TY - JOUR

T1 - Downstream evolution of the thermochronologic age signal in the Brahmaputra catchment (eastern Himalaya)

T2 - implications for the detrital record of erosion

AU - Gemignani, Lorenzo

AU - van der Beek, Peter

AU - Braun, Jean

AU - Najman, Yanina Manya Rachel

AU - Bernet, Matthias

AU - Garzanti, Eduardo

AU - Wijbrans, J.

PY - 2018/10/1

Y1 - 2018/10/1

N2 - The Namche Barwa massif in the eastern Himalayan syntaxis is characterized by very rapid exhumation and provides a significant proportion of the sediment flux carried by the Brahmaputra River. We present new detrital zircon fission-track (ZFT) and muscovite 40Ar/39Ar (MAr) data from modern sediments of rivers draining the eastern Himalaya. The cooling-age populations for both thermochronometers contain a characteristic <2 Ma signature related to the rapid exhumation of Namche Barwa, which can be traced hundreds of kilometers downstream from their source into the Brahmaputra foreland, despite dilution from downstream tributary catchments. To estimate present-day erosion in the catchments, we apply a mixing model based on linear inversion of the binned age distributions. The inversion predicts relative erosion rates in the syntaxial region that are an order of magnitude higher than those in upstream catchments, and about twice as high as those in the southern Himalayan catchments, consistent to first order with previous estimates of erosion-rate patterns in the eastern Himalaya. A comparison of the observed downstream evolution of age distributions with a sediment-flux model suggests that the ZFT signal can be explained by dilution from Himalayan tributaries only, whereas the MAr signal is also affected by selective sequestering (possibly through winnowing) of micas as they are transported downstream. Nevertheless, thermochronological ages <2 Ma provide a diagnostic signal of syntaxial exhumation in the sedimentary record of the eastern Himalaya; this study suggests the most robust signal to be recorded in the most proximal deposits with respect to the syntaxis.

AB - The Namche Barwa massif in the eastern Himalayan syntaxis is characterized by very rapid exhumation and provides a significant proportion of the sediment flux carried by the Brahmaputra River. We present new detrital zircon fission-track (ZFT) and muscovite 40Ar/39Ar (MAr) data from modern sediments of rivers draining the eastern Himalaya. The cooling-age populations for both thermochronometers contain a characteristic <2 Ma signature related to the rapid exhumation of Namche Barwa, which can be traced hundreds of kilometers downstream from their source into the Brahmaputra foreland, despite dilution from downstream tributary catchments. To estimate present-day erosion in the catchments, we apply a mixing model based on linear inversion of the binned age distributions. The inversion predicts relative erosion rates in the syntaxial region that are an order of magnitude higher than those in upstream catchments, and about twice as high as those in the southern Himalayan catchments, consistent to first order with previous estimates of erosion-rate patterns in the eastern Himalaya. A comparison of the observed downstream evolution of age distributions with a sediment-flux model suggests that the ZFT signal can be explained by dilution from Himalayan tributaries only, whereas the MAr signal is also affected by selective sequestering (possibly through winnowing) of micas as they are transported downstream. Nevertheless, thermochronological ages <2 Ma provide a diagnostic signal of syntaxial exhumation in the sedimentary record of the eastern Himalaya; this study suggests the most robust signal to be recorded in the most proximal deposits with respect to the syntaxis.

KW - detrital thermochronology

KW - mixing model

KW - present day erosion rate values

KW - Namche Barwa syntaxis

KW - Eastern Himalaya

U2 - 10.1016/j.epsl.2018.07.019

DO - 10.1016/j.epsl.2018.07.019

M3 - Journal article

VL - 499

SP - 48

EP - 61

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

ER -