The role of fluids in the structural and thermal evolution of large-scale crustal extension is not fully understood. This project will examine possible fluid-related thermal variations and the depth to which meteoric waters can penetrate in such systems. The research approach is to study the oxygen and hydrogen isotopic characteristics of fluid-rock interactions in a well-studied Cordilleran metamorphic core complex (Whipple Mountains of southeastern California). Results will be applicable to thermal and tectonic models for extensional tectonic regimes.