Abstract

The Penokean orogeny was a major early Proterozoic (1875-1825 Ma) tectonic event in the Great Lakes region. In east-central Minnesota, it is marked by multiply deformed and highly metamorphosed supracrustal rocks of the early Proterozoic Denham and Thomson Formations. Structural features similar to those in the supracrustal rocks also exist in the basement Archean (2700 Ma) McGrath Gneiss. Such features are here explained in a tectonic model consisting of southward-directed oblique subduction along the Great Lakes tectonic zone. Intense deformation occurred in the footwall of the major thrust, which marked the boundary between downgoing and overriding plates during A-type (continental) subduction. Sedimentary rocks (Thomson Formation) deposited on the footwall during loading caused by thrusting eventually became incorporated into the deformation zone. Early-formed structures related to footwall deformation are a dominantly well-developed foliation in the gneiss and isoclinal, recumbent folds with a bedding-subparallel foliation in the Denham and lower Thomson Formations. Progressive metamorphism during subduction reached the garnet zone of the amphibolite facies. Various deformation inversions show that this early phase of deformation involved extreme flattening (with Z vertical) and large amounts of extension in both the north-south and east-west directions.

Footwall deformation was followed by imbrication and accretion onto the hanging wall during uplift associated with continued compression and isostatic rebound. Later-formed structures associated with imbrication and deformation within the hanging wall consist of folding of the foliation and development of shear zones in the McGrath Gneiss and open to close, upright-to-overturned folds in the Denham and Thomson Formations. The peak metamorphic event (represented by staurolite) occurred after the later deformation at temperatures of about 470-530 °C and a minimum pressure of 3.4 kbar (minimum depth of 12.4 km). Increasing temperature associated with decreasing pressure (uplift) is explained by conductive relaxation caused by crustal thickening and erosion. This tectonic model may have more widespread implications for explaining similar structural features found in many Precambrian terranes worldwide.

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