DIGITAL MAPPING OF THE SYN-TECTONIC DAMARISCOVE DIKE INTRUSION COMPLEX OF MID-COAST MAINE

Damariscove Island, Maine is comprised of the Cape Elizabeth Formation, a schist in the Falmouth-Brunswick sequence on the eastern limb of the S-plunging Boothbay anticline, a tight upright regional F2 fold structure. Limb layers and parasitic folds are cross-cut by late Devonian aphanitic and pegmatitic granitic dikes which show variable responses to deformation depending on orientation and age. Granite intrusion patterns were mapped with RTK-GPS (Trimble 5700) and Total Stations (SpectraPrecision 608) and imported into ArcGIS (v. 9.2). Orientation data was collected with Brunton compass and positioned with handheld GPS (Trimble GeoXT). Measured, generally-vertical, granite contacts fall into 3 major strike clusters: 9°, 33°, and 85°. Strikes at 9°, when compared with mapped patterns, correlate with large dike intrusions that occur along the regional fold axis trend through the island. These N-S trending dikes are 55 m at their largest width. Strikes at 33° represent a second, smaller dike intrusion phase ranging in width from 9 to 17 m of uncertain origin. Strikes at ~85º represent smaller, folded granite dikes (1-3m in width), which are found to be roughly perpendicular to the two previous intrusion phases. Contact relations between the larger NNE-striking granites and these near orthogonal, now folded granites suggest simultaneous intrusion into both the grain-parallel fractures and the orthogonal-to-grain fractures. Continuing grain-parallel elongation is seen in the horizontal extension as expressed in WNW-ESE striking vertical quartz veins perpendicular to that grain and boudinage of the NNE-striking granite dikes. Accompanying WNW-ESE compression and shortening is reflected in the folding of the initially orthogonal-to-grain dike intrusions and the development of mica foliation within the larger NNE-striking granites. Calculations using boudin area and granite widths determined from georeferenced camera pole photos and on-screen digitizing reveal ~115% elongation. Folded dikes, digitized from georeferenced camera-pole imagery, reveal a 40-70% shortening. This provides further evidence that granitic dike patterns on Damariscove Island and other mid-coast area exposures reflect syntectonic intrusion processes, modified by continuous ductile deformation with concurrent shortening and elongation.