IMAGING THE SUBSURFACE GEOMETRY OF THE SIERRA NEVADA FRONTAL FAULT IN THE OWENS VALLEY: NORMAL/STRIKE-SLIP FAULT INTERACTIONS UNDER TRANSTENSION

Improved seismic instrumentation and interpretation, combined with intensive field investigations, have recently been applied to interpreting major transtensive ruptures such as the El Mayor-Cucapah earthquake of 2010. The results from this apparently immature fault system demonstrate strong coupling of high-angle, predominantly strike-slip faults with low-angle, predominantly normal ones, resulting in complex patterns of oblique slip.

The faults of the Sierra Cucapah form a relatively narrow zone with short faults (20 to 30 km) that interlink at changes of fault-zone strike. To what extent are the types of coupling observed in this setting carried over into more mature transtensional settings with longer, straighter faults, greater distances between faults, and much larger fault displacements? Do strike-slip and normal/oblique faults evolve into quasi-independent structures that are laterally separated, or do they remain coupled by sub-horizontal structures even at considerable distance?

We have attempted to address that question by inferring the subsurface geometries of normal and strike-slip faults along the Sierra Nevada (California) front between the Garlock Fault and Long Valley. The Southern Walker Lane (alternatively, Eastern California Shear Zone) has undergone ~100 km of transtensive spreading since the middle Miocene. Surface mapping with geometrical analysis, relocated earthquake aftershock hypocenters, gravity inversions of basement depth and other data were used to infer fault geometries. Outcrop dips of normal faults range from 75° to 30°. Faults are visibly listric in outcrop exposure. In the subsurface, some faults are clearly listric, but larger-displacement faults tend to be more planar, possibly due to footwall rebound. Normal faults in some cases clearly carry oblique slip underneath paired near-vertical strike-slip faults, showing that the strike-slip faults are subsidiary.

These observations support a model in which the complex interlinkages exhibited by the small-scale El Mayor-Cucapah fault zone are preserved with continued transtension.