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The San Andreas and Nacimiento faults of coastal California both separate granitic and metamorphic basement rocks of the Salinian block from partly coeval but contrasting Mesozoic terranes underlain by the Franciscan subduction complex. By analogy with Neogene dextral strike slip along the San Andreas fault, Cretaceous sinistral strike slip can be inferred along the Nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount. Following restoration of known San Andreas and inferred proto-San Andreas dextral displacements, reversal of about 560 km (350 mi) of postulated sinistral slip on the Nacimiento fault brings four major Mesozoic lithotectonic belts of California and Baja California into simple al gnment as subparallel terranes related to Mesozoic subduction along the continental margin.
Neogene deformation within the San Andreas transform system involved (a) elongation of the Salinian block by dextral slip along subsidiary faults that branch from the San Andreas fault, and (b) dextral rotation of crustal panels within the Transverse Ranges. Latest Cretaceous and/or earliest Paleogene dextral slip along a proto-San Andreas fault followed the San Andreas course in central California, but diverged westward in southern California. Nacimiento sinistral displacements occurred in mid-Cretaceous to early or medial Late Cretaceous time, after Cretaceous emplacement of plutons now within the Salinian block but prior to deposition of uppermost Cretaceous sedimentary sequences in central California. Available data on Mesozoic relative and absolute plate motions in the Mesoameric n region support the likelihood of Cretaceous sinistral strike slip subparallel to the California continental margin.
Paleotectonic reconstruction of crustal blocks in California and Baja California to their inferred mid-Cretaceous relative positions shows the Salinian block inserted on a bias between the flanking Mojave and Peninsular Ranges blocks. Salinian granitic rocks thus formed an interior part of the Mesozoic batholith belt, and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks. The similar Mesozoic terranes that now lie east and west of the Salinian block were then adjacent to one another west of the Sierra Nevada block. Available paleomagnetic data neither support nor preclude the reconstruction, but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents. A correct interpretation of the Nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal California.
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