Tilted plutons in the Klamath Mountains of California and Oregon

Abstract

Geologic and petrologic evidence indicates that the Ashland pluton and the Wooley Creek batholith, two major plutons of the Klamath Mountain province, have been significantly tilted subsequent to emplacement. Late Cretaceous sandstone depositionally overlies the Ashland pluton and dips approximately 25 ø toward the ENE. Contact metamorphic assemblages in diverse wall rocks surrounding the Wooley Creek batholith suggest at least 30 ø of tilt toward the WSW. Subsequent erosion has exposed at least 12 km of structural relief in the Wooley Creek batholith. Tilting was accomplished, at least in part, by thrusting of the plutons and their host rocks over low-density rocks of the western Jurassic belt, followed by regional doming of deeply buried rocks of the underthrust terrane (Condrey Mountain Schist). Diagrammatic reconstruction of the plutons to pretilt positions suggests inward chemical fractionation of the Ashland pluton but upward fractionation of the Wooley Creek batholith and comagmatic Slinkard pluton. The results of this study indicate that rootless plutons in accretionary margins can undergo significant tilting, facilitating petrologic interpretations. In addition, the study suggests that for Klamath plutons, the effects of tilting must be taken into account before meaningful paleomagnetic interpretations can be made. , 1960, 1966, 1981]. These are, from west to east in ascending structural order, the western Jurassic belt, the western Paleozoic and Triassic belt, the Central Metamorphic belt, and the eastern Klamath belt [Irwin, 1981] (Figure 1). The western Paleozoic and Triassic belt has been further subdivided [Irwin, 1972] into three terranes, each bounded by eastward dipping thrust faults. These terranes are, from west to east in ascending structural order, the Rattlesnake Creek terrane, the Hayfork terrane, and the North Fork terrane (Figure 2). The Wooley Creek batholith and Ashland pluton were intruded into rocks of the western Paleozoic and Triassic belt. The Wooley Creek batholith has yielded Pb-U ages from zircon of approximately 161 Ma [Allen et al., 1982; Barnes et al., this issue]. K-Ar dates on hornblende and biotite from the Ashland pluton range from 133 to 166 Ma and cluster near 6059 SETTING The Klamath Mountain province of northern California and southern Oregon has been divided into four major lithostratigraphic belts [Irwin

Deformation and Metamorphism of the Marble Mountain and Pony Camp Areas, Western Triassic and Paleozoic Belt, Central Klamath Mountains, Northwestern California

“…Even with conservative thermodynamic esti mates, the ~162 Ma Wooley Creek contact metamorphism apparently occurred at much higher pressures (6.5 to 8.5 kbar) (Barnes et al, 1986) than those recorded by regionally meta morphosed rocks at equivalent structural posi tions (<3.7 kbar) in the Marble Mountains. Possible magmatic epidote (Barnes et al, 1986) in the structurally lower, coeval Slinkard pluton suggests similar high pressures.…”

“…The Wooley Creek batholith is exposed approximately 10 km west of the study area. Contact metamor phic assemblages along the northeast contact of the batholith, at about the same structural level as the lower WLa and upper BCa, indicate P-T conditions of 6.5 to 8.5 kbar and 650° to 750 ° C (Barnes et al, 1986).…”

“…Possible magmatic epidote (Barnes et al, 1986) in the structurally lower, coeval Slinkard pluton suggests similar high pressures. Implications of these data generally have been downplayed in regional syntheses (e.g., Coleman et al, 1988;Hacker et al, 1995).…”

“…Although stratigraphic thicknesses and areal extents of these rocks are difficult to determine, original thicknesses probably did not exceed 1 km, which would be comparable to the volume of rocks erupted from a single relatively short lived forearc or arc eruptive center. Even including Sawyers Bar metavolcanic and meta- (Barnes et al, 1986;Helper, 1986;Coleman et al, 1988;Donato,1989).…”

“…Igneous crystallization ages for metadikes that intrude the WTrPz to the north and south of the study area are 172 ± 2 Ma (U/Pb, zircon) (Saleeby, in Hill, 1985) and 173.2 ± 3.7 Ma, 174.6 ± 1.3 Ma (Ar/Ar, hornblende) . Wooley Creek batholith contact metamorphism overprints the regional fabric and is dated at ~162 Ma (Barnes et al, 1986;Wright and Fahan, 1988). To the south of the study area, in the Sawyers Bar region, regional greenschistfacies metamorphic fabrics are overprinted by contact metamorphism of the English Peak batholith, which crystallized at about 165 Ma (Ernst, 1990;.…”