Structure, age, and tectonic setting of a multiply reactivated shear zone in the piedmont in Washington, D.C., and vicinity

Abstract

The Rock Creek shear zone is the dominant tectonic feature in the Piedmont in Washington, D.C. and adjacent parts of Maryland, has an exposed length of 25 km, and a width of up to 3 km. The shear zone is characterized by a complicated composite fabric produced by the imposition of both ductile and brittle structures as well as the reactivation, transposition, and folding of older structures during subsequent antithetic displacement. At least five main types of structural elements are discernible and include: 1) relict, medium- to coarse-grained mylonitic foliation and related structures produced by sinistral shearing under at least middle amphibolite facies conditions; 2) a ductile fault zone having an apparent sinistral displacement of at least several km and an unknown, but possibly significant component of upward throw of the east wall; 3) pervasive, fine-grained ultramylonitic foliation associated with quartz ribbons and late oblique shear bands, generated by dextral shearing under thermal conditions that appear to have progressed from middle greenschist to sub-greenschist (semi-brittle); 4) a system of oblique-(west wall up) and dextralship faults localized chiefly within a tectonic me??lange at the junction of two major strands, and whose motion spanned the ductile-brittle transition; and 5) a system of post-Cretaceous thrust faults that cut Coastal Plain rocks as young as Quaternary as well as the previously deformed crystalline rocks. The first two sets of structures are of probable Ordovician age and are thus believed to coincide with the Taconic event, which produced regional middle to upper amphibolite facies metamorphism, widespread plutonism, and extensive southwest-vergent fold phases in this area. In contrast, the dextral shearing and faulting were generated during final thermal cooling and represent the latest Paleozoic penetrative deformation that affected this area. They are very likely Alleghanian because of their great similarity to other better-dated Alleghanian structural features in the Maryland and Virginia Piedmont.