Evolution of a structural basin: Numerical modelling applied to the Dehdasht Basin, Central Zagros, Iran

Abstract

The Dehdasht Basin, a small structural basin located in the southeast of the Dezful Embayment in the Zagros fold-and-thrust belt, has a complex tectonic structure characterized by both compressional and halokinetic features. 2D numerical models are used to test how geometrical and rheological parameters affected the Miocene-Pliocene evolution of this deep basin. The analysed parameters include rates of syntectonic sedimentation and erosion, thickness and viscosity of the lower detachment (Hormuz salt) and of the upper detachment (Gachsaran evaporites) developing diapiric salt walls, salt extrusions and minibasins-growth synclines that characterize the internal structure of the Dehdasht Basin. Assuming reasonable dimensions and rheologies (0.5 km Hormuz basal detachment with moderate viscosity of 1019 Pa·s, and Gachsaran upper detachment with a minimum original thickness of 1.5 km and viscosity between 5 · 1018 and 1019 Pa·s), our models reveals that an almost intermediate ratio between the rates of surface processes and deformation well approximate the geological and geophysical observations. A local decrease in the thickness of the Hormuz salt below the Dehdasht Basin with respect to surrounding regions was of great importance for its structural evolution. We suggest that the large volume of Gachsaran evaporites presently filling the basin was partly due to their gravitational flow from the emerging surrounding anticlines into the basin. The numerical experiments also demonstrate that in a compressional setting, shortening is the main factor for the rapid initial growth of the diapirs, although, with increasing syntectonic sedimentation the effect of shortening diminishes.