A geologic history of the north-central Appalachians, part 3. The Alleghany orogeny
The north-central Appalachians occupy a critical position within the 3000+ km-long Appalachian orogen, lying southwest of the boundary between the central and northern Appalachians (CNAB). The one-billion-year-long history of tectonic activity in eastern Laurentia includes the creation and evolution of the Appalachian orogen during the Paleozoic and the Mesozoic transformation of the orogen into a passive margin during Pangea's disassembly. A most important ingredient in the evolution of the orogen was the Alleghany orogeny, which was driven by the convergence and collision between Laurentia (Laurussia) and West Gondwana (Africa). The Alleghany orogeny in the central and southern Appalachians was a decollement tectonism that involved a larger part of eastern Laurentia than had the previous three orogenies. The fundamental element was a very low-angle thrust (decollement) that originated in mid-crustal levels east of the presently-exposed Appalachians and rose westwardly to progressively higher levels in the upper crust and the supra-crustal Paleozoic section. Alleghany deformation was widely developed in the hanging-wall block (allochthon), primarily in the form of thrust faults and fold-and-thrust structures, both of which splayed upward from the basal decollement. The youngest manifestations of the Alleghany orogeny were northeast-trending strike-slip faults and dextral shear zones in the Piedmont. In the north-central Appalachians, the exposed allochthon consists of two parts: the sedimentary externides (Appalachian Plateau and Valley and Ridge provinces) and the crystalline externides (Reading Prong, Blue Ridge belt, and Piedmont province). Long, thrust-cored anticlines predominate in the sedimentary externides. A widespread layer-parallel shortening preceded the folding; it is largely coaxial with the folding but extends considerably farther to the northwest toward the craton. It is hypothesized that the folding developed in reverse order, sequentially from the northwest to the southeast. The crystalline externides are dominated by low-angle thrust faults and upright folds trending east-northeast. The first-order Valley and Ridge folds on the northwest side acted as a buttress and diverted the crystalline externides rocks north-northwestwardly, onto the topographic low area over the Anthracite region. This thrusting of the crystalline externides caused anthracitization of the coals within the Pennsylvanian rocks there. Metamorphism and magmatism were significant events during the earlier phase of the Alleghany orogeny in the southern Appalachians. Whatever magmatism and medium-to high-grade metamorphism developed in the north-central Appalachians are in the covered internides to the southeast. The Alleghany orogeny of the north-central Appalachians occurred during the Early Permian. Erosion of anticlinal crests probably began as the folds grew, with accumulation of this locally-derived sediment in the intervening synclines. A regional alluvial plain coalesced above the partially-eroded externides structures as erosion of the pre-Alleghany highland and the Alleghany hinterland mountains continued to the southeast, spreading sediment to the northwest. This erosion and northwest transport probably persisted, with diminishing intensity, throughout the remainder of the Permian and into the Mesozoic, and changed only with the beginning of crustal extension during the Late Triassic.