MULTI-PHASE TECTONIC DEVELOPMENT OF THE OUTER CONTINENTAL BORDERLAND WEST OF SAN DIEGO, SOUTHERN CALIFORNIA

Recently released multichannel seismic-reflection (MCS) data, collected by WesternGeco, Inc., reveal the sequential tectonic development of the basins, ridges and knolls that form the outer part of the California Continental Borderland west of San Diego. The earliest of three tectonic phases spanned from the middle Mesozoic to the early Miocene and involved eastward subduction of the Farallon Plate beneath the continental margin. Despite numerous MCS lines collected across the Patton Escarpment, which marks the lower slope of the ancient accretionary wedge, only a few structures stemming from this tectonic phase can be recognized. The second tectonic phase began during the middle Miocene and involved crustal extension intense enough to expose the Catalina Schist in one or more metamorphic-core complexes. The deep structural imprint of this phase is evident in numerous graben and half graben, many of which strike nearly north. In the southeastern part of the study area, east-dipping reflections from within basement may reveal an extensional detachment that transects the upper crust. Local seafloor relief over the tip of the detachment indicates Quaternary normal separation along the detachment. The third tectonic phase involved Pliocene and younger transpressional deformation of the Continental Borderland, when strike-slip faults formed that strike northwest and cut obliquely across the older extensional fabric. Where structural elements of the second and third tectonic phases intersect, the older extensional structures became inverted, so that locally uplifted graben fill forms ridges and knolls. Large northwest-trending aeromagnetic anomalies indicate basement rocks that were involved in episodic deformation. Inverted grabens approximately parallel the east side of the anomalies, and a meandering zone of stratal truncation follows the west side. Stratigraphic information from the 1975 Continental Offshore Stratigraphic Test well reveals the age and distribution of two extensive, strongly reflective horizons. Near the well the shallower horizon is the top of a 1-km thick basalt layer; elsewhere, this horizon is probably from the unconformable base of middle and/or late Miocene strata. The deeper extensive horizon is the unconformity between Paleocene and Upper Cretaceous rocks.