HOFFMANS FAULT: TECTONIC HISTORY OF A TACONIC FORELAND BASIN FAULT IN THE MOHAWK VALLEY (NYS), BASED ON CHARACTERISTICS OF FRACTURES IN UTICA SHALE CORE AND OUTCROP

The Hoffmans Fault, located in the eastern part of the Mohawk Valley, NYS, is a normal fault striking NNE (with up to 381 m throw) that offsets pC, Cambro-Ordovician, and younger units. The fault was active primarily during the Taconic Orogeny. Two unoriented cores penetrated the Utica black shale near the fault; 74-NY-12, and 75-NY-11. The core sites are located 0.55 km west and 1.73 km east of the Hoffmans Fault, respectively.

We measured characteristics of 400 veins and fractures in core NY-74-12. At 6 m to 49 m above the lower contact of the Utica, nearly all fractures were calcite-filled with apertures ranging from <0.1 mm to 5 mm. At 4 m to 7 m above the lower contact of the Utica, bitumen-coated (but otherwise unmineralized) fracture surfaces dipping 29⁰ to 60⁰ exhibit striations with oblique dip-slip motion. Rakes of the striations range from 23⁰ to 73⁰ with a mode at 56⁰. Geochemical analyses of the bitumen coated surfaces indicate a vitrinite reflectance (Ro) equivalent of 1.84%, whereas nearby in the same core graptolites and pyrobitumen have significantly higher Ro values of 2.85-3.0% and 2.8 to 4.1%, respectively. The Ro disparity may indicate different timings (e.g., fault bitumen postdates the graptolite heating event(s)) or that the conversion from the aromatic methylphenanthrene index (MPI) of the fault bitumen to Ro is only approximate.

Outcrops in Utica Shale near the fault trace display <23 cm thick, vein-filled fractures with releasing and restraining bends. These and other kinematic indicators such as striations indicate dip slip motion (some with high angle reverse motion indicated by fold structures), as well as left and (dominantly) right lateral oblique slip. The dominant vein-filled fractures outcrop in discrete zones <90 m from the main fault. The Taconic foreland basin Hoffmans Fault underwent oblique slip in addition to dip slip motion. If the oblique slip motion is Taconic aged, then the motion could be a result of escape tectonics and/or the result of the orientation of the faults with respect to S_Hmax during late-stage Taconic displacement. However, some of the kinematic indicators (such as the bitumen-coated fault surfaces) may be younger, based on lack of calcite on striations in core. In this study, no kinematic indicators provide evidence for dominantly strike-slip motion.