Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes
Published:August 01, 2011
John Wakabayashi, 2011. "Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes", Mélanges: Processes of Formation and Societal Significance, John Wakabayashi, Yildirim Dilek
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The classic mélanges of the Franciscan Complex of California comprise a variety of structural-tectonic settings and give insight into mélange forming processes and material movement patterns within subducting plate margins. Structural settings of mélanges include (1) shale matrix mélanges that occur within, cutting, and bounding coherent nappes, and (2) serpentinite matrix mélanges that occur at the structurally highest levels and commonly cut blueschist and higher grade coherent nappes. Although nappe-bounding zones may have accommodated tens of kilometers or more of movement and represent paleosubduction megathrust zones, exposures at El Cerrito Quarry in the eastern San Francisco Bay area suggest that most displacement is accommodated in a narrow zone (5 m wide in this example) of brittle faults between the nappe-bounding mélange and the coherent nappes. Intranappe mélanges, or their boundaries, accommodated smaller displacements owing to similar or identical units bounding them. Blueschist-facies sedimentary breccias present in the northwest Diablo Range range from nearly undeformed to strongly foliated rocks that appear to be similar or identical to classic foliated shale mélange matrix. Most of the breccia clasts exhibit blueschist-facies metamorphic mineral growth that predated sedimentation, indicating exhumation of source material from blueschist depths, followed by deposition and resubduction to blueschist depth. Published apatite fission-track data suggest that the source of the clasts may have been completely eroded since exhumation. Blueschist-facies sedimentary breccia on one side of a high-grade blueschist block within serpentinite-matrix mélange in southern Sonoma County suggests that the mélange originated as sedimentary serpentinite, following exhumation of blueschist and higher grade rocks early in Franciscan subduction history, prior to deposition of metaclastic rocks (absent as blocks in the mélange). The strongly foliated serpentinite matrix apparently recrystallized during resubduction to blueschist-facies depth, erasing sedimentary textures, in contrast to the unstrained and unmetamorphosed sedimentary serpentinite in the basal Great Valley Group that may be a temporal equivalent. These relationships suggest an early forearc history in which exhumed serpentinite and associated high-grade blocks were shed into the trench and into what later became the forearc basin, prior to forearc high development and significant clastic sedimentation. The occurrence of high-grade metamorphic blocks in structurally low mélanges, as well as their preferential localization along the boundaries of mélanges, also suggests sedimentary mixing. Thus, many Franciscan mélanges, including the nappe-bounding mélanges that might be expected to have accommodated large displacement, show evidence of early sedimentary mixing. Most of the block-in-matrix fabric and introduction of exotic blocks may have resulted from sedimentary processes rather than tectonic strain. In contrast, some mélanges that cut across the bedding or foliation of coherent units apparently had a diapiric rather than a sedimentary origin.