APPALACHIAN PROMONTORIES AND THE STRUCTURAL EVOLUTION OF THE OROGEN

In a classic study, Thomas (1977) proposed that Appalachian promontories and embayments were inherited from a structural framework formed during late Neoproterozoic rifting of the eastern margin of Laurentia. This framework influenced the structural evolution of the orogen during later tectonothermal events and it also provides a potential reference frame for detecting transcurrent displacement within the orogen.

Accreted terranes of the Appalachian Iapetan and peri-Gondwanan realms display structural trends concordant with promontories and embayments in native Laurentian realm rocks, indicating that during accretion, the shape of the ancient continental margin acted as a template around which accreted terranes were molded. The change in structural trends around promontories appears to have led to partitioning of tectonism during and after accretion. Late Ordovician to Silurian sinistral oblique accretion of peri-Gondwanan terranes is recorded in penetrative tectonites limited to the southern sides of the New York and St. Lawrence promontories.

Post-accretion, transcurrent motion in the orogen appears to be recorded by a displaced promontory. Along the NC-VA state line, the Virginia promontory is defined by a change in structural trends in Laurentian margin and Iapetan rocks, yet peri-Gondwanan rocks outboard of the promontory are undeflected. However, south of the promontory, in the area of the NC-SC state line, peri-Gondwanan rocks display a flexure, similar in form and scale, to that of the promontory whereas across strike, Laurentian and Iapetan rocks show no change in structural trends. This geometry suggests that the Virginia promontory was imposed upon the peri-Gondwanan rocks during Late Ordovician-Silurian accretion, but was subsequently subject to approximately 200 km of dextral displacement relative to the Laurentian margin. Mid-Paleozoic paragneisses of the Cats Square terrane lying between the Waxhaw flexure and the Virginia promontory may represent the remnant of a pull-apart basin associated with dextral displacement of the promontory. The dextral strike slip fault system responsible for this geometry may well lie buried beneath Alleghanian hinterland thrust sheets.