Abstract
AbstractAs the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (during the “early stage” of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9°N, 218.6°E, A95 = 4.8°, N = 30) and the upper portion of the reversed Osler Volcanic Group (42.5°N, 201.6°E, A95 = 3.7°, N = 59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change.
Highlights
Despite being active for more than 20 million years [Davis and Green, 1997] and resulting in the thinning of prerift crust to less than 10 km [Cannon, 1992], the 1.1 Ga Midcontinent Rift failed to dismember Laurentia
Low-temperature remanence experiments were run on representative samples and loss of remanence across the Verwey transition demonstrates that the magnetic mineralogy is dominated by low-titanium magnetite (e.g., Figure 2b). These results revealed the ability of alternating field (AF) demagnetization to isolate the characteristic remanent magnetization (ChRM) held by magnetite with relatively small and variably present overprints being effectively removed by low-field AF steps
The statistically significant difference between the populations of flow means in the lower and upper third of the stratigraphy, combined with the result that the middle third data have a mean that is an intermediate direction between the lower and upper means, supports the hypothesis that progressive plate motion was ongoing throughout the eruption of the Osler Volcanic Group with Laurentia moving to lower latitudes
Summary
Despite being active for more than 20 million years [Davis and Green, 1997] and resulting in the thinning of prerift crust to less than 10 km [Cannon, 1992], the 1.1 Ga Midcontinent Rift failed to dismember Laurentia (cratonic North America). This failure resulted in the preservation of a thick record of rift-related volcanic and sedimentary rocks that gives geoscientists insight into the development of this ancient rift. Constraining this paleogeographic change is essential for understanding rift development and for constraining late Mesoproterozoic paleogeographic reconstructions given the centrality of Laurentia’s apparent polar wander path to such efforts
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