EFFECTS OF HURRICANE FAULT ARCHITECTURE ON GROUNDWATER FLOW IN TIMPOWEAP CANYON OF SOUTHWESTERN, UTAH

Hydrologically 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 for groundwater flow. 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 flow in a fault zone. The Hurricane fault is an active, steeply dipping, normal fault which is 250 km long. It is a major intraplate boundary separating the Basin and Range and the Colorado Plateau Provinces. The Hurricane Fault exhibits about 1000 m of displacement, with a damage zone ~ 120 m wide where thermal groundwater (~40°C) and CO₂ gas discharge from the highly fractured limestone. The springs discharge at 0.25 m³/sec and the number of CO₂ gas vents along the bed of the Virgin River range from 0 to 38 vents per linear meter. Fracture densities from field measurements and photo mosaics range from 0.05 to 2.5 m/m², with a mean of 1.3 m/m², with fracture density increasing toward the fault, although the greatest number of gas vents and spring discharge occurs in an area ~300 m from the fault, with an average fracture density of ~1.7 ± 0.3 m/m². In the immediate vicinity of the fault, the number of gas vents increases yet spring discharge is low. Another zone of high spring discharge occurs approximately 400 meters from the fault and although there is a low fracture density there are several large fractures and high fracture connectivity. Although fracture density has some influence on the spring discharge, fracture connectivity combined with large through-going fractures and solutions weathering of fractures control the spring discharge.