COMPOSING A TECTONIC HISTORY OF A MÉLANGE, RATTLESNAKE CREEK TERRANE, KLAMATH MOUNTAINS, NORTHERN CALIFORNIA

The Klamath Mountains region in Northern California contains multiple terranes that have mixed geologic records creating a large-scale story. The Rattlesnake Creek Terrane (RCt) in particular is a tectonic mélange of different metamorphosed sedimentary, mafic volcanic-plutonic, and ultramafic rocks with an extensive geologic history that includes tectonism, volcanism, and accretion. The geologic setting in which the RCt initially formed is speculated to be either an endemic or exotic model. An endemic model will show evidence of continental factors playing a part in the formation of the RCt, while an exotic model will display evidence of foreign mafic samples. Field work, sandstone petrography, and detrital zircon geochronology is explored to explain this speculation of the history of the RCt.

We present new geochronologic and petrographic data from metasedimentary and metaigneous blocks in the RCt. Some blocks have significant age populations around 162 Ma, 250 Ma, 500 Ma,1200 Ma, 1400 Ma, 1700 Ma, and 2500 Ma, consistent with North American provenance. Other blocks, including mafic heavy block, have a single Early or Middle Jurassic age population with a few older ages similar to the first group. These patterns suggest that the RCt incorporated North American-derived material prior to deposition of the overlying Middle Jurassic cover sequence. Maximum depositional ages will provide constraints on the accretion history. Although the blocks are metamorphosed, petrographic point counting can help us understand the protoliths and even their tectonic settings. The ultimate goal of this study is to determine the setting in which the RCT formed to provide more accurate information about the geologic history of the Klamath Mountain Region.