Anisotropic Magnetic Susceptibility of Felsic Flows on the North Shore of Lake Superior, Minnesota

Ryan Porter

Whitman College, Walla Walla, WA 99362

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Problems/Significance: This project addresses three broad topics, the first is the practicality of using AMS to determine flow direction in rhyolites, the second is to determine if there is a difference in AMS fabric between lava flows and rheoignimbrites, the third is to determine the source vents for the Southeast “limb” of felsic flows in the NSVG. This project is significant because it should give significant insight into how to effectively use AMS on rhyolite. If successful, this project will also be significant in interpreting the regional geology of the area by helping to locate the source vents for the felsic flows in the NSVG.

Initial Thoughts: I think that the data I have right now presents several problems but also offers significant insight into rhyolite flows. Given the high viscosity of the rhyolite it seems as though magnetic material may not always orient itself as well as it would in mafic material which is less viscous. The Lakewood flow yielded the most coherent results in the center of the flow, a phenomenon I think most likely stems from its longer cooling time and lack of interference from the terrain on the bottom. The rheoignimbrites from the Palisade flow seem to have a strong low angle north/south k-max direction that is not present in the Lakewood flow. The seemingly incoherent stereonets for some of the data is open to a variety of interpretations but the even seemingly random can indicate irregularities in flow within the rhyolites.

Hypotheses: 1. AMS will successfully give flow direction for the NSVG Rhyolites 2. The Palisade Rhyolite and the Lakewood Rhyolites will all have similar flow directions when analyzed using AMS. 3. The general flow direction will be rift normal. 4. There will be a general trend in flow direction between the northeast and southwest “limbs” of the NSVG.

Introduction (kind of): The North Shore Volcanic Group (NSVG) located on Lake Superior in Minnesota is part of the 1.1 Ga Midcontinent Rift System. In the area between Silver Bay and Duluth, 10% of the NSVG is composed of rhyolite and icelandite. (Green and Fitz, 1992). While Green and Fitz (1992) have each contributed to an understanding of the petrology of the flows through chemical and field analysis, little geophysical work has been done on these flows. The anisotropic magnetic susceptibility (AMS) of these flows was measured in order to determine the direction of flow and evaluate the effectiveness of the method on measuring rhyolitic flows. The flows chosen were the Palisade Rhyolite, determined by Fitz and Green (1992) to be a rheoignimbrite and the Lakewood Rhyolites determined to be a lava flow. Samples were collected from three locations in the Lakewood flow and four locations in the Palisade Rheoignimbrite. These were determined to represent the upper, middle and lower sections of each flow. The samples were then cut into 1 inch square cubes and run through the “roly-poly” at the Institute for Rock Magnetism located at the University of Minnesota. This device spins the samples in a magnetic field at 3 different orientations in order to determine the AMS fabric for each sample. This resulted in a k-max and k-min strike and dip for each sample which was then plotted on a stereonet giving an indication as to flow direction. This resulted in a variety of orientations for the k values of each sample. The data from the “roly-poly” will now be refined and then combined with data from the northeast limb of the NSVG. This will help to identify any possible correlation between the two locations as well as gain a better understanding of AMS fabric for rhyolites. Ideally, the results from this study and a study of the area to the north will result in a broader understanding of the felsic flows throughout the region. Using AMS on rhyolites is unprecedented though it has been using on welded volcanic tuff and a few other volcanic flows before and been relatively successful. (Ellwood, 1982) (waiting on ILL). This project will help to determine the practicality of AMS in measuring rhyolitic flows as well as provide a greater understanding of the flow directions and source for the NSVG.