FRACTURE DEVELOPMENT, FLUID HISTORY, AND PALEO-OVERBURDEN OF THE MIDDLE AND UPPER DEVONIAN SEQUENCE, EASTERN PENNSYLVANIA VALLEY AND RIDGE

Fracture orientation data was collected from the Middle and Upper Devonian units along a transect across the eastern part of the Pennsylvania salient. Fractures are typically planar, and may be bed restricted or cut across multiple beds. From the hinterland toward the foreland, bedding is relatively consistent and strikes range from 063° to 083°. Fracture orientations fall into two groups, one set is perpendicular to bedding and is commonly mineralized with one or more stages of blocky to fibrous quartz. Vein thickness ranges from 1 to 15 mm. The other dominant set is subparallel to bedding strike (047° to 073°) and is rarely mineralized with blocky quartz. The orientations of the fracture sets are consistent across fold-and-thrust, suggesting that the shortening direction must have been the same throughout the development of the duplex. The fold-and thrust belt structure is a duplex of Cambro-Ordovician carbonate horses that is overlain by a passive roof cover rock sequence of Ordovician through Mississippian rocks. The deformed length of the duplex is 95 km, with a retrodeformed length of the carbonate strut of 205 km.

Fluid inclusion microthermometry of vein minerals provides information on the fluid pressure, temperature, and composition history. This data can provide insight into paleo-overburden present during deformation. Veins were sampled along the entire transect. In all veins, the quartz has abundant CH₄±CO₂ inclusions with 0 – 4% CO₂, and less common aqueous inclusions. Homogenization temperatures (T_h) vary both within single quartz crystals, and across the fold-and-thrust belt. Veins from the southern part of the belt have CH₄±CO₂ inclusions with T_h values as low as -110°C, while aqueous inclusions have a T_h of 120°- 210°C. There are also two main groups of aqueous inclusions based on salinity, 13.8 to 16.5 wt. % NaCl equiv. and 8.1 to 8.4 wt. % NaCl equiv. Coeval aqueous and methane inclusions indicate trapping pressures of 170 to 180 MPa (6.5 – 7.0 km burial). On the northern margin of the transect, at the Appalachian Structural Front (ASF), only degraded hydrocarbon inclusions are present. Based on the retrodeformed section and the preliminary fluid inclusion microthermometry data, syn-Alleghenian sediments accumulated to 5 to 6 km toward the hinterland side, tapering to <1 km at the ASF.