Oceanic faulting and hydrothermal circulation within the crustal sequence of the Josephine Ophiolite, Northwest California and Southwest Oregon, USA
Robert J. Alexander 1992
A Dissertation submitted to the State University of New York at Albany in partial fulfillment of the requirements for the degree of Doctor of Philosophy
College of Sciences and Mathematics, Department of Geological Sciences
Advisor: G.D. Harper

Based upon detailed (1:10 and 1:100) outcrop mapping and regional-scale (1:24,000) mapping of the Late Jurassic Josephine ophiolite's crustal sequence, a complex history of overlapping episodes of magmatic, structural and hydrothermal events has been documented using crosscutting relationships, structural analysis, petrography, geochemistry, and strontium and oxygen isotopic data. The following history is typical of the sheeted dike/gabbro transition zone: (1) crystallization of gabbro and later subvertical mafic dike injection; (2) amphibolite facies metamorphism; (3) extensional faulting and tilting of dikes; (4) continued brittle faulting, tilting and dike injection associated with retrograde metamorphism at greenschist facies conditions; (5) continued extensional faulting and tilting synchronous with the development of a variety of fault-controlled hydrothermal veins under decreasing temperature conditions and increasing fluid/rock ratios; and (6) rare subvertical injection of dikes which typically truncate all previous features (1-5). Structural analysis of crosscutting dikes generally reveals a consistent pattern: >90% of dikes dip in one direction and steep dikes typically crosscut tilted dikes, indicating that significant (~50º) tilting of the Josephine crust occurred at the paleo-spreading axis by ocean floor extensional faulting. Probable growth faulting in the extrusive sequence is also consistent with tilting at the rift axis. (87Sr/86Sr)initial ratios from recrystallized whole rocks, and from hornblende, epidote and prehnite separates from veins, display a systematic increase with relative age (from 0.7033 for variably altered gabbro and dikes to 0.7049 for prehnite cement in young oceanic fault rocks). Calculated oxygen isotope fluid compositions for the same suite of samples range from d18Ofluid = +5 to -1 with time, indicating a change to a seawater-dominated hydrothermal system with time, consistent with observed increased fracture permeability in outcrop (i.e. veining) due to seafloor extensional faulting. Crosscutting relationships, alteration mineral assemblages and isotopic data suggest (1) an early stage of high temperature (>450ºC) alteration with low permeability (i.e. grain-scale flow), followed by (2) a decrease in temperature (~350 to <200ºC), an increase in permeability due to faulting, and accompanied by tectonic tilting at the rift axis. The consistency of these crosscutting relationships at similar pseudo-stratigraphic levels at different localities in the Josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading (slow to intermediate).

Alexander, R.J., 1992. Oceanic faulting and hydrothermal circulation within the crustal sequence of the Josphine Ophiolite, Northwest California and Southwest Oregon, USA. Unpublished PhD dissertation, State University of New York at Albany. 324pp., +xii; 1 plate (folded map)
University at Albany Science Library call number:  SCIENCE MIC Film QE 40 Z899 1992 A54
Copies of this PhD dissertation can be ordered from Proquest UMI

        Front matter (title, table of contents, abstract, acknowledgements) - 0.65MB pdf file
        Photo pages in dissertation (colour and greyscale photos with captions): - 18.1MB pdf file

        Plate 1 - Bedrock geologic map of the Shelly Creek Quadrangle, California and Oregon
                    (uncoloured geological map, scale 1:24,000) - 4.1MB pdf file

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