TESTING EXOTIC COLLISION VERSUS ENDEMIC RE-ACCRETION MODELS FOR LATE JURASSIC (NEVADAN) DEFORMATION IN THE KLAMATH MOUNTAINS PROVINCE: AGE AND PROVENANCE OF SEDIMENTARY ROCKS IN THE RATTLESNAKE CREEK TERRANE

Two distinct models are used to explain Late Jurassic orogeny in the Klamath Mountains Province (ca. 155–150 Ma; Nevadan orogeny, sensu stricto): (1) collision of a far-traveled exotic arc and its basement (the Western Klamath terrane built on the Rattlesnake Creek terrane) against western North America, or (2) re-accretion of a plate-margin volcanic arc and its basement that were rifted from the continental margin during an earlier episode of plate-margin extension (i.e., endemic; Western Klamath terrane built on Rattlesnake Creek terrane). We analyzed detrital zircon U-Pb ages of two medium-grained lithic wacke samples from the “cover sequence” of the Rattlesnake Creek terrane in the southern Klamath Mountains (Dubakella Mtn. 15’ quad.) using LA-ICP-MS to test the exotic-collision versus endemic re-accretion models.

On ternary petrofacies diagrams, both samples plot in the arc-orogen and lithic-recycled fields. In sample 16KM001 (n=99), age modes are present ca. 168, 188, 215, and 244 Ma (Mz=22% of all ages), ca. 265, 303, 358, and 416 Ma (Pz=20%), and ca. 575, 740, 1150, 1340, 1810, and 2640 Ma (Pc=56%). In sample 16KM003 (n=114), age modes are present ca. 163 and 205 Ma (Mz=6%), ca. 280, 355, and 535 Ma (Pz=30%), and ca. 1065, 1270, 1445, and 1785, and 2600–2500 Ma (Pc=64%).

The youngest age modes require that clastic rocks of the Rattlesnake Creek terrane cover sequence are at least late Middle–early Late Jurassic in age (ca. 168–163 Ma). This is ca. 30 Ma younger than a previously interpreted Late Triassic–Early Jurassic age (ca. 206–195 Ma), allowing for a re-interpretation of the plate tectonic setting of the basin. We interpret clastic rocks of the Rattlesnake Creek terrane cover sequence (1) as representing detritus derived dominantly from older accreted terranes of the Klamath Mountains Province, with input from a young-arc source, and (2) as recording the early stages of regional Middle–Late Jurassic supra-subduction zone basin formation. The deposition of recycled Klamath Mountains-derived detritus on Rattlesnake Creek terrane basement prior to Late Jurassic deformation demonstrates that collision of an exotic terrane was not the driver for Late Jurassic deformation. Instead, Late Jurassic deformation occurred as a result of closure of the supra-subduction zone basin and re-accretion of endemic terranes.