Changing tectonic environments of the Avalon superterrane and the Nashoba terrane in Massachusetts

Abstract

The paper is composed to test the hypothesis (Skehan and Rast, 1992) that the Nashoba zone terrane of southern New England is basement to the Avalon, and that both zones are remnant blocks of the Avalonian volcanic-plutonic arc. These blocks preserve evidence for long strike-slip fault zone(s) within the volcanic arc trending parallel to an associated subduction zone, as is the case in modern arcs. We propose that the “exotic” or “enigmatic” Nashoba Formation mantling the gneissose basement core complex of the Nashoba terrane can be generally correlated with the easterly Late Proterozoic Avalonian formations that antedate the formation of the Avalonian volcanic-plutonic arc. A related objective is to evaluate the geodynamic model accounting for the anticlinorial uplift of the core complex of the Nashoba block to its present position west of the Avalon zone and southeast of the Merrimack trough. Keys to verifying these propositions are: (1) the ongoing mapping of the extensive mylonite zones of the Boston Avalon zone into the Nashoba zone; (2) the role of Taconian age tectonics in the emplacement of Ordovician-Silurian plutons in the Nashoba and the Massabesic blocks; (3) the existing basis for isotopic dating of mylonites. Additionally, a key to understanding is the role of the Alleghanian orogeny. This study has implications for understanding correlations among several Nashoba-like basement inliers of eastern New England, and also similar Late Proterozoic complexes in the Avalonian, Cadomian, and Pan-African terranes.