26-1 Exhumation-Redeposition-Subduction of High-Pressure Metamorphic Rocks in the Franciscan Complex, California: Some Constraints on Cycle Times.

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

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Abstract:

The Franciscan Complex (FC) of the California Coast Ranges records subduction, accretion, and exhumation that spanned over 150 My, starting at ca.176 Ma. Sedimentary mélanges/conglomerates and sandstones in the FC, and in the Great Valley (GV) forearc basin structurally overlying the FC, include blocks/clasts of high-pressure metamorphic rocks.  These relationships allow assessment of subduction-exhumation-deposition cycle times. Some sedimentary mélanges include high-grade blocks of amphibolite, garnet-amphibolite, and eclogite, with varying degrees of blueschist overprint.  These blocks and their coherent equivalents have yielded an ages of ca. 155-176 Ma for metamorphism above ~500°C, with later cooling ages (blueschist facies) as young as 132 Ma.  Such blocks are found in olistostromal horizons in the basal GV forearc that have depositional age of ca. 130-150 Ma indicating rapid exhumation and deposition of these high-grade rocks.  Many FC sedimentary mélanges include high-grade blocks, the oldest reported being horizons in the quartz-rich lawsonite blueschist of the Skaggs Springs schist, that has a maximum depositional age of ca. 120 Ma. High-grade blocks or clasts are found in many units, including those that accreted at ca. 112 Ma, ca. 107 Ma, ca. 104 Ma, ca. 95 Ma, and ca. 80 Ma.  Some blocks are second-cycle because they are present within blocks of sedimentary mélange that are blocks within the host mélange; similar relationships are observed at sand size in sandstones.  Two-cycle HP blocks/clasts have been observed in mélanges deposited at 104 and 80 Ma. The ca. 112, 107, and 104 Ma accreted horizons listed above are blueschist facies.  The blueschist overprint of high-grade blocks and clasts within these horizons took place prior to deposition and subsequent subduction as indicated by truncation of the internal blueschist fabrics by clast/block boundaries.  High-grade sand-sized grains are found in sandstones and matrix of those units. Lower-grade blueschist-facies rocks, including metasandstones or metabasalts are common as clasts and blocks.  Blocks similar to Skaggs Springs schist are found in units that accreted at ca. 80 Ma; these units include blocks of metasandstone similar to those that accreted between ca.112 and 104 Ma.

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