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Abstract

Profiles of slope-normal gradients of continental slopes of the U.S. Pacific margin can be related to the large-scale structural framework of the margin, and to the secondary effects of sedimentation. Tabulations of average gradient, steepest gradient, slope width and the relation of steepest segment to midslope depth define three major slope provinces.

The first two regions form the area extending from the Agua Bianca Fault Zone and its offshore extensions off northern Baja California to the Mendocino Fracture Zone. The boundary between region 1 and region 2 has been taken at Point Conception. The boundary separates 1) the outer steep-slope province off the California Borderland, the Patton Escarpment, and 2) a northern province with gentler declivities, usually because of modification by deposition. Both provinces show possible effects of the intersection of oceanic fracture zones with the margin. The northern province (region 3), from Cape Mendocino to Juan de Fuca Strait, can be subdivided into two subregions approximately at Heceta Bank, Oregon.

The major provinces correspond on the first-order level to the two main modes of margin interaction with the adjacent ocean plates, underthrusting and transform motion. Second-order factors are the location of fracture zones, and deposition.

Within the California Continental Borderland, the shallower base-of-slope depths and narrower shelves produce slopes that have lower relief and are generally steeper than the main continental slopes. These are secondarily modified by depositional processes and mass failure. There is a general increase in depth of slope base and in steepness to the south, illustrating the regional depression to the south, as well as the decrease in sediment supply in that same direction.

It is evident that regional characteristics of the morphology of slopes is a function of the structural setting and of the local sediment supply.

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