LINKS BETWEEN SURFACE WATER RUNOFF AND ACTIVE SUBSIDENCE OF PARADOX EVAPORITES IN THE NEEDLES DISTRICT, CANYONLANDS NATIONAL PARK

Active extension of brittle strata in the Needles District of Canyonlands National Park is driven by plastic flow of underlying evaporite deposits along a gentle gradient towards the Colorado River Canyon. Previous studies in the area have defined how extensional fault arrays at the surface evolve with increasing strain and how plastic flow accommodates the internal flow of evaporite deposits. Other work has defined the reorganization of stream channel networks in response to surface faulting (i.e. elongate horsts and grabens). We present initial analysis that involves mapping locations of surface water input into actively dilating faults, and compare this to the modern strain field defined by InSAR data. Our goal is to correlate locations of more rapid dissolution of actively flowing Paradox evaporites with surface water runoff and rates of strain in overlying brittle strata. Comparison of small-scale variation in the active strain field (from InSAR) with drainage networks suggests the following. Small patches of higher subsidence (a rate of 1-2mm/yr) generally correlate with high-order trunk streams (i.e. streams that drain large catchment areas) where they cross faults. In addition, patches of higher subsidence correlate well with lower-order (i.e. smaller) streams that drain in individual grabens. To a lesser extent, streams that terminate at the base of fault scarps where surface runoff is piped directly into the underlying evaporites correlate with ~ 30% of InSAR patches of higher subsidence. These correlations suggest that surface water runoff increases rates of surface subsidence by 100-200% in localized areas at the scale of individual faults. The distribution of patches of increased rates of subsidence implies that variation in strain is distributed unevenly throughout the Needles, although more patches are present in the central part of the region where average strain rates are higher. While our work suggests that infiltration of surface water runoff coincides with some areas of increased subsidence, patterns of strain over larger areas (~ 20-30% of the entire fault array) are likely driven by contributions of other processes. These factors may include variations in groundwater influx and flow throughout the entire array, or varying strength of overlying brittle sandstone.