Detrital zircon evidence of Laurentian crustal dominance in the lower Pennsylvanian deposits of the Alleghanian clastic wedge in eastern North America

Abstract

The late Paleozoic Alleghanian orogeny culminated with the continent–continent collision between Laurentia and Gondwana in the construction of the supercontinent Pangea. Within the Appalachian basin, the sedimentological record of the early stages of the continental collision is preserved as late Mississippian-age siltstones and mudstones that mark the end of carbonate production. An upward transition to coarse, quartz-rich, cratonward-prograding, fluvial deposits of Pennsylvanian age is cited to mark the initiation of continental collision and cratonward advancement of the tectonic load. Seven samples of early Pennsylvanian (early-middle Morrowan) sandstones and conglomerates from the Appalachian basin were collected along the orogen from eastern Pennsylvania to central Alabama for U–Pb dating of detrital zircons. All of the samples reveal a dominance of zircons with ages that can be attributed to crust-forming events in Laurentia. A small percentage of the detrital zircons have ages (530–680 Ma and 2000–2200 Ma) that correspond to the Pan-African/Brasiliano orogenic belts and Trans-Amazonian/Eburnian cratons typical of Gondwanan crust. Although recycling of pre-orogenic strata may disproportionately bias the detrital-zircon population toward ages of Laurentian affinity, the dominance of the Laurentian “signature” in the Appalachian basin along strike suggests that the early components of the Alleghanian orogen were composed mainly of recycled Laurentian crust. Detrital-zircon populations are not indicative of substantial incorporation of Gondwanan crust in the orogenic highlands. On the basis of the timing of dextral displacement along shear zones in the Appalachian hinterland, the early Pennsylvanian clastic deposits preserved in the Appalachian basin likely correspond to transpressional exhumation of the Laurentian margin, consistent with hypotheses of oblique collision between Gondwana and Laurentia in the late Paleozoic.