Effects of Hurricane Fault Architecture on Groundwater Flow in the Timpoweap Canyon of Southwestern, Utah Effects of Hurricane Fault Architecture on Groundwater Flow in the Timpoweap Canyon of Southwestern, Utah

by Dutson, Sarah J

Hydrogeologically important features of fault zones include undamaged country rock, the damage zone, and the core zone. Fault cores generally have low porosity and permeability, and often act as a barrier to groundwater flow. The damage zone, by contrast, consists of small faults and fracture networks, which can act as conduits. Timpoweap Canyon near Hurricane, Utah has superb exposures of the fault core and damage zone of the Hurricane Fault. Also within the canyon, springs discharge from the damage zone into the Virgin River, providing an ideal natural laboratory for the study of groundwater discharge from a fault zone.

The Hurricane fault is an active, steeply dipping, normal fault that is 250 km long, and exhibits about 2500 m of displacement. The damage zone in Timpoweap Canyon controls thermal groundwater (~40°C) and CO2 gas discharge from highly fractured limestone. Total spring discharge is 260 L/s. Approximately 4 L/s of CO2 gas also discharges with the springs. The ä2H and ä16O composition of the springs exhibits a geothermal shift from the global meteoric waterline. This suggests that the circulation depth is about 3 km below the ground surface (bgs) in basement bedrock. The CO2 gas discharging originates from either magmatic sources or from diagenesis. The fracture density in a typical damage zone decreases with increasing distance from the fault, thus spring discharge should also decrease with increasing distance from the fault. The damage zone in Timpoweap Canyon does not follow this pattern because pre-existing fractures that developed from Laramide and Sevier Orogeny stresses suppress the pattern. Collapse structures from gypsum dissolution and large fractures also control the location of spring discharge.