David W. Valentino

Late Paleozoic transcurrent tectonic assembly of the central Appalachian Piedmont

Recent investigations in south-eastern Pennsylvania and northern Maryland have demonstrated a major anastomosing strike-slip shear system. The Pleasant Grove-Huntingdon Valley shear system emerges from beneath the coastal plain cover at Trenton, New Jersey, and extends to the area west of Baltimore, Maryland, where it is overlain by the Culpepper Mesozoic rift basin. The sense of offset across this system is dextral. In the Susquehanna River region and north of the shear zone, the rocks of the Octoraro Formation contain evidence for two metamorphisms and deformations prior to strike-slip shearing, whereas south of the shear zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen. Although conclusive ages for the strike-slip deformation do not exist at this time, the timing of deformation is loosely constrained where the shear system crosscuts known Taconian structures in the Piedmont. Comparison of deformation style with other regions in the Appalachian suggests the Pleasant Grove-Huntingdon Valley shear system is related to Alleghanian transcurrent tectonics in the Piedmont. Palinspastic reconstruction of the Pleasant Grove-Huntingdon Valley shear system reveals fundamental problems in current tectonic models for the central Appalachian Piedmont. A minimum of 150 km of dextral offset is proposed for the Pleasant Grove-Huntingdon Valley shear system based on reconstruction of the Cambrian-Ordovician shelf edge between northern Maryland and southeastern New York. Displacement of this magnitude can account for the previously proposed tectonic models that portray a failed Iapetan rift block and microcontinent that contains the Baltimore Grenvillian massifs. Even though a history of early orthogonal collision is preserved within discrete structural blocks, transcurrent shearing has greatly influenced the distribution of those blocks. Models not including the strike-slip component of tectonic assembly need serious reconsideration, as evidence grows that the magnitude of orogen-parallel displacement is equal to or larger than the orthogonal component.

Late Proterozoic Rift Control on the Shape of the Appalachians: The Pennsylvania Reentrant

The Pennsylvania reentrant, the most prominent deviation in the trend of the Appalachians, is the product of Late Proterozoic rifting. The Peters Creek Formation, Pennsylvania-Maryland Piedmont, contains rift-gene rated, deep-water turbidite deposits of Late Proterozoic-Cambrian(?) age. These rocks are an extension of the Westminster terrane and lie well to the northeast of the southern Appalachian Late Proterozoic-Cambrian rift basin (Lynchburg-Chilhowee Group basin). The basin into which the Peters Creek Formation was deposited may have connected the southern rift basin with one to the north. The preservation of the Peters Creek Formation and other age equivalent units within the Pennsylvania reentrant indicates that the New York promontory acted as a buttress to Paleozoic orogenic activity.

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

Abstract During and subsequent to the late Paleozoic crustal thickening by the Alleghanian orogeny, the mid-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 1. 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 1 features locally. The character of this later collapse is directly controlled by the geometry of the local structural thickening. The non-pervasive nature of late Alleghanian extensional deformation in the Appalachian Piedmont is probably a function of the non-linearity of the late Paleozoic transcurrent fault system.