THE POST-EMPLACEMENT DEFORMATIONAL HISTORY OF GRANITE INTRUSIONS: THE QUARRIES OF DAMARISCOVE ISLAND, MAINE

Early European settlers on Damariscove Island, Maine quarried the island's granites throughout the 19th century. Analyzing the post-emplacement deformation of these granites can lead to a better understanding of how effectively geologic elements were employed in their quarrying operations. The key structural elements within fine-grained granites at Flat Ledges, one of six quarrying sites on Damariscove, were mineral foliations, brittle shear fractures with horsetail terminations, orthogonal joint sets (all of which are steeply-dipping) and horizontal sheeting joints. The granites were intruded during the later stages of deformation associated with the Acadian orogeny and can be correlated with the L. Devonian Waldoboro pluton. Deformation of the larger of these syntectonic granites by E-W compression led to the development of a weak mineral foliation oriented at about N38E. Preliminary E-W tensile stresses upon these cooled larger granites led to the intrusion of three narrow (<15 cm) pegmatite dikes striking N9E on average. Localized dextral shear possibly caused slippage along pre-existing foliation planes, forming prominent NE-SW striking (N35E) dextral shear fractures featuring asymmetric horsetail terminations. Subsequent extensions in both E-W and N-S directions caused thin, vertical, rusty, orthogonal fractures oriented approximately N1E and N84W. Horizontal sheeting joints developed as the granites were uplifted and eroded, completing the structural history. Surface ledge quarrying of granite on Damariscove is believed to have begun in the early 1800s and persisted for nearly a century. Quarried stone was used in many structures on the island and water-front access to the quarry sites allowed some granite to be exported to the mainland. Quarrying procedures included two variations on the plug and feather technique: the round hole and flat wedge methods, both of which can be seen in the remaining blocks and edges at each of the quarrying sites. The quarrymen on Damariscove opportunistically exploited each structural element of the granites as quarrying proceeded. However, the most valuable elements were undoubtedly the horizontal sheeting joints as they yielded granite blocks of a consistent thickness.