FRACTURE CHARACTERISTICS IN UTICA SHALE CORE AND THEIR RELATION TO THE TECTONIC HISTORY OF THE HOFFMANS FAULT IN THE MOHAWK VALLEY, NEW YORK STATE

The Cambro-Ordovician section in the Mohawk Valley of New York State has been an incubator for Taconic plate tectonic models as well as for black shale studies. The Hoffmans fault is one of the most significant NNE-striking normal faults in the Mohawk Valley (with up to 381 m throw), and was primarily active during the Taconic Orogeny. Two unorienetd cores penetrated the black shale of the Utica Formation near the fault; core 74-NY-12 retrieved the complete Utica section (53.6 m), whereas core 75-NY-11 retrieved an incomplete lower section of Utica. These cores are located 0.55 km west and 1.73 km east of the Hoffmans fault, respectively. By examining structural features in the Utica, including fractures, veins, cross cutting relationships, and kinematic indicators such as crystal orientations and rhombochasms, we have been able to build a more complete tectonic history of the Hoffmans fault.

We measured 400 veins and fractures in NY-74-12. At 6 m to 49 m above the lower contact of the Utica, almost all fractures were calcite filled with apertures ranging from <0.1 mm to 5 mm. At 4 m to 7m above the lower contact of the Utica, bitumen-coated (but otherwise unmineralized) fracture surfaces dipping 29⁰ to 60⁰ exhibit striations with oblique, but predominantly dip slip motion. The rakes of the striations range from 23⁰ to 73⁰ with a mode at 56⁰.

It has been proposed that several of the Taconic faults, which are generally considered to be normal faults resulting from flexural extension, also underwent oblique slip/strike-slip motion as a result of escape tectonics during Taconic times and/or as the result of the orientation of the faults with respect to S_H. If the zone of oblique slip in core NY-74-12 is Taconic-aged, then the oblique slip/strike-slip model is supported by the core data. The lack of calcite in these fractures might indicate, however, a younger age. In core 75-NY-2, which lies 12.42 km east of the Hoffmans fault, and adjacent to the Saratoga-McGregor fault, another orogen parallel synthetic normal fault, kinematic indicators support only down-dip motion. In all core examined thus far, no kinematic indicators provide evidence for dominantly strike-slip motion.