ABSTRACT: The Coast Range Ophiolite: Polygenetic Crust in a Late Mesozoic Oceanic Fore Arc
PHIPPS, STEPHEN PAUL, University of Pennsylvania, Philadelphia, PA, and GLENN J. MACPHERSON, Smithsonian Institution, Washington, DC
Stratigraphic, petrologic, and geochemical data from ophiolitic rocks in the Coast Range Ophiolite, Franciscan Complex, and basal Great Valley Group indicate that the ophiolite was largely formed and extensively modified in a young, tectonically active oceanic fore arc dominated by basement exposures.
The ophiolite is stratigraphically anomalous and variable. At some localities, its mafic section is thin or missing; this thinness may be due, in part, to later attentuational faulting, but in some areas it was a primary feature and serpentinized harzburgite tectonite was widely exposed at the sea floor early in the evolution of the fore arc. Late, arc-related magmatism cross-cuts earlier formed crust, and locally the margins of arc edifices overlie the ophiolite. Petrologic and geochemical data indicate that whereas much of the ophiolite was formed by supra-subduction-zone magmatism, it may also include small amounts of crust formed at a mid-ocean ridge, as well as ocean-island basalts erupted at a (near-axis?) seamount.
After its formation, the ophiolite was extensively faulted and locally deeply eroded. Diapiric serpentinite muds erupted into the fore arc, constructing serpentinite mud volcanoes up to 25 km across and 1 km thick that contain blocks of rock from both the hanging-wall and the subducting slab. Fractured fore arc crust on the trench inner wall slumped into the trench, introducing blocks of Coast Range Ophiolite parentage into the Franciscan subduction complex.
The Mariana fore arc exhibits many of these features and, with other modern ophiolitic fore arcs, provides a striking actualistic analog for the early evolution of the Coast Range Ophiolite and subduction zone.