CARBONIFEROUS TRANSPRESSION AND SCALE-DEPENDENT STRAIN HETEROGENEITY IN THE SOUTHERN BRONSON HILL ZONE, NEW ENGLAND AS A RESULT OF DEXTRAL OBLIQUE CONVERGENCE WITH THE CENTRAL MAINE ZONE

The Bronson Hill zone in southern Massachusetts includes a range of structures that record markedly different states of finite strain suggestive of contraction, wrenching, and extension within the middle crust. The metaplutonic Monson gneiss in the Palmer quadrangle can be largely described as a kilometer-scale, subvertical to steeply inclined slab of S>L tectonite with subvertical (locally subhorizontal) stretching lineations, and reverse/dextral kinematic indicators. The orthogneiss is bounded on the west and east by the Mount Dumplin (normal/sinistral) and Conant Brook (reverse/dextral) conjugate high strain zones characterized by metastratified cover rocks with subvertical to steeply inclined mylonitic foliations, a range of lineations, and well-developed kinematic indicators. We have not observed significant evidence of overprinting. Rather, physical relationships suggest all structures were coeval as part of a regional partitioned dextral transpression zone, where contraction and wrenching were predominantly compartmentalized within orthogneiss and bounding cover rocks, respectively. Zircon and monazite ion microprobe analysis, combined with in-situ monazite EPMA support contemporaneous deformation, constraining transpression to 350-300 Ma. Five structural domains have been delineated by fabric orientation along and across strike in the Monson gneiss. Within each domain, deformation was quasi-homogeneous and conforms to predictions of various transpressional models in terms of fabric and kinematics. Each domain has accommodated deformation by a unique monoclinic to triclinic transpression with subvertical to steeply inclined extrusion, while being displaced laterally northward along bounding high strain zones. Strain compatibility between adjacent domains was maintained by internal heterogeneous deformation, which accounts for the array of subordinate S- to L-tectonites, localized zones of mylonite, and coplanar isoclinal folds within outcrop and thin section. In turn, heterogeneity was largely controlled by rheological contrasts associated with lithologic contacts, and inherited structural anisotropy. The structural pattern observed here is a manifestation of regional middle to late Paleozoic dextral displacement between Avalon/Meguma and Laurentia.