A SEISMIC-REFRACTION AND REFLECTION STUDY ACROSS THE CENTRAL SAN-JACINTO BASIN, SOUTHERN CALIFORNIA

The San Jacinto Basin is a northwest-trending, pull-apart basin in the San Jacinto fault zone of the San Andreas fault system in southern Ca lifornia. About 24 Bm long and 2 to 4 km wide, the basin sits on a gra ben bounded by two strands of the San Jacinto fault zone: the Claremon t Fault on the northeast and the Case Loma Fault on the southwest. We present a case study of shallow structure (less than 1 km) in the cent ral basin, A 2.75-km refraction line running from the northeast to sou thwest across the regional structural tread reveals a groundwater barr ier (Offset I), Another line, bent southward and continued for 1.65-km , shows a crystalline basement offset (Offset III) near an inferred tr ace of the Casa Loma Fault, Although a basement refractor was not obse rved along the 2.75-km Line, a mismatch between the estimate of its mi nimum depth and the basement depth determined for the 1.65-km line sug gests that an offset in the basement (greater than 260 m) exists aroun d the junction of the two refraction lines (Offset II). By revealing m ore faults and subtle sedimentary structures, the reflection stack sec tions confirm the two refraction offsets as faults. Offsets I and III each separate sediments of contrasting structures and, in addition, Of fset III disrupts an unconformity. However, the sense and amount of th e offset across Offset III contradict what may be expected across the Cast? Loma Fault, which has its basinward basement down-thrown to abou t 2.5 km in the better defined southeastern part of the graben. The Ca se Loma Fault trace has been mislinked in the existing geological maps and the trace should be remapped to Offset II where the reflector dis ruptions spread over a 400-m wide zone. Our Offset III is an unnamed, concealed fault.