detailed crustal thickness variations beneath the illinois basin area implications for crustal evolution of the midcontinent we present high resolution imaging results of crustal and upper mantle velocity discontinuities across the illinois basin area using both common conversion point stacking and plane wave migration methods applied to p wave receiver functions from the earthscope oiink ozark illinois indiana and kentucky experiment the images reveal unusually thick crust up to 62 km throughout the central and southeastern illinois basin area a significant moho gradient underlies the nw trending ste genevieve fault zone which delineates the boundary between the illinois basin and ozark dome relatively thinner crust 45 km underlies most of the precambrian highlands surrounding the illinois basin and beneath the rift related structures of the reelfoot rift and the rough creek graben we consider four hypotheses to explain the presence of thick crust under the central and southeastern illinois basin crustal thickening may have been produced 1 prior to its accretion to north america around 1 55 ga and is an inherited characteristic of this crustal province 2 by underthrusting or shortening during proterozoic convergent margin tectonics around 1 55 1 35 ga 3 by late precambrian magmatic underplating at the base of older crust associated with the creation of the eastern granite rhyolite province around 1 3 ga and 4 through crustal relamination during an episode of proterozoic flat slab subduction beneath the illinois basin possibly associated with the grenville orogeny

variations in mid continent rift magma volumes consistent with microplate evolution 1 modeling of gravity data along the 1 1 ga failed mid continent rift system shows systematic patterns in magma volume between and along the rift s two arms the volume of magma increases towards the lake superior region consistent with magma flowing away from a hotspot source there the west arm experienced significantly more magmatism these patterns are consistent with a model in which the two rift arms acted as boundaries of a microplate the volume of magma along the west arm increases with distance from the euler pole indicating that it acted essentially as a spreading ridge whereas the much smaller magma volumes along the east arm are consistent with its acting as a leaky transform this view of the rift system s evolution is compatible with the rift being part of an evolving plate boundary system rather than an isolated episode of midplate volcanism

evidence and implications of an extensive early mesozoic rift basin and basalt diabase sequence beneath the southeast coastal plain regional seismic reflection profiles across the georgia and south carolina coastal plain provide a detailed view into the internal structure and stratigraphy of the buried triassic jurassic south georgia basin reflection data indicate that the basin is a complex composite of smaller individual basins which vary drastically along strike these basins are relatively wide along north south profile maximum 100 km but appear to be more restricted along strike in cross section the basins are asymmetric being typically bounded on their south sides by an interpreted north dipping master normal falult the thickness of interpreted basin fill below the cretaceous tertiary overlap also varies drastically locally reaching 6 km analysis and correlation of the reflection profiles when integrated with drill hole and offshore reflection data suggest the presence of an areally extensive basalt flow and or diabase sill sequence of probable early middle jurassic age covering a large part of the triassic jurassic south georgia basin the sequence is traceable on seismic reflection profiles across much of the south georgia basin and may extend as far as from western georgia to offshore south carolina the implied areal extent of this sequence would be more than 100 000 km 2 ranking it among the great basalt flows and or sills of the world on seismic reflection profiles over the south georgia basin the basalt diabase sequence is also important in marking a post rift unconformity which separates tilted and apparently faulted lower mesozoic rift related strata below from a thinner more widespread and generally unfaulted sequence above this configuration is interpreted to mean that basalt extrusion and or hypabyssal intrusion occurred after the main episode of late triassic early jurassic normal faulting and associated localized subsidence syn rift phase and prior to a later jurassic episode of more widespread uniform subsidence post rift phase accompanied by relatively minor normal faulting this two stage evolutionary sequence is analogous to that of many rift basins throughout the world and suggests a hypothesis that the south georgia basin was an area of incipient crustal spreading associated with the early formation of the central atlantic ocean

devils hole nevada δ18o record extended to the mid holocene abstract the mid to late pleistocene devils hole 18 o record has been extended from 60 000 to 4500 yr ago the new 18 o time series in conjunction with the one previously published is shown to be a proxy of pacific ocean sea surface temperature sst off the coast of california during marine oxygen isotope stages mis 2 and 6 the devil hole and sst time series exhibit a steady warming that began 5000 to 10 000 yr prior to the last and penultimate deglaciations several possible proximate causes for this early warming are evaluated the magnitude of the peak 18 o or sst during the last interglacial lig is significantly greater 1 per mill and 2 to 3 c respectively than the peak value of these parameters for the holocene in contrast benthic 18 o records of ice volume show only a few tenths per mill difference in the peak value for these interglacials statistical analysis provides an estimate of the large shared information variation between the devils hole and eastern pacific sst time series from 41 to 2 n and enforces the concept of a common forcing among all of these records the extended devils hole record adds to evidence of the importance of uplands bordering the eastern pacific as a source of archives for reconstructing pacific climate variability

electrical conductivity structure of southeastern north america implications for lithospheric architecture and appalachian topographic rejuvenation abstract we present the first three dimensional view of the lithospheric electrical conductivity structure beneath southeastern north america by inverting earthscope long period magnetotelluric mt data we obtain an electrical conductivity image that provides new insights into both the architecture of the appalachian orogen and the cryptic post rifting geodynamic history of the southeastern united states our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the appalachian orogen most significantly we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern piedmont and coastal plain physiographic provinces as high resistivity values in mantle minerals require cold mantle temperatures the mt data indicate that the sub piedmont thermal lithosphere must extend to greater than 200 km depth this firm bound conflicts with conclusions from seismic results the boundary between the anomalously thick resistive sub piedmont lithosphere and the relatively thin moderately conductive sub appalachian lithosphere corresponds within resolution to the modern appalachian topographic escarpment this newly recognized contrast in lithospheric properties likely has important implications for appalachian topographic rejuvenation

comment on reconciliation of the devils hole climate record with orbital forcing moseley et al reports 8 january 2016 p 165 postulate an increase in dissolved thorium isotope 230 th with depth below the water table as the explanation for the differing ages of termination ii flow of geothermal water through the devils hole caverns precludes this explanation deposition of younger secondary calcite into the initial porosity of the calcite comprising their cores is a plausible alternate explanation

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

seismic structure of the central us crust and shallow upper mantle uniqueness of the reelfoot rift abstract using seismic surface waves recorded with earthscope s transportable array we apply surface wave imaging to determine 3d seismic velocity in the crust and uppermost mantle our images span several proterozoic and early cambrian rift zones mid continent rift rough creek graben rome trough birmingham trough southern oklahoma aulacogen and reelfoot rift while ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences the reelfoot rift is unique in its association with low mantle seismic velocity its mantle low velocity zone lvz is exceptionally pronounced and extends down to at least 200 km depth this lvz is of variable width being relatively narrow 50 km wide within the northern reelfoot rift which hosts the new madrid seismic zone nmsz we hypothesize that this mantle volume is weaker than its surroundings and that the reelfoot rift consequently has relatively low elastic plate thickness which would tend to concentrate tectonic stress within this zone no other intraplate ancient rift zone is known to be associated with such a deep mantle low velocity anomaly which suggests that the nmsz is more susceptible to external stress perturbations than other ancient rift zones

crustal evolution across the southern appalachians initial results from the sesame broadband array 1 receiver functions from the earthscope southeastern suture of the appalachian margin experiment broadband deployment and u s transportable array were analyzed to constrain average crustal thickness and composition across the southern appalachians low vp vs ratios 1 69 1 72 across the carolina terrane and parts of the inner piedmont indicate that the crust has a felsic average composition the results are consistent with models of thin skinned thrusting of carolina arc fragments over laurentian basement whereas arc collision models require significant crustal modification to explain the low vp vs new crustal thickness estimates provide constraints on the extent of the blue ridge crustal root the present root may be a remnant of a broader structure formed by alleghanian thrust loading root preservation is attributed to mesozoic heating and thinning of the lower crust beneath outboard terranes leaving colder blue ridge crust largely intact however thickened crust 50 55 km across the region may also be inherited from continental collision during the proterozoic grenville orogeny

interpreting seasonal convective mixing in devils hole death valley national park from temperature profiles observed by fiber optic distributed temperature sensing 1 devils hole a groundwater filled fracture in the carbonate aquifer of the southern nevada mojave desert represents a unique ecohydrological setting as home to the only extant population of cyprinodon diabolis the endangered devils hole pupfish using water column temperatures collected with a fiber optic distributed temperature sensor dts during four field campaigns in 2009 evidence of deep circulation and nutrient export are for the first time documented the dts was deployed to measure vertical temperature profiles in the system and the raw data returned were postprocessed to refine the calibration beyond the precision of the instrument s native calibration routines calibrated temperature data serve as a tracer for water movement and reveal a seasonal pattern of convective mixing that is supported by numerical simulations of the system the periodic presence of divers in the water is considered and their impacts on the temperature profiles are examined and found to be minimal the seasonal mixing cycle may deplete the pupfish s food supplies when nutrients are at their scarcest the spatial and temporal scales of the dts observations make it possible to observe temperature gradients on the order of 0 001 c m 1 revealing phenomena that would have been lost in instrument noise and uncertainty

seaward dipping reflectors offshore the southeastern united states seismic evidence for extensive volcanism accompanying sequential formation of the carolina trough and blake plateau basin deep penetration multichannel seismic reflection profiles off the southeastern united states reveal widespread seaward dipping reflectors sdrs similar features have been imaged and sampled on other north atlantic rifted margins where voluminous volcanism has accompanied continental breakup beneath the carolina trough are two sets of sdrs one along a basement hinge zone and another seaward of the east coast magnetic anomaly axis the hinge sdrs lie beneath and apparently developed prior to a flood basalt that erupted at 184 3 ma and is marked by a prominent reflector j beneath the northern blake plateau basin only the hinge sdrs are observed but they developed after j we suggest that the inferred north to south age difference of sdr emplacement implies a heretofore unrecognized time transgressive breakup of northwest africa and north america during the early middle jurassic

low temperature thermal history and landscape development of the eastern adirondack mountains new york constraints from apatite fission track thermochronology and apatite u th he dating the adirondack mountains in northern new york state form an elongate domal exposure of mainly high grade metamor phic tectonites in a mountainous setting with topographic relief of 1 km the origin of the adirondack mountains and this relief has long been enigmatic since the adirondacks presently lie within an intracratonic setting inboard of the north american passive mar gin and far from any active plate boundaries through the application of apatite fi ssion track aft thermochronology and apatite u th he ahe dating within the eastern adirondack mountains this study provides constraints on both the thermal and ero sional effects of mesozoic passage near a hot spot and the timing of relief development aft thermochronology and ahe dating record relatively stable thermal conditions within the eastern adirondacks from the middle jurassic into the early cretaceous during the early cretaceous ca 130 120 ma the region underwent heating as sociated with progressive movement near the great meteor hotspot resulting in the temporary establishment of an elevated geo thermal gradient following regional heat ing cooling rates increased considerably ca 105 95 ma likely due to both thermal doming producing an increase in erosion rate and the relaxation of isotherms after passage near the hotspot the regional distribution of aft ages across the eastern adirondacks reveals no systematic age gradient from core to periph ery which would be expected under con ditions of persistent high relief during the decay of a crustal root over many tens to hun dreds of millions of years instead thermo chronological data suggest that the present relief developed during the late cretaceous cenozoic through plateau dissection during periodic base level changes

stratigraphy and structure of the laurentian rifted margin in the northern appalachians a low angle detachment rift system the neoproterozoic early cambrian rifted margin of eastern laurentia is framed by promontories and embayments defined by northeast trending rifts offset by northwest trending transforms a regional first order synthesis of the available stratigraphic data in the northern appalachians reveals significant along strike variations in the thickness composition age and facies of synrift and early postrift stratigraphy these variations are consistent with models for low angle detachment rift systems and allow for resolution of regional structures specific to low angle detachments including upper plate margins lower plate margins and transform faults that bound zones of oppositely dipping low angle detachments

crustal composition and moho variations of the central and eastern united states improving resolution and geologic interpretation of earthscope usarray seismic images using gravity

wave gradiometry and its link with helmholtz equation solutions applied to usarray in the eastern u s wave gradiometry is an array processing technique utilizing the shape of seismic wavefields captured by usarray ta stations to determine fundamental wave propagation characteristics we first explore a compatibility relation that links the displacement spatial gradients to seismogram displacements and velocities through two unknown coefficients a and b these coefficients are solved for through iterative damped least squares inversion to provide estimates of phase velocity back azimuth radiation pattern and geometrical spreading we show that the a coefficient corresponds to the gradient of logarithmic amplitude and the b coefficient corresponds approximately to the local wave slowness a and b vector fields are interpolated to explore a second compatibility relation through solutions to the helmholtz equation for most wavefields passing through the eastern u s we show that the a coefficients are generally orthogonal to the b coefficients where they are not completely orthogonal there is a strong positive correlation between b and changes in geometrical spreading which can be further linked with areas of strong energy focusing and defocusing we finally obtain isotropic rayleigh wave phase velocity maps for 15 periods between 20 and 150 s by stacking results from 37 earthquakes which show a velocity change along the approximate boundary of the early paleozoic continental margin we also observe two low velocity anomalies one centered over the central appalachians where eocene basaltic volcanism has occurred and the other within the northeastern u s possibly associated with the great meteor hotspot track

dense lower crust elevates long term earthquake rates in the new madrid seismic zone knowledge of the local state of stress is critical in appraising intraplate seismic hazard inverting earthquake moment tensors we demonstrate that principal stress directions in the new madrid seismic zone nmsz differ significantly from those in the surrounding region faults in the nmsz that are incompatible with slip in the regional stress field are favorably oriented relative to local stress we jointly analyze seismic velocity gravity and topography to develop a 3 d crustal and upper mantle density model revealing uniquely dense lower crust beneath the nmsz finite element simulations then estimate the stress tensor due to gravitational body forces which sums with regional stress the anomalous lower crust both elevates gravity derived stress at seismogenic depths in the nmsz and rotates it to interfere more constructively with far field stress producing a regionally maximal deviatoric stress coincident with the highest concentration of modern seismicity moreover predicted principal stress directions mirror variations observed independently in moment tensors at the nmsz and across the region

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

timing and duration of the central atlantic magmatic province in the newark and culpeper basins eastern u s a abstract new major and trace element data and 40ar 39ar plateau ages constrain the timing duration and time related geochemical evolution of the central atlantic magmatic province in the u s a newark and culpeper basins and refine correlations with basaltic lava flows from other late triassic early jurassic circum atlantic basins the precise statistically robust 40ar 39ar plateau ages were obtained on biotite and on fresh plagioclase and calculated using the latest 40k decay constants these ages are supported by a general consistency of the ca k calculated from 37ar 39ar of the plateau steps and the ca k obtained by detailed electron microprobe analyses on plagioclase phenocrysts the ages of five analyzed basalt lava flows from all three lava flow units in the newark basins and the ages of two sill samples are indistinguishable indicating a brief magmatic peak phase at 201 8 0 7 ma recalibrated 40ar 39ar plateau ages from the entire province indicate a near synchronous onset and peak volcanic activity at the triassic jurassic boundary within the circum atlantic basins from the u s a canada and morocco the early erupted magmas moroccan lower to upper basalts the fundy basin north mountain basalt and orange mountain and equivalent u s a flows yield an enriched geochemical signature e g with relatively high la yb whereas late magmas in the u s a hook mountain and hampden basalts and morocco recurrent basalt yield relatively depleted geochemical compositions low la yb a slight but significant age difference for eruption of hook mountain and hampden basalts 200 3 0 9 ma and recurrent basalts 198 2 1 1 ma is interpreted as evidence of a diachronous northward rift drift transition during break up of pangea our data indicate also a prolonged intrusive sequence that continued until about 195 ma at the palisades sill and is consistent with sporadic late camp magmatism for dykes from the south eastern u s a and for intrusions from guinea

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

neogene rejuvenation of central appalachian topography evidence for differential rock uplift from stream profiles and erosion rates abstract the persistence of topography within ancient orogens remains one of the outstanding questions in landscape evolution in the eastern north american appalachians this question is manifest in the outstanding problem of whether topographic relief is in a quasi equilibrium state decaying slowly over many millennia or whether relief has increased during the late cenozoic here we present quantitative geomorphic data from the nonglaciated portion of the susquehanna river drainage basin that provide insight into these end member models analysis of channel profiles draining upland catchments in the northern valley and ridge appalachian plateau blue ridge and piedmont provinces reveals that a large number of streams have well defined knickpoints clustered at 300 600 m elevation but not systematically associated with transitions from weak to resistant substrate cosmogenic 10 be inventories of modern stream sediment indicate that erosion rates are spatially variable ranging from 5 30 m myr above knickpoints to 50 100 m myr below knickpoints overall channel gradients normalized for drainage area scale linearly with catchment averaged erosion rates collectively regionally consistent spatial relationships among erosion rate channel steepness and knickpoints reveal an ongoing wave of transient channel adjustment to a change in relative base level reconstructions of relict channel profiles above knickpoints suggest that higher rates of incision are associated with 100 150 m of relative base level fall that accompanied epierogenic rock uplift rather than a change to a more erosive climate or drainage reorganization channel response timescales imply that the onset of relative base level change predates 3 5 ma and may have begun as early as 15 ma we suggest that adjustment of the channel network was likely driven by changes in mantle dynamics along the eastern seaboard of north america during the neogene

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

lithospheric discontinuities beneath the u s midcontinent signatures of proterozoic terrane accretion and failed rifting abstract seismic discontinuities between the moho and the inferred lithosphere asthenosphere boundary lab are known as mid lithospheric discontinuities mlds and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution in this study we used s to p converted waves recorded by the usarray transportable array and the oiink ozarks illinois indiana kentucky flexible array to investigate lithospheric structure beneath the central u s this region a portion of north america s cratonic platform provides an opportunity to explore how terrane accretion cratonization and subsequent rifting may have influenced lithospheric structure the 3d common conversion point ccp volume produced by stacking back projected sp receiver functions reveals a general absence of negative converted phases at the depths of the lab across much of the central u s this observation suggests a gradual velocity decrease between the lithosphere and asthenosphere within the lithosphere the ccp stacks display negative arrivals at depths between 65 km and 125 km we interpret these as mlds resulting from the top of a layer of crystallized melts sill like igneous intrusions or otherwise chemically modified lithosphere that is enriched in water and or hydrous minerals chemical modification in this manner would cause a weak layer in the lithosphere that marks the mlds the depth and amplitude of negative mld phases vary significantly both within and between the physiographic provinces of the midcontinent double or overlapping mlds can be seen along precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks a prominent negative sp phase can be clearly identified at 80 km depth within the reelfoot rift this arrival aligns with the top of a zone of low shear wave velocities which suggests that it marks an unusually shallow seismic lab for the midcontinent this boundary would correspond to the top of a region of mechanically and chemically rejuvenated mantle that was likely emplaced during late precambrian early cambrian rifting these observations suggest that the lithospheric structure beneath the reelfoot rift may be an example of a global phenomenon in which mlds act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath

seismic velocity structure of the crust and shallow mantle of the central and eastern united states by seismic surface wave imaging seismic surface waves from the transportable array of earthscope s usarray are used to estimate phase velocity structure of 18 to 125 s rayleigh waves then inverted to obtain three dimensional crust and upper mantle structure of the central and eastern united states ceus down to 200 km the obtained lithosphere structure confirms previously imaged ceus features e g the low seismic velocity signature of the cambrian reelfoot rift and the very low velocity at 150 km depth below an eocene volcanic center in northwestern virginia new features include high velocity mantle stretching from the archean superior craton well into the proterozoic terranes and deep low velocity zones in central texas associated with the late cretaceous travis and uvalde volcanic fields and beneath the south georgia rift which contains jurassic basalts hot spot tracks may be associated with several imaged low velocity zones particularly those close to the former rifted laurentia margin

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

arc crustal differentiation mechanisms abstract the detailed vertical compositional and thermal structure of the entire kohistan arc section is constructed using 63 p t constraints and 209 plutonic whole rock analyses the aim is to better understand the nature of the chemical differentiation in the plutonic arc crust at different crustal levels results indicate that a distinct difference in character exists between the upper and lower plutonic arc crust plutonic rocks in the lower crust 25 30 km have whole rock chemical characteristics that indicate mineral accumulation and residual melt loss in these cumulate rocks there is a systematic chemical stratification of decreasing incompatible trace elements with depth e g rb k2o in contrast the plutonic rocks of the upper crust represent mainly frozen liquids and show no systematic relationship between chemical composition and intrusion depth these results clearly indicate that magmatic differentiation in the kohistan arc dominantly occurred in the deeper arc crust 30 km consistent with the lower crustal hot zone model to understand the role of density barriers for melt stagnation in the crust and thereby for the observed chemical stratification the detailed density structure of the plutonic arc crust is compared to melt densities calculated for plutonic rocks with near liquid compositions results indicate that melts are consistently less dense than rocks and there is no evidence for a neutral buoyancy line controlling melt emplacement it is speculated that melt stagnation in the lower arc crust and subsequent chemical differentiation of a felsic upper and mafic lower continental crust is dominantly controlled by temperature and to a lesser extent to density or rheological barriers

shallow mantle velocities beneath the southern appalachians from pn phases to constrain mantle structure that might contribute to the topography of the southern appalachian mountains pn phases from regional earthquakes recorded in northern georgia by earthscope southeastern suture of the appalachian margin experiment and transportable array stations were used to solve for shallow mantle p wave velocities mantle velocities vary laterally with values of 7 6 7 8 km s beneath the higher elevations of the blue ridge terrane and northwestern flank of the inner piedmont terranes and values of 8 3 8 5 km s farther south where elevation is lower the zone of low velocity mantle could represent a source of buoyancy that helps to support the higher elevations in addition to the root of thickened crust that also exists beneath the mountains

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

anomalous seaward dip of the lithosphere asthenosphere boundary beneath northeastern usa detected using differential array measurements of rayleigh waves summary rayleigh wave phase velocities and azimuth anomalies in the period range of 30 100 s are measured for a set of four triangular arrays of broad band seismometers in coastal northeastern usa this is a region in which a strong upper mantle slow shear velocity anomaly a new england anomaly crosses the continental margin earthquakes from a wide range of directions are used to detect the variation of parameters with azimuth of propagation no lateral heterogeneity in phase velocity is detected at these periods between stations at the centre and the edge of the anomaly however large 10 20 per cent azimuthal variations occur and have a cos 1 dependence which is indicative of a dipping structure in the upper mantle corresponding azimuth variations with a magnitude of 5 are also detected this behaviour is consistent with a southeasterly n150 e dip of the lithosphere asthenosphere boundary beneath new england this dip is associated with the shoaling of the new england anomaly beneath the adirondack mountains west of the array it is opposite to the dip associated with lithospheric thickening toward the interior of the craton

3 d upper mantle shear velocity model beneath the contiguous united states based on broadband surface wave from ambient seismic noise ambient noise seismic tomography has been widely used to study crustal and upper mantle shear velocity structures most studies however concentrate on short period 50 s surface wave from ambient noise while studies using long period surface wave from ambient noise are limited in this paper we demonstrate the feasibility of using long period surface wave from ambient noise to study the lithospheric structure on a continental scale we use broadband rayleigh wave phase velocities to obtain a 3 d vs structures beneath the contiguous united states at period band of 10 150 s during the inversion 1 d shear wave velocity profile is parameterized using b spline at each grid point and is inverted with nonlinear markov chain monte carlo method then a 3 d shear velocity model is constructed by assembling all the 1 d shear velocity profiles our model is overall consistent with existing models which are based on multiple datasets or data from earthquakes our model along with the other post usarray models reveal lithosphere structures in the upper mantle which are consistent with the geological tectonic background e g the craton root and regional upwelling provinces the model has comparable resolution on lithosphere structures compared with many published results and can be used for future detailed regional or continental studies and analysis

devils hole paleotemperatures and implications for oxygen isotope equilibrium fractionation subaqueous calcite in devils hole nevada was growing continuously from slightly super saturated groundwater providing a 570 kyr long o18 paleoclimate record due to its very slow growth it has been assumed to have grown under conditions of isotopic equilibrium however its holocene o18 value is 1 5 higher than predicted by laboratory precipitation based oxygen isotope thermometer calibrations the suggestion that devils hole calcite anchors the isotope thermometer to more 18o enriched values has stirred a debate as to which paleothermometer calibration is relevant for paleoclimate and casts doubt on the validity of o18 based paleotemperatures we used clumped isotopes to test the assumptions of the devils hole alternative 18o thermometer carbonate clumped isotopes are a temperature proxy that measures the abundance of 13c18o bonds in caco3 this proxy is independent of the water composition and therefore gives independent estimates of temperatures when calcite forms at thermodynamic and isotopic equilibrium we find that devils hole water paleotemperatures were constant at 30 6 2 6 c between 27 and 180 ka similarly to the modern groundwater temperature of 32 8 34 3 c the proximity of the devils hole clumped isotope data to values expected based on modern groundwater temperatures supports the notion that devils hole calcite grew under equilibrium conditions therefore the commonly used laboratory based o18 temperature calibrations should be reconsidered the constant water temperature over the glacial interglacial cycles indicates that the long devils hole o18 record reflects only variations in the groundwater o18 values and as such represents a valuable archive of paleoclimate and isotope paleohydrology

was the midcontinent rift part of a successful seafloor spreading episode the 1 1 ga midcontinent rift mcr the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the north american craton is traditionally considered a failed rift formed by isolated midplate volcanism and extension we propose instead that the mcr formed as part of the rifting of amazonia precambrian northeast south america from laurentia precambrian north america and became inactive once seafloor spreading was established a cusp in laurentia s apparent polar wander path near the onset of mcr volcanism recorded by the mcr s volcanic rocks likely reflects the rifting this scenario is suggested by analogy with younger rifts elsewhere and consistent with the mcr s extension to northwest alabama along the east continent gravity high southern appalachian rocks having amazonian affinities and recent identification of contemporaneous large igneous provinces in amazonia

love and rayleigh phase velocity maps 5 40 s of the western and central usa from usarray data abstract continuous data recorded on more than 1600 usarray seismic stations operating in the western and central us between 2006 and 2012 are used to map phase velocities of love and rayleigh waves at short periods 5 40 s using a noise correlation technique vertical and transverse records for all station pairs separated by less than 600 km are cross correlated in 4 h long segments and the resulting spectra are stacked for the time period of station operation dispersion curves are determined from the locations of zeros in the real component of the correlation spectra using a method based on that of aki 1957 phase velocity maps expanded on a 0 25 by 0 25 pixel grid are estimated by inversion of the phase velocity measurements comparison with predicted phase velocity maps based on the crustal model crust2 0 combined with the mantle model nd08 shows good agreement at the longer periods strong slow anomalies 25 in the short period maps are geographically correlated with basins and regions of thick sedimentary cover the strength of these anomalies is not well predicted by existing crustal velocity models

preserved extent of jurassic flood basalt in the south georgia rift a new interpretation of the j horizon at the end of the triassic 200 m y ago the central atlantic magmatic province camp one of the largest igneous provinces in the world was emplaced within a very short period of time the flows sills and dikes that mark the event are predominantly preserved in triassic rift basins along the atlantic margins conventional wisdom implies that the areally largest of the camp flows is preserved in the south georgia rift a triassic rift basin buried beneath the atlantic coastal plain the extent of this flow has been mapped on the basis of a prominent seismic reflection referred to as the j horizon this seismic horizon has been used as a time marker for estimating the end of rifting in the southern united states and the beginning of seafloor spreading reanalysis of existing well and seismic data however shows that the extent of the flood basalt is limited to a few areas and that the j horizon coincides with the base of the coastal plain this reopens the question of how the camp relates to the rift drift transition of eastern north america

non plane geometries of seismic surface wavefields and their implications for regional surface wave tomography summary seismic surface waves exhibit much more complicated wavefields than is commonly assumed we are led to this conclusion after analysing 90 teleseismic events recorded at on average eight broad band stations in southern germany large amplitude and phase fluctuations across the network are observed which as we show are definitely not due to instrument response or calibration problems in order to give an impression of how surface wavefields may look in reality we fit wavefields to the observed network data using basis wavefields derived from hermite gaussian functions we show both amplitude and phase distributions of several events synthetic wavefields generated by acoustic finite difference computations in a random medium with realistic correlation length and rms velocity fluctuations support the interpretation that most of the observed wavefield anomalies have accumulated on the path of the wave from the source to the network even if no lateral heterogeneities existed in the region of the network the large scale features of the observed wavefields would remain nearly unaltered we also model the synthetic wavefields using hermite gaussian wavefields and achieve extremely good fits in view of the large anomalies of the wavefields we investigate their implications for the regional surface wave tomography in this method one infers regional structure from a set of measurements of phase traveltime between pairs of stations the basic assumption of the method is that the wavefield incident on the network be plane thus all observed phase traveltime anomalies are interpreted in terms of regional heterogeneous structure the consequence is a seemingly highly inconsistent data set we perform several tomographic experiments with realistic synthetic data sets our experiments indicate that regional surface wave tomography may work if there is an excellent coverage of paths crossing the region and if each path is sampled several times under realistic conditions that is on a sparse network with not much more than one velocity value per path the imaging power of surface wave tomography is very poor owing to the seeming inconsistency of the phase traveltimes variance reduction is generally low with an increasing number of traveltime data it further decreases while at the same time the imaging power increases the opposite behaviour is observed if the number of data is reduced variance reduction increases but imaging power decreases hence in connection with regional surface wave tomography high variance reduction indicates a lack of data rather than the goodness of reconstruction

crustal structure of the southeast georgia embayment carolina trough preliminary results of a composite seismic image of a continental suture and a volcanic passive margin new deep penetration multichannel seismic reflection data combined with refraction results and magnetics modeling support a hypothesis that the carolina trough is a mesozoic volcanic passive margin exhibiting a seaward dipping wedge and associated underplating the structure of carolina platform continental crust is consistent with the late paleozoic continental collision that produced the appalachians but imbrication has had no obvious effect on shallower structures produced by mesozoic extension and volcanism the origin of prominent magnetic anomalies crossing the southeast georgia embayment can be explained by processes attending mesozoic separation of africa and north america and is not related to a paleozoic continental suture as previously postulated

imaging crustal structure beneath the southern appalachians with wavefield migration to constrain crustal structures in the southern appalachians and the suture zone with the gondwanan affinity suwannee terrane we applied the 2d grt wavefield migration method to the scattered incident p wavefield recorded by the earthscope southeastern suture of the appalachian margin experiment and adjacent transportable array stations we resolve the root of thickened crust beneath the high topography of the blue ridge mountains and estimate its density contrast with the mantle to be only 104 20 kg m3 a weak velocity contrast across the crustal root moho is observed and may be related to an ongoing crustal delamination event possibly contributing to local tectonic rejuvenation beneath the suwannee terrane we confirm prior observations of a gently south southeastward dipping crustal suture indicating the terminal collision of laurentia and gondwana involved several hundred kilometers of overthrusting

the northern appalachian anomaly a modern asthenospheric upwelling the northern appalachian anomaly naa is an intense laterally localized 400km diameter low velocity anomaly centered in the asthenosphere beneath southern new england its maximum shear velocity contrast at 200km depth is about 10 and its compressional to shear velocity perturbation ratio is about unity values compatible with it being a modern thermal anomaly although centered close to the track of the great meteor hot spot it is not elongated parallel to it and does not crosscut the cratonic margin in contrast to previous explanations we argue that the naa s spatial association with the hot spot track is coincidental and that it is caused by small scale upwelling associated with an eddy in the asthenospheric flow field at the continental margin that the naa is just one of several low velocity features along the eastern margin of north america suggests that this process may be globally ubiquitous