Structural development of a major late Cenozoic basin and transpressional belt in central Iran: The Central Basin in the Qom-Saveh area

The Central Basin of the Iran Plateau is between the geologically better-known regions of the Zagros and Alborz Mountains. Hydrocarbon exploration in the Central Basin has revealed the details of the late Eocene–Holocene evolution of the basin based on seismic reflection data, geological field work, basin modeling, and satellite interpretation. The multistage basin history commenced with broad sag-type subsidence and isolated normal faults during Oligocene–early Miocene time. It evolved to an extensional or transtensional basin in the early-middle Miocene, with as much as 4–5 km of Upper Red Formation section being deposited in some parts of the basin during this stage. The upper part of the Upper Red Formation is associated with a change to transpressional deformation, with the development of thrusts and folds. This latest (probably middle and/or late Miocene–Holocene) deformation is transpressional, and includes a mixture of basement-involved strike-slip and thrust faults and thin-skinned folding and thrusting detached on Oligocene evaporites. Local detachment levels higher in the stratigraphy also exist. Subsidence in mini-foredeep basins and strike-slip fault bounded basins occurred during this stage, and several kilometers of Upper Red Formation were deposited in the main depocenters. Northwest-southeast– to north-northwest–south-southeast– striking dextral strike-slip to compressional faults dominate the area, with subordinate east-west and north-south fault orientations also present. These different fault sets combine in places to form major strike-slip duplex geometries. The Eocene volcanic belt (Urumieh-Dokhtar zone) along the southern margin of the basin forms a chain of massifs as much as 3 km high, the outcrops of which were exhumed by movement along major thrusts from 5–6 km depth between the middle Miocene and present day. The Central Basin–Urumieh-Dokhtar zone forms a distinctive transpressional belt that underwent a minimum of 38 km shortening between the late Miocene and Holocene. The Central Basin and the Zagros and Alborz Mountains all indicate that the onset of widespread crustal shortening in Iran occurred late (latest early Miocene or later), relative to the initial collision of the Arabia Peninsula with Eurasia during the late Eocene or early Oligocene. Uplift of the Central Basin surface from approximately sea level to 900–1000 m occurred during the middle or late Miocene, after deposition of the marine Qom Formation.