Earth and Planetary Sciences ETDs

Publication Date

8-27-2009

Abstract

Magnetic anisotropy is widely used for measuring the orientation of minerals in many rock types. The anisotropy of magnetic susceptibility (AMS) is used extensively to determine magma flow fabrics in both intrusive and extrusive igneous rocks. The anisotropy of anhysteretic remanent magnetization (AARM) allows determination of the anisotropy of ferro/ferrimagnetic minerals in igneous rocks, which have a strong control on AMS. Chapter One investigates the relationship of magma flow to local structures in the Philipsburg Batholith, a Late Cretaceous intrusion that cross-cuts the Georgetown-Princeton Thrust in the Flint Creek Range of southwest Montana. Well defined, mostly subhorizontal AMS fabrics were measured from 119 sites in the Philipsburg Batholith. Areas with steep magnetic foliations along the Georgetown-Princeton Thrust and the Bungalow Fault are interpreted as zones of magma ascent. Rising magma was emplaced along the Georgetown-Princeton thrust at the top of a ramp. 40Ar/39Ar dates from 13 biotite separates suggest that the Philipsburg Batholith was emplaced over about two millions years, with an average age of 74.6 ± 0.3 Ma. These data suggest that Sevier-age thrust belt development in SW Montana had shifted to the east by ~75 Ma. Paleomagnetic data from 28 accepted sites in the Philipsburg Batholith show 9 to 16 ± 8° of west-down tilting compared to reference directions for the North American Craton. Tilting is due to further Sevier thrust belt development to the east or uplift in the hanging wall of the Anaconda Metamorphic Core Complex to the east. Chapter Two compares AMS and AARM fabric data in the Philipsburg Batholith, two intrusions from the Central Montana Alkalic Province, and dikes from the Spanish Peaks Igneous Complex, Colorado. Rock magnetic tests show that the Philipsburg Batholith contains a large population of multi-domain magnetite. The Shonkin Sag and Square Butte Laccoliths contain a mixture of multi-domain and pseudo single-domain magnetite. Four relationships between AARM and AMS fabrics are recognized. All three intrusions have more than one AARM/AMS relationship, suggesting that multi-domain magnetite is not the only control on AARM/AMS relationships. Varying degrees of anisotropy within different size fractions of magnetite may also affect AMS and AARM fabrics.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Roy, Mousumi

Second Committee Member

Selverstone, Jane

Project Sponsors

Geological Society of America

Language

English

Keywords

Paleomagnetism, Batholith, Anisotropy, Magnetism, Susceptibility

Document Type

Thesis

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