CENOZOIC DEXTRAL SHEAR IN THE ZAGROS: WHAT WE KNOW AND SOME VIABLE MODELS

Key questions about the evolution of the Arabia-Eurasia collision include: (1) What role has dextral shear played in controlling the architecture of the high Zagros folds and thrusts?, and (2) How has dextral shear been accommodated through time? The major active dextral fault in the High Zagros is the Main Recent Fault (MRF), which probably accommodates all of the 4 +/- 2.5 mm/yr of dextral shear resolved across the Zagros by GPS studies, and offsets the Cretaceous Kuh-e-Auleh gabbro body in the NW Zagros by ~25 km. The MRF system is ~600 km long and is composed of several segments ranging from 100 to ~150 km in length. The southwestern and longest segment (Ardal segment) is generally shown as two strands that link SE to the Kazarun Fault (KF) in an enigmatic way. The NW end of the MRF is also enigmatic; it has been shown as connecting directly to the Karlova triple junction through Lake Van in E. Turkey or as a continuation of the Zagros-Bitlis suture zone which joins the Taurus thrust belt of SE Turkey and N Iraq. Our recent work confirms the young, dextral nature of the MRF and defines the MRF segments. We show that the Ardal segment is probably a single stand (rather than two) and that it locally cuts older thrust faults. This suggests a transition from an earlier dip-slip or oblique-slip kinematic regime to a strike-slip regime. Offset geomorphic markers suggest that the KF also has ~25 km of dextral offset, so most or all of MRF slip is probably transferred to the KF, with any remainder being absorbed by a fan of long-lived, SE-striking dextal faults in the western Fars salient. The ~25 km total offset of the MRF is consistent with the initiation of slip on the MRF at ~6 Ma assuming that the ~4 mm/yr of GPS derived dextral shear across the Zagros is all taken up on the MRF. This timing places the initiation of the MRF within error of the age of a known tectonic reorganization in the Middle East, which is also consistent with our inference that the dominantly strike-slip kinematic regime overprinted an earlier oblique- to dip-slip kinematic regime.