DIGITAL MAPPING TECHNIQUES TO DELINEATE LEFT-LATERAL SHEAR AT THE EASTERN CONTACT OF THE WALDOBORO PLUTON, MUSCONGUS BAY, MAINE

The 367+4 my (Knight and Gaudette 1987) Late Devonian Waldoboro Pluton is a syntectonic granite associated with the Acadian Orogeny, which intruded the deformed granofels, gneiss, schist and quartzite of the Ord. Benner Hill and Sil-Ord. Bucksport Formations of mid-coast Maine. We investigated the sinistrally-sheared eastern contact of the pluton as first reported by Sidle and Barton (1991) and our survey focused on the shores of Black, Hall, Harbor and Friendship Islands in eastern Muscongus Bay. Precision digital mapping techniques that utilized RTK GPS, handheld GPS, electronic total stations, digital outcrop photography, high-resolution digital aerial photos and GIS, were used to collect and analyze information about the deformation history of the area through detailed outcrop surface mapping that accurately depicts the geometry of deformed intrusions, veins and metamorphic foliations. Kinematic indicators found throughout the granitoids and steeply-dipping metamorphic rocks suggest dominantly layer-parallel sinistral shear, minor dextral shear, and significant layer-normal shortening. Sinistral shear is seen in isolated asymmetric boudins of quartz and granite, s-shaped asymmetric folds within the metamorphic host rock, opposing hook-shaped flanking folds adjacent to granite intrusions, left lateral offsets of quartz and granite veins along layer-parallel shear zones, and along brittle pseudotachylyte fault surfaces. Minor z-folds within the metamorphic layers, minor dextral offset of quartz and granitic veins along some layer-parallel shear zones provide possible evidence for dextral shear. Significant layer-normal shortening is seen in the tight folding of initially orthogonal granite intrusions and quartz veins. The evidence leads to a complex interpretation for the tectonic history of the eastern contact of the Waldoboro Pluton and, in turn, for the history of Norumbega shearing in this area. A possible mid-coast extrusion model for regional strain accommodation during dextral Norumbega shearing best explains the observed field evidence. In this model, a mid-coast block, bounded by the right-lateral Phippsburg shear zone to the west and the left-lateral Pemaquid-Harbor Island shear zones to the east was extruded southward out of the Norumbega zone of transpression.