DETRITAL THERMOCHRONOLOGICAL DATA DEMONSTRATE FORELAND ACCRETION AND DEFORMATION WAS UNDERWAY BY ~5MA ALONG 300 KM OF THE ACTIVELY DEFORMING NW HIMALAYAN FORELAND

The timing of accretion and subsequent deformation of the foreland has broader implications for the style, pattern and rates of shortening within the entire orogenic wedge. In the Himalayas, previous estimates of foreland accretion along the Main Frontal Thrust (MFT), based largely upon sedimentation and paleomagnetic records, are broadly bracketed as Plio-Quaternary and range from 6.3 Ma in Pakistan to 4.3-1.7 Ma between NW India and Nepal. However, a new set of detrital (U-Th)/He ages constrain the thermal exhumation history of five thrust sheets along a geologically well-constrained, NE-SW transect across the deformed NW-Himalayan foreland. Our results indicate that thrust-related exhumation was underway by ~5 Ma, roughly 3 Ma earlier than previous estimates. This implies significantly reduced shortening rates compared to previous studies. An earlier accretion age indicates either that missing shortening be accommodated elsewhere in the thrust wedge or that thrust sheets with eroded hanging wall cutoffs accommodate more shortening then previously assumed. Additionally, progressively younger (U-Th)/He exhumation ages observed toward the orogenic hinterland, as young as 4.2 Ma on the Jawalamukhi Thrust sheet, indicate that the orogenic wedge was sub-critical prior to the Late Miocene-Pliocene shift to a glacial climate. Distributed deformation thus can be reasonably interpreted as rebuilding of wedge taper following frontal accretion, which may have been amplified by enhanced erosion related to Quaternary climate oscillations.