Date of Award
Master of Science (MS)
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, Kristin A., "Geology and structure of the rocks associated with the basal (Madstone) Thrust of the Josephine Ophiolite in southwestern Oregon: evidence for a metamorphic sole" (1990). Geology Theses and Dissertations. 27.