To provide a detailed record of a relatively rare thrust surface rupture and examine its active tectonic implications, we have conducted field mapping of the surface rupture associated with the 2005 M w 7.6 Kashmir earthquake. Despite the difficulty arising from massive earthquake-induced landslides along the surface rupture, we found that typical pressure ridges and warps extend northwestward for a distance of ∼70 km, with a northeast-side-up vertical separation of up to ∼7 m. Neither the main frontal thrust nor the main boundary thrust is responsible for the earthquake, but three active faults or fault segments within the Sub-Himalaya, collectively called the Balakot-Bagh fault, compose the causative fault. Although the fault exhibits substantial geomorphic expression of repeated similar surface ruptures, only a part of it had been mapped as active before the earthquake. The location of the hypocenter suggests that the rupture was initiated at a deep portion of the northern-central segment boundary and propagated bilaterally to eventually break all three segments. Our obtained surface rupture traces and the along-strike-slip distribution are both in good agreement with results of prompt analyses of satellite images, indicating that space geodesy can greatly aid in time-consuming field mapping of surface ruptures. Assuming that the extensive fill terrace in the meizoseismal area was abandoned during 10-30 ka, we tentatively estimate the earthquake recurrence interval and shortening rate on the Balakot-Bagh fault to be 1000-3300 yr and 1:4-4:1 mm=yr, respectively. These estimates indicate that the Balakot-Bagh fault is not a main player of Himalayan contraction accommodation. Ⓔ Selected field photographs and ArcGIS files of the mapped surface rupture traces and measured vertical separations are available in the electronic edition of BSSA.Online Material: Field photographs and ArcGIS files of surface rupture traces and vertical separations.

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[1] We calculated static stress changes from the devastating M = 7.6 earthquake that shook Kashmir on 8 October, 2005. We mapped Coulomb stress change on target fault planes oriented by assuming a regional compressional stress regime with greatest principal stress directed orthogonally to the mainshock strike. We tested calculation sensitivity by varying assumed stress orientations, target-fault friction, and depth. Our results showed no impact on the active Salt Range thrust southwest of the rupture. Active faults north of the Main Boundary thrust near Peshawar fall in a calculated stress-decreased zone, as does the Raikot fault zone to the northeast. We calculated increased stress near the rupture where most aftershocks occurred. The greatest increase to seismic hazard is in the Indus-Kohistan seismic zone near the Indus River northwest of the rupture termination, and southeast of the rupture termination near the Kashmir basin.

Detailed mapping suggests a new scenario for the kinematic history of the Late Cretaceous Kohistan arc complex and for the cause of Cenozoic metamorphism and exhumation along the Indian continental margin in Pakistan. The earliest deformational phases resulted in early Cenozoic amphibolite-facies metamorphism, superposed tight to isoclinal F1/F2 folds, and development of strong penetrative rock fabric within the Indian plate in response to southwestward, and then southeastward, overthrusting of ophiolitic mélange in the Indus suture zone and West Pakistan fold belt, respectively. This deformation included the development of the Banna and Malakand thrust faults. Prograde metamorphism ended following this deformational phase, and the early structures were deformed along N-trending, open, upright, F3 folds and domes. This was followed by ESEto E-directed thrusting of the Kohistan arc complex onto the Indian continent along the dominantly brittle Kohistan fault. Thrusting ended in the late Oligocene-early Miocene when the Kohistan arc assumed its present position along the northern margin of India. South-verging structures began to form on the Indian plate for the fi rst time in the late Oligocene-early Miocene. Deformation had expanded to the foreland by the middle to late Miocene with development of the Panjal-Khairabad thrust. Neogene structures in the metamorphic zone include N-trending, eastside-up, high-angle fault zones that deform or reactivate the Kohistan fault, and NWtrending, east-side-up reverse faults, both of which are seismically active.In the tectonic scenario outlined here, shortening along the Kohistan fault occurred subsequent to metamorphism, and was directed toward the ESE-E throughout its history. The southern termination of the fault is considered to be a right-slip sidewall ramp such that the Kohistan arc never advanced much father south on the Indian plate than its present position. Broad fl exure, folding, and erosion in front of and south of the actively advancing Kohistan arc complex contributed to exhumation of the Indian plate. This differs from interpretations that state that Kohistan was thrust in a southward direction; that underthrusting beneath Kohistan was the primary cause of early Cenozoic metamorphism on the Indian plate; and that top-side-north extensional reactivation of the Kohistan fault contributed to exhumation.