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Flow Fabric Determination of Two Mesoproterozoic Midcontinent Rift Dike Swarms, Northeastern Minnesota

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Flow Fabric Determination of Two Mesoproterozoic Midcontinent Rift Dike Swarms, Northeastern Minnesota FLOW FABRIC DETERMINATION OF TWO MESOPROTEROZOIC MIDCONTINENT RIFT DIKE SWARMS, NORTHEASTERN MINNESOTA A thesis submitted to the Kent State University Graduate College in partial fulfillment of the requirements for the degree of Master of Science by Elizabeth May Fein May, 2009 Thesis written by Elizabeth May Fein B.A., Oberlin College, 2003 M.S., Kent State University, 2009 Approved by ___________________________________, Advisor Daniel Holm ___________________________________, Chair, Department of Geology Daniel Holm ___________________________________, Dean, College of Arts and Sciences Timothy Moerland ii DEPARTMENT OF GEOLOGY THESIS APPROVAL FORM This thesis, entitled Flow fabric determination of two Mesoproterozoic midcontinent rift dike swarms, northeastern Minnesota has been submitted by Elizabeth Fein in partial fulfillment of the requirements for the Master of Science in Geology. The undersigned member’s of the student’s thesis committee have read this thesis and indicated their approval or disapproval of the same. Approval Date Disapproval Date _______________________________ _______________________________ Daniel Holm Daniel Holm _______________________________ _______________________________ Donald Palmer Donald Palmer _______________________________ _______________________________ David Schneider David Schneider iii Table of Contents List of Figures…………………………………………………………………………....v List of Tables…………………………………………………………………………….vi Acknowledgements…………………………………………………………………….vii Abstract ………………………………………………………………………………….1 1. Introduction Role of dikes in continental rifting……………………………………………..3 Midcontinent Rift System tectonic setting…………………….………………6 Previous work on Midcontinent Rift System dike swarms…………………16 Research focus and hypotheses……..………………………………………21 2. Magnetic Fabric Study Technique background………………………………………………………..24 Methodology…………………………………...……………………………….32 Data Analysis…………………………………………………………………..42 Results………………………………………………………………………….47 Interpretations………………………………………………………………….58 Conclusions…………………………………………………………………….67 References Cited……………………………………………………………………....72 Appendix...……………………………………………………………………………...78 iv List of Figures 1. Tectonic rift models.…………………………………………………………………4 2. Global tectonic context for the Midcontinent Rift System.………………………7 3. Midcontinent Rift System map.………………………………...…………………..8 4. Mesoproterozoic magnetostratigraphy.………………………………………….12 5. Midcontinent Rift System evolution in the Lake Superior region.……………..14 6. Sample site location maps.……………………………………………………….19 7. The AMS ellipsoid and models of magma flow in dikes….……………………26 8. Carlton County dike swarm description.………………………………………...33 9. Duluth dike swarm description.…………………………………………………..35 10. Steps in the process of obtaining AMS data…………………………………..38 11. Normal vs. inverse AMS fabrics as functions of magnetic grain size……….45 12. Low field and high field magnetic susceptibility plots…………………………49 13. Directional AMS results, 13 Carlton County swarm dikes……………………51 14. Directional AMS results, 13 Duluth swarm dikes.……………………………..52 15. Site averaged AMS stereoplots………………………………………..………..55 16. Carlton County dike swarm magnetic lineation maps………………..……….56 17. Duluth dike swarm magnetic lineation maps…………………………………..57 18. AMS fabrics and dike properties plots………………………………………….62 19. Tectonic interpretation model of an evolving rift environment……………….65 v List of Tables 1. Dike swarm comparison……………………………..........................................17 2. Sampled dike locations……………………………………………………………79 3. Field data……………………………………………………………………………80 4. Site-averaged AMS tensor parameters……………………………………….....81 5. Site-averaged AMS anisotropy factors…………………………………………..82 6. Site-averaged AMS principal directions…………………………………………83 7. Rotated site-averaged AMS principal directions………………………………..84 8. High-field magnetic hysteresis data………..…………………………………….85 vi Acknowledgements Daniel Holm provided consistent and thoughtful guidance and a model of continuous improvement to strive for throughout every stage of this research, from proposal conception through manuscript writing and everything in between. Eric Ferré shared his extensive expertise as well as his rock magnetic laboratory facilities at Southern Illinois University, where Justin Skord performed the high field tests. France Belley and Kevin Butak were also readily helpful in the lab. Donald Palmer and David Schneider supplied significant and constructive comments during the research proposal and defense stages of this work. Stephanie Maes kindly entertained my questions and offered valuable advice. Audrey and Gordey Madson and Astrid Holm made field work luxurious by generously providing accommodations and home cooked meals in Duluth, MN. Susanna Fein and David Raybin provided love and financial support, without which this work would never have been possible. My parents also lent their combined editorial eye to the manuscript itself. Invaluable support for the completion of this work was also given by: Yonathan Admassu, Cristina Robins, David Waugh, Merida Keatts, Karen Smith, Kate Harper, Edith Fein, Carolyn Fein, Carolyn Turner Schneiderman, Carolyn Tustin- Gregory, Debbie Sellers, and Ray Leone. This thesis is dedicated to the memory of Ooky, who was with me at the start, and to Maudey, who is with me now. Funding for this project was provided by student research grants from the: Geological Society of America; Institute on Lake Superior Geology; Sigma Xi; Kent State University Graduate Student Senate; and Kent State University Department of Geology (School of Hard Rocks Mineral Resources Award). vii Summary 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 3.0×10-2 SI volume, 1 2 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. Introduction Role of dikes in continental rifting Continental rifting leads to lithospheric thinning and, subsequently, to decompression and partial melting of underlying hot, ultramafic asthenosphere to produce basaltic magma (Fig. 1A). During extension, vertical thinning of the crust via tectonic processes such as normal faulting is partially offset by vertical thickening associated with the addition of crustal mass via magmatic processes. Vertical magmatic thickening occurs both at the top of the crust through subaerial eruption of igneous material and, often simultaneously, within the lower crust through emplacement of batholiths and stocks. Continental rift settings are thus ideally suited for studying the interplay between deeper crustal plutonism and upper crustal volcanism. During continental rifting, the maximum principal
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