Metamorphic core complexes (MCC) provide a rare glimpse into thermomechanical processes in the lithosphere and play a substantial role in the evolution of the crust. The North American Cordillera contains a northwest trending line of MCCs, which have been extensively studied using bedrock thermochronology and modelling approaches to better understand extensional processes related to Cordilleran collapse. While these studies have proposed a wide variety of models to explain the timing and mechanism behind MCC formation, few have considered the syn-deformational basin record, which preserves a unique archive of sediment sources in adjacent MCC highlands. This study focuses on the Deer Lodge Valley, located in the hanging-wall of the Anaconda MCC. We utilize detrital zircon (U-Pb)-(U-Th)/He double dating in the context of stratigraphic and sedimentologic analyses, and HeFTy time-temperature modelling to reconstruct basin evolution. Stratigraphic analysis shows that the basin was dominated by deposition of coalescing alluvial fans, with sediment sourced directly from the footwall of the detachment fault. U-Pb maximum depositional ages indicate late Paleocene to early Eocene proximal basin sedimentation. (U-Th)/He analyses from U-Pb dated zircons range from 194-32 Ma; >70% of dates are Eocene. Preliminary HeFTy modelling shows a period of rapid cooling between 65-55 Ma, which is supported by short (<10 Myr) sediment lag times and inferred rapid exhumation in the MCC. Our findings support a link between MCC exhumation and basin formation. They further depict a potentially earlier period of MCC exhumation than previous work has proposed, indicating an earlier onset of extension in western Montana.