OPHIOLITES AND OCEAN CRUST: A FEW COMPOSITIONAL AND STRUCTURAL COMPLEXITIES

Ever since recognition of ophiolites as possible oceanic crust and mantle formed by oceanic spreading, a problem has existed in that few if any ophiolite mafic sequences are as thick as seismic oceanic crust. Available information from the world's oceans (e.g., Dick et al., 2006) suggest complete "Penrose-type" oceanic crust may be present along magma-rich fast and intermediate oceanic spreading centers, or even some slow-spreading ocean crust such as, say, south of Iceland, either in mid-oceanic, back-arc, or forearc settings. Magma-poor spreading centers may mostly lack mafic igneous rocks, and "Hess-type" oceanic crust (nearly completely serpentinized peridotite) may characterize some magma-poor settings.

The Coast-Range ophiolite, California, contains partial mafic sequences, geochemically chiefly of "ssz" composition, but with stratigraphic evidence for, in some places, an open ocean setting (e.g., Hopson et al., 2005, 2008). NW-SE extension directions from Coast Range serpentinite slickenfibers (L. Guenther, 2004) imply NW-SE extension in a former oceanic spreading center. Mixed "ssz" and "morb" igneous compositions and both open-ocean pelagic and volcaniclastic overlying sediments suggest a complex paleogeography, perhaps analogous with some mid-ocean ridge settings (e.g. Wanless et al., 2010)

Similar compositional and stratigraphic complexities in regions such as the neighboring "morb"-like Pindos and "ssz"-like Vourinos ophiolites, Greece, suggest similar paleogeographic complexities. Vourinos may represent a magma-poor ocean crust above a relict mantle spreading center.