2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 13
Presentation Time: 4:45 PM


BECKER, Thomas1, THOMAS, William1, GEHRELS, George2 and ARONSON, James L.3, (1)Department of Geological Sciences, Univ of Kentucky, 101 Slone Bldg, Lexington, KY 40506-0053, (2)Geosciences, Univ of Arizona, Tucson, AZ 85721, (3)Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, tpbeck2@uky.edu

The Alleghanian orogeny evidently included both dextral shear in the internides and cratonward thrusting in the foreland. The seeming mechanical paradox commonly is explained as the result of a temporal kinematic succession. Analyses of foreland detritus provide insights into the possible succession and timing of events.

A compilation of detrital-zircon and white-mica ages from Pennsylvanian and Permian sedimentary deposits within the Appalachian foreland basin reveal a subtle, but systematic, change in the source of detritus from the Alleghanian orogen. Detrital-zircon populations of both Pennsylvanian and Permian strata in the Appalachian basin are dominated by Grenville ages (950-1300 Ma) and include subordinate late Proterozoic-middle Paleozoic ages (360-570 Ma). The Pennsylvanian-age strata in the Appalachian basin have a significant (~13%) population of Archean (>2500 Ma) and Paleoproterozoic (1600-1900 Ma) detrital zircons, but these older zircons are not represented in Permian sandstones. K-Ar ages of detrital white mica from the Permian Dunkard Group range from 390.8 to 394.9 (+/- 9.2) Ma, indicating that exhumation of the Alleghanian orogen was restricted to shallow levels of the crust.

Geochronologic and petrographic data show a crude correlation between the range of detrital-zircon-age populations and the relative maturity of Pennsylvanian and Permian sandstones. Permian samples have the most restricted range of detrital-zircon ages and are the most feldspathic, suggesting a change in sediment sources through time from one dominated by recycled sedimentary cover to one dominated by primary crystalline sources. The change in composition of the sediment source may be a result of the initiation of cratonward directed thrusting of the Alleghanian crystalline hinterland in the early Permian. The change in source is consistent with a change from oblique collision and dextral shear in the internides to orthogonal collision and cratonward thrusting in the foreland.