DIGITAL MAPPING OF THE EAST HARPSWELL SYNTECTONIC GRANITE DIKE COMPLEX OF EASTERN CASCO BAY, MAINE: REGIONAL STRAIN ACCOMMODATION DURING NORUMBEGA SHEARING

The East Harpswell metamorphic group forms the Devonian Hen Cove Anticline, a NS-trending doubly-plunging F2 fold structure exposed in Eastern Casco Bay, SE of the Norumbega Fault Zone. Ragged and Yarmouth Islands lie in the hinge of the fold with Cundy Points and Rogue Island on the east limb, and Little Yarmouth on the west limb. Previously unmapped granite dikes of probable Late Devonian age, ranging from 4cm to 12m wide, representing ~16% of the outcrop surface, were digitally mapped with Total Station and RTK GPS with field data compiled in ArcMap. Structural measurements were positioned with handheld GPS. The syntectonic granite dike complex consists of two sets of intrusions that display subsequent deformation. The early intrusion set strikes NNE and lies parallel to the metamorphic host rock layering and regional stretching lineation. These granites have undergone extension parallel to their lengths and parallel to the host rock lineation as seen in cross cutting quartz veins and symmetric boudinage that accompanies symmetric fishmouth boudinage in the metamorphic layers. The younger set of intrusions strikes WNW and intruded perpendicular to the host rock layering and lineation. These granites show WNW-ESE shortening perpendicular to the metamorphic layering as evidenced by folding and foliation development in the granites. Digital camera-pole imagery allowed strain calculations for granite deformation. Elongation in granite boudinage was calculated by surface area reconstruction where total area and maximum width of mapped granite boudins were used to estimate initial length. Average elongation of layer parallel granite was 104% (78%-150%). Shortening was calculated using a line length reconstruction comparing initial length along the folded granites to final outcrop length. Average shortening of the folded granites was 49% (47%-52%). This pattern suggests a pure shear strain for post granite deformation involving layer parallel elongation and layer normal shortening representing a tightening of F2 folds. The tectonic interpretation for this part of eastern Casco Bay favors a no-simple-shear-strain block that is centered on the Hen Cove fold structure and bounded by layer parallel dextral shear zones along flanking steep limbs that developed during regional Norumbega shearing.