INVESTIGATING A BREACHED FAULT RELAY RAMP WITH SHALLOW SEISMIC AND TEMPERATURE DATA

We conducted high-resolution shallow seismic refraction and ground temperature surveys in the vicinity of a relay ramp on a left-stepping en echelon normal fault at the Borax Lake Hot Springs area of the Alvord Basin, southeast Oregon. Elevated shallow ground temperatures near the lower end of the fault relay ramp are consistent with the discharge of sub-boiling hydrothermal fluids, and appear to indicate increased permeability from fractures associated with a lower-ramp breach. In contrast, the seismic survey indicates a complex distribution of subsurface velocities ranging from 0.3 to approximately 3.2 km/s to depths of about 20 m. The observed velocity structure may reflect a complex assembly of interleaved hydrothermal precipitate (sinter), alluvium, and playa deposits. The extent to which subsurface constructional features may influence hydrothermal discharge in the vicinity of the relay ramp is unclear, and more investigation is needed to establish the relationship between the near-surface temperature distribution, the ramp breach, and the geometry of the subsurface materials.