A tectonic synthesis based on stratigraphic and structural analysis of western New England is proposed for the Ordovician Taconian orogeny. It emphasizes arc-continental collision in which ocean-floor, continental-margin, and ensialic-rift rocks were imbricated westward in a repeatedly deformed accretionary wedge. Continued compression displaced segments of the North American sialic crust to the west and deformed the earlier emplaced slices of the Taconic allochthons which were derived from the continental margin.
Critical arguments for this synthesis are (1) the west-to-east stratigraphic relations among the basal rift clastic rocks of the Dalton, Pinnacle, and Hoosac Formations of late Precambrian to Early Cambrian age; (2) the stratigraphic and sedimentological similarities between the rocks of the lower Taconic sequence and rocks in the Pinney Hollow and Underhill slices to the east and north of the Green Mountain massif; (3) the environmental similarities between the Cambrian and Lower Ordovician section of the Giddings Brook slice and the age-equivalent section in the St. Albans synclinorium; (4) the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices (groups 1 and 2) of the Taconic allochthons; (5) the presence of synmetamorphic, fault-related structures in the youngest and highest slices (group 3) of the Taconic allochthons; (6) the recognition of extensive thrust zones in the pre-Silurian eugeoclinal sequence east of the middle Proterozoic basement of the Housatonic, Berkshire, Green Mountain, and Lincoln massifs; (7) the location of the Taconic root zone within the pre-Silurian eugeoclinal sequence; (8) the recognition of numerous faults in the serpentinite belt; (9) the similarity between the rocks of the Moretown Formation and modern fore-arc basin sequences; (10) the recognition that the volcanic arc-continental complexes of the Ascot-Weedon and Bronson Hill have been displaced westward over the Moretown and/or Hawley Formations along such faults as the Bristol and Coburn Hill thrusts; (11) the allochthonous and internally imbricated nature of the North American basement in the Berkshire massif; (12) the proposition that the Housatonic, Green Mountain, and Lincoln massifs, as well as the middle Proterozoic cored domes of southeastern Vermont, are also thick sialic slices of North American basement; (13) the recognition of medium-high- to high-pressure metamorphic mineral assemblages in the pre-Silurian eugeoclinal rocks of Vermont; and (14) the recent synthesis of isotopic age data by Sutter and others (1985).
On the basis of an analysis of the foregoing arguments and relationships, a chronological sequence of seven structural sections between Albany, New York, and the Bronson Hill anticlinorium in central Massachusetts is used to depict the evolution of the Taconian orogeny. Retrodeformed distances are based on structural overlap and restoration of the Taconic slices to their depositional setting along the ancient North American continental margin. These easterly younging, diverticulated slices formed as a result of horizontal compression rather than gravity sliding. This palinspastic analysis implies the following. (1) Approximately 1,000 km of shortening has occurred during the emplacement of the Taconic allochthons and the subsequent imbrication of North American basement as thick sialic slices. Approximately 330 km of this shortening is attributed to multiple cleavage generations. (2) Repeated movement along such major surfaces as the Cameron's Line-Whitcomb Summit-Belvidere Mountain thrust zone has buried the Taconic root zone. We suggest that the northern extension of this root zone is exposed to the east of the Lincoln massif in Vermont where the Underhill, Pinney Hollow, and Hazens Notch Formations are exposed. These formations, here considered thrust slices, disappear along the Belvidere Mountain-Whitcomb Summit thrust zone as it is traced southward into western Massachusetts and western Connecticut. (3) Taconian metamorphic rocks, particularly the older medium-high-pressure rocks in northern Vermont, have been transported westward on such reactivated surfaces as the Belvidere Mountain thrust. (4) The anticlinorial form of the middle Proterozoic basement in the Green Mountain and Lincoln massifs may have resulted from fault-bend folding on deep mantle-involved thrusts that developed late in the Taconian orogeny.