T289 Folds: Causes and Consequences (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

MESOSCOPIC STRUCTURE OF THE MIXER POND GNEISS, SOUTH-CENTRAL MAINE

Paper number: T289-12
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Polyphase structural mapping complemented by recent U–Pb zircon/Lu–Hf garnet dating in the Mixer Pond Gneiss [MPG] reveals late Silurian–Early Devonian silicic magmatism and coeval[?] contractional deformation along the Liberty–Orrington belt of south-central Maine. Mesoscopic structures recorded in fine-grained granitic orthogneiss [petrologically complex MPG unit] include a variably SE-dipping gneissic foliation [Sn~20–85° dip] and shallow NE-plunging crenulation lineation [<10° plunge] associated w/ NW-vergent antiform-synform pairs. Near our ~420 Ma zircon sample locality, asymmetric folds in Sn are steeply inclined [axial surfaces >65°] and possess ~40–65° interlimb angles defining a close geometry; fold hinges are subhorizontal [~5° NE plunge] and subparallel to crenulation lineations developed on Sn. In other areas [e.g., ~402 Ma garnet locality], orthogneiss bodies are characterized by shallow NW-dips [Sn~35°]; strings of subhedral, mm-scale porphyroblastic garnet are often observed along the traces of synmetamorphic foliation. Where Sn is steeply NW-dipping [~80°], asymmetric K-feldspar porphyroclasts [rare σ-structures viewed on Sn traces] suggest tops-to-the-NE/dextral shear. Consistent w/ the MPG, kinematic indicators in adjacent country rocks [e.g., Devonian–Ordovician Passagassawakeag Gneiss] record contractional and strike-slip components of deformation. Where schistose fabrics dip moderately SW [Sn~40°], cm-scale felsic boudins occasionally observed along Sn traces support a tops-to-the NE/reverse sense of shear; conversely, parasitic folds and imbricated mineral pods associated with steep NW-dipping penetrative fabrics [Sn >70°] indicate dextral shear. In contrast to MPG structures described above, open-to-isoclinal folds in high-strain exposures of the Passagassawakeag Gneiss and other stratified metasedimentary units [e.g., Silurian–Ordovician Bucksport Formation] consistently plunge SW [~5-25°] and often contain a subvertical axial-planar crenulation cleavage [Sn+1]. Extensive U–Pb zircon dating [Gray et al., 2017], detailed 1:24000-scale geologic mapping [Pollock, in progress], and structural details reported in this study suggest that intrusive rocks of the MPG shared in the transpressional deformation history recorded along coastal Maine.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-308199

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.