DISTRIBUTED OBLIQUE-DEXTRAL TRANSPRESSION IN THE HIGH ZAGROS MOUNTAINS, IRAN

The HZ expose Cambrian to Tertiary strata of Arabian affinity that are imbricated by oblique dextral thrusts and cut by dextral strike-slip faults, both related to the oblique collision between Arabia and Eurasia. They are bounded on the southwest against the Simply Folded Belt by the High Zagros fault. On the NE, the Main Zagros reverse fault juxtaposes them against central Iran. Major High Zagros thrusts strike northwest, dip northeast, and carry ~5 km of Paleozoic-Mesozoic strata, locally including Cambrian sandstone at their bases. Although the thrust-belt architecture appears typical we find that many “thrusts” are in fact oblique-dextral faults and that significant dextral strain has been active in the High Zagros, probably since the onset of orogeny in early Miocene or late Oligocene(?) time. Earliest motion is recorded on the Main Zagros reverse fault, which controlled deposition of 3 Bakhtiyari conglomerate units near Shalamzar that are separated by unconformities. The middle unit contains early Miocene (23-17 Ma) marine fossils and pollen; the lower unit is undated but may be late Oligocene. Thus, the MZRF was active by early Miocene or late Oligocene(?) time. Apatite (U-Th)/He (AHe) ages from the two lower Bakhtiyari units are ~19-15 Ma; the younger ages in this range cannot represent detrital-grain ages (erosion in source terrain) because they postdate the youngest possible depositional age. We infer that the Bakhtiyari beds were buried by the Main Zagros thrust sheet to depths below the AHe partial retention zone, then exhumed and cooled from ~19-15 Ma in response to slip on thrusts farther southwest. Dated slip on High Zagros faults shows that the Main Zagros reverse fault is older than thrusts farther southwest, and that thrust timing also varies along strike. Southwest of Shalamzar, the Kuh-e-Lajin thrust is internal to the High Zagros in its northwest part but merges southeast with the High Zagros fault. AHe ages from basal clastic strata northwest of the intersection cluster around 18-15.5 Ma. These ages probably record activity on the Kuh-e-Lajin thrust or a deeper one, either of which may have uplifted the Shalamzar Bakhtiyari. Farther southeast, AHe ages from basal strata of the Kuh-e-Dinar segment of the High Zagros fault and from footwall syntectonic Bakhtiyari conglomerate are concordant, at ~11-8 Ma. An unroofing sequence and generally SW-directed paleocurrents from the Dinar Bakhtiyari indicate a source in the adjacent thrust sheet. The concordant ages suggest that the Bakhtiyari AHe ages are detrital-grain ages reflecting exhumation in their upper-plate source area. It appears likely that dextral transpression was distributed through the HZ in Miocene time, causing penetrative dextral S-C fabrics accommodated by pressure solution/precipitation along the Main Zagros reverse fault, oblique- to strike-slip striations on vertical and “thrust” faults, extension and salt extrusion along northerly striking fault segments, “thrusts” that climb section mainly to the southeast forming dextral duplexes in map view, and southeast-directed fold-accommodated shortening where faults end. The presently active, dextral Main Recent fault system has net offset of ~25 km and apparently developed more recently. The additional north-south convergence accommodated by this distributed oblique-dextral strain is difficult to quantify but may be large enough to account for the discrepancy inferred between plate reconstructions and dip-slip cross section reconstructions that neglect lateral slip components.