Casella, Clarence J.
M.S. (Master of Science)
Department of Geology
The Marquette Synclinorium and the Republic Trough are narrow linear belts of folded Precambrian X (middle Precambrian) metasediments. They are bordered by Precambrian W (lower Precambrian) gneisses and greenstones. The regional structural trends within both major chronostratigraphic units are nearly parallel. They are east-west in the Marquette Synclinorium and N.45°W. in the Republic Trough. However, divergent orientations of fold axes within both major chronostratigraphic units, the presence of fault- and shear-zone contacts between the two units, and other structural discontinuities clearly indicate their unconformable nature. Field evidence indicates that both belts of Precambrian X metasediments are bounded by a series of fault- or shear-zones. Total vertical displacement along these in the eastern Marquette Synclinorium exceeds 1600 feet. In the Republic Trough total vertical displacement probably approaches 6000 feet. Detailed gravity profiles, matched with theoretical curves derived from models, indicate the basement (Precambrian W) consists of a series of wedges or blocks bounded by vertical faults. Periodic movement along these faults while overlying sediments were only partially consolidated has given rise to the deformation now apparent in the Precambrian X sequences. Precambrian W blocks moved almost wholly as rigid bodies during Precambrian X time. Folds in Precambrian X sequences formed by passive movement along slaty cleavages. The cleavages are a result of tectonic dewatering of unlithified or partially consolidated sediments. Faults occurring on the limbs of folded metasediments are a result of continued stress caused by the same vertical tectonism which produced folding. Faulting and folding of Precambrian X rocks, therefore, are genetically related. Both are a result of nearly vertical differential movement of the basement complex as a series of blocks or wedges. The anisotropy present in Precambrian W rocks before Precambrian X time controlled cleavage, fold, and fault orientations produced in sedimentary sequences during Precambrian X time.
Ginzel, Marvin G., "A regional structural analysis and gravity study of the eastern Marquette Synclinorium and Republic Trough, Michigan" (1973). Graduate Research Theses & Dissertations. 521.
ix, 75 pages, 5 unnumbered pages
Northern Illinois University
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