SYNTECTONIC GRANITES AND TRANSPRESSIONAL DEFORMATION AT PEMAQUID POINT, MID-COAST MAINE

Precision digital mapping and data collection techniques were used to map outcrop structure at several sites along the Pemaquid Point shoreline of midcoast Maine. The regional bedrock consists of tightly folded Bucksport Formation and syn- to post-tectonic granites of the 367 my Waldoboro pluton. Strain partitioning during Late Devonian transpression in a broad area of regional strain accommodation related to the dextral Norumbega shear system allowed for the initial emplacement of granites perpendicular to the L2 hinge parallel stretching lineations within oblique F2 upright folds. Shearing parallel to the oblique fold limbs resulted in rotation, stretching and boudinage of these early orthogonal granites where the sense of local shearing provides constraints on modeling of regional Norumbega deformation. Asymmetric features in these exposures offer evidence for layer-parallel sinistral shear not dextral as seen in the Casco Bay area closer to the main fault. Counter-clockwise rotation during sinistral shear in the Pemaquid area generated kinematic indicators that include oblique boudin strings, rotated quartz veins and partings, and distortion of quartz fill at boudin necks. Steeply-plunging hook-shaped flanking folds demonstrate layer-parallel sinistral shear around larger boudins. The distribution of poles to metamorphic layers in stereonet helps to delineate the local SW-plunging F2 fold axes parallel to prominent stretching lineations in the rocks. Post-tectonic quartz veins, granite intrusions and quartz-filled partings during boudinage are perpendicular to these lineations. Earlier variably-deformed veins and intrusions have been rotated to oblique angles to the layering during sinistral shearing. The shear geometry as indicated by the distribution of poles for undeformed to deformed granites is constrained within the metamorphic limb layers with counter-clockwise rotation about a steeply plunging rotation axis. The sinistral antithetic shear observed at Pemaquid Point supports both the CW fold rotation with antithetic shearing model [Swanson, 1999] and a new lateral escape model for regional strain accommodation. The evidence presented here for sinistral antithetic shear in the Pemaquid area will prove pivotal in refining our understanding of Norumbega-related deformation.