Kinematics of a sigmoidal fold and vertical axis rotation in the east of the Zagros–Makran syntaxis (southern Iran): Paleomagnetic, magnetic fabric and microtectonic approaches

Abstract

The Zagros Simple Fold Belt is characterized by elongated, curved, or sigmoidal folds. The trend of these structures together with the structural style, change suddenly across the Zagros–Makran syntaxis which separates the continental collision domain of Zagros from the oceanic subduction one in Makran. This work focuses on the Minab anticline, outcropping in the easternmost part of Zagros. In order to understand the kinematics of a sigmoidal fold and underscore possible vertical axis rotations in the eastern side of the syntaxis, we performed a joint study of magnetic fabric, microtectonics and paleomagnetism of the northern termination of this fold. The two limbs have been sampled (7 sites, 134 samples) along three cross-sections corresponding to three different orientations of the fold axis. The rocks are weakly deformed fine-grained Mio-Pliocene reddish siltstones. The shortening directions deduced from both magnetic fabric analysis and microtectonic observations are consistent with each other, they are horizontal and roughly perpendicular to the local fold axis, following the torsion of the fold hinge line, and indicating a tectonic origin of the magnetic fabric. Rockmagnetic analyses (thermomagnetic curves, hysteresis loops) point to the presence of magnetite in the PSD and MD ranges as the main magnetic carriers, together with a minor contribution from hematite. Apart from a post-tilting sub-actual VRM and/or CRM (component A), paleomagnetic analyses yield mainly two pre-tilting magnetization components: Component B is carried by magnetite, spanning the intermediate to high unblocking temperature range (300 °C ≤ T ubs ≤ 580 °C). Component C has unblocking temperatures characteristic of hematite (580 °C ≤ T ubs ≤ 680 °C). Both are ante-folding, based on positive reversal and fold tests, inside each of the cross-section but also for the three sections together. However, because component C is biased by some inclination flattening, only component B is taken into account afterwards. After full tectonic correction, the site mean direction of component B calculated for 6 out of the 7 sites is not statistically different from the individual site mean directions. The intersection direction determined by small circle analysis in these six sites is: D = 18.0° I = 33.7° k = 482 alpha 95 = 2.6°. Compared to the Mio-Pliocene direction expected in this area for Africa, it corresponds to an overall clockwise rotation of the whole structure of some 18.3° ± 5.1°, consistent with the regional geodynamic context. The absence of differential paleomagnetic rotation from one cross-section to the other, and the fact that the magnetic lineation together with the shortening direction deduced from microtectonic analysis follows the fold curvature demonstrate that the torsion of this fold is not secondary but rather coeval with the fold amplification. The torsion of the fold would stem from underlying inherited Mesozoic structures and the rheological response, during the deformation, of two incompetent décollement layers undergoing differential velocities and a change of thickness, in a constant stress field direction.