The Mesozoic and early Cenozoic arc-trench system of California has been interpreted to include the Franciscan Complex as a subduction complex, the Great Valley sequence as a fore-arc basin, and the Sierra Nevada batholith and volcanic rocks in the western states as the magmatic arc. The similar ages of these elements and the stacking sequence in the Franciscan Complex (youngest to the west) are consistent with this interpretation. However, some recent studies in paleomagnetics, radiometric dating, and sandstone petrology, particularly in the Franciscan Complex, suggest that a simple model of subduction and accretion does not explain many relations.

With the growing evidence for lateral translations of microplate along the Pacific margin during the Mesozoic and Cenozoic, some authors have attempted to explain Franciscan complexities by these large-scale lateral translations. Limestone and volcanic blocks in the Franciscan melange have been shown to be allochthonous, but there is yet no definitive evidence for large-scale translations of the melange as a whole. Several problems remain unsolved, including the relation between blueschist terranes, the large time delay between deposition of some sediments and their subsequent metamorphism to blueschist facies, the relation of sandstone provenance between the fore-arc basin and subjection complex, the distribution of sedimentary facies across the entire trench slope/fore-arc region, a d the differing styles of deformation in the various Franciscan belts. Recent studies of modern and ancient arc-trench systems help explain some of these problems, but others remain enigmatic.

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