ARCHEAN UPLIFT OF A SUBPROVINCE BOUNDARY IN THE CANADIAN SHIELD, REVEALED BY MAGNETIC FABRICS

A high strain zone, reported elsewhere as a subprovince or tectonic-terrane boundary. shows evidence of uplift during a penetrative D1 event. Some outcrops reveal this by conventional L-S petrofabric observations. However, all outcrops permit measurements of the anisotropy of low field magnetic susceptibility (AMS) which here records the preferred crystallographic orientation of chlorite, biotite and amphibole. Furthermore, anisotropy of anhysteretic magnetic remanence (AARM) reflects the preferred orientation of ferromagnetic grains (magnetite and pyrrhotite). All three types of fabric show that the rocks were shortened in a north-south direction, perpendicular to the terrane boundary, and stretched upwards in a nearly coaxial strain history. A few anomalous magnetic fabrics may indicate some late D1 subhorizontal E-W extension, compatible with transpression along the boundary. We conclude that this boundary is a zone of high strain associated with the uplift of the high-grade plutonic rocks of the Winnipeg River subprovince, with only slight evidence of minor components of late transpression during the penetrative D1 fabrics.