KINEMATICS OF THE PHIPPSBURG SHEAR ZONE AT HERMIT ISLAND AND THE WOOD ISLANDS, SMALL POINT, MAINE

The Phippsburg Shear Zone is located within the Casco Bay restraining bend associated with the Norumbega fault zone in south central Maine. Mapping of the shear zone was conducted at Hermit Island during the summer of 2011 as part of an EDMAP grant. The shear zone deforms schists of the Ordovician Cape Elizabeth Formation and granites and pegmatites of Devonian age. The shear zone is part of a D₄ deformational event that proceeded regional folding associated with D₃ Acadian deformation. The shear zone has a foliation that strikes northeast-southwest and dips steeply SE along the western shore of the Phippsburg peninsula. There is a strong lineation defined by fold hinge lines and quartz rods that plunges gently south. The shear zone width extends at least two kilometers to the west of Small Point. Swanson (1999, unpub.) has documented dextral shear both regionally and locally by evaluating macroscale kinematic indicators within Casco Bay and Hermit Island. In this study, γ-shear strain within the Hermit Island section of the Phippsburg Shear Zone was calculated from the synthetic rotation of granites that are assumed to have intruded orthogonally. Minimum γ-shear strain values were calculated from 15 rotated and variably boudined granites on Hermit Island and 27 of the same on the Wood Islands. These data suggest a decrease in shear strain from south (γ = 5.14) to north (γ = 1.47) and west (γ = 9.51) to east (γ = 1.43). A NE-SW striking line separates areas of high from low shear strains and is demarked by a regional dip change from east dips in the shear zone to west dips outside of it. 2D Strain ellipses determined from elongation of boudin strings and shortening of fold trains around Hermit Island will be used to complement the shear strain calculations and further define the eastern boundary of the shear zone. The east dipping foliation, south plunging lineation, and dextral kinematic indicators suggest the Hermit Island shear zone is a dilational, type I shear zone (Fossen, 2010). The eastern shear zone boundary strikes more northerly than the shear zone as defined by Swanson (unpub.). The kinematics of Phippsburg Shear Zone at Hermit Island support the strike-slip fault bounded crustal extrusion model for the Casco Bay restraining bend as proposed by Swanson and Bampton (2009).