SUPRASTRUCTURAL-INFRASTRUCTURAL TERRANE JUXTAPOSITION AND THE EASTERN PIEDMONT FAULT SYSTEM IN THE NC-VA EASTERN PIEDMONT: DEVELOPMENT OF A DEXTRAL FREEWAY IN THE SOUTHERN APPALACHIAN HINTERLAND

In the NC-VA eastern Piedmont, eight strike-slip strands of the late Paleozoic Eastern Piedmont fault system (EPFS) and three late Paleozoic to early Mesozoic normal faults form a complex crystal plastic-brittle fault network. These faults juxtapose four late Proterozoic greenschist-facies (chlorite-biotite to local staurolite zone) suprastructural and six late Proterozoic to Early Devonian amphibolite-facies (garnet-staurolite-kyanite-sillimanite zone) infrastructural terranes that are traditionally interpreted to be parts of the Carolinia or Goochland lithotectonic domains but may possibly be linked to northern Appalachian domains.

This array of terranes is located northeast of a strike change of the EPFS from N40-50E in SC to N15-20E in the NC-VA eastern Piedmont. This dextral fault reorientation marks an Alleghanian transpressional restraining bend characterized by strike-slip duplex and half-flower structure formation, regional juxtaposition and exhumation of terranes of contrasting metamorphic grades, and localization of granite plutonism. Shear zone evolution appears to involve wrench, rotational, and contractional strain during Laurentian–Gondwanan collision and Pangea amalgamation. Normal faults mark Pangea breakup in the early Mesozoic Era and generally follow the trend of the strike-slip faults. Strands of the EPFS join suprastructural and infrastructural terranes along a series of dextral Y- and l-type shear zone junctions. While zippered shear junctions require both dextral and sinistral kinematics, data supporting sinistral slip are not reported for the NC-VA eastern Piedmont. EPFS shear strands instead appear to merge as an orogen-scale dextral freeway system that analogue modeling indicates can produce significant orogen-parallel displacement. An east-side-south ductile freeway combined with post-orogenic ductile-brittle uplift of freeway terranes may explain new U-Pb zircon geochronology indicating Early Devonian meta-igneous and metasedimentary infrastructural terranes are bound by Neoproterozoic meta-igneous and metavolcaniclastic suprastructural terranes. Geochronology indicates that infrastructural terrane metamorphism occurred at least by earliest Alleghanian orogenesis while normal faulting was active by late Permian to Triassic time.