Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota

“…It traditionally has been viewed as a distinct tectonic entity. However, the recent work of Bauer (1985Bauer ( , 1986 has demonstrated that the migmatites of this complex and the greenstone belt assemblage south of the fault share a common structural geometry and sequence. We consider the rocks on both sides of the fault zone as having evolved, at least in part, in a common tectonic regime.…”

Section: Geological Settingmentioning

confidence: 95%

Transpression in an Archean greenstone belt, northern Minnesota

Hudleston¹,

Schultz-Ela²,

Southwick³

1988

Can. J. Earth Sci.

131

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Weakly metamorphosed Archean sedimentary and volcanic rocks of the Vermilion district, northern Minnesota, occupy an east-west-trending belt between gneisses of the Vermilion granitic complex to the north and the Giants Range batholith to the south. All the measured strain, a foliation, and a mineral lineation in this belt are attributed to the "main" phase of deformation (D,). Foliation strikes parallel to the belt and dips steeply, and the mineral lineation plunges moderately to steeply east or west and is parallel to the maximum stretching direction, X, and subparallel to fold hinges. An earlier, possibly nappeforming, event (D,) left little evidence of fabric in the Vermilion district.A number of features indicate that the D, deformation involved a significant component of dextral strike-slip shear in addition to north -south compression. They include ductile shear zones with sigmoidal foliation patterns, shear bands, asymmetric pressure shadows, and the fact that the asymmetry of the F, folds is predominantly Z. Other features are more simply explained by a deformation involving simple shear. The S, cleavage is locally folded, and a new spaced cleavage developed in an orientation similar to that of the old cleavage away from the folds. We consider this the result of a process of continuous shear, with perturbations of flow resulting in folding of S, and the development of a new foliation axial planar to the folds. The same type of perturbation can lead to the juxtaposition of zones of constrictional and flattening strains, a distinctive feature of the rocks of the Vermilion district otherwise hard to account for. The strain pattern requires a north-south component of shortening in addition to shear. The D, deformation in the Vermilion district can therefore be characterized as one of transpression: oblique compression between two more rigid lithospheric blocks to the north and south.Dans le district de Vermilion, au Minnesota nord, les roches volcaniques et skdimentaires archkennes, faiblement mktamorphiskes, occupent une ceinture orientke est-ouest comprise entre les gneiss du complexe granitique de Vermilion au nord et le batholite de Giants Range au sud. Toutes les contraintes mesurkes, la foliation et la linkation minkralogique observkes dans cette ceinture sont attribukes B la phase de dtformation "principale" dksignke (D,). La foliation prtsente une direction parallble a l'orientation de la ceinture et est fortement inclinke; la linkation minkralogique plonge modkrkment a abruptement vers l'est ou l'ouest, et elle est parallble 21 la direction d'ktirement maximum, X, et subparall&le aux charnibres des plis. Une fabrique peu dkveloppke dans le district de Vermilion rksulte d'un kvenement plus ancien (D,), possiblement une formation de nappes.Les zones de cisaillement ductile avec foliation de style sigmoide, les bandes de cisaillement, les ombres de pression asymktriques et la prkdominance de l'asymetrie Z dans les plis F, impliquent dans le cas de D,, non seulement une compression nord-sud, mais en plu...

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“…8) (Southwick, 1972) and the Giants Range batholith to the south. The measured strain, a cleavage, upright folds, and a mineral lineation in this belt have been attributed to the "main" phase of deformation (D2) that followed an early nappe-forming event (D t ) (Bauer, 1985). The nappes show little evidence of a penetrative fabric (Hudleston, 1976).…”

Section: Discussionmentioning

confidence: 89%

Great Lakes tectonic zone in Marquette area, Michigan: Implications for Archean tectonics in north-central United States

Sims¹

1991

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Correlation of early recumbent and younger upright folding across the boundary between an Archean gneiss belt and greenstone terrane, northeastern Minnesota

Cited by 24 publications

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Archean geology of the northern block of the Ishpeming greenstone belt, Marquette County, Michigan

Archean geology of the northern block of the Ishpeming greenstone belt, Marquette County, Michigan

Transpression in an Archean greenstone belt, northern Minnesota

Great Lakes tectonic zone in Marquette area, Michigan: Implications for Archean tectonics in north-central United States

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