Magnetic fabric, paleomagnetic, and 40Ar/39Ar geochronologic data bearing on the emplacement of the Late Cretaceous Philipsburg Batholith, SW Montana fold-and-thrust belt

Abstract

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 fl ow 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 40 Ar/ 39 Ar 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.

Geometry and growth of syn-tectonic plutons emplaced in thrust shear zones: Insights from Abu Ziran Pluton, Egypt

“…Further, there is yet an ongoing debate about magma ascent and emplacement in regions of dominant compressional stresses which are considered to be sites of space denying conditions (e.g., Hamilton 1994;Hutton 1996; . Several field-based studies have drawn attention to the close relationship between pluton emplacement and the geometry of flats and ramps in the active thrust faults (e.g., Shimura 1992;Karlstrom et al, 1993;Toyoshima et al, 1994;Searle 1999; Kalakay et al, 2001;Musumeci et al, 2005;Naibert et al, 2010). Hutton (1996) stressed the role of sites of local dilation along thrust faults as potential sites for accommodating ascending magma, especially in the middle and upper crustal levels.…”

Internal fabrics of the Idaho batholith, USA

“…Fabric development may result from internal (e.g., flow, mixing, thermal convection) or external (regional strain) processes. Previous workers have interpreted magmatic fabrics in plutons as a record of either magmatic processes at the emplacement level (Cruden and Launeau, 1994;Tobisch and Cruden, 1995;Archanjo et al, 1999;Ferré et al, 1999;de Saint-Blanquat et al, 2001Parada et al, 2005;Stevenson et al, 2007;Naibert et al, 2010;Archanjo and Campanha, 2012;Gutiérrez et al, 2013;Morgan et al, 2013) or regional strain during syntectonic emplacement (Brun and Pons, 1981;Brun et al, 1990;Archanjo et al, 1995;Leblanc et al, 1996;Benn et al, 1997Benn et al, , 1999Benn et al, , 2001Sen et al, 2005;Mamtani et al, 2013). Solid-state (or subsolidus) fabrics in granitic intrusions are formed by crystal-plastic deformation of phases in the absence of melt.…”