STRAIN ANALYSIS OF SYNTECTONIC GRANITE INTRUSIONS DEFORMED BY NORUMBEGA SHEARING, CASCO BAY, MAINE

Casco Bay, Maine is composed of steeply-dipping and strongly-lineated metamorphic rocks and deformed granite intrusions resulting from transpression associated with shearing on the Norumbega Fault Zone. Strain partitioning during dextral transpression resulted in the emplacement of initially orthogonal granite intrusions with respect to the host rock flow layering and lineation and continued shearing resulted in CW rotation and elongation into strings of granite boudin pods oblique to the host rock flow layering. The study sites were digitally mapped using Total Stations and GPS, and analyzed in ArcView GIS. Using geometric analysis of boudin strings and foliation, surface area reconstruction of boudin pods, and a line length technique for folded intrusions, the gamma shear strain, elongation, and shortening values were determined to estimate the pure and simple shear components to the deformation affecting three sites across Casco Bay. The strain values represent minimum estimates only reflecting strain since the late syntectonic emplacement of the granite. Due to its proximity to the main Norumbega shear zone within inner Casco Bay, Merepoint shows higher strain values than Hermit Island and Wood Island farther to the east in outer Casco Bay. Merepoint produced gamma values of 11.6 and 12.6 (assuming simple shear) with elongation of 316-610%. Wood and Hermit Islands show gamma values ranging from 1.3-10.7 for simple shear and elongation values from 45-220%. These values appear to be independent of the bedrock structure, with variably dipping folded layers at Wood and steeply dipping limb layers at Hermit along the eastern flank of the Hen Cove anticline and an interpreted "Phippsburg Shear Zone". All sites also display a significant pure shear layer-normal shortening component to the deformation as seen in crumpled orthogonal granite intrusions with shortening values from 25 to 66 % that reduces the apparent simple shear values calculated from the boudin string-foliation geometry.