NW-TRENDING STRUCTURES IN SOUTHERN COASTAL MAINE: REORIENTED BY STEEPLY-PLUNGING F3 FOLDS RELATED TO REGIONAL NORUMBEGA SHEAR

The Silurian-age Kittery Formation of southern coastal Maine at Kennebunkport, Timber Island and Biddeford Pool exhibits NW-trending structures that include upright F2 fold limbs, F2 fold axes and hinge-parallel stretching lineations. These NW-trending features are perpendicular to the more typical NE-SW-trending structures that dominate farther south from Ogunquit to Kittery and just to the north across Cape Elizabeth and into Casco Bay. These NW-trending structures, gently-plunging at Timber Island and Biddeford Pool, likely had initial NE-trending orientations but with significant reorientation developing late in their deformation history. Detailed mapping along Shore Road in Kennebunkport reveals a set of 10 m-scale steeply-plunging F3 folds responsible for bed/F2 fold limb and F2 fold hinge reorientation and a larger km-scale fold with a steeply-plunging fold nose exposed at Point Arundel. These steeply-plunging F3 folds developed just prior to intrusion of local outcrop-scale granites related to the nearby Carboniferous-age Biddeford Pluton as these crosscut all structures throughout the area. Geologic maps (Hussey et al. 2016) for Kennebunkport to Biddeford show distributed strike and dip measurements for bedding that suggest a pattern of large-scale steeply-plunging Z-type folding possibly preserved in the deformation pattern of the Kittery Formation in this area. Such larger-scale Z-folding would account for the reorientation of regional fabrics and ultimately be related to regional Norumbega right-lateral shearing. Crumpling of upright beds into large scale asymmetric folds is in contrast to the fabric-parallel right-lateral shearing that is prominent to the north across Cape Elizabeth as can be seen at Two Lights State Park and Portland Head Light and to the south at Bald Head, Ogunquit and within the Rye Formation at Gerrish Island, Kittery.