HR: 1340h
AN: T23B-0544    [Abstracts]
TI: Surface and subsurface hydrothermal flow pathways at Norris Geyser Basin, Yellowstone National Park
AU: * Graham Wall, B R
EM: bgrahamwall@usgs.gov
AB: During summer 2003 at Yellowstone's Norris Geyser Basin notable changes were observed in the discharge of heat and steam, creating new thermal features, dying vegetation, and the consequent closure of trails to protect public safety. In order to interpret data collected from GPS, seismic, and temperature instruments deployed in response to the increased hydrothermal activity, a study has been undertaken to provide a more complete knowledge of the spatial distribution of subsurface fluid conduits. Geologic data, including mapped outcrops, aerial imagery, thermal infrared imagery, and subsurface core, indicate that fracture pathways in the Lava Creek Tuff (LCT) channel flow in the hydrothermal system. These data show clear evidence that NE-SW and NW-SE trending structures provide major flow pathways at Norris. By mapping fracture sets in outcrops of LCT with varied degrees of hydrothermal alteration, one can consistently identify fractures that localize hydrothermal fluid flow, alteration, and the geometry of surface thermal features. Alteration is characterized by acid leaching that quickly alters LCT mafic minerals and glassy groundmass, which in outcrop is recognized by corroded and disaggregated LCT with local secondary mineral deposition. Mapping the sequence from unaltered to altered LCT has identified vertical cooling joints as primary conduits for hydrothermal fluids. These vertical joints correlate with the NE-SW trending geomorphic expression of the LCT in this area, and parallel the adjacent caldera boundary. Horizontal fractures parallel depositional stratigraphy, and in core from drill holes Y-9 (248 m) and Y-12 (332 m) appear to initiate at collapsed vapor-phase cavities or regions of altered fiamme. Vertical fractures in the core show sequences of hydrothermal minerals locally derived from water-rock interaction that line fracture walls, characteristic of mineral deposition associated with repeat reactivation. Although the hydrothermal system is dynamic, potential flow pathways are embedded in the system. Variations in hydrothermal fluid distribution, due to changes in groundwater- or earthquake-related fluid pulses, have the potential to redistribute flow pathways through sealing or reactivation along the pre-existing fracture matrix. Throughgoing vertical fractures appear to control regions where new hydrothermal activity develops, such as at the NE-SW trending structural lineament `The Gap' which became more active in the early 1990's. This investigation provides a geologic framework for further study of variations in hydrothermal activity at Norris Geyser Basin.
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8486 Field relationships (1090, 3690)
DE: 8488 Volcanic hazards and risks
SC: Tectonophysics [T]
MN: Fall Meeting 2005