FRACTURE HISTORY OF THE WEIR MOUNTAIN SYNCLINE AND LEHIGTON ANTICLINE, EASTERN VALLEY AND RIDGE PROVINCE, PENNSYLVANIA

The Weir Mountain syncline and Lehigton anticline are located in eastern of Pennsylvania, and are the most hinterlandward structures in the Valley and Ridge province. Joint and vein orientations are best grouped upon rotation to bedding horizontal, and show four groups based on strike. A nearly orthogonal 346°±12°set and a 070°±10° set are likely related to a 340° directed shortening to form the folds which have an average bedding strike is 070°. A second nearly orthogonal 298°±10° set and a 025°±9° set may be related to an early pre-folding shortening event.

Many of the fractures are mineralized with quartz, and they were analyzed using vein petrography. Blocky quartz, fibrous quartz, and elongate blocky quartz grains were observed in various veins. There was no preference for quartz vein morphology based on structural location or vein set. An SEM equipped with Cathode Luminescence (CL) was used to further characterize the veins. Distinct grey tones were seen in the SEM-CL analysis showing the different trace element compositions of quartz. Individual crystals in blocky quartz veins showed zoned regions of a light grey luminescent early quartz interior, dark grey middle, and a late stage light grey rim of quartz. Elongate blocky quartz veins also have zoned crystals. Fibrous veins showed compositional differences in layers from their base at the host rock boundary to the center of veins. Most veins contain older zones of quartz with a light grey luminescence that was apparently brecciated and re-cemented with quartz with a dark grey luminescence.

Preliminary fluid inclusion analysis of two-phase aqueous inclusions in blocky quartz shows that both early dark grey luminescent quartz and late stage light grey luminescent quartz compositions differ in their Th and Tm values. In the Weir Mountain Syncline, the late stage quartz was observed with a higher burial depth that the early stage quartz showing that fluid compositions were changing during burial. The very early quartz in the brecciated zones contain inclusions are ragged in their morphology. These inclusions are interpreted as becoming decrepitated due to isothermal decompression resulting from a relatively fast uplift while remaining at a near constant temperature.