Arc segmentation and landscape evolution in the Bhutan Himalaya:
Insights from integrated thermochronometry, cosmogenic radionuclide and
geomorphometry studies
Abstract
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.