ANALYZING COMPLEX FOLDING USING DIGITAL MAPPING TECHNIQUES ON SALTER AND SEGUIN ISLANDS, MID-COAST MAINE

East of the highly-sheared inner Casco Bay area, at the mouth of the Kennebec River, the Ordivician age Cape Elizabeth Formation is complexly folded. The Robinhood Cove syncline, on nearby Georgetown Island, resulted from upright F2 folding modified by regional transpression on the Norumbega Fault system. Seguin and Salter Islands, south of Georgetown, display outcrop-scale parasitic folding, including tight to isoclinal anticlines and synclines as well as asymmetric S- and Z-folds, parallel to the regional SW-plunging F2 fold structure. Digital survey equipment, including total stations, Trimble 5700 RTK GPS, Trimble GeoXT GPS and a digital camera pole for low elevation outcrop photography was used to collect structural data, map deformed granites and delineate fold patterns with data displayed and analyzed using ArcGIS. Steeply-dipping metamorphic layering on Seguin Island strikes 011° and fold axes have a mean plunge and trend of 40° to 193°. At the southern tip of the island, F2 fold axes are consistent, but diverse axial plane orientations with a strike variation of 171° and a dip variation of 61° are attributed to polyclinal folding within a plunging anticlinal hinge zone, represented by the splayed axial planes, formed by flexural flow during late stage deformation. North of Seguin, Salter Island outcrops show folded granite intrusions within steeply-dipping metamorphic layers as a late stage pure shear deformation, associated with tightening of F2 folds. Symmetric scar folding of metamorphic layers adjacent to boudin structures in some layer-parallel granite have steeply-plunging axes, showing a mean plunge and trend of 89° to 328°, due to subhorizontal elongation parallel to the F2 fold axes and L2 lineations. Uniform, steeply-dipping metamorphic layers with few parasitic folds suggest Salter Island occupies a limb section of the regional fold structure between the main Robinhood Cove syncline at Georgetown and the Seguin anticlinal hinge zone farther south. The orientation of these regional folds oblique to the Norumbega Fault trend suggests widespread strain accommodation within the mid-coast area associated with early Norumbega shearing where layer-normal flattening and F2 fold tightening dominated over layer-parallel shear, as seen on Salter and Seguin Islands.