THREE-DIMENSIONAL STRUCTURE OF DEFORMED MIGMATITES AND GRANITES IN THE NORUMBEGA SHEAR ZONE SYSTEM, SOUTHERN MAINE

New roadcuts along a ramp for I-95 north in Yarmouth Maine provides a 3-D opportunity to examine structures in a single large exposure within the transpressive Norumbega Shear Zone system sensu lato, on its western margin. The inside roadcut (north) is a semicircular arc, extending 150m in an arc from the roadcut along I-95 at due north and around 180 degrees along the off-ramp clockwise ending at the due south position near the end of the ramp. The outside roadcut (east and south) curves around the on-ramp for about 90 degrees, extending approximately 250m along its arc from due east to due south. Rocks here are at the southeastern contact of the Migmatite-Granite Complex of southern Maine (see Tomascak and Solar, NEGSA 2016). With the goal of documenting structures, what defines them, and how they relate to both the Norumbega zone and the Migmatite-Granite Complex, we performed detailed mapping and thin section petrography of rock types and structures, and the contacts between rock types.

Metapsammitic rocks dominate all exposures, and are all medium- to fine-grained migmatite with mm-scale leucosomes with biotite melanosome borders. The foliation and migmatite structure are sub-parallel, and shallowly- to moderately-SE-dipping consistently throughout the exposures. The subordinate granite bodies are foliated and cm- to m-scale boudinage with E-W-trending long axes, flat and concordant to the main structure. Boudinage is locally folded asymmetrically with the migmatite structures with W-vergence in the footwall block of a W-vergent thrust. Two larger granitic dikes are irregularly-shaped and steeply E-dipping. The larger (2 m wide) is continuous with a 10-30 cm-thick sill at the top of the outside exposure. Thin sections of migmatites and boudins reveal that deformation is recorded at the microscale with similar structural aspects to those found in outcrop.

We interpret these structures to have formed during local contraction in an apparent restraining bend of the transpressive Norumbega zone. The migmatite and boudin granites are deformed in the solid-state indicating the migmatite formation and granite intrusions are much older than the latest deformation, and the cross-cutting granites post-date migmatite formation, consistent with rocks of the Migmatite-Granite Complex.