Architecture of a low-angle normal fault zone, southern Basin and Range (SE California)

Exposures of the denuded Cenozoic detachment fault system in the southern Sacramento Mountains (SE California) delimit the architecture of a regional low-angle normal fault, and highlight the evolution of these enigmatic faults. The fault was initiated ~23 Ma in quartzo-feldspathic basement gneiss and granitoids at a low-angle (<20 degrees); isostatic accommodation due to unloading and doming of the footwall continued until ~16 Ma, leading to initiation of a secondary breakaway accommodating up to 5km of additional ENE-directed slip. Minimum-relief structure contours define the fault as a continuous low-angle structure with both slip-parallel (NE) and slip-normal (NW) corrugations. Fault dip of the secondary breakaway is ~7° based on the contour map, and 10-15° measured at outcrop, flattening to <2° down dip. Slip-parallel corrugations plunge NE with wavelengths between 600m and >2km, and amplitudes up to 100m. These corrugations are continuous along their hinges for up to 3.6 km. Damage zone fracture intensity varies both laterally, and perpendicular to the fault plane (over an area of 25km2), decreasing with depth in the footwall, and varies as a function of lithology and proximity to corrugation walls. Deformation is concentrated into narrow damage zones (<4m) where gouge is developed. In contrast, thick damage zones (>100m) are found in areas where low-fracture intensity horses are corralled by sub-horizontal zones of cataclasite (up to 8m) and thick zones of epidote (up to 20cm) and silica-rich alteration (up to 1m). Sub-vertical shear and extension fractures, and sub-horizontal shear fractures/zones dominate the NE side of the core complex. In all cases, sub-vertical fractures verge into or are truncated by low-angle fractures that dominate the top of the damage zone. These low-angle fractures have an antithetic dip to the detachment fault plane. Some sub-vertical fractures become curviplanar close to the fault, where they are folded into parallelism with the sub-horizontal fault surface in the direction of transport. These field data, corroborated by ongoing microstructural analyses, indicate fault activity at a low angle accommodated by a variety of deformation mechanisms dependent on lithology, timing, fluid flow, and fault morphology.