Composition and deformation patterns of the caprock on salt extrusions in southern Iran – Field study on the Karmostaj and Siah Taq diapirs

Abstract

Two iconic salt diapirs in the Zagros Mountains in Iran – Karmostaj (Gach) and Siah Taq – are regarded as world-class examples of salt extrusions and are frequently called "salt glaciers". However, our field survey revealed that their glacier-like parts are formed by thick, variegated and deformed caprock, only locally mixed with salt. Caprock is a layer of insoluble residuum that consists primarily of dissolution breccia and gypsum surrounding blocks of non-halite lithologies of the original evaporite sequence. Deformation within the caprock was accommodated primarily by shearing along gypsum-rich zones (gypsum mylonites) that surround subdomains of dissolution breccia and folded and fragmented blocks of carbonates and siliciclastics. Ductile flow in these mylonite shear zones was accommodated by pressure solution-precipitation creep of the lath-shaped gypsum grains. The ductile shearing of gypsum was locally accompanied by increased fluid-pore pressure driven fracturing of the surrounding lithologies. Since the subsurface shape of the salt diapirs is unknown, we present three hypothetical scenarios trying to explain the extrusion process of the studied diapirs and the associated deformation history of their caprocks. The diapiric structures represent either, 1) the remnants of degraded paleoglaciers, 2) advancing salt extrusions, or 3) only slightly reactivated, wide diapirs. To test these hypothetical scenarios, a geophysical survey is required to gain insights into the subsurface structures of the exposed diapirs and salt sheets.