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Data from a number of seismic surveys in the California Coastal Ranges have shown the existence of a mafic layer in the lower crust throughout the entire ranges. The existence and geometry of this layer place significant constraints on the tectonic models for evolution of the deep plate boundary under the influence of triple junction migration. It is possible that different tectonic models are correct in different regions. Both the Pacific slab model and the asthenospheric upwelling model are inconsistent with the seismic data.