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Rates of Relative Plate Motion During the Acadian Orogeny Based on the Spatial Distribution of Black Shales

Frank R. Ettensohn
The Journal of Geology
Vol. 95, No. 4 (Jul., 1987), pp. 572-582
Stable URL: http://www.jstor.org/stable/30081087
Page Count: 11
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Rates of Relative Plate Motion During the Acadian Orogeny Based on the Spatial Distribution of Black Shales
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Abstract

Devonian-Mississippian black shales apparently were deposited in foreland basins formed in response to deformational loading in the Acadian orogen. These black shales were deposited in a succession of partially overlapping basins separated from each other by coarser clastic units, which migrated episodically south-westward in time parallel to the orogen. If foreland basins are results of an isostatic response to deformational loading in the orogen, migration of successive basins along the orogen must reflect concomitant shift of orogeny, and hence relative plate motion in the same direction. These black-shale deposits, then, not only demonstrate the diachronous nature of the Acadian orogeny, a fact previously indicated by other lines of evidence, but also enable approximation of rates of relative plate motion to be made. Using three different dating schemes, the black-shale deposits indicate Late Devonian plate motion along an 800 km segment of the orogen during approximately 11 Ma, for an average rate of relative plate motion slightly greater than 7 cm/yr. The facts that the black-shale basins do apparently track plate movement and resulting orogeny and that the along-strike component of basin migration exceeds the perpendicular-to-strike component suggest that at least Late Devonian parts of the Acadian orogeny are related to episodes of oblique convergence involving major strike-slip movement with substantial amounts of transpression.

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