STRUCTURAL AND GEOCHRONOLOGICAL EVIDENCE FOR OLIGO-MIOCENE INTRA-ARC LOW-ANGLE DETACHMENT FAULTING IN THE TAKAB- ZANJAN AREA, NW IRAN

The late Mesozoic to middle Cenozoic geodynamic evolution of Iran was dominated by northward subduction of the Neo-tethys and associated voluminous arc magmatism (e.g., Urumieh-Dokhtar and Alborz belts) and intra-arc tectonism. Several conflicting models have been proposed with respect to the number and nature of magmatic arcs, the relationship between arcs, and in particular the nature and timing of Tertiary intra-arc extension. However, the lack of geochronological and structural constraints on magmatism and intra-arc tectonism has made it difficult to critically evaluate any geodynamic models. This study presents new structural, U-Pb geochronological, and (U-Th)/He thermochronological data from basement rocks in the Takab-Zanjan area of northwestern Iran. The area is structurally dominated by NW-trending top-to-the-SW thrust faults related to late Cenozoic contraction within the Zagros-Alborz collision zone. Zircon U-Pb dating of selected granites within thrust sheets shows that the area was characterized by both Pan-African plutonism (~560 Ma) and voluminous Eocene arc magmatism (~53-35 Ma). The timing of thrust sheet exhumation is constrained by 3-4 Ma apatite (U-Th)/He ages and Plio-Pleistocene syn-contractional, coarse-clastic growth strata. Near Maneshan, an ENE-trending top-to-the-NW extensional low-angle normal fault is exposed juxtaposing a mylonitized ~25 Ma granite in the footwall against ~560 Ma granite gneisses and metamorphosed Precambrian (?) marbles and micaschists. Apatite (U-Th)/He data (~20-22 Ma) from the footwall suggest rapid extensional exhumation shortly after intrusion. The hanging wall also contains a thick sequence of Oligo-Miocene red beds, evaporites, and carbonates of the Lower Red, Qom, and Upper Red formations. Therefore, the timing of extensional faulting indicates that at least a portion of these continental to marginal marine sediments are syn-extensional in origin. These new structural and geochronological constraints provide strong evidence for the occurrence of Oligo-Miocene intra-arc extension accompanied by syn-extensional magmatism and sedimentation. Given the similarity of structural trends, it is likely that many extensional structures in north-central Iran have been obscured by omnipresent late Cenozoic contractional structures.