2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 46-8
Presentation Time: 9:00 AM-5:30 PM


THOMPSON, M.D., Geosciences Department, Wellesley College, Wellesley, MA 02481, mthompson@wellesley.edu

The Newton Quadrangle, located west of Boston, Massachusetts, is mainly underlain by 595-585 Ma Roxbury Conglomerate associated with the Brighton Igneous Suite and overlain by ≤ 570 Ma Cambridge Formation, all occupying the topographic Boston Basin. Up-faulted blocks south and west of the Basin are dominated by 610 Ma Dedham Granite and unconformably overlying 597-593 Ma Lynn-Mattapan volcanic rocks. The Mount Hope Fault interpreted as an Ediacaran normal fault active during Roxbury deposition bounds the Basin on the south. Generally anticlinal structure within the Basin is locally modulated by short-wavelength folds and cut by high angle faults. Closer analysis of these features in the forthcoming 1:24,000 Bedrock Geologic Map of the Newton Quadrangle in production at the Massachusetts Geological Survey reveals that some of this deformation significantly pre-dates traditionally assumed Alleghanian events.

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.