ABSTRACT The Appalachian Mountains in northern Vermont host a complex rock record of the tectonic evolution of eastern Laurentia, from the opening of the Iapetus Ocean to the subsequent formation of a convergent Paleozoic margin involving multiple phases of orogenesis. Prior 40 Ar/ 39 Ar studies in Vermont and northern Massachusetts have generally interpreted two major events associated with a dominantly Ordovician Taconic orogeny and a Devonian Acadian orogeny; intermediate ages were considered to reflect Taconic metamorphism and/or deformation that was “partially reset” during the Acadian orogeny. However, recent studies have documented Salinic ages in northern Vermont, aligning with multiple lines of evidence in southern Quebec for an intervening Salinic orogeny during the Silurian. This study reports integrated microstructural and 40 Ar/ 39 Ar geochronological analyses of samples collected across the Green Mountain anticlinorium in northern Vermont. The dominant S 2 and S 3 foliations are defined in thin section by predominantly white mica/quartz microlithons and aligned mica cleavage domains in schist to graphitic schist that formed under greenschist-facies conditions. Correlation of microstructures across the field area and associated 40 Ar/ 39 Ar plateau ages reveal a spatial pattern associated with microstructural development across the anticlinorium. In the eastern limb, the oldest plateau age, 457.6 ± 2.0 Ma (1σ), is interpreted to reflect the timing of formation of S 2 . The youngest plateau age, 419.0 ± 2.4 Ma, comes from the western limb of the anticline near the trace of the Honey Hollow fault, where S 2 is completely transposed by S 3 . Intermediate ages were obtained across the axis of the anticline, where S 3 is a crenulation cleavage. While the Green Mountain anticlinorium has been previously interpreted to have formed in the Devonian during the Acadian orogeny, the typical ca. 386–355 Ma ages are notably absent in the data set, except in locally disturbed spectra. The results of this work are closely aligned with published results of 40 Ar/ 39 Ar dating in southern Quebec that reflect deformation during Taconic and Salinic orogenesis. These new data, together with recently reported ages of west-directed transport on Taconic thrusts along the western Green Mountain front at ca. 420 Ma, suggest a phase of mountain building in the New England Appalachians that has been previously unreported in Vermont. The formation of the Green Mountain anticlinorium coincided with a complex tectonic interval that overlapped temporally with (1) the transition from Salinic thrusting to normal faulting, (2) magmatism attributed to slab breakoff, and (3) syntectonic deposition in the Connecticut Valley–Gaspé Basin.

ABSTRACT The Avalon terrane of southeastern New England is a composite terrane in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia, and New Brunswick, Canada, based on rock types and ages, U-Pb detrital zircon signatures of metasedimentary rocks, and Sm-Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ca. 647–643 and ca. 745–733 Ma age populations and minor zircon up to ca. 3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane, or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, Massachusetts, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, Massachusetts, and as far west as Warwick, Rhode Island, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or it may have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone.

ABSTRACT Southeastern New England is largely composed of Ediacaran granitoid and related volcanic rocks formed during the main phase of arc-related magmatism recorded in West Avalonian lithotectonic assemblages extending through Atlantic Canada to eastern Newfoundland. In situ Lu-Hf analyses presented here for zircons from the Dedham, Milford, and Esmond Granites and from the Lynn-Mattapan volcanic complex show a restricted range of εHf values (+2 to +5) and associated Hf- T DM model ages of 1.3–0.9 Ga, assuming felsic crustal sources. The most evolved granites within this suite lie in a belt north and west of the Boston Basin, whereas upfaulted granites on the south, as well as the slightly younger volcanic units, show more juvenile Hf isotopic compositions. Similar inferences have been drawn from previously published Sm-Nd isotopic signatures for several of the same plutons. Collectively, the isotopic compositions and high-precision U-Pb geochronological constraints now available for southeastern New England differ in important respects from patterns in the Mira terrane of Cape Breton Island or the Newfoundland Avalon zone, but they closely resemble those documented in the Cobequid and Antigonish Highlands of mainland Nova Scotia and New Brunswick’s Caledonia terrane. Particularly significant features are similarities between the younger than 912 Ma Westboro Formation in New England and the younger than 945 Ma Gamble Brook Formation in the Cobequid Highlands, both of which yield detrital zircon age spectra consistent with sources on the Timanide margin of Baltica. This relationship provides the starting point for a recent model in which episodic West Avalonian arc magmatism began along the Tonian margin of Baltica and terminated during diachronous late Ediacaran arc-arc collision with the Ganderian margin of Gondwana.

ABSTRACT Dinosaur tracks have been illustrated since they were first found. The earliest illustrations depicted dinosaur tracks as the work of mythical beings. With the advent of scientific inquiry into dinosaur tracks in the nineteenth century, natural explanations were sought for the fossil tracks. Illustrations of the period were relatively realistic but were influenced by then-current beliefs and were constrained by the artists’ skills and by what scientists considered salient. In the mid-nineteenth century, the first photographs were used for the scientific study of fossil tracks. Photography eliminated some limitations of artistic talent and showed complete specimens, not just aspects that were deemed salient. The ability to compare and name similar tracks from disparate authors and places became easier. Advances in photography, laser scanning, optical scanning and lidar, and the ability to manipulate images with computers, have enabled the modern synthesis of illustrating dinosaur tracks, which combines many types of images. With each advance and the adoption of newer technologies, the older methods have not been retired. Rather, we have continued to see new uses for old methods and an integration of illustrative styles. For Patrick. Your friendship and your vision will be so deeply missed.

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.