S U M M A R YWe present observations and models of the Sulaiman Range of western Pakistan that shed new light on the evolution and deformation of fold-thrust belts. Earthquake source inversions show that the seismic deformation in the range is concentrated in the thick pile of sediments overlying the underthrusting lithosphere of the Indian subcontinent. The slip vectors of the earthquakes vary in strike around the margin of the range, in tandem with the shape of the topography, suggesting that gravitational driving forces arising from the topography play an important role in governing the deformation of the region. Numerical models suggest that the active deformation, and the extreme plan-view curvature of the range, are governed by the presence of weak sediments in a pre-existing basin on the underthrusting Indian Plate. These sediments affect the stress-state in the over-riding mountain range and allow for the rapid propagation of the nose of the range and the development of extreme curvature and laterally varying surface gradients.Key words: Seismicity and tectonics; Continental neotectonics; Dynamics: gravity and tectonics; Dynamics: seismotectonics.
I N T RO D U C T I O NPlan-view curvature of geological structures and range-front topography has long been a recognized and debated feature of both ancient and active fold-thrust belts (e.g. Argand 1924;Carey 1955;Hindle & Burkhard 1999;Marshak 2004). Though poorly understood, the relationship between the size and shape of a mountain range, surface deformation, and continental rheology, is key to our understanding of mountain-building processes. As the largest active mountain ranges on Earth, much of the body of work surrounding this topic has focused on the Tibetan Plateau and the Andes (e.g. Argand 1924;Suárez et al. 1983;Dewey et al. 1988;England & Houseman 1988;Isacks 1988;Molnar & Lyon-Caen 1989;Whitman et al. 1996;Lamb & Hoke 1997;Barke et al. 2007;Royden et al. 2008). However, a full understanding of continental tectonics and rheology requires knowledge of the deformation, evolution and dynamics of not just the planet's largest and most rapidly deforming regions, but also of smaller deformation zones (e.g. Batt & Braun 1999;Macedo & Marshak 1999;Nissen et al. 2011b). A lack of published data, extremely limited geodetic coverage and difficulty of access mean there have been relatively few studies of the western part of the Himalaya-Tibetan Plateau system, where the Himalaya curve to the southwest into the lobate fold-thrust belts of Pakistan. The widest of these, the Sulaiman Range (Fig. 1a), forms a strongly curved, asymmetric lobe with ∼300 km across-strike width. This range has experienced plentiful earthquakes during the instrumental period, and has been the subject of several competing hypotheses about the behaviour and mechanics of fold-thrust belts (Humayon et al. 1991;Jadoon & Khurshid 1996;Haq & Davis 1997;Macedo & Marshak 1999;Bernard et al. 2000;Reiter et al. 2011). In this paper, we study the Sulaiman Range in order to gain insights into the ...