Alleghanian Polyphase Deformation of the Hope Valley Shear Zone, southeastern New England

Publisher Website
Google Scholar
Abstract
Structural, petrographical, and Rb‐Sr isotopic studies in the highly deformed basement rocks of southern Rhode Island have allowed the identification of the southern continuation of the Hope Valley Shear Zone (HVSZ). Previous work recognized the HVSZ as an Alleghanian tectonic boundary separating the distinct late Precambrian Esmond‐Dedham and Hope Valley terranes in southeastern New England. Two important components of the Esmond‐Dedham terrane (the late Precambrian Ponaganset gneiss and the Devonian Scituate granite gneiss) now have been traced with structural continuity into southern Rhode Island. The distribution of these gneissic units and their structures shows that the southern segment of the HVSZ has been strongly deformed, abruptly deflecting it from a NW‐SE orientation to a N‐S trend. The deflection of the HVSZ marks an abrupt structural boundary between two distinct, regionally developed mineral lineation domains: a northwestern domain with north to NNW trends (characteristic of most of the HVSZ) and a southeastern domain with NE trends. Structural relations suggest that all mineral lineations were originally parallel along the HVSZ with a north to NNW trend. The deflection formed during a subsequent phase of ductile, right‐lateral shear, producing a clockwise rotation of both lineations and the HVSZ in southern Rhode Island. Thus basement gneisses in southeastern New England have suffered polyphase Alleghanian deformation at high metamorphic grades. It has been argued that during the Late Paleozoic, southern New England was located in a structural transition zone between convergent tectonics in the central and southern Appalachians and wrench tectonics in the northern Appalachians. The structural link between these two distinct tectonic regimes is an E‐W, right‐lateral transcurrent fault. The late dextral shear deformation described in southern Rhode Island basement rocks appears to have resulted from right‐lateral movement at depth within this transcurrent fault zone.