VEIN EMPLACEMENT AND DEFORMATION IN A TRANSPRESSIONAL SETTING IN CAPE ELIZABETH, SOUTHERN MAINE

We studied the history of emplacement and subsequent deformation of quartz veins during dextral transpression in Two Lights State Park in Cape Elizabeth, southern Maine. The area consists of phyllites that are correlated with the Lower Paleozoic Kittery Formation of the Merrimack Group. Late Paleozoic deformation is expressed by NE-trending recumbent F₁ folds, NE-trending upright, gentle to open F₂ folds, and dextral transpression. Dextral transpression is associated with movement along the NE-trending Norumbega fault system and resulted in numerous Riedel shears, antithetic Riedel shears and boudinaged quartz veins in Two Lights State Park. Two sets of veins are present. One set of undeformed veins occurs at high angle with the shear zone boundary. Another set of stretched veins, with trends between 012° and 044°, is interpreted as being a result of stretching and rotation towards the 044° trending shear zone boundary. Previous work has suggested that the stretched veins intruded in the same orientation as the currently undeformed ones. Veins were emplaced orthogonally to the direction of shearing during periods of shear zone-parallel extension and rotated during intermittent periods of simple shear.

We propose that emplacement and deformation of the stretched veins may have occurred during continuous transpression. The highest stretch observed in veins that are sub-parallel to the shear zone boundary is 2.87, a much lower stretch than would be expected from simple shear deformation. We numerically modeled the stretches and orientations of the veins during progressive deformation and they are consistent with models of transpression.

It is unclear whether the stretched veins initially intruded in an orientation parallel to the set of currently undeformed veins. The currently undeformed veins are more yellow in color and thinner than the stretched veins, suggesting that the two sets of veins were emplaced under different geological conditions and perhaps in different orientations. Additionally, the stretched veins occur within ~2-8 meter wide shear zone-parallel zones, and do not cut across the entire outcrop as the undeformed veins do. This suggests that the stretched veins may have intruded at a low angle with the shear zone boundary, within the extension field of the deformation.