GENERAL SHEAR ZONES AT THE EASTERN BASEMENT-COVER CONTACT, BLUE RIDGE PROVINCE, VIRGINIA

The contact between Mesoproterozoic basement granitoids and Neoproterozoic metasedimentary cover rocks (Lynchburg and Fauquier groups) on the southeastern flank of the Blue Ridge anticlinorium in Virginia has traditionally been mapped as a southeast dipping unconformity. Some tectonic models posit that this contact frames the Neoproterozoic Iapetan hingeline between highly extended crust to the east with its attendant thick rift sequence from a thin rift sequence to the west and as such the contact could be a Neoproterozoic normal fault.

At a number of locations the contact between the basement and cover sequence is framed by greenschist-facies basement mylonite zones, 10 to 100 meters thick, that dip northwest and have down-dip mineral elongations. Kinematic indicators typically record top-to-the southeast (reverse) shear. Finite strains in XZ sections range from 4:1 to >15:1 and three-dimensional strains vary from oblate to weakly prolate. Vorticity gauges (R_s/Θ, back rotated clasts, quartz c-axes) indicate general shear (W_m = 0.3 to 0.8). Minimum ductile displacements across these zones are calculated between 50 and 700 meters. Bedding in the cover sequence near the contact is typically upright and dips to the northwest as does the penetrative foliation. Cover rocks are folded into a sequence of tight to open, southeast verging structures. At a few locations blocks of mylonitic basement are imbricated with cover rocks.

Collectively, these observations indicate that the basement-cover contact forms a northwest-dipping shear zone/fault with reverse displacement. The geometry of the basement-cover contact in the eastern Virginia Blue Ridge may have been a original Iapetan rift-structure that was rotated and reactived in the Paleozoic. Regardless of the contact’s Neoproterozoic heritage, the structure accommodated crustal shortening during the Paleozoic and is in essence an out-of-sequence backthrust likely related to movement of the Blue Ridge basement massif over ramps.