STRUCTURAL GEOLOGY OF THE NEWTON 7.5-MINUTE QUADRANGLE, MIDDLESEX, NORFOLK AND SUFFOLK COUNTIES, MASSACHUSETTS
Poles to moderately north dipping bedding in the north half of the Basin and more steeply south dipping beds in the south jointly define a N85E/13NE fold axis consistent with the “Central Anticline” in classic works of Billings and LaForge. Hinges of smaller-scale folds on the north limb of this broad arch, however, lie 20º counterclockwise of this trend, and steep S1cleavage bearing an axial planar relationship to the Central Anticline strikes diagonally across the short-wavelength folds. This pattern indicates that the latter structures developed before regional arching. Brighton intrusives cut closely spaced folds in both surface and subsurface sections, further implying that the early folding is older than ~ 585 Ma.
Steep S1 directions are pervasively overprinted by irregular partings with dips averaging 30N. Typical outcrops contain one such structure assigned to S2 where strikes vary around EW or to S3 if strikes are NW-to-NNW. Examples with intermediate orientations are also common, suggesting that the collective range of directions records progressive deformation in a dextral regime. Penetrative S2 fabric in some Roxbury clasts and mica locally parallel to S3 reflect persistent low-grade metamorphic conditions throughout low-angle cleavage formation. Whether this deformation reflects Acadian accretion of Avalonia, Late Devonian rifting or Alleghanian orogenesis remains unclear.