AN IMAGE LOG-BASED GEOMETRICAL AND TEXTURAL ANALYSIS OF A LOW-ANGLE NORMAL FAULT SYSTEM BENEATH THE F.O.R.G.E. SITE NEAR THE MINERAL MOUNTAINS, UTAH

Field-based studies of exhumed faults have provided valuable insight into the structural geometry and rock textures of fault systems, yet few studies have examined a vertical, subsurface profile through a low-angle normal fault (LANF) system. Seismic reflection data from near the Department of Energy’s Frontier Observatory for Research in Geothermal Energy (FORGE) site to the west of the Mineral Mountains, UT, reveal a low-angle, W-dipping reflector underlying the basin fill. This surface is thought to represent a LANF, but its exact structural significance is still unclear. A Formation Microimager was logged in a vertical, 8.75 inch borehole over 2240-7550 ft in the Roosevelt Hot Springs Well 58-32 within the FORGE site, providing an “outcrop view” of the basin alluvium, igneous basement rock fabrics, and the textures and structures of this LANF system.

Sub-horizontal basin strata at the top of the logged interval (2240 ft) increase in dip with depth to a mean orientation (dip/azimuth) of 15/297 (n = 47) over 2700-3150 ft. Rare faults and fractures in the basin dip 45-55° toward the E and W. At the basin-basement contact at 3176 ft is a strongly conductive feature of 0.5 ft in thickness, oriented 26/250. The only substantive low-angle, W-dipping planes are associated with cataclastic textures over 3176-3186 ft and 3212-3222 ft with mean orientations of 13/300 (n = 16) and 26/268 (n = 8), respectively. Basement fabrics define an elongated, W-E dome with no significant preferred orientation that would indicate top to the W shearing. In the basement (3176-7550 ft), faults have mean orientations of 51/270 (n = 35), 47/074 (n = 8), and 76/176 (n = 4). Cataclastic textures are prevalent over 3176-3410 ft, and are only locally developed deeper in the basement within 2-12 ft of fault planes. Conductive fractures have a range of orientations, but they dominantly dip W with a mean orientation of 39/276 (n = 1260), or are sub-parallel to faults with mean orientations of 74/183 (n = 220), 67/133 (n = 110), and 38/084 (n = 180).

Results indicate that the shallowly W-dipping seismic reflector encountered by the borehole is a LANF that primarily accommodated deformation via brittle deformation mechanisms. No obvious shear or mylonitic textures are obvious from the image log, or are too fine-grained to observe.