Abstract
The Pennsylvanian sediments of the Narragansett Basin, Rhode Island, were deformed during the Al-leghanian orogeny and record the effects of Alleghanian deformation in southeast New England without the complication of earlier structures. During this deformation the basin was intruded at its southwestern corner by the Narragansett Pier Granite, which has a crystallization age of 273 Ma. Mapping of basin metasediments along the Narragansett Bay shore in the southwestern portion of the basin reveals features formed by several episodes of deformation. Two generations of isoclinal folds along north-trending axes, F1a and F1b, are the earliest structures in the area followed by a set of small closed folds, F2, also on north-trending axes. The Fla, F1b, and F2 folds are the result of east-west closure of the basin. Minor sinistral and more significant dextral motion on faults within the Beaverhead Shear Zone produced local east-trending crenulations, S3, and open folds with north-trending axes, F4, in the study area. Correlation of these structures with structures described elsewhere in the basin allows the integration of the deformation history in the study area into that of the entire basin. Cross-cutting relationships indicate that the Narragansett Pier Granite began intrusion into the southwestern corner of the basin during the initial, sinistral shearing. Thus the crystallization age of the granite gives an approximate date for the transition from east-west shortening during basin closure to shearing along the Beaverhead Shear Zone. The date for correlative transitions in several other areas in southeastern New England and the accompanying reorientation of the regional stress field is likewise established. A model involving convergence of Africa with North America followed by counter-clockwise rotation of southeastern New England can reconcile most of the current data for the Alleghanian orogeny in this area.
Published Version
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