GNGTS 2022 - Atti del 40° Convegno Nazionale

138 GNGTS 2022 Sessione 1.3 COSEISMIC FOLDING DURING RAMP FAILURE AT THE FRONT OF THE SULAIMAN FOLD-AND-THRUST BELT M.T. Javed 1 , S. Barbot 2 , F. Javed 3 , C. Braitenberg 1 , A. Ali 4 1 Department of Mathematics and Geosciences, University of Trieste, Italy 2 Department of Earth Sciences, University of Southern California, USA 3 Centre for Earthquake Studies, National Centre for Physics, Pakistan 4 Department of Earth Sciences, Quaid-i-Azam University, Islamabad, Pakistan Abstract. The Sulaiman Fold Thrust (SFT) in Central Pakistan formed during the India- Eurasia collision in the late Cenozoic. However, the mechanics of shortening of the brittle crust at time scales of seismic cycles is still poorly understood. Here, we use spaceborne radar interferometry to analyze the deformation associated with the 2015 moment magnitude (Mw) 5.7 Dajal earthquake at the eastern boundary of the SFT. We use kinematic inversions to determine the distribution of slip on the frontal ramp and of flexural slip along active axial surfaces for the forward verging two end-member models: a double fault-bend fold system and a fault-propagation fold. In both models, a decollement branches into a shallow ramp at approximately 7 km depth with coseismic folding in the hanging wall. The Dajal earthquake ruptured the base of the foreland-vergent Boundary Thrust buried under sediment from the Indus River floodplain, representing fault-bending or fault-propagation folding near Sakhi- Sarwar fault-bend 30 km east of its surface exposure. Introduction. The kinematics of crustal deformation at fold-and-thrust belts is complicated by the interaction between faulting and folding (Poblet and Lisle, 2011). Complex fault geometry causes shortening in the hanging wall that is accommodated by flexural slip, a type of plastic deformation that results from slip on multiple bedding planes in sedimentary strata (Sathiakumar et al. , 2020). Thrust-and-fold belts can also be found at the margin of collision zones and in other transpressive settings (Lai et al. , 2006). The role of folding on the long-term build-up of topography is well explained in various tectonic environments (Mahanjane and Franke, 2014). However, the mechanical coupling between folding and faulting during seismic cycles is still poorly understood. Some observations and modeling (Sathiakumar et al. , 2020) indicate that co-seismic folding is possible, but how this deformation occurs in the crust is still poorly resolved. The western margin of the India-Eurasia collision belt, which accommodates 29 mm/yr of relative plate motion, provides an ideal setting to study the mechanics of an active fold-and- thrust belt (Reynolds et al. , 2015). The shear component is accommodated along the transform boundary of the Chaman Fault (CF) system but shortening is taken up by the transpressive Kirthar Fold Thrust (KFT) and the Sulaiman Fold Thrust (SFT). The historical earthquakes and fault plane solutions in the SFT belt are shown in Fig. 1a. The region is dominated by thrust events and a few strike-slip earthquakes (Ambraseys and Bilham, 2003, Pezzo et al. , 2014). Although the Central and Western SFT are characterized by well-developed fault-bend folding structures, the eastern SFT zone is under-developed (Jadoon et al. , 2019), experiencing only moderate-sizeNE-SWand E-Wthrust earthquakes, with a deficit of large (Mw>6.0) earthquakes along the BT in the last several decades. Large cities with population of more than 3 million are located approximately 10 to 50 km from the BT. On October 23, 2015, an Mw 5.7 earthquake occurred near Dajal, 55 km SSW of Dera Ghazi Khan, one of the most populated cities of Punjab province, Pakistan. The earthquake occurred at the eastern boundary of the SFT, providing an opportunity to study the interaction of coseismic slip and folding within an active fold-and-thrust belt. The 2015 Mw 5.7 Dajal, Pakistan earthquake occurred in a remote location, outside of any ground-based geodetic observatory. Fortunately, the deformation was captured by the spaceborne synthetic aperture radar Sentinel-1A. To document the surface deformation induced by the earthquake, we