Geophysical evidence for Miocene extension and mafic magmatic addition in the California Continental Borderland

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The California Continental Borderland is a key region for understanding the effect on crustal structure of tectonic processes associated with the initiation and elongation of the San Andreas transform system because the borderland lies near the point where the Pacific-Farallon spreading system first encountered the North American plate margin. Whereas the central and northern California margins have been extensively investigated by geophysical surveys in recent years, the crustal structure of the borderland, particularly the outer borderland, has received relatively little attention. This paper presents three gravity models that stretch from the Pacific ocean floor to shore; they provide new evidence that the crust of the entire California Continental Borderland has been extended and infiltrated by mafic melt since early Miocene time. The gravity models were constructed from interpretations of nearly 750 km of seismic reflection data, seismic refraction studies, and geologic maps, thus minimizing the nonuniqueness of the method. The evidence for extension in the models is that the crust of the accretionary complex and forearc basin that make up the borderland is only ∼22 km thick, ∼35% thinner than comparable crustal sections in central and northern California. Extension could well have been accommodated by eastward flow of accretionary-complex material during middle Miocene formation of a metamorphic core complex in the inner borderland. The gravity models also show that the accretionary complex is 100–200 kg/m 3 more dense than commonly observed to the north. This density difference may result from influx of basaltic melts due to mantle upwelling at the Rivera triple junction and beneath the highly extended inner borderland. The borderland models permit the occurrence of a discrete mafic layer that forms the base of the crust, as is seen along the central and northern California coast, but do not require such a layer9s presence anywhere except immediately inland of the shelf-slope break.