Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino-Garberville-Covelo 30′ × 60′ quadrangles, northwest California

• Published in: Tectonics (Washington, D.C.) Vol. 31; no. 1
• Main Authors: Ernst, W. G., McLaughlin, Robert J.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01.02.2012
• Subjects: Accretion; Franciscan Complex; Lithosphere; metagraywackes; Microstructure; Mineralogy; neoblastic minerals; Paleogene; Petrology; Plate tectonics; Rocks; Slabs; structure; wedging; USA, California, San Rafael Mts., Franciscan Complex; INE, USA, California; ANW, USA, Virginia, False Cape
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1029/2011TC002987

The Franciscan Complex is a classic subduction‐zone assemblage. In northwest California, it comprises a stack of west vergent thrust sheets: westernmost Eastern Belt outliers; Central Belt mélange; Coastal Belt Yager terrane; Coastal Belt Coastal terrane; Coastal Belt King Range/False Cape terranes. We collected samples and determined P‐T conditions of recrystallization for 88 medium‐fine‐grained metasandstones to assess their subduction‐exhumation histories and assembly of the host allochthons. Feebly recrystallized Yager, Coastal, and King Range strata retain clear detrital features. Scattered neoblastic prehnite occurs in several Coastal terrane metasandstones; traces of possible pumpellyite are present in three Yager metaclastic rocks. Pumpellyite ± lawsonite ± aragonite‐bearing Central Belt metasandstones are moderately deformed and reconstituted. Intensely contorted, thoroughly recrystallized Eastern Belt affinity quartzose metagraywackes contain lawsonite + jadeitic pyroxene ± aragonite ± glaucophane. We microprobed neoblastic phases in 23 rocks, documenting mineral parageneses that constrain the tectonic accretion and metamorphic P‐T evolution of these sheets. Quasi‐stable mineral assemblages typify Eastern Belt metasandstones, but mm‐sized domains in the Central and Coastal belt rocks failed to achieve chemical equilibrium. Eastern Belt slabs rose from subduction depths approaching 25–30 km, whereas structurally lower Central Belt mélanges returned from ∼15–18 km. Coastal Belt assemblages suggest burial depths less than 5–8 km. Eastern and Central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin; K‐feldspar‐rich Coastal Belt rocks were stranded along the continental edge without undergoing appreciable subduction, probably during Paleogene unroofing of the older, deeply subducted units of the Franciscan Complex in east‐vergent crustal wedges. Key Points Franciscan Coastal Belt not subducted Central Belt 15‐18 km subducted Eastern Belt at least 25 km subducted