The Himalayas have been the focus of many studies looking to identify relationships between topography, exposed geologic relationships of different rock units, and the underlying tectonic processes involved in mountain building. These studies have helped established a paradigm for the evolution of the Himalayas, which explains how patterns erosion are intrinsically linked to the evolution of faults and geologic structures underlying the range. However, this paradigm breaks down in the northwest Himalayas, where spatial patterns of topographic relief (the difference between the highest and lowest elevations in a given area), exposed rock units, and erosion rates differ substantially from those in the central Himalayas (near the brown dashed line in the figure below).

Regional maps indicating the along-strike changes in geology and geomorphology.
Geographic and geologic maps of the study area. (A) Topography and major river networks in the northwest Himalayas and southern Tibetan Plateau. (B) Regional-scale geologic units and major fault systems. Brown dashed line represents the approximate position of the transition in crustal architecture near 77°E. KF - Karakoram Fault, GCT - Great Counter Thrust, STDS - South Tibetan Detachment System, MCT - Main Central Thrust, MBT - Main Boundary Thrust, MFT - Main Frontal Thrust, KRW - Kullu-Rampur Window, AK - Almora Klippe, LK - Lansdowne Klippe, DR - Dehradun Reentrant, KR - Kangra Reentrant. (C) Tropical Rainfall Measurement Mission (TRMM) mean annual precipitation (MAP). (D) Topographic relief calculated over a 1-km window.

Previous studies have also noted two key observations from the spatial patterns of low-temperature thermochronometric dates in the northwest and central Himalayas: (1) duplex development within the Lesser Himalayan Sequence appears to have migrated westward since the late Miocene, and (2) the sharp along-strike change in the patterns of topography, exposed geology, and erosion rates can be attributed to a change in tectonic style, with the transition coinciding with the westward extent of duplex propagation. However, low-temperature thermochronometry cannot resolve the most recent patterns of erosion (i.e., within the past several million years), so it is uncertain whether duplexing has continued to migrate to the west over this period. Resolving the pattern of modern erosion rates can help detemine whether duplexing continues to migrate, which is a key test to identify the mechanism(s) that may explain these observations.

In this work, I used cosmogenic 10Be-derived denudation rates, in conjunction with published thermochronometric dates (i.e., helium and fission track dates), to better resolve the spatial patterns of erosion in the northwest Himalayas over millennial and million-year timescales and determine whether duplexing continues to migrate westward. To increase the density of denudation rate measurements across our study area, I generated an estimated erosion rate map using a calibrated, empirical relationship between catchment-averaged 10Be-derived denudation rates and ksnQ, a newly developed proxy for erosion rates..

Map of predicted erosion rates with transect locations shown.
Map of predicted erosion rates and exhumation rates derived from thermochronometric dates in the northwest and central Himalayas. Superimposed are swaths (yellow lines; solid:centerline, dashed:boundary), and regional fault systems.
Example of a transect of erossion rate estimates across different geochronometer systems.
Example swath transect displaying how thermochronometric dates, 10Be denudation rates, inferred erosion rates, and topography vary across the northwest Himalayas. Transect location is labeled “Dh” in map above.

Erosion rates mapped using the relationship between millennial denudation rates and ksnQ exhibit a similar spatiotemporal pattern to exhumation rates determined from low-temperature thermochronometric dates (see figure below), suggesting that duplexing has not migrated over the past several million years. Of the mechanisms we evaluated, only asynchronous ramp formation and advection within the orogenic wedge (Mercier et al., 2017) independently explains both the sharp changes observed in our study area and the apparent westward migration of duplexing. Our results represent a key contribution to the growing literature demonstrating the utility of ksnQ to assess regional patterns of erosion rates and emphasize the importance of accounting for spatially variable precipitation when using channel steepness as a proxy for denudation rate.

Erosion map zoomed into Rohtang Pass.
(A) Map of estimated erosion rates and exhumation rates from low-temperature thermochronometry for the transitional region between the northwest and central Himalaya. Estimated erosion rates decrease sharply from >1 mm/yr south of the Rohtang Pass to <0.4 mm/yr to the north, consistent with the pattern of observed thermochronometric dates. (B) Barbed drainages (blue) in tributaries to the Beas River are consistent with westward divide migration, which I suspect resulted from breaching of the Lesser Himalayan Sequence to the southeast.

A manuscript with additional project details is currently in the final stages of preparation. A link to the manuscript will be added to this page once it has been peer-reviewed and accepted for publication.

Penserini, Brian D., Kristin D. Morell, Vincent Godard, et al. “Modern Exhumation Patterns of the Northwest Himalayas Resolved using 10Be-derived Denudation Rates and Topographic Analyses.” (in prep).

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