STRUCTURAL ANALYSIS OF THE PHYLLONITE AT CHURCH ROAD; BERWICK, ME

The phyllonite at Church Road (Scr; formerly the Gonic Fm) in the Somersworth ME 7.5 minute quad is a strongly sheared tectonostratigraphic unit with previously unknown shear sense (Hussey et. all 2010). This research seeks to characterize the deformation and determine the shear sense in the Scr through lithological mapping, and structural analysis at the outcrop and microstructural scales.

The Shapleigh Group and Berwick Fm border the Scr to the NW and SE, respectively. Shapleigh group rocks are graded metawacke and metapelites. Outcrop-scale folding of S_n form upright S-folds that plunge NE. Locally an axial planar cleavage, S_(n+1), develops oblique to S_n in hinge areas. Exposures of Berwick Fm are limited, but where present it crops out as biotite granofels with local thin pelitic tops and common calc-silicate layers. Bedding is near-vertical along contact with the Scr and outcrop-scale folding is not recognized.

The Scr is a Ms-Qtz-Bt-Grt-St +/- Chl schist, with a strong shear fabric and multiple metamorphic growths. The dominant fabric is a near-vertical NE to ENE-striking phyllonitic schistocity. Local S-folds plunge moderately NE, parallel to S-folds in adjacent units. Where strain is more intense a late cren-cleavage developed orthogonal to the shear fabric with cren-hinge lines plunging NE parallel to S-fold hinge lines.

Thin sections cut parallel to the cren-hinge lines show the NE-striking shear fabric overprinting an earlier fabric, allowing the interpretation for the shear fabric as equivalent to S_(n+1). Grt and St overgrow the shear fabric and locally inclusion trails preserve S-folds, yet are late syn-tectonic to crenulation development. Qtz grains are fully recovered with sharp extinction and grain boundaries forming triple junctions. Lack of rotated porphyroblast or asymmetric feldspar clasts, and degree of recovery in Qtz makes shear-sense interpretation difficult in the most strongly strained layers; however, the abundant NE-plunging S-folds support simple shear with s-1 perpendicular to their hinge lines. The slightly younger and symmetric crenulation reflects foliation-parallel shortening with s-1 oriented perpendicular to it, which is nearly parallel to that interpreted for the folds. Therefore, we interpret these fabrics to record the change from simple to pure shear in a single left-lateral, east-side up event.