HR: 0800h
AN: GP21A-0122 [Abstracts]
TI: Insights Into Magma Ascent During Shallow-Level Crustal Shortening From Magnetic Fabrics of the Philipsburg Batholith, SW Montana
AU: * Naibert, T J
EM: tjn@unm.edu
AF: University of New Mexico Department of Earth and Planetary Sciences, MSC03 2040
1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Geissman, J W
EM: jgeiss@unm.edu
AF: University of New Mexico Department of Earth and Planetary Sciences, MSC03 2040
1 University of New Mexico, Albuquerque, NM 87131, United States
AB:
Latest Cretaceous development of the Sevier fold and thrust belt in SW Montana overlapped spatially with silicic
magmatism. In the fold thrust belt, large volumes of magma were emplaced well east of the main magmatic
arc, now exposed as the Idaho Batholith. Hypothesized mechanisms for emplacement of magma within the
overthrust belt often involve magma ascent along shallow, west-dipping faults. The ~ 74 Ma (K-Ar method)
Philipsburg Batholith is a 122 km2 tabular granodiorite emplaced into deformed Precambrian Belt
Supergroup through Cretaceous strata. The Philipsburg Batholith lies in the upper plate of the Georgetown-
Princeton Thrust, NW of Anaconda, Montana and cross-cuts two other previously mapped faults. Anisotropy of
magnetic susceptibility (AMS) measurements of 122 sites from the Philipsburg Batholith define magnetic
foliations and/or lineations to test magma ascent along the Georgetown-Princeton Thrust. AMS fabrics in the
Philipsburg Batholith, dominantly defined by magnetite, are generally oblate or triaxial and are typically very
consistent at the site level. Preliminary fabric data show subhorizontal foliations across most of the batholith, with
steeply dipping foliations near the margins and a minor increase in foliation dip near the inferred fault trace. The
hypothesis of magma ascent along fault surfaces will be supported if further data confirm the concentration of
relatively steep foliation orientations across the trace of the Georgetown-Princeton thrust.
DE: 1518 Magnetic fabrics and anisotropy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting