TACONIC CONVERGENCE FAULTS IN THE MOHAWK VALLEY OF NEW YORK STATE

The “Taconic” faults exposed in the Mohawk Valley in New York State have been studied for over 100 years; in the past 40 years they have been thought to be normal faults related to the Laurentian plate flexing as it descended eastward (present coordinates) into a Taconic trench. The roots of these faults can be traced northward across the Precambrian Adirondack Dome. However, to the south the pattern of faults has been poorly controlled (except for lineament studies), due to a lack of 3D seismic, tightly spaced well logs, and outcrop with exposed faults. Furthermore, many of the faults in the Mohawk Valley are poorly exposed, or not exposed at all. The result is that the motion history of these faults is generally poorly constrained. For many faults, we do not even know if they are actually normal faults (or reverse faults). The UB Rock Fracture Group and industrial associates have refined the understanding of these faults, based principally on extensive high-quality 3D seismic, as well as integration with field studies, well logs, and lineaments.

Many of the Taconic faults are reactivated Iapetan-opening faults. The timing of the major motion in the Taconic and the timing of motion cessation are extremely variable for different faults with similar strikes. In some areas, the faults that were active in Utica time primarily bound horsts (based on regional horizon levels), consistent with a compressional regime with a horizontal ~ EW-directed S_H related to final collision. Some of the NNE-striking faults exhibit releasing bends, from which we can infer slight dextral oblique slip at Trenton time and just after. The dextral motion on the NNE-striking faults is also indicative of an ~ EW-directed S_H, but the near-basal Utica age of this stress is earlier than would be suspected from present tectonic models. Could such stress imply that some of the Mohawk Valley “Taconic” faults may actually be oblique faults that sustained a high angle reverse slip component? In contrast to these faults, the Dolgeville Fault that strikes N (“neo-Taconic”) displays only down-dip motion indicators in outcrop. Post-Taconic reactivation characterizes the faults, and includes slip in the Salinic, Acadian, and possibly Alleghanian and the Atlantic opening. In fact, the Saratoga-MacGregor fault system was reactivated during the Mineral, VA seismic event in 2011.