bluuebunny/crossref_metadata_2025 huggingface dataset:
The heterogeneous Iceland plume: Nd‐Sr‐O isotopes and trace element constraints
Authors: Hemond Christophe , Arndt Nicholas T. , Lichtenstein Uwe , Hofmann Albrecht W. , Oskarsson Niels , Steinthorsson Sigurdur
Published: February 2008

We present a comprehensive set of Sr, Nd, and O isotope data and trace element concentrations from tholeiitic and alkaline lavas of the neovolcanic zones of Iceland (picrites, olivine and quartz tholeiites, transitional and alkali basalts, differentiated rocks). Variations in the oxygen isotope results allow us to distinguish two groups. The first, which comprises quartz tholeiites and more differentiated rocks usually associated with central volcanoes, has low δ18O values (+5 to +1‰) resulting from interaction with the hydrothermally altered Icelandic crust. The second group, which contains picrites, olivine tholeiites, and alkali basalts, has normal mantle oxygen isotopic compositions (δ18O = +5 to +6‰) which are thought to represent those of the mantle source. Nd isotopic compositions vary greatly, from 143Nd/144Nd = 0.51314 in picrites to 0.51295 in alkali basalts. To produce such a variation for rocks with the chemical compositions of Icelandic volcanics (147Sm/144Nd = 0.12=0.28) requires >200 m.y., a period that greatly exceeds the maximum age of Icelandic crust. Previous models, in which the Sr isotopic variations were explained in terms of evolution of crustal reservoirs, are invalidated, and mantle reservoirs with different Nd and Sr isotopic compositions are indicated. The Iceland data define a linear array in the Sr‐Nd isotope diagram which overlaps both mid‐ocean ridge basalt and oceanic island basalt fields and indicates mixing between depleted and enriched end‐members. Alkali basalts come preferentially from an isotopically and chemically enriched component of the Iceland plume, and picrites come from a more refractory, more depleted portion. Positive Sr, Rb, and Ba anomalies are present in picrites and other lavas with low trace element contents. These anomalies are not correlated with isotopic differences but are nevertheless believed to result from interaction between the parent magmas of these rocks and altered Icelandic crust. This indicates that even the most primitive Icelandic lavas have been contaminated with some crustal material.
Mechanical stability of the triangle zone: The backthrust wedge
Authors: Jamison William R.
Published: February 2008

The triangle zone arrangement of faults that commonly develops at the foreland margin of fold‐and‐thrust belts is one of the few places in this structural setting that backthrusts of significant dimensions are observed. Even in this setting, the backthrust may be a relatively ephemeral feature, as it can be cannibalized by the foreland advance of thrusting in an active fold‐and‐thrust system. One of the factors that determines the longevity of a specific backthrust and triangle zone in an active system is the mechanical stability of the backthrust wedge (the wedge of rock that forms the hangingwall to the backthrust). Upper and lower stability limits for this backthrust wedge can be determined using the critical wedge concept for a cohesive Coulomb material. The configuration of the backthrust wedge developed in triangle zones along the southern and central Alberta portion of the Canadian Cordillera agrees well with the stability limits derived for reasonable rock properties and normal fluid pressures. In contrast, a low‐taper triangle zone interpretation for northeastern British Columbia does not fall within the calculated stability limits, even when abnormally high fluid pressures and extreme rock properties are assumed. However, a similar low‐taper geometry may be achieved by the shingling of stable backthrust wedges. Upper crustal delamination and passive‐roof duplexing are both processes that serve to sustain a mechanically stable backthrust wedge configuration, suggesting that they are both viable paths for the progressive advance of the mountain front triangle zone.
Tectonic controls on rift basin morphology: Evolution of the northern Malawi (Nyasa) Rift
Authors: Ebinger C. J. , Deino A. L. , Tesha A. L. , Becker T. , Ring U.
Published: February 2008

Radiometrie (K‐Ar and 40Ar/39Ar) age determinations of volcanic and volcaniclastic rocks, combined with structural, gravity, and seismic reflection data, are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern Malawi (Nyasa) rift and to characterize changes in basin and flank morphologies with time. Faulting and volcanism within the Tukuyu‐Karonga basin began at approximately 8.6 Ma, when sediments were deposited in a broad, initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief. Extension, primarily by slip along the border fault, and subsequent regional isostatic compensation led to the development of a 5‐km‐deep basin bounded by broad uplifted flanks. Along the low‐relief basin margin opposite the border fault, younger stratigraphic sequences commonly onlap older wedge‐shaped sequences, although their internal geometry is often progradational. Intrabasinal faulting, flank uplift, and basaltic and felsic volcanism from centers at the northern end of the basin became more important at ∼2.5 Ma when cross‐rift transfer faults developed to link the Tukuyu‐Karonga basin to the Rukwa basin. Local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin's depocenter. Sequence boundaries are commonly erosional along this low‐relief (hanging wall) margin and conformable in the deep lake basin. The geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake, irrespective of paleoclimatic fluctuations.
The crystallization of lava lakes
Authors: Worster M. Grae , Huppert Herbert E. , Sparks R. Stephen J.
Published: February 2008

Thermal convection driven by the kinetic undercooling of solidification can produce compositional and textural variations in an initially uniform lava flow that is cooled predominantly from above. We show that such kinetic undercooling is sufficient to drive convection at high Rayleigh numbers even in shallow lava lakes. We then employ a theoretical model that includes the effects of convection at high Rayleigh number to make predictions of the complete evolution of the lava. We show that predictions of the rate of growth of solid crust at the surface of the lake do not differ much from predictions made by ignoring effects due to convection. However, convective motions do shorten the time for complete solidification because, when they are driven by kinetic undercooling, there is internal crystallization in addition to that occurring near the cooled upper boundary of the lava. This important effect, which is absent from purely conductive models, predicts stratification and zonation of the lava, in agreement with field observations. We also examine the effect of taking into account the heat transfer to the country rock below the lava lake, and determine how the evolution of the lava lake varies with the viscosity of the lava.
Causes and consequences of variations in faulting style at the Mid‐Atlantic Ridge
Authors: Shaw Peter R. , Lin Jian
Published: February 2008

Both volcanism and faulting contribute to the rugged topography that is created at the Mid‐Atlantic Ridge (MAR) and preserved off‐axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault‐generated topography is essential to understanding the variations in these processes and how these variations are affected by the three‐dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault‐generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small‐throw faults occur toward segment centers, while shorter, larger‐throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active faults (e.g., 10° reached 20 km from the active fault). Fault spacing is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabric in these regions.
Sr‐Nd‐Pb isotope systematics of the Banda Arc, Indonesia: Combined subduction and assimilation of continental material
Authors: Vroon P. Z. , van Bergen M. J. , White W. M. , Varekamp J. C.
Published: February 2008

We present Sr, Nd, and Pb isotope results and SiO2, Rb, Sr, Sm, Nd, U, Th, and Pb data for six active volcanoes and one extinct volcanic island distributed over the whole length of the Banda Arc. Rock types range from low‐K tholeiitic in the NE to high‐K calc‐alkaline in the SW. The volcanoes in the NE have “normal” arc signatures (87Sr/86Sr = 0.7045–0.7055, 143Nd/144Nd = 0.51273–0.51291, and 206Pb/204Pb = 18.66–18.75), whereas those in the SW have extreme values (87Sr/86Sr = 0.7065–0.7083, 143Nd/144Nd = 0.51252–0.51267, and 206Pb/204Pb = 19.28–19.43). Serua, situated in the central part, is the most anomalous volcano with regard to its Sr and Nd isotopic composition (87Sr/86Sr = 0.7075–0.7095 and 143Nd/144Nd = 0.51240–0.51260) but not with regard to Pb isotopes (206Pb/204Pb = 19.02–19.08). The inactive island of Romang in the SW overlaps the Serua trends. The volcanoes display variable within‐suite ranges in 87Sr/86Sr and 143Nd/144Nd. Large ranges (e.g., at Nila) are consistent with assimilation (10–20%) of carbonate‐bearing sediments from the arc crust. Despite the evidence for assimilation, it cannot explain all of the Sr‐Nd isotopic trends found, and Banda Arc magmas must have already obtained a “continental” signature at depth before they reached the arc crust. Within‐suite trends of Pb isotopes are virtually absent. We found an extreme range in the volcanics along the arc which coincide with a similar trend in sediments in front of the arc and consider this as strong evidence for the contribution of subducted continent‐derived material to magma sources. Bulk addition of 0.1–2% of local sediment in the NE Banda Arc, and of 1–3% in the SW Banda Arc, to an Indian Ocean mid‐ocean ridge basalt (I‐MORB) source can explain the isotopic trends; both Serua and Romang require > 5% sediment. The Pb isotopes (e.g., 207Pb/204Pb ‐ 208Pb/204Pb) also suggest changes in the mantle end‐member from I‐MORB to oceanic island basalt (OIB) source type. The latter becomes more conspicuous toward the SW and has the high 208Pb/ 204Pb characteristic of Indian Ocean (Dupal) OIBs. We hypothesize that mixing of magmas in the mantle wedge and/or in the arc crust was an important mechanism by which mantle and subducted end‐members were incorporated in the final products.
Dilatational anelasticity in partial melts: Viscosity, attenuation, and velocity dispersion
Authors: Green Douglas H. , Cooper Reid F.
Published: February 2008

Thermodynamic constraints on the geometry of melt/solid interfaces require that a texturally equilibrated partial melt respond to changes in mean normal stress through adjustment of the sizes of melt‐bearing triple‐grain junctions. This dilatational response is well‐modeled as that of a standard anelastic solid provided that the response rate is limited by solid‐state deformation processes attending the dilatation rather than by melt flow itself. Using this model, we have analyzed flexural creep data obtained for a fine‐grained partial melt comprising a MgSiO3 polycrystalline solid phase in equilibrium with a sodium aluminosilicate melt. The anelastic dilatational (or “bulk”) viscosity for this two‐phase material is nearly an order of magnitude less than its shear viscosity, the latter being determined by diffusional creep. The corresponding modulus that controls the extent of dilatation is several orders of magnitude smaller than the elastic bulk modulus of the two‐phase aggregate. Applied to the case of harmonic loading, the model predicts a substantial band‐limited dissipation spectrum for dilatation (Qk−1) tnat would be absent but for the presence of the melt. This creates a large and strong P wave absorption (Qp−1) band for the enstatite material, accompanied by substantial P wave velocity dispersion. Through this enhanced P wave attenuation, the presence of the melt phase suppresses P‐to‐S velocity ratios and produces equivalent Q values for the two modes.
Crustal structure and deep reflector properties: Wide angle shear and compressional wave studies of the midcrustal surrency bright spot beneath southeastern Georgia
Authors: Pratt Thomas L. , Mondary Joel F. , Brown Larry D. , Christensen Nikolas I. , Danbom Stephen H.
Published: February 2008

Three‐component (P, SH, and SV) expanding spread profiles (ESP), common‐midpoint profiles, and sparse three‐dimensional P wave data were collected over an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), in southeastern Georgia. Shear wave reflections from the SBS at 10.9 s (16 km depth), and possibly from the lower crust at 18.3 s (29 km depth), were recorded but required substantial source effort (stacking) and were too weak for reliable reflectivity measurements. Reflections on the ESPs delineate a 1.5‐km‐thick Atlantic Coastal Plain section whose seismic properties (Vp=2.53 km/s, Vs=1.51 km/s, Vp/Vs= 1.67) are consistent with quartz‐rich sandstones and siltstones, sitting atop a 15‐km‐thick upper crust (Vp=6.38 km/s, Vs=3.25 km/s, Vp/Vs=1.96), which in turn overlies a 15 km‐thick lower crust of slower material (Vp=6.02 km/s, Vs≈3.26 km/s, Vp/Vs≈1.84). The velocity inversion may result from underthrusting of upper crustal rocks during suturing of Florida to North America. Amplitude‐versus‐offset analyses, combined with an earlier reflection polarity test and waveform modeling, indicate that the SBS originates from a thin (∼80–120 m), high‐impedance layer, most likely a mafic dike or tectonically emplaced ultramafic body.
Dynamics of diffusive bubble growth in magmas: Isothermal case
Authors: Prousevitch A. A. , Sahagian D. L. , Anderson A. T.
Published: February 2008

We have conducted a parametric study and developed a new cell model describing diffusion‐induced growth of closely spaced bubbles in magmatic systems. The model accounts for (1) the effects of advection of melt resulting from bubble growth, and its affect on the local concentration profile; (2) dynamic resistance of the viscous melt during diffusive growth; (3) diffusion of volatiles in response to evolving concentration gradients; (4) mass balance between dissolved volatiles and gas inside the bubble; (5) changes in the equilibrium saturation concentration at the bubble‐melt interface; (6) total pressure within the bubble consisting of ambient, surface tension, and dynamic pressures. The results of this study reveal that bubble growth depends strongly on ambient pressure, volatile oversaturation in the melt, and diffusivity coefficients, but only weakly on bubble separation and initial bubble radius. Increased volatile oversaturation increases growth rate to the point at which it actually reduces time for complete bubble growth. This counterintuitive result is due to significant advective volatile flux toward the bubble interface during growth. Viscosity controls growth dynamics only for cases of high viscosity (>104 Pa s). The documentation of the evolution of gas fraction in the melt and bubble wall thickness as a function of time makes it possible to estimate bubble disruption thresholds which bear on volcanic eruption mechanisms. Model results can be applied to the larger‐scale problem of magmatic degassing in terms of bubble coalescence, flotation and the development of foams in magma chambers and vent systems, and ultimately to the dynamics of eruption mechanisms.
Zircon U‐Pb geochronology of the Zambales and Angat Ophiolites, Luzon, Philippines: Evidence for an Eocene arc‐back arc pair
Authors: Encarnación John P. , Mukasa Samuel B. , Obille Eligio C.
Published: February 2008

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thick and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U‐Pb ages at 44.2 (±0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U‐Pb ages of 45.1 (±0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Aksitero Formation. No age difference is discerned between the arc‐like Acoje block and MORB‐like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (±0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike‐pillow lava‐sediment sequence south‐southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Madre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophiolite implies that there are two ophiolitic basements exposed in the Southern Sierra Madre. The relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suggests that the Zambales‐Angat ophiolite represents a preserved Eocene back‐arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales‐Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.
Continuous monitoring of high‐temperature fumaroles on an active lava dome, Volcán Colima, Mexico: Evidence of mass flow variation in response to atmospheric forcing
Authors: Connor Charles B. , Clement Bradford M. , Song XiaoDan , Lane Sammantha B. , West‐Thomas Jennifer
Published: February 2008

Rapid, multichannel monitoring of fumaroles on Volcán Colima, Mexico, provides new insight into the time‐scales and magnitudes of fumarole temperature variation. Temperatures in five fumaroles, all located along a single fracture cutting the summit lava dome of the volcano, were monitored at 20‐min intervals between May 1991 and May 1992. Measurements were made using a programmable data logger deployed near the fumarole field, and data were radiotelemetered to a nearby volcano observatory at regular intervals. Mean fumarole temperatures varied between 350°C and 550°C. Statistical analysis of these time series shows that significant diurnal variation occurs in each fumarole. Magnitudes of these daily fluctuations are generally between 25°C and 50°C, although larger‐amplitude variations occur, especially in cooler fumaroles. Simultaneous monitoring of atmospheric pressure at the fumaroles indicates that these variations in temperature are inversely correlated with barometric pressure. These observations indicate that fumarole temperatures respond to atmospheric forcing. A numerical model developed to explore the dependence of fumarole temperature on mass flow demonstrates that many aspects of observed temperature variation are accounted for by mass flow variation, resulting from small changes in barometric pressure. The relationship between mass flow and fumarole temperature is nonlinear: the response of fumarole temperatures to a given change in mass flow is greatest in fumaroles with low mass flow (and cool temperatures). The nature of this dependence is little affected by fumarole geometry for the cases considered. Continuous measurement of fumarole temperatures may be an effective means of monitoring local mass flow on volcanoes. At Volcán Colima, average temperatures changed by less than 100°C during the 1‐year sampling period. During and immediately following effusive activity, changes in degassing were abrupt and inconsistent along the length of the fracture. Following this period, temperatures decreased gradually, and there was a higher degree of correlation between fumaroles. The method described here represents a substantial improvement over traditional fumarole‐monitoring techniques because subtle variation can be quickly identified using standard statistical techniques, and the method provides regular information about thermal activity on a volcano, minimizing the hazards normally associated with the collection of these data on a regular basis.
A geodynamic model of mantle density heterogeneity
Authors: Ricard Yanick , Richards Mark , Lithgow‐Bertelloni Carolina , Le Stunff Yves
Published: February 2008

Using Cenozoic and Mesozoic plate motion reconstructions, we derive a model of present‐day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle. The thermal buoyancy of these slabs is estimated from the observed thermal subsidence (cooling) of oceanic lithosphere. Slab velocities in the upper mantle are computed from the local convergence rate. We assume that slabs cross the upper/lower mantle interface and continue sinking into the lower mantle with a reduced velocity. For a velocity reduction factor between 2 and 5, our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other. We have also computed a synthetic geoid from our density model. For a viscosity increase of about a factor of 40 from the upper to lower mantle, our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95% and explains 84% of the observed geoid assuming that the model C21 and S21 terms are absent due to a long relaxation time for Earth's rotational bulge. Otherwise, 73% of the geoid variance is explained. The viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle, since downwelling velocities are expected to scale roughly as the logarithm of viscosity (loge 40 = 3.7). These results show that the history of plate tectonics can explain the main features of the present‐day structure of the mantle. The dynamic topography induced by this heterogeneity structure consists mainly of about 1‐km amplitude lows concentrated along the active continental margins of the Pacific basin. Our model can also be used to predict the time variation of mantle heterogeneity and the gravity field. We find that the “age” of the geoid, defined as the time in the past before which the geoid becomes uncorrelated with the present geoid, is about 50 m.y. Our model for the history of the degree 2 geoid, which is equivalent to the history of the inertia tensor, should give us a tool to study the variations in Earth's rotation pole indicated in paleomagiietic studies.
Melting and subsolidus relations of SiO2 at 9–14 GPa
Authors: Zhang Jianzhong , Liebermann Robert C. , Gasparik Tibor , Herzberg Claude T. , Fei Yingwei
Published: February 2008

Experiments were performed on amorphous silica at temperatures of 1000°–2850°C and pressures of 9–14 GPa using a 2000‐t uniaxial split‐sphere multianvil apparatus (USSA‐2000). The melting curve of coesite was extended from 8 to 13.7 GPa, and the melting temperatures indicate a slope that is almost independent of pressure. Melting of stishovite was observed at 2850°C and 14 GPa. The triple point for coexisting coesite, stishovite, and melt was located at 2800°C and 13.7 GPa. The coesite‐stishovite phase transition was determined between 2000° and 2800°C and is described by the equation P (GPa) = 5.8(4) + 0.0028(2)T(°C). The resulting dP/dT slope is more than twice that determined in the in situ X ray study of Yagi and Akimoto (1976). The present coesite‐stishovite boundary cannot be predicted by existing thermodynamic data (e.g., Fei et al., 1990), which suggests some uncertainties in the extrapolation of heat capacities of coesite and stishovite at high temperatures. Thermodynamic extrapolations of the metastable melting curves of quartz and coesite show a strongly negative slope and thus are consistent with the proposal that the room temperature amorphization of quartz and coesite results from crossing of the metastable extension of their melting curves.
Hydrological signatures of earthquake strain
Authors: Muir‐Wood Robert , King Geoffrey C. P.
Published: February 2008

The character of the hydrological changes that follow major earthquakes has been investigated and found to be dependent on the style of faulting. The most significant response is found to accompany major normal fault earthquakes. Increases in spring and river discharges peak a few days after the earthquake, and typically, excess flow is sustained for a period of 6–12 months. In contrast, hydrological changes accompanying pure reverse fault earthquakes are either undetected or indicate lowering of well levels and spring flows. Strike‐slip and oblique‐slip fault movements are associated with a mixture of responses but appear to release no more than 10% of the water volume of the same sized normal fault event. For two major normal fault earthquakes in the western United States (those of Hebgen Lake on August 17, 1959, and Borah Peak on October 28, 1983), there is sufficient river flow information to allow the magnitude and extent of the postseismic discharge to be quantified. The discharge has been converted to a rainfall equivalent, which is found to exceed 100 mm close to the fault and to remain above 10 mm at distances greater than 50 km. The total volume of water released in these earthquakes was around 0.3 km3 (Borah Peak) and 0.5 km3 (Hebgen Lake) Qualitative information on other major normal fault earthquakes, in both the western United States and Italy, indicates that the size, duration, and range of their hydrological signatures have been similar. The magnitude and distribution of the water discharge for these events are compared with deformation models calibrated using seismic and geodetic information. The quantity of water released over a time period of 6–12 months suggests that crustal volume strain to a depth of at least 5 km is involved. The rise and decay times of the discharge are shown to be critically dependent on crack widths, and it is concluded that the dominant cracks have a high aspect ratio and cannot be much wider than 0.03 mm. Using the estimated depth to which water is mobilized, the modeled crack size, and the measured volumes of water expelled, it is concluded that even at distances of 50 km from the earthquake epicenters, cracks must be separated by no more than 10 or 20 m. In regions of highest discharge nearer the earthquake epicenters, separations of 1 or 2 m are required. These results suggest that water‐filled cracks are ubiquitous throughout the brittle continental crust and that these cracks open and close throughout the earthquake cycle. The existence of tectonically induced fluid flows on the scale that we demonstrate has major implications for our understanding of the mechanical and chemical behavior of crustal rocks.
Three‐dimensional mantle convection beneath a segmented spreading center: Implications for along‐axis variations in crustal thickness and gravity
Authors: Sparks David W. , Parmentier E. M. , Morgan Jason Phipps
Published: February 2008

Segmentation and along‐axis variations within individual segments indicate the inherently three‐dimensional nature of mantle up welling and melting beneath oceanic spreading centers. Numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center. The experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher‐viscosity mantle half‐space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets. Buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitudes of the buoyant and plate‐driven components of mantle flow are controlled by the spreading rate and mantle viscosity, with buoyant flow more important at lower spreading rates and viscosities. Buoyant flow beneath the spreading axis amplifies along‐axis variations in upwelling near a ridge‐transform intersection, and distributes the variations along the entire spreading axis. Buoyant flow may thus be responsible for the more three‐dimensional character of slow spreading centers. Away from the spreading axis, thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along‐axis wavelength controlled by the prescribed thickness of the asthenosphere. However, the position and stability of rolls are influenced by the segmentation geometry. In cases where the spreading center geometry does not allow a stable configuration of rolls, the flow is time‐dependent. Along‐axis variations in upwelling cause variations in melt production, which imply large variations in crustal thickness that dominate the surface gravity signal. The crustal thickness distributions implied by these numerical experiments produce bulls‐eye‐shaped negative mantle Bouguer anomalies centered over spreading segments, as observed at several spreading centers. The amplitude of the anomaly increases with decreasing spreading rate.
Space geodetic measurement of crustal deformation in central and southern California, 1984–1992
Authors: Feigl Kurt L. , Agnew Duncan C. , Bock Yehuda , Dong Danan , Donnellan Andrea , Hager Bradford H. , Herring Thomas A. , Jackson David D. , Jordan Thomas H. , King Robert W. , Larsen Shawn , Larson Kristine M. , Murray Mark H. , Shen Zhengkang , Webb Frank H.
Published: February 2008

We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north‐south, and a semimajor axis of approximately 3 mm/yr oriented east‐west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (“repeatability”), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 ± 1, 5 ± 1, 5 ± 1, and 2 ± 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the “San Andreas discrepancy” can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1–2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.
Variations in magma supply rate at Kilauea Volcano, Hawaii
Authors: Dvorak John J. , Dzurisin Daniel
Published: February 2008

When an eruption of Kilauea lasts more than 4 months, so that a well‐defined conduit has time to develop, magma moves freely through the volcano from a deep source to the eruptive site at a constant rate of 0.09 km3/yr. At other times, the magma supply rate to Kilauea, estimated from geodetic measurements of surface displacements, may be different. For example, after a large withdrawal of magma from the summit reservoir, such as during a rift zone eruption, the magma supply rate is high initially but then lessens and exponentially decays as the reservoir refills. Different episodes of refilling may have different average rates of magma supply. During four year‐long episodes in the 1960s, the annual rate of refilling varied from 0.02 to 0.18 km3/yr, bracketing the sustained eruptive rate of 0.09 km3/yr. For decade‐long or longer periods, our estimate of magma supply rate is based on long‐term changes in eruptive rate. We use eruptive rate because after a few dozen eruptions the volume of magma that passes through the summit reservoir is much larger than the net change of volume of magma stored within Kilauea. The low eruptive rate of 0.009 km3/yr between 1840 and 1950, compared to an average eruptive rate of 0.05 km3/yr since 1950, suggests that the magma supply rate was lower between 1840 and 1950 than it has been since 1950. An obvious difference in activity before and since 1950 was the frequency of rift zone eruptions: eight rift zone eruptions occurred between 1840 and 1950, but more than 20 rift zone eruptions have occurred since 1950. The frequency of rift zone eruptions influences magma supply rate by suddenly lowering pressure of the summit magma reservoir, which feeds magma to rift zone eruptions. A temporary drop of reservoir pressure means a larger‐than‐normal pressure difference between the reservoir and a deeper source, so magma is forced to move upward into Kilauea at a faster rate.
Geodetic measurement of deformation in the Ventura Basin Region, southern California
Authors: Donnellan Andrea , Hager Bradford H. , King Robert W. , Herring Thomas A.
Published: February 2008

We have measured the deformation in the Ventura basin region, southern California, with Global Positioning System (GPS) measurements carried out over 4.6 years between 1987 and 1992. The deformation within our network is spatially variable on scales of tens of kilometers, with strain rates reaching 0.6 ± 1 μrad/yr in the east‐central basin. Blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower (0.06 ± 1 μrad/yr to the south). We also observed clockwise rotations of 1°–7°/m.y. Shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and GPS measurements give consistent, though less precise, results. The geodetic rates of shortening across the basin and Western Transverse Ranges are lower than those estimated from geological observations, but the patterns of deformation from the two methods agree qualitatively.
Effects of vertical distribution of water vapor and temperature on total column water vapor retrieval error
Authors: Sun Jielun
Published: February 2008

The physically based total column water vapor retrieval algorithm of Wentz (1992) is tested for sensitivity to realistic vertical distributions of temperature and water vapor. The spatial pattern of systematic retrieval error of total column water vapor due to this sensitivity is simulated using the European Center for Medium Range Weather Forecasts (ECMWF) monthly averaged temperature and humidity fields. Effects of liquid water and ice are not considered. The estimated systematic error is within 0.1 g cm−2 over about 70% of the global ocean area; systematic errors greater than 0.3 g cm−2 are expected to exist only over a few well‐defined regions, about 3% of the global oceans, assuming that the global mean value is unbiased. Over subtropical and middle latitudes the largest biases occur in regions with strong temperature inversions, such as the northwest Pacific during summer. The warm temperature structure is significantly different from the globally averaged temperature structure used implicitly in the retrieval algorithm. The largest biases in the tropics appear to be confined to regions with horizontal wind convergence and large amounts of water vapor in the lower troposphere. The large amounts of water vapor cause saturation of the 22‐GHz channel by water vapor, which increases the sensitivity of the retrieval algorithm to discrepancies introduced by simplifications in radiative transfer equation that forms the basis of the Wentz algorithm.
Thermal energy conservation in icebergs and tracking by temperature
Authors: Løset Sveinung
Published: February 2008

Calculations using a two‐dimensional numerical model which simulates the heat balance and temperature distribution of icebergs show that the temperature in the central region of an iceberg is almost unaffected by the thermal conditions imposed on its boundary. Hence the original temperature of the iceberg at the time of calving is retained in its core owing to the insulating quality of the ice. When correlated with glacioclimatic conditions, this temperature conservation may be used to backtrack an iceberg to its parent ice cliff. Nine different icebergs were manually temperature‐profiled in the Barents Sea during late winter 1988–1990. A scenario is considered wherein one of these icebergs under the forces of winds and currents is assumed to set off from Franz Josef Land in late autumn 1987. The numerical heat balance model is used to compute the temperature distribution, heat balance, and ablation of this iceberg as it drifts through the Barents Sea. The calculations show that the heat balance of the part of the iceberg exposed to air has marginal influence on the ablation and thus on the expected lifetime of the iceberg. Ablation of the submerged body also occurs during winter, when the water temperature is well below the melting point of glacial ice. This is caused by freezing‐point depression leading to an ablation of about 0.5 m per month of the underside and sidewalls of the submerged body. For water temperatures above 0.6°C the ablation results in a steep temperature gradient reaching the core temperature 3 m into the ice. With a tolerance of 0.4°C the temperature is virtually free of any thermal boundary influence 12 m into the ice. The simulated and measured temperature profiles of the iceberg are compared with measurements of the thermal regime of Austfonna, Nordaustlandet, and the Churlenis Ice Dome on Franz Josef Land. The model calculations indicate that the ice cliffs on Svalbard can be excluded as parent ice cliffs for these icebergs and that the core temperature of the bergs appears to be a conservative tracer.
Numerical modeling of an underwater Bingham plastic mudslide and the waves which it generates
Authors: Jiang Lin , LeBlond Paul H.
Published: February 2008

Underwater landslides are a common source of tsunamis in coastal areas. Theoretical and experimental studies on this problem, however, are scarce because of the difficulties in modeling and observing a submarine slide and the associated tsunami. To study this problem, we present a numerical model that simulates the coupling of a Bingham plastic mudslide on a gentle uniform slope with the surface waves which it generates. A formulation of the dynamics of the problem is presented, where the landslide is treated as an incompressible Bingham plastic flow and the water motion is assumed irrotational. The long‐wave approximation is adopted for both water waves and the mudslide. Dispersion of waves and potential turbulent mixing are not considered. The resulting differential equations are solved by a finite difference method. We present the numerical results for successive profiles of the mud surface, the horizontal velocity distributions of the slide, the evolution of the surface elevations, and the distributions of the particle velocity of the water motion. Three major parameters dominate the magnitude of the waves (if the volume of the mud is fixed): the density of mud, the yield stress of the mud, and the depth of water at the mudslide site. Because of the yield stress, the slides stop on the slope when the shear stress exerted on the bottom become smaller than the yield stress. The Bingham plastic behavior of the mud significantly reduces the extent and the speed of the mudslide and also the magnitude of the surface waves generated. Comparison of the solution of the Bingham plastic mudslide model with a snow flow test is presented. The solution of a viscous fluid model has been derived as a special case of the present solution.
Spectral characteristics of time‐dependent orbit errors in altimeter height measurements
Authors: Chelton Dudley B. , Schlax Michael G.
Published: February 2008

A mean reference surface and time‐dependent orbit errors are estimated simultaneously for each exact‐repeat ground track from the first two years of Geosat sea level estimates based on the Goddard Earth model (GEM)‐T2 orbits. Motivated by orbit theory and empirical analysis of Geosat data, the time‐dependent orbit errors are modeled as 1 cycle per revolution (cpr) sinusoids with slowly varying amplitude and phase. The method recovers the known “bow tie effect” introduced by the existence of force model errors within the precision orbit determination (POD) procedure used to generate the GEM‐T2 orbits. The bow tie pattern of 1‐cpr orbit errors is characterized by small amplitudes near the middle and larger amplitudes (up to 160 cm in the 2 years of data considered here) near the ends of each 5‐ to 6‐day orbit arc over which the POD force model is integrated. A detailed examination of these bow tie patterns reveals the existence of daily modulations of the amplitudes of the 1‐cpr sinusoid orbit errors with typical and maximum peak‐to‐peak ranges of about 14 cm and 30 cm, respectively. The method also identifies a daily variation in the mean orbit error with typical and maximum peak‐to‐peak ranges of about 6 cm and 30 cm, respectively, that is unrelated to the predominant 1‐cpr orbit error. It is suggested that the two daily signals arise from daily adjustments of the drag coefficient in the GEM‐T2 POD procedure. Application of the simultaneous solution method to the much less accurate Geosat height estimates based on the Naval Astronautics Group orbits concludes that the accuracy of POD is not important for collinear altimetric studies of time‐dependent mesoscale variability (wavelengths shorter than 1000 km), as long as the time‐dependent orbit errors are dominated by 1‐cpr variability and a long‐arc (several orbital periods) orbit error estimation scheme such as that presented here is used. The accuracy of POD becomes more important for studies of larger‐scale variability.
Modeling of topographic effects on Antarctic Sea ice using multivariate adaptive regression splines
Authors: De Veaux Richard D. , Gordon Arnold L. , Comiso Joey C. , Bacherer Nadine E.
Published: February 2008

The role of seafloor topography in the spatial variations of the southern ocean sea ice cover as observed (every other day) by the Nimbus 7 scanning multichannel microwave radiometer satellite in the years 1980, 1983, and 1984 is studied. Bottom bathymetry can affect sea ice surface characteristics because of the basically barotropic circulation of the ocean south of the Antarctic Circumpolar current. The main statistical tool used to quantify this effect is a local nonparametric regression model of sea ice concentration as a function of the depth and its first two derivatives in both meridional and zonal directions. First, we model the relationship of bathymetry to sea ice concentration in two study areas, one over the Maud Rise and the other over the Ross Sea shelf region. The multiple correlation coefficient is found to average 44% in the Maud Rise study area and 62% in the Ross Sea study area over the years 1980, 1983, and 1984. Second, a strategy of dividing the entire Antarctic region into an overlapping mosaic of small areas, or windows, is considered. Keeping the windows small reduces the correlation of bathymetry with other factors such as wind, sea temperature, and distance to the continent. We find that although the form of the model varies from window to window due to the changing role of other relevant environmental variables, we are left with a spatially consistent ordering of the relative importance of the topographic predictors. For a set of three representative days in the Austral winter of 1980, the analysis shows that an average of 54% of the spatial variation in sea ice concentration over the entire ice cover can be attributed to topographic variables. The results thus support the hypothesis that there is a sea ice to bottom bathymetry link. However, this should not undermine the considerable influence of wind, current, and temperature which affect the ice distribution directly and are partly responsible for the observed bathymetric effects.