The structure and activity of the offshore Newport-Inglewood - Rose Canyon zone of deformation

The offshore Newport-Inglewood- Rose Canyon zone of deformation (NI-RC) is a 106 km-long, linear zone of strike-slip and thrust faults, and folds that extends from Newport Beach to La Jolla. It can be separated into the Newport-Inglewood zone (NI) and the Rose Canyon zone (RC) at overlapping, left-stepping fault splays north of Encinitas. The NI-RC is a flower structure that becomes positive between San Onofre and Oceanside where a major basement anomaly and left-step in the NIRC occur. The San Onofre-Oceanside fault merges with the NI-RC trend near San Onofre and then strikes SSE beyond shelf-break. Since its inception during early-middle Miocene time there has been at least 4.25 km of right-lateral displacement along the NI-RC and up to 1 km of vertical movement on the basement surface. With only three left-/right-stepping fault-trace breaks over 1 km in width it is suggested that the offshore NI-RC is an immature strike-slip feature. Using high resolution analog profiles, digitally processed seismic reflection data and seismicity, two distinct fault segments of the offshore NI-RC zone are defined. These two segments control the position and trend of shelf-break along the inner margin of the continental borderland. 1) Dana Point segment (Laguna Beach to southeast of San Onofre, 40 km). A right-stepping zone with activity decreasing southward to San Onofre, where the latest activity was during the middle Holocene. The southern terminus of this segment is a 2 km-wide, left-stepping break southeast of San Onofre. 2) Oceanside segment (southeast of San Onofre to La Jolla, 66 km). A major, 2 km-wide, left-stepping break marks the northern terminus of this segment near San Onofre. A small microearthquake cluster is located east of the left-step. A 40� asperity at La Jolla marks the southern terminus. North of Encinitas, overlapping, left-stepping fault splays that average 3.5 km width, separate the NI that extends northward, from the RC that extends southward. The landward splay (NI) forms a compressional ridge that deflected ancient streams - this documents Late Quaternary activity. A major "blind" thrust ramp has been mapped from San Mateo Point to Oceanside. It is associated with a 2 km left-step in the NI-RC, a 20 km-long synclinal fold subparallel to the NI-RC and a microseismic earthquake cluster shoreward of the NI-RC. Extending upward from the thrust ramp into the overlying Neogene sequence, a series of fault -propagation folds and thrusts indicate ongoing compressional tectonics. These folds and thrusts are associated with 100 meters of surficial bowing of the continental slope. Vertical slip rates of these thrusts are 0.08- 0.5 mm/yr for the Pliocene to Quaternary time interval and 0.01 mm/yr for the Quaternary. The thrust ramp may be the southern extension of the TorranceWilmington fold and thrust belt of Hauksson (1990). It is suggested that compressional deformation along the NI-RC is a direct result of changes in North American versus Pacific plate motions that occurred between 4-2 Ma. Deformation was focused at apparent basement discontinuity near the left-stepping break that separates the Dana Point and Oceanside segments. A detached block or flake resulted. Focal mechanism studies and seismic reflection profile analyses indicate that right-lateral faulting, with a minor reverse component, dominates thrusting, along the NI-RC zone. An abrupt decrease in seismicity and the presence of compressive structures suggests that the Torrance-Wilmington fold and thrust belt extends at least as far south as Oceanside. The NI-RC may mark the eastern boundary of this fold and thrust belt. Based upon work completed by Leminski and Brown (1988), within the San Andreas transform system the NI-RC may be dislocated at depth by a zone of decollement- this would explain the close association of strike-slip and compressional tectonics along the NI-RC and suggest that the NI-RC is a strike-slip fault in an intraplate setting.