Pliocene-Pleistocene Diastrophism of Santa Monica and San Pedro Shelves, California Continental Borderland

Abstract

Data from geophysical profiling and from jet and dart coring have been used to study the late Cenozoic history of the Santa Monica and San Pedro Shelves. Deformed strata commonly crop out on the outer shelf and consist chiefly of shale belonging to the Monterey Shale and the Repetto Formation (middle Miocene through lower upper Pliocene). In contrast, undeformed sands and silts, primarily beneath the inner shelf, represent the Pico and San Pedro Formations (uppermost Pliocene through middle Pleistocene) as well as unnamed upper Pleistocene and Holocene deposits. Together with the Palos Verdes Hills and the Lasuen Knoll, outer-shelf strata have been folded into five northwest-trending en echelon anticlinoria. One anticlinorium is in a structural depression directly north of Canyon and has been buried beneath undeformed sediments. This depression facilitated initial development of the canyon during late Pleistocene time. An east- ortheast-striking termed the Redondo Canyon fault, separates the platform anticlinorium from the Palos Verdes Hills anticlinorium. Formation of the anticlinoria and the Canyon fault is attributed to convergent dextral shear along the northwest-striking Palos Verdes fault after early Pliocene time. Although displacement along the Palos Verdes fault has continued in San Pedro Bay through late Pleistocene time, the absence of large vertical separation in the upper 300 m of strata in Santa Monica Bay north of the platform anticlinorium indicates that the fault has been relatively inactive there at least since the beginning of late Pleistocene time. These observations infer that the late Quaternary uplift of the Palos Verdes Hills was decoupled from the Santa onica shelf along the Canyon and may have occurred because the Santa Monica Shelf restrained the relative northwestward movement of the Palos Verdes Hills block. It is suggested that late Pliocene and early Pleistocene diastrophism in the California continental borderland was characterized by convergent dextral shear accompanied by en echelon folding. This contrasts with the middle and early late Miocene history of widespread faulting and large lateral displacements proposed by other investigators. The transition in the style and locus of deformation between 12 and 3 m.y.B.P. may be related to a change in the orientation of the transform boundary between the North American and Pacific plates, changing slip rates along the San Andreas and, possibly, differences in slip rates along the San Andreas north and south of the Transverse Ranges.