FRACTURE, VEIN AND FAULT INTERRELATIONSHIPS WITHIN THE UTICA SHALE, MOHAWK VALLEY, NEW YORK: IMPLICATIONS FOR TECTONIC MODELS

The Ordovician section in the Mohawk Valley of New York State contains complex fracture systems that formed in response to tectonic far field stresses and local fault influences. Eight fracture sets have been identified in the Mohawk Valley through the characterization of over 2000 fractures measured primarily within the Utica Shale. Sets strike N/S, NNE, NE, ENE, E/W, WNW, NW and NNW. Highlights evident from the compilation of fracture characteristics include 1) fracture frequencies vary directly with fault throw and inversely with lateral distance to nearby faults, 2) vein-filled fractures indicate that ENE and E/W trends sustained right lateral motion whereas WNW and NNW trends sustained left lateral motion, and 3) NNE sets predate NE, ENE, E/W, and WNW sets based on abutting and crosscutting relationships.

Mohawk Valley field data support tectonic models for the Taconic Orogeny that begin with the formation of N and NNE-striking fault systems within a syndepositional extensional basin. Extension facilitated the development of a dominant NNE-striking vein-filled fracture set in the Utica Shale. Extension was followed by a westward directed σ_Hmax resulting in younger ENE and WNW-striking conjugate shear fracture sets and E/W-striking mode 1 fracture sets. The west-directed σ_Hmax is consistent with earlier suggestions that final Taconic collision resulted in the formerly normal faults converting to high angle reverse faults. ENE right lateral vein-filled shear fractures are also consistent with Taconic escape tectonics related to the New York promontory. Rarely observed E/W and NNW vein-filled fracture sets with inferred conjugate shear indicate that a later NW-oriented σ_Hmax developed within the basin. This conjugate shear set may have resulted from the NW-directed collisional phase of the Alleghanian Orogeny.