Signatures of Deformation Related to the Plio-Quaternary State of Stress in the Mahneshan- Mianeh Basin (NW Iran)

Since the last two decades, the inversion of fault kinematics combined with geodetic and geophysical data has led to the better understanding of the geodynamics of the Arabia – Eurasia collision, as well as the tectonic history of Iran. These researches indicate that dynamics of ongoing deformation in the most parts of the Iranian plateau and surrounding deformation belts is controlled by the NNE- to NE-trending Arabia – Eurasia convergence. This general pattern of deformation has been affected by local- and regional-scale geodynamic elements (e.g., the rigid South Caspian Basin, Persian and Anatolian extruding blocks) producing complex stress and strain fields in distinct tectonic zones such as northwestern Iran and the region surrounding the South Caspian Basin. Accordingly, two drastically different stress fields prevail in either sides of the North Tabriz fault such that the NW-trending horizontal compression in the northern part changes into the NE direction in the southern regions; the type and boundary of this change remain unknown. Every kinematic and structural investigation in NW Iran would help to determine the boundary condition and characteristics of this significant geodynamic complexity. The Mahneshan–Mianeh Cenozoic basin, located at the transition of the two aforementioned tectonic provinces, is a key area providing crucial data for this geodynamic issue. This area, which is also called “Folded Miocene Belt”, is a NW-trending fault-bounded narrow basin developed in the hinterland of the NW-trending Zagros orogen. The Late Miocene detritic sedimentary sequence of the Upper Red Formation constitutes the dominant outcrops of the basin. The sequence is unconformably overlaid by Late Miocene conglomerate to Pliocene-Quaternary deposits. Main geological structures of the basin are NW-trending detachment folds evolved in the fault-related folds in hanging wall of shallow reverse faults. This structural assemblage has been produced and evolved by different stress states during the Late-Miocene compressional tectonic regime.
The original data presented in this paper are based on the remote-sensing analysis of satellite imageries and structural field surveys including the study of folding stages, folding interference patterns and faulting trends. These are complemented by the measurement of folds geometry and fault kinematics data collected in 25 sites throughout the area of interest. The inversion of fault kinematics data allowed us to investigate the post-Miocene tectonic regimes affecting the area. The sorting and separation of the kinematics data has been done considering relative age relationships of fault planes and their striations (e.g. using cross-cutting and superposition relationships). FCALC software is used for the inversion analysis, with especial attentions to the rules of data separation, rotation and geological considerations recommended in fault inversion processes.
The inversion of fault slip data related to the youngest tectonic regime leads to the determination of the present-day state of stress characterized by a NE-trending maximum horizontal compression. The penultimate Pliocene – Quaternary state of stress (paleostress field) was investigated after the removing of kinematic signatures of the youngest tectonic regime. The Pliocene – Quaternary paleostress state is characterized by a compressional tectonic regime with ~N138E direction of maximum horizontal compression, which is compatible with the results of previous researches in surrounding areas.
The study of surface folding patterns and geometry of folds indicates superposition of the Late-Miocene NW-trending fold set by a younger sub-vertical NE-trending fold set. This second folding stage is especially developed in northern and central parts of the basin, in Pliocene – Quaternary geological units. Superposition of these folding geometries results in the formation of basin and dome and boomerang interference patterns, which are clearly visible in satellite imageries. The penultimate Pliocene – Quaternary stress field (NW-trending compression) is responsible for the formation of the last NE-trending folding stage. This paleostress field was changed into a modern compressional stress state characterized by a NE-trending regional compression. The modern stress state of the Mahneshan – Mianeh basin is well consistent with the overall direction of the Arabia – Eurasia convergence and clearly discriminates the ongoing kinematic characteristics of the area of interest from the region to the north of the North Tabriz fault. This puts the boundary of these distinct tectonic domains to the vicinity of the North Tabriz fault.