STRUCTURAL GEOMETRY OF THE NITTANY ANTICLINE, CENTRAL PENNSYLVANIA

The Nittany Anticline is a highly deformed duplex system formed during the Late Paleozoic Alleghenian Orogeny, and lies on the outer arc of the Pennsylvania salient. The structure is the most forelandward in the Valley and Ridge province. When seen in map view the anticline is noticeably less curved relative to the rest of the Valley and Ridge province, although the northeast terminus and southwest terminus of the structure curve approximately 20° toward the east and south respectively. Using field data, published well data, basement depth data and bed thicknesses from the Centre County region, a series of eleven line-balanced cross sections were created, spanning roughly 100 km along strike. The sections are connected into a three-dimensional representation of the structure using 3D Move software to verify lateral continuity of stuctures. The duplex is characterized by 2.0 to 2.7 km thick horses of Cambro-Ordovician carbonate rocks overlying a décollement with a gentle southeast dip in the Cambrian clastic rocks.

Within the carbonate duplex, the primary detachment is within the Ordovician Reedsville Formation. The structure of the anticline is dominated by a fault-propagation style fold with an overturned frontal limb that terminates on either end of the structure into broad open plunging anticlines. The overturned portion sits above a structural duplex of a duplicate or triplicate of carbonate horses. Data from Mobil #1 Long well requires that the overturned portion of the anticline to be sitting above a ramp going through the Ordovician to Devonian sequence, as the Silurian Tuscarora Fm. was penetrated underneath the carbonate section. Based on surface contact data, it is necessary for the frontal limb of the overturned fold to be highly faulted, even though there are no faults mapped. These faults are most likely cryptic within the poorly exposed and folded Devonian clastic rock sequence. On average, the total structural shortening along the Nittany Anticline in approximately 50%. Previous fluid inclusion work suggests that there was little Permian syn-tectonic load over this structure, which may have facilitated the development of the duplex.