Fig. 2

Conceptual cross section of fault-controlled geothermal upwelling in the western USA (modified from Richards and Blackwell 2002). Geothermal upwelling from depth occurs along steeply dipping faults, but the location(s) of surficial geothermal features (e.g., fumaroles, hot springs) or shallow temperature anomalies may not necessarily indicate the location of the fault(s) that host upwelling. (1) Fluids upwell directly to the surface and effuse along the trace of the fault and/or (2) intersect a shallow aquifer, masking the thermal/geochemical signature, and flow out distal to the trace of the upwelling fault. (3) Upwelling fluids are capped by impermeable sedimentary layers in the subsurface, leaving no expression of geothermal fluids at the surface at all, (4) or upwelling fluids charge a permeable stratigraphic reservoir in the subsurface