THE ANATOMY OF A SHEAR ZONE: EVIDENCE FOR CONSTRICTIONAL DEFORMATION AND CRUSTAL THICKENING WITHIN THE NEVADAN-AGE CONDREY MOUNTAIN SHEAR ZONE

Detailed geologic mapping at the 1:200 scale was completed along a transect that crosses an Upper Jurassic shear zone within the Klamath Mountains Province of northern California. The Condrey Mountain Shear Zone (CMSZ) juxtaposes the amphibolite grade migmatitic gneiss of the Gold Flat Amphibolite (Rattlesnake Creek terrane) above the epidote amphibolite grade schists of the outer Condrey Mountain schist (CMS) along a domed thrust contact. This inverted metamorphic gradient is not a unique feature in the Klamath Mountains, with the first “upward increase” in grade/structural complexity having been observed by Greg Davis in the Central Metamorphic belt some fifty years ago.

Structural datasets obtained along this transect and in complementary locations elucidate a complex relationship between the RCt and CMS. These structures include the parallel alignment of linear fabrics that plunge moderately west including 1) plagioclase and amphibole stretching lineations within dm-scale boudinaged L-tectonite lenses of the gneissic-migmatitic RCt, 2) the long axes of the those boudins, 3) fold-hinges of sheath folds, and 4) localized development of rods whose long dimension is parallel to the dominant trend of other linear features. There is no obvious structural or lithological discontinuity that defines this contact, rather the CMSZ comprises a thick, approximately 1.5 km mélange zone containing localized discordant contacts between gneiss and schist and a zone of brecciated clasts suspended in a schistose matrix. Planar fabrics within the hangingwall are folded about an axis that plunges moderately west while planar fabrics within the footwall rocks have not been folded.

The position of the CMSZ at the base of an overly thickened crustal mass during terrane amalgamation and thrust loading resulted in a widening of the zone of deformation. Additional heat from the adjacent 158 Ma Slinkard pluton and partial melting of the amphibolites within the shear zone at the base of the hangingwall focused flow and the localization of the constrictional fabrics that define much of this domain, although similar structures are observed away from plutons. We interpret the inverted metamorphic gradient as the product of synchronous thrust-loading and initial doming of the “hot” RCt above the “cold” CMS in Late Jurassic time.