STRUCTURAL SIGNIFICANCE OF L TECTONITES IN PART OF THE WESTERN HAYFORK TERRANE, KLAMATH MOUNTAINS, CALIFORNIA

The formation of L tectonites is little understood and scarcely studied, but it is likely an important part of plastic deformation in the crust. I present a case study of a km-wide domain of L tectonites in the Pigeon Point high-strain zone (PPHSZ) in the Klamath Mountains, California. The PPHSZ developed under greenschist- and local amphibolite-facies conditions. It cuts metasedimentary and mafic metavolcanic rocks and accommodates top-to-the-W displacement coupled with zone-normal contraction and transport-parallel elongation. Detailed mapping, cross-section reconstructions, and strain analyses show that L tectonites formed in a narrow corridor in the apex of the convex-upwards, lens-shaped PPHSZ. Outside of the domain of L tectonites, the zone is characterized by plane-strain conditions, and flattening strains are not exposed. These observations indicate that the PPHSZ records bulk strain in the constrictional field. Quartz c-axis fabrics collected from L tectonites of micaceous quartzite define asymmetric double girdles centered about the lineation. Biotite and muscovite c-axis fabrics form diffuse girdles centered about the lineation with weak maxima that define a foliation not visible in the hand specimens.

In my preferred kinematic model a component of strike-parallel, transport-perpendicular contraction was localized in the domain of L tectonites. The convex-upwards shape of the upper high-strain-zone boundary is the only obvious mechanism that can account for the localization of this component of deformation and the resulting formation of L tectonites in the PPHSZ. Plastic deformation appears to have been catalyzed by local magmatic heating, and the domain of L tectonites is centered about a local zone of amphibolite-facies metamorphism associated with syntectonic mafic/ultramafic composite dikes. This local increase in temperature is inferred to have caused the upwards doming of the high-strain-zone boundary. Otherwise, rheological and metamorphic transitions did not affect the localization and/or partitioning of constrictional deformation. This deformation geometry developed in response to thrust loading associated with regional contractional deformation and terrane amalgamation during the mid-Jurassic, and it represents flow from beneath a major thrust plate.