The effects of plate margin inhomogeneity on the deformation pattern within west-Central Zagros Fold-and-Thrust Belt

- In this contribution, we benefited from gravity, topography, GPS, seismic, structural and kinematics data to constrain: (1) the Dezful behaves as an indenter, (2) there is a gradation in the tectonic regime from thrust in foreland to the strike-slip in hinterland; (3) inhomogeneous plate margin rigidity can change the deformation pattern and tectonic regime by change of obliquity and escape of the upper crust to the salients as free mechanical boundary and, in this case, plate convergence direction may not be the first-order constraint; (4) ductile shear in viscous lower crust facilitates the strain partitioning and escape; (5) indentation caused the deformation to be concentrated in a HZB as out-of-sequence thrust faulting in addition to propagating toward the foreland; 6) the MFF is a diachronous fault figuring out the Zagros orocline that its activities changed in time and space; 7) thin- and thick-skinned tectonics are consistent but they do not follow a constant configuration: in recesses the deformation is followed by thin-skinned tectonics in foreland and thick-skinned in hinterland but in salients both are active in foreland.

Zagros Folded and thrust Belt (ZFTB) is a Cenozoic mountain belt in SSW of Iran and northeast of Arabia that resulted from closure of Neo-Tethys and collision between Arabia passive margin and Central Iran active margin. At this tectonic grain of Alp-Himalayan chain, change of the tectonic regime, degree of basement involvement, and character of accommodation of the deformation, in time and space, has been under debate. The results show that the deformation along the Zagros is not uniformly accommodated. Although the researchers have already paid attention to basement involvement in addition to the sedimentary cover rocks, little has been shown on the role of the basement's rigidity and lower crust's rheology on the deformation pattern along and across the belt. This study tries to address these questions in terms of inversion of slip vector of basement earthquakes, measurement of fault-slip data on the cover rocks, geomorphic and field observations, and existing geodetic and gravity data, with especial focus on Dezful embayment and surrounding area. The results show the Dezful embayment plays an indenter role. It, with the other reentrants, changes the obliquity of Arabian plate convergence, facilitates the escape of the upper crust toward free mechanical boundaries in salients, controls the slip-rate of Zagros Main Recent Fault at the rear of the belt and contributes in forming Zagros Fold-and-Thrust Belt as an orocline. The viscous lower crust helps the strain partitioning by providing a ductile shear base under the rigid upper crust.