Cenozoic tectonic evolution and paleomagnetic rotation in Iran

Abstract

To provide insights on the late Mesozoic-Cenozoic paleogeographic and tectonic evolution of Iran, I present new paleomagnetic, anisotropy of magnetic susceptibility (AMS) and structural data from the Upper Jurassic Bidou and Garedu Formation of Central Iran and the Miocene Upper Red Formation (URF) of Central Iran and Alborz Mountain, which we used in conjunction with published paleomagnetic data. Palemagnetic results demonstrate two main episodes of counter-clockwise (CCW) vertical axis rotations that occurred in Central Iran in the Late Jurassic–Early Cretaceous and after the Middle– Late Miocene. Paleomagnetic inclination values indicate that, during the Late Jurassic, the Central-East-Iranian Microcontinent (CEIM), consisting of the Yazd, Tabas, and Lut continental blocks, was located at low latitudes close to the Eurasian margin and the CEIM was oriented WSW–ENE, with the Lut Block bordered to the south by the Neo-Tethys Ocean and to the southeast by the Neo-Sistan oceanic seaway. Subsequently, the CEIM underwent the first significant counter-clockwise (CCW) rotation during the Early Cretaceous with an average amount of ~30°. This rotation may have resulted from the northward propagation of the Sistan rifting-spreading axis during Late Jurassic–Early Cretaceous, or to the subsequent (late Early Cretaceous?) eastward subduction and closure of the Sistan oceanic seaway underneath the continental margin of the Afghan Block. The second episode of counter-clockwise (CCW) vertical axis rotations in Central Iran, occurred after the Middle–Late Miocene with an average amount of ~20°. Paleomagnetic results show that crustal blocks bounded by sets of N-S right lateral strike slip faults are rotated CCW to accommodate NNE-SSW shortening related the convergence between Arabia-Eurasia during the Cenozoic. The paleomagnetic results of URF sediments in the Northern sector of Arabia-Eurasia collision zone (Alborz Mountain) document clockwise, counter-clockwise and clockwise rotations along vertical axis for the western, central and eastern arms of the Alborz thrust-fault system, respectively. My results suggest orocline bending mechanism for the origin of the curved Alborz Mountains, which acquired most of its curvature in the last 8 Myr. Most likely, the bending processes caused by relative motion between the stable and rigid blocks (Caspian Sea and Central Iran) within Arabia-Eurasia collision zone. In the Southern part of Lut Block (Central Iran), opposite vertical axis rotations have been recorded in the Middle-Upper Miocene units cropping out along the Northern and Southern parts of Shahdad thrust-fold system which may due to an orocline bending mechanism acquired most of its curvature after deposition of Upper Red Formation in the Late Miocene. I used the integration of AMS and structural data for understanding the continental deformation in collision zone of Arabia-Eurasia. At the tectonic boundary between the Lut and the Tabas blocks of Central Iran, the results of Garedu Red Beds, outcropping in the core of a NNE–SSW oriented syncline in the northern Shotori Range, confirm that the Garedu Red Beds syncline is a transected fold which formed as a consequence of right-lateral transpressional tectonics. It may have related to the shortening phase which caused the closure of the small oceanic basins around Central Iran, as a result of the motion of the Arabian Plate and the closure of the Neotethys oceanic basin. My results in the Ferdows Thrust-fold system (Lut block), show two sets of the magnetic fabrics. In most cases magnetic fabric, acquired when the bedding were still horizontal, is related to different degrees of LPS shortening and in some cases magnetic fabric has been developed as a consequence of later shortening related to the activity of the thrust system responsible of the recent earthquakes in the area. These results appears in agreement with the overall tectonic history of the Ferdows thrust–fold system which is characterized by two distinct phases of fold growing, marked by a sharp angular unconformity between the middle-upper Miocene URF and the overlying alluvial and fluvial Quaternary deposits.