The Late Cretaceous (ca. 75 Ma) Philipsburg Batholith in SW Montana provides an excellent setting to examine the emplacement mechanisms of large-volume silicic plutons in the Sevier fold-and-thrust belt of the western U.S. Cordillera. Magnetic fabrics, determined using the anisotropy of magnetic susceptibility from 119 sites and anisotropy of anhysteretic remanent magnetization data from a subset of these sites, suggest that the Philipsburg Batholith is a tabular body and that its emplacement involved subhorizontal magma flow controlled by local thrust faults, including the Georgetown-Princeton thrust, which served as conduits for magma ascent. Field relations and magnetic fabric data, which are typically well characterized at the site level and dominantly subhorizontal except near major thrust faults, suggest that the Philipsburg Batholith was emplaced within a fault-bend fold at the top of a ramp in the Georgetown-Princeton thrust system. New 40Ar/39Ar age spectrum data indicate that the entire Philipsburg Batholith was emplaced rapidly at ca. 74.8 ± 0.1 Ma. Paleomagnetic directional data indicate that the Philipsburg Batholith has experienced ∼9°–16° ± 8° of west-side-down tilt (tilt estimate and associated error are based on a new approach described herein) since emplacement in the Late Cretaceous. The data do not support any internal deformation of the batholith since emplacement and suggest that Sevier-age thin-skinned deformation in the Flint Creek Range had ceased by ca. 75 Ma. The modest magnitude of observed tilt could have occurred during subsequent Cenozoic extension. The inferred mode of emplacement of the Philipsburg Batholith may typify how large volumes of silicic magma are emplaced in shallow, fold-and-thrust belt settings.
- Received 28 October 2009.
- Revision received 19 February 2010.
- Accepted 8 April 2010.
- © 2010 Geological Society of America