DEVELOPMENT OF AN OROGENIC PLATEAU DURING MID-CRUSTAL FLOW IN THE CORDILLERAN HINTERLAND: EVIDENCE FROM FOOTWALL ROCKS OF THE ANACONDA METAMORPHIC CORE COMPLEX OF WESTERN MONTANA

The Anaconda metamorphic core complex exposes a crustal cross section through mid- to upper-level continental crust in a region where Cordilleran arc plutonism formed an east-protruding magmatic corridor that overlapped in space and time with foreland contraction. We synthesize our detailed mapping, structural analysis, U-Pb geochronology, thermochronology and thermobarometry to document the geometry and timing of deformation, magmatism and metamorphism. Regionally, Belt Supergroup through Cretaceous stratigraphic thickness exceeds 12,000m. Within the core complex footwall, this sequence underwent amphibolite-facies metamorphism, extreme tectonic attenuation, and nappe-style folding during quartz diorite–granodiorite plutonism at ca. 78–75 Ma. Thermobarometry gives pressures of 4.6–6.0 kbar for peak metamorphism and deformation. Late Cretaceous plutons intruded as laccoliths within sedimentary sequences and along thrust faults. Magmatic and sub-solidus fabrics in late, shallow plutons (e.g., Boulder batholith) are weak, despite emplacement during regional contraction. Deeper level plutons show well developed fabrics (e.g., Storm Lake, quartz diorite stocks) oriented parallel to high temperature fabrics in wall rocks. Regional scale isoclinal-recumbent folds (e.g., Fishtrap Creek nappe) developed within Belt metasediments are accompanied by an axial planar foliation and extreme thinning along fold limbs. Thinning is particularly prominent in metapelites and calc-silicates. Upright, west-verging folds deformed earlier-formed nappes and foliation. We hypothesize that weak fabrics in shallow plutons resulted from extensional/neutral strain environments in the upper, rigid parts of an orogenic plateau. West-verging folds are interpreted as “backfolds” that developed at the interface between a viscous mid-crustal channel and a rigid semi-passive lid. Stratigraphic attenuation, isoclinal-recumbent folds and highly strained plutons represent high-temperature lateral flow within what was a particularly thick, “orogenically fertile” stratigraphic section. Concurrent attenuation, isoclinal folding, backfolding, and silicic magmatism are consistent with mid-crustal channel flow and orogenic plateau models proposed for many ancient and ongoing orogenic belts.