bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations in ridge morphology and depth‐age relationships on the Pacific‐Antarctic Ridge
Authors: Marks Karen M. , Stock Joann M.
Published: February 2004

Adjacent segments of the Pacific‐Antarctic ridge display significantly different morphologies and depth‐age relationships over seafloor younger than 36 Ma. The spreading corridor southwest of Fracture Zone XII is characterized by a rift valley and an usually small subsidence constant of 226±13 m/m.y.½, while the two spreading corridors immediately northeast of Fracture Zone XII have an axial high and a subsidence constant consistent with the global average. This abrupt variation in ridge morphology is not usually characteristic of medium‐rate spreading centers, nor is such an abrupt variation expected of adjacent ridge segments that are spreading at the same rate. We suggest that a thermal anomaly beneath the ridge may influence the first‐order effects of spreading rate and lithospheric cooling enough to produce the observed rift valley and axial high and the different subsidence constants. Although we are not certain what would produce the thermal anomaly here, we speculate that when the spreading rate on the Pacific‐Antarctic ridge increased from slow to intermediate rates since 20 Ma, so did the need for materials for accretion, which may be supplied in part by along‐axis asthenospheric flow from hotspots or a hot region to the northeast. A sufficient supply of hot asthenosphere may still be lacking in the ridge segment with the axial valley to the southwest, leaving it cooler and starved for accretionary materials.
Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri
Authors: Sun Weiwei , Jackson Mike
Published: February 2004

Magnetic carriers in remagnetized early Paleozoic carbonates of the North American midcontinent were studied in extracts and in situ, using rock magnetism and scanning electron microscopy (SEM). SEM observation of extracts and in situ samples shows that the dominant magnetic particles are large irregular grains and framboids (10–20 μm in diameter) and individual spherical and euhedral particles (0.2–1 μm). Comparing the saturation remanence of the bulk rock and extracts indicates that the extracts only account for ∼10% of the saturation remanence of the bulk rock. Most of the remanence carriers were lost during the dissolution and separation procedure, especially the finer‐grained carriers. The framboids and the individual submicron euhedral magnetic particles observed by SEM thus make a fairly minor contribution to the magnetic remanence of the remagnetized carbonates. Hysteresis loops of the extracts are very similar to those reported by Suk et al (1993), with saturation remanence to saturation magnetization (Jr/Js) ratios from 0.1 to 0.13 and remanent to bulk coercivity (Hcr/Hc) ratios ∼4. However, the loops for the “nonmagnetic” residue are strongly “wasp‐waisted”, and Hcr/Hc ratios are very high, similar to whole rock specimens of remagnetized carbonates (Jackson, 1990). This suggests that bulk rock and “nonmagnetic” residue contain very fine (superparamagnetic (SP) and single domain (SD)) magnetic particles which were not recovered by the magnetic separation procedure. Low‐temperature experiments support the conclusion that the principal remanence carriers in the remagnetized carbonates are SD magnetite and that SP magnetite plays an important part in the unusual bulk magnetic properties of these rocks. Owing to their dissolution, the actual natural remanent magnetization bearing particles cannot be observed petrographically.
Modeling wide‐angle seismic data for crustal structure: Southeastern Grenville Province
Authors: Zelt C. A. , Forsyth D. A.
Published: February 2004

A modeling methodology for obtaining two‐dimensional (2‐D) crustal structure from wide‐angle seismic data is applied to data from the southeastern Grenville Province. Pre‐modeling steps include (1) assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal‐to‐noise ratio, (2) using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity, and (3) identifying data unsuitable for 2‐D modeling. The goal of the travel time inversion‐amplitude modeling approach is to obtain a minimum‐structure and minimum‐parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide‐angle data. Each step of a layer‐stripping procedure involves a series of inversions in which a one‐dimensional or simple starting model is improved with additional velocity and/or interface nodes until a satisfactory trade‐off between travel time fit, parameter resolution and complete ray coverage of all source‐receiver pairs is achieved. Using zero vertical‐velocity gradient layers and head waves during preliminary first‐arrival inversion can (1) decrease the number of intermediate models, (2) allow greater lateral heterogeneity to be imaged, and (3) simplify incorporation of amplitude modeling constraints into the final model. Using amplitude‐distance curves allows quantitative modeling of the relative amplitude and offset variations of phases. Discrepancies between observed and calculated reflection amplitudes are used to infer fundamental, non step‐like velocity changes at layer boundaries. Later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that “float” within the model without an associated velocity structure. These reflectors provide a spatial image like that obtained from vertical‐incidence reflection data, as opposed to a velocity image. The model of Grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data. A crustal‐scale zone of wide‐angle reflectors with an average easterly apparent dip of 13° defines a major Grenvillian terrane boundary.
Seismic structure of the northeastern Japan convergent margin: A synthesis
Authors: Hasegawa Akira , Horiuchi Shigeki , Umino Norihito
Published: February 2004

Many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern Japan arc and its relationship to seismic and volcanic activity. Spatial distributions of the depths to the Conrad and the Moho discontinuities, estimated from shallow earthquake data and seismic explosion data, show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the Japan Sea and the Pacific Ocean. The Pn velocity has a lateral variation; it is as low as ∼7.5 km/s beneath the land area, while that beneath the Japan Sea and the Pacific Ocean is 8.0–8.2 km/s. It changes abruptly at the transition zones, which are located along the coastlines. Precise structure and location of the subducted Pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate. The Pacific plate is composed of a thin (∼5 km) low‐velocity upper layer and a thick high‐velocity lower layer, its total thickness being 80–90 km. The upper plane seismicity of the double seismic zone is confined to the thin low‐velocity upper layer, which probably corresponds to the subducted former oceanic crust. The lower plane seismicity lies at the middle of the high‐velocity lower layer, and the lower half of the plate below it is incapable of generating earthquakes. The shallower portion of the upper surface of the plate beneath the Pacific Ocean, along which major seismicity with low‐angle thrust faultings is actually occurring, is also located by seismic observations on land and in the sea. The Pacific plate subducts at an extremely low angle of ∼5° for the first ∼25‐km depth, and then the dip steepens rather abruptly to ∼30°. Normal‐fault type events at the top of the plate have not been detected in the portion where the downward‐bending is the largest, but have been ditected near the trench axis, where it is rather small. Tomographic inversions for seismic velocity structure clearly delineate the inclined high‐velocity Pacific plate with a thickness of 80–90 km and low‐velocity zones in the crust and the mantle wedge beneath active volcanoes. Seismic attenuation tomography also shows similar zones of low‐Q value beneath active volcanoes, although its spatial resolution is much lower. The low‐velocity zones with 2–6% velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100–150 km in the mantle wedge, and are approximately parallel to the dip of the underlying Pacific plate. These low‐velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the Earth's surface, corresponding to a portion of the subduction‐induced secondary mantle wedge flow.
Late Paleozoic remagnetization in limestones of the Craven Basin (northern England) and the rock magnetic fingerprint of remagnetized sedimentary carbonates
Authors: McCabe Chad , Channell J. E. T.
Published: February 2004

Samples of Lower Carboniferous Chatburn and Pendleside limestones collected from 27 sites in the Craven Basin (northern England) carry a syn‐Hercynian remagnetization residing in magnetite. The paleomagnetic directions are all of reversed polarity consistent with a Kiaman Superchron remagnetization. The partially (62%) tilt corrected mean direction has the minimum correlation statistic implying a synfolding magnetization. The resulting pole appears to be Late Carboniferous in age, and there is no indication of any magnetization component that predates the remagnetization event. The magnetization appears to be of the same type as the synfolding remagnetizations documented in several Appalachian carbonate units. Jackson (1990) reported that remagnetized Appalachian carbonates have distinctive hysteresis properties, and we have obtained similar hysteresis loops from remagnetized Paleozoic carbonates from the Craven Basin (England), the Great Basin (Nevada), Alaska, and New York State. The hysteresis loops are all “wasp‐waisted” and yield highly anomalous hysteresis ratios, compared to data from synthetic or “typical” magnetite‐bearing samples. In contrast, unremagnetized limestones from Italy show hysteresis behavior which is more typical for magnetite‐bearing rocks (Channell and McCabe, this issue). These results suggest that remagnetization in magnetite‐bearing carbonates can be readily detected on the basis of hysteresis properties, prior to a full‐scale paleomagnetic investigation.
Measurements of the strain field associated with episodic creep events on the San Andreas Fault at San Juan Bautista, California
Authors: Gladwin M. T. , Gwyther R. L. , Hart R. H. G. , Breckenridge K. S.
Published: February 2004

Over the past 6 years, repeated strain steps recorded on a tensor strainmeter installed close to the San Andreas fault trace at San Juan Bautista in California correlated closely with episodic creep events registered on a nearby creepmeter. The strain events were remarkably similar in character, were of about 1 hour duration, and were followed within hours to days by creep events of 2–6 days duration. The episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data. We postulate that the additional stress transferred to this region by the Loma Prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region, thus accounting for the increased frequency of the creep/strain events following the Loma Prieta earthquake.
Crystallization history of the 1984 Mauna Loa lava flow
Authors: Crisp Joy , Cashman Katharine V. , Bonini Jennifer A. , Hougen Sarah B. , Pieri David C.
Published: February 2004

During a 3‐week eruption in 1984, Mauna Loa produced vent lavas that increased in crystallinity from <1 to 30%, and 27‐km‐long flows that increased in crystallinity as they moved downstream. We examined the crystallization history of these lavas using crystal size distribution (CSD) analysis to study the rates of crystallization, viscosity increase, and latent heating. Typical average growth and nucleation rates were 5×10−9 cm s−1 and 5 cm−3 s−1 for microphenocrysts (20‐ to 500‐μm size crystals nucleated in the rift zone) and 5×10−8 cm s−1 and 5 × 104 cm−3 s−1 for microlites (1‐ to 20‐μm size crystals nucleated in the channel). These crystallization rates are high compared with those found in other CSD studies of igneous rocks, probably due to highly nonequilibrium conditions brought on by rapid degassing in the rift zone and cooling in the lava channel. Growth and nucleation rates decreased with time at the vent and with distance downstream. The maximum downstream total crystallinity measured is 39% (25% microlites, 14% microphenocrysts) in a quenched sample 14 km from the vent. Growth and nucleation rates cannot be calculated for postemplacement samples, but they place upper limits of 53–58% on the amount of crystallization in the channel 9–20 km from the vent. Crystallization could have been mostly responsible for the 105‐fold downstream increase in apparent viscosity, although degassing and increasing incorporation of solid lava fragments also contributed. Another effect of crystallization on the lava flow was the sizeable latent heating (0.01 J g−1 s−1 over the first half of the flow length, if the crystallinity of downstream quench samples is representative of the hot fluid core), which may have been counteracted by entrainment of cooler material. Measurements of crystallization are shown to be crucial in the study of lava flow emplacement dynamics.
40Ar/39Ar age constraints for the Jaramillo Normal Subchron and the Matuyama‐Brunhes geomagnetic boundary
Authors: Izett Glen A. , Obradovich John D.
Published: February 2004

Our mid‐Pleistocene 40Ar/39 Ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope, climate record calibration of the astronomical timescale proposed by Johnson (1982) and Shackleton et al. (1990). 40Ar/39Ar ages of a normally magnetized rhyolite dome in the Valles caldera, northern Mexico, yielded a weighted‐mean age of 1.004±0.019 Ma. A K‐Ar age of 0.909±0.019 Ma for this rock by Doell and Dalrymple (1966) was the linchpin for the recognition and calibration of the Jaramillo Normal Subchron (JNS). Other 40Ar/39Ar ages from the Valles caldera and 40Ar/39Ar ages of Ivory Coast tektites indicate that the JNS began at about 1.11 Ma and ended before 0.92 Ma, probably near 0.97 Ma. The Matuyama‐Brunhes boundary occurred between 0.79 Ma and 0.76 Ma on the basis of 40Ar/39Ar sanidine ages from (1) three reversely magnetized rhyolite domes of the Valles caldera (0.793±0.018 Ma, 0.794±0.007 Ma, and 0.812±0.023 Ma) and pumice (0.789±0.006 Ma) from the reversely magnetized Oldest Toba Tuff of Sumatra and (2) pumice (0.764±0.005 Ma and 0.757±0.009 Ma) from the lower and upper units of the normally magnetized Bishop Tuff. The age of the boundary may be close to 0.77 Ma as deduced from rates of sedimentation in ancient Lake Bonneville, Utah.
P wave velocity across a noncoaxial ductile shear zone and its associated strain gradient: Consequences for upper crustal reflectivity
Authors: Rey Patrice F. , Fountain David M. , Clement William P.
Published: February 2004

In order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone, we determined P wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile (≈ 27 cm long) through a transition zone and its associated strain gradient. The ductile shear zone, developed within an aplitic leucogranite, was sampled from a kilometer‐wide ductile transcurrent fault in the northern French Massif Central. Strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation; shear strain (γ) systematically increases from zero in the undeformed protolith to ≈ 30 in the mylonite. The transition zone thickness (T) is about 30 cm, and the mylonite thickness (M) is about 10 cm. The amount of quartz and mica increases relative to feldspar toward the mylonite, indicating that a mineralogical composition change accompanied mylonitization. Mica and quartz developed a strong crystallographic preferred orientation (CPO). In the least strained domain, seismic anisotropy is low and mean Vp is 6 km/s at 600 MPa. Anisotropy increases up to 10% and Vp decreases up to 5.35 km/s for propagation normal to the mylonite foliation through the transition zone. This systematic velocity change correlates with the increasing γ through the transition zone and can be directly related to the CPO of mica and the increase in volume percent mica within the mylonite zone. These results indicate that velocity and anisotropy gradients may, in some cases, be associated with ductile shear zones and that mylonite boundaries may not represent first‐order discontinuities. The reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength (λ) and on the T/M ratio. Synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio T/M increases. We show that layers with second‐order boundaries (velocity gradients in transition zones) are only seismically detectable within a narrow thickness range. Extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is, to a first approximation, scale independent. In granitic domains, ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m. Development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low‐temperature conditions in the granitic upper crust. Indeed, low‐temperature strain gradient may explain the high seismic reflectivity of the upper crust in the Scandinavian Caledonides, whereas high‐temperature strain gradient may explain, in part, the relative transparency of the European Variscan upper crust.
Three‐dimensional inversion of regional P and S arrival times in the East Aleutians and sources of subduction zone gravity highs
Authors: Abers Geoffrey A.
Published: February 2004

Free‐air gravity highs over forearcs represent a large fraction of the power in the Earth's anomalous field, yet their origin remains uncertain. Seismic velocities, as indicators of density, are estimated here as a means to compare the relative importance of upper plate sources for the gravity high with sources in the downgoing plate. P and S arrival times for local earthquakes, recorded by a seismic network in the eastern Aleutians, are inverted for three‐dimensional velocity structure between the volcanic arc and the downgoing plate. A three‐dimensional ray tracing scheme is used to invert the 7974 P and 6764 S arrivals for seismic velocities and hypocenters of 635 events. One‐dimensional inversions show that station P residuals are systematically 0.25–0.5 s positive at stations 0–30 km north of the Aleutian volcanic arc, indicating slow material, while residuals at stations 10–30 km south of the arc are 0.1–0.25 s negative. Both features are explained in three‐dimensional inversions by velocity variations at depths less than 25–35 km. Tests using a one‐dimensional or a two‐dimensional slab starting model show that below 100 km depth, velocities are poorly determined and trade off almost completely with hypocenters for earthquakes at these depths. The locations of forearc velocity highs, in the crust of the upper plate, correspond to the location of the gravity high between the trench and volcanic arc. Free‐air anomalies, calculated from the three‐dimensional velocity inversion result, match observed gravity for a linear density‐velocity relationship between 0.1 and 0.3 (Mg m−3)/(km s−1), when a 50‐km‐thick slab is included with a density of 0.055±0.005 Mg m−3. Values outside these ranges do not match the observed gravity. The slab alone contributes one third to one half of the total 75–150 mGal amplitude of the gravity high but predicts a high that is much broader than is observed. The inclusion of upper‐plate velocity anomalies predicts the correct width of the anomaly, 100–150 km, where the anomaly is most positive. Because the forearc gravity high is continuous along the entire Aleutian arc and is found in most arcs globally, high upper‐plate forearc velocities are suspected to be a common feature of the upper plate of most subduction zones. The forearc mass excesses appear to be sustained by upward regional flexure of the upper plate that partly balances the depression of the lower plate at the trench, thus elevating high‐density and high‐velocity material. Thus a part of the downward flexure of the subducting plate is regionally compensated by shallow positive mass anomalies in the upper plate, and the strength of the upper plate helps generate the forearc gravity highs.
Effects of temperature on friction: Constitutive equations and experiments with quartz gouge
Authors: Chester Frederick M.
Published: February 2004

A state‐variable constitutive relation that can describe the dependence of friction on temperature at or near steady state conditions is presented. The relation is derived from existing state‐variable relations used to describe velocity dependence and the assumption that the micromechanisms of friction are thermally activated and follow an Arrhenius relationship. The relation adequately describes the transient and steady state friction behavior displayed in sliding experiments on 1.5‐mm‐thick layers of fine‐grained (<100 μm diameter) quartz gouge. Gouge layers were sheared up to 10 mm between rough steel surfaces at a constant effective confining pressure of 20 MPa and under room‐dry or water‐saturated conditions in a servo‐controlled triaxial apparatus. In each experiment, velocity was stepped between 4, 0.4, and 0.04 μm/s at constant temperature, and temperature was stepped between 24°, 57°, and 82°C at a constant velocity of 0.04 μm/s. Coefficient of friction was calculated from measurements of sample strength using corrections for all apparatus effects including the temperature and velocity dependent strength of the metal sleeves used to isolate the sample and of the graphite‐lubricated interface that allowed lateral slip of the sample halves. Experiments indicate that (1) an abrupt increase in temperature induces a transient friction response similar to that induced by a step decrease in velocity, (2) the transient friction response is relatively symmetric for steps up and steps down in temperature, and (3) the characteristic slip distance for friction to evolve to steady state after step changes in temperature is the same as after step changes in velocity. The apparent activation energy determined for wet quartz gouge at the test conditions is 89±23 kJ/mol. This value is consistent with models of friction involving failure of contact junctions by subcritical crack growth and time dependent strengthening of contact junctions by either increasing the true area of contact through subcritical cracking or by increasing the quality of contact junctions through healing.
Sound velocity measurements in liquid iron by ultrasonic interferometry
Authors: Nasch P. M. , Manghnani M. H. , Secco R. A.
Published: February 2004

Compressional wave velocity Vp has been measured in liquid iron in the temperature range 1823–1883 K and in the frequency range 5.4–11.8 MHz for the first time using ultrasonic interferometry. The value of Vp at the melting temperature is 3820±3 ms−1 which is comparable to previous measurements all using time‐of‐flight methods. The temperature derivative of velocity, dVp/dT, in this temperature range (−0.72±0.07 m s−1 K−1 ) and the lack of dispersion observed in this frequency range are in agreement with previous studies. Our calculated value of the relaxed (adiabatic) bulk modulus K0 at the melting temperature is 102 GPa. Estimate of the average longitudinal relaxation time τl indicates that the liquid iron is in a relaxed state in the present frequency range. Accurate measurement of attenuation is not yet possible with our present apparatus, but typical interferograms indicate a very low attenuation, which is qualitatively consistent with the prediction from viscoelastic theory and seismological determinations of attenuation of Earth's outer core.
Quantitative estimation of granitoid composition from thermal infrared multispectral scanner (TIMS) data, Desolation Wilderness, northern Sierra Nevada, California
Authors: Sabine Charles , Realmuto Vincent J. , Taranik James V.
Published: February 2004

We have produced images that quantitatively depict modal and chemical parameters of granitoids using an image processing algorithm called MINMAP that fits Gaussian curves to normalized emittance spectra recovered from thermal infrared multispectral scanner (TIMS) radiance data. We applied the algorithm to TIMS data from the Desolation Wilderness, an extensively glaciated area near the northern end of the Sierra Nevada batholith that is underlain by Jurassic and Cretaceous plutons that range from diorite and anorthosite to leucogranite. The wavelength corresponding to the calculated emittance minimum λmin varies linearly with quartz content, SiO2, and other modal and chemical parameters. Thematic maps of quartz and silica content derived from λmin values distinguish bodies of diorite from surrounding granite, identify outcrops of anorthosite, and separate felsic, intermediate, and mafic rocks.
Seismic velocities and inferred porosities in the accretionary wedge sediments at the Cascadia margin
Authors: Yuan T. , Spence G. D. , Hyndman R. D.
Published: February 2004

Large offset multichannel seismic reflection data from the subduction zone margin off Vancouver Island define the geometry, internal structure, and deformation style of the Cascadia accretionary prism. A very detailed seismic velocity analysis has been performed to quantitatively assess the velocity structure and thus to infer porosity variations and pore fluid expulsion across the deformation front. Within the sediments of Cascadia Basin approaching the deformation front, and within the frontal thrust zone of the accretionary prism, velocities increase landward probably as a result of sediment consolidation associated with horizontal compression. A remarkable conclusion is that more than one third of the initial pore fluid content of the incoming sediments is lost by the time they are incorporated into the wedge. In contrast, a pronounced velocity decrease is evident in the lower slope region 15–20 km landward of the deformation front where the wedge sediment thickness is more than doubled compared with the section in Cascadia Basin. The low velocity is explained by underconsolidation resulting from rapid horizontal shortening and vertical thickening of the sediment column, accommodated by displacements along thrust faults or by distributed deformation. As interred from the velocity data, sediment porosities are high in this lower to midslope region of the wedge. The thickening of the wedge results in sediment elements being moved to greater depths landward with little porosity loss and velocity change. In the rapidly thickening section the overburden load stress increases faster than the sediments can respond by fluid expulsion and overpressures likely result.
Vent and nonvent faunas of Cleft segment, Juan de Fuca Ridge, and their relations to lava age
Authors: Milligan Barry N. , Tunnicliffe Verena
Published: February 2004

The distribution and composition of the ridge crest fauna of Cleft segment, southern Juan de Fica Ridge, are examined in the context of geological setting. The concept that faunal characteristics should reflect lava age and morphology was tested with information from vent animal collections and from photographs of nonvent fauna. Numbers of both vent and nonvent species are low compared to other sites in the region. The young age of the segment, relative eruptive instability, terminal position on the ridge, and current patterns may influence species accumulation. Recently eruptive lavas, 2 to 6 years old, were already colonized by several vent species but only two sessile nonvent animals; search and recruitment capabilities of vent species appear to be highly developed. Lava age and morphology do not appear responsible for observed differences in species composition of vent fauna between samples. In contrast, nonvent assemblages showed abundances that reflect lava ages, although the basalt morphology complicated the pattern. That gross geological setting does not explain vent faunal characteristics suggests that factors such as water chemistry and reproductive behavior need closer investigation.
High‐temperature deformation of diopside crystal: 3. Influences of pO2 and SiO2 precipitation
Authors: Jaoul Olivier , Raterron Paul
Published: February 2004

Single crystals of gem quality diopside (with Fe/(Ca+Mg+Fe) ≃ 0.02) were deformed in a dead load apparatus under controlled oxygen partial pressure (pO2), in the range 8×10−14–2×10−9 MPa, at two temperatures T1 = 1100°C and T2 = 1200°C. The aim of these experiments was to investigate the sensitivity of diopside creep rate to pO2 at these two temperatures. T1 and T2 are on both sides of a critical temperature Tc ≃ 1130°–1140°C at which the activation energy E* of the creep rate decreases (from 442 to 48 kJ/mol) with rising temperature (Raterron and Jaoul, 1991) when siliceous microdroplets (≃0.1 μm in size) form (Ingrin et al., 1991). Specimens were deformed with axial compressive stress σ (110–143 MPa) along [010]; with this setting, the {110}1/2〈a±b〉 slip systems are symetrically activated, and strain rates
are in the range 2×10−8–2×10−7 s−1. At T1 and under low pO2, we find
for samples that lack SiO2‐rich precipitates in the host. At T2 and at the highest pO2 explored,
becomes insensitive to pO2 for samples that contain SiO2‐rich precipitates in the matrix. Electrical conductivity σe shows similar sensitivities to pO2 (Huebner and Voigt, 1988). We propose a point defect model based on the chemistry of nonstoichiometric compounds with cationic vacancies and ferric iron Fe3+ as majority point defects. The model predicts the critical values of Tc and pO2c beyond which the increasing abundance of the majority point defects promotes SiO2 precipitation. Tc and pO2c values are interdependent; they are also functions of Fe content in diopside and of its initial nonstoichiometry. This model offers an explanation of the pO2 dependencies of point defects concentrations as well. A comparison with experimental
and σe sensitivities to pO2 suggests that interstitial divalent cations, which are minority defects, control electrical transport and diffusion‐assisted dislocation glide. The model also shows that the occurrence of SiO2 precipitation does not necessarily imply a supersilicic starting material.
Segregation of subducted oceanic crust in the convecting mantle
Authors: Christensen Ulrich R. , Hofmann Albrecht W.
Published: February 2004

Subducted oceanic crust, transformed into dense mineral assemblages at high pressure, may gravitationally segregate at the bottom of the convecting mantle, for example, the D″ layer. Here it could be stored for a long enough time to develop an “enriched” isotopic signature, before it is recycled in mantle plumes and hence control the geochemical character of hot‐spot basalts. We study both the geodynamical and geochemical aspects of this hypothesis in two‐dimensional numerical convection models, in which plate motion is imposed by a velocity boundary condition. About 250,000 tracer particles are used to identify the basalt fraction in the oceanic crustmantle system. High‐, average‐, and low‐tracer densities indicate basalt or eclogite, peridotite, and harzburgite, respectively. The tracers are negatively buoyant to account for the density differences between the rock types. At surface divergence zones, crust formation is simulated by extracting tracers and transferring them into a thin layer at the surface. The tracers carry a certain amount of the relevant nuclides of the U‐Pb and Sm‐Nd systems, which are fractionated between the basalt tracers and a second species of residue tracers during crust formation. Using reasonable parameter values, we find that of the order of 1/6 of the subducted crust accumulates in pools at the bottom, which reside underneath thermal plumes. After 3.6 Ga, the [207Pb]/[204Pb], [206Pb]/[204Pb], and [143Nd]/[ 144Nd] ratios in various parts of the model cover the observed HIMU‐MORB range. A systematic study of the influence of control parameters on the results indicates that the amount of segregation and the diversity of isotope ratios (1) increases strongly with Rρ, the ratio of chemical to thermal buoyancy; (2) decreases moderately with the Rayleigh number Ra, where Ra = 106 is the highest value that we employed; (3) increases strongly with the degree of temperature dependence of the viscosity; and (4) is not very sensitive to the partitioning between internal and bottom heating. The largest uncertainty in applying our model results to the Earth lies in the lack of accurate density data for conditions at the core‐mantle boundary. We conclude that, if our density estimates are correct, segregation and reentrainment of subducted crust is of fundamental importance for the dynamics and chemistry of mantle plumes.
Inversion forPandSwave attenuation structure, Long Valley caldera, California
Authors: Ponko Stefan C. , Sanders Christopher O.
Published: February 2004

PandSwave amplitude data from digital, three‐component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three‐dimensional attenuation structure in the upper crust of Long Valley caldera, California. Differential attenuation (∂Q−1) values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios. The spectral ratios are inverted for ∂Q−1using the LSQR algorithm. Zones of high, positivePwave ∂Q−1(greaterPwave attenuation) are observed at a depth of 4–5 km beneath the east flank of Mammoth Mountain and at 6–8 km beneath the southwestern edge of the resurgent dome. A region of positiveSwave ∂Q−1is seen at 7–8 km beneath the southern edge of the resurgent dome, whereas at 6–7 km, theSwave results are near average. The region of positive S wave ∂Q−1at 7–8 km beneath the resurgent dome may indicate the roof of a magmatic system. The positivePwave ∂Q−1at 4–5 km beneath Mammoth Mountain and 6–8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water. These zones may represent some of the source regions for the Long Valley geothermal system.
Thermodynamics of thin fluid films
Authors: Hess Paul C.
Published: February 2004

When fluids are confined within narrow spaces, their phase equilibria, structural, and dynamical properties become quantitatively and qualitatively different from their bulk values. The conditions obtained at thermodynamic equilibrium between a thin film and bulk fluid are derived in this paper. It is shown that the film thickness and film tension are new thermodynamic parameters. Because the thin film has properties distinct from that of the bulk fluid, it is possible for fluid to exist along planar boundaries even for systems characterized by non zero dihedral angles. For a stable thin film to exist, it is necessary that the “disjoining pressure” be positive or replusive. An analysis of the disjoining pressure for a basalt melt‐peridotite system indicates that a thin basalt liquid film may exist at interphase boundaries.
Upward flow of magmatic fluids from the Old Woman granodiorite, Old Woman Mountains, southeastern California
Authors: Morrison Jean , Hoisch Thomas D.
Published: February 2004

Isotopic compositions, mineral equilibrium, and field relations at the contact between the midcrustal Cretaceous Old Woman granodiorite and Paleozoic carbonates indicate that water‐rich, silica‐saturated magmatic fluids were transported upward, away from the pluton, across an impermeable 30‐ to 40‐m thick marble which caps the granodiorite, to higher structural levels along a complex network of hydrologically induced fractures. Within the fractures, fluids reacted to form symmetrical radiating splays of wollastonite with minor amounts of diopside, vesuvianite, and quartz. In many cases, pegmatites are found in the center of these calc‐silicate skarns. Cross‐cutting pegmatites and wollastonite veins in the aureole indicate that during late stages of crystallization of the granodiorite there were multiple episodes of fluid expulsion. Above the marble layer at higher structural levels, magmatic fluids flowed both laterally and vertically, interacting with lithologies in a more pervasive manner. Values of δ18O for calcite in the vein skarns average 11.8‰ and pegmatite whole rock silicate δ18O values average 9.4‰. Thus oxygen isotopic compositions are consistent with a magmatic origin for the skarn‐forming fluids. Away from the vein skarns, values of δ18O for the capping marble range from 18.7 to 22.1‰ (avg. = 21‰) and values of δ18C range from −3.8 to −3.0‰ (avg. = −3.4‰). The high δ18O values provide evidence that the marble largely retained its premetamorphic isotopic composition, indicating that fluids from the granodiorite did not flow pervasively across the unit. Lithologies at higher structural levels show evidence of more pervasive interaction with magmatic fluids: forsterite‐bearing calc‐silicates have δ18O values down to 11.8‰ and coarse‐grained vesuvianite‐ and wollastonite‐bearing skarns have δ18O values of ∼13‰.
Diapiric ascent of magmas through power law crust and mantle
Authors: Weinberg Roberto Ferrez , Podladchikov Yuri
Published: February 2004

There has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust. We argue here that this is mainly because the country rocks have previously been assumed to be Newtonian, and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten. The ascent velocity of diapirs is calculated through an equation with the form of the Hadamard‐Rybczynski equation for the rise of spheres through Newtonian ambient fluids. This well‐known equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid. These correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of Newtonian drops through power law fluids. The equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks. The velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth. The depth of solidification of the diapir is systematically studied as a function of the geothermal gradient, buoyancy of the body, solidus temperature of the magma, and rheological parameters of the wall rock. The results show that when the wall rock behaves as a power law fluid, the diapir's ascent rate increases, without a similar increase in the rate of heat loss. In this way, diapirs rising at 10 to 102 m/yr can ascend into the middle or upper crust before solidification. Strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates. The thermal energy of the diapir is used to soften the country rock only at late stages of ascent. The transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures.
Japan Sea, opening history and mechanism: A synthesis
Authors: Jolivet Laurent , Tamaki Kensaku , Fournier Marc
Published: February 2004

The respective tectonic effects of back arc spreading and continental collision in Asia are considered either as two independent processes or as closely interrelated. Extrusion tectonics assumes that the opening of the South China Sea and the left‐lateral motion along the Red River fault are geometrically linked in a pull‐apart manner. This model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated. In the case of the Japan Sea, we can show without ambiguity that back arc opening was controlled by large intracontinental strike‐slip faults which can be easily understood as effects of the India‐Asia collision far from the indenter. The Japan Sea opened in the early Miocene in a broad pull‐apart zone between two major dextral strike‐slip shear zones. The first one extends from north Sakhalin to central Japan along 2000 km, it has accommodated about 400 km of finite displacement. Deformation along it varies from dextral transpression in the north to dextral transtension in the south. The second is between Korea and SW Japan and has accommodated a smaller displacement of about 200 km. The extensional domain in between lies in the back arc region of Japan. Distributed stretching of the arc crust resulted in the formation of most of the Japan Sea, while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the Japan Basin. The first oceanic crust was formed in a small triangle based on the eastern shear zone, and spreading propagated westward inside the pull‐apart region. Timing of oceanic crust formation, of formation of the dextral shear zones and of block rotation in between, as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history. This evolution is discussed by comparison to other manifestations of the arc and back arc activity, such as the history of sedimentation and volcanism. The paper then suggests that the collision of India can have tectonic consequences as far north as Japan and Sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion.
On landscape self‐organization
Authors: Rigon Riccardo , Rinaldo Andrea , Rodriguez‐Iturbe Ignacio
Published: February 2004

A new quantitative characterization of landscape‐forming processes in the general framework of self‐organized criticality and of fractal analyses is proposed. The coupled processes considered are threshold‐independent hillslope evolutions and threshold‐dependent fluvial transport phenomena. From a body of experimental and theoretical evidence we argue that geomorphological thresholds, principles of minimum energy expenditure and concepts of self‐organized criticality are of crucial importance for the understanding of the basic general mechanisms which govern landscape evolution. This paper considerably extends both the theoretical framework and the empirical evidence for a recently developed theory which incorporates the above general principles. The modeling of landscape evolution by principles of self‐organization is accomplished through the introduction of diffusion processes operating mainly on the hillslopes and the coupling of these processes with the fluvial evolution of the network previously studied through principles of self‐organized criticality. The effects of spatial variability of surface erodibility are investigated under the general framework of random space functions with a correlation structure. Finally, a fractal analysis of the characteristics of the resulting landscape is performed and compared with recent results from real landforms to suggest the relationship of landscape fractal dimensions with the underlying landscape‐forming processes.
Ship and satellite observations of chlorophyll stocks in interacting cyclone‐anticyclone eddy pairs in the western Gulf of Mexico
Authors: Biggs Douglas C. , Müller‐Karger Frank E.
Published: February 2004

When anticyclonic eddies shed by the Loop Current of the Gulf of Mexico reach the western margin of the gulf, they influence the surface circulation over the continental slope and rise. Of particular interest is the generation of cyclone (cold‐core)‐anticyclone (warm‐core) pairs when aging Loop Current eddies interact with the continental margin. In this paper we describe the physical and biological characteristics of these cyclone‐anticyclone pairs. Our objective was to determine how eddy pairs affect the distribution of phytoplankton in the region and how satellite ocean color measurements are applicable to tracing of the eddies. We present shipboard data collected between 1980 and 1982 on the hydrography, chlorophyll stocks, and nutrient concentrations of eddy pairs in the western Gulf of Mexico and compare these data with coastal zone color scanner (CZCS) images collected during the time frame of the cruises. Surface pigment concentrations followed a seasonal cycle, with low concentrations (0.05–0.1 mg m−3) found within cyclones and anticyclones from April through early November and higher concentrations (>0.1 mg m−3) found in the winter. CZCS pigment concentrations were locally high in the flow confluence of cyclone‐anticyclone pairs. The CZCS imagery shows that some cyclone‐anticyclone geometries transport high‐chlorophyll shelf water seaward at least 100–200 km off‐shelf.
Mass and heat budgets in the East Australian current: A direct approach
Authors: Ridgway K. R. , Godfrey J. S.
Published: February 2004

We present a technique which uses the large set of expendable bathythermograph data that are available in the waters off Eastern Australia to correct sampling errors in the mass transport function calculated from the much more limited number of deep hydrology stations. This technique utilizes the close correlation between temperature at a single depth and subsurface steric height, found in these waters, to perform the correction. After corrections are applied, it is found that a mass transport function for the top 2000 dbar can be used to estimate the net geostrophic outflows from the Tasman and Coral seas; it balances Ekman inflows to within −0.4±2.0 Sv (1 Sv = 1.0×106 m3 s−1). The mass transport closure allows a much improved determination of the distribution of individual transport components into the region to be obtained, including a robust estimate of the net southward transport through the whole width of the Tasman Sea (including the East Australian Current (EAC)) of 9.4 Sv, through a section at 28°S between the coast and 171°E. A more direct determination of this Tasman Sea transport, which uses annual mean alongshore currents on the shelf to infer mass transport on the slope, provides an independent confirmation. Mass budgets between density surfaces are also examined; for the deeper layers we find a closure of the mass budget within 10% of the net inflow (a maximum misclosure of 0.9 Sv), suggesting little diapycnal mixing. The geostrophic inflow upwells as it flows south in the EAC at a rate of about 3 Sv at a 250 m depth. The net transport balance along the path also allows the first determination of the heat transport associated with the mean mass transport into the Tasman and Coral seas to be obtained (we have not considered any contributions from eddy heat transport). There is a small net inflow of heat transport to the region of 0.13×1015 W, with more heat entering the Tasman Sea than the Coral Sea. The net upward heat flux of 24 W m−2 (heat loss to the atmosphere) is consistent with values given in climatologies obtained from direct surface flux calculations. The results from a Sverdrup‐Munk model agree broadly with the the observed pattern of the mass transport function, particularly the location of the boundary currents, although the model transports are generally higher.