Caprock matters: Surface deformation patterns of salt diapirs using high-resolution Persistent Scatterer Interferometry and geological field observations, Karmostaj and Siah Taq salt diapirs, Zagros mountains, southern Iran

Abstract

Salt diapirs are prominent features in the Zagros fold-and-thrust belt displaying complex interplay between the buoyant forces driving the rock salt to the surface, distribution of the caprock and erosion. The caprock represents the solid residue of salt dissolution in the apical part of the diapir. Understanding the dynamics of subaerial spreading of salt in salt glaciers therefore requires knowledge about the spatiotemporal surface deformation and the underlying controlling factors. However, the contemporary vertical surface-deformation pattern across salt diapirs is difficult to detect and interpret along disciplinary boundaries. Therefore, our goal is to analyse the active surface-deformation patterns of diapir-caprock systems in the Zagros, where diapirs and their caprocks are well exposed and accessible for field mapping. We primarily integrate high-resolution Persistent Scatterer Interferometry (PSI) and field mapping. We used Persistent Scatterer Interferometry (PSI) to obtain the highest available spatiotemporal resolution (on the range of mm/yr, ~12-day repeat cycle) of the vertical surface-deformation pattern across the Karmostaj and Siah Taq salt diapirs for which previous knowledge from detailed geological field mapping is available. Both diapirs contain a thick and deformed caprock layer on top or in the surroundings of the salt cupolas and are located 20 km south from the Lar city in southern Iran. We analysed the PSI-data using Sentinel-1 images acquired between October 2014 and December 2018 using the German Aerospace Center’s (DLR) Integrated Wide Area Processor (IWAP). First, the time-series analysis of the deformation signal in the line-of-sight is investigated for seasonal effects correlations, such as precipitation and heat. Second, the line-of-sight signal is split into vertical and horizontal components. In the next step, geological observations from field mapping provide the context to interpret the geodetic data. Preliminary results indicate that the deformation identified from PSI signal decreases outwards from the apical part for salt diapirs with thick caprock, as the caprocks and other residuals get relatively thicker above the underlying viscous salt. We postulate that the extra load the caprock body exerts on the underlying ductile salt drives subsidence in the crestal portion of the diapir. Understanding the spatiotemporal deformation pattern helps to recognize the impact of dissolution/erosion mechanisms and the distribution of caprock on salt diapirs. Therefore, careful analysis of geodetic signals, which requires contextual integration with geological field observations, reveals the influence of caprock on salt movement.