Quantifying salt extrusion rates in active emergent salt diapirs from Persistent Scatterer Interferometry data and salt budget estimates: Finu Diapir in Zagros (Iran) as a case study

In emergent salt diapirs, the volume of the salt at the surface represents an equilibrium state between the salt supplied from the underlying orifice and that lost by dissolution and erosion at the surface. Therefore, the salt volume at the surface represents the surplus of the salt supply over the dissolution and erosion neglecting any volume changes from the decompaction. Both dissolution and erosion processes occur at or near surface and can be estimated, while estimating salt supply remains challenging because it occurs at depth. At the surface, the salt moves away from the central dome above the diapir’s orifice toward the flanks by gravity spreading in the direction of the slope.

In this work, the salt volume change at the surface was estimated using Persistent Scatterer Interferometry (PSI) data to estimate the salt supply into the surface. While the salt tends to move in the direction of slopes, maximum deformation signal can be detected from the PSI data where the slope direction aligns with the line-of-sight (LOS) of satellite in the east- and west-facing slopes of salt diapirs. The salt deformation was estimated from both the east-west and up-down components of the decomposed PSI data along an east-west trending swath profile across the symmetrical Finu salt diapir to obtain salt gain (surplus) along this profile. During the PSI processing, areas with surface erosion and dissolution (sinkholes) were mostly filtered out, and the relevant salt loss was already accounted when analyzing the salt deformation.  However, the down (subsidence) signal of the analyzed PSI data in nearly horizontal areas in the flanks of the Finu diapir, where salt movement is expected to be horizontal, is assumed to be associated with the subsurface salt dissolution. The used PSI data cover a period of four years from October 2014 to December 2018.

Our results show that salt area surplus is c. 14 m² a^-1 along the profile crossing the Finu diapir. With the profile length of 4.6 km, the surplus rates along it are c. 3.1 mm a^-1. The average subsidence rate on the diapirs flat flanks, which is supposed to be associated with the subsurface dissolution, was estimated at c. 1.0 mm a^-1. Thus, the total supply rate along the profile is c. 4.1 mm a^-1. Along the profile length, these rates mean that a total section area of c. 19 m² a^-1 of salt is added to the profile. Considering the semi-circular and symmetrical shape of the diapir and assuming uniform salt supply rates, the total volume of the salt delivered to the surface is in the order of c. 70,000 m³ a^-1 from the underlying orifice. The orifice is approximately circular with a diameter of c. 1.7 km and an aperture covering c. 2.3 km² area. This means the salt is extruded at a rate of c. 30 mm a^-1 averaged over the period of four years.