Comparison of Geomorphic Expression of the Garlock Fault with the Structural Expression of the Fault in a Trench

RM 211

To better understand the relationship between the shallow subsurface structure of the Garlock fault and its expression at the surface, I conducted geological and geomorphological investigations at a paleoseismic trench and an alluvial fan surface 240 m to the east of the trench along a linear segment of the fault. The age of the fan surface is comparable to the age of the sediments exposed 0.5 – 1.0 m below the surface in the trench. Aerial images of the fault on the alluvial fan and trench exposures of the subsurface both show a linear depression (graben) along the southern edge of the fault zone and a linear ridge (horst) along the northern edge of the fault zone. Grabens are only present locally along the fault and likely have formed at step-overs between en echelon strands of the fault. The presence of left-stepping en echelon fault strands along this left-lateral fault explains both the localized presence of grabens and the fact that fault strands within the trench are oriented 8-13 degrees more easterly than the fault zone as a whole. The data from this study suggests that geomorphic characteristics of faulting at the surface are typically expressed and controlled by the fundamental behavior of individual fault strands at depth within the active fault zone of the Garlock fault.