The influence of pattern grading on bodice fit and style sense
The western Piedmont of Pennsylvania is underlain by the Octoraro and Peters Creek Formations, and these formations were juxtaposed during Late Paleozoic dextral strike-slip shearing. North of the shear zone, the Octoraro Formation contains 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. A minimum of 150 km of orogen parallel dextral offset is proposed for the shear system based on palinspastic reconstruction of the Cambrian-Ordovician shelf edge between northern Maryland and southeastern New York.
The Peters Creek Formation consists of three lithofacies: 1) graded metasandstone beds, 2) meta-quartz-pelite, and 3) massive metasandstone lenses within the graded bedded sequences. The occurrence of interlayered greenstone in lithofacies 1) suggests rift related deposition. These rift clastics consist of two submarine turbidite-fan systems defined by thick sequences of interlayered feldspathic metasandstone and schist, separated by a region underlain of quartz-schist. Comparison of the Peters Creek Formation with known Iapetan rift clastics in the central Appalachians of Virginia suggests the Peters Creek deposits are also related to Iapetan rifting.
Transpressional structural models have been applied to oblique convergence tectonics, with the coeval development of contractional and transcurrent structures. Late Paleozoic post-Taconian deformation in the north-central Appalachian Piedmont of Pennsylvania and Maryland is characterized by two stages of dextral transpression. Stage one comprises a map-scale ductile conjugate shear zone pair (the Rosemont and Crum Creek shear zones) that developed at amphibolite facies. These conjugate shear zones were later overprinted, during stage two, by greenschist facies dextral shear zones that flank broad upright antiforms. Conjugate shear-pair criteria were applied to these structures to constrain the paleo-principal compressive stress orientations. During stage one σ1 and σ3 were shallowly plunging, with σ2 steeply plunging. During stage two σ1 and σ2 were shallowly plunging, with σ3 steeply plunging. The structural evolution and associated change in stress array suggests unroofing during transpression, consistent with the decrease in metamorphic grade. Post-transpressional deformation produced a pair of conjugate cleavages in the lower Susquehanna River region, and determined orientations of the principal compressive stresses suggest Late Paleozoic extension, possibly related to gravitational collapse.
Previously published orthogonal collision and tectonic assembly models for the north-central Appalachian Piedmont are incompatible with the new data. Oblique collision tectonics resulted in complex dextral transpressional deformation and large orogen parallel displacement of crustal blocks. Tectonic models that do not include the transpressional orogen component of the tectonic history should be seriously reconsidered.