Abstract

Duplex structures several hundred metres thick bounded by marker beds exhibiting basalt-chert association have been found in the Franciscan complex near Pacheco Pass, central Diablo Range, California. The terrane was regionally metamorphosed and deformed under blueschist facies conditions at 150 °C and 7–8 kbar. Detailed mapping of the basalt-chert sequence has clarified the change of environment from mid-oceanic ridge to subduction zone. Eastward convergence of duplexing indicates eastward subduction of the oceanic plate during Early Cretaceous time. The presence of duplexing and a reconstruction of oceanic plate stratigraphy recorded within the underthrust unit demonstrate the accretionary origin of protoliths of the Franciscan coherent unit, which has long been believed to have an in situ origin, interpreted as either near-trench volcanism or accreted terranes in which “geosynclinal-type” volcanism occurred.

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