The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises melange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Melanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimenta...

Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales

Oxygen, hydrogen and carbon isotope studies of the franciscan formation, Coast Ranges, California☆

Ultramafic blocks within mud-matrix melange of the Franciscan Complex, California, preserve a series of metasomatic mineral zones generated by infiltration of Si-rich hydrous fluids during subduction. We describe the petrology and geochemistry of the metasomatic zones and compare them to current model predictions for the metasomatism of the mantle wedge by subduction zone fluids. Fluid flow affected a Cr-spinel lherzolite protolith to form first serpentinite, then a talc-dominated rock, and finally an amphibole-rich assemblage. A diverse suite of accessory minerals in the amphibole-rich zone (titanite + clinozoisite + zircon + apatite) suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. Oxygen isotopic evidence suggests that the ultramafic blocks equilibrated with metasomatic fluids during serpentinization and that subsequent reactions occurred in equilibrium with these fluids in a temperature range of 450–500 °C. Whole-rock geochemistry indicates mobility of many elements into and out of the blocks during metasomatism, including elements such as Ti which are currently considered to have low solubilities in such fluids.

Taken as a whole, the blocks appear to preserve the metasomatic structure of the slab-mantle interface in subduction zones and imply that the chemistry of slab-derived fluids is modified as they pass through these metasomatic zones in the mantle wedge. Our results suggest that the primary composition of subduction zone fluids is not likely reflected by arc magmas. Instead, we propose that arc magmas are derived from regions of the mantle fluxed by fluids residual to the metasomatic processes we observe.

Constraints on the petrologic structure of the subduction zone slab-mantle interface from Franciscan Complex exotic ultramafic blocks

The San Gregorio—Hosgri fault trend is a component of the San Andreas fault system on which there may have been about 115 kilometers of post-early Miocene right-lateral strike slip. If so, right slip on the San Andreas and San Gregorio-Hosgri faults accounts for most of the movement between the Pacific and North American plates since mid-Miocene time. Furthermore, the magnitude of right slip on a Paleogene proto-San Andreas fault inferred from the present distribution of granitic basement is reduced considerably when Neogene-Recent San Gregorio-Hosgri right slip is taken into account.

https://science.sciencemag.org/content/sci/199/4325/179.full.pdf

Evidence for 115 kilometers of right slip on the san gregorio-hosgri fault trend.

Concepts of rarity in vascular plant species, with special reference to the genus calochortus pursh (liliaceae)

The Sur fault zone is a complex series of high-angle and low-angle faults that juxtapose a northeast block of competent crystalline rocks against a southwest block of generally sheared Franciscan rocks. The Franciscan rocks are an assemblage of sandstone, siltstone, greenstone, and chert. This assemblage can be divided into a northwestern terrane characterized by unmetamorphosed sheared and unsheared units containing detrital K-feldspar and a southeastern terrane with a pervasive dip-slip tectonite fabric throughout, metamorphic mineral assemblages indicative of high-pressure, low-temperature metamorphism, and no K-feldspar. A lawsonite isograd in Franciscan sandstone crosses the southeastern terrane. K-Ar dates indicate a minimum age of Late Cretaceous for high-pressure metamorphism of the Franciscan.

During mid- to Late Cretaceous time, accumulation of Franciscan rocks was closely followed by subduction and high-pressure, low-temperature metamorphism. Low-pressure, high-temperature metamorphism and plutonism occurred concurrently in the Salinian block. In Late Cenozoic time, the San Andreas fault probably offset the Sur-Nacimiento fault from the Great Valley subduction zone.

Wyatt G. Gilbert

Papers

Franciscan Rocks near Sur Fault Zone, Northern Santa Lucia Range, California