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LATE CENOZOIC REACTIVATION OF BASEMENT FAULTS IN CENTRAL ALBORZ RANGE, IRAN

Ali Yassaghi

Abstract

The Alborz range, surrounding the south Caspian basin, forms a composite orogenic belt and has suffered shortening and uplift through several tectonic events from the Late Triassic Cimmerian orogeny to the present day stage of convergence between the Arabian and Eurasian Plates. The central Alborz, comprises main ly Tertiary rock formations, is an intensively deformed portion in which deformation is accommodated on major reverse faults. These faults are transversely crosscut by several NE- and NW- trending strike-slip faults. Surface and subs urface investigation in this study provides insight into the significant of these faults on structural evolution of the range throughout Tertiary. The left-lateral ki nematics of the NE-trending fa ults were interpreted using fault slip data and the curved geometry on axial traces of the range main folds as well as focal mechanism of the recent earthquake. The sinistrally offset of the basement contour lines in aeromagnetic map of the study area is considered to show basement nature of the faults, which are in acute angle of almost 15 degree to the major magnetic lineament. Therefore, they are interpreted to develop as synthetic Riedel shears on the cover sequences during reactivation of the basement fault. This reactivation is deduced to occur since Pleistocene when the south Caspian basin block rotated and attained a SSW movement relative to the central Iran. This rotation was in favor for reactivation of the NNE-trending basement faults resulted in development of several generation fractures on overlaying cover. Consequently, the basement transverse faults reactivation are also responsible for the recent deformation and are the main sources for recent earthquakes in the range in which their geometry and mechanism are differ from the range major reverse faults.

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