A Structural Investigation and Tectonic Interpretation of the Penokean Orogeny: East-Central Minnesota
1986-07
View/Download File
Persistent link to this item
Statistics
View StatisticsJournal Title
Journal ISSN
Volume Title
Title
A Structural Investigation and Tectonic Interpretation of the Penokean Orogeny: East-Central Minnesota
Authors
Published Date
1986-07
Publisher
Type
Thesis or Dissertation
Abstract
The Archean McGrath Gneiss and overlying Early Proterozoic Denham Formation have undergone similar conditions of deformation related to the Penokean orogeny. Petrographic analysis indicates dominantly ductile deformation under moderate-high temperature/moderate pressure conditions. The McGrath Gneiss has a poorly-developed to extremely well-developed east-west foliation and horizontal lineation. It is locally folded and anastomosing shear zones crosscut the foliation. The Denham Formation comprises a variety of rock types (quartzite, marble, schist, metaarkose, pillow basalt, and volcaniclastic rocks) and directly overlies the McGrath Gneiss. Deformation involved the formation of an early pervasive bedding-subparallel foliation similar to that of the overlying Thomson Formation. Late east-west trending folds have axial surfaces that dip steeply to the north and south. An east-west, horizontal mineral lineation is also pervasive throughout the Denham Formation. The Denham Formation has been metamorphosed to the staurolite zone of the amphibolite facies. The peak metamorphic temperature occurred immediately (?) after the formation of the later structures. Biotite-garnet geothermometry give temperature conditions associated with the later deformation of ca. 530°c. Almandine composition gives pressure conditions on the order of 4.5 kb (17 km). The rocks of the Denham Formation were deposited during rifting of the Great Lakes tectonic zone. This extensional period was followed by the Penokean orogeny and involved subduction of the granite-greenstone terrane beneath the gneissic craton to the south. Deformation associated with subduction formed the pervasive foliation and the lineation of the McGrath Gneiss and isoclinal, recumbent folds with a bedding-subparallel foliation in the Denham and Thomson Formations. Rocks located stratigraphically higher up in the Thomson Formation were deposited late in the deformational history and therefore were not incorporated with subduction. This explains the presence of a tectonic front with highly deformed rocks to the south and less deformed rocks to the north. Various strain analyses (accounting for a geologically reasonable volume loss and/or rotation of the later deformation) in the overlying Thomson Formation show that the early deformation involved extreme flattening (with Z vertical) and large amounts of extension in both the N-S and E-W directions. The formation of a tectonic fabric involving extension east-west and north-south requires kinematics during subduction (the early deformation) to be oblique to the plate boundary, and therefore relative plate motion to be oblique rather than normal as is usually interpreted. Early structures, formed during tectonic burial, were deformed by later more disharmonic structures related to imbrication of the footwall into the hangingwall during uplift. Continued compression involved basement shortening and the formation of shear zones. Basement involvement during the later deformation formed tight, overturned folds in the Denham Formation and may have contributed to the differing styles of folding in the Denham and Thomson Formations. Increasing temperature with decreasing pressure is explained by conductive relaxation caused by crustal thickening and erosion.
Description
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Daniel Keith Holm in partial fulfillment of the requirements for the degree of Master of Science, July 1986.
Related to
Replaces
License
Series/Report Number
Funding information
Isbn identifier
Doi identifier
Previously Published Citation
Suggested citation
Content distributed via the University Digital Conservancy may be subject to additional license and use restrictions applied by the depositor. By using these files, users agree to the Terms of Use. Materials in the UDC may contain content that is disturbing and/or harmful. For more information, please see our statement on harmful content in digital repositories.