Movement along Shear Zones of the Northwestern Superior Boundary Zone

The Superior Boundary Zone in northern Manitoba separates the Neoarchean Pikwitonei Granulite Domain of the Superior Province to the southeast from the Paleoproterozoic amphibolite- and lower grade Trans-Hudson Orogen to the northwest. Previously, movement along the Superior Boundary Zone has been interpreted in terms of a promontory model, in which the Superior Province moved to the northwest to collide with rocks of the Trans-Hudson Orogen. This model explains sinistral, southeast-side-up movement along the 030°-trending SSW segment (Setting Lake shear zone) and dextral, southeast-side-up movement along the 050°-trending ENE segment (Assean Lake shear zone) of the promontory.

Northwest-side-up dextral movement along 050°-trending shear zones of the central segment between the SSW and ENE segments is inconsistent with the promontory model. Furthermore, north-side-up dextral movement along the 110°-trending Aiken River shear zone along the ENE segment within the Superior Province remains unexplained. Thus, a simple promontory model is insufficient to explain the movements along all shear zones. Shear zones were either active at different time periods or the Superior Boundary Zone may be a zone of macroscopic brecciation, rather than a zone of rigidly moving domains. The latter would explain the presence of zones of brecciation along the central segment of the promontory.

At least part of the dextral, southeast-side-up movement on the Assean Lake shear zone occurred after ~1.84 Ga, based on a U–Pb zircon age of a deformed aplite. Movement may or may not have been concurrent with <1.77 Ga sinistral and subsequent southeast-side-up movement along the Setting Lake shear zone. Based on crosscutting relationships, movement on the Assean Lake shear zone outlasted movement on the Aiken River shear zone. Both shear zones probably also experienced earlier, Late Archean movement. The timing of movement along various shear zones is currently further being constrained by U–Pb and ⁴⁰Ar/³⁹Ar geochronology.