A CASE FOR ALLEGHANIAN 'MINI' INDENTER TECTONICS IN THE SOUTHEASTERN NEW ENGLAND APPALACHIANS

A model involving indenter tectonics is proposed in order to explain the rapid change in the effects of the late Paleozoic Alleghanian orogeny (leading to the formation of Pangea) in the southeastern New England Appalachians. Evidence for Alleghanian deformation and high-grade metamorphism is widespread in CT and RI, but decreases rapidly towards northeastern MA. In the latest Devonian-Carboniferous Narragansett and Norfolk basins (which were only affected by the Alleghanian orogeny), the metamorphic grade decreases within as little as ~50 km from the kyanite/sillimanite zone in southern RI to below chlorite grade in eastern MA. Also, K/Ar and ⁴⁰Ar/³⁹Ar mineral cooling ages in the Avalon terrane decrease from northeastern MA to southern CT and RI, consistent with a strong Alleghanian overprint to the south. Northeast of MA, evidence for Alleghanian deformation diminishes even further and is mostly localized along shear zones, such as the Norumbega fault system in Maine and the Minas fault zone in Nova Scotia.

The Alleghanian deformation and changes within southeastern New England are too far north to be explained by the indenter proposed previously as having caused the Alleghanian deformation and metamorphism in the southern Appalachians. However, it may be explained by a ‘mini’ indenter. In southeastern MA, a westward transport direction of Gondwana relative to Laurentia during the Alleghanian orogeny is interpreted based on northerly-trending folds and thrusts in the Narragansett Basin and part of the Merrimack belt. In the Avalon terrane immediately south of the Putnam terrane in southeastern CT, the northerly trend changes into a west-trending zone with predominantly dextral structures. 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. Based on existing plate tectonic reconstructions, the indenter may have been part of the Anti-Atlas region of Morocco, and the dextral shear zone could have been continuous with the Atlas Paleozoic transform zone or South Meseta shear zone of northwest Africa. Likewise, the Avalon terrane of southeastern New England may have acted as a Laurentia-based promontory, causing the deformation immediately north of this shear zone.