EXTENSION OF THE ANACONDA METAMORPHIC CORE COMPLEX: LOW-TEMPERATURE THERMOCHRONOLOGY FROM THE FOOTWALL OF THE ANACONDA DETACHMENT, SOUTHWEST MONTANA

The Anaconda Metamorphic Core Complex (AMCC) is located ~200 km west of Bozeman, Montana and is the easternmost in a group of Cordilleran metamorphic core complexes formed during Eocene time. Core complex formation has been attributed to variable causal mechanisms, generally aligning more closely with one of two end-member explanations: (1) initial crustal extension caused by a change in regional tectonic stress, followed by magmatism and pluton emplacement driven by decompression, or (2) initiation of magmatism leading to onset of extension, with pluton emplacement providing the necessary thermal weakening and buoyancy to drive extension along a low-angle normal fault. We applied geochronologic and thermochronologic dating techniques to determine the dynamics of AMCC formation in the context of the two end-member models. Zircon (U-Th)/He analysis was conducted for four samples from footwall rocks of the Anaconda detachment, yielding 18 interpretable dates. Dates range from 41.38 ± 0.56 Ma to 24.73 ± 0.33 Ma for single crystals, mainly clustering between 38-28 Ma. These new data supplement previously published higher- (Ar-Ar) and lower-temperature (apatite fission track) data that record 53-39 Ma and 40-20 Ma cooling, respectively. Zircon U-Pb crystallization ages indicated temperatures >900 °C until 65-60 Ma. HeFTy inverse modeling was used to test a range of potential thermal histories against our data. Time-temperature paths generated in HeFTy favored initially rapid cooling through 450-300 °C by ~45 Ma, approximately 15 Myr after pluton emplacement in the AMCC footwall. Therefore, we interpret that pluton emplacement was the driver of core complex formation rather than a result of footwall decompression. Based on our data and thermal modeling, we interpret that the AMCC formed between 45-25 Ma, making it one of the earliest-formed core complexes in the North American Cordillera. Our data is consistent with the previously observed correspondence between the Eocene trench-ward magmatic sweep and core complex extension. The potential dynamic link between extension and magmatism explains why core complexes are expected to track slab rollback.