DETRITAL RECORD OF SYNCONVERGENT EXTENSION IN THE ANACONDA METAMORPHIC CORE COMPLEX

Metamorphic core complexes (MCC) are a critical component in our understanding of the dynamic and thermomechanical processes that thin the lithosphere and play a significant role in the differentiation of the crust. The North American Cordillera contains a northwest trending line of MCCs, which have been extensively studied using a variety of bedrock thermochronologic techniques. An often-overlooked dataset in determining the timing and driving mechanisms behind MCC formation is the synextensional basin record, which preserves a unique archive of source region evolution and the initial surface response to high-magnitude, localized extension. We present new (U-Th)/He thermochronology and HeFTy inverse thermal history models on U-Pb dated detrital zircon samples from the Deer Lodge Valley, a synextensional basin situated in the hanging wall of the Anaconda MCC. We report 63 detrital zircon (U-Th)/He analyses ranging from 613 Ma – 11 Ma, 67% of grains producing Paleocene-Eocene cooling ages, and > 50% of grains yielding lag times < 10 m.y. HeFTy inverse thermal history models show all samples remain below the zircon partial retention zone (PRZ) until ca. 70 Ma, then rapidly cooled between ca. 70 – 55 Ma. When compared to preexisting basement thermochronologic data from the southern Anaconda MCC footwall, multiple thermochronometric systems cooled through the PRZ in the northern Anaconda MCC footwall prior to the previously recognized onset of exhumation and extension ca. 55 Ma. In this interpretation, an earlier period of exhumation-related cooling ca. 62 – 55 Ma is due to focused thermal weakening of overthickened crust within the hinterland following the emplacement of multiple Cretaceous-Paleocene batholiths in the MCC footwall. This period of cooling and exhumation is syncontractional with eastward propagation in the Helena Salient, and aligns with other early phases of cooling within MCCs located in Idaho, Montana and British Columbia.