bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress directions in eastern North America determined to 4.5 km from borehole elongation measurements
Authors: Plumb Richard A. , Cox John W.
Published: February 2008

Stress directions have been determined to depths of 4.5 km in eastern North America from borehole elongation measured by dipmeter calipers in 47 wells. The average maximum horizontal stress directions are eastern Canada, N54°E ± 7°; Appalachian Basin, N58°E ± 8°; and the Illinois Basin, N89°W ± 5°. Stress directions determined for the Illinois and Appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions. New results from eastern Canada and the Appalachian Basin show that stress directions are everywhere consistent with the Midcontinent stress province. These data suggest that the maximum horizontal stress does not rotate from the ENE Midcontinent trend to NW near the Atlantic coast. A directional analysis of hydraulic fractures, petal‐centerline fractures, and natural fractures from the Appalachian Basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures, centerline fractures, and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures. These observations indicate that borehole elongations provide reliable stress orientations.
Isotope geochemistry of Pacific Mid‐Ocean Ridge Basalt
Authors: White William M. , Hofmann A. W. , Puchelt H.
Published: February 2008

We report Sr, Nd, and Pb isotope ratios of 51 mid‐ocean ridge basalts (MORB) from the Juan de Fuca Ridge, the Gorda Ridge, the East Pacific Rise (EPR) and the Galapagos Spreading Center (GSC). The results confirm the general isotopic homogeneity of Pacific MORB, though high Pb isotope ratios (206pb/204pb up to 19.24) occur in basalts from the Easter microplate, reflecting the influence of the Easter hotspot. There is no evidence in our data of a “Dupal” geochemical anomaly in the mantle beneath the EPR in the southern hemisphere. Sr, Nd, and Pb isotope ratios are all correlated with one another, but correlations involving 87Sr/86Sr are poorer than other correlations, indicating that Rb‐Sr fractionation in the sub‐Pacific mantle has to some degree been decoupled from Sm‐Nd, U‐Pb, and Th‐Pb fractionation. Small but significant isotopic variations occur in MORB on regional scales of 1–100 km. These variations are, however, of lesser amplitude than those observed on larger scales. Small‐scale isotopic variations indicate either that large magma chambers do not exist beneath the EPR or that they are not efficient homogenizers of melts passing through them. On regional scales, Pb isotope ratios are more highly correlated than in Pacific MORB overall. We interpret these correlations as isochrons which record the age of events that created the regional mantle heterogeneities. These ages range from 1.4 to 2.4 Ga. Regional coherence of isotopic compositions indicates that the MORB source region consists of integral volumes which may be related to convection patterns. A comparison of the mean isotopic compositions of Pacific spreading ridges with means of Indian and Atlantic MORB shows that Pacific MORB have generally lower 87Sr/86Sr than MORB from the Mid‐Atlantic Ridge (MAR) and Indian Ocean ridges but have generally lower 143Nd/144Nd and higher 206pb/204pb than the MAR. Presently available data do not support the relationship of isotopic diversity and mean isotopic composition to spreading rate suggested by others. Differences between isotopic compositions and between isotopic diversity appear to be inherent features of the mantle beneath the Pacific, Indian, and Atlantic ocean basins.
Epeirogeny and eustatic sea level changes as inferred from Cretaceous shoreline deposits: Applications to the central and western United States
Authors: Sahagian Dork
Published: February 2008

Uplift and subsidence histories of continental margins are by now reasonably well understood in the context of plate tectonics, but the epeirogenic movements of continental interiors have been neither well documented nor properly understood. Epeirogenic movements are important to recognize when reconstructing paleogeography and may yield information regarding mantle dynamics and the amplitudes, wavelengths, and rates of lithospheric flexure. Sediments deposited at or near sea level can be used as a worldwide datum against which to measure postdepositional epeirogenic movements. The present elevations of shoreline sediments of epicontinental seas deposited during sea level highstands at particular times in the past define a warped surface documenting subsequent epeirogeny of continental interiors. When the amplitude of sea level rise during transgression is small relative to the observed epeirogenic motions, the diachronous shoreline deposits represented in the entire transgressive surface can be used as a datum in the analysis, thus greatly augmenting the available data and areal coverage. If a “stable” frame of reference can be defined, a eustatic sea level correction can be applied to every continent. The elevation of the upper Cenomanian (mid‐Cretaceous) shoreline gives little evidence of warping, tilting, or uplift in central cratonic North America but indicates that a large area of the western United States, including the Colorado Plateau, has undergone more than 2 km of post‐Cenomanian epeirogenic uplift.
Fast tidewater glaciers
Authors: Meier M. F. , Post Austin
Published: February 2008

Some iceberg‐calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets. No temperate tidewater glaciers are known to have floating termini, but many polar and subpolar tidewater glaciers do. The fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed, which may approach zero, and the longitudinal back stress on the terminus. The terminus boundary condition (the calving relation) is imperfectly known yet is vital to the dynamics of these glaciers. Calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested; the calving relations for floating termini are virtually unknown. This, together with the imperfect understanding of basal sliding, inhibits confidence in our understanding of the stability of these glaciers. Columbia Glacier (Alaska) is an instructive example because observations have been made on the major changes in its geometry, calving rate, and dynamics that have occurred in less than 10 years. The calving flux has increased more rapidly than the glacier flux, causing thinning and retreat; as a result, the ice velocity has increased markedly. The short‐term velocity changes relate to changes in back pressure (ice recession, tidal changes) and the flux of water injected to the bed. These results have relevance to the mechanisms of basal sliding, glacier surges, and the stability of marine ice streams.
A theory of glacier surges
Authors: Fowler A. C.
Published: February 2008

We propose a model of glacier flow that is capable of explaining temperate glacier surges. The laws of conservation of mass and momentum are supplemented by the prescription of a sliding law that gives the basal shear stress τ as a function of the basal velocity u and the effective pressure N. The effective drainage pressure N is determined by a simple study of the subglacial hydraulic system. Following Röthlisberger, we determine N = NR for the case of drainage through a single subglacial tunnel. Alternatively, following Kamb, we find that the corresponding theory for a linked‐cavity drainage system yields N = NK < NR. Furthermore, the stability of each drainage system depends on the velocity u, such that for large enough u, there is a transition from tunnel to cavity drainage. Consequently, one can write N = N(u). We then find that the sliding law τ = τ(u) is multivalued, and hence so also is the flux/depth relation Q = Q(H). An analysis of the resulting system of equations is sketched. For large enough accumulation rates, a glacier will undergo regular relaxation oscillations, resembling a surge. The surge is triggered at the point of maximum stress; from this point two hydraulic transition fronts travel up and down glacier to calculable boundary points. The speed of propagation is the order of 50 metres an hour. At these fronts, the tunnel drainage system collapses, and a high water pressure cavity drainage system is installed. This activated zone has high velocities and quickly relaxes (surges) to a quasi‐equilibrium state. This relaxation is much like opening a sluice gate, in that a large wave front propagates forward. Behind this wave front, the velocity can decay oscillatorily, and thus the flow can be compressive. We conclude with some discussion of the effects of seasonal variation and of prospects for the current theory's applicability to soft‐bedded glaciers.
Evidence from Rayleigh wave data for magma in an upper crustal dike in the Albuquerque‐Belen Basin of the Rio Grande Rift, New Mexico
Authors: Schlue J. W. , Hostettler K. K. , Edwards C. L.
Published: February 2008

We have found evidence for a dikelike structure in the middle of the Albuquerque‐Belen basin of the Rio Grande rift from our interpretation of two‐station Rayleigh wave amplitude and phase velocity data. Two‐dimensional finite element modeling suggests that the structure is about 200 m thick, extends vertically from a depth of about 2½ km to at least 20 km, and contains material characterized by a shear wave velocity of about ½ km/s. Simple physical arguments suggest that the length of the structure may be at least 28 km long. Nearby high heat flow values, historical swarmlike seismicity, recent (0.14 Ma) volcanism, and an extensive, sill‐like magma body at midcrustal depths all suggest that the structure is a magma‐filled dike. Source characteristics and seismometer response limit our reliable data to the frequency band of 0.08 Hz to 0.250 Hz; thus we cannot tell whether the dike is solely an upper crustal feature or whether it extends through the crust and (in)to the upper mantle.
New observations of the shallow seismic structure of young oceanic crust
Authors: Purdy G. M.
Published: February 2008

The results are presented of three new experiments carried out on the Mid‐Atlantic Ridge (MAR) near latitude 23°N using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust. The data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography. These 1‐ to 2‐km‐long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200–300 m of the young igneous crust. One experiment was carried out over the site of hole 648B of the Ocean Drilling Program on a small volcano within the median valley of the MAR. The seafloor velocity was observed to be 2.1 km s−1 underlain by an approximately linear velocity gradient of 4 s−1. Given that we know the crust at this location consists of fresh basalt lavas with laboratory‐measured velocities in excess of 5.8 km s−1, porosities at the seafloor of as high as 30–50% are inferred. The two other experiments were located over 7‐m.y.‐old crust near Deep Sea Drilling Project site 395 west of the MAR, separated by only 14 km laterally but by over 1400 m in water depth. The experiment positioned in the ∼100‐m‐thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable. The velocity of the uppermost basaltic basement was 4.1 km s−1, and the velocity gradient was less than 0.5 s−1. This ∼2 km s−1 difference in velocity between the two sites is assumed to be a consequence of age‐related modifications to the physical properties of the crust. If the primary change is in only the total porosity, then a 15–20% reduction is required to explain these observations. This seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data (e.g., off‐axis volcanism, secondary mineral deposition, tectonic compression). A combination of many processes acting in unison would be necessary to produce such a decrease in porosity. If the geometry of the cracks and voids were modified in just the right way, then no change in total porosity is required to explain the velocity difference. These on‐bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor's geologic features. They produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements.
Pyrite: Shock compression, isentropic release, and composition of the Earth's core
Authors: Ahrens Thomas J. , Jeanloz Raymond
Published: February 2008

New shock wave data (to 180 GPa) for pyrite (FeS2) shocked along (001) demonstrate that this mineral, in contrast to other sulfides and oxides, does not undergo a major pressure‐induced phase change over the entire pressure range (320 GPa) now explored. (This is probably so because of the initial, low‐spin 3‐d, orbital configuration of Fe+2). The primary evidence which indicates that a large phase change does not occur is the approximate agreement of the shock velocity when extrapolated to zero particle velocity, 5.4 km/s, with the expected zero‐pressure bulk sound speed of pyrite (5.36 to 5.43 km/s on the basis of previous ultrasonic data). Pyrite displays a prominent elastic shock (or Hugoniot elastic limit) of 8 ± 1 GPa. The velocity of the elastic shock approaches 8.72 km/s with decreasing shock pressure, the longitudinal elastic wave velocity. As shock pressure increases, the elastic shock velocity approaches 9.05 km/s and the elastic shock becomes overdriven for shock pressures greater than about 120 GPa. Analysis of release isentrope data obtained via the pressure‐particle velocity buffer method indicates that buffer particle velocities in all experiments are from 1.7% to 20% greater than expected for a Grüneisen ratio given by 1.56 (V/Vo)1.0. This discrepancy appears to result from volume increases upon pressure release of 0.04% to 4.5% which may result from shock‐induced partial melting. The normalized pressure, finite‐strain formalism for reducing Hugoniot data is extended to take into account initial porosity and shock‐induced phase transitions. A least squares fit to the present and previous shock data for pyrite yields an isentropic bulk modulus, Ks, of 162 ± 9 GPa and a value of dKs/dP = 4.7 ± 0.3. This is close to the 145 ± 3 GPa bulk modulus observed ultrasonically. If the slight discrepancy in zero‐pressure modulus is taken into account in the normalized pressure finite‐strain formalism, a zero‐pressure density of the shock‐induced high‐pressure phase having a density some 2% to 3% less than pyrite is inferred to occur in the high‐pressure shocked state. We suggest from this result, the release isentrope results, and limited phase diagram data that the Hugoniot states probably correspond to material which is partially to completely melted. Using the above derived equation of state and previous shock wave data for iron, both the seismologically determined density and bulk modulus distribution in the outer core are fit to models with various temperature distributions and varying weight percent sulfur. Good agreement between the shock wave derived equation of state and the density/bulk modulus relations of the liquid outer core are obtained for temperatures of ∼3000 K at the core/mantle boundary extending to 4400 K at the outer core‐inner core boundary. For this thermal model a calculated sulfur content of 11 ± 2% is obtained.
Abundance and distribution of ultramafic microbreccia in moses rock dike: Quantitative application of mapping spectroscopy
Authors: Mustard John F. , Pieters Carle M.
Published: February 2008

Moses Rock dike is a Tertiary diatreme containing serpentinized ultramafic microbreccia (SUM) located on the Colorado Plateau in Utah. Field evidence indicates that SUM was emplaced first followed by breccias derived from the Permian strata exposed in the walls of the diatreme and finally by complex breccias containing basement and mantle‐derived rocks. SUM is primarily found dispersed throughout the matrix of the diatreme. We examined the nature of SUM abundance and spatial distribution using data collected remotely by the Airborne Imaging Spectometer (AIS). The minerals serpentine, gypsum, and illite as well as desert varnish and the lithologies SUM and various sandstones were identified from the AIS data. Six end‐members (SUM, two types of sandstone, gypsiferous soil, clay‐rich soil, desert varnish) were chosen to represent the dominant lithologies of the surface in Moses Rock dike region. Spectra of these end‐members were used in an intimate mixing model to deconvolve the AIS spectral data into surface abundance coefficients for each component using a nonnegative least squares inverse algorithm. The results of this calculation of surface composition are consistent with field observations and investigations. SUM distribution and abundance in the matrix of the diatreme were examined in detail, and two distinct styles of SUM dispersion were observed. One style is characterized by high to moderate SUM abundance surrounded by halos of lesser SUM abundance. The dispersion halos grade steeply into regions of the matrix with little to no SUM. The second style is characterized by moderate SUM abundances and broad dispersion halos with SUM distributed across the entire width of the diatreme. These dispersion styles are consistent with emplacement of the dike as a fluidized solid‐volatile system where SUM, which is emplaced early, becomes reincorporated into the turbulent flow of the diatreme by abrasion and comminution by paniculate matter carried along in the eruption. This eroded material is then dispersed throughout the matrix by eddy diffusion. The first style represents an early arrested phase of the eruption sequence, while the second style represents a more mature phase. Distributions of the second style probably indicate the location of channels where flow was concentrated during later stages of eruption. Since both styles are observed at the same level of erosion, this indicates that the duration of eruption varied along strike at Moses Rock dike.
Earthquake source mechanisms and transform fault tectonics in the Gulf of California
Authors: Goff John A. , Bergman Eric A. , Solomon Sean C.
Published: February 2008

We have determined the source parameters of 19 earthquakes in the Gulf of California from an inversion of long‐periodPandSHwaveforms. Fifteen of the earthquakes, on transforms between the Cerro Prieto fault and the Tamayo Fracture Zone, are characterized by right‐lateral strike‐slip faulting, and all but one of these likely reflect the relative motion between the North American and Pacific plates. Slip vectors from these events constitute an improved data set for determination of North American‐Pacific motion. Most centroid depths are poorly resolved because of trade‐offs between depth and source time function. Both the ranges in acceptable centroid depths and the average fault width estimated from cumulative moment release during this century are compatible with the hypothesis that seismic slip does not extend below about 10‐km depth, or the approximate depth of the nominal 800°C isotherm, along the principal oceanic transforms of the gulf. Two normal‐faulting earthquakes during a swarm in the northern gulf have centroid depths of 6 ± 3 and 3 ± 2 km, consistent with earthquake centroid depths in other settings (Arctic, northern Red Sea) where plate separation has carried continental rifting nearly to completion. A thrust‐faulting earthquake near the Tres Marias escarpment has a centroid depth of 11 ± 2 km and probably reflects relative motion between the Rivera and North American plates. The transition from oceanic transform faulting to continental transform faulting in the northern gulf is marked by a broadening of the zone of deformation and a 10° change in the strike of the principal fault accommodating plate motion. Fault kinematics in the transition zone may be understood in terms of an unstable FFF triple junction, with the Agua Bianca and San Miguel faults acting to transfer a portion of the plate motion to faults west of the San Andreas system and with the Delfin and Wagner basins forming as a direct result of slip on the three fault systems meeting in the junction region.
An alternative method of orbit improvement for a Doppler‐tracked satellite: A step toward Earth gravity model refinement
Authors: Lansard Erick , Biancale Richard
Published: February 2008

An alternative method of orbit and gravity field improvement for Doppler‐tracked satellites is presented here, in view of the prospect of future geodetic and altimetric missions (Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)/SPOT 2, the Ocean Topography Experiment (TOPEX)/Poseidon, ERS 1). Owing to a refined analysis of the Doppler residuals generated by a classical orbit computation, it is successively possible (1) to improve the computed spacecraft orbit with interesting by‐products such as improved tracking station coordinates and Doppler reference frequencies, (2) to recover the disturbing forces due to a mismodeling of the geopptential model (i.e., regional gravity anomalies above the stations, in line‐of‐sight direction, and at satellite altitude), and (3) to globally refine the mismodeled gravity field from a dense set of this regional information. The paper deals with the first two steps with emphasis on step 1. The more classical geodetic problem of step 3 will be the subject of a future paper. Simulation results for Seasat and DORIS are presented and discussed: With a simulated random noise of 0.3 cm/s, orbit error is recovered with an accuracy of a few decimeters. With a noise level of 0.3 mm/s, orbit and force errors are recovered with an rms accuracy better than 0.2m and 0.2 mGal at 800 km altitude. This is very encouraging for future Doppler positioning systems like DORIS.
Heat flow across Cascadia Basin near 47°N, 128°W
Authors: Moran J. E. , Lister C. R. B.
Published: February 2008

Some 200 previously unreported heat flow measurements were made along a flow line crossing the Juan de Fuca Ridge crest near 47°N. All but one of the multipenetration stations were on the east flank of the ridge, in the thickly sedimented Cascadia Basin. The accuracy of individual penetrations remains low due to the poor penetrability of the basin turbidites, but the multiplicity of the measurements provides a statistical estimate of precision at most stations. The raw heat flow means of the multipenetration stations do not decrease regularly with increasing distance from the spreading center; the lowest measurements are near 70 km from the ridge crest, well onto the turbidite sediment fill of the basin. A basement outcrop near these measurement sites appears to cause the low heat flow by ventilating hydrothermal circulation beneath the sediments. In the eastern part of the basin the sedimentation correction to the observed heat flow becomes significant, and, if the filling of the basin occurred mostly in the Pleistocene, the correction brings those measurements close to the theoretical heat flow of cooling lithosphere. Extrapolated basement temperatures reach nearly 200°C beneath the eastern basin, a region where the magnetic anomalies become noticeably weaker than those closer to the ridge. They do not, however, disappear entirely, as might be expected from some laboratory measurements of blocking temperature.
Depth of faulting during the 1968 Meckering, Australia, Earthquake sequence determined from waveform analysis of local seismograms
Authors: Langston Charles A.
Published: February 2008

Waveforms for 11 foreshocks and 48 aftershocks of the Ms 6.8 Meckering earthquake recorded at the World‐Wide Standard Seismograph Network station Mundaring are analyzed to determine the depth distribtion of faulting during this unusual intraplate earthquake sequence. Clear depth phases including sP and Rg are seen in the local Seismograms at distances of 60–95 km and are studied using synthetic seismograms computed using generalized ray theory and wave number integration techniques. Large Rg/S ratios seen on the vertical component short‐period data for many events imply source depths less than 2 km. The short‐period P waveform contains the best depth estimator in the form of sP so that depth can be estimated to within an uncertainty of about 1 km for most events. The foreshocks cluster at less than 2 km depth, and most aftershocks occur within 1 km of the surface. A few aftershocks occur as deep as 7 km. These results are consistent with a previous teleseismic waveform study in which faulting was inferred to start near the surface at 1.5 km depth with rupture proceeding downward and not exceeding 6 km depth. These results, coupled with previous stress studies in the Australian shield and models of crustal strength, show that faulting in a cold shield area is a near‐surface phenomenon and implies that most of the crust is too strong to be fractured.
Helium, radon, and hydrothermal circulation associated with the Carnmenellis radiothermal granite of southwest England
Authors: Gregory R. G. , Durrance E. M.
Published: February 2008

The Carnmenellis granite of southwest England has higher than average radioelement concentrations and a heat flow greater than 100 mW m−2, more than twice the United Kingdom average. Measurements were made of 4He concentrations and 222Rn activities in surface waters over the granite and its immediate host rocks. High values (greater than 48 ppb 4He, 160 c/min 222Rn) for either or both gases are interpreted as indicating discharge of groundwater from the upwelling limbs of hydrothermal convection cells. 4He has a spatial distribution which agrees well with the observed variations of heat flow within the pluton, but 222Rn is also influenced by lithology and depth of groundwater flow. Although the distribution of convection cells is partly determined by the presence of NW‐SE faults, there is close similarity with theoretical models of groundwater circulation. It is concluded that 4He and 222Rn surveys provide a simple but effective method for mapping areas of modern hydrothermal activity.
Trace element geochemistry of volcanic gases and particles from 1983–1984 eruptive episodes of Kilauea Volcano
Authors: Crowe Bruce M. , Finnegan David L. , Zoller William H. , Boynton William V.
Published: February 2008

Compositional data have been obtained for volcanic gases and particles collected from fume emitted at the Pu'u O'o vent on the east rift zone of Kilauea volcano. The samples were collected by pumping fume through a filter pack system consisting of a front stage particulate filter followed by four base‐treated filters (7LiOH). Particles and condensed phases are trapped on the particulate filter, and acidic gases are collected on the treated filters. The filters are analyzed for 30 elements by instrumental neutron activation analysis. Fume samples were collected from the Pu'u O'o vent for two eruptive episodes: (1) 7 days after episode 11 (cooling vent samples) and (2) the waning stage of episode 13 (active vent samples). Additional samples were collected by aircraft from the gas plume released during the lava fountaining phase of episode 17 (aircraft samples). Element concentrations in the vent gases were > 104 μg m−3 for S, Cl, and F. Enrichment factors (EFs) for the volcanic fume versus the source magma were calculated using the volatile element Br as the reference element for normalization and the U.S. Geological Survey standard BHVO‐1 as the magma standard. This removes the ash dilution effect obtained by using an ash constituent (Al, Sc, or Mg) as the reference element. Bromine‐normalized EFs (× 105) range from 101 to 102 for Na, K, and Cu; 102 to 105 for Zn, W, Sb, In, Ir, Ag, F, and As; and > 105 for Au, Cd, Re, Cl, Se, and S. The highest enrichment factors are for aircraft samples collected during the most gas‐rich phase of an eruption cycle. Metal and volatile‐element data form two groups: (1) elements showing little or no variation in abundance ratios with sample type (group 1: Cl, Br, and Re) and (2) elements that show significant variation in abundance ratios by sample type (group 2: Zn, W, Sb, In, Ir, Au, and Cd). Bivariate plots of elements of the first group versus elements of the second group separate by sample type. The separation corresponds to samples collected during eruptive activity versus samples collected during repose periods. Monitoring trace metal ratios in volcanic fume could provide an additional tool for predicting volcanic eruptions. The F/Cl ratio of cooling vent samples is higher than those of active vent or aircraft samples, and the ratio is inversely correlated with EFs for most volatile metals.
The composition and petrogenesis of the lower crust: A xenolith study
Authors: Rudnick R. L. , Taylor S. R.
Published: February 2008

Granulite facies lower crustal xenoliths from a single basaltic vent (Hill 32) in the McBride volcanic province, north Queensland, Australia, illustrate the extreme lithologic diversity of the deep crust. These xenoliths are dominantly mafic, but intermediate and felsic granulites represent a significant proportion (∼20%) of the xenolith population. All xenoliths have high‐grade mineralogies and generally well‐equilibrated textures, as well as decompression features which are indicative of derivation from deep crustal levels (0.7–1.0 GPa). Major and trace element chemistry for 12 xenoliths, chosen to span the observed lithologic range, are used to constrain the petrogenesis of these rocks. In the smaller, layered samples, metamorphic differentiation may lead to nonrepresentative sampling. However, in most instances such secondary processes can be identified and the original chemical characteristics delineated. The mafic xenoliths formed through a variety of processes, including crystallization of mafic magma, crystal accumulation from mafic and felsic magmas, and partial melting of intermediate compositions to yield a mafic residuum. The two analyzed intermediate xenoliths are metasediments, based upon high alumina to alkali ratios and rare earth element patterns, whereas the two felsic xenoliths have compositions similar to igneous rocks. Comparisons of noncumulate and/or restite xenoliths with unmetamorphosed rock types show that K and Rb are variably depleted whereas Th and U are strongly depleted, in all rock types. The large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains, suggesting similar processes were operative. Suggestions of an anomalously Ba‐ and Sr‐rich lower crust are not supported by the data. By using the observed lithologic proportions of xenoliths at Hill 32, and the chemical analyses presented here, a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions. The weighted mean composition is mafic, and formed through a combination of basaltic underplating, crystal accumulation, tectonic underplating, and partial melting. No simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in Phanerozoic continental margin settings.
Hunting for Paleoclimatic Periodicities in a Geologic Time Series With an Uncertain Time Scale
Authors: Park Jeffrey , Herbert Timothy D.
Published: February 2008

An 8‐m section of a mid‐Cretaceous (Albian) deep‐sea sediment core from Piobbico, Italy contains ∼2 cycles/m compositional oscillations. As postdepositional chemical migration in the core is weak, the relative amount of carbonate is a primary indicator of Cretaceous paleoclimate variability. We report results of spectral analysis of two series, CaCO3wt % measurements taken at roughly 2 cm intervals and a more densely sampled photodensitometer record of light‐dark variation. The uncertainty in the age‐depth relation (i.e., unknown fluctuations in sedimentation rate) hampers a detailed assessment of the suggested eccentricity correlation. We fit for long‐term (time scale greater than 75 kyr) fluctuations in sedimentation rate by tracking the local frequency modulation of the dominant harmonic in the data series, assuming the true input signal to be monochromatic. We estimate uncertainty in the observed local frequency by means of a statistical jackknife. We fit a smoothed cubic spline to the set of discrete frequency modulation estimates, specifying a χ2=Nmisfit criterion, whereNis the number of frequency estimates. From this analysis we infer fluctuations of a factor of two in the sedimentation rate, which occur predominately on a time scale of 400 kyr. We generate “tuned” series using several candidate age‐depth relations from the carbonate and densitometer series. The tuned carbonate series returns greater correlation with the modern Milankovitch periodicities. The 2 cycles/m energy resolves principally into two harmonic components whose frequency ratio corresponds (to an observational accuracy of 1–2%) to that of the 97.0‐ and 127.6‐kyr eccentricity oscillations. At frequencies lower than 2 cycles/m, we find harmonic oscillations whose phase correlates with the “beating” envelope of the 2 cycles/m energy in a manner consistent with the modern Milankovitch insolation. We also observe a sinusoidal oscillation of lower amplitude with period (scaled by those of the presumed eccentricity oscillations) of 39.2 ± 1.1 kyr, which agrees with the period of the modern obliquity cycle after correction for the effects of tidal friction. Oscillations corresponding to Milankovitch precessional frequencies are not established, owing possibly to low signal level and unknown short‐term deposition fluctuations. The tuned densitometer series recovers few new periodic features, although its inferred age‐depth relation correlates well with that of the carbonate series. We calibrate the depth variable downcore to the orbital cycles to estimater=5.0 ± 0.15 m/m.y. as the effective sedimentation rate.
Reflection Seismic Investigations of the Siljan Impact Structure, Sweden
Authors: Juhlin C. , Pedersen L. B.
Published: February 2008

Reflection seismic measurements were carried out during 1985 in the area of the Siljan impact structure of Sweden. A total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies. Standard stacked sections have been interpreted in terms of current impact models. The transient crater diameter is found to be approximately 22 km, considerably smaller than current estimates based on interpretation of surface geology. The structural uplift is estimated to be 3 km at the 5‐ to 8‐km level, and the material flow necessary to support the uplift is mainly horizontal rather than vertical.
Modeling of surface deformation in volcanic areas: The 1970–1972 and 1982–1984 crises of Campi Flegrei, Italy
Authors: Bianchi R. , Coradini A. , Federico C. , Giberti G. , Lanciano P. , Pozzi J. P. , Sartoris G. , Scandone R.
Published: February 2008

Campi Flegrei is a densely inhabited volcanic area within a caldera, 12–14 km in diameter, immediately west of Naples, Italy. In 1970–1972 and in 1982–1984, Campi Flegrei experienced considerable ground surface deformation producing a bulge about 3.5 m high at its peak. In the two inflation episodes the point of maximum uplift did not change, coinciding with the caldera center, about which the displacements exhibited radial symmetry. The 1982–1984 uplift was accompanied by considerable seismic activity. Gravity changes have been interpreted as reflecting the occurrence of a subvertical magmatic intrusion. Volcanological, petrological, and geophysical data point to the existence of a shallow magma chamber just beneath the center of the caldera. We assume that the observed surface displacements are the response of an essentially elastic medium to a pressure increase in the magma chamber. Static deformation is calculated by using a finite‐element method, assuming axial symmetry around a vertical axis through the caldera center. The role played by various components of the model, geometry, boundary conditions, elastic properties, is analyzed. It is shown that models assuming uniform elasticity are inadequate. Therefore the major volcanotectonic features as well as the influence of temperature and pressure on the elastic properties are taken into account. The results of various more complex models are presented. It is suggested that a progressively fractured zone, whose size and shape depend also on the possible occurrence of magmatic intrusions, may explain the observed surface displacements and the other data collected at Campi Flegrei. It is suggested that the 1970–1972 uplift was possibly accompanied by larger‐scale changes in the stress field.
On the accuracy of marine gravity measurements
Authors: Wessel Pål , Watts Anthony B.
Published: February 2008

We have assessed the accuracy of Lamont‐Doherty Geological Observatory's global marine gravity data bank by examining the crossover errors (COEs) at intersecting ship tracks. More than 63,000 COEs were found, having a standard deviation of 22.43 mGal (mGal = 10−5 m s−2). We use the COEs to find and remove linear drifts and DC shifts present in the data set. This adjustment reduces the standard deviation to 13.96 mGal. COEs generally decrease with latitude, which we attribute to uncertainties in the Eötvös correction. High COEs occur in areas of high gravity gradients. These two features point to poor navigation as the principal source of error in marine gravity surveys. COEs have generally been decreasing during the last two decades, reflecting improvements in instrumentation and quality of navigation. A comparison of the shipboard gravity data to Seasat derived gravity revealed a 9‐mGal bias in the terrestrial data, which probably reflects uncertainties in choice of reference field. The adjusted data set was used to generate a gravimetric geoid for the NW Atlantic Ocean. By removing this geoid from the Seasat sea surface heights, a residual “geoid” was obtained. A special feature of this map is an elongate ENE trend that appears to correlate with the edge of the Gulf Stream.
Crustal structure of the Columbia Plateau: Evidence for continental rifting
Authors: Catchings R. D. , Mooney W. D.
Published: February 2008

Data from a 260‐km‐long seismic refraction profile provide a detailed look at the crustal structure beneath the central Columbia Plateau (CCP). The CCP profile, centered on the Pasco Basin in eastern Washington, trends approximately N50° E between the towns of Wasco, Oregon, and Warden, Washington. The upper crust above the crystalline basement consists of between 5 and 12 km of the Columbia River Basalt Group (CRBG) and underlying sediments. The CRBG is laterally variable in thickness, ranging from 3 to 6 km over the length of the CCP seismic profile, but is thickest near the center of the Columbia Plateau. A thick, but laterally varying, accumulation of sediments underlies the CRBG with the thickest accumulation of sediments (5.0 km/s) occurring beneath the Pasco Basin in what we interpret to be a graben. Sediments there extend from the base of the CRBG to basement (6.3 km/s) at depths between 10 and 12 km. We recognize two layers between the basement and the upper mantle with seismic velocities of 6.8 and 7.5 km/s, located at 18‐ and 25‐km depths, respectively. The 6.8‐km/s layer thins beneath the graben, where it has apparently been replaced by the 7.5‐km/s layer, creating the characteristic geometry and velocity of the “rift pillow” layer previously observed in some other recognized continental rifts. The Mono (8.4 km/s upper mantle) is located at about 40‐km depth; an apparent localized, low‐velocity layer is observed within the upper mantle at a depth of about 50 km. Thus the deep crustal and upper mantle structure beneath the central Columbia Plateau is complex and is atypical of normal deep continental crust. The presence of an upper crustal graben and lower crustal rift pillow, as inferred from the seismic refraction data, combined with gravity, earthquake, electrical, and geologic data, suggest that Eocene (and possibly later) continental rifting occurred prior to deposition of the CRBG.
Convective partial melting: 1. A model for the formation of thick basaltic sequences during the initiation of spreading
Authors: Mutter John C. , Buck W. Roger , Zehnder Carolyn M.
Published: February 2008

Stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure/temperature environment where partial melting occurs. The amount of melt liberated is, however, insufficient to account for the thickness of igneous crust emplaced near the continent‐ocean boundary at some passive margins. These igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide. Seismic studies also suggest that the transition from continental to oceanic crust on these “volcanic margins” is relatively abrupt. Their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading. To account for these observations, we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle. Deep, hot mantle material is transported upward by the convection, and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling. Numerical calculations show that “convective partial melting” can provide substantial additions to melting due to passive upwelling alone. Using this model, we propose an evolutionary scheme for “volcanic” passive margins, emphasize the contrast with “nonvolcanic” margins, and describe conditions which might lead to the development of both types.
Adaptation of back projection tomography to seismic travel time problems
Authors: Humphreys Eugene , Clayton Robert W.
Published: February 2008

A back projection method of reconstruction is adapted to invert seismic travel time data for velocity structure. Adaptations are made so that the inhomogeneous and anisotropic ray sets and the three‐dimensional geometries commonly dealt with in seismic experiments can be handled with greater success. Jacobi iteration, deconvolution, and ray weighting work well in augmenting the basic back projection method to produce a well‐focused image. These methods succeed by amounts that depend on the quality of the ray coverage. Also, the ability to reconstruct an accurate image when the data include moderate amounts of noise is shown to be good. Comparison of inversions produced with back projection tomography and with damped least squares indicate that the two methods are comparable in their ability to reconstruct an image of the actual structure. The back projection approach, however, is much more computer efficient. In practice, this allows for the construction of more detailed inversions.
Simultaneous inversion of regional wave spectra for attenuation and seismic moment in Scandinavia
Authors: Sereno Thomas J. , Bratt Steven R. , Bache Thomas C.
Published: February 2008

Frequency‐dependent regional wave attenuation is estimated for continental paths to the NORESS array in Norway. Regional Lg and Pn spectra from 186 events at ranges between 200 and 1400 km and local magnitudes between 1.1 and 4.8 are inverted for both seismic moment and apparent attenuation. The Lg spectra were inverted between 1 and 7 Hz, and the Pn spectra were inverted between 1 and 15 Hz. The method uses both the spectral and spatial decay of observed signal amplitudes to separate source and path contributions. The assumptions include the geometric spreading rate and the source spectrum to be uniquely defined by its long‐period level. Most events considered have local magnitudes less than 3.0, so the source corner frequencies are near or beyond the upper limit of the inverted bandwidth. The Q results, particularly for Lg, are therefore not very sensitive to the details of our source parameterization. The inversion parameters are source moment (for each event), a constant relating corner frequency and moment for the entire data set, and two parameters describing a power law frequency dependence of Q in the region. For fixed source and spreading assumptions the inversion defines clear trade‐offs among model parameters. These trade‐offs are resolved by adding the constraint that the separately derived source parameters for Lg and Pn are consistent. The “preferred” estimates for the apparent attenuation are QLg(f) = 560f0.26 and QPn(f) = 325f0.48. These Q values correspond to assumed geometric spreading rates of r−0.5 for Lg and r−1.3 for Pn. For fixed Lg spreading, the Pn spreading rate is constrained by requiring that the relative Lg amplitude for earthquakes and explosions of the same moment be consistent with well‐supported results from previous empirical studies. The relationship between the inverted seismic moment values and local magnitude is generally consistent with values from near‐field studies. Since magnitude does not enter the inversion, this result lends considerable support to the derived Q models. Whatever the physical interpretation of the results, they certainly provide an accurate parameterization of observed amplitude spectra in this region. This is valuable for representing wave propagation in the region, and it provides important data for assessing the event monitoring capabilities of small regional networks.
Seismicity of the Caribbean‐Nazca Boundary: Constraints on microplate tectonics of the Panama region
Authors: Adamek Scott , Frohlich Cliff , Pennington Wayne D.
Published: February 2008

There are at least three kinds of models that have been used to explain plate motions and boundaries in the Panama region. These models suggest that the major Caribbean‐Nazca boundary is (1) a thrust fault along which the Panama Deformed Belt has developed, (2) a left‐lateral strike‐slip fault along the southern margin of Panama, or (3) a diffuse array of mostly thrust faults which cause internal deformation to occur throughout Panama. In this paper, we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events. Hypocenters of 13 events beneath the Panama Deformed Belt suggest that overthrusting of Panama upon the Caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3. The nature of seismic faulting along the Panama Deformed Belt is complex and supports the premise that slow north‐south convergence loads the cold thick Caribbean lithosphere and causes it to fail seismically. Epicenters of 64 events along the Panama Fracture Zone indicate that this north–south trending rightlateral transform fault is splayed north of 6°N. Overall, the seismicity throughout suggests that the Panama arc can be treated as a microplate, in accordance with models 1 or 2. Similarly, a possible second microplate may exist south of Panama, but if so it does not affect regional plate motions significantly. The strongest evidence for model 3 occurs in the easternmost and westernmost portions of Panama, where the diffuse seismicity prevents us from resolving specific microplate triple‐junction boundaries.