Controls on fault geometry during early stages of extension in the Larkspur Hills, northwest Basin and Range
Detailed analyses of normal faults in the Larkspur Hills, CA-NV, northwest Basin and Range, offer insight into factors controlling normal fault initiation, growth, and distribution. N-trending faults in the southern portion of the study area share trends of major range-bounding structures and Pliocene linear volcanic vents; in contrast, NNW- and NNE- trending faults dominate further north and into south-central Oregon. Stress analyses and comparison with experimental and field data suggest that preexisting structures control faults in the northern Larkspur Hills, while faults form perpendicular to σ3 in the southern hills. The change in fault orientations is abrupt, occurring across a major NNE fault. A regional transition is thus captured within the Larkspur Hills, suggesting they overlie a structural boundary at depth that separates isotropic crust from crust with a pre-existing NW-trending fabric. This has implications for better understanding of local and regional structural controls on subsurface hydrothermal flow paths.
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