FRACTURES, VEINS, AND FAULTS: CHARACTERISTICS OF BRITTLE DEFORMATION IN THE UTICA SHALE, MOHAWK VALLEY, NEW YORK STATE

Brittle deformation characteristics in the eastern Mohawk Valley, New York State vary among outcrops as a function of location within the Taconic foreland basin stratagraphic sequence, the rheology of the lithologic units at the time of deformation, and on the timing of tectonic events. Major structural variations within the Indian Castle Formation, the Flat Creek Formation, the Dolgeville Formation and the Schenectady Formation include drag fold morphology on normal faults with an absence of fracture intensification domains (FIDs), normal faults with accompanying FIDs, brecciated veins and transtensional shear interpreted from vein-filled rhombochasms. The variety of structural features within the Utica Shale suggests a complex tectonic history of formation and reactivation of faults in the Mohawk Valley.

Approximately 1700 fracture and vein measurements were taken from 26 outcrops covering an area between Dolgeville and Schenectady, New York State. Variations in the strike of fracture sets include sets that strike parallel and sub-parallel to major faults in the vicinity of the field sites and sets that maintain a strike that is orthogonal to these same faults. Veins observed in the field area exhibit Mode I opening throughout the western field sites within the Indian Castle Formation and show an increase in transcurrent motion moving east into the Flat Creek Formation. Right lateral and left lateral shear distribution varies between ENE, EW, and WNW sets with vein counts peaking at 70° for right lateral shear and 290° for left lateral shear. The motion on these vein sets suggests the formation of conjugate shear zones with σ₁ oriented E-W. Maximum E-W compression with the possible contemporaneous formation of fracture sets striking orthogonal to faults supports the hypothesis that normal faults were reactivated as thrusts within the Taconic basin.