RECORD OF PALEOZOIC TO MIDDLE JURASSIC TECTONICS IN THE RATTLESNAKE CREEK TERRANE, KLAMATH MOUNTAINS, NORTHERN CALIFORNIA

The Klamath Mountains in northern California and Oregon were formed by a series of Paleozoic and Mesozoic terrane accretions on the western margin of Laurentia. One of the most complex of these is the Rattlesnake Creek terrane. Interpreted to have formed in a fracture zone, the serpentinite-matrix mélange became the basement of a Late Triassic to Early Jurassic arc. A cover sequence of volcanic, volcaniclastic, and sedimentary rocks are deposited on top of the mélange, incorporated as blocks, and cut by 207-195 Ma dikes. Near Dubakella Mountain, the cover sequence was deposited in arc-parallel grabens later overprinted by westward thrusting. Although now separated by a west-verging thrust fault, the 177-168 Ma metasedimentary and metavolcanic rocks of the Western Hayfork terrane are interpreted to have been deposited on the Rattlesnake Creek terrane. Only a few coeval plutons are exposed. The Rattlesnake Creek terrane cover sequence is generally interpreted as an intraoceanic arc which accreted to Laurentia in the Jurassic. The interpretation of the Western Hayfork terrane has varied more because only part of the tectonic system is exposed. A recent study showed that a 174 Ma pluton intruding Laurentia to the east is geochemically similar to the Western Hayfork terrane deposited proximal to subduction of a young, hot slab. The additional observation of quartz-rich sediments means that the Western Hayfork terrane formed above an east-dipping subduction zone on the western margin of Laurentia, not in an intraoceanic arc.

The Rattlesnake Creek terrane and its cover sequence have the potential to record deformation associated with subduction initiation, island arc accretion, and overthrusting. We present preliminary field work and petrography from the Rattlesnake Creek terrane and cover sequence just south of the Oregon border. Compared to exposures farther south, the terrane is generally higher grade and consists of ultramafic, serpentinite, and limestone blocks within metasedimentary and metavolcanics rocks. Foliations are generally consistent with westward thrusting, but mesoscale and microscopic fabrics show multiple episodes of deformation. Geochronology will help constrain the timing of these deformational events to unravel the complex history of the Rattlesnake Creek terrane.