Segmentation of the Main Himalayan Thrust Revealed by Low‐Temperature Thermochronometry in the Western Indian Himalaya

Abstract

Despite remarkable tectonostratigraphic similarities along the Himalayan arc, pronounced topographic and exhumational variability exists in different morphotectonic segments. The processes responsible for this segmentation are debated. Of particular interest is a 30-to 40-km-wide orogen-parallel belt of rapid exhumation that extends from central Nepal to the western Himalaya and its possible linkage to a midcrustal ramp in the basal décollement, and the related growth of Lesser Himalayan duplex structures.Here we present 26 new apatite fission track cooling ages from the Beas-Lahul region, at the transition from the Central to the Western Himalaya (~77°-78°E) to investigate segmentation in the Himalayan arc from a thermochronologic perspective. Together with previously published data from this part of the orogen, we document significant lateral changes in exhumation between the Dhauladar Range to the west, the Beas-Lahul region, and the Sutlej area to the east of the study area. In contrast to the Himalayan front farther east, exhumation in the far western sectors is focused at the frontal parts of the mountain range and associated with the hanging wall of the Main Boundary Thrust fault ramp. Our results allow us to spatially correlate the termination of the rapid exhumation belt with a midcrustal ramp to the west. We suggest that a plunging anticline at the northwestern edge of the Larji-Kullu-Rampur window represents the termination of the Central Himalayan segment, which is related to the evolution of the Lesser Himalayan duplex.

Neogene Kinematic Evolution and Exhumation of the NW India Himalaya: Zircon Geo‐ and Thermochronometric Insights From the Fold‐Thrust Belt and Foreland Basin

“…Along strike comparison of zircon (U‐Th)/He cooling ages across the Himalaya of NW India. The position of the Pliocene high exhumation zone is marked by a red dashed line (Eugster et al, ). The locations of each transect is denoted in Figure .…”

“…The Kullu‐Rampur window marks a zone of recent rapid exhumation attributed to LH duplexing, orographic precipitation, and fluvial erosion (Thiede et al, ; Vannay et al, ). West of the Kullu‐Rampur window in the Chamba region, the absence of a midcrustal ramp and movement along a flat décollement likely resulted in uplift primarily along the frontal part of the system, where both precipitation and exhumation are currently focused (Deeken et al, ; Eugster et al, ; Thiede et al, ). This dynamic relationship highly influenced the diversity of exposed LH strata from east to west across NW India.…”

“…This northward decrease in ZHe ages is due to a northward shift in exhumation after 16 Ma related to the development of iLH duplexing at depth. Farther to the west in the Chamba region where the MBT is still active along a distinct frontal ramp (Thiede et al, ), an opposite northward trend is observed in ZHe and apatite fission track ages, such that cooling ages increase to the north away from the MFT (Figure a; Deeken et al, ; Schlup et al, ; Eugster et al, ). These opposite trends in cooling ages observed likely reflect a combination of differing structural styles (i.e., iLH duplexing to the east and the lack of midramp structures to the west near Chamba) and differences in where orographic induced monsoonal precipitation is focused (i.e., precipitation is focused primarily along frontal range in Chamba whereas focused precipitation persists farther north of the frontal system along the Sutlej River and to the east; Deeken et al, ; Thiede et al, ).…”

“…It is unclear what primarily drove this shift in structural styles between that observed within and east of the Kullu‐Rampur window, and that observed in the Chamba region. Recent work has recognized that the westward decreasing convergence rate (associated with the arcuate shape of the Himalaya) coupled with potential thinning of Proterozoic cover units toward the northwest may explain the relatively low exhumation experienced within the interior of the Chamba region; however, neither of these can predict the sharp transition observed in structural style and exhumation to the east (Eugster et al, ). Some mechanism or combination of mechanisms must be present in order to partition deformation to northern duplex structures.…”

“…Integration of bedrock ZHe and U‐Pb data with detrital zircon U‐Pb and ZHe provenance analysis of the foreland basin provides direct constraints on Himalayan exhumation during progressive thrusting, and these data may reveal spatial‐temporal patterns in kinematically induced bedrock exhumation. Resolving the exhumational evolution of the Himalaya from the Kullu‐Rampur window and to the east is essential to understand potential controlling mechanisms for the sharp contrast in exhumation and structural style observed from the Chamba region in the west to Nepal in the east (e.g., Eugster et al, ; Stübner et al, ).…”