COMPARING PATTERNS OF LOW ANGLE SHEAR FRACTURING AND JOINTING IN THE APPALACHIAN PLATEAU AND VALLEY-AND-RIDGE OF PENNSYLVANIA AND NEW YORK

The Marcellus shale is a hydrocarbon bearing siliciclastic black shale that underlies much of the Northeastern United States. This formation, the oldest of the Devonian aged Hamilton group, was deposited in the foreland basin of the Acadian orogeny and then deformed during the Alleghanian orogeny. Today, the Marcellus outcrops in two distinct structural settings: the Appalachian Plateau and the Valley and Ridge province. Although, deformation occurred under different conditions in these provinces, fracturing is a pervasive and penetrative form of deformation in both.

Previous studies have focused on extension joints that are observed in the Marcellus and throughout the Appalachian basin, but little work has been done on shear fractures (wrench and wedge faults). Here, we present data from outcrop found in the Appalachian plateau of New York and on the northern limb of the Nittany Anticline, which marks the transition to the Valley and Ridge province in northern Pennsylvania. In both regions, low angle shear fractures are observed that indicate both N-S and E-W shortening.

In the Valley and Ridge, bedding is tilted into folds of multiple scales associated with south dipping thrust faults. Additionally, there is bed-parallel shearing evidenced by fibrous slickenlines on bedding surfaces. Low angle shear fractures observed in tilted Valley and Ridge beds show a variety of dips. Some fractures dip 15˚-35˚ while others show similar orientations only after bedding dips are rotated to horizontal, resulting in a “smearing” of dip angles. Thus, low angle faults preceded folding and continued to form during folding. In the gently dipping beds of the Appalachian plateau, there is little rotation of bedding and low angle shear fractures consistently dip 17˚-22˚. Additionally, Scanning Electron Microscopy shows slickenlines indicating dip slip shearing on these fractures. We compare the structures in the two areas in terms of their orientation and timing with respect to other structures in the area.