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 thermomechanical feedbacks between erosion by the Yarlung River and growth of a crustalscale 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 40 Ar/ 39 Ar 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 thermomechanical 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.

Timing and mechanism of the rise of the Shillong Plateau in the Himalayan foreland

“…Transhimalayan detritus in dated Siwalik sediments of the eastern Himalaya shows that the Yarlung and Brahmaputra were connected by 13 Ma; connection through the Siang River ( Fig. 1) was established by at least 7 Ma Chirouze et al, 2013;Cina et al, 2009;Govin, 2017;Lang and Huntington, 2014;Lang et al, 2016).…”

<sup>10</sup>Be systematics in the Tsangpo-Brahmaputra catchment: the cosmogenic nuclide legacy of the eastern Himalayan syntaxis

“…Other detrital proxies such as thermochronological ages of sediment grains could be affected similarly, i.e., the young cooling ages found in the NBGPm (e.g., Zeitler et al, 2014) could be transferred to the typically studied sand fraction of the sediment load further downstream only after coarse landslide material is abraded. The population of zircon fission track (FT) ages (Enkelmann et al, 2011) and 39 Ar / 40 Ar ages (Lang et al, 2016) was measured in three Tsangpo-Brahmaputra river sediment samples from downstream of the NBGPm to the Himalayan front (Fig. 8).…”

Late Miocene-Pleistocene evolution of India-Eurasia convergence partitioning between the Bhutan Himalaya and the Shillong Plateau: New evidences from foreland basin deposits along the Dungsam Chu section, eastern Bhutan

“…Sedimentary archives of the Neogene‐Quaternary Siwalik Group preserved in the Himalayan foreland basin have been extensively documented along the Himalayan arc from Pakistan to Nepal, providing valuable information on mountain building in space and time, past organization of drainage networks, and paleoclimate [e.g., Najman , , and references therein]. Recent studies have also focused on sections of the Siwalik Group in eastern India, in the states of West Bengal [ More et al ., ] and Arunachal Pradesh [ Chirouze et al ., , ; Lang et al ., ]; however, there is still a lack of information in the frontal Bhutan Himalaya, leaving about 400 km along the strike of the orogen of undocumented foreland basin deposits regarding ages, thicknesses, sedimentary facies, and depositional paleoenvironments.…”