Early Onset of Franciscan Subduction

Abstract

The Franciscan subduction complex of California is considered a type example of a subduction-accretion system, yet the age of subduction initiation and relationship to the tectonic history of western North America remain controversial. Estimates for the timing of Franciscan subduction initiation are largely based either indirectly on regional tectonic arguments or from the ages of high-grade blocks within mélange. Many of the high-grade blocks record counterclockwise pressure-temperature paths with early amphibolite overprinted by later eclogite and blueschist; however, their origin and significance with respect to subduction initiation have been debated. In contrast, some high-grade blocks show evidence for clockwise pressure-temperature paths and an early eclogite assemblage overprinted by later amphibolite. Zircon U-Pb ages from inclusions in garnet and Lu-Hf estimates of initial garnet growth ages from these samples record early eclogite metamorphism at~176 Ma. Matrix zircon U-Pb ages and Lu-Hf estimates of final garnet growth ages record a barroisite-amphibolite assemblage overprint of eclogite at~160 Ma. Combined with petrologic data and existing geochronology, the data suggest that (1) Franciscan subduction was underway by no later than 180 Ma, (2) continuous subduction metamorphism occurred for at least 100 Ma, and (3) Franciscan subduction initiation predated the formation of the overlying Coast Range Ophiolite, supporting models that form the ophiolite above an east dipping Franciscan subduction zone.

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

“…(2) In the Franciscan complex of California, despite N10,000 km of eastward subduction, rock recovery was limited. It is characterized by small volumes returned shortly after subduction inception (~175-170 Ma; Anczkiewicz et al, 2004;Mulcahy et al, 2018; Fig. 4d), of high temperature eclogites (high-grade blocks; Cloos, 1985;Ukar and Cloos, 2014;Wakabayashi, 2015) or slightly more coherent terranes (e.g.,Ward Creek, Goat Mountain, Willow Spring, Skaggs Springs; Wakabayashi, 2015).…”

Subduction initiation and early evolution of the Easton metamorphic suite, northwest Cascades, Washington

“…Franciscan subduction began ca. 180 Ma (Mulcahy et al, 2018), and the ages of many early Franciscan amphibolites (ca. 175-155 Ma; e.g., Ross and Sharp, 1988;Anczkiewicz et al, 2004;Mulcahy et al, 2014Mulcahy et al, , 2018 overlap with hornblende cooling ages from the Easton metamorphic suite (167-165 Ma).…”

“…Geochronology of Franciscan rocks records continuous subduction metamorphism from 180 Ma until ca. 80 Ma (Anczkiewicz et al, 2004;Wakabayashi and Dumitru, 2007;Ukar et al, 2012;Ernst, 2015;Mulcahy et al, 2018). The structural and metamorphic evolution of early Franciscan subduction, however, is complicated by the fact that early high-grade rocks commonly occur as blocks in a younger lower-grade mélange (Cloos, 1985;Wakabayashi, 1992;Ukar et al, 2012), and only locally as coherent slabs at high structural levels that are faulted against younger lower-grade units (e.g., Wakabayashi and Dumitru, 2007).…”

Forearc strike-slip displacement as an alternative to subduction erosion, with examples from Mexico and California (sinistral Nacimiento fault)

“…Figure 3 illustrates that these high-grade rocks could have been incorporated into the Franciscan Complex as the Farallon plate began its descent beneath the accreted arc terranes around 155 Ma, following the Nevadan orogeny. Complex pressuretemperature paths (as discussed by Mulcahy et al 2018) would certainly occur during this process. The significant aspect of this model for the current contribution is that the suture zone between the colliding arcs is now found in the western foothills of the Sierra Nevada (Schweickert 2015) and the Peninsular Ranges (Dickinson and Lawton 2001), where it is onlapped by the youngest part of the post-Nevadan forearc basin, as discussed later.…”

“…Confidence in this model for the Nevadan orogeny has been strengthened by documentation of oceanic slabs in the mantle, consistent with subduction of the Mezcalera plate between the colliding arcs (Sigloch and Mihalynuk 2017), as proposed by Moores (1998) and Dickinson and Lawton (2001) for all of southwestern North America. Claims that Franciscan subduction initiated as early as 180-165 Ma (e.g., Wakabayashi 2015;Mulcahy et al 2018; and references therein) are based on the ages of high-grade metamorphic rocks within the Franciscan Complex. Figure 3 illustrates that these high-grade rocks could have been incorporated into the Franciscan Complex as the Farallon plate began its descent beneath the accreted arc terranes around 155 Ma, following the Nevadan orogeny.…”