Paleomagnetism of the Middle Proterozoic Mount Nelson Formation: evidence for a regional remagnetization event in the Late Precambrian of the Cordillera

Chemical and thermal demagnetization of 92 specimens from the Mount Nelson Formation (uppermost Purcell Supergroup) in the Purcell Mountains of southeastern British Columbia yields two distinct directional groups. MN-A, found by thermal and chemical demagnetization, has a mean direction of D = 274°, I = 19° (α 95  = 10°, tilt corrected), which corresponds to a pole position at 156 °E, 10 °N, MN-A resides in fine-grained hematite and is similar to other overprint directions reported from middle and upper Belt–Purcell strata in the Glacier National Park – Clark Range area. Genesis of this overprint is related to some regional geological process, most probably the Goat River Orogeny [Formula: see text]. Direction MN-B, which is found after acid leaching of more than 250 h, has a mean direction of D = 331°, I = 47°(α 95  = +4°, tilt corrected), corresponding to a pole at 119 °E, 59 °N. The MN-B pole is significantly different from all poles previously reported from the Belt–Purcell Supergroup (ca. 210 °E, 20 °S). As the age of the MN-B pole is unconstrained the significance of this marked discrepancy is at present uncertain.