ABSTRACT Synthesis of the Ordovician Taconic orogeny in the northern Appalachians has been hindered by along-strike variations in Laurentian, Gondwanan, and arc-generated tectonic elements. The Dashwoods terrane in Newfoundland has been interpreted as a peri-Laurentian arc terrane that collided with the Laurentian margin at the onset of the Taconic orogeny, whereas along strike in New England, the Moretown terrane marks the leading edge of peri-Gondwanan arcs. The peri-Laurentian affinity of the Dashwoods terrane hinges on the correlation of its oldest metasedimentary rocks with upper Ediacaran to Lower Ordovician rift-drift deposits of the Laurentian Humber margin in western Newfoundland. Here, we report U-Pb dates and trace-element geochemistry on detrital zircons from metasedimentary rocks in the southern Dashwoods terrane that challenge this correlation and provide new insights into the Taconic orogeny. Based on age and trace-element geochemistry of detrital zircons analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), we identified ca. 462–445 Ma sedimentary packages with a mixed provenance consisting of Laurentian, Gondwanan, and arc-derived Cambrian–Ordovician sources. These deposits overlap in age with Upper Ordovician strata of the Badger Group of the Exploits subzone, which also contain Laurentian detritus. We infer dominantly east-directed transport of Laurentian detritus from the Taconic collision zone across a postcollisional arc–back-arc complex at ca. 462–455 Ma followed by dominantly west-directed transport of detritus from the Red Indian Lake arc at ca. 455–445 Ma. Our analysis of zircon inheritance from Dashwoods igneous rocks suggests that 1500–900 Ma Laurentian crystalline basement of the Humber margin is an unlikely source of Dashwoods inherited zircon. Instead, a more cosmopolitan Laurentian inheritance may be best explained as sourced from subducted Laurentian sediment. Our results demonstrate that the sampled metasedimentary units from the southern Dashwoods terrane do not correlate with rift-drift strata of the Humber margin as previously proposed, nor with the basement of the Moretown terrane; yet, these Middle to Upper Ordovician successions suggest the potential for an alternative plate-tectonic model in which the Taconic orogeny may have been initiated by collision of Gondwanan arc terranes that closed the main tract of the Iapetus Ocean along the Baie Verte–Brompton Line.

ABSTRACT The Berkshire massif in western Massachusetts is one of several external basement massifs in the New England Appalachians. The Day Mountain thrust is a segment of the western frontal thrust of the Berkshire massif that carried Mesoproterozoic basement gneisses and unconformably overlying cover rocks of the Neoproterozoic (?) Dalton Formation and Cambrian Cheshire Quartzite over the Cambrian to Ordovician Stockbridge Formation. The basal unit of the Dalton Formation is a distinctive deformed quartz-pebble conglomerate. We made 27 strain estimates at 18 locations using the deformed conglomerate to investigate the strain field in the Day Mountain thrust sheet and test the plane-strain model of thrust emplacement. Although the strain ellipsoids vary from prolate to oblate shapes over distances as small as 200 m, and the orientations of the principal directions of strain range widely, a remarkably simple strain pattern, broadly consistent with simple shear, emerges when the strain data are plotted on contoured stereograms. The preferred orientation of the maximum elongation direction plunges gently and approximately coincides with the west-northwest transport direction of the thrust sheet, the preferred orientation of the intermediate principal strain axis is nearly horizontal and perpendicular to the transport direction, and the preferred orientation of the short axis plunges steeply. Most of the strain ellipsoids fall in the prolate field, which is indicative of constrictive flow, especially in the northern part of the thrust sheet. We suggest that the steep gradients in three-dimensional strain type were caused by flow of the more ductile conglomerate over an irregular surface of relatively rigid basement rocks, which were little affected by Paleozoic deformation. The constrictive flow conditions that dominate the strain field in the northern part of the thrust sheet may reflect the irregular paleotopography of the unconformity surface and/or a lateral ramp oriented at an oblique angle to the transport direction that impeded west-northwest–directed thrusting.

Geologic maps and cross-sections effectively summarize the structural geology of a region, but they can be difficult for non-geologists to interpret. Textbooks and interpretive guides commonly integrate maps and cross-sections into static perspective block diagrams to help novices visualize basic concepts in geology. The inherent power of block diagrams, however, is dramatically increased by software such as Google SketchUp, a free downloadable program, which can create interactive 3-D models of a region. The stand-alone models can be Rotated, Panned, and Zoomed by the user and exported for animations. An efficient way to create block diagrams is to combine the individual strengths of dedicated GIS software with SketchUp, and merge the results into a single 3-D model. Effective 3-D block diagrams drape a geologic map on a digital elevation model and show how the map and cross-sections connect at the topographic surface. Creating block diagrams in such a way that portions of the map between cross-section planes are independent segments gives the user flexibility to make portions of the map invisible. By “turning off” parts of the surface, it is possible to sequentially reveal multiple cross-sections. 3-D block diagrams help students and non-specialists visualize geologic structures. Once created, the 3-D block diagrams can be quickly edited by substituting alternate images of geologic maps and cross-sections. Thus they provide an elegant approach for comparing different interpretations of a region. Combined with tools available in SketchUp, they also provide geologists with a valuable resource for assessing the geometric plausibility of geologic cross-sections.

The New England Appalachians contain two north-south–trending sets of gneiss domes. The western belt, which includes the Chester dome, contains 13 domes that expose either 1 Ga Laurentian basement rocks or ca. 475 Ma rocks of the Shelburne Falls arc. The eastern belt contains 21 gneiss domes cored by either 600 Ma crust of possible Gondwanan affinity or ca. 450 Ma rocks of the Bronson Hill arc. Domes in both belts are surrounded by Silurian and Early Devonian metasedimentary rocks, which were deposited in two north-south–trending basins before the Acadian orogeny. The Chester dome in southeastern Vermont, the main focus of this study, is an intensively studied, classic example of a mantled gneiss dome. Lower Paleozoic units around the Chester dome are dramatically thinner than they are elsewhere in southern Vermont, and are locally absent. A strong spatial correlation between the highly attenuated mantling units and highly strained, mylonitic rocks suggests the presence of a ductile, normal-sense shear zone. Garnet-bearing rocks in the core of the dome record metamorphism during decompression of 2–3 kbar, whereas rocks above the high-strain zone were metamorphosed during nearly isobaric conditions. Strain markers and kinematic indicators suggest that extension occurred during northward extrusion of lower- to middle-crustal wedges of Proterozoic and Ordovician quartz-feldspar–rich gneisses below and up into a thick tectonic cover of Silurian mica-rich metasediments that had been transported westward in large-scale nappes. If the ductile, normal-sense shear zone was responsible for synmetamorphic decompression, as we propose, extrusion occurred at ca. 380 Ma.

Tectonic models of the Taconian orogeny in western New England must account for the rapid metamorphism of the Taconic klippen after thrusting. The most likely source of heat for this metamorphism is an overlying hot thrust sheet of accretionary wedge material, which overrode the continental margin of ancient North America, culminating in a continent-island arc collision. Thermal calculations indicate that rapid conductive heat transfer from such a sheet is possible. The dimensionless Peclet number suggests that conductive heat transfer is faster than, or operates at rates comparable to, advective heat transfer due to thrusting over a distance of at least 6 km from a thrust surface. Thus, syntectonic heating of footwall rocks below a major thrust surface is important and must be taken into account in tectonic models. The continental margin thrust system (CMTS) in western New England may have formed as a set of duplexes under a main roof thrust separating the CMTS from the overriding thrust sheet of accretionary wedge material and above a main floor thrust along which the CMTS was transported over autochthonous continental margin rocks. Thrust sheets in this system are composed of Middle Proterozoic Grenville basement and/or upper Proterozoic to Middle Ordovician cover rocks of a western shelf sequence or eastern slope-rise sequence. According to this duplex model, thrust faults tended to develop sequentially toward the foreland, in the transport direction. The relative timing of thrusting and metamorphism is an important constraint on tectonic models, but metamorphism is not a reliable datum with which to compare the timing of events in different parts of the thrust belt. As an example, synmetamorphic thrusting in the eastern internal part of the belt may have preceded brittle faulting to the west near the foreland. P-T paths of rocks from different thrust sheets separated by major faults will be qualitatively different, and detailed petrologic studies to determine and compare P-T paths from different thrust sheets may be useful in identifying faults along which the greatest displacement has occurred.