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Thermal Fluid and Fault Interactions at the Intersection of Two Faults, Agua Caliente, California
SCEC Fall Meeting 2011 (2011)
  • Rebekah Wood, Utah State University
  • James Evans, Utah State University
Agua Caliente Springs lies at a unique intersection between the NNW-trending Elsinore fault and the 40° northeast-dipping, likely inactive West Salton detachment fault; it provides an opportunity to study damage zone geometry, fault behavior in crystalline rocks, a left-stepover, microseismicity, and the influence of thermal fluids on rock deformation. The Elsinore fault bounds the northwestern flank of the Tierra Blanca Mountains with strike-slip and normal motion; the detachment fault wraps around the northernmost portion of the mountains. Damage along the Elsinore ranges in thickness from a narrow slip plane to > 100 m along the eastern flank of the Tierra Blanca Mountains. Subsidiary faults trend northeast and southeast, and slip orientations vary from normal to strike-slip horizontal motion. Thermal fluids (~30°C) emerge at the intersection of the West Salton detachment and Elsinore faults actively alter the 94 Ma La Posta tonalite pluton, already fractured and crushed during fault slip, to a fine-grained white to orange powder through mineral re-equilibration. Grain sizes decrease with closer proximity to the faults. Fault cores contain thin dark green zones of chlorite ± epidote, and fault surfaces are coated with a thin layer of the same. Origin of the mineralization may be from reworked biotite crystals. We present water chemistry data from the hot springs at Agua Caliente in conjunction with geochemical and petrographic analysis of the surrounding rock. Water analyses include cation and anion measurements, bicarbonate, stable isotopes, tritium, and a multi-month recording of spring conductivity, water level, and temperature fluctuations. Cation geothermometry shows the fluids are enriched in Na, Ca, Mg, K, and Si from broken down quartz, plagioclase, and orthoclase. Water level and temperature data are compared to seismicity during the logging interval; temperatures so far have diurnal fluctuations indicating air temperature plays a larger role than anticipated for the subsurface fluids. Conductivity also displays daily cycles. We propose a larger scale map of the intersection of the two faults and the continuation of the Elsinore farther south showing the current extent and probable growth of the damage and alteration as more slip occurs. Spring flow increases post seismic events, and we believe by monitoring fluid chemistry and comparing seismicity along the faults we will see precursors to and effects from fault motion.
Publication Date
Summer September 12, 2011
Citation Information
Rebekah Wood and James Evans. "Thermal Fluid and Fault Interactions at the Intersection of Two Faults, Agua Caliente, California" SCEC Fall Meeting 2011 (2011)
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