Transpression in an Archean greenstone belt, northern Minnesota

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...

Transpression and transtension zones

“…If, however, the wrench non-coaxial component is kinematically partitioned into boundary-parallel highstrain zones, vertical lineations may switch back into sub-horizontal orientations. Possible field examples of such switching behaviour have been described by Hudleston et al (1988), Holdsworth (1989) and Tikoff & Greene (1997). Conversely, in transtension, the stretching direction is always horizontal, but the shortening direction switches from vertical to horizontal where ct < 20 ~ with increasing finite strain (McCoss 1986).…”

“…Exhumed examples of middle-crustal transpression zones display similar configurations except that the blocks may be internally deformed and the block-bounding structures are now ductile shear zones (e.g. Hudleston et al 1988;D'Lemos et al 1992). The nature of transpressional and transtensional structures in the weak ductile lower crust and much stronger ductile upper mantle is uncertain but is of some importance if the coupled model proposed by Molnar (1992) for zones of oblique convergence and divergence is correct.…”

Evolution des failles de la sous-province de l Abitibi

“…Ces failles sont donc des failles de chevauchement sur lesquelles se sont superposés d'importants mouvements horizontaux dextres (Hudleston et al 1988;Devaney et Wiiiams 1989;Williams 1990;Benn et al 1992;Sawyer et Benn-1993). …”

“…La discontinuité de Duvan (voir section 1.5.3 et figure 5) (Hudleston et al 1988;Borradaile et al 1988;Lafrance 1989), ce qui n'est pas le cas pour la discontinuité de Lyndhurst. (Dimroth et al 1983a;Chown et al 1992 (Hocq 1990;Chown et al 1992; Dimroth et al (1982;1983a;1983b) Finalement, une compression nord-sud provoque du chevauchement le long de failles est-ouest.…”

Tectônica da conexão do Cinturão Ribeira com a Faixa Araçuaí - Divisa Rio de Janeiro/Espírito Santo

“…Desde sua definição original (Harland 1971, Sanderson e Marchini 1984 A importância dos regimes oblíquos, transpressionais e transtrativos, é reconhecida em diversos cinturões orogênicos modernos e antigos , Dewey et al 1988, como nos cinturões variscano e caledoniano na Europa e na Groenlândia (Holdsworth e Strachan 1991, Dias e Ribeiro 1994, Leblanc et al 1996, Strachan et al 2001, nos cinturões arqueanos no Canadá e nos EUA (Hudleston et al 1988, Borradaille et al 1993 Braathen et al 2002, Johnson 2006, entre outros). Nos Alpes e nos Himalaias, essas estruturas podem se relacionar com o transporte lateral de massa b, Pêcher 1991, processo favorecido durante a convergência oblíqua de placas, onde a tectônica transpressiva ocorre desde o início da colisão.…”