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Topographic Forcing of Active Salt Structures in Canyonlands, Utah: Insights From InSAR Mapping and Mechanical Modeling

Abstract

The Needles District of Canyonlands, Utah contains coupled extensional faults, salt diapirs and a sinuous anticline that all grow in response to gravitational stresses driven by erosion along the Colorado River and its tributaries. Interferometric Synthetic Aperture Radar (InSAR) and 3D numerical modeling are used to map strain in the region and define how it relates to salt structures and patterns of surface deformation. InSAR results from ERS satellite scenes taken between 1992 and 2000 show line-of-sight (LOS) deformation rates of ~1–3 mm/yr both in the grabens and further south, where extension is accommodated by the Imperial Valley fault. These rates are similar to that measured by a creepmeter installed across the Imperial Valley. Analysis of Envisat InSAR data from 2006–2010 shows higher LOS displacement rates in the grabens of up to 5 mm/yr and LOS rates of ~1–3 mm/yr in the area to the southwest. Envisat data could thus be marking an increase in displacement rates over time. However, these results contain uncertainty because they indicate higher rates of subsidence in tributaries of the Colorado River Canyon, where uplift should dominate. 3D numerical models produced using FLAC3D are used to test how plastic flow of evaporites and brittle extension of overburden are coupled during deformation. Topographic models of the region were made using a 50 m resolution digital elevation dataset. Topography was overlaid onto a strain softening, mohr coulomb rheology to represent the 400 m thick sequence of strata that overlie the salt. A viscous, flat lying salt layer with a thickness of 340 m comprised the model beneath the overburden. The models were run for 2,000 years to test how topography currently drives deformation. Results show the sensitivity of strain to topography in the region. In particular, displacement is focused to the northwest, directly toward the river canyon in the grabens region. However, in the area around the Imperial Valley fault, the geometry of the river canyon appears to direct strain to both the north and west, which may be related to the change in strike in faults in this area. Additionally, salt deformation in the models show overburden gliding as the dominant deformation mechanism, not salt flow, consistent with previous studies. Comparison between the strain rate of the models and InSAR suggests they are similar, further demonstrating that overburden gliding can accommodate the observed strain.