Late Miocene Deformation Kinematics Along the NW Zagros Fold-Thrust Belt, Kurdistan Region of Iraq: Constraints From Apatite (U-Th)/He Thermochronometry and Balanced Cross Sections

摘要

Apatite (U-Th)/He (AHe) thermochronometric results are integrated with geologic cross sections, structural relationships, and stratigraphic data to reconstruct the growth of the NW Zagros orogenic belt in the Kurdistan region of Iraq. Prolonged exhumation is documented across the belt with deformation advances and retreats from similar to 14 Ma onward. After in-sequence propagation of deformation during middle to late Miocene times, preserved growth strata and AHe data show a deformation retreat by latest Miocene time (similar to 5 Ma). In the NW Zagros, the Phanerozoic succession contains two principal decollements in Lower Triassic and middle Miocene units. The Triassic strata are interpreted as the main decollement for a thin-skinned system that was dominant during most of the Cenozoic. By similar to 8-5 Ma, the fold-thrust belt shifted to basement-involved deformation in association with growth of the mountain front flexure and reactivation of frontal structures. The shift from thin-skinned to a hybrid thin- and thick-skinned mode of shortening may reflect variations in the mechanical behavior of the upper crust and the presence of inherited basement discontinuities. On the basis of two NE-SW balanced cross sections spanning the NW Zagros, the estimated total minimum horizontal shortening is similar to 18.2 km (6%) in the central and similar to 16 km (7%) in the southern sectors of the Kurdistan region of Iraq. These findings suggest that the evolution of the NW Zagros orogenic belt was likely driven by the mechanical stratigraphy of the sedimentary cover, inherited basement discontinuities, and the dynamic and thermomechanical effects of potential slab breakoff and lithospheric mantle delamination events.