Timing of clay mineralization in the Central Appalachians and its extended foreland; challenging the orogenic fluid expulsion ("squeegee") hypothesis.

Radiometric dating of authigenic, low-temperature clays in folded Central Appalachian rocks of Pennsylvania and Maryland and extended foreland in New York show distinct patterns of ages. Extended foreland Ar-encapsulation ages preserve a narrow window of ages of 305-318 Ma, whereas folded Appalachian rocks gives ages in a much younger range of 239-273 Ma. These young ages reflect mineralization during post-peak orogeny exhumation, while foreland ages coincide with mineralization during peak Alleghanian deformation in the Appalachians. Stable isotopic analysis of dated clay samples shows that surface fluid contributed the larger relative proportion of mineralizing fluid volume across the entire region, and was primarily responsible for fluid-mediated growth of illitic material. H isotopes of fluids (δD), calculated from measured clay compositions at a fractionation temperature range of 120-160ºC, are -54 to -70 ‰ in the foreland and -51 to -87 ‰ in folded rocks. These values are distinct from modern waters, but match predicted meteoric δD values for late Paleozoic Pangea (Poulsen et al., 2007). The preservation of slightly more negative values in the fold belt may reflect greater elevation during meteoric fluid infiltration. Whereas mineralization ages in the extended foreland match peak Alleghanian deformation, fluids signatures do not support the hypothesis of expulsion of laterally far-traveled, deep-orogenic fluids (the "squeegee" hypothesis; Oliver, 1986). Moreover, much younger clays within the frontal fold-thrust belt show that surface fluids continued to mix with lesser volumes of formation fluids and metamorphic fluids into the latest Paleozoic, as the Appalachian Orogen eroded and exhumed after final collision.