COMPARATIVE STRUCTURE, BASEMENT COVER RELATIONS, TECTONIC HISTORY OF MESOPROTEROZOIC BASEMENT MASSIFS OF THE WESTERN NEW ENGLAND, VERMONT TO NEW YORK

The “Grenville” 1.4 Ga to 950 Ma polycyclic basement of western New England from north to south occurs in Green Mountain massif(GM) and the Chester- Athens domes(CD), Berkshire and Housatonic massifs, Hudson Highlands(HH) and Manhattan prong(MP), and small outliers in the Hudson Valley. Previous mapping of the Mesoproterozoic Berkshire and Green Mountain (GM) basement (Zen and others,1985 ; Ratcliffe and others, 2011) and more recent studies of the (HH) gneiss complex and (MP) complex indicate all are age-correlatives of the eastern Adirondack “Grenville” and that the HH gneisses and the Fordham Gneiss share the same cover, and are coextensive. Facies distribution of the unconformable Neoproterozoic to Cambrian cover sequence indicates deposition on an irregular rifted margin. Taconic orogenic shortening and internal deformation of the massifs vary systematically from north to south. In Vermont the (GM, CD) preserve the E-W Shawinigan structural grain of the Adirondacks in a single slab. The Berkshire massif (BM) contains multiple levels of west- vergent ductile thrusts and folds but preserves earlier Grenville grain. To the west a lower level basement is exposed in a small window. The HH has west- verging thrusts and internal basement- cover detachments, right-oblique shear zones and sub-vertical contractional shear zones that characterize the early deformation. A late episode of east-verging thrusts and easterly overturned regional folds caused the repetition of MP basement and cover. Local discontinuous detachments occur along the limbs of the overturned to isoclinal folds. The folded and refolded map pattern of basement and cover in the MP is better explained by refolding of earlier regional folds systems than by stacked fold- or thrust-nappes. A late component of NNW shortening and right lateral shear may explain the present map patterns. The resulting composite structure of the HH and MP resembles that of a deep crustal flower structure. Comparison of structural features of the massifs and sequence of deformational events suggests that the maximum internal deformation occurs in the Berkshire massif, and decreases northward toward Champlain recess, and southward towards the New York recess. Structural inversion of a rift promontory during Taconic arc collision is proposed as a working model.