VARIATION IN GEOMETRY OF THE APPALACHIAN ALLOCHTHON ACROSS THE PENNSYLVANIA SALIENT-NEW YORK RECESS

Building on his 1977 paper, Dr. William Thomas interpreted the Pennsylvania embayment and the New York promontory as representing upper-plate, Iapetos rift margins offset by the New Jersey transform fault (Thomas, 1993). Subsequent Paleozoic orogenesis deformed the Neoproterozoic to Ordovician rift-drift rocks into the Pennsylvania salient and New York recess, known locally as the Reading Prong and Great Valley sequences of Pennsylvania and New Jersey.

The structural style of the Appalachian allochthon varies along-strike from the Pennsylvania salient to the New York recess, despite the upper-plate characterization of the original rifted continental margin. Stratigraphic position of the basal decollement varies along strike from salient to recess. Large, recumbent, fold-nappe structures in the Pennsylvania salient evolve into northwest verging, imbricate thrust slices with associated duplexes and fault-bend folds in the New York recess. Displacement of the Appalachian allochthon is interpreted as decreasing across the salient-recess transition. This paper will address potential reasons for allochthon variation across the salient-recess transition despite the upper plate inheritance shared by both.