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S U M M A R YWe investigate the depth of faulting and its connection with surface folding in the Zagros Simply Folded Belt of Iran. Our focus is a sequence of earthquakes (M w 5.7, 5.5, 5.2, 5.0, 4.9) that struck the Fin region, in the south-eastern Simply Folded Belt, on 2006 March 25. Modelling ground displacements measured with radar interferometry, we find that either N-or S-dipping model reverse faults can reproduce the observed fringe patterns. Despite the uncertainty in fault orientation, we can constrain the vertical extents of rupture to between a top depth of ∼5-6 km and a bottom depth of ∼9-10 km, consistent with the ∼8 km centroid depth of the largest earthquake. We suggest that the faulting ruptured the thick 'Competent Group' of Paleozoic and Mesozoic conglomerates and platform carbonates, which makes up the lower part of the sedimentary cover. The rupture probably terminated within the Precambrian Hormuz salt at its base, and the Cretaceous Gurpi marls at its top. These mechanically weak layers act as barriers to rupture, separating faulting within the Competent Group from deformation in the layers above and below. The pattern of coseismic surface uplift is centred on the common limb of the Fin syncline and Guniz anticline, but is oblique (by 20 • ) to the trend of these open, symmetric, 'whaleback' folds, and also overlaps a section of the Fin syncline axis. These observations suggest that locally, surface folding is decoupled from the underlying reverse faulting. Although the Fin syncline and Guniz anticline are symmetric structures, some other nearby folds show a strong asymmetry, with steep or overturned southern limbs, consistent with growth above N-dipping reverse faults. This suggests that the Simply Folded Belt contains a combination of forced folds and detachment folds. We also investigate the distribution of locally recorded aftershocks in the weeks following the main earthquakes. Most of these occurred at depths of ∼10-30 km, with a particularly high concentration of events at ∼20-25 km. These aftershocks therefore lie within the crystalline basement rather than the sedimentary cover, and are vertically separated from the main rupture. This study confirms earlier suggestions that earthquakes of M w 5-6 are capable of being generated within the thick 'Competent Group' of Paleozoic and Mesozoic sediments, as well as in the basement below the Hormuz Salt Formation.

Determining the relationship between folding and faulting in fold and thrust belts is important for understanding the growth of geological structures, the depth extent of seismic slip, and consequently, the potential earthquake hazard. The 2013 M w 6.2 Khaki-Shonbe earthquake occurred in the Simply Folded Belt of the Zagros Mountains, Iran. We combine seismological solutions, aftershock relocations, satellite interferometry, and field observations to determine fault geometry and its relationship with the structure, stratigraphy, and tectonics of the central Zagros. We find reverse slip on two along-strike, southwest dipping fault segments. The main shock rupture initiated at the lower northern end of the larger northwest segment. Based upon the hypocenter and rupture duration, slip on the smaller southern segment is likely aseismic. Both faults verge away from the foreland, toward the high-range interior, contrary to the fault geometries depicted in many structural cross sections of the Zagros. The modeled slip occurred over two mutually exclusive depth ranges above 10 km, resulting in long (∼16 km), narrow rupture segments (∼7 km). Aftershocks cluster in the depth range 3-14 km. This indicates reverse slip and coseismic shortening occurred mostly or exclusively in the sedimentary cover, with slip distributions likely to be lithologically controlled in depth by the Hormuz salt at the base of the sedimentary cover (∼10-12 km), and the Kazhdumi Formation mudrocks at upper levels (∼4-5 km). Our findings suggest lithology plays a significant role in the depth extent of slip found in reverse faults in folded belts, providing an important control on the potential size of earthquakes.

International audienceWe present a synthesis of recently conducted tectonic, global positioning system (GPS), geomorphological and seismic studies to describe the kinematics of the Zagros mountain belt, with a special focus on the transverse right-lateral strike-slip Kazerun Fault System (KFS). Both the seismicity and present-day deformation (as observed from tectonics, geomorphology and GPS) appear to concentrate near the 1000 m elevation contour, suggesting that basement and shallow deformation are related. This observation supports a thick-skinned model of southwestward propagation of deformation, starting from the Main Zagros Reverse Fault. The KFS distributes right-lateral strike-slip motion of the Main Recent Fault onto several segments located in an en echelon system to the east. We observe a marked difference in the kinematics of the Zagros across the Kazerun Fault System. To the NW, in the North Zagros, present-day deformation is partitioned between localized strike-slip motion on the Main Recent Fault and shortening located on the deformation front. To the SE, in the Central Zagros, strike-slip motion is distributed on several branches of the KFS. The decoupling of the Hormuz Salt layer, restricted to the east of the KFS and favouring the spreading of the sedimentary cover, cannot be the only cause of this distributed mechanism because seismicity (and therefore basement deformation) is associated with all active strike-slip faults, including those to the east of the Kazerun Fault System