BEDROCK GEOLOGY OF THE EAST BROOKFIELD 7.5 MIN. QUADRANGLE (EBQ), SOUTH-CENTRAL MASSACHUSETTS: STRUCTURAL AND PETROLOGIC EVIDENCE FOR THE NATURE OF REGIONAL DEFORMATION AND KINEMATICS OF THE SOUTHERN CENTRAL MAINE ZONE

Across-strike shortening from Maine to Connecticut is the most conspicuous orogen-scale feature of the New England Appalachians. Mapping (1:24000) and structural analysis in the EBQ and adjacent quads demonstrate that shortening in the southern Central Maine zone was accommodated by orogen-parallel elongation and orogen-normal flattening and thrusting. Map units are Rangeley Fm. (Sil-Bt-Grt schist and gneiss, Bt-Pl±Grt granofels; thin marble layers); Paxton Fm. (Bt-Pl±Grt granofels; calc-silicate granofels; Bt-Sil-Grt schist); tonalitic Bt orthogneiss (Bt-Pl-Qtz±Grt); migmatitic felsic and mafic gneisses and amphibolite; and leucopegmatites (10-m-scale thickness). Foliations (primarily compositional layering, leucosomes in the plane of flattening, Bt SPO, flattened quartz and feldspar grains) throughout the EBQ is remarkably consistent (average Rangeley S = 014 + 16 /38 + 12 W; average Paxton S = 018 + 19 /25 + 12 W; average orthogneiss S = 014 +24/24 + 13 W). Strike-parallel lineation is defined primarily by Sil in the Rangeley schist; dip parallel lineations are defined by Bt/Qtz/Fsp streaks in orthogneiss and locally Sil in Rangeley. No map- or outcrop-scale folds are observed; m-scale intrafolial, tight to isoclinal folds of leucosomes/pegmatites and marble/calc-silicate layers with eastward vergence predominate. Fold hinge lines are parallel to strike-parallel lineations suggesting strike-parallel constriction. This is supported by m-scale pinch-and-swell zones elongated parallel to strike in folded Rangeley, and leucosome boudinage. Meter-scale high strain zones and cm-scale mylonitic layers occur within orthogneiss and pegmatite, locally expressed as zones of dextral S-C or S-C-C’ shear fabrics that trend NNE-SSW. Pegmatites and leucosomes in all units are flattened parallel to foliation and boudinaged on vertical strike parallel and dip parallel faces. Marked flattening and formation of S-tectonites occurs in metasedimentary units at margins of pegmatites. All units exhibit dextral asymmetry viewed on planes perpendicular to foliation and parallel to lineation (top ESE or top NNE). The occurrence of bimodal lineations in the EBQ is similar to those observed in the Central Maine-Bronson Hill boundary zone in the Palmer quad. 25 km west and the Winchendon quad. at the MA-NH border.