Thermal Spring System Plumbing across a Major Normal Fault: Pah Tempe, Utah, USA

Abstract

Abstract Faults, including their cores and associated fracture zones, can play an important role in controlling the flow of ground water. Pah Tempe Hot Springs at Timpoweap Canyon in southwestern Utah provides excellent exposures of the damage zone of a major normal fault and associated groundwater discharge. This includes numerous point discharges of ~40°C Na-Cl water into the Virgin River along a ~500 m stretch of the footwall damage zone of the Hurricane fault, part of the greater Wasatch-Hurricane fault system. An impermeable fault core and fine-grained rocks in the hanging wall impede cross-fault flow and force thermal waters to rise to the surface. The spectacular canyon-wall exposures permit an investigation of a fractured bedrock vadose zone as an analogue to subjacent aquifers. Photogrammetry (visual and infrared) enabled the mapping of discrete inflow locations, as well as permeable discontinuities in the canyon walls. Geochemistry, including reaction-path models, defined plausible chemical evolution pathways of thermal waters, whereas high-resolution geophysical surveys provided information on subsurface disruption and the depth to bedrock beneath Virgin River channel alluvium. The synthesis of these approaches indicates that permeable pathways in fractured carbonate rocks exhibit heterogeneous patterns. Some rock volumes contain abundant permeable fractures, bedding-plane partings, and solution cavities. Yet, within such volumes, one fracture may be permeable, whereas another only a few cm away may not. On a large scale (tens of meters), entire volumes of bedrock may be impermeable. Geophysical studies can shed light on gross fracture patterns in the bedrock aquifer but are incapable of showing which features accommodate flow. Patterns of permeability in the thick vadose zone may be the best predictor of subjacent flow paths in the aquifer. Thermal waters represent the leakage of deep brines or the mixing of Virgin River underflow with deep brines. Carbonate rocks exhibit self-organization where stochastic distribution of fracture apertures channel flow into wider features that channel additional flow. In turn, these features self-widen through solution weathering. By contrast, siliciclastic and basement rocks are not nearly as prone to dissolution. Thus, fluxes are partitioned into north-south fractures that will tend to have aperture in this east-west extending region. The results of this study help improve the conceptualization of flow systems near fault zones, and the methods employed may be a template applicable to the study of other systems.