A CONCEPTUAL MODEL OF FLUID CIRCULATION IN A NORMAL FAULT SYSTEM

We investigated near-surface fluid circulation flow paths in the step-over region of an active, en echelon normal fault using in-situ measurements of ground temperature. Our investigations were conducted at the Borax Lake geothermal area in the Alvord Basin of southeast Oregon, where approximately 175 geothermal springs outline the trace of a fault system striking approximately N15ºE.

We measured ground temperatures at a 5 m resolution over a 100×100 m area in the step-over region of the Borax Lake fault. The data show elevated temperatures due to upwelling of geothermal fluids that clearly delineate the subsurface locations of the two fault splays. Temperature profiles perpendicular to the plane of the fault show a steep rise crossing a fault trace from west to east, and more slowly declining temperatures to the east. We attribute this asymmetric profile to geothermal discharge rising along high-permeability pathways in the damage zones on the eastern sides of the fault cores. Subsurface flow is blocked to the west by the low-permeability core; therefore, discharge slowly cools and moves eastwards away from the fault through the shallow subsurface. Discharge in the area between the two splays appears to be blocked by the down-gradient fault core, and may move downwards and northwards along the damage zone flanking the eastern fault splay.

Previous investigators have characterized faults as being either high-permeability conduits, barriers to flow, or some combination of conduit and barrier depending on the distribution and extent of development of the fault core and damage zones. Our data suggest that the Borax Lake fault acts as a barrier to flow perpendicular to the plane of the fault, and as a conduit for flow parallel to the fault plane. These findings are consistent with prior investigations, but provide a more detailed picture of fluid circulation in an active fault zone than has previously been available.