Blueschists in the Franciscan Complex of California: Petrotectonic constraints on uplift mechanisms

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Abstract
High-pressure/low-temperature metamorphic rocks in the Franciscan Complex of western California are primarily found as small blocks in mud-matrix melange and as extensive coherent, bedded sheets or slabs. The highest-pressure rocks, the high-grade sodic and/or calcic amphibole-epidote-garnet-omphacitic pyroxene-bearing blueschists, amphibolites, or eclogites, are found as blocks (typically meters to a few tens of meters across) in the melanges. Low-grade schistose blueschists are found as small blocks in melanges and in extensive coherent belts. Many Franciscan greenstones and metagray-wackes in both the melanges and coherent tracts locally contain sodic pyroxene + quartz, lawsonite, and/or aragonite but are neither blue nor strongly schistose. Petrotectonic constraints on models for uplift and preservation of Franciscan high-pressure/low-temperature metamorphic rocks include: (1) high-grade blueschist and eclogite blocks found in mud-matrix melange terranes underwent cooling under high-pressure conditions and were once immersed in serpentinite; whereas lower-temperature/high-pressure rocks found both as blocks in melanges and extensive coherent tracts are typically only incipiently recrystallized and show no evidence of former immersion in serpentinite; (2) extensive tracts of schistose blueschists are largely in fault contact with the base of the overriding plate in northern California, whereas both the high-grade blocks and the jadeitic pyroxene + quartz-bearing coherent units in northern California are not juxtaposed against the overriding plate; (3) uplift of both blueschist blocks in melange and extensive coherent tracts typically occurred without thoroughly penetrative strain; (4) high-grade blocks in mud-matrix melanges must have been displaced oceanward and upwards from beneath the base of the overriding plate; (5) synsubduction uplift of Franciscan blueschists to depths less than 10 kilometers or so is indicated by the lack of retrograde greenschist facies alterations, the widespread preservation of aragonite, and the present exposure of blueschists where subduction continues off northernmost California and southern Oregon; and (6) the scarcity of high-pressure metamorphic detritus in Franciscan sediments indicates that large tracts of blueschists were not exposed in California by synsubduction erosion.

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