Transpression in slates along the margin of an Archean gneiss belt, northern Ontario—magnetic fabrics and petrofabrics

Abstract

Archean slates along the northern boundary of the Quetico subprovince of the Superior Province show marked variations of the structural facing direction within coplanar primary cleavage surfaces. These are interpreted as being due to sheath folds near the faulted east-west boundary of the slates with the metavolcanic Wabigoon Belt. Sheath folds with axial traces nearly parallel to the belt boundary are macroscopic, primary, and isoclinal and are believed to result from pervasive dextral transpression of the northern margin of the Quetico subprovince. Magnetic fabrics confirm the presence of a single penetrative flattening tectonic microfabric and considerable north-south shortening on the scale of hand specimens and outcrops. The magnetic fabric is due to the alignment of metamorphic sheet silicates, with a subordinate contribution due to the preferred dimensional orientation of detrital magnetite grains. In some low-strain environments at the hinges of sheath folds and at the sandy bases of graded beds relict sedimentary magnetic fabrics are preserved. However, a tectonic magnetic fabric is usual, with flat susceptibility ellipsoids parallel to axial planes and with variably oriented maximum susceptibility directions. To the south, farther into the interior of the Quetico subprovince, the primary folds become more homoaxial and the metamorphic grade rises rather steeply, and still farther south, small-scale polyphase deformation becomes evident especially where the rocks are remobilized.Les schistes ardoisiers archkens qui affleurent le long de la limite nord de la sous-province de Quetico de la province du Sup6rieur pdsentent des variations remarquables de la direction du regard structural dans les surfaces coplanaires du clivage primaire. Elles sont interprktkes cornme ktant dues A des plis en fourreau p k s de la faille a la limite est-ouest des schistes ardoisiers contre la zone mktavolcanique de Wabigoon. Les plis en fourreau avec leurs traces axiales quasi parallkles avec la limite de la zone sont macroscopiques, primaires et isoclinaux et semblent rksulter d'une transpression dextre pBnBtrante de la bordure nord de la sous-province de Quetico. Les orientations magnttiques corroborent la prksence de rien qu'une seul microfabrique d'aplatissement tectonique pknktrative et de rktdcissement considkrable au n o d -sud not6 A l'tchelle des sptcimens cueillis ?i la main et des affleurements. Une fabrique magnktique est produite par un alignement des phyllosilicates mktamorphiques, et elle est accompagnke d'une orientation pkfkrentielle de la forme des grains dktritiques de rnagnktite. Des reliques de fabriques magnBtiques skdimentaires sont prkservtes dans les charnikres des plis en fourreau oh les forces de contrainte Btaient faibles et dans les sables de fond des lits granoclassks. Quoi qu'il en soit, l'existence d'une fabrique magnktique tectonique est commune avec des ellipsoides de susceptibilitk de forme aplatie parallklement aux plans axiaux et avec des directions changeantes de la susceptibilitk maximale...

Transpression in an Archean greenstone belt, northern Minnesota

“…A better anology may be the subduction-related oblique accretion of island arcs and microcontinents or progradation of an arc-trench system (Hoffman 1987). One thing that is becoming clear, however, is that large strike-slip motions, most commonly dextral, are common within the Superior Province (e.g., Poulsen 1986;Borradaile et al 1988). For personal use only.…”

A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada

“…However, the orientation and style of F, folds vary, apparently, from northwest facing recumbent and isoclinal (Hudleston 1976;Sawyer 1983;Bauer 1985) to sideways facing and neutral (Kehlenbeck 1984), to variable upwards, downwards, and sideways facing within a single region (Borradaile 1982). Some variability may be due to the presence of sheath folds developed during dextral transpressive deformation (Borradaile et al 1988). Consistently north-facing stratigraphic top indicators have been used to infer stratigraphic younging direction and hence relative age with respect to adjacent greenstone sequences (i.e., Wawa beneath Quetico beneath Wabigoon) (MacDonald 1939).…”

“…6). Oblique convergence could account for transpressional structures (Corfu and Stott 1986;Hudleston et al 1988;Borradaile et al 1988), dextral wrench zones (Poulsen 1986;Williams 1987c), and transcurrent boundary faults. However, the thermal regime does not appear to be appropriate for low-P metamorphic development.…”

Evolution des failles de la sous-province de l Abitibi

“…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.…”