Paleomagnetism: a Potential Avenue for Mapping the Allochthon Boundary Thrust and for Understanding the Development of the Grenville Orogen.

English Abstract:

Paleomagnetic data are presented for the ~2.47 Ga River Valley anorthosite (RV) and for 1.23 Ga Sudbury dykes (SD) which lie in the Grenville Province (GP) between the Grenville Front (GF) and the Allochthon Boundary Thrust (ABT). Both rock types yield high H_c/T_ub remanences with shallow down to the ESE directions. Their age is no more than ~ 980 Ma based on Ar-Ar plateaux on hornblende from the anorthosite. S of the ABT Dunlop & co-workers find multi-component remanences but one (A) with the highest H_c/T_ub has a steep up direction to the WNW. It may have an age close to the mean of U-Pb ages on titanite (T_b ~ 600-650°C) that range from ~1030±30 Ma, N=20 determinations in the Central Gneiss Belt (CGB) to 1100 ± 60 Ma, N=27, in the Central Metasedimentary Belt (CMB) (Tohver et al 2006). A large difference in magnetization direction across the ABT suggests that paleomagnetic data represent a potential tool in mapping this thrust fault. S of the ABT, Ar-Ar hornblende ages are similar to those closer to the GF; for the CGB the mean is 980±23 Ma (N=22); in the CMB, 970±40 Ma for N=23 (Tohver et al. 2006). They may date magnetic overprints, many of which are similar or antipolar to SD and RV. The SD and RV poles lie on, but the A pole lies to the west of, the Laurentian APWP. The A pole and the mean SD - RV pole lie on a great circle ~normal to the GF, and could suggest that the ABT is a thrust along which Grenville rocks, already magnetized in response to an earlier collisional uplift and cooling, have been transported to the NW. Further NW movement followed, utilizing deeper thrusts, one of which defines the GF, along which uplift and cooling produced the RV and SD poles. The collision causing A may have been the earliest (ca 1.2-1.3 Ga) of four which define the Grenville orogen. Collectively, they may have been responsible for transporting rocks of the allochthon 4000+/-1000 km to the NW, a distance based on that between the A and the mean RV-SD poles.