Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino-Garberville-Covelo 30′ × 60′ quadrangles, northwest California
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....
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Published in | Tectonics (Washington, D.C.) Vol. 31; no. 1 |
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Main Authors | , |
Format | Journal Article |
Language | English |
Published |
Washington
Blackwell Publishing Ltd
01.02.2012
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Subjects | |
Online Access | Get full text |
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Summary: | 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 |
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Bibliography: | limited NSF funding - No. NSF EAR 0948676 to Marty Grove istex:B6872F5D8A891EDEF9FBF7937D6E086643037214 ArticleID:2011TC002987 ark:/67375/WNG-8VXNKCXW-R Tab-delimited Table 1.Tab-delimited Table 2.Tab-delimited Table 3.Tab-delimited Table 4.Tab-delimited Table 5.Tab-delimited Table 6.Tab-delimited Table 7.Tab-delimited Table 8.Tab-delimited Table 9.Tab-delimited Table 10.Tab-delimited Table 11. ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0278-7407 1944-9194 |
DOI: | 10.1029/2011TC002987 |