CRUSTAL CONSTITUENCY OF THE ALLEGHANIAN OROGEN AS REFLECTED BY RADIOMETRIC SEDIMENTARY PROVENANCE PROXIES

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.