CONTROLS ON FAULT GEOMETRY DURING EARLY STAGES OF EXTENSION IN THE LARKSPUR HILLS, NORTHWEST BASIN AND RANGE

As extensional provinces mature, the earliest stages recording normal fault growth and development are either buried or lost to erosion. Where early stages are preserved, detailed analyses of fault geometries offer insight into factors controlling their distribution. The Larkspur Hills, at the NW margin of the Basin and Range, is such a location. Here, extension decreases northward from 4-9% to ≤ 2%, thus faults are in various early stages of development, offering an ideal place to examine fault initiation, growth, and relationship with coeval (3-8 My) basalt flows. Understanding surface faults is critical to determine subsurface geometries, and could be used to predicatively model interactions of buried basin faults that influence geothermal fluid flow.

To analyze fault geometry, we grouped faults in the Larkspur Hills into domains bordered by two major (>100 m offset) NNE-trending faults. All domains have minor offset faults, but differ as follows: 1) A few segments trend NNW, and one major fault trends N; 2) numerous, close-spaced, faults trend N; tilt and offset decrease eastward; 3) short faults; orientation, dip, and tilt vary; bounded by major W, E, and SSE dipping faults; 4) numerous NNW close-spaced faults, offset and tilt decrease to the NNW; 5) few faults, orientation and dip direction vary, wide un-faulted areas, and little to no block tilting; 6) only two small NNW fault segments evident, bordered by a major N trend fault offsetting basalt and bedrock.

The NNE faults are not ideally oriented for slip with current E-W extension, so may represent basement structures exploiting fractures in the basalt cover; strain may have localized on these pre-existing weaknesses in otherwise strong bedrock in domains 1 and 6. By contrast, 2 – 4 may have variable underlying units; located along a basin margin, they likely contain sedimentary layers. The combination of these weak layers and the fractured basalts may result in greater fault density, which may stem from large basement structures in domains 2 – 3 that form an accommodation zone. All domains have faults trending NNW, so may reflect a pre-existing regional fabric that dominates 4 due to fewer proximal major structures. Domain 5 hosts the least amount of extension. Overall, the strongest influence on fault geometries seems to be pre-existing structures.