Abstract

The Jurassic Josephine Ophiolite of northwest California and southwest Oregon is one of the largest and most complete ophiolites in North America and clearly formed in a suprasubduction-zone setting. The geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation. Basalts rich in Fe and Ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite. They have MORB (mid-oceanic-ridge basalt) affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between IAT and MORB (IAT = island-arc–tholeiite). This late Fe-Ti–MORB suite was erupted while the ophiolite was undergoing the last half of ∼50° of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and upper-mantle peridotite related to freezing of the axial magma chamber. Fe-Ti basalts are characteristic of propagating spreading centers on mid-ocean ridges and in at least one backarc basin. Geochemical, structural, stratigraphic, and regional geologic constraints suggest formation of the Josephine Ophiolite by propagation of a spreading center into rifted island-arc lithosphere that is preserved along the margin of the ophiolite. Another possible indication of propagating-rift tectonics is the presence of off-axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fallout from a second propagating spreading center. The Lau Basin provides modern analogues, including spreading centers propagating into rifted arc lithosphere, into older backarc ocean crust, and across the arc into the forearc.

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