Development and Evolution of the Rattlesnake Creek Terrane, Klamath Mountains, Northern California

The basement of the Rattlesnake Creek terrane (RCT) in the Klamath Mountains is a mélange of metamorphosed sedimentary and igneous blocks. Recent work shows that the overlying RCT cover sequence has a North American provenance but formed after accretion to the continental margin, so it is unclear if the basement mélange formed exotic or endemic to North America. This study presents petrography and zircon geochronology from RCT metasedimentary blocks and crosscutting intrusions. The southernmost RCT preserves both Early Jurassic and Middle-Late Jurassic cover sequence deposits and records continental clasts and 33% pre-Mesozoic zircons at ~201 Ma, effectively none at ~191 Ma, and 79–90% from 168 to 163 Ma. During active magmatism 207–193 Ma, the RCT was receiving continental sediment, inconsistent with a distant intraoceanic arc. We interpret that the RCT subduction zone formed proximal to North America in the Late Triassic and that there was a sediment pathway to the RCT at ~201 Ma. During Middle to Late Jurassic rifting and subsequent Nevadan compression, the cover sequences were dismembered and incorporated into the mélange by tectonic and sedimentary processes. The age and provenance of metasedimentary deposits in the RCT is inconsistent with west-dipping subduction models in the Klamath Mountains region.