SYN-OROGENIC CHANGES IN OVERBURDEN THICKNESS AND DEFORMATION CONDITIONS IN THE CENTRAL APPALACHIANS: EVIDENCE FROM FLUID INCLUSIONS

Pore fluid pressure and temperature within a deforming fold-and-thrust belt are constantly changing as a result of changes in overburden. The overburden of a particular stratigraphic interval may increase due to syn-orogenic sedimentation in a foreland basin far from the advancing thrust front, at the toe of an emergent thrust, in a piggy-back basin within the fold-and-thrust belt, or by the emplacement of an overriding thrust sheet. Overburden may decrease due to syn-orogenic erosion of growing folds and/or thrust sheets being transported to a higher structural level across a thrust ramp. These processes vary temporally and regionally across the fold-and-thrust belt such that a unit that is experiencing burial in one part of the belt, may be experiencing denudation in another. Calcite and quartz vein minerals commonly contain aqueous brine and methane-carbon dioxide fluid inclusions that record these changes in deformation conditions. The following summarizes the changes in overburden during the hinterland to foreland development of the central Appalachian fold-and-thrust belt: 1) Emplacement of the North Mountain thrust (NMT) system resulted in significantly increased overburdens due to syn-orgenic deposition at the toe of the thrust and/or actual overriding of the thrust sheet. Nearest the thrust, restored post-Pennsylvanian overburdens ranged from 3.0 to over 6.0 km. These values decreased steadily into the foreland, such that at 50 to 60 km from the NMT, there was no significant post-Pennsylvanian overburden. 2) Growth of the Cacapon Mt. – Adam Run anticlinorium resulted in syn-folding denudation and removal of 4.0 to 6.0 km of overburden in the NMT footwall region. 3) Thrust duplication of the Cambro-Ordovician carbonate section during the formation of the Nittany anticlinorium resulted in syn-thrusting erosion, and removal of 1.0 to 4.0 km of Paleozoic section. 4) Much of the eroded material from the Nittany anticlinorium was deposited in the Appalachian Plateau region, resulting in post-Pennsylvanian overburdens of 2.5 to 4.0 km at the Allegheny Front, and deceasing steadily toward the foreland. Post-Pennsylvanian overburdens in the vicinity of the Chestnut Ridge anticline were approximately 2.0 to 2.5 km.