GEOLOGIC BEDROCK MAPPING AND STRUCTURAL ANALYSIS OF THE WESTERN HALF OF THE PETERSHAM 7.5-MINUTE QUADRANGLE, CENTRAL MASSACHUSETTS: EVIDENCE FOR TRANSPRESSION ALONG THE BRONSON HILL-CENTRAL MAINE BOUNDARY

Bedrock geologic mapping and structural analysis reveal the distribution of lithologic units and the orientation of tectonic fabrics and structures in the western half of the Petersham 7.5’ quadrangle, western Mass. The area is underlain by three main lithologic units (W to E: Ordovician Monson granitic orthogneiss, Silurian Rangeley migmatitic paragneiss, and Devonian Hardwick tonalitic orthogneiss). Their tightly folded contact zones strike north to south through the entire quadrangle. An undated (Devonian?), generally unfoliated qtz diorite (10-100 m wide) intrusion occurs within the Rangeley along strike through the entire map area. The dominant fabric present in both orthogneisses is a shallow to moderately west-dipping foliation defined by quartzofeldspathic streaks, Bt and/or Hbl mineral grain orientation, and planar amphibolite layers in the Monson. In the strongly migmatitic Sil-Grt-Bt-Pl-Kfs Rangeley paragneiss, a subvertical foliation and a subhorizontal lineation (both striking north to south) are present. The foliation is defined by parallelism of leucosomes and mica grain orientation; the lineation is defined by the orientation of acicular Sil grains. Recumbent isoclinal outcrop-scale folds (axes parallel to Sil lineation) within the Rangeley near lithologic contacts, have been refolded on the cm to m scale, and are visible in vertical east to west striking faces. Asymmetric structures are only visible in one location in the map area (near the Rangeley-Hardwick contact), include asymmetric folds and S-C and S-C-C’ fabrics, and are most prominent on horizontal faces. They indicate localized sinistral displacement along a north to south striking axis parallel to unit boundaries and tectonic fabrics in the area. Evidence for vertical and lateral extrusion of the Monson Orthogneiss as observed in the Palmer area (Massey and Moecher, 2013) is absent in the Petersham area. Instead, the primary strains are E-W shortening and N-S boundary compatible extension. Shortening is expressed in the planar foliation fabrics present in all three units; elongation is expressed in the lineation most prevalent in the Rangeley formation. The abundance of leucosome and large garnet porphyroblasts in the Rangeley adjacent to the Hardwick and diorite suggest regional metamorphism was driven by magmatic heat.