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Degree

MS, Kent State University, College of Arts and Sciences / Department of Geology, 2009.

Abstract

Dike structures represent evidence of planar conduits along which magma is transferred via flow in the upper crust. This study documents regional-scale igneous flow patterns in two Midcontinent Rift System (MRS) mafic dike swarms by measuring anisotropy of magnetic susceptibility (AMS) fabrics as a proxy for magmatic fabrics. The 1100 Ma MRS stretches about 2,300 km across the central North American continent and comprises one of the thickest packages of igneous and sedimentary rocks in the world. The Carlton County (CC) and Duluth dike swarms, located in and around Duluth, MN, are proximal but distinct in strike pattern, age, and chemical composition. The subparallel, reverse polarity (older) CC dikes intrude Paleoproterozoic metagreywackes, whereas the more irregularly striking, normal polarity (younger) Duluth dikes intrude MRS volcanic rocks. The dikes in both swarms appear massive and lack visible flow structures, making traditional, macroscopic fabric measurement impossible. AMS, defined in this study by the preferred orientations of magnetic mineral grains, provides for sensitive delineation of fabrics in apparently isotropic rocks. Using a Kappabridge KLY4-S susceptibility bridge at field intensity 300 A/m, a significant measure of magnetic fabrics was achieved for 32 oriented block samples from 26 dikes (13 from each swarm). The bulk magnetic susceptibility (Km) for all 530 cubes (an average of ~17 per sample) yielded a mean value of 3.0x10^-2 SI volume, meaning that the magnetic signal is robust and likely dominated by ferromagnetic phases. The mean corrected degree of magnetic anisotropy (Pj) for both swarms is 1.036 ± 0.006 with a range of 1.002-1.142. A plot of Pj versus Km also suggests that ferromagnetic phases control the AMS signal in all samples.

The principal directions cluster well at most sample sites. Site-averaged directional results for the CC dikes indicate mostly normal AMS fabrics with subvertical to steeply inclined magnetic lineations that cluster relatively consistently. In contrast, site-averaged directional results for the Duluth dikes are more complex and indicate mostly inverse AMS fabrics, with the interpretable normal sites preserving oblique to the SW magnetic lineations. The inverse AMS fabrics in the Duluth swarm rocks may be a result of the influence of tiny single-domain magnetite grains or may be attributable to the influence of a more complex stress-state during emplacement within the rift axis.

The AMS data indicate vertical regional magma flow in the off-axis CC dike swarm and more complex subvertical to oblique SW to NE regional flow for the on-axis Duluth swarm. Vertical dike emplacement is predicted above a proposed 500-km-radius plume head, consistent with the CC swarm results. However, local variations in stress state may have led the on-axis Duluth swarm to deviate from this model. This study provides evidence that a single long-lived regional magma source potentially fed both sets of intrusions as the MRS evolved through time via vertical to oblique migration of rift melts from depth.

Subject Headings

Geology

Keywords

Midcontinent Rift; continental rift; dikes; anisotropy of magnetic suscetibility; AMS

Committee / Advisors

Daniel Holm, PhD (Advisor)
Donald Palmer, PhD (Committee Member)
David Schneider, PhD (Committee Member)

Pages

92p.

Document number: kent1232051971
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=kent1232051971

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