PROGRESSIVE FORELANDWARD TECONIC LOADING DURING GROWTH OF THE CENTRAL APPALACHIAN FOLD-AND-THRUST BELT: EVIDENCE FROM FLUID INCLUSIONS

The Late Paleozoic central Appalachian fold-and-thrust belt is rich in syntectonic quartz and calcite veins that are pervasive throughout the orogen. Fluid inclusion microthermometry and barometry of these temporally-controlled veins provides information on deformation conditions during fold-and-thrust belt deformation and estimates of syntectonic overburdens. When the paleo-overburden values derived from the aqueous brine and methane-carbon dioxide fluid inclusions are plotted on either a current regional cross-section or a palinspastic reconstruction of the fold-and-thrust belt, a there is no consistent pattern. However, when the same data are related to sequentially deformed sections, patterns of thrusting-related tectonic loading emerge.

Several major tectonic loading events punctuate the over 165 km of Latest Carboniferous to Permian shortening of the central Appalachian fold-and-thrust belt from the Blue Ridge through the Appalachian Plateau: 1) Emplacement of the Blue Ridge thrust system created a syn-tectonic wedge of rock that was up to 7 km thick above the current Massanutten synclinorium and tapered to <3 km thick, 30 km toward the foreland. 2) Emplacement of the North Mountain thrust system resulted in significantly increased overburdens in the eastern Valley and Ridge province. Nearest the thrust, syn-tectonic overburden was up to 7 km thick. This wedge tapered to a thickness of <3 km at 40 km from the thrust. 3) Growth of the Cacapon Mt. – Adam Run anticlinorium resulted in significant denudation and the formation of a syn-folding wedge of sediment that tapered from 4 km, to <2 km at 45 km from the anticlinorium. 4) Thrust duplication of the Cambro-Ordovician carbonate section during the formation of the Wills Mountain anticlinorium and concurrent syn-thrusting denudation resulted in the deposition of at least 4 km of Permian sediment at the Allegheny Front, which deceased to 2.5 km at 70 km from the Allegheny Front.

Each of these tectonic loading events triggered an episode of major thrust propagation as pore-fluid pressures were elevated along detachment horizons. In addition, as a fold-and-thrust belt deformed, these temporal and spatial changes in overburden had direct implications for a variety of geologic processes, including diagenesis, hydrocarbon generation, and fluid migration.