FRACTURE CONTROL OF HYDROTHERMAL FEATURES ALONG THE NORRIS-MAMMOTH CORRIDOR, YELLOWSTONE NATIONAL PARK

During Fall 2002, acquisition of 3-5 micron, night-time, airborne thermal infrared (TIR) imagery with high spatial resolution advanced the Park's mapping and monitoring of active thermal features along the Norris-Mammoth Corridor including Norris Geyser Basin. Norris Geyser Basin is a dynamic geyser basin with thermal disturbances affecting active thermal features along natural fractures. Existing thermal features align with fractures and newly-formed thermal features also develop along natural fractures.

The calibrated, TIR mosaic of the Norris-Mammoth Corridor from Norris Geyser Basin to Roaring Mountain clearly shows the major N, NE, and NW fractures controlling the flow of hydrothermal fluids. These major trends influence the orientation of streams and valleys along the Norris-Mammoth Corridor and within Norris Geyser Basin. Tantalus Creek, the major creek draining Norris Geyser Basin, follows N and NW-trending fractures and appears to be offset at prominent zones of NE-trending fractures. After the summer 2003 thermal disturbance, new thermal features formed along a fracture. Curious circular areas of hydrothermal fluid flow also are apparent on the thermal infrared mosaic within Norris Geyser Basin.

Within the Norris area, excavations for Yellowstone's infrastructure allowed an examination of natural fractures within the Lava Creek B Tuff. At these excavations, orientation of the natural fractures document near vertical fractures with the following trends: N to NNE (11-20^o, 30^o), NE (50^o, 55^o), WNW (255^o, 290^o), NW (330^o) and NNW (345^o). Within fractures, instruments measured dangerous levels of gases (CO, C0₂ and H₂S). Bleached white rock or yellow-brown to orange-brown colors on the surface of the natural fractures indicated vertical and near horizontal movement of hydrothermal fluids. Sub-parallel, NE-trending (50^o) fractures were the longest fractures exposed in the excavations and were spaced about 5-10 cm apart in zones of intense fracturing. In summary, major N, NE (50^o) and NW (330^o) natural fractures control the flow of hydrothermal fluids observable at various scales, over a wide geographic area, at the outcrop, on the landscape, and on airborne TIR imagery.