No abstract

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...

The Minnesota River Valley subprovince of the Superior Province is an Archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east-northeast-trending linear geophysical anomalies. Two of the block-bounding zones are verified regional-scale shears. The geological nature of the third boundary has not been established. Potential-field geophysical models portray the boundary zones as moderately north-dipping surfaces or thin slabs similar in strike and dip to the Morris fault segment of the Great Lakes tectonic zone at the north margin of the subprovince. The central two blocks of the subprovince (Morton and Montevideo) are predominantly high-grade quartzofeldspathic gneiss, some as old as 3.6 Ga, and late-tectonic granite. The northern and southern blocks (Benson and Jeffers, respectively) are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late-tectonic plutons. A belt of moderately metamorphosed mafic and ultramafic rocks having some attributes of a dismembered ophiolite is partly within the boundary zone between the Morton and Montevideo blocks. This and the other block boundaries are interpreted as late Archean structures that were reactivated in the Early Proterozoic. The Minnesota River Valley subprovince is interpreted as a late accretionary addition to the Superior Province. Because it was continental crust, it was not subductible when it impinged on the convergent southern margin of the Superior Craton in late Archean time, and it may have accommodated to convergent-margin stresses by dividing into blocks and shear zones capable of independent movement.

High-precision U-Pb ages have been obtained for high-grade gneisses, late-kine-matic to postkinematic granitic plutons, and a crosscutting mafi c dike of the Archean Minnesota River Valley tectonic subprovince, at the southern ramparts of the Superior craton of North America. The antiquity of the Minnesota River Valley terranes is confi rmed by a high-precision U-Pb zircon age of 3422 ± 2 Ma for a tonalitic phase of the Morton Gneiss. Voluminous, late-kinematic monzogranites of the Benson (Ortonville granite) and Morton (Sacred Heart granite) blocks yield identical crystallization ages of 2603 ± 1 Ma, illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the Minnesota River Valley terranes. Postkinematic, 2591 ± 2 Ma syenogranites and aplitic dikes in both blocks effectively constrain the fi nal penetrative deformation of the Minnesota River Valley subprovince. Monazite growth from 2609 to 2595 Ma in granulitic paragneisses of the Benson and Montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism. Neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the Mesoarchean continental Minnesota River Valley terranes to the southern margin of the Superior Province, along the western Great Lakes tectonic zone. Subsequent Paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion, expressed in the Minnesota River Valley by major WNWtrending tholeiitic diabase dikes dated at 2067 ± 1 Ma, only slightly younger than the structurally and geochemically similar 2077 ± 4 Ma Fort Frances (Kenora-Kabetogama) dike swarm of northern Minnesota and adjoining Canada.