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The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges

Abstract The 162 Ma Josephine ophiolite, NW California and SW Oregon, consists of harzburgite tectonite (>800 km2), cumulates, high-level gabbro, a sheeted dyke complex having a consistent dyke orientation over hundreds of square kilometres, and pillow lavas. The ophiolite is conformably overlain by hemipelagics which grade upward into synorogenic turbidites. Open folding of the ophiolite and sediments provides ideal conditions to reconstruct the structural geometry of oceanic features in the ophiolite. Episodic axial magma chambers, structural extension, and episodic hydrothermal circulation is indicated by: (1) the local presence of both highly fractionated and very primitive lavas and dykes, (2) thick talus(?) breccias, and (3) multiple stacked massive sulphide deposits, each overlain by up to 5 m of mudstone within the pillow lavas. The effects of structural extension are evident from: (1) a 50° tilting of the entire crustal sequence except for the uppermost lava flows, (2) oceanic normal and transfer faults, and (3) shear zones within the harzburgite consisting of peridotite (± talc) mylonite and high-temperature serpentine mylonite. A regionally extensive, subhorizontal serpentine mylonite zone within the upper c. 1 km of the harzburgite formed at c. 500°C and may represent an oceanic detachment fault. The 50° tilting of the entire crustal sequence occurred prior to eruption of the uppermost lava flows and implies extreme attenuation (c. 100%) at the spreading ridge. This extension would account for the present thin crustal sequence of the Josephine ophiolite (c. 3 km thick). It is important to note that where deep faulting occurs the actual oceanic crustal thickness, as defined by seismic velocities, is likely to coincide with the lower limit of hydrothermal serpentine alteration in the harzburgite tectonite. Subseafloor hydrothermal alteration resulting from flow of discharging fluids is especially localized along brittle oceanic faults in the Josephine ophiolite. Seafloor massive sulphide deposits in the pillow lavas and mineralized stockworks in the upper dyke complex apparently formed by discharging black smoker fluids localized along oceanic fault zones. Relatively late oceanic faults have been observed in the sheeted dyke complex and are extensively altered to quartz + sulphides + sericite. They are tentatively interpreted as off-axis faults and may represent pathways for lower temperature fluids which vented to the seafloor and formed metalliferous sediments 8–23 m above the uppermost pillow basalts of the ophiolite. Although the regional setting and geochemistry of the Josephine ophiolite indicate that it formed in a suprasubduction zone setting, the following features suggest that it may provide a useful structural analogue for slow-to intermediate-spreading mid-ocean ridges: (1) episodic magmatism, (2) extensive faulting and talus deposits, (3) normal faulting extending into the upper mantle, (4) fault-controlled hydrothermal discharge, and (5) large-scale tilting of the entire crustal sequence. The high degree of structural extension distinguishes the Josephine ophiolite from the Oman and Bay of Islands ophiolites, which probably formed at fast-spreading centres.