A Ahmadi-Torkmani; M.R Ghassemi
Abstract
The present research uses precise field data to provide a balanced cross-section of the Mahneshan area, and investigate nature of depth distribution of its major structures. Our structural studies indicate that the Mahneshan and Anguran faults are two major faults, which penetrate deep into the crust ...
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The present research uses precise field data to provide a balanced cross-section of the Mahneshan area, and investigate nature of depth distribution of its major structures. Our structural studies indicate that the Mahneshan and Anguran faults are two major faults, which penetrate deep into the crust and cause a considerable amount of horizontal shortening in the area. In a more specific way, the Anguran fault roots deep into the middle crustal levels of about 21 km, and thrusts the whole Phanerozoic sequence and even parts of the Precambrian basement rocks over the younger strata. We believe that the abovementioned thrust originates not from a low-competency decollement plane, but from a ductile shear zone in deep crust. Evidences for development and conditions of such shear zone are present in the Precambrian basement rocks of the Anguran fault’s hanging-wall. We suggest that the decollement surface for the Mahneshan thrust, which is located in the shallower depths (13 km), is related to probable occurrence of evaporitic materials equivalent to the Hormoz Series beneath the Kahar Formation. Syn-sedimentary deformation within the Qom Formation in the hanging-wall of the Anguran thrust, as well as other evidences present in Neogene deposits of the area suggest that the thrust fault has been active since Oligocene. Restoration of displacements across the Anguran fault, and comparing the results with inception age for the fault suggests that the Anguran fault has been active with a slip rate of about 1 mm/yr. The structural features in the Mahneshan area indicate that thick-skinned faulting along with thin-skinned tectonics have resulted in a considerable amount of thickening of the crust in the region; this observation is in accordance with abovementioned characteristic of the crust in the Sanandaj-Sirjan zone.
M. Rahgoshay; I. Monsef; H. Shafaii Moghadam; M. Mohajjel
Abstract
Petrofabric, structural and geochemical study of the ultramafic tectonites in the Khoy ophiolitic complex suggest that these tectonites including low-temperature NW-SE shear zones cut the high-temperature to medium-temperature NE-SW mantle flow direction. Microstructures in these tectonites, record a ...
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Petrofabric, structural and geochemical study of the ultramafic tectonites in the Khoy ophiolitic complex suggest that these tectonites including low-temperature NW-SE shear zones cut the high-temperature to medium-temperature NE-SW mantle flow direction. Microstructures in these tectonites, record a fabric transition from oblate porphyroclastic and oblong porphyroclastic textures (related to the high- and medium-T deformations in mantle sections) to mylonitic textures (with low-T deformation in the shear zones). The study of olivine LPO patterns in high- and medium-T deformation samples of mantle shows slip on the (010) [100] high-temperature-low stress and (0kl) [100] moderate-temperature systems (up to 1000o) while olivine LPO patterns in the low-T deformation samples within the shear zones indicate gliding along (001) [100] low-temperature slip system (800-900o). Spinels in these peridotites show high variations in Cr number (10 to 90) and Mg number (50 to 90). Cpxs rich in Cr suggest a low degree of partial melting in these peridotites. The very variable composition of the spinels may be the result of partial melting process and recrystallization of these minerals in the mantle lithosphere during the detachment phase and the development of the shear zones.
