized that thermo-mechanical feedbacks between erosion by the Yarlung River and growth of a crustal-scale antiform may have locally sustained exhumation rates exceeding 5 km/m.y. during the late Pliocene and Pleistocene. However, young (younger than 3 Ma) cooling histories from syntaxial bedrock samples restrict interpretations of the timing and mechanism initiating feedback development. To extend this record of landscape evolution, we reconstructed an exhumation history since the late Miocene from analysis of detrital minerals in Himalayan foreland basin deposits. We combined magnetostratigraphy, detrital white mica 40Ar/39Ar thermochronology, and coupled zircon U-Pb and fission-track geothermochronology from a 4.6-km-thick stratigraphic section proximal to the eastern syntaxis. We used a simple thermal model to interpret the combined provenance and lag-time data set, concluding that rock exhumation rates in the core of the syntaxis increased by a factor of 5–10 in the late Miocene and have sustained extremely rapid exhumation rates (>5 km/m.y.) since 5 Ma. This onset significantly postdates the first appearance of Tibetan detritus in the Himalayan foreland, suggesting that thermo-mechanical feedbacks sustaining rapid exhumation are unrelated to river integration. Instead, such feedbacks may develop where large, antecedent rivers sustain elevated erosion rates across a region of enhanced rock uplift. Compilation of similar data sets across the Himalaya demonstrates extraordinary syntaxial exhumation histories, potentially resulting from peculiar geodynamics at these orogenic margins.