Taconic Allochthon Research Articles

Arc and slab-failure magmatism of the Taconic Orogeny, western New England, USA

Abstract During the 1970s, geologists considered that the Upper Ordovician Taconic Orogeny represented the collision of Laurentia with the Ammonoosuc arc, now largely exposed on the Bronson Hill anticlinorium. Subsequently, several researchers noted that magmatic rocks which intrude and overlie the Ammonoosuc arc are younger than the c. 455–451 Ma Taconic Orogeny. This led them to hypothesize that a Middle Ordovician collision was followed by westward-dipping subduction beneath the amalgamated Laurentian–Ammonoosuc zone to produce the younger arc rocks. In this model, the Taconic allochthons and foredeep were produced later in a retro-arc setting above westward-dipping subduction. However, those models prove inadequate due to the lack of ash beds, foredeep sedimentation and deformation on the Laurentian platform prior to the Upper Ordovician Taconic Orogeny. Here, we resolve the dilemma by recognizing that the magmatic rocks, which post-date the 455–451 Ma Taconic Orogeny, are not arc rocks but, instead, typical post-collisional slab-failure rocks as old as 450 Ma, with Sr/Y > 10, Sm/Yb > 2.5, Nb/Y > 0.4 and La/Yb > 10. Thus, in New England and western New York, the Upper Ordovician Taconic Orogeny represents the collision of the Ammonoosuc arc with Laurentia followed by slab failure of the descending plate.

Tectonics and foreland basin development at the leading edge of the Humber Arm Allochthon, western Newfoundland, Canadian Appalachians

The thrust front of the Northern Appalachians involves Ediacaran rift-related structures of the Laurentian margin that were re-worked in Taconic (Ordovician), Salinic (mainly Silurian) and Acadian (mainly Devonian) deformation events. Much of the thrust front is concealed under the Gulf of St. Lawrence, but Port au Port Peninsula, in western Newfoundland provides a cross-section through the Laurentian passive margin, overlying foreland basins, and the Humber Arm Allochthon. A large area of foreland basin strata is present in areas formerly mapped as Humber Arm Allochthon. Abrupt thickness changes and an abundance of fault-scarp-derived limestone conglomerate indicate that deposition was influenced by block faulting and differential subsidence in a basin undergoing active flexural extension. A coarsening upward trend in the Goose Tickle Group represents a transition from a distal to a proximal sediment source as the Humber Arm Allochthon was emplaced westwards. Goose Tickle Group separates two packages of allochthonous rocks. The West Bay Thrust Sheet, the lower package, represents the leading edge of the Taconic allochthon. Its timing of emplacement is constrained by Darriwilian 3 graptolites both above and below the thrust sheet. Its present-day configuration results from a combination of processes including thrusting, extension by gravity spreading, gravity gliding, and subsequent erosion of material that was deposited in the Goose Tickle Group. The structurally higher Lourdes Thrust Sheet is an out-of-sequence structure associated with Acadian (Devonian) orogenesis. High-angle faults show a protracted history of movement that includes early Taconic flexural extension, Acadian inversion, and later Carboniferous or Mesozoic strike-slip motion. The leading edge of the Humber Arm Allochthon was influenced by both thin-skinned and thick-skinned tectonics throughout the development of the Appalachian orogen.

Bridging the gap between the foreland and hinterland II: Geochronology and tectonic setting of Ordovician magmatism and basin formation on the Laurentian margin of New England and Newfoundland

Ordovician strata of the Mohawk Valley and Taconic allochthon of New York and the Humber margin of Newfoundland record multiple magmatic and basin-forming episodes associated with the Taconic orogeny. Here we present new U-Pb zircon geochronology and whole rock geochemistry and neodymium isotopes from Early Paleozoic volcanic ashes and siliciclastic units on the northern Appalachian margin of Laurentia. Volcanic ashes in the Table Point Formation of Newfoundland and the Indian River Formation of the Taconic allochthon in New York yield dates between 466.16 ± 0.12 and 464.20 ± 0.13 Ma. Red, bioturbated slate of the Indian River Formation record a shift to more juvenile neodymium isotope values suggesting sedimentary contributions from the Taconic arc-system by 466 Ma. Eight ashes within the Trenton Group in the Mohawk Valley were dated between 452.63 ± 0.06 and 450.68 ± 0.12 Ma. These ashes contain zircon with Late Ordovician magmatic rims and 1.4 to 1.0 Ga xenocrystic cores that were inherited from Grenville basement, suggesting that the parent magmas erupted through the Laurentian margin. The new geochronological and geochemical data are integrated with a subsidence model and data from the hinterland to refine the tectonic model of the Taconic orogeny. Closure of the Iapetus Ocean by 475 Ma via collision of the peri-Gondwanan Moretown terrane with hyperextended distal fragments of the Laurentian margin is not clearly manifested on the autochthon or the Taconic allochthon other than an increase in sediment accumulation. Pro-foreland basins formed during the Middle Ordovician when these terranes were obducted onto the Laurentian margin. 466 to 464 Ma ashes on the Laurentian margin coincide with a late pulse of magmatism in both the Notre Dame arc in Newfoundland and the Shelburne Falls arc of New England that is potentially related to break-off of an east-dipping slab. Following slab reversal, by 455 Ma, the Bronson Hill arc was established on the new composite Laurentian margin. Thus, we conclude that Late Ordovician strata in the Mohawk Valley and Taconic allochthon of New York and on the Humber margin of Newfoundland were deposited in retro-foreland basins.

Bridging the gap between the foreland and hinterland I: Geochronology and plate tectonic geometry of Ordovician magmatism and terrane accretion on the Laurentian margin of New England

U-Pb dates on magmatic and detrital zircon from samples in the hinterland of the Taconic orogen place new constraints on the timing and plate tectonic geometry of terrane accretion and magmatic arc activity. The Moretown terrane, a Gondwanan-derived exotic block, extends from the Rowe Schist-Moretown Formation contact in the west to the Bronson Hill arc in the east. Arc-related plutonic and volcanic rocks formed above an east-dipping subduction zone under the western leading edge of the Moretown terrane from approximately 500 to 475 Ma, until collision with hyperextended distal fragments of Laurentia, represented by the Rowe Schist, at 475 Ma. Magmatic arc rocks formed during this interval are primarily located in the Shelburne Falls arc, although some are also located in the Bronson Hill arc to the east. Metasedimentary rocks in the Shelburne Falls arc contain detrital zircon derived from mixing of Gondwanan, Laurentian, and arc sources, suggesting that the Moretown terrane was proximal to Laurentia by 475 Ma. Explosive eruptions at 466 to 464 Ma preserved in the Barnard Volcanic Member of the Missisquoi Formation in Vermont and as ash beds in the Indian River Formation in the Taconic allochthons may record slab-breakoff of subducted lithosphere following collision of the Moretown terrane with distal Laurentian crustal fragments. Between 466 and 455 Ma a reversal in subduction polarity lead to a west-dipping subduction zone under Laurentia and the newly accreted Moretown terrane. Magmatic arc rocks in the Bronson Hill arc formed above this west-dipping subduction zone along the eastern trailing edge of the Moretown terrane at approximately 455 to 440 Ma. The western boundary of Ganderia in New England is east of the Bronson Hill arc, buried beneath Silurian and Devonian rocks deformed during the Acadian orogeny.

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland, USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (∼1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.

Late‐stage orogenic loading revealed by contact metamorphism in the northern Appalachians, New York

AbstractPelitic schists from contact aureoles surrounding mafic–ultramafic plutons in Westchester County, NY record a high‐P (~0.8 GPa) high‐T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km2) gabbroic body, consists of quartz–plagioclase–biotite–garnet–sillimanite–ilmenite–graphite–Zn‐rich Al‐spinel. Both K‐feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk‐rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620–640 °C and 0.5–0.6 GPa. In the aureole sample, WDS X‐ray chemical maps show distinct Ca‐enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet–biotite Mg–Fe exchange equilibria on inclusions and adjacent garnet host interior to the high‐Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P–T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P–T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P–T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism (c. 435 Ma). In the absence of contact metamorphism‐induced recrystallization, it is likely that this regional‐scale loading would remain cryptic or unrecorded.

Taconic Orogeny in Pennsylvania: A ∼15 –20 m.y. Apennine-style Ordovician event viewed from its Martic hinterland

In Pennsylvania, the Taconic Orogeny lasted from ∼461 to ∼443 Ma as Cambro-Ordovician slope deposits were deformed into mountains edging the Laurentian craton at the same time that materials from an adjacent deep-water basin were being transported ∼50 –70 km across a carbonate platform into foreland basins. This paper focuses on shelf-edge hinterland features, mostly the Martic Zone as a folded, stack of imbricate thrust sheets of slope materials that corresponds to Vermont's Taconic Mountains and Southern Quebec's zone of Taconic allochthons. Work of the last century is summarized, corrected, and combined with a new ∼450 Ma radiometric date and fluid inclusion data from the Pequea Mine within the Martic Zone. These and abundant new graptolite and conodont dates in the foreland paint a revised Pennsylvania picture differing from the northern Taconic areas. Differences are: (1) transport of very large allochthonous masses of deep-water material, the Dauphin Formation, far across the carbonate platform, and (2) deformation migrating progressively across that platform during a ∼15 –20 m.y. period, incorporating it and its foreland cover into alpine-scale, recumbent folds and thrusts. The scenario has many analogies to Italy's modern Apennine Mountains minus the Latian volcanics.

K–Ar age constraints on the evolution of polydeformed fold–thrust belts: The case of the Northern Appalachians (southern Quebec)

Precise geometric and temporal constraints are essential for understanding the evolution of fold–thrust belts. The lack of isotopic dating for shallow faults in the Northern Appalachian Taconic allochthons limits the interpretation of their structural evolution. New and more precise kinematic constraints have been obtained using a structural study that combined field work with the mineralogical, morphological and K–Ar isotopic analysis of clay-rich fractions (<0.2 and 1–2 μm) from fault rocks and host lithologies in selected fault zones within the southern Quebec allochthons. The validated K–Ar data define four age groups at ca. 490, 465–450, 410 and 360 Ma, which are, respectively, referred to as D1 and D2 Taconic deformation pulses, D3 Late Silurian–Early Devonian deformation, and D4–5 Acadian deformation. The recognition of discrete contractional pulses during the Taconic Orogeny suggests that folding and thrusting were initiated during Early Ordovician time (ca. 490 Ma) and culminated with the out-of-sequence imbrication of thrust stacks during Middle (ca. 465 Ma) to Late Ordovician time (ca. 450 Ma). The Silurian–Early Devonian subsidence (ca. 410 Ma) of the Appalachian foreland along wrench and normal fault systems occurred simultaneously with D3 hinterland-directed tectonic transport at the boundary between the external and internal Humber zones. Late Devonian Acadian imbrications (D4) in the Taconic allochthons of the Quebec Appalachians are reported here for the first time. The combination of a D4 contractional pulse and Silurian–Early Devonian D3 back-thrust faulting caused the exhumation of low-grade metamorphic rocks affected by an Early Devonian thermal event. Across the Laurentian margin, the Acadian contraction evolved from Middle Devonian deformation in the internal metamorphic domain to Late Devonian deformation in the allochthons. The relaxation of Acadian tectonic stress is characterized by D5 normal faults and related hydrothermal activity. The recognition of an Acadian contraction event in the southern Quebec Appalachians challenges the traditional view of a gradual NW-directed progression of faulting in the Taconic thrust belt, and allows for a new reconstruction that bridges the Appalachians of mainland Canada and Newfoundland.

Snake Hill — reconstructing eastern Taconic foreland basin litho- and biofacies from a giant mélange block in eastern New York, USA

Exotic lithofacies and faunas have long been known from Snake Hill, eastern New York, USA. The faunally diverse, sandstone-dominated Upper Ordovician succession at Snake Hill sharply contrasts with surrounding tectonized sparsely fossiliferous distal shale. Re-examination of the Snake Hill section shows that it is a storm- and wave-dominated near-shore facies with a benthic fauna analogous to that of the younger Lorraine Group (Ashgillian) of central New York, and to that of the upper Martinsburg Group (upper Caradocian) of eastern Pennsylvania. Orthograptus ruedemanni Chron graptolites indicate that the Snake Hill succession is older than the surrounding tectonized, deep-water shale ( Climacograptus spiniferous Chron). Snake Hill is best interpreted as a parautochthonous block in mélange originally deposited close to the shoreline of the emergent Taconic accretionary prism. Because the Snake Hill succession is sandstone-dominated, it is inappropriate to refer mudstone-dominated facies that underlie the western margin of the Taconic allochthon in the Hudson River valley region to the Snake Hill “Shale,” as has been done in the past. The thick (ca. 150 m), lithologically distinct succession at Snake Hill is therefore referred to as the “Snake Hill Formation.” The Snake Hill Formation is the only known example of proximal, near-shore facies deposited on the western side of the outer Taconic arc, and represents easternmost deposition in the Taconic foreland basin. The Snake Hill Formation is a unique occurrence, and thus is restricted to its type locality at Snake Hill, New York.

Geochemistry of Cambro‐Ordovician Sedimentary Rocks of the Northeastern United States: Changes in Sediment Sources at the Onset of Taconian Orogenesis

Trace element and Nd isotopic compositions of Late Cambrian to Middle Ordovician passive margin metasediments from the northernmost Taconic Allochthon of the northeast United States indicate a provenance consistent with a two‐component mixture of an evolved Grenville source and a rift source generated during the Late Proterozoic opening of the Iapetus Ocean. Initial ϵNd values for these preorogenic deposits range from −11.3 to −14.6 ($$\mathrm{average}\,=-13.0$$ ). Samples of overlying mid‐Ordovician Taconian sediments exhibit initial ϵNd values between −6.1 and −9.8 ($$\mathrm{average}\,=-8.5$$ ). The trace element and Nd isotopic compositions of these synorogenic samples are consistent with a mixture of a less evolved Grenville Province source and recycled preorogenic passive margin deposits. No mantle‐derived (ophiolitic or andesitic) source is unambiguously recognized in the synorogenic samples, consistent with published geochemical studies of other Taconian foreland sequences from this region. Sedimentary rocks from the northern Appalachians record a clear change in provenance at ca. 458 Ma, coincident with the onset of Taconian orogenesis and similar to patterns observed in other sedimentary sequences throughout North America.

Late Shortening and Extensional Structures and Veins in the Western Margin of the Taconic Orogen (New York to Vermont)

Abstract Along the western margin of the Taconic orogen in New York and Vermont, undeformed quartz‐calcite veins commonly occur in the belt of melange that formed beneath the westward‐advancing Taconic Allochthon during the Middle‐Late Ordovician Taconic orogeny. The veins have mineral slickenfibers recording either reverse or normal slip. In New York, the reverse‐motion veins recording the latest phase of shortening are crosscut by the normal‐motion veins and faults. The shortening indicated by the reverse‐motion veins is correlated with the convergence along the Champlain thrust in Vermont, which is also crosscut by a significant, strike‐parallel normal fault. Fluid inclusion data from the veins, complemented by stable isotope data, lead to a reconstruction of the sequence of events in the context of a cooling of the fluids, which is consistent with crosscutting relationships among the veins. Following cessation of the convergence, there was regional extension of the western margin of the Taconic orogen...

Mollusc fauna of the Early Cambrian Bastion Formation of North-East Greenland

A diverse mollusc fauna from the Bastion Formation (Early Cambrian, middle Dyeran Stage) of North-East Greenland includes fifteen species (thirteen helcionelloids and two bivalves), adding considerable detail to the known fossil record of Early Cambrian molluscs from Laurentia. The occurrence of secondarily phosphatized shell surfaces together with phosphatic internal moulds in acid resistant residues allows new morphological details to be observed in several taxa. The fauna shows affinity to contemporaneous faunas from the Taconic allochthon of the eastern United States, but also to mollusc faunas of South Australia, China and Europe. The following new helcionelloid taxa are described: Capitoconus inclinatus n. gen. and n. sp. Capitoconus artus n. sp., Figurina groenlandica n. sp. and Latouchella ostenfeldense n. sp.

Geochemistry of the metamorphosed Ordovician Taconian Magmatic Arc, Bronson Hill anticlinorium, western New England

The Bronson Hill anticlinorium (BHA) in western New England is a north- to northeast-trending belt of gneiss domes containing a metamorphosed stratified sequence of Ordovician, Silurian, and Devonian sedimentary and volcanic rocks. These overlie mostly late Ordovician intrusive igneous rocks. The intrusive rocks, exposed in the dome centers, are dominated by felsic gneisses of tonalitic to granitic compositions. They are the exposed parts of a composite batholith that developed along the axis of the Taconian volcanic arc. The Fourmile Gneiss of the Pelham dome, and the Monson Gneiss of the Monson dome, covered in most detail here, are calc-alkaline, dominantly tonalitic to granodioritic gneisses having 61–78% SiO 2. These are divided into two geochemical groups. The most abundant rocks have low Sr (∼50–200 ppm), high Y (∼10–50 ppm), low Al 2O 3, high FeO t, commonly negative Eu anomalies, and have flat to somewhat concave upward MREE and HREE patterns. These rocks were probably derived from melting of plagioclase–pyroxene–amphibole granulites in the deep crust. Rocks of the subordinate high-Sr group are similar in composition to adakites and have high Sr (∼300–600 ppm), low Y (∼1–13 ppm), higher Al 2O 3, lower FeO t, commonly no Eu anomalies, and are strongly depleted in HREE. These rocks were probably derived from a pyroxene–amphibole–garnet ± plagioclase source at higher pressure. Amphibolites are low in abundance in the Fourmile and Monson Gneisses. Most are a low-Nb type (<6 ppm Nb), compositionally similar to typical calc-alkaline island arc basalts. These were probably derived by melting of spinel lherzolite in the mantle wedge under the arc. Gabbroic anorthosite and ultramafic rocks also occur in the Monson dome and appear to be cumulates from magmas similar to the low-Nb amphibolites. A high-Nb (6–12 ppm) amphibolite group was identified and is unique to the Monson Gneiss. These rocks are transitional between calc-alkaline and alkaline basalts, are strongly LREE-enriched (La is 46–440 times chondrite), are enriched in strongly incompatible elements, and two of seven samples have normative nepheline. A broader survey of available analytical data from BHA felsic gneisses shows that gneiss compositions vary from dominantly tonalitic and granodioritic in the southern BHA to dominantly granitic from the Croydon and Mascoma domes (west-central New Hampshire) northward. This variation suggests that the southern part of the arc exposed in the BHA was based on mafic crust, whereas the northern part was based on intermediate to felsic crust. Examination of radiometric dates of Taconian igneous rocks, and biostratigraphic constraints in the Taconic allochthons and in the Taconian foreland, are consistent with a model in which the BHA gneisses are the youngest igneous components of the Taconian arc proper. Collision of this composite arc with Laurentia reached a conclusion in the latest Caradoc or Ashgill of the late Ordovician or in the very earliest Silurian. Younger plutons continuing into the Silurian (e.g., Highlandcroft in part) may be related to magmas generated during post-collision delamination or detachment of the subducted Iapetus oceanic slab.

Dating cleavage formation in slates and phyllites with the 40Ar/39Ar laser microprobe: an example from the western New England Appalachians, USA

Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high‐spatial‐resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.

Albite porphyroblasts with sigmoidal inclusion trails and their kinematic implications: an example from the Taconic Allochthon, west-central Vermont

Microstructural analyses of albite porphyroblasts in phyllite of the Taconic Allochthon were conducted in order to determine the origin of the sigmoidal inclusion trails within the porphyroblasts and to evaluate if and when the porphyroblasts rotated during the deformation history. Three cleavages ( S 2, S 3, and S 4) with similar strike are present in the phyllite. The albite porphyroblasts are characterized by inclusion-rich cores and inclusion-free rims, and the boundary between the cores and overgrowths is characterized by relatively straight and parallel zones of concentrated rutile. The sigmoidal inclusion trails within the cores are inferred to be trapped microfolds. The zones of concentrated rutile are interpreted to be truncation planes that formed during the development of S 3. In contrast to the tight orientation distribution of S 3, the truncation planes show a wide distribution pattern. The mean orientations of S 3 and the truncation planes also differ. These orientation data indicate that the porphyroblasts have not only rotated with respect to S 3 but also to the geographic reference frame. In addition, the growth of the porphyroblasts was influenced by differentiation associated with S 3 crenulation cleavage formation; the porphyroblasts grew preferentially within microlithon domains where chlorite, a reactant mineral for the porphyroblasts, is abundant.

U-Pb zircon age of metafelsite from the Pinney Hollow Formation; implications for the development of the Vermont Appalachians

The Pinney Hollow Formation of central Vermont is part of a rift-clastic to drift-stage sequence of cover rocks deposited on the Laurentian margin during the development of the Iapetan passive margin in Late Protero- zoic to Cambrian time. Conventional U-Pb zircon data indicate an age of 571 5 Ma for a metafelsite from the Pinney Hollow Formation. Geochemical data indicate that the protolith for the metafelsite, now a quartz-albite gneiss or granofels, was rhyolite from a source that was transitional between a within-plate granite and ocean-ridge granite setting and probably came through partially distended continental crust. The transitional setting is consistent with previous data from metabasalts in the Pinney Hollow Formation and supports the idea that the source magma came through continental crust on the rifted margin of the Laurentian craton. The 571 5 Ma age provides the first geochronologic age from the rift-clastic cover sequence in New England and establishes a Late Proterozoic age for the Pinney Hollow Formation. The Late Proterozoic age of the Pinney Hollow confirms the presence of a significant mapped thrust fault between the autochthonous and para-autochthonous rocks of the cover sequence. These findings support the interpretation that the Taconic root zone is located in the hinterland of the Vermont Appalachians on the eastern side of the Green Mountain massif. INTRODUCTION The Pinney Hollow Formation of central Vermont is part of a para-autochthonous eugeoclinal rift-clastic to drift-stage sequence deposited on the Laurentian margin during the development of the Iapetan passive margin in Late Proterozoic to Cambrian time. The Pinney Hollow Formation consists of metamorphosed pelitic and psammitic sedimentary rocks and lesser amounts of predominantly mafic volcanic rocks exposed east of Middle Proterozoic basement gneiss and schist in the Green Mountains of Vermont (fig. 1). Prior to plate tectonic models, the cover rocks were considered to be an eastward-dipping and -younging homoclinal sequence of coherent stratigraphy (Doll and others, 1961). In the past 15 years, many faults have been identified in the ''eastern cover sequence'' throughout Vermont. As a result, the Pinney Hollow Formation is now considered part of a fault-bounded lithotectonic belt with its own internal stratigraphy (Walsh and Ratcliffe, 1994a, b; Walsh and Falta, 1996a, b, and in press; and Ratcliffe, Walsh, and Aleinikoff, 1997). Similarities between the metasedimentary rocks of the eastern cover sequence and metasedimentary rocks in the lower parts of the Taconic allochthons have long been recognized. These lithic similarities led early workers to suggest that the source of the Taconic slices was in the eastern cover sequence on the east side of the Green Mountain massif in Vermont and the Berkshire massif in Massachu- setts (Keith, 1932; Prindle and Knopf, 1932; Osberg, 1952). More recently, Stanley and Ratcliffe (1985) postulated that the Taconic root zone was exposed at the western edge of the Pinney Hollow, Underhill, and Hazens Notch Formations at the latitude of the northern end of the Taconic allochthons. Stanley and Ratcliffe (1985) assigned these three formations to lithotectonic belts called the Pinney Hollow and Underhill slices and interpreted the western limit of these slices as the root zone for the Taconic allochthons. The geochemistry of the mafic volcanic rocks in the eastern cover sequence, from alkalic to transitional within-plate basalts in the west to MORB-like basalts in the east, has been linked to the relative original geometry of the ancient margin. In this model, west to east

Tectonic and regional metamorphic implications of the discovery of Middle Ordovician conodonts in cover rocks east of the Green Mountain massif, Vermont

Middle Ordovician (late Arenigian - early Caradocian) conodonts were recovered from a dolostone lens in carbonaceous schist 30 m below the base of the Pinney Hollow Formation in the Eastern Cover sequence near West Bridgewater, Vermont. These are the first reported fossils from the metamorphic cover sequence rocks east of the Green Mountain, Berkshire, and Housatonic massifs of western New England. The conodonts are recrystallized, coated with graphitic matter, thermally altered to a color alteration index (CAI) of at least 5, and tectonically deformed. The faunule is nearly monospecific, consisting of abundant Periodon aculeatus Hadding? and rare Protopanderodus. The preponderance of Periodon and the absence of warm, shallow-water species characteristic of the North American Midcontinent Conodont Province suggest a slope or basin depositional setting. The conodont-bearing carbonaceous schist is traceable 3 km southeast to the Plymouth area, where it had been designated the uppermost member of the Plymouth Formation, previously regarded as Early Cambrian in age. The age and structural position of the carbonaceous schist above dolostones of the Plymouth Formation but below the Pinney Hollow Formation (upper Proterozoic and Lower Cambrian?) suggest that this unit may be correlative or time transgressive with the Ira Formation, which underlies the Taconic allochthons in the Vermont Valley. Such a correlation supports the concept of placing the western limit of the root zone of the Taconic allochthons beneath the Pinney Hollow Formation. An approximate absolute age assignment for the conodont-bearing rock is between 470 and 454 Ma. This suggests that dynamothermal metamorphism during the Taconian orogeny on the east flank of the Green Mountains was younger than early Caradocian, which is in accord with the middle Caradocian age of the Ira Formation west of the Green Mountain massif.

Foreland crustal structure of the New York recess, northeastern United States

A new structural model for the northeast part of the Central Appalachian foreland and fold-and-thrust belt is based on detailed field mapping, geophysical data, and balanced cross-section analysis. The model demonstrates that the region contains a multiply deformed, parautochthonous fold-and-thrust system of Paleozoic age. Our interpretations differ from previous ones in which the entire region north of the Newark basin was considered to be allochthonous. The new interpretation requires a substantial decrease in Paleozoic tectonic shortening northeastward from adjacent parts of the Central Appalachian foreland and illustrates the common occurrence of backthrusting within the region. During early Paleozoic time northern New Jersey consisted of a Taconic orogenic foreland in which cover folds (F1) involved lower Paleozoic carbonate and flysch overlying Middle Proterozoic basement. F1 folds are open and upright in the foreland and more gently inclined to recumbent southeastward toward the trace of the Taconic allochthons. F1 structures were cut and transported by a fold-and-thrust system of the Alleghany orogeny. This thrust system mostly involves synthetic faults originating from a master decollement rooted in Proterozoic basement. Antithetic faults locally modify early synthetic overthrusts and S1 cleavage in lower Paleozoic cover and show out-of-sequence structural development. The synthetic parts of the regional thrust system are bounded in the northwestern foreland by blind antithetic faults interpreted from seismic-reflection data. This antithetic faulting probably represents Paleozoic reactivation of Late Proterozoic basement faults. Tectonic contraction in overlying cover occurred by wedge faulting where synthetic and antithetic components of the foreland fault system overlap. S2 cleavage in the Paleozoic cover stems from Alleghanian shortening and flattening and commonly occurs in the footwall of large overthrust sheets. Paleozoic structures in Proterozoic basement include fault blocks bounded by high-angle faults and low- to moderate-angle shear zones that locally produce overlying cover folds. Broad and open folds in basement probably reflect shear-zone displacement of subhorizontal foliation. Our cross-section interpretations require limited involvement of lower Paleozoic cover folds in the footwalls of major overthrust faults. Palinspastic restoration of F1 folds produces an arched passive-margin sequence. The tectonic contraction for the Valley and Ridge province and southeastern Pocono Plateau is about 25 km, and tectonic wedge angles are 8°–11°.

Oldest shelly fossils from the Taconic Allochthon and late Early Cambrian sea-levels in eastern Laurentia

Continental slope facies in eastern New York have the most diverse small shelly faunas known in Laurentian late Early Cambrian strata. Diversity of a lowerElliptocephala asaphoidesassemblage, the oldest body fauna in the Taconic allochthon, reflects turbidity current exhumation, transport, and deposition of phosphatized fossil hash in a proximal facies of the Browns Pond Formation. Phosphatization is linked to the first of twoOlenellusChron intervals with a thickened dysaerobic water mass on the slope. This interval of increased rate of sea-level rise corresponds to development of extensive carbonate belts and onlap of dark shales (e.g., Forteau and lower Kinzers formations) on the east Laurentian shelf.The assemblage includes two trilobites, archaeocyathan fragments, echinoderm debris, the first edrioasteriod from east Laurentia, and 26 small shelly fossil species. New taxa areAsperconellanew genus andStenotheca taconicanew species (helcionellids);Mackinnonia obliquanew species (pararostroconch);Conotheca laurentiensisnew species,Insolitothecanew genus, andPetasotheca minutanew genus and species (hyoliths); andPlinthokonion?psamminonnew species (agglutinated problematicum).

Finite strain heterogeneity and volume loss in slates of the Taconic Allochthon, Vermont, U.S.A.

Slates of the Taconic Allochthon have ellipsoidal reduction spots which have been used to determine the finite state of strain at seven sites. The strain is highly heterogeneous at all scales, ranging from several centimeters to many kilometers. Variations in X Y ratios is small but variations in Y Z ratios is large and this pattern is seen at all scales. The heterogeneity can be used to determine a strain path, which is nearly horizontal on a Flinn diagram. We propose that uniaxial flattening associated with late stage cleavage development was accommodated by volume loss, and that the most highly strained sites have experienced an average 55% volume loss, at a minimum. Explaining our results without volume loss requires a subhorizontal extension and creates strain compatibility problems. Consideration of the strain history of these rocks suggests that folding-related strains have been modified by later cleavage-related events, the last of which was uniaxial flattening accommodated by volume loss. The finite strains are reflective almost exclusively of the latest, cleavage producing strains.