Eocene and Oligocene sedimentary rocks of the northern Channel Islands, San Nicolas Island, Santa Monica Mountains, Simi Hills, and northwest Peninsular Ranges of southern California include conglomerate beds characterized by the distinctive Poway clast suite of gray and red siliceous tuffs. The beds containing Poway clasts in the offshore islands and the Santa Monica Mountains are separated from beds containing similar clasts in the Peninsular Ranges by blueschist and greenschist of Santa Catalina Island and the Los Angeles basin; these schists are basement rocks unrepresented in the clasts.

The offshore islands taken together and the Santa Monica Mountains–Simi Hills are interpreted as being two separate blocks or small plates which broke away from the Peninsular Ranges ∼16 m.y. ago. These blocks originally comprised the upper plate of a large thrust fault (the southern California subduction zone of Hill, 1971) over blueschist and greenschist. Boundaries of these blocks are identified in part by the narrow belt of Miocene San Onofre Breccia along their edges, in part by basement contrasts in wells in the Los Angeles basin, and in part by the 400-fm isobath on tne western side of Santa Cruz basin. The island block drifted relatively westward for 3 m.y. during Miocene time at a rate of ∼5.3 cm per yr, thereby opening up the inner basins of the southern California borderland. If a palinspastic translation of the Santa Monica Mountains 90 km east is combined with a 155- to 160-km eastward translation of the island plate, the paleocurrent indicators of the Poway-bearing sequence radiate from a point east of the Santa Ana Mountains near Lake Elsinore. Northwest, west, and southwest from this point, the percentage of conglomerate decreases and, in the marine rocks, water depth at the time of deposition increases. The sedimentary body thus defined is called the Poway fan and submarine cone, analogous to the Mississippi delta and cone but now fragmented by Miocene rifting. The palinspastic restoration also places a north-trending lower Miocene shoreline in the Santa Monica Mountains on a line with a similar shoreline in the Santa Ana Mountains, and it places lower Miocene rocks of shallow-marine facies in the western Santa Monica Mountains adjacent to rocks of similar age and facies on Santa Cruz Island.

Presumably, the drifting of these small blocks represents a brittle separation of Peninsular Ranges basement and attenuation of the underlying blueschist and greenschist accompanying east-west spreading of the underlying lithosphere. The east-west relative motion necessary to explain the distribution of these small Poway-bearing microplates suggests that the motion was entirely within the North American plate and not a consequence of the northwest-southeast relative motion between the Pacific and North American plates. The southern California borderland may have formed as an intracontinental interarc basin as the East Pacific Rise and the West Coast subduction zone were annihilated at the western edge of the continent, and the northern boundary of the borderland may represent an intracontinental transform structure analogous to the Garlock fault as postulated by Davis and Burchfiel (1973).

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