Year of Publication

2011

Degree Name

Master of Science (MS)

Document Type

Master's Thesis

College

Arts and Sciences

Department

Earth and Environmental Sciences (Geology)

First Advisor

David P. Moecher

Abstract

Bedrock geology of the East Brookfield quadrangle (EBQ), based on new 1:24000 scale mapping, consists of an interfolded sequence of Rangeley Fm. and Paxton Fm. metasediments, intruded by Early Mississippian tonalitic to granitic orthogneisses and underlain in the western half by folded orthogneisses of unknown age. Pervasive flattening strains have created planar, generally NNE-SSW striking and consistent moderately west dipping foliations. Generally strike-parallel stretching lineations and boudinage structures, with rarer dip parallel stretching lineations, reflect extrusion of CMZ metasediments and orthogneisses with a flattening component. Mapping in the EBQ demonstrates that a transition between deformational mechanisms of the southern Central Maine Zone is located there. Evidence for previously mapped faults was not observed during mapping. Alternatively, map scale folds terminating in the East Brookfield quadrangle likely accommodated the across-strike shortening and repetition of lithologic units of the CMZ during Acadian orogenesis. Aeromagnetic patterns and outcrop data approximate a map scale, east-verging and west-dipping, shallowly north-plunging isoclinal fold consisting of interlayered mafic and felsic gneisses. Constrictional and flattening strains documented in the deformation features of the East Brookfield quadrangle suggest that it is located on/near the eastern margin of the zone of transpression approximated by the Bronson Hill gneiss domes. Outcrop- to meter-scale folds with hinge axes sub-parallel to parallel with regional lineations suggest constrictional deformation in a transpressional regime. Deformation features associated with map-scale folding, extrusion/extension, and flattening of CMZ lithologies in the East Brookfield quadrangle support a regime of transpressional deformation caused by oblique convergence of Avalonian terrane with Laurentia. U-Pb ages and outcrop structural relationships suggest that regional flattening and folding was syn- to late-orogenic in nature and Carboniferous or younger in age. All previous structural fabrics are transposed by late regional flattening. Lithologies of the East Brookfield quadrangle exemplify the nature of the Acadian granulite-facies high, displaying a peak metamorphic assemblage of Crd + Grt + Sil + Kfs that has undergone retrograde reactions to produce Bt + Sil. Retrograde metamorphism and deformation occurred during strike-parallel extrusion of CMZ lithologies. Retrograde minerals define kinematic/shear sense indicators that record an overall top to the NNE or ENE asymmetry on vertical faces and dextral west side north on horizontal surfaces. Progressive deformation of CMZ lithologies through transpressional mechanisms is proposed as an alternative to the sequential development of Acadian deformational features in three separate stages.

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