England Appalachians Research Articles

The Piermont allochthon revisited and redefined at its type locality: Discussion

[Timms (2004)][1] has presented a very restricted interpretation of an important structural feature in the New England Appalachians that I originally termed the Piermont allochthon and interpreted as a far-traveled Acadian thrust sheet ([Moench et al., 1987][2]; [Moench, 1990][3]). As described

Paleostress Analysis of Atlantic Crustal Extension in the Quebec Appalachians

Abstract A paleostress analysis using normal fault slip data and Jurassic dike trends has been conducted in the Quebec and New England Appalachians. Stress tensor analysis of fault slip by a numerical inversion method has distinguished homogeneous and directionally consistent E‐W‐ and NW‐SE‐oriented extensional stress regimes in the Quebec Appalachians and St. Lawrence Lowlands. The directions of extension compare well with the mean direction of extensional stress inferred from the Jurassic dolerite dike trend in New England. Both extensions are interpreted as the result of rift‐related stress associated with the separation of North America and Africa, early in the Late Triassic–Early Jurassic fragmentation of Pangea. The paleostress field configuration is assumed to be related to the direction of divergent plate motion. The N‐S and NE‐SW Late Proterozoic faults that bordered the Quebec Reentrant and Taconian‐Acadian structures of the Quebec Appalachians appear to have played a significant role in the dis...

Constraining the relative strengths of high-grade metamorphic rocks using foliation refraction angles: an example from the Northern New England Appalachians

Foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite-facies metaturbidites in the Presidential Range of the White Mountains, in eastern New Hampshire. An early-formed foliation, developed during km-scale nappe folding, consistently displays larger bedding-foliation angles in metapelitic units than metapsammitic units, suggesting that the metapelitic units had higher effective viscosities during this deformation. We collected bedding-foliation angle measurements from a combination of outcrops and locally-derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades. Our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers, which we attribute to the presence of large (up to 15 cm long), abundant (up to 30 vol.%) effectively rigid andalusite porphyroblasts in the metapelitic layers. We present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types, address the practical limitations we encountered, and suggest this method is an easy way of estimating the strength contrast between different rock types. Finally, we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis.

Lower to Middle Ordovician evolution of peri-Laurentian arc and backarc complexes in Iapetus: Constraints from the Annieopsquotch accretionary tract, central Newfoundland

The Annieopsquotch accretionary tract in Newfoundland is composed of a series of west-dipping structural panels, each containing remnants of ophiolitic and arc-backarc complexes of Laurentian affinity formed during the Ordovician closure of Iapetus. Panels were transferred from an upper-plate to a lower-plate setting during their Middle to Late Ordovician accretion to the Laurentian margin and become progressively younger eastward. Geochronological data indicate a complex and rapid history of generation and accretion of peri-Laurentian suprasubduction zone rocks. The rapid changes in tectonic environments and the complexity of the relationships are analogous to the complex arc-backarc relationships observed in the western Pacific today. The recognition of the peri-Laurentian provenance of these units based on stratigraphy, geochronology, isotopes, and geochemistry defines the position of the Red Indian Line, the fundamental suture zone in the northern Appalachians, but more importantly enables the development of a realistic tectonic model for the Annieopsquotch accretionary tract involving both thrust and sinistral transcurrent displacements. The oldest and most inboard unit in the Annieopsquotch accretionary tract is the Annieopsquotch ophiolite belt (ca. 480 Ma), which marks the initiation of subduction outboard of the Laurentian margin. The Lloyds River ophiolite complex (ca. 473 Ma) preserves a fragment of younger, more mid-ocean-ridge–like backarc-oceanic crust than the adjacent, structurally overlying Annieopsquotch ophiolite belt. The Lloyds River ophiolite complex originated as a backarc to the Buchans Group (ca. 473 Ma) ensialic bimodal calc-alkaline arc. The panels containing the Annieopsquotch ophiolite belt and Lloyds River ophiolite complex were stitched and overlain by ensialic arc rocks of the Otter Pond Complex (ca. 468 Ma) immediately after their accretion to composite Laurentia together with the structurally underlying Buchans Group. The youngest, structurally lowest two panels comprise the elements of the Red Indian Lake group (465–460 Ma), which record the opening of a backarc basin and the subsequent establishment of a bimodal ensialic calc-alkaline arc sequence. The observed relationships indicate that the Annieopsquotch accretionary tract was generated above a single west-dipping subduction zone outboard of the Laurentian margin over ∼20 m.y. Accretion mainly took place in two stages at ca. 468 and 450 Ma, which correspond with the collision between Laurentia and the Dashwoods ribbon continent and the collision with the peri-Gondwanan Victoria Arc along the Red Indian Line, respectively. Both collisions form part of the Taconic orogeny. The latter, Late Ordovician collision terminated the relatively rapid closure of the main Iapetan tract. The proposed model is similar to the correlative tracts in the British and Irish Caledonides, and may encourage a new look at the New England Appalachians.

On the development of gneiss domes

The New England Appalachians contain some of the first documented gneiss domes. The classic domes of southeast Vermont are typical of these structures in that they appear to have formed by doming of both the gneissosity in basement gneisses and the dominant matrix schistosity in the overlying rocks, after these foliations had formed. However, the three matrix foliations (other than bedding) present in these rocks, which include the one parallel to all compositional layering, post-date dome development. The domes formed as upright anticlines after deposition of the overlying Lower Palaeozoic sediments and volcanics on a Precambrian granitic or gneissic basement. This occurred during orogenesis driven by horizontally directed compressional forces with interspersed periods of relatively coaxial gravitational collapse, rather than through early nappe development followed by folding or diapiric granite emplacement, or crustal extension and upwards bowing of a necked portion of the crust. In spite of numerous subsequent deformations, the domes are very old structures that have been little modified at the level of current crustal exposure. The dominant process during each deformation was reactivation of the compositional layering or gneissosity on at least one limb of the dome. This destroyed earlier developed crenulation cleavages that lay oblique to the compositional layering and commonly prevented new ones from developing. Consequently, a schistosity parallel to bedding in the overlying rocks, and the gneissosity below, formed and was intensified by reactivation on at least one limb of the dome during all subsequent deformations. Nappe development, which has commonly been proposed as the origin of foliation parallel to bedding in the rocks overlying gneiss domes, is not necessary and there is no evidence for such a deformation history preserved within either the bedding or matrix foliations that have been folded around the Chester and Athens Domes of southeast Vermont. Subhorizontally dipping foliations that formed during collapse stages of orogenesis have mainly been the product of relatively coaxial deformation rather than of the highly non-coaxial deformation that accompanies nappe development. The fold to the west of the Chester and Athens Domes, which has classically been regarded as the nose of the major nappe, formed with an upright rather than shallowly dipping axial plane.

The Piermont allochthon revisited and redefined at its type locality

Regional lithostratigraphic and structural mapping of the Piermont allochthon of northwestern New England at its type locality indicates that the Foster Hill fault, the supposed sole thrust, is a stratigraphic boundary. Rather, the allochthon lies above the Bean Brook fault, covers a much smaller area than suggested previously, and is limited to the vicinity of Piermont, western New Hampshire. The allochthon, which primarily consists of rocks correlated with the Silurian age Rangeley Formation of Maine, has horizons of metamorphosed polymictic conglomerate at its base. Rocks directly below the allochthon are correlated with the metasedimentary, Ordovician-aged Albee Formation and contain abundant metamorphosed dykes and sills that do not cross the sole detachment of the allochthon. Analysis of macro- and mesoscale structures indicates local preservation of fold hinges and overprinting foliations oblique to and later than those previously described and has allowed the deformation history to be significantly extended by several events. Emplacement of the allochthon early in the deformation history at metamorphic conditions caused NW-SE elongation lineations of clasts in conglomerate at the base of the allochthon. The relict foliations and fold hinges are commonly preserved below the allochthon, suggesting that these rocks have had a more protracted deformation history relative to rocks in the allochthon. The new interpretation of the Piermont area provides a framework to build a more thorough tectonic history for this part of the New England Appalachians.

Tectonic evolution and significance of Silurian – Early Devonian hinterland-directed deformation in the internal Humber zone of the southern Quebec Appalachians

In the southern Quebec Appalachians, the early tectonic history of the Laurentian margin (Humber zone) comprises foreland-propagating, northwest-directed thrust faulting, nappe emplacement, and regional prograde metamorphism in response to the obduction of large ophiolitic nappes during the Taconian orogeny. In the internal Humber zone, this event is dated at 462 ± 3 Ma (late Middle Ordovician), which is interpreted to represent the timing of near-peak Taconian metamorphism. Superimposed hinterland-directed structures are accompanied by retrograde metamorphism and consist of back thrusts and normal faults, which respectively delimit the northwestern and southeastern limbs of the Sutton and Notre-Dame mountains anticlinoria, both salient structures of the internal Humber zone of southern Quebec. Geochronologic data on the timing of hinterland-directed deformation vary from 431 to 411 Ma. Two tectonic models are presented and discussed, which may account for the Silurian – Early Devonian evolution of the Laurentian margin: (1) back thrusting and syn- to post-compressional crustal extension in response to the tectonic wedging of basement-cored duplexes inducing delamination of supracrustal rocks; (2) tectonic exhumation of the internal Humber zone by extensional collapse. Evidence for Silurian – Early Devonian extensional tectonism in the Humber zone provides the basement infrastructures necessary for the creation and the onset of sedimentation in the Gaspé Belt basins (e.g., Connecticut Valley – Gaspé synclinorium). Several structural, metamorphic features in the internal Humber zone of the northwestern New England Appalachians yield analogous characteristics with those of southern Quebec and may have shared a similar Silurian – Early Devonian tectonic evolution.

Geochemistry and tectonic setting of metabasic rocks of the Gneiss Dome Belt, SW New England Appalachians

Acadian structural domes in the Connecticut Valley Zone in western Connecticut expose polydeformed, amphibolite and upper amphibolite facies pre-Silurian rocks correlative with the Notre Dame subzone (western Dunnage zone) of Williams et al. [Geologic Survey of Canada Paper 88-1B, 1988, pp. 91–98] of the Canadian Appalachians. We have analyzed the geochemistry of 99 metabasic rocks from the Cambrian-Lower Ordovician Rowe Schist (RS), the Arenig or older Taine Mountain Formation (TMF), and the overlying Arenig Collinsville Formation (CF) to identify the tectonic setting of mafic volcanism in the Gneiss Dome Belt (GDB) in western Connecticut and southwestern Massachusetts. The metabasites record epidote-amphibolite to lower amphibolite facies metamorphism and exhibit equilibrium microstructures and only limited retrogression. We used our data to test whether the GDB represents part of a Taconic volcanic arc in southwestern New England. RS and TMF metabasites that can be interpreted as dikes range in composition from subalkaline basalt to basaltic andesite and exhibit overall arc-like signatures. Their REE patterns at 10–25× chondrite are mildly LREE enriched with (La/Yb) N=1.50–1.94. Enriched HFSE from Th to Ce and lack of negative Ta–Nb anomalies indicate that the RS and TMF metabasites experienced some degree of crustal contamination. The majority of CF metabasites range from basaltic andesite to subalkaline basalt. The CF metabasites are geochemically diverse and have a wide range of Ti/V ratios characteristic of boninites and low-Ti arc tholeiites, arc tholeiites, ocean floor basalts, and ocean island and alkali basalts. Based on their geochemical characteristics, CF metabasites have been subdivided into three major types: (1) boninitic and low-Ti IAT metabasites (20% of the analyzed samples), (2) arc-like metabasites (27%), and (3) MORB-like and MORB/WPB transitional metabasites (53%). Boninitic CF metabasites are characterized by extremely depleted HFSE, enriched LILE, and distinctive U-shaped REE patterns below the 10× chondrite level. Arc-like CF metabasites have moderately depleted HFSE from P to Yb and relatively enriched LILE; REE patterns at 10–25× chondrite are mostly mildly LREE enriched with (La/Yb) N=1.68–2.12. HFSE from Th to Ce are relatively enriched indicating a small degree of crustal contamination in these metabasites. MORB-like CF metabasites display signatures characteristic of BABB; they have flat, weakly enriched HFSE and moderately enriched LILE typical of evolved MORBs. REE patterns at 15–35× chondrite are flat to slightly LREE enriched with (La/Yb) N=1.08–1.49. On tectonic discriminant diagrams, the majority of MORB-like CF metabasites define MORB-WPB trend. Four samples with the most elevated REE patterns at 20–100× chondrite and moderately LREE enriched have a transitional MORB/WPB character. The tectonic position and geochemical signatures of the RS and TMF dikes suggest that they represent extension-related magmatism, either in an arc or forearc setting, prior to the establishment of Arenig- to Caradoc-aged magmatism in this part of western New England. The coexistence of boninitic, arc-like, and MORB-like compositions is consistent with a backarc setting as the eruptive environment of the CF metabasites in the GDB in western Connecticut and southwestern Massachusetts as predicted by Crawford et al.'s [Earth, and Planetary Science Letters 54, 1981, 346–356] model of boninite generation. Whether the initial rifting and opening of this backarc basin occurred in the forearc or as intra-arc splitting remains unclear. MORB-like compositions among the CF metabasites are well represented and denote a more evolved stage of this backarc basin development. Boninitic and arc-like CF metabasites conclusively prove a supra-subduction zone setting, but the polarity of this subduction remains equivocal. Our results indicate that Taconic accretion in western New England Appalachians was more complex than previously proposed and likely involved forearc, arc, and backarc complexes.

RELATIONSHIPS OF COTICULE GEOCHEMISTRY TO STRATIGRAPHY IN THE PERRY MOUNTAIN AND MEGUNTICOOK FORMATIONS, NEW ENGLAND APPALACHIANS

Coticules (garnet-rich quartzites) are chemically distinctive lithologies of controversial origin. They generally occur in pelitic schists, amphibolites, cherts, and quartzites as thin layers or discontinuous lenses in Paleozoic formations in the New England Appalachians and other orogenic belts. Geochemical data were collected to characterize coticules and their host rocks from the Middle Silurian Perry Mountain Formation of southern New Hampshire (near Rochester) and western Maine (near Rangeley), and the Cambrian Megunticook Formation of coastal Maine (Camden and Calais). Relative to their host rocks, the coticules are enriched in Fe, Mn, and P, and depleted in Ti and alkalis. Electron-microprobe analyses of garnet show higher spessartine and lower almandine components relative to garnet in the host. Coticules are light- REE -enriched and heavy- REE -depleted. They have concentrations between 10 and 100 times those of chondrites, generally lack a Ce anomaly, and have a negative Eu anomaly. Major-element compositions and REE profiles argue strongly against a hydrothermal origin for these rocks, and do not obviously support a hydrogenous (diagenetic) origin such as Mn–Fe formation. Rare-earth-element patterns differ as much within a local area as for widely separated samples within a single formation. All three samples from the Megunticook Formation are nearly identical, supporting stratigraphic correlation. But they are also like some of the Perry Mountain Formation samples, so that coticule compositions for a formation are not unique.

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.

Geochemistry of Early Devonian calc-alkaline plutons in the Merrimack Belt: implications for mid-Paleozoic terrane relationships in the New England Appalachians

A series of northeast-trending plutons extending from northeastern Massachusetts to southeastern Maine intruded the metasedimenlary rocks of the Merrimack belt. The Early Devonian Dracut, Sweepstakes, Island Pond. Exeter, and Webhannet plutons are metaluminous and, with the exception of the granitie Webhannet pluton, are dominantly mafic to intermediate in composition. The plutons are calc-alkaline in character and have major, minor, and trace clement compositions typical of magmas generated at destructive plate margins. These characteristics include mid- to high-K contents, enrichment of LII.E, LREE, Ba. and Sr. and negative Nb and Ta anomalies.
 
 Whole-rock chemical data indicate that the plutons of the Merrimack belt arc similar in every measured geochemical parameter to ca. 400 Ma mafic to intermediate rocks of the New Hampshire Plutonic Suite. These similarities suggest that both groups of plutons were emplaced within the same magmatic arc and belong to the same magmatic suite. A magmatic suite common to both the Merrimack belt and Central Maine terrane suggests that the two lithotectonic zones were proximal to each other at ca. 400 Ma. Trace element differences between Merrimack belt and Sharpners Pond rocks suggest (hat the Putnam-Nashoba terrane represents a separate arc.
 
 Located in the Central Maine terrane of New Hampshire, the Rochester pluton is geochemically distinct from the Siluro-Devonian plutons of the Merrimack belt and New Hampshire Plutonic Suite. The Rochester pluton has alkaline affinities but retains overall calc-alkaline features. High concentrations of incompatible elements (K, Ti, P, Ba, Rb, Zr) in the Rochester pluton are markedly similar to those observed in the 360 Ma Hardwick Tonalite of Massachusetts. The strong geochemical correlation between the Rochester and Hardwick plutons implies derivation from the same magmatic event and a common, Late Devonian origin. We suggest that these plutons may have originated in response to pull-apart rifting related to late-stage Acadian transpression.
 
 RÉSUMÉ
 Une série de plutons orientée vers Ie nord-est s'élendant du nord-est du Massachusetts au sud-est du Maine, fait intrusion dans les roches métasédimentaires de la ceinture de Merrimack. Les plutons du Devonien inférieur Dracut Sweepstakes. Island Pond. Exeter et Webhannet constituent des intrusions métalumincuses et, mis à part le pluton granitique Webhannet, ils ont une composition en prédominance mafique à intermédiare. Les plutons ont une conformation calco-alcalinc et leur composition du point de vue des principaux éléments présents, de ceux présents en quantité restreinie et des éléments traces est représentative des magmas produits aux frontièrs de plaques destructives. Leurs caractéristiques component nolamment une teneur movenne à élevée en potassium, un enrichissement en LILEen éléments de terres rares légères. en BA et en SR. de mème que des anomalies négatives de Nb et Ta.
 
 Les données de la roche totale révèlent que les plutons de la ceinture de Merrimack sont semblables. sous le rapport de chacun des paramètres géochimiques mesurés. aux magmas mafiques à intermédiaires d'il y a environ 400 Ma du cortège plutonique du New Hampshire. Ces similarités permeltent de supposer que les deux groupes de plutons ont été insérès à l’intérieur du mème arc magmatique et qu'ils appartienment au mème cortège magmatique. L'existence d'un cortège magmatique commun à la ceinture de Merrimack et au terrane de Central Maine permet de supposer que les deux zones lithotectoniques étaient proximales I'une de 1'autre il y a environ 400 Ma. Les différences par rapport aux éléments traces entre les magmas de la ceinture de Merrimack et ccux de Sharpners Pond laissent supposer que le terrane de Putnam-Nashoba rcprisenie un arc distinct
 
 Le pluton Rochester, situé dans le terrane de Central Maine du New Hampshire, est géochimiquement distinct des plutons siluro-dévoniens de la ceinture de Merrimack et du cortège plutonique du New Hampshire. Le pluton Rochester présente des affinités alcalines tout en conservant ses carectéristiques calco-alcalines générales. Les concentrations élevées d'éléments incompatibles (K, Ti, P, Ba, Rb, Zr) dans le pluton Rochester sont manifestement semblables à celles observers dans la tonalite d'il y a 360 Ma de Hardwick, au Massachusetts. La correlation géochimique prononcée entre les plutons Rochester et Hardwick suppose qu'ils découlent du mème phénoméne magmatique et qu'ils ont une origine commune remontant au Dévonien supérieur. Nous pensons que ces plutons tirent probablement leur origine d'une réaction a une distension d'écartemcnt apparentée à une transpression acadienne tardive.
 
 Traduit par la rédaction

Ammonium partitioning and nitrogen-isotope fractionation among coexisting micas during high-temperature fluid-rock interactions: examples from the New England Appalachians

Despite recent advances in the field of N-isotope geochemistry, our understanding of the behavior of this element in the solid earth remains limited by a lack of fundamental information regarding the partitioning of ammonium and isotopic fractionation of N among coexisting mineral and fluid phases. Study of N behavior in regionally metamorphosed rocks provides the opportunity to assess intermineral NH 4 + partitioning and N-isotope fractionation among coexisting micas during metamorphism and affords an application of the N system as a tracer of high-T fluid-rock interactions. Analyzed mica samples range in δ 15N air from +3.3 to +11.9‰, and contain 9 to 1820 ppm N. The outcrop at Townshend Dam, Vermont, allows examination of N behavior across-strike on a relatively small scale, and samples from western Maine demonstrate the effect of varying metamorphic conditions on N behavior in metapelites. Δ 15N bt-w.mica(δ 15N biotite-δ 15N white-mica) ranges from −0.9 to +2.7‰ (for all samples from both suites, mean = +0.36‰, with 1σ = 0.79‰), with samples containing a separate paragonite white-mica phase showing the greatest range (−0.12 to +1.02‰; mean = 0.58‰, 1σ = 1.03‰). Thirteen samples containing only Na-poor muscovite (six from Townshend Dam, seven from Western Maine) have mean Δ 15N bt-w.mica of 0.07‰ (1σ = 0.41‰). In both suites, biotite nearly always contains more N than coexisting white mica, but N w.mica/N bt also shows some significant scatter (mean N w.mica/N bt = 0.46, with 1σ = 0.34). The thirteen samples, containing only a Na-poor, muscovitic white-mica phase, have mean N w.mica/N bt = 0.39 with 1σ = 0.26, similar to that reported by others for other metamorphic suites containing only muscovite as the white-mica phase. There is no obvious suggestion of equilibrium N-isotopic fractionation among coexisting micas at epidote-amphibolite to amphibolite-facies metamorphic conditions, although NH 4 + appears to partition systematically among coexisting biotite and white mica. Significant scatter in both Δ 15N bt-w.mica and NH 4 + partitioning (conceivably the result of differential closure to exchange during cooling or of retrograde replacements) could, however, obscure observation of small equilibrium intermica fractionations related to the characteristics of the interlayer sites in which NH 4 + resides. Samples most unlike the mean in both Δ 15N bt-w.mica and N w.mica/N bt contain abundant chlorite, some of which is likely retrograde, based on petrographic observations [chl/(chl+bt) >0.3]. Thus, retrograde replacement of biotite by chlorite may have been accompanied by fluid-mineral N-isotope exchange, perhaps involving the production of fine-grained, retrograde N-bearing white mica observed petrographically for some samples. It is also possible that sampling at scales greater than those of N-isotope equilibrium domains (e.g., across fine interlayers) results in some scatter because of varying relative modal proportions of the two micas in adjacent fine interlayers. Although further investigation of the extent of retrograde reequilibration of mica N systematics is warranted, the observed lack of systematic N-isotope fractionation among coexisting micas, and the reasonably systematic NH 4 + partitioning data for these phases provide important preliminary constraints for attempts to model N-isotope behavior in fluid-rock systems. These results, and other attempts to calibrate N-isotope fractionation through field studies, point to the conspicuous lack of experimentally determined mica-fluid N-isotope fractionation factors.

Thinning of the upper mantle during late Paleozoic Appalachian orogenesis

Across the Appalachian orogen of New England, the splitting of core-refracted shear waves from a wide range of arrival directions indicates the presence of two nearly uniform horizontal layers of anisotropic upper mantle. The anisotropy in the lower layer has a fast axis nearly parallel to the absolute motion of the North American plate and thus is attributed to basal shear as the plate plows through asthenospheric mantle. The anisotropy of the upper layer is inferred to be a fossil fabric, residing in lithospheric mantle. The finite extension direction of the upper fabric is subhorizontal and oriented normal to the local trend of the Appalachian orogen. The upper fabric is consistent over a broad region beneath and west of the New England Appalachians, which indicates that it formed after Devonian closure of the Iapetus ocean, probably during or after the Paleozoic Acadian and Alleghany orogenies. Tectonic scenarios for synconvergent or postconvergent extension, developed for Tibet, predict rapid surface uplift and increased heat flow due to lithospheric thinning, consistent with coeval late orogenic mantlederived magmatism in both the northern Appalachians and Morocco.

Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburn Falls arc: Comment and Reply

Research Article| April 01, 1999 Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburn Falls arc: Comment and Reply Nicholas Ratcliffe; Nicholas Ratcliffe 1U.S. Geological Survey, Reston, Virginia 22092, USA Search for other works by this author on: GSW Google Scholar Willis E. Hames; Willis E. Hames 2Department of Geology, Auburn University, Auburn, Alabama 36849, USA Search for other works by this author on: GSW Google Scholar Rolfe S. Stanley; Rolfe S. Stanley 3Department of Geology, University of Vermont, Burlington, Vermont 05405, USA Search for other works by this author on: GSW Google Scholar Paul Karabinos; Paul Karabinos 4Department of Geology, Williams College, Williamstown, Massachusetts 01267, USA Search for other works by this author on: GSW Google Scholar Scott D. Samson; Scott D. Samson 5Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Search for other works by this author on: GSW Google Scholar J. Christopher Hepburn; J. Christopher Hepburn 6Department of Geology and Geophysics, Boston College, Chestnut Hill, Massachusetts 02467, USA Search for other works by this author on: GSW Google Scholar Heather M. Stoll Heather M. Stoll 7Departamento de Geologia, Universidad de Oviedo, 33005 Oviedo, Asturias, Spain Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (4): 381–382. https://doi.org/10.1130/0091-7613(1999)027<0381:TOITNE>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Nicholas Ratcliffe, Willis E. Hames, Rolfe S. Stanley, Paul Karabinos, Scott D. Samson, J. Christopher Hepburn, Heather M. Stoll; Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburn Falls arc: Comment and Reply. Geology 1999;; 27 (4): 381–382. doi: https://doi.org/10.1130/0091-7613(1999)027<0381:TOITNE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Short-lived continental magmatic arc at Connemara, western Irish Caledonides: Implications for the age of the Grampian orogeny

U-Pb geochronological data from the Connemara region of Ireland indicate that continental arc magmatism along the southern margin of Laurentia was short-lived, lasting only from ca. 475 to 463 Ma. Previous work has demonstrated that intrusive activity in Connemara was broadly synchronous with Grampian mid-crustal deformation and upper amphibolite facies metamorphism of Neoproterozoic–lower Paleozoic Dalradian Supergroup rocks. The two oldest intrusions, the Currywongaun and Cashel–Lough Wheelaun gabbros, have U-Pb zircon ages of 474.5 ± 1.0 and 470.1 ± 1.4 Ma, respectively, whereas the U-Pb xenotime age of the postdeformational Oughterard granite is 462.5 ± 1.2 Ma. Thus, the implied age of the Grampian orogeny in Connemara is substantially younger than generally acknowledged, but consistent with other age constraints from Ireland and Scotland. A revised age for the Grampian orogeny helps to solve a long-standing controversy about the age of contractional deformation and arc magmatism in Dalradian Supergroup rocks of the British Isles. This event now demonstrably postdates Neoproterozoic–lower Paleozoic rift deposits and Laurentian passive margin sediments, a finding consistent with deposition of Dalradian Supergroup rocks in an extensional environment at or near the Laurentian margin. The Grampian orogeny is also now demonstrably synchronous with the Taconian orogeny of the northern Appalachians. In Middle Ordovician time, the Laurentia-Iapetus plate boundary was characterized by ophiolite obduction, arc-continent collision, and associated short-lived subduction beneath Laurentia from at least the New England Appalachians to the Irish Caledonides.

Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc

Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Shelburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.

Petrologic and 40Ar/39Ar geochronological constraints on the middle to late Paleozoic thermotectonic history of the southern Connecticut Valley zone, New England Appalachians

New 40 Ar/ 39 Ar data from The Straits Schist and interlayered formations in the southernmost Connecticut Valley zone of the northern Appalachian orogen, together with published 40 Ar/ 39 Ar and U-Pb data, are used to account for relatively rapid Devonian (post-Acadian) cooling of gneisses and schists near the Waterbury dome and slower Devonian through Permian cooling in structurally higher schists south of the dome. Metamorphic hornblende 40 Ar/ 39 Ar cooling ages range from 374 Ma 10 km south of the Waterbury dome to 324 Ma 15 to 40 km south of the dome. Muscovite 40 Ar/ 39 Ar cooling ages also decrease to the south, from 318 to 254 Ma. The ages from the southernmost Connecticut Valley zone are as much as 50 and 100 m.y. younger than hornblende and muscovite, respectively, from the Waterbury dome. The difference in cooling histories between the Waterbury dome and southern Connecticut Valley zone is interpreted to be a result of relatively slower cooling in the southern Connecticut Valley zone following Acadian metamorphism and/or a static Pennsylvanian (Alleghanian) thermal event that reset the muscovite K/Ar system, promoted growth of fine-grained monazite, and resulted in partial melting of the crust beneath the southern Connecticut Valley zone. Such a thermal event would have been contemporaneous with Alleghanian orogenesis in Appalachian terranes in southeastern New England, but the mechanism for generating a Pennsylvanian thermal pulse is still unknown.

Variability of crustal attenuation in the northeastern United States from Lg waves

High‐quality, digital seismograms from eight pairs of collocated earthquakes in the northeastern United States were analyzed to determine accurate source spectrum corner frequencies. This was accomplished by applying the empirical Green's function method to regional Pg and Lg (or Sg) phases recorded by vertical component seismographs of the U.S. National Seismographic Network (USNSN) and the Lamont‐Doherty Cooperative Seismographic Network (LCSN) stations. The frequency band used was 0.5–16 Hz for USNSN and 1–30 Hz for LCSN records. The source spectrum corner frequencies for the eight larger earthquakes of the event pairs (magnitudes between mb(Lg) = 2.5 – 4.1) range from about 4.3 to 16.3 Hz. Based on the comer frequencies obtained independently from the empirical Green's function analysis, Sg or Lg wave displacement amplitude spectra up to 30 Hz were used to determine the crustal average Q factors along 87 event‐station paths. These paths crossed diverse tectonic features in the northeastern United States and were in the epicentral distance range of 41 to 1394 km. We found that within the northeastern United States, the crustal average QLg we obtained was frequency dependent and showed spatial variability which correlated fairly well with the major tectonic features in the region. Our attenuation measurements indicated low Lg attenuation in the Adirondack Mountains with exposed Precambrian Grenville basement with QLg = 905 f0.40, high Lg attenuation in the central Appalachian Province with QLg = 561–586 f0.46–0.47, and an intermediate Lg attenuation in northern New England Appalachians with Q = 705 f0.41.

Late-Taconian and pre-Acadian history of the New England Appalachians of southwestern Connecticut

Research Article| April 01, 1995 Late-Taconian and pre-Acadian history of the New England Appalachians of southwestern Connecticut J. H. Sevigny; J. H. Sevigny 1Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100 Search for other works by this author on: GSW Google Scholar G. N. Hanson G. N. Hanson 1Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100 Search for other works by this author on: GSW Google Scholar Author and Article Information J. H. Sevigny 1Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100 G. N. Hanson 1Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1995) 107 (4): 487–498. https://doi.org/10.1130/0016-7606(1995)107<0487:LTAPAH>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. H. Sevigny, G. N. Hanson; Late-Taconian and pre-Acadian history of the New England Appalachians of southwestern Connecticut. GSA Bulletin 1995;; 107 (4): 487–498. doi: https://doi.org/10.1130/0016-7606(1995)107<0487:LTAPAH>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New U-Pb age determinations and field observations combined with published field relations refine the late Taconian and pre-Acadian history of the New England Appalachians of southwestern Connecticut. Foliation within the Camerons Line shear zone was developed after 514 ± 11 Ma but prior to 445 ± 1.5 Ma. This structure probably experienced renewed movement in the Early Silurian (437 ± 2 Ma), as suggested by new xenotime growth in protomylonite. Metasedimentary and metavolcanic rocks east of Camerons Line were deposited, were isoclinally folded, and reached temperatures in excess of about 550 °C before emplacement of small dioritic plutons of the 453 ± 3 Ma Brookfield plutonic series. Igneous titanites in Brookfield plutons cooled below U and Pb diffusion (e.g., 525 ± 25 °C) in the latest Ordovician to earliest Silurian and were not reset in the Acadian. The Newtown gneiss is a composite magmatic complex composed of quartz diorite (446 ± 2 Ma), granodiorite (436 ± 2 Ma), and granite (438 ± 2 Ma). Low 207Pb/204Pb feldspar isotopic compositions and 1150 Ma zircon inheritance in the Newtown gneiss and Brookfield plutonic series are consistent with interaction with Grenvillian crust or crustally derived material.We propose that the Newtown and Harrison gneisses, and possibly the Brookfield plutonic series, make up the plutonic roots of a Late Ordovician to Early Silurian (454–438 Ma) magmatic arc complex generated during subduction along the eastern North American continental margin. Magmatic activity was coeval with that in the Bronson Hill anticlinorium to the east (ca. 454–442 Ma). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Orogenic evolution of the New England Appalachians of southwestern Connecticut: Discussion and reply

Research Article| September 01, 1994 Orogenic evolution of the New England Appalachians of southwestern Connecticut: Discussion and reply RICHARD L. KROLL; RICHARD L. KROLL 1Department of Geology and Meteorology, Kean College of New Jersey, Union, New Jersey 07083 Search for other works by this author on: GSW Google Scholar J. H. SEVIGNY; J. H. SEVIGNY 2Department of Earth and Space Sciences, State University of New York; Stony Brook, New York 11794 Search for other works by this author on: GSW Google Scholar G. N. HANSON G. N. HANSON 2Department of Earth and Space Sciences, State University of New York; Stony Brook, New York 11794 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD L. KROLL 1Department of Geology and Meteorology, Kean College of New Jersey, Union, New Jersey 07083 J. H. SEVIGNY 2Department of Earth and Space Sciences, State University of New York; Stony Brook, New York 11794 G. N. HANSON 2Department of Earth and Space Sciences, State University of New York; Stony Brook, New York 11794 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (9): 1229–1231. https://doi.org/10.1130/0016-7606(1994)106<1229:OEOTNE>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RICHARD L. KROLL, J. H. SEVIGNY, G. N. HANSON; Orogenic evolution of the New England Appalachians of southwestern Connecticut: Discussion and reply. GSA Bulletin 1994;; 106 (9): 1229–1231. doi: https://doi.org/10.1130/0016-7606(1994)106<1229:OEOTNE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.