Northeastern Section - 50th Annual Meeting (23–25 March 2015)

Paper No. 3
Presentation Time: 8:00 AM-12:00 PM


SWANSON, Mark T., Geosciences, University of Southern Maine, Gorham, ME 04038,

Precision digital surveying of brittle structures cutting Devonian Gouldsboro granite at Schoodic Point, Acadia National Park has focused on the long-famous mafic to felsic dike intrusions as well as a newly-discovered zone of brittle strike-slip faulting referred to as the Schoodic Point Fault Zone. Survey-grade RTK GPS rovers and electronic total stations were used to delineate the exact geometry of fracturing and faulting associated with the extensional dike intrusion and early stage fault zone development preserved within these exposures. Mapping has covered ~80 separate dike intrusions from cm to several meters in width and includes early pluton-related aplite and rhyolite porphyry dikes as well as the more common rift-related aphyric and porphyritic basalt and diabase dikes. Pluton-related mafic dikes have been reported but not recognized at this time. Orientations for both the pluton- and rift-related dikes are dominantly NE-SW indicating that the regional extension direction remained fairly consistent from Late Paleozoic to Early Mesozoic time. However, significant variation in dike orientation exists with additional NNE-SSW, and generally younger NW-SE and NNW-SSE oriented dike sets. Nearly 500 individual fault segments have been surveyed including both right- and left-lateral strike slip faults in an overall NW-SE zone of right-lateral strike-slip fault development that was initiated prior to nearly all of the dike intrusions. The faults display zones of spectacular breccias with coarse angular fragments in a finer grained darker matrix that formed during early stage fault zone development within the granite. Minor horizontal striations and offset of at least one aplite dike and other earlier faults show strike-slip displacements in the cm to 10’s of cm range with possible meter scale displacements for the largest zones where smaller fault segments have merged together. This zone of faulting displays a Riedel shear structure with dominantly right-lateral R-shears at low angles to the zone and minor left-lateral conjugate R’-shears at high angles to the overall zone. Projected connections between mapped fault sections reveal at least four 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.