Magmatism and Extension in the Anaconda Metamorphic Core Complex of Western Montana and Relation to Regional Tectonics

• Published in: Tectonics (Washington, D.C.) D.C.), 2021-09, Vol.40 (9), p.n/a
• Main Authors: Howlett, Caden J., Reynolds, Aislin N., Laskowski, Andrew K.
• Format: Article
• Language: English
• Subjects: Cenozoic; Cordilleran tectonics; core complex; Crustal thickness; Dating techniques; Earth crust; Eocene; Evolution; flat slab; Geochronology; Geochronometry; Geologic mapping; Igneous rocks; Lithosphere; Magma; Palaeocene; Paleocene; Plate tectonics; Plutons; Radiometric dating; Removal; Rock; Rocks; Tectonics; Uplift; Volcanic activity; western United States; Zircon
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1029/2020TC006431

Metamorphic core complexes (MCCs) are a product of crustal extension, but their dynamics are still debated. Early research suggests that the formation of MCCs in the western United States was due to gravitational collapse of crust that had been thickened during Cordilleran orogenesis. However, the instability of overthickened crust alone cannot explain the diachronous formation of core complexes with a strong spatial dependency, as there was relatively uniform crustal thickness along strike of the Cordillera. For this reason, there is an interest in what role other lithospheric processes (such as subducted slab removal) play in the evolution of MCCs. We investigate the role of such processes by determining the temporal relation between magmatism and extension in the Anaconda MCC (AMCC) of western Montana. Geologic mapping, zircon U‐Pb geochronology, and zircon (U‐Th)/He thermochronology reveal that the initiation of extension in the AMCC in the Eocene (∼53 Ma) began at least 3 Myr after the emplacement of voluminous Paleocene two‐mica plutons. We interpret that the AMCC is an example of a core complex that was primed for extension by magmatic thermal weakening and suggest that foundering of the Farallon flat slab and the onset of the ignimbrite flareup in western Montana was responsible for the initiation of AMCC extension. An updated compilation of MCC cooling ages and Cenozoic volcanic activity across the western United States supports previous interpretations that the removal of Farallon oceanic lithosphere likely initiated MCC exhumation in some regions. Plain Language Summary Horizontal extension of the Earth's crust can lead to topographic uplift and uncover rocks from depths of 10–30 km (the middle crust). The resulting geologic feature, consisting of ductilely deformed rocks underlying a low‐angle normal fault, is known as a metamorphic core complex (MCC). MCCs are important because they record properties of crust that would otherwise be unreachable. Despite their importance, the mechanisms that cause core complexes to form are not well understood. In this study, we use field and radiometric dating techniques to determine the role that magmatism (intrusion of molten rock into the crust) served in the evolution of the Anaconda MCC in western Montana. We also compile data from the western United States, which suggest that the removal of a previously subducted tectonic plate from beneath North America caused some core complexes to form. Key Points Vol