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

hydrodynamics of steep streams with planar coarse grained beds turbulence flow resistance and implications for sediment transport the hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows energetic subsurface flow and macro scale form drag from immobile boulders and channel and bed forms heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro scale form drag however little work has explored steep river hydrodynamics in the absence of complex bed geometries here we present theory for the vertical structure of flow velocity in steep streams with planar rough beds that couples surface and subsurface flow we test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes 0 4 30 experimental flows have a nearly logarithmic velocity profile far above the bed flow velocity decreases less than logarithmically towards the bed and is non zero at the bed surface velocity profiles match theory derived using a hybrid eddy viscosity model in which the mixing length is a function of height above the bed and bed roughness subsurface flow velocities are large 1 m s and follow a modified darcy brinkman forchheimer relation that accounts for channel slope and shear from overlying surface flow near bed turbulent fluctuations decrease for shallow rough flows and scale with the depth averaged flow velocity rather than bed shear velocity flow resistance for rough planar beds closely matches observations in natural steep streams despite the lack of bed or channel forms in the experiments suggesting that macro scale form drag is smaller than commonly assumed in stress partitioning models for sediment transport

regional wave propagation in new england and new york we validate and improve 1d velocity models of the two main crustal provinces in the northeastern united states neus using seismograms from the 20 april 2002 m 5 au sable forks earthquake which is the largest earthquake in the region to be recorded by multiple recently deployed good quality regional broad band stations to predict and mitigate the effects of future earthquakes in the north eastern united states more information is needed regarding both the local earthquake sources and how seismic waves travel through the region we investigate the source and regional wave propagation for the au sable forks earthquake the earthquake epicenter is located near the boundary of two distinct geological provinces the appalachian new england and grenville new york we use a forward modeling approach to study the waveforms recorded at 16 stations located within 400 km of the epicenter we generate synthetic seismograms using the frequency wavenumber method testing several published models for the two provinces several models per form well at low frequencies 0 1 hz we refine these models and generate two alternative 1d crustal models for intermediate frequencies 1 hz of engineering interest our new grenville model performs better than previously published models for all six source station paths modeled in that province according to goodness of fit statistics variance reduction and correlation coefficient our alternative appalachian model improves the fit of synthetics to data for five of the ten paths modeled in that province from the results we identify two specific sources of wave field complex ities that should be investigated in future studies of earthquake ground motions in neus 3 azimuthal anisotropy in the appalachian province and complex wave paths along the boundary between the two provinces

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

bryozoan beds in northern italy as a shallow water expression of environmental changes during the oligocene isotope event 1 abstract shifts in carbonate producing biotic communities in the geological record provide evidence of past environmental changes in the neritic realm the shallow marine calcare di nago formation exposed in the san valentino section northern italy covers the late eocene and earliest oligocene the succession is characterized by the occurrence of light dependent biota such as coralline algae and larger benthic foraminifera in the uppermost part of the section however the fossil association is dominated by bryozoans which are filter feeder organisms this ca 12 m thick interval locally contains up to 86 bryozoans while coralline algae as well as larger benthic foraminifera are absent coralline algae and nummulitid foraminifera recover in the upper part of the bryozoan beds whereas orthophragminids do not recover the gradual disappearance of larger foraminifera and coralline algae within the bryozoan dominated deposits is coeval with a pronounced positive shift in 13c based on its biostratigraphic position this positive shift is interpreted to be linked to the positive shift in 13c recognized in deep sea records shortly above the eocene oligocene boundary which in turn is associated to the positive shift in 18o leading to the oi 1 oligocene isotope event 1 cooling phase total phosphorus content increases in the bryozoan beds suggesting enhanced nutrient supply to the neritic ecosystem this phosphorous peak is coeval with the globally recognized increment in ocean productivity around the oi 1 and 13c positive shift thus disappearance of light dependent biota and the dominance of bryozoans in the platform carbonates studied are interpreted to result not necessarily from a deepening of the depositional environment but from the combination of lower sea surface temperatures and the deterioration of underwater light conditions on account of elevated turbidity in surface waters resulting from enhanced primary productivity as bryozoan beds occur in several italian localities around the eocene oligocene boundary they are interpreted to represent the regional expression of neritic carbonate depositional systems to global environmental changes occurring at the dawn of an ice house earth

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

determination of seismic energy from lg waves seismic energy is a physical concept related to broadband information on the source radiation this is different from seismic moment which is related to low frequency information improved estimates of seismic moment and radiated energy can improve the scientific framework for interpreting observed seismicity in terms of physical source processes this study evaluates the utility of regional lg waves the dominant feature of seismograms for regional earthquakes for the estimation of radiated seismic energy the rms lg which is defined as the root mean square of lg wave ground velocity within a certain group velocity range is proportional to the square root of lg wave radiated energy flux we find that rms lg is a good quantity for estimating the energy release of earthquakes because a large fraction of the total seismic energy is radiated as lg waves we scale the network averaged rms lg value to a reference distance of 100 km for a set of 52 earthquakes in the northeastern united states using an empirically derived relation between rms lg and epicentral distance we apply the source parameters estimated for these earthquakes to boatwright s deceleration source model and calculate the total seismic energy expected to have been radiated by each event assuming it can be modeled by this source we then obtain for earthquakes in the northeastern united states the scaling of the total radiated seismic energy of earthquakes with network averaged rms lg values from synthetic seismograms it is found that different velocity structures have an effect on the efficiency of lg wave radiation using measurements of rms lg made from synthetics we find that the vertical strike slip source tends to radiate less lg wave energy than other source mechanisms manuscript received 4 february 1998

effect of fluid slippage on eddy growth and non darcian flow in rock fractures abstract fluid flow in rock fractures is usually theoretically and numerically investigated on the premise of no slip boundary condition bc however fluid slippage at rock surface is naturally present in some circumstances that might lead to a violation of no slip bc how and to what extent slip bc affects non darcian fracture flow remains poorly constrained this study systematically investigated the slippery flow behaviors in rock fractures under sequentially increasing pressure gradients two competing mechanisms impacting the apparent permeability k due to fluid slippage were revealed k was not only enhanced by fluid slippage at fracture walls but also was reduced by the accelerated eddy growth due to enhanced velocity field the increment in k caused by slip velocity initially dominated over the decrement caused by eddy growth during this competing process but this dominant situation could reverse given a sufficiently large pressure gradient with fully developed eddies moreover the slippery fluid flow behavior was tested to be very sensitive to the slip length based on our sensitivity analysis therefore a reasonable estimation of the slip length would be crucial to accurately determine the effects of fluid slippage this study deepens our understanding of fluid flow in rock fractures when slip bc is present and might provide theoretical guidance for organic pollution remediation oil recovery and geological carbon sequestration in fractured reservoirs

adirondack appalachian crustal structure the cocorp northeast traverse reflection surveys from northeastern new york and central vermont the first elements of a cocorp seismic traverse of grenville and appalachian terranes in the northeastern united states indicate that an unusual zone of layered reflections characterizes the lower crust beneath the eastern adirondacks and that grenville age basement of the green mountains may actually be allochthonous perhaps thrust over paleozoic shelf rocks the top of the adirondack deep layered sequence is found at about 20 km beneath the marcy meta anorthosite massif it appears to dip about 10 northwestward and wedge out against more horizontal reflections at about 28 km this intracrustal sequence may correspond with a previously inferred increase in electrical conductivity possible interpretations range from its being a layered igneous complex to its representing a tectonically buried metasedimentary assemblage although graphitic schists and iron or sulfide segregations in such an assemblage might contribute to its electrical characteristics the problematical issue of water in the lower crust if and how it got there and remained is again raised if not resolved other intrabasement features include major east and west dipping reflections possibly corresponding to faults arched reflections that may represent folds a well defined lens shaped zone virtually devoid of reflections and possible moho reflections unequivocal sub moho reflections are absent superimposed at shallower levels on the eastern part of the traverse are appalachian structures reflections down to about 4 5 km beneath the taconic mountains are interpreted to represent paleozoic shelf sediments of the middlebury synclinorium underlying the taconic allochthons possibly correlative although less well defined reflections appear to extend in disrupted fashion beneath the precambrian rocks of the green mountains suggesting a major basement overthrust the inferred decollement for this overthrust steepens rapidly beneath the east flank of the green mountains east dipping reflections at the east side of the green mountains may represent another major thrust zone involving basement and overlying metasedimentary slices the basement overthrusting suggested by this study is similar in certain respects to the style if not the extent of that demonstrated by previous cocorp surveys in the southern appalachians

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

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

enhancement of denitrification in permeable carbonate sediment due to intra granular porosity a multi scale modelling analysis abstract it has recently been hypothesized that bulk denitrification rates in carbonate sands may be enhanced by reactions occurring in the intra granular pores cracks and crevices we tested this hypothesis using a series of flow and reactive transport models spanning from the pore scale mm to the continuum scales 10 cm bedforms pore scale simulations solved the coupled navier stokes and brinkman equations and represented flow through reactor experiments previously performed on coral reef sands the results revealed that intra granular transport and reactions can explain over all denitrification enhancement a sensitivity study with a single grain diffusive transport model showed that in the majority of cases the resultant increase in denitrification was not coupled to nitrification within a single grain only for large grain diameters of 2 and 4 mm was coupled nitrification denitrification important in most cases coupled nitrification denitrification instead arose as conditions became more reducing along a flow path as is the case in quartz sands without intragranular pores an intra granular reaction rate based on a single grain model was incorporated into a continuum scale darcy flow and reactive transport model for a rippled sand bed where porewater flow is driven by the turbulent current over the ripple the results of the darcy scale model suggest that intra granular pores increase the amount of slow flowing areas within which coupled nitrification denitrification can occur we conclude that the complex advective flow field does not strongly inhibit denitrification enhancement by carbonate sand grains as it does in silica sands thus intra granular reactions may enhance bulk denitrification in carbonate sediment with porous grains under natural advective conditions

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

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

crustal structure of the southeastern grenville province northern new york state and eastern ontario the grenville province exposes an oblique cross section through mid lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the grenvillian orogeny 1 1 ga the southeastern grenville province is divided into two subterranes by the carthage colton mylonite zone a 110 km long lineament characterized by intense ductile shear and igneous intrusion which separates the amphibolite facies metasediments of the central metasedimentary belt to the west from the granulite facies metaplutonics of the central granulite terrane to the east the recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the grenville province raises questions concerning the deep structure of these subterranes and in particular the means by which the mid lower crustal rocks exposed in the grenville province were emplaced seismic refraction wide angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern grenville province a travel time inversion for velocity and interface depth was applied to the seismic data together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern grenville province in the central metasedimentary belt the upper crust is characterized by velocities in the range 6 3 6 4 km s and a poisson s ratio of 0 26 0 01 which are attributed to quartzofeldspathic rocks farther east in the central granulite terrane upper crustal velocities of 6 55 km s and a poisson s ratio of 0 28 0 01 are associated with the marcy anorthosite the seismic homogeneity of the upper crust in the region of the carthage colton mylonite zone suggests that this boundary is a shallow feature limited to the upper 2 3 km of the crust the deep crustal structure of the southeastern grenville province is characterized by two discrete and laterally discontinuous seismic interfaces in the central metasedimentary belt the top of the lower crust is delineated by an eastward dipping interface at 24 28 km depth in the central granulite terrane prominent en echelon reflections referred to as the tahawus complex form a gently arched dome at 17 22 km depth interpretation of the tahawus complex as a zone of layered mafic cumulates is supported by its high velocity 7 1 km s and poisson s ratio 0 27 0 02 the lower crust is characterized by a velocity of 7 0 7 2 km s and an anomalously high poisson s ratio of 0 30 0 02 which are representative of pyroxene garnet granulites in contrast velocities of 6 8 7 0 km s are modeled beneath the central granulite terrane and appear to signify a lateral change in composition the moho lies at 44 45 km depth and is characterized by pronounced en echelon reflection segments suggesting compositional interlayering around the crust mantle boundary the velocity of the upper mantle is 8 0 8 2 km s an anomalous upper mantle layer with a reversed velocity of 8 6 km s dips eastward from 50 to 60 km depth beneath the southeastern grenville province our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the grenvillian orogeny are preserved in the mid lower crust as a layered cumulate body tahawus complex and in the upper mantle as an eclogitic lens possibly delaminated from the overthickened crust during uplift of the southeastern grenville province

tectonic synthesis of the taconian orogeny in western new england a tectonic synthesis based on stratigraphic and structural analysis of western new england is proposed for the ordovician taconian orogeny it emphasizes arc continental collision in which ocean floor continental margin and ensialic rift rocks were imbricated westward in a repeatedly deformed accretionary wedge continued compression displaced segments of the north american sialic crust to the west and deformed the earlier emplaced slices of the taconic allochthons which were derived from the continental margin critical arguments for this synthesis are 1 the west to east stratigraphic relations among the basal rift clastic rocks of the dalton pinnacle and hoosac formations of late precambrian to early cambrian age 2 the stratigraphic and sedimentological similarities between the rocks of the lower taconic sequence and rocks in the pinney hollow and underhill slices to the east and north of the green mountain massif 3 the environmental similarities between the cambrian and lower ordovician section of the giddings brook slice and the age equivalent section in the st albans synclinorium 4 the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices groups 1 and 2 of the taconic allochthons 5 the presence of synmetamorphic fault related structures in the youngest and highest slices group 3 of the taconic allochthons 6 the recognition of extensive thrust zones in the pre silurian eugeoclinal sequence east of the middle proterozoic basement of the housatonic berkshire green mountain and lincoln massifs 7 the location of the taconic root zone within the pre silurian eugeoclinal sequence 8 the recognition of numerous faults in the serpentinite belt 9 the similarity between the rocks of the moretown formation and modern fore arc basin sequences 10 the recognition that the volcanic arc continental complexes of the ascot weedon and bronson hill have been displaced westward over the moretown and or hawley formations along such faults as the bristol and coburn hill thrusts 11 the allochthonous and internally imbricated nature of the north american basement in the berkshire massif 12 the proposition that the housatonic green mountain and lincoln massifs as well as the middle proterozoic cored domes of southeastern vermont are also thick sialic slices of north american basement 13 the recognition of medium high to high pressure metamorphic mineral assemblages in the pre silurian eugeoclinal rocks of vermont and 14 the recent synthesis of isotopic age data by sutter and others 1985 on the basis of an analysis of the foregoing arguments and relationships a chronological sequence of seven structural sections between albany new york and the bronson hill anticlinorium in central massachusetts is used to depict the evolution of the taconian orogeny retrodeformed distances are based on structural overlap and restoration of the taconic slices to their depositional setting along the ancient north american continental margin these easterly younging diverticulated slices formed as a result of horizontal compression rather than gravity sliding this palinspastic analysis implies the following 1 approximately 1 000 km of shortening has occurred during the emplacement of the taconic allochthons and the subsequent imbrication of north american basement as thick sialic slices approximately 330 km of this shortening is attributed to multiple cleavage generations 2 repeated movement along such major surfaces as the cameron9s line whitcomb summit belvidere mountain thrust zone has buried the taconic root zone we suggest that the northern extension of this root zone is exposed to the east of the lincoln massif in vermont where the underhill pinney hollow and hazens notch formations are exposed these formations here considered thrust slices disappear along the belvidere mountain whitcomb summit thrust zone as it is traced southward into western massachusetts and western connecticut 3 taconian metamorphic rocks particularly the older medium high pressure rocks in northern vermont have been transported westward on such reactivated surfaces as the belvidere mountain thrust 4 the anticlinorial form of the middle proterozoic basement in the green mountain and lincoln massifs may have resulted from fault bend folding on deep mantle involved thrusts that developed late in the taconian orogeny

crustal velocity and moho topography in central new hampshire the white mountain batholith in central new hampshire is a major plutonic topographic and gravimetric feature superimposed on the ne sw trending regional pattern of accreted terranes in northeastern north america compressional and shear wave arrival times from a two dimensional seismic array experiment are used to constrain a one dimensional crustal velocity model and moho topography for central new hampshire our deployment a piggyback on the ontario new york new england refraction experiment onynex recorded 10 explosive sources using 20 three component digital seismometers the crust can be modeled as 34 5 km thick and with three layers having linear gradients without midcrust discontinuities for this model p velocities are 6 1 6 2 km s in the upper crust and 6 2 6 8 km s in the lower crust s velocities are 3 5 3 7 km s in the upper crust and 3 7 3 9 km s in the lower crust because moho reflections comprise our principal constraints on lower crustal velocities there is a tradeoff between lower crustal velocity and crustal thickness in particular an average crustal thickness of 39 40 km reported elsewhere in new england is not precluded by our data the poisson s ratio for the crust varies between 0 23 0 26 in the lower range of values reported for crustal minerals our dataset is consistent with 2 lateral variation in the upper crust underlying the new hampshire and white mountain plutonic series but larger variations deeper than 15 km if modeled in terms of moho topography pmp traveltime residuals and pmp p residuals suggest thicker crust 36 37 km under the white mountain batholith with thinner crust 33 34 km to the east and southwest of the white mountains such topography of the moho correlates well with both the surface elevation and the bouguer gravity anomaly in the region gravity data do not favor the simplest alternative model a thick low velocity granitic lower crust beneath the batholith using a simplified density model roughly 30 35 of the gravity anomaly in the white mountain region can be modeled as the result of the inferred moho depression requiring only a 5 km low density root for the white mountain batholith the short wavelength of the moho topography is consistent with a very low flexural rigidity for the lithosphere 1021 nt m suggesting that the plutons were intruded into a weak lithosphere a rapid transition from positive to negative pmp residuals falls along a line that parallels the onshore extension of the kelvin seamounts and may correspond to the line of weakness hypothesized by sykes 1978 to have been tectonically active during the opening of the north atlantic ocean at 100 200 ma

wide angle deep crustal reflections in the northern appalachians summary seismic refraction data collected in the northern appalachians provide an unusual opportunity to use wide angle reflections to examine the lower crust and upper mantle the pmp phase clearly identified on several hundred records has been used to construct an isopach map of the crust which shows a stepwise regional thickening of the crust beneath the axis of the appalachians the data have been examined to times of up to 40 s two way travel time to investigate the possibility of coherent upper mantle reflections such as those recently observed by birps on near vertical data northwest of britain although substantial coherent energy appears after the pmp phase synthetic seismogram modelling shows that all of these arrivals are explicable by s phases converted phases and multiples from within the crust we conclude that in this region of the northern appalachians we have not detected any significant regionally extensive reflecting horizons within the upper mantle

the role of eddies inside pores in the transition from darcy to forchheimer flows 1 we studied the role of intra pore eddies from viscous to inertial flows in modifying continuum scale flow inside pores flow regimes spanning reynolds numberre 0 to 1350 are divided into three zones one zone follows darcy flow and the other two zones describe non darcy or forchheimer flow during viscous flows i e re 1 and their growth leads to the deviation from darcy s law and the emergence of forchheimer flow manifested as a characteristic reduction in the apparent hydraulic conductivity ka the reduction in ka is due to the narrowing of the flow channel which is a consequence of the growth in eddies the two zones of forchheimer flow correspond to the changes in rate of reduction in ka which in turn are due to the changes in eddy growth rate since the characteristics of forchheimer flow are specific to pore geometry our results partly explain why a variety of forchheimer models are expected and needed for different porous media

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

the acadian thermal history of the merrimack synclinorium in new hampshire the acadian structural history of the merrimack synclinorium involves a phase of nappe emplacement and thrusting at 400 m y b p followed by a second thickening event before 370 m y b p at the end of this phase the present erosion surface was buried at pressures of 6 to 8 kbar and subsequently experienced peak metamorphic temperatures in the range of 500 c to 650 c the thermal history of the acadian metamor phism of the merrimack synclinorium is examined in a well studied area of central new hampshire to determine the conditions required for the high grade metamorphism of the region constraints are provided by metamorphic pressure estimates by information on the erosional history and by present day heat flow determinations in the synclinorium one dimensional heat transfer calculations show that the high grade metamorphism could readily have been produced during thermal relaxation of the thickened continental crust under conditions of normal continental heat flow these conditions also permit the me

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