STRAIN ANALYSIS OF DIGITALLY MAPPED SYNTECTONIC GRANITE INTRUSIONS ON SALTER ISLAND, MID-COAST MAINE

Syntectonic granites intruded into steeply-dipping layered metamorphic host rocks on Salter Island, mid-coast Maine have undergone significant layer-normal shortening and layer-parallel elongation subsequent to emplacement ~375-385 Ma (Hussey, 1988). Island host rocks consist of tightly-folded schist and amphibolite of the Cape Elizabeth Formation with a consistent upright limb orientation of 009°, 88°. Symmetric boudinage in these limb lithologies shows that strain experienced on Salter Island was dominated by pure shear with layer-normal flattening and layer-parallel extension. Horizontal extension parallel to F2 fold limbs and L2 hinge parallel lineations is seen in layer-normal veining by quartz veins and pegmatitic granite intrusions as well as boudinage of some layer-parallel granite producing steeply-plunging necklines and symmetric scar folding in the adjoining layers. This late stage horizontal extension direction plunges 2° to 192° as determined from the mean pole to cross-cutting quartz veins. Shortening normal to this extension direction is seen in the tight folding of granites and quartz veins intruded across the metamorphic layers. The axes of these folds have two dominant orientations that plunge 32° to 188° and 61° to 005°. The gently-plunging granite fold axes are parallel to the metamorphic fold axes, and may represent earlier intrusions in non-vertical orientations. The steeply-plunging granite fold axes reflect the deformation of late vertical intrusions emplaced during a period of horizontal layer-parallel extension and accompanying layer-normal shortening. Minimum strain estimates for these deformed granites used surface area reconstruction of granite boudin pods to estimate layer-parallel elongation, and line length reconstruction of folded granites to estimate layer-normal shortening. Estimates for minimum layer-parallel extension on Salter Island averaged ~170% (60-300%) while estimates for minimum layer-normal shortening averaged ~64% (30-89%). This suggests that these syntectonic granites exploited cross fractures perpendicular to a regional horizontal extension direction oblique to the trace of the Norumbega Fault Zone and were deformed by fold flattening during regional strain accommodation related to continued Norumbega shearing.