AN INTEGRATED PALEOMAGNETIC, ROCK MAGNETISM, ANISOTROPY OF MAGNETIC SUSCEPTIBILITY, STRATIGRAPHIC AND STRUCTURAL STUDY OF MIOCENE SYNOROGENIC RED BEDS FROM NW IRAN: INSIGHT INTO THE GEOLOGIC EVOLUTION OF THE INTERMONTANE TAROM BASIN AT THE LINKAGE BETWEEN THE WESTERN ALBORZ MOUNTAINS AND THE NORTHERN MARGIN OF THE IRANIAN PLATEAU

Abstract

Sedimentary basins represent first-order geological archives that can provide information on the evolution of the sediment source regions, the growth and the erosional history of adjacent topography, as well as the evolution of the depositional settings and the paleoenvironmental conditions. Sedimentary basin can be found in diverse geodynamic settings including continental collision zones, where they can host thick piles of sediments deposited over time scales of 105 to 107 years. This comprises foreland areas, orogenic plateaus interiors and intermontane areas at the transition between the plateau and its foreland, or within mountain belts. In this study, I focus on the intermontane Tarom Basin at the transition between the Iranian Plateau (IP) and Alborz Mountains. This basin was filled with synorogenic red beds that allow investigating puzzling aspects of this region such as the timing and mechanisms of basin development, and the growth of the adjacent plateau margin and Alborz Mountains. To tackle this issue, I performed an integrated paleomagnetic, rock magnetic, anisotropy of magnetic susceptibility (AMS), geochronologic, magnetostratigraphic, structural, sedimentologic, and sediment provenance analysis on three major sedimentary sections exposed in the Tarom Basin. This approach allowed documenting for the first time the termination of arc volcanism in this region at 38-36 Ma, an age for the synorogenic red beds of ~16.5 to 7.6 Ma, the occurrence of an early stage of Late Eocene deformation followed by higher magnitude of deformation at least from ~16.5 Ma, a complex evolution of the drainage system with several episodes of efficient and reduced to absent fluvial connectivity with the Caspian Sea, changes in the depositional setting and in the sediment source area or in the sediment routing system, the occurrence of post 7.6 Ma intrabasinal deformation event which had a localized impact in the magnetic fabric of sediments and finally, the recognition of localized Middle-Late Miocene orogen perpendicular extension, which was most likely caused by gravitational instability of the basin margin rather than regional lithospheric stretching. Overall, my data indicate that the northern basin margin accommodated a greater magnitude of deformation and exhumation than the southern one, which experienced limited deformation and erosional exhumation and hence must have been uplifted via deep-seated processes. Finally, my data suggest that the Tarom Basin cannot have been an integral part of the IP because my results does not indicate the presence of a formerly elevated basin likewise in the plateau interior