THE LOCATION AND RELATIVE ACTIVITY OF ELSINORE FAULT ZONE SPLAYS, SOUTH SHORE OF LAKE ELSINORE, RIVERSIDE COUNTY, CALIFORNIA

At Lake Elsinore (Riverside County), the Elsinore Fault Zone (EFZ) forms a ~2-km wide, right-oblique, transtensional, pull-apart tectonic basin bordered by the active (Holocene) Glen Ivy North and Glen Ivy South faults on the north and the Willard and Wildomar faults on the south. Immediately south of Lake Elsinore, the structural relationships and relative activity of these faults have heretofore been poorly constrained owing to a lack of geomorphic expression and to a ~10-m thick cover of late Pleistocene and Holocene lacustrine and fluvial (San Jacinto River) distal fan and deltaic deposits. Now, however, interpretations of data from new 20 to 30-m deep cone penetrometer test soundings and continuous borings, from seismic refraction logs, from soil-stratigraphic documentation of unbroken paleosols and other stratigraphic markers exposed in up to 12-m deep trenches, and from several internally consistent radiocarbon dates and related rates of fine-grained sedimentation, we determine that last displacement of pull-apart faults in a subsurface, ~60-m wide zone, occurred about 33 to 39 ka ago. The subsurface faults are right stepping and decrease in displacement to the south. We interpret these faults as the bifurcating and "dying out" southern extension of the Glen Ivy North fault, which has demonstrable Holocene offset some 18 km to the north. Accordingly, most neotectonic slip on the south side of Lake Elsinore is now likely taken up by the Wildomar fault zone, expressed geomorphically by the nearby transpressional horst of Rome Hill and by escarpments along the east side of the Elsinore Trough at Murrieta and Temecula. Accordingly, previous southward projections and inferred Holocene activity of the Glen Ivy faults on the east and south side of Lake Elsinore now appear to be unfounded.