Nicholas M. Ratcliffe

Tectonic synthesis of the Taconian orogeny in western New England

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 Cameron9s 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.

40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England.

40Ar/39Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mountain massifs, as well as 40Ar/39Ar and K-Ar mineral and whole-rock ages from Paleozoic metamorphic rocks, suggest that the thermal peak for the dominant metamorphic recrystallization in western New England occurred 465 ± 5 m.y. ago (Taconian). Although textural data indicate a complex metamorphic-tectonic history for Paleozoic rocks, no evidence in rocks at least as high as kyanite grade dictates an Acadian age for the Barrovian metamorphism. Available 40Ar/39Ar and K-Ar data suggest that the low-grade metamorphism and cleavage formation in Taconic allochthons and the higher-grade metamorphism and emplacement of the Berkshire massif allochthon are Taconian. 40Ar/39Ar age data from a poorly defined terrane beginning near the east margin of the Green Mountain massif and extending along the eastern one-third of the Berkshire massif as far south as Otis, Massachusetts, suggest that the area has been retrograded during a metamorphism that peaked at least 376 ± 5 m.y. ago (Acadian). Available age and petrologic data from western New England indicate the presence of at least three separate metamorphic-structural domains of Taconian age: (1) a small area of relict high-pressure and low-temperature metamorphism in northern Vermont (T-1 domain), (2) a broad area in Vermont and eastern New York of normal Barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient (T-2 domain), and (3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks extending from near the Cortlandt Complex northeastward through Dutchess County, New York, to the Berkshire massif in western Massachusetts (T-3 domain). Areas of maximum metamorphic intensity within the T-3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting (Berkshire massif) or from recumbent folding (Manhattan Prong) of remobilized North American continental crust in the later stages of the Taconic orogeny.

Low-angle extensional faulting, reactivated mylonites, and seismic reflection geometry of the Newark basin margin in eastern Pennsylvania

Low-angle 25° to 35° dips have been determined for the border fault of the Newark basin near Riegelsville, Pennsylvania, on the basis of a Vibroseis profile and two continuously cored drill holes across faults at the basin margin. A group of moderately strong planar reflections in a zone 0.5 km thick in gneiss and carbonate rocks of the footwall block coincides with the updip projection of imbricate fault slices and mylonites associated with the Musconetcong thrust system of Drake et al. (1967). Contrasts in acoustic impedance among mylonitic dolostone and mylonitic gneiss and their protoliths, determined from measurements on samples from a third cored hole, are sufficiently large to account for reflections seen in the footwall block. Analysis of drill core and surface outcrops supports the conclusion that low-angle extensional faulting in the early Mesozoic was localized by reactivation of Paleozoic imbricate thrust faults in the basement rocks. Extension in the northwest-southeast quadrant was approximately perpendicular to the strike of the ancient thrust faults in eastern Pennsylvania. The data presented here are the most explicit three-dimensional information obtained thus far in the eastern United States in support of the concept of fault reactivation in controlling formation of early Mesozoic extensional basins.

Cortlandt-Beemerville magmatic belt: A probable late Taconian alkalic cross trend in the central Appalachians

A belt of alkalic to alkalic-calcic pipes, lamprophyres, and intrusive complexes of probable Late Ordovician age extends from the Peach Lake–Croton Falls area of Westchester County, New York, about 100 km S80°W to Beemerville, New Jersey. The rocks are intruded across a Taconic dynamothermal belt ranging from sillimanite grade on the east to chlorite grade on the west. Foliated Middle Ordovician and older rocks containing slaty cleavage or coarser grained metamorphic fabrics are crosscut by dikes that are posttectonic to late syntectonic relative to the Taconian dynamothermal events. Analysis of thrust faults and fold structures within this belt suggests that this cross-grain belt of alkalic to weakly alkalic rocks intruded steeply dipping brittle fractures trending N40° to 50°W, N35° to 60°E, and N70° to 80°E and along more ductile zones trending N20° to N70°E. This plutonic activity was restricted to a discrete east-west belt. The clear posttectonic to syntectonic nature of the dikes and the spatial and temporal association with Ordovician tectonism suggest that magmatic activity may be related to fracturing of mantle rock at the junction of two salients of Taconic age that intersect in the New York recess. The tectonic setting of the intrusive belt is unusual because it is associated in space and time with compressional tectonics of the Taconic orogeny, rather than with extensional rifting.

Basaltic rocks in the Rensselaer Plateau and Chatham slices of the Taconic allochthon: Chemistry and tectonic setting

Tholeiitic to transitional alkalic basalt and basaltic tuff form widely separated but distinctive units within the Nassau Formation of late Proterozoic or Early Cambrian age, within the Rensselaer Plateau and Chatham slices of the Taconic allochthon in eastern New York State and western Massachusetts. Examination of all known occurrences of these basaltic rocks and detailed mapping of the enclosing strata indicate that these basalts are submarine lava flows and water-transported tuffaceous deposits restricted to the lower third of the stratigraphic section in both slices. In each slice, the basalts thicken to the west along with thickening and coarsening of enclosing graywacke beds (Rensselaer Graywacke Member of the Nassau Formation) and thin eastward into pillow basalt and tuffaceous basalt that is associated with fine-grained graywacke and thinly laminated purple and green turbidites of the Nassau Formation. These high-TiO 2 , low-MgO basalts resemble very closely late Proterozoic basalts and feeder dikes of the Catoctin Formation of the Blue Ridge in Virginia and late Proterozoic metadiabase dikes in the northern Reading Prong in New York, both of which intrude pre-Iapetan, eastern North American basement. Major-element and rare-earth chemistry and geology also establish a correlation between the Taconic metabasalts and metabasalts preserved in the Tibbit Hill Volcanic Member of the Pinnacle Formation that unconformably overlies middle Proterozoic basement, in central Vermont. Palinspastic reconstruction places the Chatham slice oceanward of the Rensselaer Plateau slice that contains abundant coarse-grained detritus of probable eastern North American Proterozoic basement These relations suggest a fault-bounded submarine basin (Nassau-Rensselaer basin) located near the continental margin that was fed by submarine fan deposits. Submarine morphology of the distributing fan complex may have channelized the basalts. The association of the tholeiitic to transitional alkalic basalts with interpreted marine-fan deposits suggests that the volcanism occurred after Iapetan rifting and thermal subsidence over tectonically thinned sialic crust, perhaps near the time and place of eventual separation.