DIGITAL MAPPING OF THE BEDROCK GEOLOGY OF LANES ISLAND AND WOLFE'S NECK: EVIDENCE FOR REGIONAL OVERTHRUST DEXTRAL SHEARING IN THE NORUMBEGA FAULT SYSTEM, INNER CASCO BAY, MAINE

Lanes Island and Wolfe's Neck are located, in inner Casco Bay, Maine, on the NW side of Flying Point Fault, the main strand of the Norumbega Fault System, and are part of the Ordovician Falmouth-Brunswick Sequence as the Nehumkeag Pond Formation, a plagioclase-quartz-biotite gneiss. These metamorphic layers at ~N44E46SE are host to two orientations of probable late Devonian syntectonic granite dikes; NE-SW layer-parallel dikes and younger NW-SE cross-cutting dikes, ranging in thickness from <1-18m, averaging ~3m. Four NE-SW basalt dikes, <1m thick, were also found. Over 30 granitic dikes were mapped with both Total Stations and RTK GPS with data compiled in ArcMap. Images of complex granite intrusions were taken with a 6-14m camera pole to capture detailed geometry. The majority of the granitic dikes were pegmatitic; however, a few fine grained dikes were also seen. Evidence for layer parallel and dike parallel elongation of the early set of granite intrusions consists of cross-cutting quartz veins and boudinage, some distinctly asymmetric. Layer-parallel dikes also show strong E-plunging stretching lineation (23°N78E), mylonitic fabrics and asymmetric Z-folds that suggest oblique shear on SE-dipping layers in a dextral thrust geometry. Small scale, asymmetric Z-folds are also present in the metamorphic host rock layers and result in a prominent S-plunging fold axis lineation (46°S17W) that is approximately perpendicular to the E-plunging stretching lineation due to shear. Generally younger, cross dikes were emplaced perpendicular to the E-plunging shear/stretching lineation due to strain partitioning effects and show signs of subsequent deformation as folded dikes and cross dike foliation. The cross dikes also show effects of rotation due to shear as paired asymmetric flanking folds of metamorphic layering against the more rigid granite. Stereonet plot of poles to cross granites shows a reconstructed rotation axis at 30°S11E that is perpendicular to the shear/stretching lineation and matches the Z-fold axis in the host rock. Overall, the two sets of granite dikes, the nature of the subsequent deformation, and the orientation of the shear/stretching lineation in the granite and fold axis lineation in the host rock reveal an early overthrust dextral shearing history for the NW side of the Norumbega shear system.