The solid-body tilt of deformed paleohorizontal planes: application to an Archean transpression zone, southern Canadian Shield

Abstract

The solid-body tilt (SBT) of deformed paleohorizontal planes is equal to the angle between their unstrained top direction and an upward-pointing vertical axis. Kinematic models of important tectonic processes such as transpression and ductile thrusting have specific SBT requirements. These models can be tested, in ancient and modern orogens, by determining the actual SBT of strata, early magmatic sills and late horizontal intrusions with relict primary layering. Classical transpression models predict that, except for sheath structures, all folds in bedding be upright and horizontal, i.e. that SBT values be close to zero within the hinge zones. Moreover, all passive folding is accomplished by amplification of asperities and caused by the pure-shear component of the transpression tensor. Nine SBT analyses were made in the Archean crust of northwest Ontario to test the transpression hypothesis north of the Quetico-Wabigoon boundary, western Superior Province, Central Canada. The nine SBT values do not discredit the hypothesis that the Quetico Fault marks an Archean transpression zone which included the Wabigoon rocks adjacent to the Quetico metasediments.