Contrasts in Thermal Maturity within Terranes and across Terrane Boundaries of the Franciscan Complex, Northern California

The Franciscan subduction complex of northern California includes several tectonostratigraphic units. We have used the technique of vitrinite reflectance to document levels of thermal maturity for the Central Belt and the Yager, Coastal, and King Range terranes. Spatial variations in thermal maturity help define the thermal structure of each Franciscan terrane, and each terrane displays a different geometry of thermal structure. These differences are controlled by the style of structural deformation, the overprinting effects of deformation following metamorphism, and thermal overprinting caused by hydrothermal circulation. Anomalies along and contrasts across terrane boundaries help establish the relative timing and the structural mechanisms of terrane amalgamation. For example, localized anomalies are evident along the Eel River fault (Central-Yager boundary), and these increases in thermal maturity may be due to shear heating, hydrothermal discharge, or conductive heat transfer across the fault. Cross-cutting relationships also exist between isoreflectance contours and terrane boundaries; these discordant geometries were caused by post-amalgamation hydrothermal overprinting of the King Range terrane and by late-stage fault dislocation along the Yager-Coastal boundary.