Rolfe S. Stanley

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.

Modeling groundwater recharge and flow in an upland fractured bedrock aquifer

This article describes a system dynamics model to simulate recharge and flow mechanisms in a fractured bedrock aquifer in northwestern Vermont. The model was constructed to aid in the interpretation of data collected as part of a stable and radiogenic isotope study of groundwater. Use of the model guided data collection and analysis throughout the course of the study, leading to the derivation of equations describing temporal and spatial changes in recharge and flow mechanisms in the study area. Copyright

The evolution of mesoscopic imbricate thrust faults—an example from the Vermont Foreland, U.S.A.

Abstract An outcrop of a thin (30 cm) bed of micrite surrounded by thicker sequences of well-cleaved, calcareous shale in the Ordovician flysch of western Vermont records a complex history of imbricate faulting and associated folding. The calcareous shale has been shortened by pressure solution. Cross-cutting relations among the floor, ramp and roof faults indicate that faulting progressed from the hinterland (east) to the foreland (west). All the ramp faults developed from arrays of W-climbing enechelon extension fractures. All other faults were controlled by the weaker bedding planes that surround the micrite, and are found at various levels in the shale. Each fault is marked by layers of highly twinned, sparry calcite and black carbonaceous shale selvedge. Simple shear along the floor thrust has rotated the S1 cleavage toward the foreland and produced, along with volume change, a pressure-solution cleavage in the fault zone. All cleavages have progressively developed from the hinterland to the foreland during folding and faulting of the micrite. Layer-parallel shortening measured in both rock types is between 11 and 16%.

Environment of Deformation, Monkton Quartzite, Shelburne Bay, Western Vermont

A 700-m 2 outcrop of Monkton Quartzite, located 275 m above the Champlain thrust and 760 m south of the Shelburne Bay cross fault, contains two generations of fractures, two generations of wrench faults, and en echelon fractures in sinistral and dextral arrays. Feather fractures are associated with some of the faults. Microscopic structures include planes of hematitic inclusions, recrystallized quartz veins with prograde chlorite, unfilled fractures, and quartz deformation lamellae. All large- and small-scale structures were produced by two relatively simple stress fields that were orthogonally superimposed so that the directions of σ 1 and σ 3 were interchanged from the first to the second generation. Quartz deformation lamellae developed only during the first event. Evidence based on the small angle of shear failure from the deduced position of σ 1 , experimental compression tests on the Monkton Quartzite, the angle between the c axis and the pole to deformation lamellae ( c ∧⊥dl) in deformed quartz, and the assemblage quartz-hematite-chlorite suggest that deformation at Shelburne Access Area developed in a shallow crustal environment (less than 1,830 m) as a result of a long period of stress in a large area; this stress system underwent local modification toward the end of the period. Structures of the first generation probably formed under decreasing lithostatic pressure and increasing rates of strain. The first-generation wrench faults are correlated with the Shelburne Bay cross fault, which offsets the north-trending trace of the Champlain thrust. The inferred direction of σ 1 is remarkably similar for all these structures, regardless of scale. The wrench faults are thought to be part of the Acadian orogeny of Middle to Late Devonian age.

Dolomite and Quartz Lamellae Stress and Strain Analysis in a Multiply-Deformed Terrane, West-Central Vermont

The methods of dynamic analysis for inferring principal stress axes from quartz, calcite, and dolomite petrofabrics (Carter and Raleigh, 1969) have been well developed during the last four decades through the availability of experimental equipment for rock deformation under controlled temperatures and pressures (Griggs, 1936) and the modern universal microscope stage (Emmons, 1943). Magnitudes of the stresses cannot be predicted from the methods they describe, however. Although Jamison and Spang (1976) proposed a method to infer differential stress from twinned carbonate aggregates, strain is in reality the only parameter that can be properly measured in deformed rocks. Well-known procedures for measuring finite strain in racks can be applied only to highly deformed rocks and require speculation about the undeformed state of the rock fabric (Ramsay, 1967). Groshong (1972) has recently developed a least-squares fit strain-gage technique which can be used to calculate the orientations and magnitudes of principal strain axes for small strains in deformed rocks containing mechanically twinned minerals. Groshong (1974) experimentally checked his method using twinned calcite aggregates, but he suggested that it could be applied to other minerals.

Mechanical Analog Calculator for Three-Dimensional Rotation Problems

A mechanical analog calculator (Geomac) has been developed that can be used for rotation programs in such fields as structural petrology, mineralogy, and paleomagnetism. The device consists of a group of concentric circles and arcs on which the problem is reconstructed, and the solution is found. In terms of precision, speed, and ease of operation, the calculator is thought to be superior o t stereographic solutions and preferable to digital computer programs for problems involving a small number of data.