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.