EVIDENCE FOR SIGNIFICANT DEXTRAL FAULTING WITHIN THE HIGH ZAGROS, IRAN

The High Zagros (HZ) in SW Iran is a NW-trending belt of folded and thrust-imbricated rocks bounded by the Main Zagros Reverse Fault (MZRF) and the Sanandaj-Sirjan zone to the NE and the High Zagros Fault (HZF) and Simply Folded Belt (SFB) to the SW. For decades the HZF, which juxtaposes Paleozoic rocks in its hanging wall with the Cenozoic sediments of SFB in its footwall has been considered to be a thrust fault with a dextral oblique component. Our research shows little clear physical evidence of dip slip thrust faults, and furthermore, there is little seismic evidence for shallow thrusting. Our preliminary field observations, interpretation of geological maps, and geometric analysis of the HZ suggest that other faults of the zone have similar geologic and tectonic histories. These faults are dominantly dextral oblique, and juxtapose PZ sections and Tertiary sediments: Kuh-e-Gari, Kuh-e-Robat, Oshtoran kuh, Bazoft, Zard kuh, Gereh (Dinarun), Sarkhun, Kuhrang, Ardal, Dinar (Dena), Sepidan in the HZ region, and the Gahkom, Faraghun and Khosh kuh in the southeastern HZ. Data supporting SE-directed dextral slip throughout the HZ include: (1) common horizontal slickenlines on vertical and subvertical fault planes along Kuhrang fault, (2) several small horst-graben features on the SW sides of faults bounding the NE edge of the SFB, suggesting zones of extension and pull apart next to the fault, (3) extrusion of Cambrian Hormuz salt along Bazoft, Zard kuh, Kuhrang, Dena, Sepidan, Gahkom and Khosh kuh faults in segments that strike N-S, suggesting SE dextral pull apart, and (4) E- to NE-striking, dip-slip (thrust) terminations of the southeastern ends of HZ faults, which accommodate N-S to NW-SE shortening. The E-W striking, dip-slip parts of HZ faults commonly are associated with up to four E- or NE-trending asymmetric anticlines at the uplifted hanging wall of each fault segment. Such folds record ~1-4 km of SE-directed shortening of the HZ sedimentary sequence. Considered together, these data and observations suggest that all of the major faults of the HZ have absorbed significant dextral strain, which may postdate an earlier, dip-slip dominated phase of deformation.