PALEO-FLUID HISTORY OF THE NITTANY ANTICLINE, VALLEY AND RIDGE PROVINCE OF PENNSYLVANIA

The Nittany anticline is the most forelandward fold in the outer arc of the Pennsylvania Salient. It is cored by a Cambro-Ordovician carbonate horse with a leading-edge fault-propagation-style fold and highly-faulted, steep to overturned frontal limb. Fluid inclusion microthermometry and stable isotope analysis of mineral-filled fractures (veins) from the Ordovician limestones provides insight on the paleo-fluid system and associated fluid-fracture interconnectivity during deformation. The veins primarily strike northwest and consist of one or more stages of blocky calcite, dolomite, and rare quartz. Oxygen and carbon stable isotopes were analyzed for 31 host rock and 70 vein samples. Fluid inclusions were analyzed for 10 veins. The samples are separated into three areas based on structural position; the southern fold plunge-out, the western overturned limb, and the Linden culmination.

Fluid inclusion microthermometry provides insight into trapping conditions, including pressure, temperature and fluid chemistry. Most inclusions are two-phase aqueous inclusions that have ice-melting temperatures of -24.2 to -9.5 °C (13.4 to 25.2 wt. % NaCl equiv. salinity). Eutectic values range from -47 to -39 °C and indicate CaCl₂ - NaCl basinal brines. Homogenization values for the southern fold plunge-out range from 123 to 145 °C and reflect a trapping depth of 4.2 – 6.4 km at lithostatic conditions. In the western overturned limb, homogenization values have a very wide range (80 to 167 °C) and reflect multiple trapping events. Maximum trapping depth is 7.3 km. Based on the current preserved stratigraphic section through the Mississippian, little to no Pennsylvanian - Permian synorogenic sediment was deposited over the Nittany anticline.

Vein isotopic and fluid inclusion data indicate that there was moderate fluid connectivity during the formation of the Nittany Anticline. Samples from the southern plunge-out have δ¹⁸O vein isotope values that are 2-3 ‰ lower than the host rock, while δ¹³C values show little to no difference from the host. The veins in the western overturned limb have δ¹⁸O values ~1 ‰ greater than host rock and δ¹³C values 1-1.5 ‰ lower. Two samples from the Linden culmination, which have veins similar to each other (approx. δ¹⁸O 1.5 ‰ lower, δ¹³C 3.0 ‰ lower than the host rock).