Shortening rate and Holocene surface rupture on the Riasi fault system in the Kashmir Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge

摘要

New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The >60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6-7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14-7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6-7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11-12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%-50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal decollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal decollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw > 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir.