Geotectonics of the cratonic margin from paleomagnetism of the Middle Proterozoic Aldridge (Prichard) Formation and Moyie sills of British Columbia and Montana

Abstract

The Precambrian Aldridge (Prichard) Formation crops out at or near the base of the Middle Proterozoic Belt-Purcell Supergroup in the Purcell anticlinorium, and the Moyie sills intrude the Lower and Middle Aldridge Formation. Paleomagnetic analysis was done on specimens from 27 sites in layered argillites of the Aldridge and from 30 sites in the Moyie metagabbros using AF and thermal (Th) demagnetization methods. Fold and contact tests show that no primary remanence components were isolated. However, three post-folding metamorphic components were found: A was found in 9 argillite (AF, 22.9°. 64.1°, α₉₅ = 9.8°) and 11 sill sites (AF, 35.2°, 62.0°, α₉₅ = 7.1°); B was found in 8 argillite (AF, 320.4°, 52.0°, α₉₅ = 11.8°) and 10 sill sites (Th, 303.6°, 56.9°, α₉₅ = 7.8°); and C was found in 19 argillite sites (AF, 209.0°, 3.1°, α₉₅ = 6.0°). All three components include normal and reversed sites. The A, B, and C poles are discordant with the positions expected for known metamorphic or intrusive events, but correction for a simple 37° clockwise rotation leads to concordancy of the A pole with the ≈ 175 Ma position, B with the ≈ 810 Ma position, and C with the ≈ 1400 Ma position on the APWP for the North American craton. These ages coincide with the known Laramide, Goat River, and East Kootenay orogenies. Evidently, the terrane housing the Purcell Anticlinorium: (1) has always been part of the North American craton with little or no translation; (2) was folded and metamorphosed in the East Kootenay orogeny prior to 1400 Ma; (3) was thermally overprinted in the Goat River orogeny; and (4) was thermally overprinted and then rotated ≈ 37°±12° clockwise as Terrane I of the Cordillera impacted from the southwest against the craton during the mid-Mesozoic Laramide orogeny.