STRUCTURAL INVESTIGATION OF THE NORTH MOUNTAIN THRUST ZONE THROUGH BEDROCK MAPPING OF THE ORKNEY SPRINGS (VA) 7.5’ QUADRANGLE

The North Mountain Thrust Zone (NMTZ) is an enigmatic structure in the central Appalachians that prior workers have interpreted as both the sole thrust of the Blue Ridge anticlinorium and as part of a duplex accommodating the change in stratigraphic level of the décollement, feeding displacement to the foreland along a basal Cambrian décollement that steps up to an Ordovician shale horizon. In this study, we present new 1:24,000 scale mapping of the Virginia portion of the Orkney Springs 7.5’ quadrangle to address the role of the North Mountain Thrust Zone (NMTZ) in transitioning from basement involved thick-skinned deformation of the Blue Ridge Province to thin-skinned deformation in the Valley and Ridge Province. Cambrian-Devonian carbonate and siliciclastic strata are folded and faulted, forming northeast trending (30°-44°) structures that record deformation from the Late Paleozoic Alleghenian Orogeny. The northeast striking NMTZ comprises several splay faults that juxtapose folded horses of the Cambrian-Ordovician passive margin carbonates above Ordovician-Devonian siliciclastic rocks of the Taconic and Acadian foreland basins in the footwall to the west. In the footwall of the NMTZ, the Ordovician-Devonian strata is folded into the overturned Supin Lick Syncline and Adams Run Anticlinorium, forming the Alleghenian floor sequence. The north central portion of the map area is characterized by north trending (~5°-10°) tight to isoclinal folding of the Silurian-Lower Devonian systems on the shared upright limb of the Supin Lick and Adams Run structures. These structures, which are not documented on existing maps of scale < 1:24,000, are subparallel to a north trending cross fault that offsets the western limb of the Supin Lick Syncline and may suggest a transpressional component to strain along the NMTZ. We use newly collected structural data and updated mapping to constrain the structure of a thrust duplex that accommodates a vertical step in the stratigraphic level of the NMTZ from the sole thrust of the Blue Ridge Province to the décollement in the Valley and Ridge Province. Our results will help to better understand the kinematic transition from thick-skinned basement involved deformation to thin-skinned foreland fold-thrust belts in collisional orogens.