Apparent polar wander paths and the close of late Archean crustal transpression, northern Ontario

Abstract

Lamprophyre dikes of the southern Superior Province of the Canadian Shield crosscut Archean structures, including a late Archean unconformity and its overlying younger Archean Timiskaming sedimentary and volcanic rocks. The dikes also cut the single schistosity that formed in these rocks. Nevertheless, the lamprophyre dikes show a weak internal tectonic fabric that is approximately parallel to the schistosity in the bedrock and oblique to dike walls. The consistent orientation of this internal tectonic fabric has been recognized in every lamprophyre dike using anisotropy of magnetic susceptibility (AMS). Thus the lamprophyre dikes provide a useful chronological marker, trapping the last pulse of Archean tectonism. Unfortunately, our previous attempts to compare the lamprophyre paleopoles with those of the age‐calibrated apparent polar wander path (APWP) failed because it was not possible to isolate separate young and old components of magnetization. That failure occurred due to the overlap of blocking temperatures or coercivities for the different characteristic vector components [Borradaile et al., 1994a]. Using new cores from the same specimens, here we distinguish successfully between consistently oriented vector components by using low‐temperature demagnetization (LTD) before thermal demagnetization. Although the paleopoles appear well defined, comparisons with published APWPs for the Superior Province from late Archean [Irving, 1979; Dunlop, 1983; Symons et al., 1994] to early Proterozoic [Buchan et al., 2000] are difficult. Moreover, the earlier parts of the paths are progressively less precise and complete. Nevertheless, A component remanences for the dikes give a paleopole near the path at ∼2650 Ma. For a Timiskaming volcanic breccia with a U‐Pb age of 2696 Ma the A component paleopole may lie between 2600 and 2650 Ma. B component remanences yield paleopoles in the NW Pacific region, incompatible with any proposed Archean path but perhaps representing some much younger Proterozoic remagnetization event (e.g., Keweenawan, ∼1100 Ma).