Northeastern Section - 40th Annual Meeting (March 14–16, 2005)

Paper No. 21
Presentation Time: 8:00 AM-12:00 PM


SWANSON, Mark T., Department of Geoscience, Univ of Southern Maine, Gorham, ME 04038,

The Harbor Island Fault Zone in Muscongus Bay, Maine is located within vertically-layered metamorphic inliers and adjacent granites within the eastern margin of the Late Devonian (367 my) Waldoboro Pluton. This fault zone has developed a 0.5 km wide array of N15E striking layer-parallel brittle strike-slip fault surfaces that produced pseudotachylyte fault veins and injection veins due to friction melting during paleoseismic slip events. The brittle faults and related outcrop surface features were mapped using survey-grade RTK GPS and electronic total stations with georeferenced photomosaics compiled in GIS. The brittle fault array consists of over 50 digitally-mapped pseudotachylyte fault veins typically only a few mm thick, some of which have been traced for up to 62 m in outcrop length. The slip surfaces have been tracked from island to island for a collective fault zone length of about 3 km. Local elongate pull-apart reservoirs can be up to 2 cm thick and 12 cm in length with a few decimeter-scale pockets of melt-matrix breccias. Vertical lateral injection veins are typically only a few cm long and up to 1.7 cm wide with one orthogonal injection vein 0.9 cm wide and 28 cm long. One unique horizontal injection vein is exposed that links adjacent layer-parallel slip surfaces with an estimated length of 1.0 m. Kinematic indicators include a meter-long contractional duplex structure and a single 20 cm long sidewall ripout that indicate sinistral strike-slip. Measured displacements from offset pre-existing quartz veins and granite intrusions are also sinistral strike-slip and range from 0.23-0.53 m. About 12 m of sinistral displacement is also accommodated on one of two 3-4 cm thick cataclasite zones. Most of the smaller-displacement layer-parallel brittle faults represent single slip events and are surprisingly simple in overall structure with few linking structures between adjacent layer-parallel slip surfaces. This suggests friction welding as an effective strain hardening mechanism for repeated stick-slip deformation. These late brittle sinistral faults combined with an earlier ductile sinistral shear history characterize the deformation at the eastern contact of the Waldoboro Pluton which in turn is related to the regional dextral transpressional deformation adjacent to the Norumbega fault zone.