ONSET AND SEQUENCE OF THRUSTING IN THE HIGH ZAGROS (HZ) MOUNTAINS, IRAN, FROM FOSSILS AND APATITE (U-TH)/HE AGES

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 SW against the Simply Folded Belt by the High Zagros fault (HZF). On the NE, the Main Zagros reverse fault (MZRF) juxtaposes them against central Iran. Major HZ thrusts strike NW, dip NE, and carry ~5 km of Paleozoic-Mesozoic strata, locally including Cambrian sandstone at their bases.

Dated slip on three HZ thrusts shows that the MZRF is older than thrusts farther SW, and that thrust timing also varies along strike. Near Shalamzar, the MZRF controlled deposition of 3 Bakhtiyari conglomerate units 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 SW. SW of Shalamzar, the Kuh-e-Lajin thrust is internal to the High Zagros in its NW part but merges SE with the HZF. AHe ages from basal clastic strata NW 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 SE, AHe ages from basal strata of the Kuh-e-Dinar segment of the HZF 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 in the HZ in Miocene time, with the presently active, mainly dextral fault system developed later.