Deep crustal structure and flexure of the Arabian Plate Beneath the Zagros collisional mountain belt as inferred from gravity observations

Abstract

The Zagros mountain belt region, a Neogene continental collisional zone between Arabia and Iran that is characterized by a suture, a folded belt, and a foreland basin, is unusual among active mountain belts in that its 6–12 km thick sedimentary cover is shortening predominantly by folding while the basement is apparently thickening along numerous high‐angle reverse faults. We have used over 9000 gravity measurements in the region in conjunction with limited seismological observations and geological cross sections of the uppermost 10 km of crust to infer the deeper crustal structure beneath the region. Crustal models constrained by the Bouguer anomalies and other seismological and geological data are characterized by a Moho that dips about 1° to the northeast beneath the folded belt and increases in dip to about 5° near the Main Zagros Thrust (MZT); the Moho depth increases from 40 km beneath the leading edge of the foreland basin (the Mesopotamian foredeep and Persian Gulf) to as much as 65 km beneath the MZT. Alternative models that incorporate a subducted oceanic crust that is attached to the underthrusting Arabian crust deviate little in their Moho configurations from the above simpler models. Negative isostatic residual anomalies are interpreted to indicate local overcompensation beneath the foreland basin and near the folded belt and beneath the MZT. We show that underthrust sedimentary rocks of the converging margins along the suture zone may account for the negative isostatic residual anomalies near the MZT. However, to explain the rest of the isostatic anomalies requires other forces to be acting on this collisional plate boundary. For example, elastic flexure models required a combination of topography and subsurface loads or downward force (the origin of which is not clear) to approximate the Bouguer or isostatic residual anomalies associated with the foreland basin and the folded belt. The topography load of the Zagros mountain belt is insufficient to cause the required deflection of the underthrusting Arabian plate. A combination of isostatic, elastic flexure, and horizontal compression forces acting on the edge of the Arabian craton and the transitional lithosphere of central Iran appears to best model the crust of the Zagros region. We propose a model in which the lower crust of the converging Arabian plate located beneath the Zagros is being shortened and thickened plastically between the seismogenic, rigid upper crust and a rigid uppermost mantle lid. The mantle lid, probably decoupled from the lowermost crust, un der thrusts the Iranian crustal blocks. Thus confined from below, the horizontally compressed, plastic lower crust hydraulically depresses the Moho and raises the faulted and folded, brittle upper crust in isostatic equilibrium.