The origin of changes in structural style across the Bandar Abbas syntaxis, SE Zagros (Iran)

Abstract

The Bandar Abbas syntaxis marks the transition between the Zagros collisional belt and the Makran subduction-related prism and constitutes a major structural feature within the Alpine–Himalayan mountain belt. Within the foreland part of the Zagros (the Zagros Fold Thrust Belt, ZFTB), the transition is marked by a sudden change in structural trends and style and a rapid decrease in seismicity and salt diapirism from west to east of the syntaxis. The depth to decollement and lithology affect the geometry and size of the folds on each side of the syntaxis, while different mechanical coupling along the basal decollement affects the degree of internal rock deformation. Thus the short and broad folds of the western side of the syntaxis are decoupled along the 8-km deep Hormuz evaporites, visible at surface in numerous salt diapirs. Strong seismicity in this area suggests that the basement is involved in the deformation. By contrast, the long and narrow folds of the eastern side of the syntaxis appear to be controlled by frictional slip along a 6-km deep detachment. The absence of seismicity suggests that the basement is not involved in the deformation in this area. We propose that the basal decollement for these structures corresponds to the eroded top of the NNE–SSW trending Oman Mesozoic nappes resulting from Late Cretaceous obduction of Tethys on the Arabian margin. These structures are visible onshore in the northern tip of the Oman peninsula and seen in offshore seismic profiles continuing northward beneath the Strait of Hormuz. Finally, map view observations show that the Main Zagros Thrust and the Zendan Fault—separating the ZFTB from the internal zones of the orogen—are shallow low angle thrust faults rather than steeply dipping faults.