Structure of the intraplate eastern Palmyride Fold Belt, Syria

摘要

The Palmyride Fold Belt is an aulacogen within the Arabian plate linked to the Levantine continental margin, which contains >10 km of Phanerozoic sediments. Inversion of this Mesozoic basin began during Late Cretaceous time, although most deformation was clearly post-Oligocene. Recent field mapping shows that almost all the structures have resulted from a phase of northwest-southeast compression followed by major dextral transpression. Early northwest-southeast compression resulted in northeast-southwest-aligned fold axes in the central and southeast Palmyrides, many of which were subsequently rotated by progressive dextral simple shear during Neogene to Recent time. The north-south-aligned sinistral Dead Sea fault is the major wrench fault aligned at 45° to the maximum compressive stress. The conjugate east-west-striking dextral Jhar fault and the northwest-southeast-aligned Bishri fault have accommodated limited dextral shear and clockwise rotation of northeast-southwest-aligned folds (Jebels Mrah, Ash Shaer). In the northern Palmyrides, dextral strike-slip faults aligned along Reidel fractures (R1) cut the flanks of earlier domal structures (Jebel Abu Rajmein). Subordinate antithetic R2 Reidel shears aligned north-northeast-south-southwest are trans-tensional sinistral strike-slip faults showing minor extension that cut through the earlier domal structures (for example, Ras' al Hawa depression in Jebel Abu Rajmein). Northwest-southeast-aligned faults (for example, Jebel Bilas cross-trend faults) show northeast-southwest extension at right angles to the Palmyride shortening direction. This extension parallels the major Euphrates graben trend in eastern Syria and northwestern Iraq and also parallels some aligned Pliocene-Pleistocene volcanic rifts south of Damascus and in the Jebel Druze (Jebel Arab) region of southern Syria and northern Jordan. A regional Palmyride strain ellipse is presented to explain the geometry of all these structures. The Ad Daww depression is a northeast-southwest-aligned transpressional basin containing around 2 km thickness of Tertiary sediments. The basin is bounded to the west by a sinistral shear couple along the Cherrife structural trend in the central Palmyrides, along the north by the dextral Jhar fault, and along the southeast by the compressional southeast Palmyrides. In the southeast Palmyrides, structural style is one of a series of northeast-southwest-aligned narrow anticlines in the Mesozoic platform sediments with upright to north-west-dipping, southeast-facing axial planes. Shortening amounts are very small in the eastern Palmyrides (1 km) and taken up almost entirely by folding, but these increase slightly toward the west to probably 20 km in the Cherrife region. Tight box folds and slightly asymmetric folds in the southeast Palmyrides have a localized basal detachment along the Triassic gypsum horizon, which in some examples flows into the fold core (Jebel Hayyan). There is no evidence for a major basal detachment underlying the Palmyrides. All folding and faulting can be related to disharmonic folding of the Mesozoic sediments above a Triassic evaporite sequence and subsequent dextral transpression. The lack of overthrusting explains the absence of any foreland basin caused by lithospheric flexuring on the Rutbah platform. The major Palmyride deformation in Miocene-Pliocene times was concomitant with the opening of the Red Sea, with the closing of the Bitlis-Zagros suture zone in eastern Turkey and central Iran, and with the main sinistral strike-slip motion along the Dead Sea fault.