TIME-VARYING ACCRETION, NONACCRETION, AND HIGH-PRESSURE METAMORPHISM IN THE FRANCISCAN SUBDUCTION COMPLEX, FROM THE INITIATION OF SUBDUCTION UNTIL CA. 80 MA

Subduction at the Franciscan trench began ca. 169-162 Ma, based on ⁴⁰Ar/³⁹Ar and Lu-Hf ages of very rare garnet amphibolite exotic blocks and intact minor slabs interpreted to represent disaggregated fragments of a metamorphic sole formed at the base of the overriding plate. There may be a significant spread in sole ages, perhaps reflecting diachronous initiation of the new subduction zone. Rare eclogite blocks, blueschist blocks, and the Skaggs Spring Schist yield ⁴⁰Ar/³⁹Ar and/or Lu-Hf ages from 158 to 132 Ma, evidence of continued subduction-induced high-pressure metamorphism for tens of millions of years following the initiation of subduction.

Based on its position at the top of the Franciscan structural stack immediately below the overriding plate, the South Fork Mountain Schist (SFMS) was apparently the first metasedimentary blueschist-facies unit of substantial size to be accreted into the Franciscan. The SFMS contains 137 Ma oceanic radiolarian (meta)cherts deposited atop oceanic crust (MORB) and yields 121 Ma 40Ar/39Ar metamorphic ages, indicating it was accreted surprisingly late in the history of the complex, probably at ca. 123 Ma. The Valentine Spring Formation and parts of the Yolla Bolly terrane were accreted sequentially soon thereafter, most likely at about 120 and 111 Ma, respectively.

The volume of Franciscan rocks that are older than the SFMS is extremely small, whereas the volume that is slightly younger is large. This suggests that the accretion of the SFMS marks a major transition from predominately nonaccretionary to accretionary conditions in the Franciscan subduction zone. Only about 25% of modern forearcs are accretionary whereas 75% are nonaccretionary (including subduction erosion), so transitions between nonaccretionary and accretionary states are to be expected. The various predominately mafic, early high-grade blocks, etc., apparently represent very sparse accretion during a fundamentally nonaccretionary period. The transition into an accretionary regime at ca. 123 Ma was essentially synchronous with the end of the Early Cretaceous magmatic lull in the Sierra Nevada magmatic arc, with the waning of Early Cretaceous faulting and warping within the Great Valley forearc basin, and possibly with changes in plate motion vectors of the Pacific and Farallon plates.