FRACTURE AND FLUID HISTORY OF THE NORTHERN SECTION OF THE NORTH MOUNTAIN THRUST SHEET, SOUTHERN PENNSYLVANIA AND MARYLAND

The northern portion of the North Mountain thrust sheet sits along the transition between the eastern Valley and Ridge, and the Great Valley of southern Pennsylvania and Maryland. In this area, the North Mountain thrust sheet terminates into a highly fractured and faulted anticline. Mineralized fractures and faults cemented with calcite were sampled from the Cambrian Conococheague Fm. through the Ordovician Martinsburg Fm. Stable isotopic values for δ¹⁸O and δ¹³C from calcite and host rock samples were analyzed to constrain the history of how the fractures and fluids behaved within the rock layers before and after large scale deformational events.

Host rock samples have isotopic values ranging from: -9.5 to -5.8 ‰ δ¹⁸O and -1.82 to 2.11 ‰ δ¹³C. Vein calcite samples have isotopic values ranging from: -12.1 to -5.8 ‰ δ¹⁸O and -2.36 to 1.87 ‰ δ¹³C. The values for the vein samples are nearly identical to the values for the host rocks except for two sites which have larger differences (> 1.0 ‰) in their isotopic values. The two sites with larger differences in isotopic values are locations with macro scale faults (10s meters displacement) and indicates that macroscale faults exhibit a more open fluid system behavior, and sites with mesoscale or no faults at all exhibit a closed fluid system behavior. The majority of fractures are perpendicular to bedding strike, are parallel to σ₁, and are pre-folding in origin.

Preliminary fluid inclusion microthermometry of two-phase aqueous inclusions from near fault zones in the Ordovician St. Paul Group gives Th values of 148°C to 156°C with salinities of ~1.5 wt. % NaCl equiv. The stratigraphically lower Cambrian Conococheague Fm. near a fault zone has two populations of inclusions with Th values of 92°C and 145°C respectively, and salinities of 22.1 wt. % NaCl equiv. and 11.0 wt. % NaCl equiv. respectively. The extremely low salinity values for the St. Paul Group indicates possible influx of either meteoric or metamorphic – derived fluids. This could be due in part to the fractures opening parallel to the fold axis and faulting to allow for the fluid influx. The Th values indicate trapping depths of about 5.1 to 6.9 km (20° - 25°/km geothermal gradient). Using published stratigraphic thickness values this indicates that there was little to no post – Mississippian rocks present during deformation.