Dating Deformation in the Palmer Zone of Transpression, Central Massachusetts: Temporal Constraints on Models for Progressive Deformation in the Middle Crust

Abstract

The Palmer Zone of Transpression (PZoT) in the Bronson Hill zone of central Massachusetts is an expression of late Paleozoic oblique convergence between Avalonia and Laurentia. The steeply W-dipping, N-S trending PZoT is defined by bounding high strain zones (Mt. Dumplin on west, Central Maine and Conant Brook on east) of opposing shear sense enclosing the Monson orthogneiss. Research was designed to establish the timing of deformation to test the hypothesis that strain in transpressional systems occurs contemporaneously. An understanding of the timing of deformation in this zone could elucidate the mechanisms that formed the zone and contribute to a greater overall understanding of fabric evolution in transpressional systems. U-Pb SIMS ages of zircon in deformed pegmatites in Monson orthogneiss and Paxton and Rangeley paragneisses are 369 to 364 Ma. Plutons that contain all fabrics associated with progressive transpression – lineations ranging from subhorizontal (initial) to steeply-plunging parallel to dip (final) – mark the maximum age of deformation. Ages range from 360-355 Ma in the Hardwick tonalite, Wachusett tonalite, West Warren diorite, Walker Mountain orthogneiss, and Nichewaug diorite, and indicate transpression in the PZoT initiated after ca. 355 Ma. Monazite from Rangeley paragneisses and schists selected in the context of petrofabrics and syn-deformational mineral assemblages yield U-Th-Pb EPMA chemical ages that indicate dextral transpression occurred continuously from 355 to 315 Ma. Monazite chemical ages also indicate that sinistral lateral displacement in the west-bounding Mt. Dumplin high strain zone initiated ca. 325 Ma and lasted for roughly 45 m.y., ending around 280 Ma. The data for the Central Maine zone, Conant Brook shear zone, and Greenwich syncline support the idea of contemporaneous deformation across the compartmentalized zones of a transpressive system (355-315 Ma), while the ages for the Mt. Dumplin high strain zone indicate sinistral lateral displacement overlapped dextral transpression by about 10 m.y. but generally continued later in time (325-280 Ma).