APPALACHIAN OROGEN STRUCTURAL STYLES IN THE APPALACHIAN FORELAND BASIN: TECTONIC IMPLICATIONS OF NEOACADIAN AND YOUNGER FAULT-BEND FOLDS AND OTHER ANTICLINES OBSERVED IN SEISMIC REFLECTION DATA IN PENNSYLVANIA AND WEST VIRGINIA

Fault-bend folds that typify the Appalachian fold and thrust belt also occur in anticlines immediately NW of the Appalachian (Allegheny) Front in the “High Plateau” of the Appalachian foreland basin. U-Pb age dates of syntectonic veins in a core near one of the anticlines yield two isochrons with ages (317 and 332 +/- 20 my) between traditional Neoacadian and Alleghanian time, suggesting that the late Alleghanian folds hide older structural phases.

The forelimb of fault-bend folds and the associated thrust are recognized in 2D seismic data based on the acute angle between steeply-dipping reflectors in the forelimb above the inferred thrust flat and flat-lying reflectors below the inferred thrust flat. The fault bend folds generally are westerly-directed, unlike the asymmetric salt/mudrock cored anticlines farther northwest that verge east. Fault-bend fold examples in units above the Devonian Onondaga include structures in the Negro Mountain, Laurel Hill, Fayette and (southern extension of the) Hiram anticlines. Chestnut Ridge anticline displays a triangle zone. A west-verging train of fault-bend and fault-propagation folds affect the Trenton beneath the Negro Mountain anticline.

Both the Arches Fork and the Wolf Summit anticlines in WV appear to be partly the result of basin inversion; normal faults of the Rome trough underwent later compression and horizontal shortening, resulting in basement blocks backing up the formerly normal faults. About 1.75% shortening across the basement fault blocks would account for the vertical structural relief and shortening observed at the Trenton horizon. Motion occurred in Alleghanian, Taconic, Salina, and possibly Neo/Acadian times.

A broad, low amplitude anticline in WV results from a sheet (or tongue) of Salina units that squeezed west out of a syncline. Sediment infill indicates a Neoacadian age. Other asymmetric evaporite/mudrock cored anticlines are spectacularly displayed in a newly reprocessed 3D seismic survey; these too have variably-aged Neoacadian infills. Based on cooling ages in the Blue Ridge, salt mobilization was promoted by crustal flexure and instability stemming from plate loading by the Blue Ridge. The early fault-bend folds in the basin may be related to ongoing transpressional motion of the Potomac Terrane and possibly terranes farther east.