Seismotectonics and the pattern of active deformation from collision to subduction in the Zagros – Makran transition zone

The transition between the active Makran subduction and the Zagros continental collision exhibits significant differences on either side, making the Zagros-Makran Transition Zone (ZMTZ) a natural laboratory to study seismotectonic processes. This study investigates the active deformation patterns and their links to deep seismogenic structures, aiming to define the main active structures of the ZMTZ. We focus on seismicity of the Goharan area, the only cluster in the region, with its main event (Mw = 6.2) occurring on May 11, 2013. The Hypocentroidal Decomposition Algorithm used for multiple event relocation analysis. By incorporating data from 46 seismic stations belong to different seismic networks, we were able to minimize the azimuthal gap and reduce the potential biases in location. The relocation results reveal an east-west lineament in the Goharan cluster, consistent with InSAR observations. The seismicity (Ml > 4) recorded from 2006-2021 by 44 permanent broad-band seismic stations of the Iran and Oman network was used to perform the moment tensor inversion using the probabilistic inversion method. Also, a stress inversion of the focal mechanisms was performed to acquire the present-day stress regime and fault planes from nodal planes of the earthquake focal mechanisms of the region. The obtained results provided Quaternary stress tensors, revealing the active stress field and fault mechanisms in the region. High-resolution satellite imagery, combined with geological and seismological data reveal that active deformation is mainly accommodated by a series of ENE-trending sinistral faults, which are in close interaction with almost N-S dextral faults. These conjugate fault networks intersect pre-Quaternary structures and are independent of structural processes that directly affect the MZP and Makran subduction zones. ENE-trending sinistral faults are accompanied by steep structural steps in the North Makran thrust boundaries and correspond to the locations where the inner Makran zone narrows westward to its wedge despair. These observations indicate that (1) sinistral faults west of 61°E have been active since the initial formation of the Makran wedge, (2) interacting with the dextral conjugate series, these Quaternary structures accommodate part of NNE-SSW shortening due to the convergence of the Arabia–Lut block, transferring some deformation northward without major folding or thrusting, and (3) the existence of these strike-slip faults in this part of the Makran wedge define distinct seismotectonic zones capable of hosting moderate to large continental earthquakes, with a significant impact on seismic hazard. This study reveals that structural interactions in complex tectonic settings can produce deformation patterns not predicted by classical geological models for the region. The surface deformation in the ZMTZ is influenced by two main deep structures; Arabian underthrusting and the Makran subduction zone. Integrating seismological and geological results provides a new description of the ZMTZ as a triple-junction area between Makran, Zagros, and Central Iran. The outcomes of this project can contribute to a better understanding of crustal deformation in similar cases.