Himalayan Strain Accumulation 100 ka Timescales

Abstract:

Crustal scale fault systems and tectonostratigraphic units in the Himalaya can be traced for 2500 km along strike. However regional studies have shown that there is variability in the location and rate of strain accumulation which appears to be driven by Main Himalayan Thrust (MHT) geometry and convergence obliquity. GPS illuminates the modern interseismic strain rate and the historical record of great earthquakes elucidates variations in strain accumulation over 10³ years. To connect these patterns with the 10⁶ year structural and thermochronometric geologic record we examine normalized river channel steepness (k_sn), a proxy for rock uplift rate, which develops over 10⁴ – 10⁵ years. Here we present a k_sn map of the Himalaya and compare it with bedrock geology, precipitation, the historic earthquake record, GPS, seismicity, and seismotectonic models. Our map shows significant along strike changes in the magnitude of channel steepness, the areal extent of swaths of high k_sn channels, and their location with respect to the range front. Differences include the juxtaposition of two narrow (30 – 40 km) range parallel belts of high k_sn in west Nepal and Bhutan coincident with MHT duplexes and belts of microseismcity, with a single broad (70 km) swath of high k_sn and microseismicity in central and eastern Nepal. Separating west and central Nepal a band of low k_sn crosses the range coincident with the West Nepal Fault (WNF) and the lowest rate of microseismicity in Nepal. To the west the orogen is obliquely convergent and has less high k_sn channels, while the orthogonally convergent region to the east contains the highest concentration of oversteepened channels in the Himalaya supporting the idea that the WNF is a strain partitioning boundary. The syntaxes are characterized by locally high channel steepness surrounded by low to moderate k_sn channels consistent with the hypothesis that rapid exhumation within the syntaxes is sustained by an influx of lower crust.