Late Paleozoic partitioned transpression and heterogeneous extrusion of granitic Monson orthogneiss, Appalachian Orogen, southern New England, USA

Appalachian orogenesis consisted broadly of three Paleozoic collisional phases (Taconian, Acadian, Alleghanian); the kinematics of each phase remain to be fully elucidated. Detailed mapping and structural analysis in central Massachusetts reveal the details of kinematics associated with middle to late Paleozoic convergence of the Bronson Hill (BHT) and Central Maine (CMT) terranes, in that part of the orogen coincident with the greatest shortening perpendicular to strike. Fabrics, structures, and geochronology in the Monson gneiss of the BHT (south-central Mass.), are consistent with continuous Late Paleozoic partitioned dextral transpression, as opposed to multiple tectonic episodes beginning in the Devonian (nappe-backfold-dome stages of previous workers). The Monson is bounded on the east by the dextral Conant Brook high strain zone (HSZ) and on the west by the sinistral Mt. Dumplin HSZ (MDHSZ). Between these HSZs, the Monson is an S>L tectonite with two lineation modes; farther north it is an L>S tectonite with a single lineation mode. Fabrics in the Monson and bounding lithologies across the zone of deformation are characterized by steep to sub-vertical foliations, a heterogeneously distributed bimodal suite of stretching lineations (moderate SSW mode and near-vertical mode, parallel to foliation dip, in Monson, representing coaxial and non-coaxial components; SSW mode only in MDHSZ, representing non-coaxial simple shear), and local closed to isoclinal folds with hinge lines sub-parallel to lineations. Kinematic partitioning, partially unconfined and inclined boundaries that allowed for vertical and lateral extrusion, and oblique simple shear are the primary elements of this zone and suggest heterogeneous flow with an overall triclinic symmetry. We suggest the consistency of fabric across the zone and its boundaries is an artifact of deformation partitioning due to lithologic heterogeneity, and stabilization of the sub-horizontal lineation (manifestation of non-coaxial component) by lateral (northward) extrusion of the Monson. Preliminary U-Pb zircon and monazite geochronology indicates that deformation occurred between 330 to 300 Ma. Lateral mid-crustal orogen-parallel flow is consistent with Late Paleozoic kinematics elsewhere in the BHT.