Geophysical investigations of the midcontinent rift in eastern Lake Superior
Abstract
Integrated interpretations of potential-field and GLIMPCE and industry seismic reflection data in eastern Lake Superior reveal the structural and stratigraphic complexity of the Midcontinent Rift (MCR) in this region. An algorithm has been developed to accommodate spatially varying direction and magnitude of magnetization within a single magnetic source. Structural attitudes derived from seismic reflection sections are used to rotate either normal or reversed polarity remanent vectors. The rotated remanent magnetization is vectorially added to the induced magnetization, resulting in a structurally corrected direction of total magnetization. This algorithm is used in both forward and inverse modeling procedures. Results of modeling in eastern Lake Superior suggest that the greater percentage of the Keweenawan basalt flows are reversely polarized and that the thickness of normally polarized flows decreases along strike of the rift from Keweenaw Peninsula to the southeastern lake shore. In addition, the Koenigsberger ratios for these rocks are consistently low, generally ranging from 1 to 3, and are confirmed to reach depths in excess of 20 km. A prominent positive magnetic anomaly strikes southwest across the trend of the rift from the vicinity of Michipicoten Island. There is a gravity minimum corresponding to this magnetic anomaly that is apparently due to a relatively strongly magnetized, low density rock unit of limited areal extent. This unit is believed to be a felsic igneous body of late-middle to upper Keweenawan age. Magnetic models are constrained by seismic reflection and gravity data. Reflection sections image the Keweenawan volcanic rocks as strong continuous reflectors reaching thicknesses of 10-20 km. Gravity models suggest clastic sedimentary rocks up to several kilometers thick overlay the volcanic rocks in localized depressions. Spatially extensive anticlinal features, reverse faults, and related drag folds imaged by the reflection data and modeled by the potential-field data attest to the influence of a late-stage compressional event. Deep crustal seismic data used to constrain gravity models provide evidence of anomalously dense lower crust beneath the MCR.
Degree
Ph.D.
Advisors
Hinze, Purdue University.
Subject Area
Geophysics|Geology
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