MICROSTRUCTURAL ANALYSIS OF THE MARTIC FAULT

The Martic fault and its associated damage zone in Lancaster County, Pennsylvania provide a valuable case study for understanding the behavior of thrust faults formed during the Taconic Orogeny. The Martic Fault is a 440-million-year-old major thrust fault (Valentino, 2004) that exposes Cambrian-aged rock formations that have been thrust over Ordovician age schist. This study examines the architecture of the fault zone, and the microstructures found within the fault zone. We also address the role of the fault zone in facilitating geologic fluid flow.

In this study, we mapped and sampled the fault zone and the rocks in the hanging wall and footwall along two transects, one perpendicular to the fault strike and one along the fault strike. Thin sections of samples were evaluated with a petrographic microscope and scanning electron microscope to determine the microstructures present within the rock and evaluate the deformation mechanisms within the fault zone. Petrographic analysis shows that samples taken further from the fault are characterized by low temperature deformation mechanisms like mechanical twinning and dissolution creep. Samples taken closer to the fault show evidence of higher stress deformation mechanisms like dislocation glide and dislocation climb, as quartz grains show evidence of both undulous extinction and subgrain formation. Samples close to the fault zone are also characterized by extensive veining and the development of multiple orientations of foliation. These results are consistent with previous work which shows that the Martic fault had multiple phases of deformation and metamorphism.