DIGITAL STRAIN ANALYSIS OF SYNTECTONIC GRANITE INTRUSIONS IN THE NORUMBEGA SHEAR ZONE, INNER CASCO BAY, MAINE

Schists and gneisses of the Cushing Formation in the core of the Merepoint Antiform are the host rock for syntectonic granite intrusions in the inner Casco Bay region within a 3.5 km wide zone of ductile shearing associated with the Flying Point section of the Norumbega Fault Zone. Trimble 5700 RTK GPS, hand held GeoXT GPS and an 8 meter digital camera pole assembly were used in mapping these deformed granits. The RTK GPS yielded high precision (+/- 1 cm) measurements used to create polygons and lines that delineate the geometry of the deformed granites as well as points for geo-referencing the digital camera pole imagery. The GeoXT GPS (+/- 50 cm), in conjunction with a Brunton compass, were used to gather orientation data for foliations, lineations and fold axes along with recording the location of hand-held outcrop photos. Orientations of structural features were plotted in stereonet diagrams and show steeply-dipping metamorphic layers with an average orientation of 046^o, 57^o and strong SW-plunging lineations at 22^o to 212^o. Maps and photos of asymmetric kinematic indicators (Z-folds, boudinage and pseudo-S folds) were analyzed in ArcGIS to estimate strain involved in the regional shearing. Beta angles, the angle between the CW-rotated boudin strings and the foliation (assuming initial emplacement orthogonal to metamorphic layering), resulted in minimum gamma values (cot (b), assuming simple shear) that ranged from 3-52 and averaged ~18. Calculated values differ due to variations in host rock competency, thickness and the timing of intrusion. Surface area reconstruction in boudin strings was used to estimate elongation (assuming initial intrusion geometry and a simple shear model). Elongation values ranged from 24-879% and averaged ~347%. The layer-normal shortening component calculated by line length reconstruction and surface area reconstruction ranged from 44-84% and averaged ~72%. This layer-normal shortening contributes to reorientation in the simple shear model and increases the apparent gamma values measured. Gamma values, without the layer-normal component, range from 1 to 10 with an average of ~ 4.5. Distributing the average minimum gamma values across the 3.5 km wide host rock suggests a minimum of ~16-63 km of right-lateral displacement for this inner Casco Bay shear zone.