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Most of the structural patterns revealed by bathymetric and magnetic mapping of the oceanic crust west and south of southern California and Baja California are inherited from the time of crustal accretion on the northern East Pacific Rise. Since the Oligocene this spreading center has separated the Pacific plate from several narrow east-flank plates, and their changing motions are recorded by reorientation of spreading axes, appearance and disappearance of spreading-center offsets, and varying speeds and directions of migration of the offsets. Parts of most of the east-flank plate fragments avoided subduction by being captured by the Pacific plate, and survive along the continental margin, surrounded by fossil spreading centers and transform faults and by a fossil trench alongside the North American continent. Many volcanoes have been superimposed on the mainly fault-produced primary structure. Some of these seamounts are in short rows built close to spreading axes, especially near sites of maximum upwelling to the accreting plate boundary. Other seamounts belong to the trail of a major hotspot, which produced large alkali-basalt cones episodically from 20 to 5 Ma, and may also have built rows of smaller tholeiitic shields aligned with the main alkali-basalt chain. This hotspot may now be affecting continental crust of the peninsula. The revised, more detailed geologic history of the oceanic crust also has implications for Californian continental geology, especially for helping to explain patterns of subaerial volcanism and the temporal and spatial distribution of strike-slip faulting.

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