Bedrock Geologic Map of the West Half of the Petersham 7.5 Minute Quadrangle, Central Massachusetts

The area of the Petersham quadrangle is underlain by the Bronson Hill-Central Maine terrane boundary, which has been shown by prior UK EDMAP student mapping projects to be a zone of Early Carboniferous, mid-crustal dextral transpression and lateral extrusion. Previous workers interpreted that zone, which is the contact between the Monson orthogneiss and Rangeley metapelitic gneiss, as the Brennan Hill thrust, an originally west directed (east dipping) ductile thrust that has since been rotated by back folding (now west dipping). The Monson-Rangeley contact and BH-CM zone and associated rocks underlie the west half of the quadrangle. Bedrock mapping and structural analysis at a range of scales in the context of detailed bedrock mapping in the western Petersham quadrangle will be the basis for testing the hypothesis that the BH-CM boundary zone is a zone of transpression with a shallowly plunging lateral displacement and not a thrust. Monazite U-Pb geochronology, guided by X-ray mapping to identify monazite growth zones, will permit testing if the timing of deformation is contemporaneous with deformation along strike to the south. The primary deliverable will be a bedrock geologic map at 1:24000 scale and cross sections of the west half of the map area. The EDMAP grant will support Lucas Rohrer, a M.S. degree candidate at UK.