Holocene shortening rates and seismic hazard assessment for the frontal Potwar Plateau, NW Himalaya of Pakistan: Insights from Be-10 concentrations on fluvial terraces of the Mahesian Anticline

摘要

We present the results of a structural neotectonic survey undertaken on the Mahesian Anticline in the frontal Himalaya of Pakistan. This anticline resulted from the folding of Precambrian to Tertiary layers that was controlled by a thrust and a backthrust, interacting in a complex way. Four generations of fluvial terraces formed by the Jhelum River and two tributaries have been distinguished on the SE flank of the anticline. Two of these terraces, T2 and T3, have been left hanging by fold development and have been dated by cosmogenic Be-10 to 6.5 +/- 0.2 ka and 3.3 +/- 0.7 ka, respectively. From such ages an uplift rate of similar to 10 mm/y was determined for the Holocene. That uplift is induced by a shortening rate of similar to 10 mm/y. We highlight that the Mahesian Anticline and Frontal Salt Range Thrust, together with the Kalabagh western lateral ramp and the Jhelum eastern lateral ramp, delineate the active tectonic boundary of the Potwar Plateau. This thrust sheet moves above the salt level without out-of-sequence deformation. Moreover, the small but significant difference between the long-term deformation rates (8.4 mm/y) and the geodetic velocities (2-5 mm/y) detected for the Central Potwar Plateau seems to be linked to episodic spurts of accelerated creeping of the thick salt level, triggered by earthquakes located to the north on the deep (>15 km) part of the MHT. In addition, the large difference between the long-term deformation rates and the geodetic velocities (less than 2 mm/y) reported for the eastern Potwar Plateau seems to be linked to the accumulation of a slip deficit around asperities formed where the salt is missing. This deficit may be recovered during earthquakes potentially as great as Mw 7. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.