SALINIC TO NEOACADIAN DEFORMATION IN THE MIGMATIZED CENTRAL MAINE BELT OF WESTERN MAINE

Detailed bedrock mapping coupled with new geochronology in the southern part of the Gilead 7.5’ Quadrangle in Western Maine has revealed at least three phases of Salinic through Neoacadian deformation. The geology of the study area is dominated by the migmatized Silurian Rangeley, Perry Mtn. (?), and Smalls Falls Formations of the Central Maine Belt (CMB), which are intruded by quartz diorites from the Piscataquis Volcanic Arc, two-mica granites, and pegmatite. All of the metasedimentary rocks are stromatic migmatites, part of the Migmatite-Granite Complex (Solar and Tomascak, 2016). The geochronology (Wheatcroft et al., 2017, this meeting) brackets the cycle of deposition, metamorphism, migmatization, and deformation to between circa 435 Ma. to 352 Ma.

D₁ is represented by cryptic pre-metamorphic faults that offset and truncate the stratigraphic units. Pre-metamorphic faults are observed outside of the study area in a contiguous section to the north. These faults are likely Salinic in age and developed synchronous with deposition or circa 435 Ma..

D₂ deformation is characterized by nappe-scale, isoclinal folding of unknown vergence where bedding, S₀, is parallel to schistosity, S₂. Only a few F₂ folds are present in the study area and in these places bedding, S₀, is antiparallel to S­₂ schistosity. The gray schists and quartzites above Bog Brook in the study area preserve this fabric relationship and suggest the presence of a macroscale F₂ hinge zone. The extensive migmatization has obscured most of the D₂ fabrics that are likely Early Acadian in age.

D₃ deformation is characterized by numerous open, reclined, upright to overturned, macroscopic folds with limbs striking 245, 87 and 345, 62, a calculated inter-limb angle of 83°, and a hinge line trend and plunge of 55, 60. Mesoscopic D₃ folds of the composite S₀/S₂ fabrics are common but of diverse fold orientations due to the migmatization. The S₃ axial planar cleavage is characterized by a zonal crenulation in the F₃ mesoscale folds. The stratigraphic age assignment supported by lithologic correlation and new detrital zircon geochronology suggests the stratigraphy is inverted due to D₂  isoclinal folding. As such the D₃ folds are best characterized as antiformal synclines and synformal anticlines and are likely of Late Acadian or Neoacadian in age (pre-352 Ma.).