Analogue models are used to study the mechanical relationship between basement strike-slip faults and salt diapirs. Displacement along a strike-slip fault in 1 g models resulted in extension along pre-existing jogs and the formation of oblique extensional faults, where reactive diapirs were triggered in some models. In the centrifuge models, prescribed cuts, simulating pre-existing structures, were reactivated during simple shear deformation of the models, resulting in formation of pull-apart basins, which were intruded by diapirs. The models show that because of the low ratio of salt to overburden thickness in the Zagros (0.15-0.35), it is unlikely that diapirs have formed solely, if at all, as a result of movement along basement strike-slip faults. Two mechanisms are suggested. First, pre-kinematic thin overburden and continuous movement along a releasing bend in the cover units may have triggered some of the diapirs in the Zagros, which were later downbuilt to their current geometry by additional sedimentation. Second, movement along the strike-slip faults (e.g. the Kazerun and Mangarak faults) induced oblique movement along NW-SE Zagros structures (folds or thrusts) resulting in the formation of pull-apart basins where diapirs were eventually intruded. Fault plane solution of shallow earthquakes supports the second scenario, which is also in agreement with previous interpretations that some of the salt diapirs associated with basement faults are younger than Zagros shortening and young southwestwards.
S U M M A R YThe Alborz Mountains are forming a ∼100-km-wide east-west trending orogenic belt that stretches 2000 m across northern Iran south of the Caspian Sea. The Alborz Mountains consist of salt-bearing Neogene sediments, which are folded and cut by faults. Global positioning system studies indicate N-S directed shortening across the Alborz range, which is accommodated by right and left-lateral strike-slip along ESE-WNW and ENE-WSW trending faults, respectively. A 20 km × 10 km × 03 km sheet of salt extruded over the central plateau of Iran arising at the front of the advancing Alborz Mountains. The extruded salt forms the Eyvanekey plateau between Eyvanekey and Garmsar, which is now known, as the Garmsar Salt Nappe.To get more insights in the evolution of the Garmsar Salt Nappe, analogue modelling has been carried out using PDMS as salt analogue and sand as analogue for the brittle overburden. The structures produced consist of folds and thrusts, which were formed while the salt analogue PDMS was rising up. The modelling results are compatible with our interpretation that the deformation front of the Alborz Mountains advanced SSW when overriding a salt sequence in the Garmsar area.Depth estimations using the gravity and magnetic fields suggest that the salt in the Garmsar Salt Nappe extruded from a depth less than 2000 m.
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