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 s outhern Virginia. Traditionally, this boundary has been viewed as a co mformable contact between infrastructural Milton rocks and suprastruct ural Carolina rocks. In contrast, our work indicates that shallowly di pping, sillimanite gneisses of the Milton terrane comprise the footwal l of the ductile shear zone whereas moderately to steeply-dipping gree nschist to amphibolite facies Carolina terrane gneisses and metavolcan ics form the hangingwall. The Hyco shear zone has a sharply arcuate su rface 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 i ndicate dominantly dextral strike slip. Along the Virginia segment, th e zone is moderately to shallowly east-dipping, with structures indica ting 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 th e 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 o f the shear zone in conjunction with geological, geophysical, and isot opic contrasts across the zone negates established views that the Milt on rocks are a comformable, higher grade portion of the Carolina terra ne. These contrasting features, instead, are remarkably similar to tho se found along the central Piedmont suture, the tectonic boundary betw een the Piedmont and Carolina zones from South Carolina to central Nor th Carolina. We propose that the Hyco shear zone is the northern equiv alent of this structure; thus, our new data on the Hyco shear zone hav e significant regional implications concerning the nature, timing, and kinematics of this zone boundary.