DEFORMATION OF LACUSTRINE SHORELINES AS A TOOL FOR UNDERSTANDING FAULT INTERACTION AND RELAY RAMP BREACHING, CATLOW VALLEY FAULT, SE OREGON

We attribute the warping of Quaternary lacustrine shorelines along two breached relay ramps to deformation associated with slip on fault segments within the Catlow Valley normal fault array in southeast Oregon. The purpose of this study is to investigate along strike differences in the location of the greatest amount of shoreline deformation along breached relays in this segmented fault system. We examined three well-preserved shorelines along two segments within the Catlow Valley fault system, a ~ 65 km long segmented normal fault located in the northwestern Basin and Range. The three shorelines (upper, middle and lower) were identified and mapped along the two segments using a combination of 1 m digital orthophotos, a 10 m digital elevation model (DEM), and field observations. The shorelines were mapped along a distance that includes two relay ramps that have been breached by linking faults. One relay is clearly at a more advanced stage of breaching than the other as indicated by relative throw on the linking structures. Results show that upwarping on the upper shoreline is spatially coincident with downwarping on the lower shoreline along most of the mapped reach. Deformation of the middle shoreline cannot be clearly associated with deformation on either of the other shorelines, which suggests it lies very close to the fault trace. The greatest vertical distance between the upper and lower shorelines occurs within 1 km of the site of fault segment linkage in both relays. At the less advanced stage of segment linkage, the largest vertical distance between the upper and lower shorelines is located on the abandoned fault tip within the relay ramp, whereas, at the advanced stage, it is located outside of the fault overlap. Additionally, one deformed terrace shows that one of the ramps has tilted about 5 degrees toward its base but shows no rotation towards either the collective footwall or hanging wall. These findings suggest that the relative maturity of the breaching structures controls the locations of maximum shoreline warping.