Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 21-4
Presentation Time: 9:00 AM-6:00 PM


GATES, Katie M., YOSHINOBU, Aaron S., BARNES, Calvin G., DAILEY, Shane R. and LEIB, Susan E., Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053

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.