ABSTRACT
Many ophiolites have amphibolite at their base which contains a
sharp inverted grade of metamorphism. These metamorphic soles are
thought to have formed during the detachment and emplacement of
the ophiolite. In addition to the sharp inverted grade of
metamorphism, other characteristics of metamorphic soles include
1). a highly strained peridotite unit of the hanging wall which
has concordant structures with underlying amphibolite and 2). an
underlying melange sequence and continental rocks.
The Madstone thrust displaces the Josephine peridotite (Josephine
ophiolite-162 Ma) onto amphibolites and underlying deformed
gabbros of the Chetco Intrusive Complex. The rocks associated with
the Madstone thrust do not display typical characteristics of
other metamorphic soles. Structural data, however, indicates
structures above and below the Madstone thrust are concordant, and
geochronology, which dates the deformation as Nevadan, indicates
that these rocks are associated with the emplacement of the
Josephine ophiolite.
In the hanging wall of the Madstone thrust, 20-40 meters of high-T
serpentinite mylonite occurs along the base of the Josephine
peridotite instead of the typical highly strained peridotite
mylonite unit in other soles. These serpentinites have lineations
that trend north-northeast and are structurally concordant with
the underlying amphibolites. The serpentinites show metasomatism
which probably resulted from the interaction with fluids derived
from the underlying amphibolite. In the foot wall, the
amphibolites are 200-300 meters thick and show two generations of
folds having hinges parallel to a north-northeast strectching
lineation. The amphibolites display a grain-size reduction and
asymmetric fabrics indicative of mylonites formed by progressive
simple shear.
The sense-of-shear criteria for the serpentinite mylonite and the
amphibolite, which are structurally concordant, indicate thrusting
of the Josephine ophiolite toward the north-northeast over the
Chetco Intrusive Complex. Thrusting continued in a north-northeast
direction during retrograde metamorphism as indicated by
sense-of-shear criteria in the phyllonite (retrogressed
amphibolite) next to the Madstone thrust. Also, the lower contact
of the amphibolite with the Chetco Intrusive complex is intrusive
and syntectonically deformed along with the amphibolite.
Various conditions during the emplacement are as follows: 1)
preliminary geochemical data suggest that the metamorphic sole is
not related to the Josephine ophiolite and may be related to the
root rocks of the Chetco Intrusive complex. 2) a geochronological
study (Harper and others, 1989) suggests that the metamorphism and
deformation occurred during the Nevadan Orogeny and indicates
cooling from ~450ºC at 153 Ma, intrusion of the pegmatite at
150 Ma, and cooling to ~350 at 153 Ma, intrusion of the pegmatite
at 150 Ma, and cooling to ~350ºC at 146 Ma. 3) Preliminary
geothermometry and geobarometry indicate relatively low P/T
metamorphism compared to other metamorphic soles. 4) Preliminary
d18O measurements indicate that metamorphic fluids were present
during serpentinization.
The metamorphic sole and regional geologic setting of the
Josephine ophiolite is distinct from other ophiolites. The sole
has no apparent inverted metamorphic gradient, it is lower in
overall temperature. The Josephine ophiolite was thrust over an
active magmatic arc rather than obducted onto a continental
margin. In addition, the ophiolite and overlying Galice Formation
were thrust beneath North America along the roof thrust (Orleans
fault), which produced regional greenschist metamorphism.
Geochronologic and structural studies indicate that the basal
Madstone thrust and the roof thrust were both active at
150±1 Ma, but the thrusting direction along the roof thrust
appears to have been west or northwest. The north-northeast
thrusting direction along the basal thrust is nearly parallel to
the inferred paleogeographic trends and almost 90º to
northwest thrusting directions along the roof thrust. On possible
interpretation is that the lineations formed perpendicular to the
thrusting directions. Another more plausible interpretation is
that the back arc basin in which the Josephine ophiolite formed
was imbricated due to oblique subduction. In this interpretation,
then, the north-northeast directed thrusting would be related to a
flattened strike-slip intra-arc wrench fault.
Grady, K.A., 1990. Geology and structure of the rocks associated
with the basal (Madstone) Thrust of the Josephine Ophiolite in
southwestern Oregon: evidence for a metamorphic sole. Unpublished
MSc. thesis, State University of New York at Albany. 160 pp.,
+xiii; 1 folded plate (map)
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 1990 G73
thesis (scanned
text) - 13.9MB pdf file
Plate 1 - Geology
and structures of the Chetco Lake area
coloured geological map (scale 1:6,700) 7.6MB
pdf file
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