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

Abstract

Metamorphic core complexes (MCCs) are domal geological structures that result from the exhumation of the mid-to-lower crust (e.g., Crittenden et al., 1980). These structures are significant because they represent locations of extreme crustal extension and provide illuminating windows into the thermomechanical properties of Earth's lithosphere (e.g., Platt et al., 2015;Whitney et al., 2013). First described in the western United States, MCCs consist of a ductilely deformed metamorphic-plutonic footwall separated from a brittlely deformed hanging wall by a low-angle normal fault (detachment fault) (Armstrong, 1982;Coney, 1980;Wernicke, 1981). Despite their widespread occurrence and tectonic significance, the origin of core complexes-specifically the regional tectonic processes that facilitate footwall exhumation from mid-crustal depths-remains controversial (e.g., Konstantinou et al., 2013). Several orogen-scale dynamic models have been proposed for MCC formation in the North American Cordillera, including: (a) a change in plate motions, (b) dynamic processes of the downgoing slab (e.g., slab rollback), and (c) late or post-orogenic collapse due to overthickened continental crust (e.g., Whitney et al., 2013). In the North American Cordillera, MCCs form an N-S trending belt that traces a pre-extensional lithospheric welt, where crustal thicknesses and

Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

“…Model results indicate the termination of rapid cooling likely associated with basement uplift by ∼50 Ma (Figure 7), approximately coincident with a regional shift from compressional to extensional deformation (e.g., Foster & Mark Fanning, 1997; Foster et al., 2001, 2010; Howlett et al., 2021). This transition in tectonic regime is associated with a widespread phase of volcanism (i.e., ignimbrite flare‐up; Best et al., 2016).…”

Where did the Arizona‐Plano Go? Protracted Thinning Via Upper‐ to Lower‐Crustal Processes

“…30 and 20 Ma (e.g., Gottardi et al., 2020). The onset of detachment faulting gets younger northward from northern Mexico (from 35 to 20 Ma) central Arizona and southward from south‐eastern California (from 24 to 20 Ma) to central Arizona (Gottardi et al., 2020; Howlett et al., 2021, and references therein). The Catalina MCC is controlled by the Catalina detachment fault (Figure 2; Dickinson, 1991), with the earliest fault initiation dated at ca.…”

Supplemental Material: Metamorphic Core Complex Dichotomy in the North American Cordillera Explained by Buoyant Upwelling in Variably Thick Crust

“…Because each MCC does not expose igneous rocks spanning the complete Late Cretaceous to mid-Cenozoic history, or such rocks have not yet been analyzed, we merged this dataset to elucidate regional trends in the igneous record. Although these are geographically distinct systems-spanning ~42 to 46°N latitude-the integrated dataset shows consistent trends of (1) a negative draw down in the Late Cretaceous-consistent with significant partial melting at this time, (2) a ɛHfT pull up in the Eocene during post-flat slab rollback, and (3) Eocene-Oligocene drawn downs during protracted crustal melting, melt mobilization, and assimilation (as discussed in Konstantinou et al, 2013 andHowlett et al, 2021). Furthermore, we can predict temporal variations between these systems based on the record of post-Laramide rollback volcanism and biotite-muscovite argon ages, which shows a ~15 Myr difference in ages, younging southward.…”

“…Data was compiled from the following sources. The hinterland data is comprised of three broad locations: (1) the Idaho Batholith complex (Gaschnig et al, 2011), (2) the Anaconda metamorphic core complex of western Montana (Howlett et al, 2021), and (3) the Albion-Raft River-Grouse Creek metamorphic core complex of northwest Utah (Konstantinou et al, 2013). Because each MCC does not expose igneous rocks spanning the complete Late Cretaceous to mid-Cenozoic history, or such rocks have not yet been analyzed, we merged this dataset to elucidate regional trends in the igneous record.…”