Exhumational variability within the Himalaya of northwest India

In the Himalaya of Chamba, NW India, a major orographic barrier in front of the Greater Himalayan Range extracts a high proportion of the monsoonal rainfall along its southern slopes and effectively shields the orogen interior from moisture-bearing winds. Along a ~ 100-km-long orogen perpendicular transect, 28 new apatite fission track (AFT) and 30 new zircon (U–Th)/He (ZHe) cooling ages reveal marked variations in age distributions and long-term exhumation rates between the humid frontal range and the semi-arid orogen interior. On the southern topographic front, very young, elevation-invariant AFT ages of < 4 Ma have been obtained that are concentrated in a ~ 30-km-wide zone; 1-D-thermal modeling suggests a Plio–Pleistocene mean erosion rate of 0.8–1.9 mm yr− 1. In contrast, AFT and ZHe ages within the orogen interior are older (4–9 and 7–18 Ma, respectively), are positively correlated with sample elevation, and yield lower mean erosion rates (0.3–0.9 mm yr− 1). Protracted low exhumation rates within the orogen interior over the last ~ 15 Myr prevailed contemporaneously with overall humid conditions and an effective erosional regime within the southern Himalaya. This suggests that the frontal Dhauladar Range was sufficiently high during this time to form an orographic barrier, focusing climatically enhanced erosional processes and tectonic deformation there. Thrusting along the two frontal range-bounding thrust, the Main Central Thrust and the Main Boundary Thrusts, was initiated at least ~ 15 Ma ago and has remained localized since then. The lack of evidence for localized uplift farther north indicates either a rather flat décollement with no ramp or the absence of active duplex systems beneath the interior of Chamba. Exhumational variability within Chamba is best explained as the result of continuous thrusting along a major basal décollement, with a flat beneath the slowly exhuming internal compartments and a steep frontal ramp at the rapidly exhuming frontal range. The pattern in Chamba contrasts with what is observed elsewhere along the Himalaya, where exhumation is focused in a zone ~ 150 km north of the orogenic front. In the NW Himalaya, preserved High Himalayan Crystalline nappes and Lesser Himalayan windows alternate on a relatively small scale of < 100 km; these alternations are closely correlated with the pattern of exhumation. Although the spatial distribution of high-exhumation zones varies considerably between individual Himalayan sectors, all of these zones are closely correlated with locally higher rock-uplift rates, sharp topographic discontinuities, and focused orographic precipitation, suggesting strong feedbacks between tectonically driven rock uplift, orographically enhanced precipitation, and erosional processes.