bluuebunny/crossref_metadata_2025 huggingface dataset:
Volcanic eruptions and solar activity
Authors: Stothers Richard B.
Published: February 2008

The historical record of large volcanic eruptions from 1500 to 1980, as contained in two recent eruption catalogs, is subjected to detailed time series analysis. Two weak, but probably statistically significant, periodicities of ∼11 and ∼80 years are detected. Both cycles appear to correlate with well‐known cycles of solar activity; the phasing is such that the frequency of volcanic eruptions increases (decreases) slightly around the times of solar minimum (maximum). The weak quasi‐biennial solar cycle is not obviously seen in the eruption data, nor are the two slow lunar tidal cycles of 8.85 and 18.6 years. Time series analysis of the volcanogenic acidities in a deep ice core from Greenland, covering the years 553–1972, reveals several very long periods that range from ∼80 to ∼350 years and are similar to the very slow solar cycles previously detected in auroral and carbon 14 records. Mechanisms to explain the Sun‐volcano link probably involve induced changes in the basic state of the atmosphere. Solar flares are believed to cause changes in atmospheric circulation patterns that abruptly alter the Earth's spin. The resulting jolt probably triggers small earthquakes which may temporarily relieve some of the stress in volcanic magma chambers, thereby weakening, postponing, or even aborting imminent large eruptions. In addition, decreased atmospheric precipitation around the years of solar maximum may cause a relative deficit of phreatomagmatic eruptions at those times.
An abrupt change in ridge axis gravity with spreading rate
Authors: Small Christopher , Sandwell David T.
Published: February 2008

The global mid‐ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate. Fast spreading ridges have axial highs with little bathymetric relief and low‐amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium. Slow spreading ridges have large axial valleys bounded by rugged topography (Macdonald, 1982) and large axial gravity troughs indicating that they are dynamically maintained. While this variation in ridge axis morphology with spreading rate has been observed, it has not been analyzed in a comprehensive manner. Moreover, it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate. Such observations would provide important constraints on models of ridge axis dynamics. Vertical deflection profiles collected by the Geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate. In this study, we have analyzed 44 Geosat profiles over ridges with spreading rates ranging from 14 to 155 mm/yr. In agreement with previous studies, we find that slow spreading ridges (<60 mm/yr) usually have high amplitude gravity troughs (40–100 μrad = 40–100 mGal), while fast spreading ridges (>70 mm/yr) are characterized by low‐amplitude ridge axis highs (∼15 μrad). Unexpectedly, we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60–70 mm/yr. Ridge axis gravity signatures are highly variable for rates less than 65 mm/yr and very uniform at higher rates. The transition of the gravity signature appears to be more abrupt than the transition of the topographic signature, suggesting an abrupt change in the style of isostatic compensation with spreading rate. Published models of ridge axis dynamics do not explain this sharp transition.
Uplift, unbuckling, and collapse: Flexural history and isostasy of the Wind River Range and Granite Mountains, Wyoming
Authors: Hall Michelle K. , Chase Clement G.
Published: February 2008

Two‐dimensional gravity and flexural models constrain the structure and isostatic compensation of the Wind River Range and Granite Mountains, Wyoming. These ranges are asymmetric, basement‐cored Laramide uplifts. Their large positive isostatic gravity anomalies imply that the topography of the uplifts is not locally compensated. For the Wind River Range, gravity data are consistent with thrusting along a listric fault which flattens and soles out at 30 km depth with no detectable offset on the Moho. We model the large Bouguer gravity high over the range with high‐density lower crustal rocks emplaced over lower‐density middle crustal rocks along the Wind River thrust. The high positive gravity anomaly over the Granite Mountains can be modeled as an extension of the South Pass greenstone belt from the southern Wind River Range eastward beneath the Emigrant Trail thrust. A crustal root, as required in local (Airy) compensation, for either range requires unreasonably high‐density material in the upper crust to obtain the observed anomaly. Combined gravity and flexural models show that a flexural rigidity at least 2.55×1022 N m (maximum deflection of 2 km of the Moho) is required to satisfy the gravity data. Our results are consistent with models of uplift due to horizontal compression and support of the resulting topography by lithospheric strength. An otherwise puzzling episode of Oligocene uplift of the Wind River Range can be explained as “unbuckling” (decrease of deflection of lithosphere) accompanying release of Laramide compressional stresses. Flexurally supported uplifts will collapse if the lithosphere is weakened. A number of Laramide uplifts have collapsed, primarily east‐west trending structures such as the Granite Mountains, the southern end of the Wind River Range, and the Uinta Mountains. We propose that collapse has occurred along zones of established weakness in the crust in response to north‐south extension.
An S‐to‐P converted phase recorded near Long Valley/Mono Craters Region, California
Authors: Ammon Charles J. , Zucca John , Kasameyer Paul
Published: February 2008

We examine and model the arrival time of a large secondary seismic arrival recorded in the Long Valley/Mono Craters region of east central California. Zucca et al. (1987) and Peppin (1987) both previously reported on different features of this same arrival. Using both arrays of sources and receivers, we demonstrate that the arrival in an S‐to‐P converted phase as first suggested by Lewis and Peppin (1988). Back projection of the observed travel times allows us to constrain the location of the converting material to somewhere within a southeast dipping zone between 7 and 16 km depth and within 5 km on either side of the northwest topographic margin of the caldera. The analysis demonstrates the power of source and receiver array combinations when analyzing seismic arrivals in complicated environments.
Regional seismic event location with a sparse network: Application to eastern Kazakhstan, USSR
Authors: Thurber C. , Given H. , Berger J.
Published: February 2008

Three‐component data from a sparse three‐station seismic network in eastern Kazakhstan, surrounding the Soviet nuclear test site, have been analyzed to determine location estimates for regional events recorded by two or three stations. Included among these events are the September 1987 chemical explosions whose locations are known. Locations are calculated using arrival times of P and S phases and arrival azimuths from first P. Location uncertainties are estimated using a combination of a priori and a posteriori data uncertainties. A layered P wave velocity model adapted from Soviet Deep Seismic Sounding surveys is employed for calculating travel times, and two S models are tried. Location results for the chemical explosions are excellent, even if only two stations are used: absolute location errors are less than 10 km, and estimated 90% confidence uncertainties are only a few kilometers. The data are also adequate to determine correctly their depth (i.e., focus at the surface). The other regional events include numerous suspected mine blasts and two earthquakes from the Tien Shan. The calculated locations of the latter events agree well with a teleseismic location for one of them, falling within a belt of regular seismic activity. Nearly all of the presumed blasts can be associated with mapped mines, and we have been able to identify the source areas for two sets of blasts in high‐resolution satellite images. “Before and after” photographs allow us to identify specific active mines. Our location estimates agree quite well with the observed active mines.
Perturbation analysis of convective instability of oceanic lithosphere and initiation of subduction zones
Authors: Ogawa Masaki
Published: February 2008

A linear perturbation analysis is made on convective instability of the oceanic lithosphere to understand the occurrence of subduction on the Earth. The oceanic lithosphere is modeled by the upper part of top thermal boundary layer in a convecting Newtonian temperature‐dependent viscosity fluid. Thickening of oceanic lithosphere with its age due to cooling at the surface is modeled by thickening of the thermal boundary layer with time; its thickening rate
is a measure of the rate with which temperature in the thermal boundary layer decreases with time and hence is a measure of the rate with which viscosity in the thermal boundary layer increases with time. The linear perturbation analysis shows the possible excitation of plural eigenmodes to give rise to an instability in the thermal boundary layer depending on a threshold value Rc of the local Rayleigh number R of the thermal boundary layer. Further, Rc is shown to be a linearly increasing function of
, which is treated as a free parameter. From estimates of R and
appropriate for the major oceanic lithosphere (denoted as Robs and
obs, respectively), I find that Rc(
= 0) < Robs < Rc(
=
obs). The flow pattern of mantle convection depends strongly on which eigenmodes are excited by the convective instability in thermal boundary layer: (1) When R is lower thanRc, only the fundamental and the first higher modes at most are excited. The resulting mantle convection manifests itself as the secondary convection. (2) When R is higher than Rc, the modes higher than the first higher mode are also excited. The resulting mantle convection involves lithospheric instability, leading to large local deformation in the oceanic lithosphere and finally to subduction of the oceanic lithosphere. This result implies that (1) subduction does not begin newly in the major oceanic lithosphere at present, since lithospheric instability is suppressed by thickening of the major oceanic lithosphere, neither by its high viscosity nor by its buoyancy, (2) subduction will newly begin in the major oceanic lithosphere due to lithospheric instability in future when the oceanic lithosphere becomes so old that its thickening rate
decreases to sufficiently small value. The latter result suggests that lithospheric instability is an important agent that maintains multiplate tectonics on the Earth.
U‐Th‐Pb partitioning behavior during partial melting in the upper mantle: Implications for the origin of high Mu Components and the “Pb Paradox”
Authors: Meijer Arend , Kwon Tsung‐Tak , Tilton George R.
Published: February 2008

The partitioning behavior of U, Th, and Pb in the upper mantle has been investigated through the analysis of ultramafic nodules found in lava flows at two localities in the southwestern United States. Secondary (i.e., pedogenic) components were distinguished from primary (igneous) components on the basis of Nd and Sr isotope ratios. Most of the primary components have Nd, Pb, and Sr isotope ratios within the ranges observed in oceanic basalts. The 238U/204Pb (mu) ratios of residual (i.e., type IA) clinopyroxene separates analyzed in this study are generally high (60–120) and show an excellent positive correlation with 147Sm/144Nd ratios. Because the latter are consistently greater than the chondritic ratio and because clinopyroxene contains most of the U in the nodules, we suggest this mineral is a “high‐mu” component in the upper mantle. Calculated “whole rock” U contents, based on analyses of purified separates of the major silicate minerals, are consistent with recent estimates for mid‐ocean ridge basalt sources. Clinopyroxene/melt partition coefficients, based on the assumption that the lavas bearing the nodules are the melts with which the nodule minerals last equilibrated, decrease in the order
, consistent with available experimental data. Sulfide is inferred to be a major Pb‐bearing phase in the nodules. Bulk partition coefficients for Pb, Th, and U during melting are inferred to depend primarily on the abundance of clinopyroxene and sulfide in the residual mantle. On the basis of the model presented, the “Pb paradox” is explained as a direct consequence of melt depletion from the upper mantle.
The role of water ice clouds in the Martian hydrologic cycle
Authors: James Philip B.
Published: February 2008

A one‐dimensional model for the seasonal cycle of water on Mars has been used to investigate the direction of the net annual transport of water on the planet and to study the possible role of water ice clouds, which are included as an independent phase in addition to ground ice and water vapor, in the cycle. The calculated seasonal and spatial patterns of occurrence of water ice clouds are qualitatively similar to the observed polar hoods, suggesting that these polar clouds are, in fact, an important component of the water cycle. A residual dry ice cap in the south acts as a cold trap which, in the absence of sources other than the caps, will ultimately attract the water ice from the north cap; however, in the presence of a source of water in northern mid‐latitudes during spring, it is possible that the observed distribution of vapor and ice can be in a steady state even if a residual CO2 cap is a permanent feature of the system.
Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites
Authors: Johnson Kevin T. M. , Dick Henry J. B. , Shimizu Nobumichi
Published: February 2008

A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near‐fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America‐Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n = 0.002–0.05); depletion of Ti (300–1600 ppm), Zr (0.1–10 ppm), and Sr (0.1–10 ppm); and fractionation of Zr relative to Ti (Ti/Zr = 250–4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in ≤0.1% increments to a total of 5–25% melting, a process very close to Rayleigh (fractional) melting; batch melting of a LREE‐depleted source cannot account for the observed trace element concentrations in abyssal peridotites. The shapes of some REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Trace element concentrations in calculated integrated fractional liquids approximate the composition of primitive ocean floor basalts, consistent with postsegregation aggregation of small increment melts produced over a depth and melting interval.
Magnetic grain size and viscous remanent magnetization of pelagic clay
Authors: Yamazaki Toshitsugu , Katsura Ikuo
Published: February 2008

Unfossiliferous pelagic clay (red clay) cores from the Pacific generally have unstable remanent magnetization except for tens of centimeters to several meters below the surface. The origin of this instability has been considered to be viscous remanent magnetization (VRM). We conducted a rock‐magnetic study of a pelagic clay core from the South Pacific, which has the unstable‐to‐stable transition at about 1.5 m below the surface. Using the suspension method of Yoshida and Katsura (1985), we determined downcore variation of complete alignment magnetization (CAM) and magnetic moment distribution (geometric mean moment, mG, and log standard deviation, log a) assuming a lognormal distribution. Mean magnetic grain diameter was calculated from the mG using the saturation magnetization of magnetite, 4.8×105 A M−1, on the assumption that all the magnetic grains are single‐domain magnetites of spherical shape. This assumption is supported by thermomagnetic analyses, isothermal remanent magnetization (IRM) acquisition experiments, and the ratio of CAM to saturation IRM The grain diameter ranges from about 0.02 to 0.15 μm and decreases with depth below about 1.5 m. The derived grain size holds even when the magnetites in the sediments suffered oxidation because the saturation magnetization of maghemite is close to that of magnetite. The frequency dependence of magnetic susceptibility showed a downward increase in the amount of superparamagnetic grains. On the other hand, the magnetic viscosity acquisition coefficient normalized by the CAM increases with depth. We conclude that magnetic grain size can control the magnitude of the secondary magnetization, VRM, of pelagic clay. It has been proposed that the major source of pelagic clay is atmospherically transported dust. Intensified global atmospheric circulation by the change of climate would raise the grain size of the magnetic minerals of the eolian component, which can cause the unstable‐to‐stable transition of the remanent magnetization.
Magnetite diagenesis in marine sediments from the Oregon continental margin
Authors: Karlin Robert
Published: February 2008

The magnetochemistry of sediments from the Oregon continental margin is examined to determine the effects of iron‐sulfur diagenesis on the paleomagnetic record. Magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites. These processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities, systematic changes in other rock magnetic properties, and increases in solid phase sulfur concentrations with depth. Hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size. Magnetite abundances decrease downcore from initial values of about 0.1%, and nominal grain diameters lie within a narrow pseudosingle domain range of 0.08 to 0.6 μm. A first‐order surface area reaction model, dA/dt = ‐kA, is proposed to explain the magnetite dissolution mechanism, where A is the total magnetite surface area and k is the rate constant. The solution of this equation predicts that the surface area and concentration decrease exponentially, and the concentration, in addition, depends on grain size. Application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area. Despite extensive magnetite destruction, magnetic directions in such sediments appear to reliably record long‐wavelength trends of the geomagnetic field.
Paleomagnetism and Neogene clockwise rotation of the Northern Cady Mountains, Mojave Desert of southern California
Authors: MacFadden Bruce J. , Woodburne Michael O. , Opdyke Neil D.
Published: February 2008

The early to middle Miocene Hector Formation of the northern Cady Mountains consists of about 400 m of syntectonic alluvial fan and volcaniclastic sediments deposited in the central Mojave desert. Paleomagnetic samples were collected from 60 sites throughout this unit. Samples from each site were treated using both alternating field and thermal demagnetization. These studies and isothermal remanent magnetization experiments indicate that the natural remanent magnetization (NRM) is complex and that characteristic directions can be carried in either high coercivity or continuous unblocking temperature spectra depending upon the sample. A positive reversals test and concordant directions of the alternating field (AF) and low and high temperature components suggest a stable magnetization. Although a Unique magnetostratigraphic correlation to the time scale is difficult, the section studied may represent chrons C6B to C5C. After screening of individual components and calculation of site means using Fisher statistics, 48 of the original 60 sites were used to calculate an overall formation mean direction, i.e., declination 18.6°, inclination 45.4° (α95 = 6.0°). Using a 20‐m.y. paleomagnetic pole for North America [Irving and Irving, 1982] the Hector Formation has undergone 20.6°±7.6° (R±ΔR) of clockwise rotation around a vertical axis. This amount of rotation is considerably less than reported for some early Miocene rocks elsewhere in the Mojave Block. Least squares regression of the 48 site mean declinations versus stratigraphic position indicates no trend throughout the section, suggesting that there was no syndepositional rotation of this unit. The late Cenozoic structural history of the Mojave Block is considerably more complex than was originally believed, and currently available paleomagnetic data indicate that many blocks within this region have experienced different degrees of rotation. There were at least two phases of tectonic rotation in this region: (1) early Miocene (about 25 Ma), probably related to the initiation of extension within the Mojave Rift; and (2) post middle Miocene‐Pliocene (between about 16 Ma to the present), possibly related to movement along principal structural features such as the San Andreas fault zone. The exact timing of the younger phase of tectonic activity cannot be better constrained until paleomagnetic data become available for late Cenozoic rocks within the Mojave Block.
Temperatures, heat flux, and frictional stress near major thrust faults
Authors: Molnar Peter , England Philip
Published: February 2008

We examine the two‐dimensional advective and conductive transport of heat in a region of thrust faulting. Both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor,
below what they would be with the same heat sources but in the absence of advection. In this expression zƒ is the depth to the fault, V is the slip rate, δ is the component of the dip of the fault in the direction of underthrusting, κ is thermal diffusivity, and b is a dimensionless factor that is essentially equal to one for most forms of heating. Initial changes in temperatures near the fault are given by
. These simple formulae are successful because of the neglible influence of lateral conduction of heat, at least for slip rates of a few mm/yr or more. Two time constants govern the transition from the initial change in temperature to steady state: t1 = uƒ/V, where uƒ is the distance along the fault in the direction of underthrusting from the surface to the depth in question, and
, where zƒ is the depth to the fault. When elapsed times exceed the sum of these two time constants, temperatures differ from their steady state values by only about 10 percent. The numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted. Putain. An application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 MPa and are greater than 30 MPa. Calculations of temperatures appropriate for the Himalaya suggest that shear stresses of 100 MPa on the Main Central Thrust probably are required to account for the Tertiary granites of the region, if melting took place after slip began on the thrust. Similarly, the cut‐off in seismicity at a depth of about 15 km in the Himalaya, if due to temperatures exceeding 350° to 450°C, implies a deviatoric stress close to 100 MPa.
Drag on the LAGEOS satellite
Authors: Rubincam David Parry
Published: February 2008

The average drag on the LAGEOS satellite appears to be well understood. It consists of three types of drag: Yarkovsky thermal, neutral particle, and charged particle. Yarkovsky thermal drag depends on the LAGEOS spin axis position. The spin axis in the decade since launch appears to have stayed near where the engineering telemetry data put it at apogee kickmotor separation: 22° colatitude and 313° east longitude. This result is based on fitting more than 11 years of along‐track acceleration data to a thermal model. The Center for Space Research at the University of Texas has obtained a similar answer. For this position, Yarkovsky thermal drag accounts for about 70% of the observed drag. Neutral particle drag accounts for about 14% of the drag for a hydrogen number density of 5×109 m−3. Proton drag accounts for 12 per cent at a number density of 3×109 m−3 and a temperature of 5000 K in the plasmasphere for a satellite potential of −1 V. Charged helium and oxygen give another 4%, so that charged particle drag totals 16%. All together the three types of drag account for all of the observed average drag. Hence there is closure: the particle environment at LAGEOS altitude, the drag models, and the observed drag are all consistent with each other. Less well understood are the fluctuations in LAGEOS's along‐track acceleration when the orbit intersects the Earth's shadow.
Shallow structure and deformation along the San Andreas Fault in Cholame Valley, California, based on high‐resolution reflection profiling
Authors: Shedlock Kaye M. , Brocher Thomas M. , Harding Samuel T.
Published: February 2008

The mapped active traces of the San Andreas fault are separated by a 1‐km‐wide right‐stepping offset in Cholame Valley. The geometry of this offset, defined in other strike‐slip systems as a releasing bend or a dilational jog, has resulted in the formation of a pullapart basin. Various researchers have inferred that this offset served as a rupture terminus for earthquakes on both strands of the San Andreas fault (1966 Parkfield and 1857 Fort Tejon); thus, this en echelon offset may represent a barrier to the propagation of rupture between two segments of the San Andreas fault. We collected 18 km of high‐resolution seismic reflection data specifically designed to image the San Andreas fault zone in the shallow crust surrounding this offset. Four short profiles (≤ 3.3 km long) of Mini‐Sosie reflection data (1 s of two‐way travel time ≈ 1.5 km deep) were collected perpendicular to the San Andreas fault; three of these profiles were tied by a 7‐km‐long profile that trended northwest through Cholame Valley, subparallel to the San Andreas fault. A zone of incoherent energy, narrow at the surface but widening with depth, underlies the mapped active traces of the San Andreas fault and abruptly terminates shallow reflections on both sides. The reflection profiles and available well data indicate that west of the mapped active traces of the San Andreas fault the shallow subsurface structure of the crust consists of thin (≤400 m thick), offset packages of reflections, laterally coherent on the scale of tens of meters, overlying deformed clastic sedimentary rocks. East of the San Andreas fault, the structure of the shallow crust in southern Cholame Valley is characterized by thick packages of reflections, laterally coherent on the scale of kilometers, overlying the Franciscan complex. All of the strata east of the fault (within Cholame Valley) dip toward the San Andreas fault and the offset, into an approximately 1‐km‐deep sedimentary basin abutting the south strand of San Andreas fault. The offset in Cholame Valley is characterized by a gentle downwarping of sediments into the offset, the presence of many small faults and discontinuous reflections (consistent with areally distributed shear) between the traces of the main fault, localized subsidence abutting the main strike‐slip fault, the formation of a basin, near the offset, that is about as deep as the jog is wide, and the southward propagation of the deformation associated with the offset. Strain field modeling based on simple geometries of the San Andreas and associated faults successfully predicts the general features of the observed topography and subsurface structure of southern Cholame Valley, including subsidence and basin formation near the offset, a discontinuous San Andreas fault plane, and at least one fault in southeastern Cholame Valley.
Laboratory studies of the electrical conductivity of silicate perovskites at high pressures and temperatures
Authors: Li Xiaoyuan , Jeanloz Raymond
Published: February 2008

The electrical conductivities of two silicate perovskites and a perovskite‐magnesiowüstite assemblage, all having an atomic ratio of Mg to Fe equal to 0.88/0.12, have been measured with alternating current and direct current (DC) techniques at simultaneously high pressures and temperatures. Measurements up to pressures of 80 GPa and temperatures of 3500 K using a laser‐heated diamond anvil cell demonstrate that the electrical conductivity of these materials remains below 10–3 S/m at lower mantle conditions. The activation energies for electrical conduction are between 0.1 and 0.4 eV (0.1–0.2 eV most probably) from our data, and we ascribe the conduction in these perovskites to an extrinsic electronic process. Our new measurements are in agreement with a bound that was previously obtained from DC measurements for the high PT conductivity of perovskite‐dominated assemblages. The results show that the electrical conductivity of (Mg0.88Fe0.12)SiO3 perovskite differs significantly from that of Earth's deep mantle, as inferred from geophysical observations.
Paleomagnetic evidence for Post‐Jurassic stability of southeastern Mexico: Maya Terrane
Authors: Guerrero Jose C. , Herrero‐Bervera Emilio , Helsley Charles E.
Published: February 2008

The tectonic evolution of southeastern Mexico has been a subject of major controversy, not only in regard to past geometry but also in the timing of proposed geological events as well. For the past 10 years, most, if not all, investigators agree that the Gulf of Mexico Basin was formed by Late Jurassic time and that the Maya Terrane was in its current location prior to the Cretaceous. In order to gain further insight into the drift history of the Maya Terrane we have undertaken a paleomagnetic study of the uppermost Jurassic‐Lower Cretaceous (Tithonian‐lower Neocomian?) San Ricardo Formation in southeastern Mexico, at 93.7°W, 16.8°N. The sampling site is located east of the Isthmus of Tehuantepec, on the southwest side of the Maya block, at the base of the Yucatan Peninsula. A suite of 133 samples was collected in stratigraphic succession from a 114‐m‐thick sequence of red shales and sandstones near Cintalapa, Chiapas, Mexico. After progressive thermal demagnetization of all samples at six steps from 350°C to 630°C, 89 samples were selected for final paleopole analysis on the basis of their magnetic stability. Four different polarity intervals were observed, the sequence being from bottom to top: N, R, N, R which assists in the assessment of the reliability of the observations. The mean pole position obtained, 160.0°E, 69.8°N, agrees with the mean pole position of the upper part of the Morrison Formation of Colorado, a unit of virtually identical age. These results indicate that no discernible rotation or displacement of the Maya block has occurred since at least early Neocomian times.
Sulfide composition and microthermometry of fluid inclusions in the Leg 111 sheeted dike section of Ocean Drilling Program Hole 504B, Costa Rica Rift
Authors: Schöps Dietmar , Herzig Peter M.
Published: February 2008

Hole 504B of the Ocean Drilling Program is dedicated to the study of crustal structure and hydrothermal processes in 5.9‐m.y.‐old oceanic basement. Continuing the work of previous legs, hole 504B was extended 212.3 m to a total depth of 1562.3 m below seafloor (bsf) during leg 111 in 1986. Quartz‐sulfide veins occur at a depth of 1369–1388 m bsf in basalts of the sheeted dike complex. The ore minerals are predominantly pyrite, less chalcopyrite, rare Corich Cu‐Fe‐S phases, and a thiospinel (linnaeite/carrollite). Microprobe analyses yield a high Co content in zoned vein pyrites (>8 wt %) as well as in the Cu‐Fe‐S phases (>5 wt %). Up to 35.8 wt % Co was detected in the thiospinel. A Co/Ni ratio of >100 distinguishes the vein pyrite from pyrite in the basaltic wall rock and from pyrite formed as an alteration product of olivine (Co/Ni <5). The Co/Ni ratios correlate positively with Cu and negatively with As. Co‐rich, nonstoichiometric Cu‐Fe‐S sulfides in chalcopyrite are interpreted as metastable phases which have been quenched at a high temperature and prohibited from exsolution of the stable products chalcopyrite and pyrite. Fluid inclusions in quartz from the quartz‐sulfide veins are two‐phase and vary from liquid‐ to vapor‐dominated. Their salinities range from 4.2 to 7.2 wt % equivalent NaCl and average 5.5 wt %. Pressure (360 bars) corrected average filling temperatures vary from 271° to 408°C with a maximum of 486°C. This is consistent with calculated quartz formation temperatures for a single quartz separate (+4.2‰ δ18O) using oxygen isotope thermometry. The δ18O value of the hydrothermal fluid was determined to be +1.7‰. The temperature data indicate fluid alteration of the sheeted dikes at about 350° to 500°C. The maximum homogenization temperatures intersect the liquid/vapor two‐phase boundary above the critical point of seawater. Thus phase separation could have occurred before or during the formation of the mineralized veins and the alteration of the sheeted dike sequence.
Shear velocity and intrinsic Q structure of the shallow crust in southeastern New England from Rg wave dispersion
Authors: Saikia Chandan K. , Kafka Alan L. , Gnewuch Scott C. , McTigue John W.
Published: February 2008

In this study, we analyzed 0.5–2.0 s period Rayleigh waves (Rg) generated by quarry and construction blasting in southern New England (CNE). We investigated group velocity dispersion and attenuation of the observed Rg waves. The paths crossing the Hartford Rift basin (HRB) show an obvious correlation between geology and Rg dispersion. The entire region in the southeastern New England comprising a wide range of geological structures and rock types from the Bronson Hill Anticlinorium to the Avalonian Terrane can be represented as one dispersion region. Therefore the relationship between lateral changes in geologic structures mapped on the surface and Rg dispersion is not as straightforward as might be expected for a best fitting flat‐layered model of the shallow crust. The shear wave velocities appear to vary between 2.55 and 3.63 km/s within the upper 2.5 km except for the central HRB where the variation is between 2.12 and 2.7 km/s. Intrinsic Q structure is considered to be the primary means for the loss of energy in the shallow crust and was analyzed by modelling the waveforms of several of the observed seismograms. For this aspect of our study, we used a modal summation of Rayleigh waves assuming a far‐field radiation approximation. The observed seismograms were dominated primarily by contributions from the fundamental mode, but higher modes were also included in the synthesis of the waveform. We were unable to model the absolute amplitudes of the waveforms because of the problems with the instrument calibration. It is clear, however, that to predict correct waveforms, the shear wave Q values in the upper few tenths of a kilometer of the crust must be about an order of magnitude smaller than Q values at the depth of 1–3 km which is of order of 100–250.
Underthrust sediments, fluid migration paths, and mud volcanoes associated with the accretionary wedge off Costa Rica: Middle America Trench
Authors: Shipley Thomas H. , Stoffa Paul L. , Dean Donald F.
Published: February 2008

Styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments. We estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the Middle America Trench near Costa Rica. Reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion. Within 4 km of the deformation front, where overburden is <800 m, more than half of the total water in the pore spaces may have been expelled. The rate of fluid transfer from the underthrust section is highest in the zone 0–4 km arcward of the deformation front. The clarity of the reflection associated with the décollement is exceptional and shows a phase reversal, relative to the seafloor. This high‐amplitude phase‐reversed reflection ends about 4 km arcward of the deformation front, suggesting a rapid reduction in the density and/or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position. Fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m×1000 m. A reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge/slope cover boundary and may mark a fluid conduit. Locally, high‐amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow. Some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section. Near the trench where fluid expulsion rates are highest, water may exit to the seafloor seaward of the downslope pinch‐out of the slope section.
Hydrogeochemistry of the Northern Barbados Accretionary Complex Transect: Ocean Drilling Project Leg 110
Authors: Gieskes Joris M. , Vrolijk Peter , Blanc Gerard
Published: February 2008

Detailed studies of the major element geochemistry, oxygen and hydrogen isotope ratios of pore fluids, and the 87Sr/86Sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern Barbados accretionary complex. These processes include (1) alteration of volcanic ash buried in the Pleistocene‐Pliocene sediment column; (2) alteration of underlying basalts of layer 2 of the oceanic crust; (3) movement of fluids from deep in the accretionary complex along fault zones (particularly the décollement) and minor permeable layers; these fluids from deeper in the complex are characterized by low chloride concentrations and increased δ18O(H2O) values, presumably as a result of dehydration of smectite interlayers; and (4) mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids.
water flow in the Barbados Accretionary Complex
Authors: Le Pichon Xavier , Henry Pierre , Lallemant Siegfried
Published: February 2008

We present a simple analytic kinematic model of the Barbados accretionary complex assuming a porosity depth law and deformation by pure vertical shear. The model is applied to two sections, a northern one with a 900‐m‐thick basin sedimentary column near 16°N and a southern one with a 4.5‐km‐thick one to the south. This simple model is compatible with growth of the Barbados accretionary complex since 50 Ma assuming a constant subduction rate and a constant basin sedimentary column equal to the present one. This conclusion is not affected by reasonable changes in the porosity depth law. It suggests that the totality of the entering sedimentary column is either accreted or subcreted to the wedge. The accretion velocity decreases dramatically with age and its variation should be taken into account in any quantitative model. The average strain rates show remarkably different distributions between the two sections, the thinner wedge having a much higher rate in a narrow area at the toe. We then use the model to compute the sediment grains flow, the fluid flow, and their thermal effects. These effects are small and do not exceed 5% of the heat flow in the absence of other internal flows. In contrast the thermal effect of the burial of the slab can result in a lowering of the surface heat flow by as much as 40%. The temperature at the base of the wedge on its arc side is close to 300°C on the southern section and half that amount on the northern one. This high temperature could account for the absence of large earthquakes to the south and their presence to the north. The amount of water discharged, if collected by faults over the whole toe area, is unable to account for some of the manifestations observed over the toe or seaward of the toe in the Barbados area. We suggest two possible other sources. One, for the southern section, is the Orinoco fan area. The other, for the northern section, might be seismic pumping at the limit of the seismogenic zone, along the backthrusts between the accretionary wedge and the forearc basin.
Densities and porosities in the oceanic crust and their variations with depth and age
Authors: Carlson R. L. , Herrick C. N.
Published: February 2008

We have used drilling and downhole logging results (from Deep Sea Drilling Project holes 395A and 418A), measured properties of core samples from the seafloor and from ophiolites, and seismic velocity structure models of the oceanic crust to constrain empirical (statistical) relationships between in situ densities, porosities, and seismic velocities and to estimate densities and porosities in the oceanic crust, with the following principal results. Our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration (or metamorphic grade) than to its igneous structure: the transition from layer 2a (3.7±0.4 km s−1) to 2b (5.2±0.4 km s−1) occurs within the extrusive pile; the top of layer 2c (6.2+0.2 km s−1) is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products; ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies. A comparison of reprocessed and corrected downhole logs from holes 395A and 418A indicates that in situ densities and fractional porosities differ by 0.2 Mg m−3 and 0.07, respectively. These differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust. Statistical relationships between in situ density, porosity and slowness are highly linear, and indicate that the density/slowness relation estimated from laboratory samples [pb = (3.81±0.02)–(6.0±0.1)S] applies to in situ properties as well. We obtained a similar empirical relationship for porosities from the hole 418A log data [Фf = −(0.35±0.03) + (2.37±0.15) S; r2 = 0.84 S.E. = 0.015]. Applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities: layer 2, 2.62–2.69 Mg m−3, 0.10–0.12 and layer 3, 2.92–2.97 Mg m−3. The estimated average density of the oceanic crust is 2.86±0.03 Mg m−3. Densities and porosities estimated from a seismic crustal structure model for old Atlantic crust agree very well with the logging data from hole 418A, suggesting that in situ densities and porosities can be estimated from seismic velocities. High density and porosity gradients are characteristic of the upper oceanic crust; increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth. Based on the seismic properties of layer 2a, we estimate that the layer thins at an approximate rate 20 to 45 m m.y.−1 while the velocity increases by 0.040 km s−1 m.y.−1. The corresponding rates of change of density and porosity are 0.016 ± 0.009 Mg m−3 m.y.−1 and −0.005±0.003 m.y.−1, respectively.
Midcontinent rift volcanism in the Lake Superior Region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin
Authors: Nicholson Suzanne W. , Shirey Steven B.
Published: February 2008

Between 1091 and 1098 Ma, most of a 15‐ to 20‐km thickness of dominantly tholeiitic basalt erupted in the Midcontinent Rift System of the Lake Superior region, North America. The Portage Lake Volcanics in Michigan, which are the youngest MRS flood basalts, fall into distinctly high‐ and low‐TiO2 types having different liquid lines of descent. Incompatible trace elements in both types of tholeiites are enriched compared to depleted or primitive mantle (La/Yb = 4.3–5.3; Th/Ta = 2.12–2.16; Zr/Y = 4.3–4.4), and both basalt types are isotopically indistinguishable. Sr, Nd, and Pb isotopic compositions of the Portage Lake tholeiites have 87Sr/86Sri ≈ 0.7038, εNd (1095 Ma) ≈ 0±2, and μ1 ≈ 8.2. Model ages with respect to a depleted mantle source (TDM) average about 1950–2100 Ma. Portage Lake rhyolites fall into two groups. Type I rhyolites have Nd and Pb isotopic characteristics (εNd (1095 Ma) ≈ 0 to −4.7; μ1 ≈ 8.2–7.8) consistent with contamination of tholeiitic rocks by 5–10% Archean crust. The one type II rhyolite analyzed has Nd and Pb isotopic compositions (εNd (1095 Ma) ≈ −13 to −16; μ1 ≈ 7.6–7.7) which are consistent with partial melting of Archean crust. Early Proterozoic crust was not a major contaminant of MRS rocks in the Lake Superior region. Most reported Nd and Pb isotopic compositions of MRS tholeiites from the main stage of volcanism in the Lake Superior region and of the Duluth Complex are comparable to the Nd and Pb isotopic data for Portage Lake tholeiites. The isotopic enrichment of the MRS source compared to depleted mantle is striking and must have occurred at least 700 m.y. before 1100 Ma. There are two likely sources for such enriched MRS tholeiitic magmatism: subcontinental lithospheric mantle enriched during the early Proterozoic or enriched mantle derived from an upwelling plume. Subcontinental lithospheric mantle alone as a source cannot be ruled out strictly on isotopic grounds, but melting of enriched lithosphere potentially ranging in age from 3.6 to 1.8 Ga would have been less likely to generate either the homogeneity or the volume of MRS tholeiites. Decompression melting of an upwelling enriched mantle plume in a region of lithosphere thinned by extension could have successfully generated the enormous volume (850×103 km3) of relatively homogeneous magma in a restricted time interval.