Tilting and transpression of an Archaean anorthosite in northern Ontario

Abstract

An Archaean anorthosite complex crops out over more than 300 km 2 at the junction between an Archaean metasedimentary accretionary prism (Quetico Belt) and a volcanic greenstone terrain to the north (Wabigoon Belt). The anorthosite is feebly deformed and weakly foliated. However, anisotropies of low field susceptibility (AMS) and anhysteretic remanence (AARM) define consistent magnetic fabric directions extending over subareas >25 km 2 in extent. Whereas magnetic fabrics cannot define strain magnitudes, they normally provide reliable indications of finite strain orientations, even where the fabrics developed by stress-controlled nucleation during a progressive noncoaxial strain history. The field schistosity S 1, being the oldest foliation, rotated closest to the shear plane. AMS foliation due to preferred crystallographic orientation of silicates and pyrrhotite as well as preferred dimensional orientation of magnetite lies at a greater angle to the shear plane; because it formed late then S 1 had less time to rotate. The few measurements of the youngest AARM foliation caused by magnetite and pyrrhotite suggest that it may lie at a still greater angle to the shear plane. Thus, S 1 and succeeding magnetic foliations provide a cryptic shear-sense indicator. They reveal relative motions along the terrain boundary that include transpression due to NW–SE shortening causing dextral E–W shear with a north-side-up component. Palaeomagnetic data confirm a progressive northwards steepening of the anorthosite sills that is consistent with northwards subduction of the accretionary prism to the south.