DIGITAL MAPPING TECHNIQUES USED TO CORRELATE LEFT LATERAL SHEAR WITH THE EMPLACEMENT OF THE WALDOBORO PLUTON, MUSCONGUS BAY, MAINE

Digital mapping techniques and low elevation aerial imagery were used to develop an interactive data base to understand the interaction between the intrusion of the Late Devonian Waldoboro Pluton and the surrounding amphibolite facies gneisses in island exposures of East Muscongus Bay, Maine. The Harbor Island field area consists of isolated metamorphic wall rock parallel to the eastern contact of the Waldoboro pluton with layer-parallel to near-parallel boudinaged vertical granite dike intrusions (0.02-2.70 m wide) as well as larger (0.05-4.00 m wide) crosscutting oblique-to-layer folded granite intrusions. Geologic features mapped with RTK GPS provided control points for geo-referencing the outcrop photo mosaics. A detailed map of the outcrop was created by on-screen digitizing of geologic features in georeferenced camera-pole photo mosaics using editing tools in ArcMap. Structural measurements and photographs of kinematic indicators were positioned with handheld GPS. The digitized map allows viewing of any or all outcrop features at any scale in their correct position and orientation. Kinematic indicators associated with the early layer-parallel granites and metamorphic host rocks are indicative of ductile left lateral shear in the Harbor Island Fault Zone and include: asymmetric flanking folds along the contact of the intrusions, boudinage of elongated granite with asymmetric scar folds, isolated pods with stair stepping tails, and ‘S' folds in the layer-parallel granites and metamorphic layering. The larger oblique-to-layer crosscutting granites show evidence for layer-normal shortening (30-60%) and layer-parallel elongation (75-97%). Fanning of metamorphic foliation around the fold noses of the oblique-to-layer granites and contortion of foliation on the inside of the granite folds are also indicative of layer-normal shortening. Lack of shear evidence associated with the folded crosscutting granites is attributed to layer-normal shortening due to simple ballooning of the adjacent pluton and supports the premise that the emplacement of the Waldoboro pluton helped lockup strike-slip movement along the regional north-northeast trending Harbor Island fault zone.