SYNTECTONIC GRANITE INTRUSIONS AND THE NORUMBEGA FAULT SYSTEM, CASCO BAY, MAINE

Syntectonic granitic intrusions during late transpressive Norumbega strike-slip shearing in outcrops of Casco Bay, Maine were digitally mapped using total stations and GPS, and were analyzed using Arcview (GIS), and GeoPlot stereonet program. Merepoint, adjacent to the main Norumbega shear zone, consists of steeply-dipping and strongly lineated Cushing Formation. To the east farther from the main shear zone the Cape Elizabeth and Scarboro Formations are part of the S-plunging Hen Cove Anticline with variably-dipping folded layers at Wood Is. and steeply-dipping limb layers at Hermit Island. The granite intrusions within these rocks are coarse grained, often pegmatitic with related veins of quartz, tourmaline, andalusite, and garnet. At all locations strain partitioning during transpression resulted in initial intrusions orthogonal to the shear direction followed by CW rotation and elongation due to continued dextral strike-slip shearing. Kinematic indicators include orthogonal veins as foliation-boudin partings filled with quartz and andalusite that illustrate CW forward rotation during shear as well as small scale dextral shear zones, steeply-plunging Z-folds associated with tourmaline alteration haloes, high angle antithetic kink bands and flanking asymmetric folds adjacent to larger granite pods, all indicating dextral shear. Merepoint intrusions occur only as oblique strings of asymmetric boudin pods at small angles (less than 5°) to the shear direction. Wood Island intrusions are up to 30m in width at variably oblique angles, 6-70°, while Hermit Island intrusions are up to 4m in width, at generally smaller oblique angles, 8-29°. These oblique granite intrusions have experienced elongation parallel to their lengths as a result of shearing and reorientation, expressed as internal quartz vein partings and extreme necking and separation into boudin pods. Crumpled cross granites and chevron folding in local hinge zone beds suggest a significant layer-normal shortening component to the shearing deformation. The formation of initially orthogonal granites due to strain partitioning provides an effective extensional mechanism for magma emplacement during transpressive deformation.