CHANNEL FLOW HYPOTHESIS FOR K-PG MAGMATISM IN WESTERN MONTANA AND IDAHO

Pluton emplacement within convergent tectonic settings has been a fertile topic of research for decades. In the absence of in-situ melting of the upper crust (anatexis), how do large batholiths find space in a crustal environment that is undergoing orogenic compression and shortening? This question takes on direct relevance in western Montana and Idaho where major composite plutonic centers include the Idaho, Boulder and Pioneer batholiths, and magmatism was broadly synchronous with crustal shortening and thickening between 85-55 Ma. These plutonic centers define a corridor of Cretaceous-Paleogene magmatism that spans west-central Montana from the Bitterroot Lobe of the Idaho batholith on the west, to the Castle Mountains and Crazy Mountains Basin on the east. However, unlike mid-crustal magmatic systems exposed in the Idaho batholith, emplacement of plutons further east occurred within evolving fold-and-thrust salients and the foreland basin at relatively shallow depths (1-10 km), mostly as thin (meter- to kilometer-scale) tabular sheets or laccoliths. Many intrusive bodies spatially coincide with major thrust zones or are superjacent to major thrust faults. Field relationships in southwest Montana show a clear spatial correlation between plutons and the tops of major footwall thrust ramps. On a regional scale, the diachronous (eastward-younging) and progressively shallower levels of intrusion suggests a direct coupling with the evolving fold-thrust belt through processes similar to channel flow. Synconvergent channel flow is a thermo-mechanical model widely applied to the Himalaya and other orogens. Lateral stress gradients drive extrusion or viscous flow of mid-crustal rocks and melt towards an orogenic foreland, dynamically coupling the deep metamorphic/plutonic hinterland to the shallow foreland fold-thrust belt (Godin et al., 2006). Channel flow is a viable mechanism to consider for the K-Pg “flat-slab” magmatic arc environment of western Montana, whereby lateral crustal stress gradients induced by the thickened hinterland of Montanaplano, coupled with erosional feedbacks driven by seasonal monsoons from the Western Interior Seaway, helped to create a magmatically-impregnated orogenic wedge.