DIGITAL STRAIN ANALYSIS OF SYNTECTONIC VEINS AND INTRUSIONS, EASTERN CONTACT OF THE WALDOBORO PLUTON, MUSCONGUS BAY, MAINE

The eastern contact of the L. Dev. Waldoboro Pluton in Muscongus Bay, Maine, exemplarizes syntectonic granite intrusion in a transpressional environment associated with regional oblique fold development due to shearing adjacent to the Norumbega fault zone. The field area was studied using Trimble RTK GPS and total stations, low-altitude digital photography and GIS to document the geometry of syntectonic intrusions on this eastern contact. The resulting set of digital outcrop surface maps and photos formed a database for strain analysis using tool functions in ArcMap. Vertically-layered metamorphics with a N12°E strike along this contact zone show layer-parallel horizontal elongation (NNE-SSW) that formed cross-cutting granite and quartz-filled fractures (WNW-ESE). Accompanying layer-normal shortening buckled the initially planar intrusions, forming tightly folded structures with vertical fold axes. Extension and boudinage of the rotated limbs of these deformed intrusions was associated with steeply-plunging scar folds in the adjacent rock and the development of quartz-filled partitions. Analysis of crumpled veins and intrusions yielded a mean stretch value of 0.45 ± 0.05, with a greatest shortening value of 0.24. This layer-normal shortening and accompanying layer-parallel elongation also rotated randomly-oriented feldspar phenocrysts in coarsely- porphyritic granite toward the regional extension direction. Analysis of phenocryst orientations using the R_f/Φ method determined the strain ellipse’s axial ratio to be 1.45 reflecting the late stage regional flattening of these granites. Asymmetric hook-shaped flanking folds formed in the less-competent metamorphic rocks adjacent to some reoriented granite intrusions as well as asymmetric boudin pods also suggest CCW rotation and significant layer-parallel sinistral shear. The overall strain can be best understood as a combination of both layer-normal shortening (with layer-parallel elongation) and sinistral strike-slip shearing. These strain estimates are similar to values determined for nearby Pemaquid Point, both of which, are lower (and of opposite shear sense) than values for Casco Bay in closer proximity to the fault and attest to the importance of layer-normal shortening in the regional Norumbega deformation.