GEOTECHNICAL IMPLICATIONS OF ALLEGHANIAN OVERTHRUST ARCHITECTURE ALONG THE YELLOW SULPHUR-CATAWBA TRANSVERSE RAMP NEAR BLACKSBURG , VA

USGS-DMME- STATE MAP Geologic studies along the I 81 corridor in SW VA have delineated several branches within the Pulaski Thrust System. The Salem branch of the Pulaski Thrust System is separated from the Main Pulaski fault system to the SW by the Yellow Sulphur – Catawba transverse ramp. Pulaski and Salem fault branches are both known for well-exposed poly-phase folds in a hundred meter thick, strata-bound structural chaos zone localized near the base of the thrust system. Recent structural analysis indicates a generation of early-formed, reclined to recumbent folds have a consistent NW-SE orientation rotated perpendicular to the leading edge of the thrust sheet. This rigid body rotation of tightly-folded Elbrook limestone fold cores appears to reflect metamorphic conditions of channel flow in the basal Elbrook zone as the detachment was imbricated and ascended some 300 meters across the NW trending Catawba Transverse ramp while buried beneath the crystalline Blue Ridge thrust sheet. Excavation of new cuts combined with diamond drill holes associated with development across the transverse structure between Ironto and Christiansburg-Blacksburg has reaffirmed a basic understanding of critical engineering properties discussed in a series of published field guides that have revisited this area for more than 30 years.

1. Construction design problems develop due the unpredictably structured greenschist facies strata exposed in rugged terrain within narrow rights of way.
2. Variable chemical composition of carbonate and phyllitic breccia may lead to loss of cohesive rock owing to dissolution in sulfide rich zones and fractured limestone fold cores.
3. Individual faults and fold cores in the detachment zone rotated to cross-highway trend can lead to many more structural reversals and structural discontinuities than a “normal” Valley and Ridge setting.
4. Recent core and outcrop studies suggest late stage extensional fracturing may be synergistic with actively developing deep epikarst detachment structures that parallel a recently discovered active microseismic zone along the SW margin of the Roanoke Valley.