Date of Award

2019

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Geophysics (MS)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

Aleksey Smirnov

Committee Member 1

Chad Deering

Committee Member 2

James DeGraff

Abstract

One of the most prominent structural features associated with the ~1.1 Ga Midcontinent Rift (MCR) system is the >350 km long Keweenaw Fault that bisects the Keweenaw Peninsula, separating the MCR-related Portage Lake Volcanics (PLV) and the younger Jacobsville Sandstone (JS). The fault trend is NE-NNE over most of its length, but changes to an easterly direction along the shore of Bête Grise Bay near the end of the peninsula. Conventionally, the Keweenaw Fault has been considered to be a continuous reverse (dip-slip) fault formed by inversion of an original rift-bounding normal fault during the Grenville Orogeny. However, recent mapping shows that the fault in this area is not a single continuous feature but instead is a fault system consisting of ENE and ESE-trending segments with substantial strike-slip movement. This segmented fault geometry could have resulted in local folding of PLV and JS strata adjacent to the fault segments. To test this hypothesis, a paleomagnetic investigation was conducted on samples of PLV basaltic flows from eight sites in the Lake Medora and Fort Wilkins map quadrangles. The sites represent the opposite flanks of a proposed anticline with an ESE-trending axis. All eight sites yielded reliable and consistent site-mean directions of characteristic remanent magnetization (ChRM). A paleomagnetic fold test conducted on these sites showed that after unfolding the ChRM directions are similar to the paleomagnetic direction expected from unfolded PLV rocks. Data from two sites also suggest rotations around a vertical axis consistent with strike-slip movement. Paleomagnetic directions obtained from three additional sites with brecciated PLV basalt and JS sandstone as well as a clastic dike of JS cutting PLV, are randomized within each site. These randomized directions provide additional evidence that paleomagnetic data from the PLV basalts were not affected by a later remagnetization event. Overall, the paleomagnetic results support the hypothesis of fault-induced folding of PLV strata in the study area. In addition, this research demonstrates that paleomagnetism represents a useful tool to investigate local structural deformation within the MCR system.

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