Melange Diapirs into the Cambria Slab: a Franciscan Trench Slope Basin near Cambria, California

Abstract

Near Cambria, California, a Franciscan trench slope basin of Late Cretaceous age is intruded by two diapirs of blueschist block-bearing, mud-matrix melange. The rise of the diapirs caused major upturning of the bedded strata within a few hundred meters of the contact and complex interfingering of mud-matrix melange with basin sediments near the contacts. At one locality, a fragment of basin sandstone is observed partially detached from the wall rock. The mud-matrix melange is highly mobile, and the intrusive character during diapirism is clearly analogous to that of magma in igneous intrusions. Diapirism may be responsible for large-scale intercalation of slope and slope basin sediments with subduction generated melange at actively convergent plate margins.