DEFORMATION IN THE DAMAGE ZONE OF THE HURRICANE FAULT AND THE CONTROL OF THERMAL WATER DISCHARGE INTO THE VIRGIN RIVER, UTAH, AS REVEALED BY HIGH-RESOLUTION SEISMIC SURVEYS

The Hurricane fault is a major structure that separates the Basin and Range Province from the Colorado Plateau transition zone. The fault is active and generated a M5.8 earthquake as recently as 1992. In Timpoweap Canyon, Utah, the Virgin River has cut a ~200 m deep canyon into the footwall of the fault, providing spectacular exposures of fracture systems and minor-offset faults above stream level in limestone members of the Toroweap Formation.

Associated with the footwall damage zone of the fault are discharges of 0.6 to 0.7 m³/s of ~42°C Na-Cl thermal water. δ¹⁸O values of thermal waters clearly indicated oxygen exchange with the wall rock, suggesting circulation depths of >3-5 km, and indicating open fractures exist well below the thickness of Phanerozoic sedimentary cover.

In order to tie thermal water discharges to subsurface structure, we have obtained high-resolution P-wave and horizontally polarized S-wave reflection profiles to image the fault architecture (in general) as well as small-offset subsurface faults. The resolution and data quality of the S-wave profile is high, yielding considerable detail of the subsurface structure in the footwall.

The stream has been logged and contoured in terms of pH, T, conductivity, and Cl^- abundances in order to document inflow locations and fluxes. Major thermal inflows are spatially correlated with offset reflectors in S-wave profiles. Ongoing photogrammetry is designed to synthesize the distribution of open fractures and small-offset faults in the canyon wall with discharge locations and subsurface faults. This will lead to an enhanced understanding of the distribution of fracture permeability within the damage zones of normal faults.