THE GRANITE BOUDINAGE STRAIN RECORD IN DEFORMED MIGMATITES IN THE NORUMBEGA SHEAR ZONE SYSTEM, SOUTHERN MAINE

Road cuts along both sides of a highway ramp in Yarmouth Maine provide 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. This study is part of a larger effort to record the complete tectonic history of this region (see Tomascak and Solar, NEGSA 2016). Rocks here are metatexite migmatites of medium- to fine-grained metapsammitic schist at the southeastern contact of the Migmatite-Granite Complex of southern Maine where about 10% of the outcrops are lens-shaped granite bodies are up to 10-15 cm thick and concordant to the shallowly SE-dipping migmatite/fabric structure. The outside road cut (east and south, the focus of this study) curves around the ramp for about 90 degrees, approximately 250m along its arc from due east to due south. This arcs across the SE-dipping structure providing 3D views of the structures and an opportunity for 3D minimum strain quantification.

The granite within the boudins have solid-state (subsolidus) fabrics indicating subsolidus strain is important to the boudinage strain history. Field data is confirmed at the micro-scale in thin sections of migmatites and boudins; plastic deformation is recorded at all scales with similar structural aspects. The granite boudins range from cm- to m-scale with generally E-W-trending long axes, and are strongly oblate (S>L). Where asymmetric and/or folded, boudins show consistent W-vergent kinematics, consistent with the W-vergent thrust fault in the outcrop. We systematically documented the lengths, widths and axial attitudes of granite boudins and ‘pinched-and swelled’ granite lenses. The outcrop surface attitude was recorded at each station in order to project field-measured dimensions along multiple orientations in order to find principal strain axes, and produce a 3D map of the structures. A detailed analysis of the boudinage led to a quantifiable amount of strain recorded during deformation. Results indicate apparent flattening strain with the maximum principal stretch axis shallowly E- to NE-plunging, and the apparently plane of flattening subconcordant to the migmatite structure.