tectonic history of the lunksoos composite terrane in the maine appalachians several ordovician volcanic terranes are preserved between the paleozoic north american margin and the avalon terrane in the northern appalachians of maine they have been interpreted as remnants of an island arc on the basis of their current position in the appalachians or as ocean islands near the margin of avalon on the basis of paleontologic data a study of one of these terranes the lunksoos terrane in northern maine was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution paleomagnetic study of the pre caradoeian pre late ordovician stacyville volcanics revealed three components of magnetization the final magnetization removed d 90 i 36 95 11 8 is interpreted to be primary and yields a paleolatitude of approximately 20 s however a late ordovician remagnetization cannot be excluded the stacyville volcanics now overlie a cambrian and lower ordovician sequence the grand pitch and shin brook formations that is believed to have been deposited at the southern side of iapetus possibly near the margin of avalon during the middle and late ordovieian avalon was at paleolatitude s higher than 40 s while the volcanics were either formed or remagnetized at a paleolatitude of 20 s the pre caradocian stacyville volcanics therefore could not have been deposited directly on the grand pitch shin brook sequence but must have been tectonically emplaced the rockabema diorite which intrudes the entire cambro ordovician sequence constrains the timing of this emplacement to be pre late early silurian these relationships indicate that the lunksoos area is a composite terrane within the maine appalachians the tectonic emplacement scenario is unnecessary if the grand pitch and shin brook formations were not located on the south side of iapetus during the ordovician this alternative however poses conflicts with the interpretation of faunal affinities

geochemistry and provenance of the middle ordovician austin glen member normanskill formation and the taconian orogeny in new england the austin glen member of the upper middle ordovician normanskill formation is a sandstone shale ysch succession deposited in the foreland of the taconian orogen petrographic major and trace element and nd pb isotopic data provide substantial constraints on its provenance lack of k feldspar and paucity of plagioclase in addition to the dominance of sedimentary rock fragments indicate that the source was dominated by recycled sedimentary components major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same no evidence of an ophiolitic or volcanic component was observed interpretation of nd isotopic characteristics are complicated by a partial resetting of the nd isotope system at about the time of sedimentation but indicate that the provenance of the austin glen member had a long term history of light rare earth element lree enrichment average tdm 1a8 ga furthermore nd isotopic compositions are extremely homogeneous end 8a1 0a6 1 s d n 23 at 450 ma the approximate depositional age indicating either a single source or very well mixed sources 207pb 204pb ratios are variable but within the range of pb isotopic compositions typically described as grenvillian the range of 207pb 204pb is greater than expected for the range of 206pb 204pb and suggests an additional component of pb possibly introduced during diagenesis the immediate source of the austin glen member may have been the accretionary prism that developed as older sediments of the laurentian margin were scraped off the basin oor incorporated within the accretionary prism and shed into the basin no evidence indicating the arrival of an undifferentiated island arc or continental fragment during the taconian orogeny has been found the data acquired during this study can be explained almost exclusively by grenville province source components but with possible additional contributions from older laurentian terranes and late proterozoic rift volcanics that are not readily quanti ed but likely to have been minor accordingly we conclude that the taconian orogeny in new england involved either 1 a continental arc that involved exclusively laurentia 2 collision of a continental block with identical geochemical characteristics as laurentia or 3 essentially no detritus from any exotic colliding block island arc or continental fragment reached the foreland basin at the time of austin glen deposition

crustal structure of the western new england appalachians and the adirondack mountains we present an interpretation of the crustal velocity structure of the new england appalachians and the adirondack mountains based on a seismic refraction wide angle reflection experiment in eastern north america extending from the adirondacks in new york state through the northern appalachians in vermont and new hampshire to central maine modeling of the eastern portion of the profile within the new england appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5 5 to 6 2 km s a midcrustal velocity of 6 4 km s and a lower crustal velocity of approximately 6 8 km s crustal thickness increases from 36 km beneath maine to 40 km in vermont little evidence is seen for structures at depth directly related to the white mountains or the green mountains a major lateral velocity change in the upper and mid crust occurs between the appalachians and the adirondacks this boundary projecting to the surface beneath the champlain valley dips to the east beneath the green mountains and extends to a depth of 25 km below the eastern edge of the connecticut valley synclinorium in vermont the tahawus complex a series of strong horizontal reflections at 18 24 km depth beneath the adirondack highlands is seen to dip eastward beneath vermont upper crustal rocks in the adirondack mountains have poisson s ratios of 0 28 0 01 that can be correlated with the marcy anorthosite pois son s ratios of 0 24 0 01 calculated for rocks of the connecticut valley synclinorium indicate a siliceous upper crust in vermont the lower crust is considered to be best represented by intermediate to mafic granulites a high poisson s ratio 0 26 0 27 tends to support a mafic lower crust in the new england appalachians this seismic refraction wide angle reflection experiment provides further evidence for the obduction of the allochthonous western appalachian units onto grenvillian crust above a zone of detachment that penetrates at least to midcrustal depths and was the locus of successive paleozoic thrusting

acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians lombard a d van der pluijm b a and van der voo r 1991 acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians tectonophysics 186 279 291 samples were collected from ten sites of the late cambrian early ordovician jim pond formation for paleomagnetic study stepwise thermal demagnetization reveals three separable components of magnetization component i is typically removed by 350ac it is subparallel to the present day field 354o 76 vs 342 72o at the site location 45 3on 289 4 e and is considered to be a recent partial overprint component ii without tilt correction is a south southeasterly and shallow direction mean 165 o o k 31 4 a s5 8 6 o that is removed over an intermediate temperature range 350 600 c component iii without tilt correction is a northeasterly and shallow upward direction mean lo 24o k 21 5 ass 7 3 and is removed over the highest temperature range 480 to 69ooc though not statistically significant for components ii and iii the precision parameter k decreases and the as5 increases when tilt correction is applied suggesting that both are post folding magnetizations component ii without tilt correction has a corresponding paleomagnetic pole located at 43on 130 e dp dm 4 3 8 6 o which falls near the late carboniferous segment of the laurentian apparent polar wander path apwp component iii without tilt correction has a corresponding pole located at 32 n 98 e dp dm 4 7o 7 8 which falls near the lower middle devonian segment of the apwp we conclude that the jim pond formation has undergone two paleozoic remagnetization events one in the early to middle devonian and a second one in the late paleozoic the ages of these remagnetizations coincide with the timing of major erogenic activity in the area i e the acadian and alleghenian respectively the remagnetization event associated with the acadian pulse can be recognized in other paleomagnetic investigations in the northern appalachians

paleomagnetism of the pennington mountain terrane a near laurentian back arc basin in the maine appalachians paleomagnetic studies of volcanic terranes in the appalachians provide quantitative data on the ordovician paleogeography of the iapetus ocean new paleomagnetic results from submarine volcanics of the middle to late ordovician winterville formation of maine further constrain the evolution of iapetus ten sites yield a tilt corrected direction of d i 327 21 95 9 3 k 27 9 the corresponding paleomagnetic pole calculated for the average site location 46 8 n 291 3 e is located at 26 n 148 e d p 5 d m 10 a 95 7 7 a silurian conglomerate frenchville formation test is inconclusive due to a strong present day field overprint which obscures any primary remanence the presence of a positive tilt test however supports the conclusion that the characteristic magnetization of the winterville formation is a prefolding primary magnetization an ordovician paleolatitude of 111 5 for the pennington mountain terrane of northern maine is indistinguishable from that of the laurentian margin 15 20 during the middle to late ordovician the paleolatitude of the pennington mountain terrane is also similar to but slightly more equatorial than that of the previously studied bluffer pond formation of the nearby munsungun terrane 18 9 we conclude that the ordovician pennington mountain and munsungun volcanic terranes of maine were formed and acquired their characteristic magnetization near the laurentian margin the paleolatitudes of these terranes are also similar to that obtained from the stacyville volcanics of the more outboard lunksoos terrane 20 8 but they contrast strongly with the ordovician miramichi terrane of northern new brunswick that yielded a paleolatitude of 51 21 16 these results from maine support the presence of one or more backarc basins adjacent to the laurentian margin during the middle to late ordovician

suspect terranes in the north american margins and relative sea levels sea level trends deduced from tide gauges show considerable variation along strike both on the convergent western and divergent eastern edges of the north american plate that portion of the variation unrelated to quaternary glaciation and deglaciation may reflect the differential reaction to extension and sediment loading in the east and in the west subduction and translation of terranes welded onto the north american craton these autonomous blocks appear to impart distinct signatures on records of relative sea levels suggesting promise in deducing complex terrane topography of other margins using tide gauge data uncertainties in these deductions can be reduced only with expanded tide gauge coverage or by application of new geodetic techniques very long baseline interferometry or differential global positioning system

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

the northeast kingdom batholith vermont magmatic evolution and geochemical constraints on the origin of acadian granitic rocks five devonian plutons west charleston echo pond nulhegan derby and willoughby that constitute the northeast kingdom batholith in vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution the batholith consists of metaluminous quartz gabbro diorite and quartz monzodiorite peraluminous granodiorite and granite and strongly peraluminous leucogranite contents of major elements vary systematically with increasingsio 40 and have small negative eu anomalies the strongly peraluminous willoughby leucogranite has unique trace element abundances and ratios relative to the rest of the batholith including low contents of hf zr sr and ba low values of k rb 80 164 th ta 9 rb cs 7 40 k cs 0 1 0 5 ce pb 0 5 4 high values of rb sr 1 18 low to moderate ree contents and light ree enriched patterns with small negative eu anomalies flat ree patterns with large negative eu anomalies are found in a small hydrothermally altered area characterized by high abundances of sn up to 26 ppm rb up to 670 ppm li up to 310 ppm ta up to 13 1 ppm and u up to 10 ppm there is no single mixing trend fractional crystallization assemblage or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the northeast kingdom batholith the plutons originated by mixing mantle derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks 50 by mass and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals

regional differences in pb isotopic compositions of feldspars in plutonic rocks of the northern appalachian mountains u s a and canada a geochemical method of terrane correlation feldspar pb isotopic compositions from paleozoic tonalitic to leucogranitic caledonian plutons from the northern appalachian mountains of the u s a and canada range from nonradiogenic to radiogenic relative to the evolution curve of stacey and kramers 1975 pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons plutons with the least radiogenic pb isotopic signatures northern group have intruded allochthonous rocks closest to the north american craton and plutons with the most radiogenic pb southern group have intruded avalonian basement feldspars from plutons in the central group have intermediate pb isotopic compositions between the northern and southern groups and do not define an isotopically unique group the predominant source of the felsic magmas was the continental lithosphere as indicated by the general pb isotopic similarity of the plutonic feldspars with either grenville or chain lakes basement in the case of the northern group and with avalonian basement rocks in the case of the southern group the source region for plutons in the central group may have contained a mixture of grenvillian and avalonian components plutons in the northern appalachians are as a group more evolved in their pb isotopic compositions relative to caledonian plutons in the british isles

late neoproterozoic to permian tectonic evolution of the quebec appalachians canada abstract due to the unique location of the quebec appalachians in a re entrant of the paleo north american margin and to the fact that the area largely escaped late paleozoic deformation this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere despite diachronicity both along and across strike of the orogen five tectonic events of regional significance are recognized 1 the earliest event is mainly recorded by neoproterozoic to early ordovician normal faults associated with extension of the laurentian craton and the establishment of a passive continental margin 2 the second event the ordovician taconian orogeny is marked by foreland directed deformation and corresponds to the principal deformational and metamorphic events recorded by the laurentian margin 3 the third event early salinic is early silurian it is restricted to the southern part of the quebec appalachians and consists of hinterland se directed thrusts folds and retrograde metamorphism 4 the fourth event late salinic is recorded by regional scale late silurian to early devonian normal or transtensional faults and is associated with the formation of the connecticut valley gaspe trough and 5 the last event the devonian acadian orogeny affects with variable intensity all the pre carboniferous lithologies this tectonic synthesis implies that the subdivision of the peri laurentian oceanic tracts proposed in other segments of the appalachians should be applied with caution in quebec and that the structural style the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen in particular the similarities of the thetford mines and boil mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with the maximum allochtony hypothesis i e the assumption that the southern quebec ophiolites are rooted east of the chain lake massif a geodynamic model involving the taconian obduction of a large ophiolitic nappe s the docking of a peri gondwanan gander medial new england terrane in early salinic the formation of large sedimentary basins connecticut valley gaspe and merrimack troughs due to trench rollback and or delamination during the late salinic event and collision at the outer margin of composite laurentia during the acadian orogeny can account for the protracted structural and metamorphic history of the quebec appalachians

the extension of grenville basement beneath the northern appalachians results from the quebec maine seismic reflection and refraction surveys from 1983 to 1984 the united states geological survey and the geological survey of canada undertook a series of seismic experiments to determine the crustal structure of the appalachian mountain belt in maine and southeast quebec these experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high resolution refraction wide angle reflection profiles both parallel and perpendicular to strike here the data recorded in quebec and the northwest part of maine are interpreted in conjunction with existing reflection profiles from quebec synthetic seismogram calculations in laterally varying media are used to model the refraction wide angle reflection data while reprocessing migration and attribute plotting are used in the interpretation of the reflection data a major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the st lawrence lowlands southeast to about 25 km depth beneath the southeast edge of the chain lakes massif this zone of reflections is interpreted as a master decollement separating autochthonous grenville basement from overlying allochthonous rocks of the appalachian orogen including foreshortened miogeoclinal rocks the remnants of one or more magmatic arcs the connecticut valley gaspe synclinorium and the chain lakes massif in the vicinity of the chain lakes massif extension has caused grenville basement to be thinned by as much as 40 reflections from the decollement show strong layering and evidence for basement faulting associated with the guadeloupe and other faults normal incidence and wide angle reflections are observed to be almost continuous beneath the baie verte brompton line and connecticut valley gaspe synclinorium indicating that the baie verte brompton line is confined to the upper crust the connecticut valley gaspe synclinorium is characterized by p wave velocities of about 5 3 km s 1 and is shown to be a shallow structure with a maximum depth of about 3 km rock velocities immediately below the connecticut valley gaspe synclinorium are typical of chain lakes material 6 1 km s 1 a new model for the tectonic development of the region is proposed in which all pre silurian units between the baie verte brompton line and the southeastern edge of the chain lakes massif are allochthonous and confined to the upper crust we infer that the chain lakes massif volcanics of the ascot weedon formation and associated deeper crustal rocks were thrust over grenvillian basement as a composite unit and may now underlie parts of the connecticut valley gaspe synclinorium

geologic constraints on archeological interpretations of a late wisconsinan site at varsity estates calgary alberta abstract a corollary of j chlachula s interpretation that chipped quartzite cobbles at the contact of lake calgary sediments and underlying till represent an early late wisconsinan lithics industry is the conclusion that laurentide and cordilleran ice did not coalesce in late wisconsinan time contrary to that conclusion are 1 nw se oriented streamlined landforms in the foothills area which could have been created only by coalescent ice flow along with numerous radiocarbon dates which indicate that laurentide ice did not advance to central and western alberta before late wisconsinan time 2 cosmogenic 36 cl dates of 12 17 ka on the foothills erratics train indicating late wisconsinan coalescence 3 an estimated thickness of 1000 m or more of late wisconsinan coalesced ice in western calgary based on mapped laurentide limits 100 km south of calgary furthermore recently discovered rare shield clasts in the till at the varsity estates and silver springs sites suggest that the till was deposited by ice at least partly of laurentide origin as laurentide ice retreated lake calgary would have immediately inundated the site allowing no time or space for human cobble chipping paleomagnetism indicates that the lake sediments at the varsity estates site rotated 25 degrees to the nw after deposition presumably by rotational slumping slumping may have been syndepositional or due to channel cutting during periods of lake drainage and refilling neither scenario either supports or contradicts the archeological interpretation it is possible that the broken and chipped clasts interpreted to be artifacts were created by natural processes

seismic velocities in the shallow crust of western new england and northern new york the ontario new york new england refraction study o nynex provides a set of high quality data for a tomographic study of an intraplate continental region with low seismic activity this study focuses on the region crossed by the united states part of the o nynex profile tomographic inversion of the travel time data is used to construct a three dimensional model of the p and s wave seismic velocity distribution within the shallow crust of this region velocity structure is resolved in the top 15 km of the crust with the most detailed picture obtained for the depth range between 1 and 5 km the upper crust of the proterozoic grenville province is found to have a p wave velocity 0 2 to 0 3 km sec higher than the paleozoic appalachians the area of anorthositic intrusions within the adirondack mountains is characterized by p wave velocities 0 3 km sec higher and s wave velocities 0 1 km sec higher than the average values for the grenville province this high velocity anomaly extends to the depth over 5 km possibly as deep as 16 km the velocity structure of central vermont is found to be closer to that of the grenville province than to the rest of the appalachians central new hampshire shows relatively small lateral variation of seismic velocity no strong correlation is seen between velocity anomalies and intrusive bodies of the white mountains magma series

depth discrete specific storage in fractured sedimentary rock using steady state and transient single hole hydraulic tests abstract a method is presented for obtaining depth discrete values of specific storage ss from single hole hydraulic tests in fractured rock boreholes using straddle packers 1 5 17 m test intervals low flow constant head ch step tests analyzed using the thiem method provide transmissivity t values free from non darcian error short term constant rate pumping tests 0 5 2 h analyzed using the cooper jacob approximation of the theis method provide s from the hydraulic diffusivity using the darcian t value from the ch step test this synergistic use of two types of hydraulic tests avoids the common source of error when pumping tests injection or withdrawal are conducted at higher flow rates and thereby induce non darcian flow resulting in the underestimation of t other errors such as well bore storage and leakage can also substantially influence s by causing a shift in the time axis of the cooper jacob semi log plot in this approach the darcian t values from the ch step tests are used in the analysis of the transient pumping test data for calculating s throughout the pumping test using the cooper jacob approximation to minimize all of the aforementioned errors resulting in more representative s values the effect of these non idealities on the measured drawdown is illustrated using the theis equation with the darcian t and s values to calculate drawdown for comparison to measured data the ss values for tests in sandstone obtained from this approach are more consistent with confined aquifer conditions than values derived from the traditional cooper jacob method and are within the range of field and lab values presented from a compilation of literature values for fractured sandstone 10 7 10 5 m 1 this method for obtaining ss values from short interval straddle packer tests improves the estimation of both k and ss and provides opportunity to study their spatial distribution in fractured rock

kinematic history of the bass river complex nova scotia cadomian tectonostratigraphic relations in the avalon terrane of the canadian appalachians

paleomagnetism of the ordovician bluffer pond formation paleogeographic implications for the munsungun terrane of northern maine the munsungun area of northern maine is an early paleozoic terrane within the central mobile belt cmb of the northern appalachians the bluffer pond formation of the munsungun terrane is composed primarily of pillow basalt and gabbro with minor intercalated sediments and has a maximum thickness of 1230 m this unit is of early late ordovician caradocian age based on paleontological data and folding most likely occurred during the devonian acadian orogeny this region has been metamorphosed to subgreenschist facies samples for paleomagnetic study have been obtained from 15 sites of pillow basalt and seven sites of gabbro within the bluffer pond formation natural remanent magnetization nrm directions are similar to the expected present day field direction and are interpreted to be strongly affected by a viscous overprint detailed stepwise thermal demagnetization fails to adequately resolve a partial overprint direction with intermediate blocking temperature but stable endpoints combined with great circles do isolate a characteristic high temperature component carried by magnetite as indicated by unblocking temperatures and rock magnetic experiments after tilt correction the formation mean calculated from pillow basalt sites only is d 339 i 33 k 12 95 14 n 11 with a corresponding paleopole of 23 n 133 e p 9 m 16 calculated for an average site location of 46 5 n 291 e when both pillow basalt and gabbro sites are used to calculate a tilt corrected formation mean the result is d 339 i 28 k 12 95 12 n 14 and this characteristic component passes a tectonic tilt test the calculated paleopole 26 n 134 e p 7 m 13 from pillows and gabbros falls near but not on the ordovician apparent polar wander path segment for north america and yields a paleolatitude 18 9 s similar to that of the lunksoos terrane which is located farther to the southeast within the cmb of maine the laurentian margin at this time was located at paleolatitudes of 15 20 s the paleogeographic implication of this result is that the munsungun terrane was located near the laurentian margin within the iapetus ocean and this near continent location is consistent with a marginal or back arc basin tectonic environment for the extrusion of the bluffer pond volcanics

a comparison of reflection and refraction processing and interpretation methods applied to conventional refraction data from coastal maine abstract in 1984 the u s geological society collected 850 km of refraction data in lines parallel and perpendicular to regional strike of the appalachian orogen in maine and southeastern quebec a 100 km long strike line in coastal maine west of penobscot bay is here interpreted on the basis of both refracted and reflected phases techniques used in seismic reflection processing including normal movement velocity corrections and common mid point stacking were applied to the refraction data in order to enhance the visibility of reflected phases the geometries of the most laterally continuous reflections as defined by these techniques can be correlated with reflections seen on a crossing near vertical reflection profile conventional ray tracing analysis of refracted and wide angle reflected phases in the refraction data constrains the velocity structure to about 14 km depth and provides evidence for high veiocity material 6 5 km sec between 4 and 7 km depth and for lower velocities

eocambrian paleomagnetism of the boston basin evidence for displaced terrane a paleomagnetic study of the late precambrian roxbury formation consisting of extrusives and clastic sediments has yielded a steeply downward directed and pre folding characteristic magnetization d i 219 71 95 5 k 311 paleopole at 13 n 267 e n 4 sites which indicates a significantly higher paleolatitude 55 than would be expected if the boston basin were part of the equatorial north american craton in the latest precambrian and earliest paleozoic this characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test a ubiquitous post folding late paleozoic overprint is present in nine sites d i 183 14 95 8 k 42 with a paleopole at 41 s 285 e the pre folding magnetization resides in hematite which is inferred to have formed during early oxidation of the rocks the high stability of this hematite may have prevented its magnetization from being reset during the late paleozoic chemical event responsible for the magnetic overprint the boston basin has a marked geological similarity to the avalon basement terranes in nova scotia and newfoundland as well as the armorican massif in france and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity a likely paleolocation is near the northwestern margin of gondwana which was located at the southpole in the latest precambrian and early cambrian

no regional anisotropic domains in the northeastern u s appalachians a fabric frozen within the continental lithosphere should lead to regional anisotropic domains with close correlation between tectonics seismic anisotropy and shear wave travel time delays alternatively seismic anisotropy could be maintained by active deformation in the sub lithospheric mantle this scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting surface tectonics and travel time delays a coherent pattern of shear wave splitting is observed in the northeastern united states covering large parts of new york and new england spanning the appalachian orogen and areas underlain by the grenvillian basement the direction of fast shear wave propagation as measured from core refracted sks skks and pks phases varies systematically with event back azimuth at all sites examined and is fit well by two layers of anisotropy with subhorizontal symmetry axes relative travel time delays of shear waves that sample the region vary over 100 km length scale and correlate with surface geology tomographic images suggest lateral shear velocity variations 3 including a slow anomaly beneath the grenvillian basement exposed in the adirondack mountains there is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting we therefore conclude that in the northeastern us the concept of regional anisotropic domains i e distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation does not apply

diffusion creep and partial melting in high temperature mylonitic gneisses hope valley shear zone new england appalachians usa field petrographic microstructural and isotopic studies of mylonitic gneisses and associated pegmatites along the hope valley shear zone in southern rhode island indicate that late palaeozoic deformation c 275 ma in this zone occurred at very high temperatures 650 c high energy cuspate lobate phase boundary microstructures a predominance of equant to sub equant grains with low internal lattice strain and mixed phase distributions indicate that diffusion creep was an important and possibly predominant deformation mechanism field and petrographic evidence are consistent with the presence of an intergranular melt phase during deformation some of which collected into syntectonic pegmatites rb sr isotopic analyses of tightly sampled pegmatites and wall rocks confirm that the pegmatites were derived as partial melts of the immediately adjacent isotopically heterogeneous mylonitic gneisses the presence of syntectonic interstitial melts is inferred to have permitted a switch from dislocation creep to melt enhanced diffusion creep as the dominant mechanism in these relatively coarse grained mylonitic gneisses 200 500 lm syn deformational grain size a switch to diffusion creep would lead to significant weakening and may explain why the hope valley shear zone evolved into a major regional tectonic boundary this work identifies conditions under which diffusion creep operates in naturally deformed granitic rocks and illuminates the deformation processes involved in the development of a tectonic boundary between two distinct late proterozoic avalonian basement terranes

regional geochemical and isotopic variations of northern new england plutons implications for magma sources and for grenville and avalon basement terrane boundaries examination of 36 devonian to pennsylvanian plutons within and bordering the central maine terrane of north central new england shows that the plutons display regional variations in mineral and bulk rock compositions and in oxygen and sulfur isotopic values these variations principally correspond to the regional structural trends of the central maine terrane plutons located on that terrane9s southeast flank adjacent to the massabesic gneiss complex on the terrane9s northwest flank adjacent to the bronson hill anticlinorium and in the northeastern part of the northeast kingdom batholith of vermont have biotites with relatively low al concentrations 3 0 cations p f u per formula unit feldspars with higher an contents up to an 55 and lower 18 o 7 9 and higher 34 s average of 0 bulk rock values compared to plutons within the central parts of the central maine terrane these characteristics are most compatible with metaigneous or metavolcaniclastic sources as shown by others the plutons of vermont and northwestern maine preserve grenvillian signatures whereas those of southeastern new hampshire specifically the granite at milford new hampshire and maine sebago phillips have avalonian signatures within the interior of the central maine terrane of new hampshire and western maine and in the connecticut valley trough of vermont are distinct regions characterized by peraluminous plutons with high 18 o values 11 low 34 s values 25 al rich biotites 3 3 cations p f u and an poor plagioclases 30 these data are most consistent with metasedimentary sources or at a minimum the plutons were heavily contaminated by central maine terrane or connecticut valley trough metasedimentary rocks these peraluminous plutons do not have the unique signatures of the underlying precambrian basements several metaluminous plutons in the central maine terrane along the new hampshire maine border separate two regions of peraluminous plutons these metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the central maine terrane indicative of less paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the central maine terrane the metaluminous plutons also have trace element characteristics identical to those of plutons in vermont and northwestern maine that have a known grenville basement terrane source we suggest that the metaluminous plutons also had sources in the underlying grenville basement and that the grenville avalon boundary cuts diagonally across northern new england because the plutons with grenvillian signatures are adjacent to plutons with avalonian signatures the sebago batholith and phillips pluton we suggest that there is no medial new england basement in this region of new england some peraluminous two mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks the components of which were deposited in an anoxic basin such as the smalls falls formation of the central maine terrane but many other plutons have compositions that are inconsistent with this model we suggest that the compositions of many new hampshire plutonic suite plutons indicate greater heat input than that available from u enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust

the source and transport of arsenic in a bedrock aquifer new hampshire usa the geographic distribution of elevated groundwater as concentrations in a fractured silicate bedrock aquifer in central new hampshire correlates with the presence of pegmatites which border late devonian granites and intrude metasedimentary rocks as concentrations in the pegmatites averaged 9 6 mg kg which is much higher than the associated granites 0 24 mg kg and metasedimentary rocks 0 8 mg kg as was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low as concentrations and pegmatites with high as concentrations arsenic behaves similarly to b which was also concentrated into these late stage fluids arsenopyrite feass with an oxidation reaction rim of scorodite feaso4 2h2o was observed in aquifer materials elevated as concentrations have been observed to occur in other new england locations in pelitic metasediments intruded by anatectic plutons it is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating as in crystalline aquifer materials which can then cause local as enrichment of groundwaters groundwater as concentrations ranged from 26 nmol l to 5300 nmol l with a median value 210 nmol l that is more than 30 times higher than the median for groundwaters from all of new hampshire 6 5 nmol l high chloride concentrations 1 mmol l resulting from road salt contamination of recharge waters suggest that groundwaters are most likely young 670 nmol l or 50 g l have very low fe concentrations 7 samples with low as 7 fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb as at ph 7 fe oxyhydroxide formation is slow and depends on dissolved o2 availability however the resultant fe oxyhydroxides have a positive net surface charge and appear to adsorb as readily

the crustal structure in central maine from coherency processed refraction data in 1984 the u s geological surcvey and the canadian department of energy mines and resources conducted an extensive seismic refraction experiment the quebec maine transect project as part of this project a 180 km long seismic refraction profile was recorded along the axis of the central maine synclinorium high near surface velocities of 6 0 6 2 km s made a conventional refraction analysis including first arrivals uninformative below about 10 km peak signal frequencies around 30 hz recorded at all offsets and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections to allow a reliable deep structural interpretation a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals in addition interpretation of a coincident wide angle reflection study provided constraints on the deep crustal velocities and structure the crust beneath the central maine synclinorium can be described by three major structural units the upper crust is interpreted as the metasedimentary rocks of the central maine synclinorium with velocities that range from 6 2 to 6 3 km s regions of relative lower velocity 6 0 6 1 km s within the upper crust are interpreted as granitic intrusions low velocity regions modeled at 2 7 km depth with velocities around 5 4 km s are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence at 12 15 km depth the base of the synclinorium appears alternately as a strong reflector indicating the presence of layering and also as a weak reflector especially ne of the sebago pluton a 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6 4 to 6 8 km s the crust thickens from 36 to 40 km going from refraction experiment the quebec maine transect project as part of this project a 180 km long seismic refraction profile was recorded along the axis of the central maine synclinorium high near surf ace velocities of 6 0 6 2 km s made a conventional refraction analysis including first arrivals uninformative below about 10 km peak signal frequencies around 30 hz recorded at all offsets and a trace spacing of 800 m led to severe spatial aliasing effects for secondary arrivals on the record sections to allow a reliable deep structural interpretation a coherency processing technique was applied to the data to enhance phase coherent secondary arrivals in addition interpretation of a coincident wide angle reflection study provided constraints on the deep crustal velocities and structure the crust beneath the central maine synclinorium can be described by three major structural units the upper crust is interpreted as the metasedimentary rocks of the central maine synclinorium with velocities that range from 6 2 to 6 3 km s regions of relative lower velocity 6 0 6 1 km s within the upper crust are interpreted as granitic intrusions low velocity regions modeled at 2 7 km depth with velocities around 5 4 km s are interpreted as seismic evidence for folding and doubling within the upper crustal metasedimentary sequence at 12 15 km depth the base of the synclinorium appears alternately as a strong reflector indicating the presence of layering and also as a weak reflector especially ne of the sebago pluton a 4 km thick boundary layer at 24 km depth is interpreted as a major intracrustal shear zone in which the velocity increases from 6 4 to 6 8 km s the crust thickens from 36 to 40 km going from the ne to the sw end of the profile surface rocks at the sw end of the profile have a metamorphic depth of burial of at least 15 km indicating that the crust below the central maine synclinorium may have been thickened to at least 55 km during paleozoic collisions

slant stacks of refraction data from maine effects of lateral variations in velocity structure abstract we discuss some of the advantages and pitfalls associated with p analysis of wide angle profiles recorded over terranes characterized by significant lateral heterogeneity we construct composite slant stacks for a pair of reversed shots recorded along the axis of the merrimack synclinorium in southwest maine and invert picks for bounds on averaged one dimensional slowness depth structure extremal bounds on total crustal thickness derived from shot point 12 are 40 to 43 km average crustal velocities for individual slowness models corresponding to the extremal depths range from 6 5 to 6 6 km sec predicted normal incidence two way travel times for the moho lie between 12 2 and 13 2 sec the utility of one dimensional inversion of p data is largely in estimating the maximum resolving power of the data set this is information that forward modeling approaches do not provide comparisons of slant stacks of reversed shot gathers with intercept times predicted for a two dimensional model derived by ray tracing highlight some of the pitfalls associated with interpretation of p data in terms of one dimensional structure in some cases the principal arrival may become multi valued resulting in misidentification of phases and compounding biases in estimates of average crustal parameters slant stacks can be highly useful however in refining two dimensional models derived from travel times because of the sensitivity of ray parameters to dip and lateral velocity gradients we use slant stacks in conjunction with the original shot gathers to refine our estimate of the configuration of the moho beneath southwest maine our final model shows a step in the moho that is consistent with variations in normal two way travel times inferred from analysis of postcritical fan shots this abrupt change in crustal thickness may mark the northeast extent of significant underthrusting of avalon crust beneath medial new england basement during the acadian orogeny the preservation of offsets in moho that were formed during paleozoic compression is consistent with the limited amount of rifting experienced by this portion of the orogen during the opening of the atlantic

evidence for a ridge subduction event in the ordovician rocks of north central maine modern and past ridge subduction events are characterized by the intrusion of midocean ridge basalt morb magmas into an overlying accretionary prism the fi eld relationships and trace element geochemistry of ordovician mafi c igneous rocks of the weeksboro lunksoos and munsungun anticlinoria of north central maine indicate that they resulted from such an event the bean brook gabbro intrudes the hurricane mountain melange and other related sedimentary strata of continental derivation the gabbro and associated dry way volcanics have morb trace element chemistries while the bluffer pond and stacyville volcanics are more enriched e morb all of which indicate derivation from a mid ocean ridge on petrogenetic diagrams mafi c samples plot in morb fi elds or when th is used as a discriminator in arc fi elds along trends that originate from morb fi elds and extend toward the composition of upper continental crust these trends are consistent with the presence of silicic and metasedimentary xenoliths in the dry way volcanics and bean brook gabbro and indicate the magmas were not subduction products but were contaminated by th rich upper continental crust the nearby chain lakes massif likely represents the basement to the chain lakes microcontinent and the geographic relationship and ages of the massif hurricane mountain melange and dry way bean brook magmatic rocks indicate northwestdirected subduction modern coordinates on the southeastern margin of the chain lakes microcontinent within the early to middle ordovician taconic ocean subduction at this boundary probably terminated because of the ridge subduction analogous to neogene california

panafrican to hercynian deformations in the mauritanides and tectonic significance of gravity anomalies abstract the mauritanides rokelides chain which extends along the western margin of the west african craton is characterized by roughly northward structural trends and at least for the northern two thirds eastward thrusting of large amplitude the style and timing of deformation change from south to north along the chain in sierra leone guinea and southeastern senegal the rokelides the main event is attributed to the panafrican orogeny apparently with only limited thrusting the belt of gravity highs associated with that part of the orogen is interpreted as a west dipping suture zone in northeastern senegal and central mauritania this panafrican event is masked by caledonian and hercynian events which resulted in strong folding and far travelled thrusting the positive gravity anomaly closely parallel to the belt records the trace of the collision of two continental blocks a taconian event in northern mauritania and south morocco the age of emplacement of large nappes is hercynian and their geodynamic interpretation requires that a trench or ocean has closed farther west presumably beneath the eastern u s atlantic coastal plain

contrasting p t t paths thermochronologic evidence for a late paleozoic final assembly of the avalon composite terrane in the new england appalachians strongly contrasting pressure temperature time paths for the avalon composite terrane and the structurally overlying putnam nashoba zone in eastern new england obtained from thermochronologic and thermobarometric data are best explained by a late paleozoic underthrusting of cover rocks by the avalon composite terrane we present new ar and u pb thermochronologic data that show that in the southern hope valley zone permian 280 ma anatectic metamorphic conditions of 700 c and 6 kbar were quenched by relatively rapid cooling 12 c m y and exhumation 0 5 km m y for 40 m y in contrast peak metamorphic conditions in the putnam nashoba zone predate silurian intrusions and slower cooling 3 5 c m y began at about 400 ma one dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the permian metamorphism of the avalon composite terrane because of the absence of high grade alleghanian metamorphism in rocks overlying the avalon terrane we conclude that high grade alleghanian metamorphism in the avalon terrane occurred east of rocks now overlying it and that significant motion between avalon and this cover occurred after peak alleghanian metamorphism similarly contrasting metamorphic histories between avalon inliers willimantic window massabesic complex gneiss pelham dome and their cover rocks reveals the regional significance of this boundary the core rocks all show permian cooling but the cover rocks show post acadian cooling ages decreasing from east to west to the pelham area where hornblende ages in avalon and cover differ by only 35 rather than 80 m y model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years consequently underthrusting of avalon is constrained to be middle mississippian or younger because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover core rocks of the pelham dome must have been relatively close to this leading edge thus carboniferous to permian underplating from a generally eastward direction best explains these thermochronologic relationships

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

along strike acadian structural variations in the quebec appalachians consequence of a collision along an irregular margin the quebec appalachians have been shaped by two major orogenies the middle to late ordovician taconian and the middle devonian acadian thrust faults and nappe structures characterized the taconian deformation throughout the quebec appalachians whereas structural styles pertaining to the acadian orogeny differ from southern quebec to the gaspe peninsula structural analysis of late ordovician to middle devonian supracrustal rocks shows two different acadian deformational regimes strike slip tectonics in the gaspe peninsula and thrust or dip slip tectonics in southern quebec in southern quebec acadian regional deformation produced folds which vary from upright to overturned and tight to isoclinal from the nw to the se major faults e g la guadeloupe are northwestward directed thrust faults marked by highly ductile shear zones outlined by mylonite in the gaspe peninsula structural trend is ne and major e w dextral strike slip faults transect this trend folds are open and upright inclined and tighter near the major faults where they have a clockwise rotation like the regional cleavage major faults e g grand pabos follow corridors delimiting high strain zones where the fabrics developed are indicative of a ductilebrittle regime of deformation in the temiscouata region major structural features of both southern quebec and the gaspe peninsula are recognized structures of this region consist of ene trending dextral strike slip faults and high angle wnw verging reverse faults parallel to major nne folds these structures reflect the transition from purely horizontal movement along major strike slip faults of the gaspe peninsula to vertical movement along thrust faults in southern quebec acadian structural variations within the quebec appalachians are interpreted in terms of a continental collision of gondwana along the irregular margin of laurentian and its taconian accreted terranes

diachronous supracrustal extension in an intraplate setting and the origin of the connecticut valley gaspe and merrimack troughs northern appalachians in the appalachians of mainland canada and new england silurian early devonian rocks are preserved in the connecticut valley gaspe and merrimack troughs and rest unconformably or in fault contact with older rocks belonging to laurentia and to gander avalon respectively the silurian early devonian rocks consist of marine clastic deposits with subordinate carbonates lava flows and terrestrial deposits the origin of these sedimentary basins is still poorly understood metamorphic ages and structures in the laurentian margin major unconformities and syn sedimentary normal faulting in both troughs argue for significant crustal extension during deposition the connecticut valley gaspe and merrimack troughs are separated by inliers of pre ordovician to ordovician rocks which we interpret as silurian basement highs that would have been buried in devonian times to form a composite sedimentary basin volcanic rocks are widely distributed in time and space in both basins and are mostly subalkaline within plate tholeiites which is consistent with a tectonic setting involving crustal extension rather than with subduction granitic plutons are abundant in the merrimack trough and attest to high temperatures at mid crustal levels it is suggested that crustal extension was responsible for the formation of both basins and that heating of the lower crust in the merrimack trough during the silurian was the result of synorogenic collapse likely triggered by delamination at the laurentia medial new england boundary delamination of the subducted slab and the upwelling of the asthenosphere would have caused 1 isostatic uplift and formation of basement highs 2 magmatism in the lower crust and regional scale contact metamorphism in the upper crust and 3 collapse of metamorphic terranes and the formation of subsiding sedimentary basins