BEDROCK MAPPING OF THE WINCHENDON (1:25,000) QUADRANGLE (MA-NH): EVIDENCE FOR DISCONTINOUS DEFORMATION ALONG THE BRONSON HILL-CENTRAL MAINE BOUNDARY ZONE

• Main Author: O'Brien, Timothy M.
• Format: Others
• Published: UKnowledge 2009
• Subjects: New England|Bronson Hill - Central Maine boundary|Retrograde Metamorphism|Structure|Tectonics; Geology
• Online Access: http://uknowledge.uky.edu/gradschool_theses/615; http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1619&context=gradschool_theses

Bedrock geology of the Winchendon quadrangle is divided into two zones based on structures and lithology. The eastern zone consists of tightly folded Silurian Rangeley and Paxton metasediments and pegmatites. Planar, NNE-SSW striking, W dipping foliations are characterized by alternating phyllosilicate-rich and leucosome layers. Fold axes of tight to isoclinal upright and recumbent folding are parallel to SSW-NNE shallowly plunging sillimanite, quartz and muscovite lineations. In the western zone Rangeley schists were intruded by Devonian Hardwick and Coys Hill and Fitzwilliam plutons. Planar NNE-SSW striking foliations dip shallowly to steeply west. Fold axes of tight-to-isoclinal asymmetric to recumbent folds plunge shallowly to steeply SSW, parallel to elongate quartz and feldspar lineations. Fabrics in both portions developed in by greenschist facies mineral assemblages Asymmetric porphyroclasts and S-C/C’ fabrics from both areas display dextral asymmetry. Lateral extrusion as the result of pure-shear dominated E-W shortening and N-S extension is recorded by structures that exhibit strong contractional strains, accompanied by stretching fabrics. A 2-3 kilometer wide belt of deformation named the Bronson Hill-Central Maine Boundary Zone (BHCMBZ) correlates with the Conant Brook Shear Zone. This creates a zone of deformation that widens to the north and separates Ordovician plutons of the Bronson Hill zone from units of the Central Maine zone. North of Winchendon, retrograde deformation is absent and Acadian metamorphism and structures are preserved. This creates an inconsistency in along-strike deformation the length of the inferred terrane boundary. A larger terrane, composed of the Bronson Hill and Central Maine zones is proposed to account for the discontinuous deformation along strike.