Mapping dikes and faults on the Schoodic Peninsula, Acadia National Park

Swanson, Mark

From the abstract: "Digital mapping of dikes and faults that cut the Devonian Gouldsboro granite on the Schoodic Peninsula, Acadia National Park was conducted by undergraduate student survey teams from the University of Southern Maine, 2013-2018. Their work provides a unique detailed perspective of the long famous dike intrusions as well as the new Schoodic Point Fault Zone. RTK GPS and total stations were used to delineate the geometry of fracturing and faulting associated with dike intrusion and the early stage fault zone development preserved within these exposures. Mapping has now covered ~150 separate dike intrusions, from less than 1 cm to over 10 meters in width, from Pond Island on the west, across Big Moose Island and Schoodic Point, and Little Moose Island to the Blueberry Hill Pullout on the east. Mapped dikes include early pluton-related felsic aplite and rhyolite porphyry dikes as well as the more common mafic basalt and diabase dikes. The distinction between older pluton-related and younger rift-related mafic dikes is still not clear at this time. Dike orientations are dominantly NE-SW with additional NNE-SSW, and generally younger NW-SE and NNW-SSE oriented dike sets. Lengthening the mapped dikes in their strike directions gives a more realistic view of how these myriad of dikes have sliced across the Schoodic Peninsula. In addition, over 900 short small-scale fault segments have been surveyed in a NW-SE zone over 400 m in length across the western side of Schoodic Point. Faulting was initiated prior to nearly all of the dike intrusions. Faults are marked by patches of coarse breccias, thin zones of cataclasite, sidewall ripout geometries and rare horizontal striations. Most offsets of the earliest aplite dikes and other earlier faults are in the cm to 10s of cm range with possible meter scale displacements for larger zones where smaller fault segments have merged together. Projected connections between mapped fault sections reveal several left-stepping en echelon right-lateral strike-slip segments that are frozen at an early stage in the process of linkage to a through-going fault zone. Lengthening of fault segments that would have occurred with further displacement suggests what a mature through-going fault geometry would look like for a more developed Schoodic Point Fault Zone."