LINEATION PATTERNS AND ASSOCIATED KINEMATICS IN CENTRAL NEW ENGLAND: UNDER-REPORTED RESOURCE FOR INTERPRETATION OF OROGENIC EVOLUTION

Lineations in central New England show intriguing swirl-like patterns near the Pelham dome and a near-orthogonal overprint pattern to the east in the Silurian of the Merrimack belt. Interpretation of distribution, related shear sense and relative timing of the patterns provides clues to kinematic evolution of the orogen. Strong N-S mineral lineations and elongations in the Pelham dome, quasi-parallel to fold axes, were first interpreted as intersection lineations or produced by “rolling” from movement lines normal to fold axes. Later these were recognized as bulk elongation lineations, parallel to the orogen and associated with pervasive N-over-S shear (Ashenden & Robinson, Reed & Williams). On E and W flanks of the dome, lineations define swirls which bend away from the dome northward; then back to the south. The E swirl was first interpreted as “Acadian”. Now geochronology (Tucker, Gromet) suggests the pattern was produced in at least three episodes, Acadian (405-385 Ma), Neo-Acadian (370-355 Ma), and Late Pennsylvanian (300-295 Ma), the last dominating the Pelham dome. In the Merrimack belt, N-S to NE-SW trending lineations and folds overprint earlier transverse E-W lineations. Pluton relations and geochronology indicate both trends are superimposed on or synchronous with peak Neo-Acadian metamorphism. Relations between these lineations and associated kinematics are well characterized within the Conant Brook shear zone, where both appear to have formed under granulite- to upper-amphibolite-facies conditions. The earlier transverse lineation is associated with top-E kinematics; the later orogen-parallel lineation with dextral shear. Strain partitioning from thin section to map scale during the later deformation allowed preservation of transverse lineation and fabrics within lower strain zones. Strain partitioning may provide an analogy for interpreting swirl patterns on flanks of the Pelham dome. The dominant orogen-parallel lineations, once thought the product of a single “dome” stage, apparently formed during Late Pennsylvanian in the Pelham dome, during Neo-Acadian in the Merrimack belt, central Massachusetts, and in “true” Acadian in New Hampshire, and suggest significant potentially long-lived components of transpression or transtension during orogenic evolution.