A POTENTIAL SOLUTION TO APPARENTLY CONTRASTING ALLEGHANIAN TECTONIC HISTORIES ALONG THE STRIKE OF THE APPALACHIANS IN EASTERN MA, RI AND CT

In the Appalachians of eastern MA, RI and CT, evidence for the late Paleozoic Alleghanian orogeny (formation of Pangea) is well developed in southern CT, RI and southeastern MA, but less well preserved in northeastern MA. Alleghanian metamorphism was generally higher grade, and deformation was more intense towards the south in CT and RI, than in northeastern MA. Also, K/Ar and ⁴⁰Ar/³⁹Ar cooling ages are younger to the south. In Maine and Canada, the Alleghanian overprint is largely absent. Evidence is mostly localized along shear zones, such as the Norumbega fault system in Maine and the Minas fault zone in Nova Scotia. In the southern Appalachians, the Alleghanian orogeny is well preserved. These contrasting geological histories along the strike of the orogen have led to controversial interpretations on the nature of the Alleghanian orogeny in southeastern New England. A unifying indenter tectonic model may provide a solution for these variations in southeastern New England.

In southeastern MA, 010°-trending folds and thrusts in the latest Devonian-Carboniferous Narragansett Basin suggest a westward motion of Gondwana relative to Laurentia during the Alleghanian orogeny. However, these structures change towards the southwest into a west-trending zone with predominantly dextral structures in the Avalon terrane in southeastern CT. This geometry is consistent with an indenter model, in which the dextral structures resulted from westward motion of an indenter immediately south of New England. The structures are interpreted as mid-crustal ductile equivalents to dextral north-trending structures in Myanmar (e.g. the Sagaing fault zone) as a result of northward motion of the India indenter to the east. Similar to the dextrally sheared zone in the Avalon terrane, the west-trending Minas fault zone in Nova Scotia also experienced dextral movement during the Alleghanian orogeny. These shear zones may have acted as transform faults along a convergent zone between Gondwana and Laurentia. Equivalent structures have been recognized in northwest Africa. Indenter models have been proposed previously to explain salients in the Appalachian foreland fold-thrust belt. The model presented here involves an additional indenter that affected southern New England, but did not produce a salient in the Appalachian foreland belt.