LATE JURASSIC DOMING AND CONSTRICTIONAL DEFORMATION IN THE CENTRAL KLAMATH MOUNTAINS, CA AND OR

Terrane-bounding shear zones hold the potential to record events related to juxtaposition of arc and related oceanic complexes, and inform us of the timing, temperature, depth, and kinematics of these punctuated, continent-growing events. In the Mesozoic terranes of the Klamath Mountain Province in California and Oregon, these shear zones extend 100s of km and define the distinctive arcuate nature of the province. The trace of the Nevadan-age Orleans thrust is ~700 km and is estimated to account for ~70, and perhaps as much as 110 km of east-directed shortening, during which the ca. 162 Ma Josephine marginal basin was closed. Deposition of the pre- and syntectonic Galice Formation continued through ca. 150 Ma. Exposed just to the east of the Orleans thrust is a structural window defined by the marginal Condrey Mountain Shear Zone (CMSZ).

The CMSZ exposes the greenschist-grade Middle Jurassic Condrey Mountain schist outer unit (CMSo) beneath the amphibolite to locally granulite-grade ophiolitic mélange of the Rattlesnake Creek terrane (RCt). Exposures of the CMSZ are best preserved within steam-polished outcrops along the Scott River, where the gneissic, locally migmatitic Gold Flat amphibolite (GFA) of the RCt records a strong L>S fabric parallel to layering, and hinges of recumbent isoclinal folds. Ultramafic blocks within the RCt have focused some of the constrictional fabrics into boudin-hinge zones and compositional layers in the GFA also appear boudined, with their long axes parallel to lineation. Similar structures to those observed in the GFA occur along the N and NE margin of the CMSZ. Leucosomes within the amphibolites within the CMSZ are ca. 155 Ma (southwest, GFA) and ca. 157 Ma (north, upper Applegate river). The age of doming of both units is currently placed in the Neogene based on uplift/tilting of Miocene strata to the NW and E of the CMS window. However, the late Jurassic melting event combined with our fabric data imply the shear zone is part of an earlier constrictional doming event resulting from underplating and uplift of the CMSo. We propose that changes in slab dip and morphology were the main driving force for doming and Nevadan orogenesis at this latitude.