Asynchronous extensional collapse of a transpressional orogen: The Alleghanian central Appalachian Piedmont, USA

During and subsequent to the late Paleozoic crustal thickening by the Alleghanian orogeny, the mid-Atlantic Atlantic Appalachian Piedmont underwent an asynchronous two-stage extensional collapse over a 100 + Ma interval. Extensional deformation features are found throughout the Piedmont but are especially well displayed in the Philadelphia area to the east and along the Susquehanna river to the west. As formerly dextral transpressional structures moved from restraining to releasing bends, synorogenic transtensional release occurred (stage 1). Because the transition from transpressional to transtensional regimes is a local phenomenon that can occur throughout the strike-slip event, the development of the extensional structures is asynchronous in stage I. Alleghanian crustal thickening in the region is expressed as large antiforms and domes. Stage 2 extension occurred as a result of late syn- to post-orogenic gravitational collapse of the antiforms or domes. Although the extensional structures produced within each of the stages are distinct, the transitions between each of them are not. Features of both stages include extensional crenulation cleavages, ductile normal faults, brittle normal faults, and metamorphic evidence of rapid decreases in temperature during deformation. The metamorphic assemblages of the deformation features show that the transition from compressive deformation to extension occurred during the same thermal event. This transition, however, is not synchronous everywhere. The stage 1 transtensional features appear to have occurred under a variety of conditions and at different times throughout the dextral strike-slip (transpressional) system. Stage 2 gravitational collapse features occurred in areas of crustal thickening but overlap stage l features locally. The character of this later collapse is directly controlled by the geometry of the local structural thickening. The non-pervasive nature of the late Alleghanian extensional deformation in the Appalachian Piedmont is probably a function of the non-linearity of the late Paleozoic transcurrent fault system.