LATE PALEOZOIC DEXTRAL TRANSPRESSION DRIVEN BY OBLIQUE CONVERGENCE, SOUTHERN BRONSON HILL TERRANE, MASSACHUSETTS

Bedrock mapping in the Palmer (MA) quad., microscopic to mesoscopic structures, and regional structural and lithologic relationships constitute the basis for a deformation history driven by oblique convergence and bulk regional dextral transpression, characterized by inheritance and amplification of lithologic strength contrasts, and heterogeneity and kinematic partitioning at all observable scales. The highly attenuated Monson orthogneiss (S to L-S to L tectonite) exhibits evidence for simultaneous northward lateral extrusion and vertical extrusion relative to the stiff Belchertown pluton and Glastonbury orthogneiss on the west and paragneisses of the Central Maine terrane (CMT) on the east. Steep foliations, tight to isoclinal folds of foliation, and two sets of stretching lineations indicate the Monson deformed by strain partitioning. Amonoosuc-Partridge Fms. in the bounding sinistral Mount Dumplin high strain zone (MDHSZ, west side) are S-C tectonites with a component of steep normal motion. Fold and porphyroclast asymmetries and two Sil lineations in paragneiss tectonites at the CMT boundary indicate dextral shear with a steep reverse component. The Belchertown formed a rigid mass except marginally against the MDHSZ where it is highly tectonized, locally producing a sinistral S-C fabric, and along its southern margin where it is an S>L tectonite/orthoschist thrust over the northward flowing Glastonbury. Shortening perpendicular to strike and simultaneous extrusion parallel to strike are the result of oblique convergence between the CMT (and underlying Avalon Composite terrane) and the Laurentian margin, with BHT lithologies accommodating high strains in the intervening zone of transpression. Geochronology indicates deformation progressed from 330-290 Ma. Dextral transpression affected an existing infrastructure of Ordovician granitic plutons and Ordovician-Devonian metastratified ‘cover'. Extrusion of the Monson (“doming”) was contemporaneous with convergence and flow of CMT paragneisses beneath the BHT (“backfolding”). Although fold nappes in the Ordovician through Devonian metastratified sequence mantling gneiss domes are demonstrable north of the Monson (along strike to the north), we see no need to invoke backfolding as a component of the BHT tectonic history in MA and CT.