Crust-Forming Process: Basement and Basements in the Southern Appalachian Orogen

Abstract

Basement is traditionally considered crust generated during an earlier cycle of tectonic activity. In the Appalachian orogen, Grenvillian (1.0–1.2 Ga) rocks commonly and correctly are thought of as the Laurentian cycle basement precursor to the Paleozoic Appalachian Wilson cycle tectonism. Grenvillian massifs are concentrated along the western margin of the Taconian metamorphic core throughout the orogen, but also occur in several windows in the internides. Three major events, however, formed the Appalachians: the Taconian-Penobscottian (Ordovician), Acadian (Devonian-Early Carboniferous), and Alleghanian (Late Carboniferous-Permian) orogenies. Each of these events involved terrane accretion or continent-continent collision that produced deformation, metamorphism, and plutonism affecting large portions of the orogen, and thus created new crust that serves as basement for each succeeding event. In New England, the amount of crust created by the Acadian orogeny is much greater than that produced by the Taconian orogeny, although this is difficult to estimate because of the amount of high-grade overprint by the Acadian and Alleghanian in southern New England. The Alleghanian orogeny produced an areally smaller metamorphic over print, but produced extensive dextral strike-slip faults and several large stepover basins in the Canadian Maritimes and the Narragansett basin (and several very small basins) in southern New England. In the southern and centralAppalachians, the Taconian and Acadian were also major crust-forming events that created new crust. This early and middle Paleozoic crust was deformed by Carboniferous dextral (ductile) strike-slip faulting related to early oblique collision, uplifted, cooled, and then transported as much as 400 km continentward as a mostly cold slab—the Blue Ridge-Piedmont megathrust (BRP) sheet—during late Alleghanian head-on collision of North America and Africa. A metamorphic overprint occurs only along the trailing edge of the BRP sheet, but plutonism is much more extensive. The southern and central Appalachians are thus a thrust-dominated collisional orogen produced by westward advance of a composite Middle-Late Proterozoic, early to middle Paleozoic basement that pushed the Paleozoic foreland in front of it. Each Paleozoic event thus produced crust that became basement for subsequent events until the Appalachian Wilson cycle was completed and the supercontinent of Pangaea formed. Appalachian and all earlier crust thus became basement for Late Triassic and younger sedimentation as the modern Atlantic Wilson cycle began.