The Hyco shear zone in North Carolina and Southern Virginia: Implications for the piedmont zone-Carolina zone boundary in the southern Appalachians

The recently recognized Hyco shear zone is a first-order structure in the southern Appalachians that separates sharply contrasting rocks of the Carolina and Milton terranes in north central North Carolina and southern Virginia. Traditionally, this boundary has been viewed as a comformable contact between infrastructural Milton rocks and suprastructural Carolina rocks. In contrast, our work indicates that shallowly dipping, sillimanite gneisses of the Milton terrane comprise the footwall of the ductile shear zone whereas moderately to steeply-dipping greenschist to amphibolite facies Carolina terrane gneisses and metavolcanics form the hangingwall. The Hyco shear zone has a sharply arcuate surface trace: it trends east-northeast in the area of Hyco Lake, North Carolina, and abruptly changes trend to north-northeast near the North Carolina-Virginia state line. The zone is moderately to steeply south-southeast-dipping along the Hyco Lake segment, and structures there indicate dominantly dextral strike slip. Along the Virginia segment, the zone is moderately to shallowly east-dipping, with structures indicating thrusting of the Carolina terrane over the Milton terrane. Timing constraints on synkinemtic plutons indicate that the shear zone is an early Alleghanian feature. The geometry, kinematics, and timing of the shear zone are compatible with the interpretation that the Hyco Lake and Virginia segments of the zone represent lateral and frontal ramps, respectively, in an overall Alleghanian thrust regime. Recognition of the shear zone in conjunction with geological, geophysical, and isotopic contrasts across the zone negates established views that the Milton rocks are a comformable, higher grade portion of the Carolina terrane. These contrasting features, instead, are remarkably similar to those found along the central Piedmont suture, the tectonic boundary between the Piedmont and Carolina zones from South Carolina to central North Carolina. We propose that the Hyco shear zone is the northern equivalent of this structure; thus, our new data on the Hyco shear zone have significant regional implications concerning the nature, timing, and kinematics of this zone boundary.