Abstract
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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