WHEN DID FORMATION/ACCRETION OF THE FRANCISCAN COMPLEX BEGIN AND WHEN DID IT END?

Scattered igneous activity typified the Triassic Californian margin, but an emergent Andean-type Sierra Nevada-Klamath arc began to form attending underflow of oceanic lithosphere at ~170 Ma. This calcalkaline belt supplied debris to Mariposa-Galice proximal overlap sequences by ~165-160 Ma. Subduction also produced HP/LT eclogites, garnet-blueschists, and amphibolites along the convergent junction; these high-grade metamafic tectonic blocks recrystallized at ~170-155 Ma, but most returned surfaceward during the Cretaceous. At the end of Jurassic time, the Klamath salient migrated ~200 km westward. The earliest Cretaceous evidently marked a step-out of the convergent plate junction directly seaward of the Klamath salient, but southward trapping pre-existing oceanic crust-capped lithosphere as ophiolitic basement of the Great Valley Group (GVG). At ~145 Ma, arc detritus of Middle and Late Jurassic igneous age began to accumulate in the GVG forearc and Franciscan trench. Relatively continuous GVG and tectonized Franciscan clastic sequences share an Andean-type arc provenance and time of onset of sedimentation. Because GVG strata were deposited on the stable North American plate, protected from surface and subcrustal erosion, initiation of sedimentation in the forearc also signals beginning of deposition in the trench during earliest Cretaceous time, ~25 m.y. after initial construction of the landward arc. The most voluminous sedimentation and accretion of coeval Franciscan and GVG rocks took place during ~125-85 Ma. The youngest Sierran granites have ~85 Ma crystallization ages, reflecting loss of the magmagenic zone beneath northern and central California. This suggests regionally restricted Late Cretaceous termination of subduction and trench obliteration, or more likely, subhorizontal oceanic plate underflow during the Laramide orogeny. Feebly zeolitized Upper Cretaceous and Lower Tertiary Franciscan clastic units of the Coastal Belt, derived from the extinct arc, were affected by low-pressure burial metamorphism but were not deeply subducted. The ~170-155 Ma HP/LT blocks formed during construction of the emergent arc and proximal Mariposa-Galice units significantly predate onset of Franciscan trench deposition. Is it appropriate to call these metabasaltic blocks Franciscan?