POLYPHASE STRUCTURAL EVOLUTION ALONG THE SOUTHERN CONTINUATION OF THE TEHAMA-COLUSA MÉLANGE, NORTHERN CALIFORNIA COAST RANGES

The eastern Coast Ranges of northern California record a multi-stage deformation history throughout the transition from a convergent to a transform continental margin. Origins of the serpentinite matrix mélange that marks the interface between the Franciscan and lower Great Valley Group are enigmatic, possibly in part due to a complex overprinting deformation history. We present new geologic mapping between Clear Lake and Lake Berryessa, focusing on the Coast Ranges serpentinite belt, the Great Valley Group, and the modern Bartlett Springs fault system. Based on similar textures and block affinities, the serpentinite matrix mélange in this region likely correlates to the Grizzly Creek and Tehama - Colusa mélanges to the north. Folding and shearing of fossil-bearing Great Valley Group sedimentary blocks into the mélange suggests reactivation and polyphase deformation. We decipher at least three stages of post-formation deformation of the serpentinite mélange. These include: 1) contractional deformation, which produced fold sets in Great Valley Group sedimentary blocks and klippe, as well as in the Coast Range Ophiolite; 2) extension, characterized by high-angle, down-to-the-east offset bringing the serpentinite mélange and Coast Range Ophiolite up in the footwall; and 3) a final transition to transpression, uplift, and the formation of a dextral-sense "hook" structure. New LiDAR mapping of Quaternary river terraces and stream profile analysis along Cache Creek tributaries suggest ongoing uplift. This work highlights the multi-stage deformational history recorded by the serpentinite matrix mélange and stresses the importance of rheological contrasts for localizing deformation during stages of plate boundary evolution.