Geology Of The Elsinore Fault Zone San Diego Region PDF Download

The Elsinore fault zone in eastern San Diego and western Imperial Counties, California is an active member of the San Andreas fault system which displays an abundance of tectonically produced features such as right-laterally offset and deflected streams, offset channel walls, shutter ridges, notches, benches, and fault scarps. The total amount of right-lateral strike separation across the fault could be as great as 8 km but is probably 4 or 5 km. Much of the motion is being taken up by drag folding south of the fault arid by drag folding and possible rotation north of the fault in the Sawtooth Ridge area. Abundant evidence for right-lateral strike-slip faulting is present along the Elsinore fault within the Peninsular Ranges and on the floor of the Imperial Valley. However, near the boundary between these two provinces, in the Carrizo Valley-Tierra Blanca Mountain area, evidence for horizontal motion is greatly reduced while there is abundant evidence for dip-slip faulting. The horizontal motion in this area may be masked by a large normal component, rotation of blocks and motion being distributed between more than one fault. The geomorphically most active stretch of the Elsinore fault within the study area lies along the southwest front of the Coyote Mountains on the floor of the Imperial Valley. Detailed studies here indicate repeated and frequent episodes of dominantly dextral strike-slip faulting during the Holocene. Offset alluvial channels, channel walls and bars suggest slip events of about 1-2 meters each with the last two events averaging about 1.4 meters each. Soils developed on faulted alluvium were assessed in terms of soil morphology and carbonate accumulation to provide temporal constraints on the ages of the surface. Virtually no soil development of the most recently faulted deposits indicate a major earthquake within the last several hundred years with several additional events recorded in other late Holocene deposits. A clear distinction is evident between soils formed before and after the change from woodland to desert scrub communities which occurred between about 8,000-12,000 yr BP in the deserts of the southwestern U.S. Thus, Holocene alluvial fan channel deposits which are offset 41 m along the southwest front of the Coyote Mountains yield a Holocene slip-rate of about 3.4-5.5 mm per year and a recurrence interval for large events of about 350 " 125 years.