OROGEN PARALLEL DUCTILE EXTENSION ATTENDING TRANSPRESSION IN THE BRONSON HILL-CENTRAL MAINE BOUNDARY ZONE, MA-NH: RESULTS FROM BEDROCK MAPPING IN THE WINCHENDON 1:25000 QUADRANGLE

Jones et al. (1997) coined the term ‘lateral extrusion’ in which stretching occurred in the horizontal direction coincident with regional shortening of a transpressional regime causing the deformation zone to lengthen. This model is proposed for the Bronson Hill-Central Maine boundary. A 2-3 km-wide belt of deformation consists of highly strained Coys Hill, Hardwick and Fitzwilliam plutons interfolded with Rangeley schists and gneisses. High-strain features include increased flattening of a NNE-SSW striking foliation and grain-size reduction at the margins of the plutons, accompanied by a consistent SSW plunging Sil stretching lineation in Rangeley. Included within the zone is the “Templeton Tectonic Mélange” consisting of tightly and recumbently interfolded Hardwick and Rangeley gneisses and leucogranites, S-C/C’ fabrics, and 1-30 cm tectonic clasts of pegmatite and tonalite that result from NNE dextral simple shear. East of the high-strain zone, deformation is recorded by structures that illustrate strong flattening strains, accompanied by Sil and Qtz-rod stretching fabrics in the Rangeley and Paxton. Constrictional structures include strong foliations, tight isoclinal folding with hinge lines parallel to SSW-plunging Sil lineations, fold tightening and stacking, S-C/C’ fabrics and outcrop scale pinch-and-swell structures. Rangeley schists often enclose 1-10 meter scale boudinaged granite pegmatite lenses with foliated. Foliations, lineations and S-C/C' fabrics of Rangeley and Paxton metasediments are defined by greenschist facies assemblages (fabric-forming, aligned chlorite and fine-grained muscovite). Geometries of structures and deformation gradients suggest that subhorizontal shortening and lateral stretching were major components of deformation of the study area. Pure-shear dominated, bulk shortening is accommodated by continuous fold tightening and stacking of folds. Lateral extrusion as the result of pure-shear dominated constriction is recorded by structures that illustrate strong flattening strains, accompanied by stretching lineations and elongate pegmatite and calc-silicate pods parallel to the fold axes. This indicates significant extension parallel to the orogen and the formation of folds with axes parallel to the maximum stretch.