Permian bulk shortening in the Narragansett Basin of southeastern New England, USA

Abstract

Pennsylvanian age sediments within the Narragansett Basin allow the effects of Alleghanian deformation to be distinguished from the Taconic and Acadian orogenies that affected the rocks to the west. Three-dimensional microstructural analyses of both matrix foliations and structures preserved in porphyroblasts define two extended periods of deformation and metamorphism about two differently-trending foliation intersection/inflection axes in porphyroblasts (FIAs). SSW–NNE-trending FIAs (set 1) formed first followed by WSW–ENE-trending FIAs (set 2). The SSW–NNE FIA trends from the south-central zone lie parallel to regional folds in the entire southern graben. The changes in FIA trends reveal that the direction of maximum bulk shortening changed from WNW–ESE to NNW–SSE during amphibolite facies metamorphism within the Central Zone of the basin. They reveal a more varied history than the simple kinematic framework of solely W–E directed bulk shortening that was previously suggested.