Paleomagnetism of the Neoproterozoic Chuar Group, Grand Canyon Supergroup, Arizona: implications for Laurentia’s Neoproterozoic APWP and Rodinia break-up

Abstract

Abstract Selected parts of the Galeros and Kwagunt Formations, of the Neoproterozoic Chuar Group (ca. 800–740 Ma) exposed in the eastern Grand Canyon, Arizona, yield paleomagnetic data that are consistent with a primary origin. Both formations are dominated by dark mudrock with multiple, hematite-cemented fine sandstone and mudstone horizons that yield magnetizations of west declination and shallow inclination (or antipodes). The grand mean magnetization for the older Galeros Formation, determined by giving unit weight to each of 16 sites, is Dec=265.5°, Inc=5.1°, α95=8.5°, κ=19.8 and yields a paleopole of 2°S, 163°E (dp=4.3; dm=8.5). The younger Kwagunt Formation provides a grand mean magnetization of Dec=280.8°, Inc=30.6°, α95=8.4°, κ=38.6 (three sites of normal polarity and six sites of reverse polarity), determined by giving unit weight to each of the nine sites, and yields a paleopole of 18°N, 166°E (dp=5.2; dm=9.4). The NRM of about a third of the collection, mainly from coarser-grained hematitic sandstones, is dominated by a north-directed, moderate positive inclination magnetization with laboratory unblocking temperatures up to 680 °C and is interpreted as a recent overprint. We argue that the east–west and shallow magnetization in the Chuar Group strata, although similar to early Paleozoic magnetizations for North America, is primary because both groups of data pass fold tests and because Chuar strata lie in angular unconformity below the flat-lying Middle Cambrian Tapeats sandstone. Furthermore, we infer that the Chuar Group paleopoles, together with other North American Neoproterozoic results, are fully consistent with a tight counterclockwise loop in the south Pacific in the apparent polar wander path for North America during the Neoproterozoic. This new path represents a drift history that positions the central part of the Rodinia supercontinent at or near the equator at ∼800 Ma and slightly younger time. Penecontemporaneous with Chuar deposition, extension began along the Cordilleran margin of Laurentia. This large-scale extensional event culminated in the rift-to-drift transition of Laurentia away from East Gondwana or Asian (South China, Siberia) blocks by ca. 750 Ma.