THE GRANITE BOUDINAGE STRAIN RECORD IN DEFORMED MIGMATITES IN THE NORUMBEGA SHEAR ZONE SYSTEM, SOUTHERN MAINE - PART 2: STRAIN QUANTIFICATION

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, at the southeastern contact of the Migmatite-Granite Complex of southern Maine (see Tomascak and Solar, 2016, NEGSA). About 10% of the outcrops are lens-shaped boudins of granite bodies, up to 10-15 cm thick and sub-concordant to the shallowly SE-dipping migmatite/fabric structure. Part 1 of this study was focused on the strain recorded at the outcrops scale, specifically in the metatexite migmatites of medium- to fine-grained metapsammite, and the main features of the boudins (Morales et al., 2017, GSA). We recorded the boudin structural record for Part 2 as found within the outside road cut (east and south) that curves around the ramp for about 90 degrees, 250m along an arc from due east to due south providing 3D views of the structures and an opportunity for 3D minimum strain quantification.

Boudins are cm- to m-length granitic rock bodies with internal solid-state fabrics along the flat aspect (sub-parallel the the migmatite structure), consistent with subsolidus strain after migmatite formation, and syn- to post-boudinage formation. Asymmetric and/or folded boudins show consistent W-vergent kinematics. Outcrop structural data is confirmed in thin sections of the host migmatites and the boudins with deformed feldspar and quartz aggregates and melanosome biotite aggregates. The boudinage long axes are a few cm- to a m-long with a general E-W trend, and part of what define strongly oblate S>L minimum strain ellipsoids. In the field, we systematically documented length, widths and axial attitudes of the boudins in several 2-D outcrop surfaces, at 20 stations. At each station the outcrop surface attitude was recorded in order to project the field-measured dimensions along multiple orientations in order to find the principal strain axes, and to produce 3D maps of the structures. The results are consistent throughout the outcrop with an apparent plane of flattening strain along the migmatite structure, with the maximum principal stretch axis shallowly E- to NE-plunging consistently.