INVERTED METAMORPHIC GRADIENTS AND CRYPTIC CONTACTS SURROUNDING THE ENIGMATIC CONDREY MOUNTAIN DOME, KLAMATH MOUNTAIN PROVINCE, CA AND OR

The Klamath Mountain Province (KMP) is an accretionary province of Paleozoic to Mesozoic age, broadly defined by four main thrust-bounded terranes that generally young from east to west and dip shallowly east. Disrupting this sequence is the 650 km² Condrey Mountain Dome (CMD), a structural window exposing an outer section (OS) of greenschist rocks and an inner section (IS) of transitional greenschist-blueschist rocks in the center. A cryptic fault separates these schists. Exposures along the Scott River preserve evidence of the contact relations between overlying amphibolite-grade rocks of the Rattlesnake Creek terrane (RCt) and rocks of the CMD. Previous workers have described the contact as a thrust fault, a folded thrust fault, and a thrust fault later modified by normal faulting. Shear sense indicators offer a range of interpretations, from no sense to top-to-the-east.

On-going mapping and structural analysis provide the following constraints: 1) the contact between the RCt and underlying CMD is defined by an inverted metamorphic gradient from amphibolite to epidote amphibolite to greenschist facies down section over a ~2.5 km map distance; 2) down section rocks in the inverted gradient include the 156-159 Ma Slinkard pluton and enclosing, structurally lower migmatitic amphibolitic gneiss, underlain by upper epidote amphibolites and lower chlorite stilpnomelene schists of the OS; 3) migmatization was accompanied by (currently) down-to-west shearing along west-dipping shear bands. A new U/Pb (zircon) age from leucosomes in the shear zone is 155.32 ±0.30 Ma. 4) We equivocate about the exact placement of the contact between “high-grade” RCt and “low-grade” CMD. Currently, we favor placement at the base of migmatitic amphibolites, on top of epidote-amphibolite rocks. Tentative structural restorations imply that the currently west-dipping shear zone was east-dipping at the time of emplacement of high- on low-grade rocks. We speculate that the OS protolith may be a duplexed sliver of the ~170 Ma Western Hayfork arc and that the IS may reflect an early remnant of the Franciscan accretionary complex, as previous workers have suggested.