Transpression, deformation partitioning, and extrusion in the Appalachian orogen, Southern New England, U.S.A.

摘要

The tectonic evolution of the south-central New England Appalachians is based on a three-stage Alpine-style model, founded primarily on inferred tectonostratigraphy. However, the lack of overprinting relationships that this model predicts, sparse detailed geochronology, and the similarity of lithologies between neighboring lithotectonic zones significantly impair this model. Transpressional deformation resulting from oblique convergence in the northern Appalachians likely extends along strike to the south and would offer a more consistent mechanism for the style of deformation. This study incorporates multi-scale mapping and structural analysis, and detailed geochronology as a test of the Alpine model and the role of transpression.;Detailed mapping in Massachusetts documents a range of structures that record markedly different states of finite strain suggestive of contraction, wrenching, and extension. Fabrics and structures suggest flattening strains were predominantly accommodated within the Monson gneiss. Shear zones along the margins of the Monson gneiss indicate conjugate kinematics of sinistral/normal in the west-bounding Mt. Dumplin high strain zone and dextral/reverse in the east-bounding Conant Brook shear zone. Observations within map-scale sub-domains agree with various models of pure-shear dominated transpression. The Monson gneiss and bounding shear zones accommodated both vertical and lateral extrusion. Compatibility between adjacent domains was maintained by heterogeneous deformation at all scales of observation.;Within the Mt. Dumplin high strain zone, a range of garnet microstructures reveal protracted retrograde deformation. Aggregates of microgarnet are characterized by a bulk CPO symmetrical to the fabric. These aggregates accommodated a significant proportion of flattening facilitated by grain boundary sliding, grain rotation, and pressure solution. However, the CPO could not have developed in this manner, but was the result of older, higher temperature dislocation creep, recovery, and recrystallization.;All observations constitute the basis for a deformation history driven by regional dextral transpression and kinematic partitioning. Although geochronology yields a range of ages from 375--300 Ma, transpression, extrusion, and exhumation at ca. 330--300 Ma were responsible for the most intense fabrics and structures now recognized. The overall mechanism for transpression is attributed to the continued oblique convergence of Avalon/Meguma, possibly in response to the onset of collision with Gondwana.;KEYWORDS: Transpression, Deformation Partitioning, Extrusion, Garnet Deformation, New England Appalachians