United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia

The Mesoproterozoic (1.47-1.40 Ga) Belt basin in western North America contains a thick succession of siliciclastic strata dominated by sediment derived from source areas that lay to the west and southwest of the present basin. The age and identity of this source area is important for reconstruction of the Mesoproterozoic history of western North America. Over 400 new SHRIMP U-Pb ages for detrital zircons, supplemented by Ar-Ar ages of detrital muscovite from sandstones within the Belt basin, document distinctive provenances of the basin fill; the predominant sediment influx came from a western craton, whereas subordinate input came from Laurentian sources to the east and south of the basin. During deposition of the lower two-thirds of the Belt basin, the western source was composed predominantly of Proterozoic rocks with 1920-1460 Ma ages; abundant grains have 1610-1500 Ma ages. These latter ages are unrepresentative of Laurentian sources, falling within the North American magmatic gap (1610-1490 Ma), but are widespread in central and eastern Australia and Baltica, the former being the most likely candidate for a former fluvial connection to western North America. The presence of detrital-zircon grains (1480-1440 Ma) with ages contemporary with sedimentation in the Belt basin suggests that the western source area hosted active magmatism during Belt sedimentation. The Missoula Group in the upper Belt records the sedimentologic effects of basin reorganization, which introduced sediment from a new southwestern source that, in contrast to earlier Belt history, resembles crust of the southwestern United States and lacks grains within the North American magmatic gap. Evidence for extension is widespread in the Belt basin in the form of mafic volcanic rocks, rapid rates of sedimentation, and intrabasinal syndepositional faults. The predominant contribution of detrital material from the western source area throughout Belt deposition, in concert with the large-scale facies patterns and evidence for rapid rates of subsidence, is taken to suggest that the Belt basin represents an extensional basin with a tectonically active western side. Although a wholly extensional environment has been suggested by previous workers, we suggest that an alternative model, wherein the Belt basin developed within an extensional domain along a collisional/convergent plate margin, analogous to Tethyan sedimentary basins such as the Black and Caspian Seas, is equally viable and paints a different picture of western North America at ca. 1450 Ma.

Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): Another piece in the pre-Rodinia paleogeographic puzzle

An evolutionary model of the western churchill province and western margin of the superior province in Canada and the North-Central United States

Study of the northern Saskatchewan–District of Mackenzie segment of the Snowbird tectonic zone suggests that fragments of relatively stiff mid-Archean crust, possibly arc related, have controlled t...

Striding-Athabasca mylonite zone: implications for the Archean and Early Proterozoic tectonics of the western Canadian Shield

To facilitate a comparative study of approaches and algorithms for two-dimensional interpretation of magnetotelluric (MT) data, a dataset of thirty-five sites, called COPROD2, was distributed to the electromagnetic induction community. The data are from stations along a 400 km east-west profile in southern Saskatchewan and Manitoba, Canada, crossing the thick Paleozoic sediments of the Williston basin. Within the basement beneath the sediments lies one of the world's longest and most enigmatic crustal conductivity features-the North American Central Plains (NACP) conductivity anomaly. Also, at the eastern extremity of the profile is a second basement anomaly (TOBE) interpreted to be associated with the Thompson Nickel Belt at the Superior-Churchill boundary. The MT data were corrected for static shifts, and only the ofd diagonal impedance data together with the transfer function data, and their errors, were made available. These MT data are of wide bandwidth (384 Hz-1820 s) and high quality (impedance errors typically <2%), and require sophisticated modelling and inversion in order to extract as much meaningful information as possible from them. A challenge for those interpreting these data is that there is a very small, but critical, response in the B-polarization data to the presence of the basement anomalies. In this introductory paper, the previous electromagnetic, and other geophysical, studies of the NACP anomaly, and its tectonic setting within the Trans-Hudson orogen, are described. Representative data from the COPROD2 data are illustrated, and models derived by various groups are shown. Comparisons of these models reveals that a single measure of misfit is an inadequate description of how well a model fits a dataset; one must compare the model fit to the data at virtually each datum to avoid trends in the misfit residuals. Additionally, reliable and consistent error and static shift estimates are essential in order to obtain high resolution images of the Earth's conductivity structure. Finally, the COPROD2 data highlight the sensitivity in B-polarization mode data to breaks in the resistive layer, whereas the E-polarization data sense regions of enhanced conductivity.

The COPROD2 Dataset: Tectonic Setting, Recorded MT Data, and Comparison of Models

To determine the extent of reworked Archean crust in the western Churchill Province, we have examined Sm–Nd crustal residence ages of basement cores in southern Alberta and southwestern Saskatchewan along with crustal residence ages calculated for other Sm–Nd data available from the Churchill Province. The deep drill hole samples from the Interior Platform give Sm–Nd crustal residence ages that average 2.8 Ga. Granulites from northeastern Alberta and composite gneisses from northern Saskatchewan also provide Archean crustal residence ages. These data demonstrate that the presence of reworked Archean crust in the western Churchill Province is more widespread than has been confirmed previously. We suggest that Sm–Nd crustal residence ages are valuable estimates of crust formation times, especially in orogenic areas where other isotope systems have been disturbed.

C. D. Frost

Papers

Nd evidence for extensive Archean basement in the western Churchill Province, Canada