Paleostress evolution of Paleozoic rocks in the Eastern Alborz (Shahroud-Aliabad Section)

Alborz, as a part of Himalaya-Alp range has formed by convergence between Iranian microplate and Eurasia since Late Triassic. The stratigraphy and tectonic setting of the Alborz Mountain is not same from west to east and so the range is divided to 3 parts, East, Central and west Alborz. Facieses studies on Alborz and Central Iran reveal a passive margin with epicontinental shelf deposits during Paleozoic Era. Several tectonic regimes are effected to the Alborz Mountain during its evolution, extension in Paleozoic and both extension and compression regime in Mesozoic and Cenozoic time. To reveal the tectonic regime and its orientation in Paleozoic time, those faults, which were active during sedimentation and not reactivated by later events are used for our paleostress analysis. The study area is located in East Alborz, between Shahroud and Aliabad, where Camberian, Ordovician, Devonian, Carboniferous and Permian rocks have good and large outcrops. The paleostress analysis were done by measurements of fault planes and slickenlines as well as fault kinematics, using FSA (Fault Slip Analysis) software. Primary results indicate almost N-S extension direction for all periods of Paleozoic Era. Rotation of the Gondwana continent from Precambrian period to carboniferous period, Alborz and Central Iran as a part of the continent, as well as rotation of the Hercynian blocks from time of separation of the Gondwana continent and move toward the Eurasia continent until the collision of time are calculated for the analysis of stress field for each periods. Based on amount of continent and block rotation, the primary data are restored to the first position, along the vertical axes. In such situation, investigates show that orientation of main tensile stress in the Cambrian period was EW. During Ordovician time changed to NW-SE. the main extension orientation in the Devonian time was N-S and for both Carboniferous and Permian periods a NE-SW direction is calculated for the extension regime.