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Description
The left-lateral Santa Rosa Island fault cuts east-west across Santa Rosa Island and is the western end of the east-west striking Transverse Ranges frontal fault system that forms the 200 km-long southern boundary of the Transverse Ranges. To the east, faults within this zone include the active Santa Cruz Island fault and the active Malibu Coast-Santa Monica-Hollywood fault system that trends onshore beneath metropolitan Los Angeles. Subsurface investigation of the Santa Rosa Island fault in Cañada Tecolote and on the eastern side of the island reveal faulted late Pleistocene and Holocene stratigraphy. In the Cañada Tecolote exposure faults break over 4 m past a clay bed dated at 9.7 ka and nearly 1 m past cultural material, to within less than 1 m of the ground surface. The exposure on the eastern side of the island, near Box Canyon, reveals faults breaking late Pleistocene eolian sand to within less than 0.5 m of an overlying Indian midden, where they offset the base of a buried A-horizon, the uppermost part of which is dated at 1.16 ka. All of the major streams and drainage basins on Santa Rosa Island are left-laterally deflected where they cross the fault, as are most of the minor drainages. There is a systematic increase in the amount of deflection with increasing drainage size, indicating that the deflections are a result of lateral slip along the fault. Several of the largest drainages that have been offset by the fault are incised into a 170-230 m-high marine terrace on the north side of the island. An uplift rate of ~0.17 mm/yr has been established for a lower terrace in this area. Using this rate, the higher terrace is estimated to be about 1.2 ± 0.2 Ma in age. Cañada Tecolote and Arlington Canyon are offset about 1-1.3 km, indicating a long-term left-lateral slip rate of about 1 mm/yr. These new data are important in understanding the role of the Santa Rosa Island and related faults in partitioning slip along the largely compressive western Transverse Ranges. Further, the newly determined Holocene activity, long-term slip rate, and estimated recurrence interval of 5 ka for the fault indicate a strong connection to the Santa Cruz Island fault and the Malibu Coast-Santa Monica-Hollywood fault system. This suggests that these faults may rupture together, indicating the possibility of a magnitude 7.7 earthquake for this frontal fault system.
