bluuebunny/crossref_metadata_2025 huggingface dataset:
Systematics of and postulated controls on submarine hydrothermal solution chemistry
Authors: Von Damm K. L.
Published: February 2008

The chemical compositions of seafloor hydrothermal solutions from the Galapagos Spreading Center, 21°N and 13°N on the East Pacific Rise, the Southern Juan de Fuca Ridge, and the Guaymas Basin, Gulf of California, are examined for systematic variations. Several elemental ratios, elemental products, and empirical geothermometers show much consistency among the various areas, in spite of large absolute differences in elemental concentrations. The results suggest that most of the major cations in these solutions are solubility controlled with respect to a similar phase assemblage at all of the sites. Examination of the solution chemistry in conjunction with experimental results and seafloor rock alteration data permits identification of phases which may be important in controlling the solution chemistry. As potassium appears to be solubility controlled, its use to calculate water/rock ratios is inappropriate. Lithium appears to be a better element to use in these calculations. A model to explain the observed solution chemistries as a result of three‐component mixing is discussed. All of the solutions are postulated to be either the result of mixing between a seawater hydrothermal solution which has not undergone a phase separation and is hence gas containing, a gas‐rich vapor phase, and a gas‐poor brine phase, or any one of these individual components alone. The vapor and brine phases are the result of phase separation, which may have occurred at some time in the past. Essentially all of the reported data fall within the field defined by these three components. The solutions apparently have had sufficient time after the mixing event(s) to reequilibrate with the rock. This is true for quartz as well as other chemical components. The best test of this model is chloride, together with gas and isotropic data, as the model predicts that solutions with elevated chlorinities will be depleted in gases and enriched in 18δO, and low chlorinity solutions will have elevated gas contents and lower 18δO.
Hydrothermal alteration processes at midocean ridges: Experimental and theoretical constraints from Ca and Sr exchange reactions and Sr isotopic ratios
Authors: Berndt Michael E. , Seyfried William E. , Beck J. Warren
Published: February 2008

Diabase and basalt were reacted with Na‐Ca‐K‐Cl fluids of seawater chlorinity at 375–425°C, 375–400 bars, and fluid/rock mass ratios of 0.5–1.0 to assess the role of basalt chemistry and texture on Sr and Ca mobility during high‐temperature hydrothermal alteration. An additional experiment, utilizing an 84Sr spiked fluid, was performed to help quantify reaction rates of processes affecting Sr mobility. The experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges. Alteration of basalt and diabase is characterized by formation of tremolite‐actinolite‐smectite‐chlorite and clinozoisitic epidote‐smectite‐chlorite, respectively. Diabase alteration produced dissolved Sr/Ca ratios similar to those observed for ridge crest hot spring fluids, whereas alteration of a mostly crystalline basalt produced significantly lower ratios. This observation supports the premise that the Sr/Ca ratios observed in vent fluids may be produced during deep‐seated reaction of the hydrothermal fluids with diabase dikes and/or gabbro at relatively low fluid/rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures (400°C) leads to formation of a mineral with low DSr/Ca, Possibly hydrothermal plagioclase. Results of the 84Sr spiked experiment indicated that only 4% of the Sr in basalt was mobilized after 800 hours of reaction despite the fact that B and Li were nearly quantitatively leached. It is thus suggested that B and Li are good indicators of the amount of fresh rock encountered by fluids, while Sr concentration and isotopic data can be used to estimate the degree of alteration that the Sr‐bearing primary minerals have undergone. The degree of alteration of subsurface reservoirs may be estimated by dividing the fluid/rock ratio obtained for highly mobile elements by the fluid/rock ratio obtained from path‐integrated Sr isotope and concentration data. Based on this method, for example, it can be estimated that only 5% of the primary Sr‐bearing minerals encountered by fluids in the subsurface at EPR 21°N are actually converted to secondary phases.
Seismic constraints on the morphology of deep slabs
Authors: Fischer Karen M. , Jordan Thomas H. , Creager Kenneth C.
Published: February 2008

Residual sphere images from deep earthquakes not only detect the presence of slab‐associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle. We have compared travel times from deep events in the Kuril and Mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth. We assume that the details of the deformation (whether the slab buckles, imbricates, fragments, etc.) are averaged out along the ray paths, and hence our models constrain the scale, not the mode, of slab thickening. The deep event travel times are best fit by undeformed models, but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects. Although the Mariana times are consistent with advective thickening factors of 5 or more, factors larger than 3 are ruled out by the Kuril data. For all models examined, the data require that slab material extends to depths of 900–1000 km. Global tomographic models and regional studies which delineate high‐velocity anomalies in the lower mantle beneath zones of Cenozoic subduction are consistent with our results, as is recent work on pulse distortion by slab gradients. Comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur, it is largely aseismic.
Late Neogene and Quaternary tectonics associated with northward growth of the San Andreas Transform Fault, northern California
Authors: Kelsey Harvey M. , Carver Gary A.
Published: February 2008

Strain patterns within the forearc at a convergent margin adjacent to a passing fault‐fault‐trench triple junction show a systematic evolution. We describe late Neogene and Quaternary deformation in northern coastal California in the last 3 m.y. associated with the migration of the Mendocino triple junction. Deformation in the North American forearc near Humbolt Bay consists of a 30‐km‐wide zone of on‐land contraction with a narrower zone of translation to the east. The zone of subaerial contraction is part of the relatively wide, southernmost extent of forearc contraction within the Cascadia subduction zone. Net Quaternary northeast‐southwest contraction across forearc thrust faults is at least 7.9 km, and minimum fault slip rates on the six major thrust faults are 0.8–2.3 mm/yr. Net right slip within the zone of translation is a minimum of 3 km. South of the forearc, translation is predominant in the North American plate and is accommodated along two major right‐lateral fault zones of the San Andreas transform boundary, the Lake Mountain, and Garberville fault zones. Discontinuous exposures of late Neogene sediments along these faults near Covelo and Garberville, respectively, indicate that crustal contraction, expressed by thrust faults and associated folding, predated translational deformation at each site. Inception of contraction near Covelo was related to inception of internal deformation of the Gorda plate at 3 Ma. This contraction, due to coupling of the convergent plates, migrated northward in the forearc in advance of and at the rate of migration of the triple junction. Present‐day deformation near Humboldt Bay thus reflects contraction associated with subduction of the Gorda plate and translation farther east associated with oblique convergence at the plate boundary. As the triple junction migrates, the strike‐slip faults of the San Andreas transform boundary will extend to the north‐northwest into the present forearc, and forearc strike‐slip faults will become faults of the San Andreas system. Contractional structures near Humboldt Bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south.
Observations of high‐frequencyPwave earthquake and explosion spectra compared with ω−3, ω−2, and sharpe source models
Authors: Walter William R. , Brune James N. , Priestley Keith F. , Fletcher Jon
Published: February 2008

Observations of 10‐, 20‐, and 30‐HzPwave spectral amplitudes from earthquakes and explosions are compared with theArchambeau[1968, 1972] earthquake model featuring aPwave falloff of ω−3beyond the corner frequency, a modifiedBrune[1970, 1971] earthquake model with ω−2falloff, and theSharpe[1942] explosion model which has a ω−2falloff. The Archambeau and Sharpe models have been, in part, the basis of a proposal byEvernden et al.[1986] that high‐frequency (≈30 Hz) seismic energy could provide an effective solution to the problem of detection and identification of low‐yield coupled and fully decoupled underground nuclear explosions. The observations of earthquakes show an increase in spectral amplitude with moment approximately in agreement with the ω−2falloff model and, for larger moments, in disagreement with the ω−3model. Comparison of theoretical and actual seismograms narrow‐band filtered at 30 Hz shows that in part the increase in spectral amplitude of earthquakes is due to the complex and long duration of the rupture process and not because of an increase in an impulsive first arrival like that characteristic of an explosion. The 30‐Hz amplitudes for explosions show much scatter, and many events have a spectral falloff greater than the ω−2predicted by the Sharpe model. Whether this is due entirely to attenuation or is the actual source spectrum is not determined. High stress drop earthquakes are predicted to have larger spectral amplitudes than the Sharpe model. Thus any discrimination technique using high‐frequencyPwave spectra should probably take into account differences in pulse shape and amplitude in the time domain.
The competition between percolation and circulation in a deformable porous medium
Authors: Scott David R.
Published: February 2008

The equations governing the flow of a liquid in a viscously deformable porous medium are reexamined. Darcy's Law governs the “segregation,” the relative motion of the solid and liquid phases by percolation; the Stokes equation governs the “circulation,” the flow of both phases without relative motion. Terms describing the interaction between the segregation and circulation modify the familiar forms of these equations. In a previous contribution the approximation of omitting the circulation was adopted, and circular solitary wave solutions were found to be stable in two dimensions. Here we remove that approximation and present numerical experiments and analysis that indicate the continued existence of circular solitary waves. The consequences of these results in understanding the behavior of partially molten regions of the Earth's mantel is discussed. Regions of higher porosity may ascend either as diapirs (solid and liquid together) or as magmons (solitary waves, with segregation of solid and liquid). We argue that the characteristic length scale of magmons will always be superimposed on larger scale variations in porosity.
Ray tracing for earthquake location in laterally heterogeneous media
Authors: Virieux Jean , Farra Veronique , Madariaga Raul
Published: February 2008

We propose a technique for the fast solution of ray tracing in three‐dimensional laterally heterogeneous media. Analytical solutions for both ray tracing and paraxial ray tracing for a medium with an arbitrary gradient of the square of slowness are used to develop a systematic procedure to construct exact solutions. Complex heterogeneous media are divided into tetrahedral finite elements inside which the square of the slowness has a simple linear distribution. Arbitrarily complex media may be studied by judicious choice of the elements. We develop appropriate boundary conditions for interfaces with zeroth or first‐order velocity discontinuities. Two‐point ray tracing is performed by a Newton method based on paraxial ray theory. With a few iterations the ray trajectories through the source and the observer are calculated with the same overall speed as with ray bending. The paraxial method has the additional advantage that it can be used to identify caustics and to separate travel time branches. We used the previously described method to replace the ray tracing algorithm of the HYPO71 program, probably the most commonly used earthquake location program. The ray tracing routine may be easily modified to adapt it to other earthquake location environments. With the modified HYPO71 program, we relocated the aftershock sequence of the November 23, 1980, Irpinia (Italy) earthquake. The introduction of a low‐velocity zone proposed by different authors shifts aftershock locations to the NE. This horizontal displacement of hypocenters is not very sensitive to the thickness of the low‐velocity zone.
Flexural behavior of the continental lithosphere in Italy: Constraints imposed by gravity and deflection data
Authors: Royden Leigh
Published: February 2008

A procedure is developed that systematically matches observations of plate flexure and gravity anomalies at simple convergent boundaries to the bestfitting two‐dimensional flexural geometry obtained by a thin elastic sheet of specified elastic strength. The procedure allows analytical solution for three independent variables, one of which is the initial (preflexure) elevation of the reference surface and two of which are related to the vertical forces and bending moments that act on the plate. Application to four profiles constructed across the Apennine convergent system in Italy shows that the observed depth of the basal Pliocene surface (which marks the base of the foredeep basin) and the observed Bouguer gravity anomalies can be fit by a two‐dimensional elastic plate model with an effective elastic thickness of about 20 km on three profiles and about 10 km on the fourth profile. The initial elevation, prior to flexure, is determined to have been about 1000 m below sealevel on all profiles. Large bending moments and/or vertical shear forces are required to act at the effective plate ends in order to maintain the observed deflection (moments between 0.5×1017 and 3×1017 N, and shear forces between + 3×1012 and −3×1012 N/m). Gravity modelling indicates that, on the two northernmost profiles, these terminal forces can not be supplied by a static mass at crustal depths and suggests that, on all four profiles, the terminal forces are supplied by anomalously dense material present at depths of 75–150 km beneath and west of the west coast of Italy. This dense material probably corresponds to westward‐subducted lithosphere of the Adriatic plate, extending from the northern Apennines to the southern Apennines and Calabria. This study suggests that in subduction systems, like the Apennines, that are characterized by backarc extension, forces acting on subducted lithosphere at depth can be expressed at the surface in the geometry of the foredeep basin that forms adjacent to the subduction zone.
Generation of high pore pressures in accretionary prisms: Inferences from the Barbados Subduction Complex
Authors: Shi Yaolin , Wang Chi‐Yuen
Published: February 2008

Recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values. Indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes. In this study the mechanisms for generating excess pore pressures is investigated by focusing on the Barbados Ridge Complex which is an end‐member of sediment‐rich accretionary prisms. Modeling is carried out using a two‐dimensional finite element procedure, with constraints provided by the following data: Deep Sea Drilling Project results on stratigraphy and lithology, seismic results on sediment structure and thickness, regional geology on tectonic history, and laboratory measurements on mechanical properties of sediments. Coupling among the equations for fluid flow, heat transfer, and sediment deformation is made throughout the computation. The results of the modeling predict that the principal decollement beneath the Barbados Ridge Complex is a zone of high pore pressure, high porosity, and reversal in seismic velocity. These characteristics may be responsible for the mechanical weakness of the decollement, which allows continuous subduction of sediments below it, and for its seismic signatures. Results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms. The direct effect of tectonic compression is of secondary importance, and the effect due to temperature increase with subduction is of little significance. Thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs. Rich sediment supply, low permeability, high subduction angle, and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms.
Intergranular distribution of pore fluid and the nature of high‐angle grain boundaries in limestone and marble
Authors: Hay R. S. , Evans B.
Published: February 2008

Experimental measurements of dihedral angles formed at the junction of calcite/fluid interfaces with high‐angle grain boundaries suggest that the two fluids used in these experiments, water and carbon dioxide, do not wet high‐angle grain boundaries. Four types of carbonate samples were used: Solnhofen limestone, Oak Hall limestone, synthetic calcite bicrystals fabricated in a dry environment at 880°C and a carbon dioxide atmosphere of 0.1 MPa with 5‐MPa uniaxial stress, and synthetic calcite bicrystals fabricated in a wet environment at 600°C with a confining pressure of 120 to 170 MPa and a pore water pressure 15 to 50 MPa lower. The ratio between grain boundary energy and surface energy obtained from measurements of dihedral angle (Θ) along grain boundary grooves was 0.46 ± 0.24 for dry bicrystals grooved at 760° to 880°C in 0.1 MPa carbon dioxide, and 0.75 ± 0.15 for those grooved at 450° to 500°C in 200 MPa H2O. Measurements of Θ from pore/grain boundary junctions in the limestones and wet synthetic bicrystals also suggest the ratio of grain boundary energy to surface energy is about 1. Using this evidence and published values of calcite surface energy, the energy of high‐angle grain boundaries is roughly estimated to be 80 mJ/m2 at 20°C. About 60 boundaries were examined using lattice fringe imaging, a high‐resolution electron microscope technique. In all cases where satisfactory images were obtained, there was no evidence of a second phase along the boundary. These results imply that waterrich pore fluids in natural carbonates will tend to form isolated pores along grain boundaries and three‐and four‐grain junctions; interconnected networks, including fluid‐wetted grain boundaries and three‐grain junctions, are probably unstable. Therefore local mass transport in unjointed carbonates is probably dominated by the properties of semicoherent grain boundaries, and not by pore‐fluid properties.
Middle Proterozoic compressional orogen in northwestern Canada
Authors: Cook Frederick A.
Published: February 2008

Evidence from seismic reflection profiling, drill holes, potential field measurements and regional geology shows that an east verging Proterozoic thin‐skinned thrust and fold belt is situated beneath the Interior Platform of northwestern Canada. Balanced cross sections and associated palinspastic reconstructions provide constraints on the extent of the shortening in the thrust belt and suggest that at least 50–90 km of compression can be accounted for. Although the Proterozoic stratigraphy in the subsurface is not well known, the cross sections are constrained essentially by seismic reflection geometry. Shortening occurred on subhorizontal detachment(s) that lie above the autochthonous (Hudsonian?) basement. Seismic reflection data from the area west of the major thin‐skinned thrust structures show that the Proterozoic strata there can be traced for up to 100 km and are characterized by broad, open folds with wavelengths of 10 km or more. Uniform potential field anomalies and subhorizontal seismic reflections further indicate that there are no major crustal boundaries (such as sutures) in the area between the thrust belt and the Campbell uplift near Inuvik, Northwest Territories, that were associated with the thrusting. The detachment(s) that carried the folded and thrust faulted allochthons must therefore extend for a considerable distance to the northwest of the thrust belt. New deep crustal seismic reflection data from the Campbell uplift, however, show evidence for crustalpenetrating Proterozoic structures that dip northwestward and that were probably reactivated during late Paleozoic compression and Mesozoic extension. Taken together, the various data thus indicate that the area from Great Bear Lake to the Campbell uplift (about 400 km) is underlain by a mid‐Proterozoic decollement above which the compressional deformation took place, and that the deformation extended to the north side of the present Campbell uplift where steeply north dipping structures indicate that the underlying autochthonous basement is characterized by a ramp structure that was probably associated with the southern edge of a Proterozoic basin.
Plastic instabilities: Implications for the origin of intermediate and deep focus earthquakes
Authors: Hobbs Bruce E. , Ord Alison
Published: February 2008

Adiabatic or catastrophic plastic shear has been reported in metals, polymers, and metallic glasses. The phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner. The driving force for the plastic instability is the stored elastic strain energy of the loading system, and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding, although the precise mechanism is different. It is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones, the earthquake events being the result of instabilities in material undergoing plastic flow. It is argued that for a particular strain rate there exists a critical temperature, TC, which is depth dependent; for temperatures below TCthe material is strain rate softening and, for a soft enough loading system, may undergo catastrophic plastic shear. For temperatures above TCthe material is strain rate hardening and is always stable during plastic shear. The cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material, and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km. The precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the TCgradient. In particular, a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 MPa above 300 km to a maximum of ≈ 50 MPa below 300 km. The differences in foci distribution between subduction zones such as Tonga, New Hebrides, and Peru result from minor differences in the geothermal gradients within the slabs. The model predicts the development of triple seismic zones high in the slab, double seismic zones down to approximately 300 km, and single seismic zones down to ≈ 800 km. Such a distribution is to be expected of relatively young, cool slabs; as the slab heats up, the seismic activity retreats up the slab. The paper only proposes a deformation mechanism for earthquake generation, it does not address the stress field within the slab but only the distribution of strength. Thus the distribution of focal plane mechanisms is not considered, only the locations where earthquakes due to plastic instabilities are possible. The absence of earthquakes does not necessarily mean that the slab does not exist, it only means that the slab is too hot to undergo plastic instability. This means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab (i.e. > circa 3°C km−1).
Secular variation and the origin of geomagnetic field reversals
Authors: Merrill R. T. , McFadden P. L.
Published: February 2008

Although there has been growing interest in suggestions that geomagnetic reversals may be initiated by external sources, we show that the present evidence favors a mechanism of internal origin for at least the vast majority of reversals. Several lines of evidence indicate that the dipole and quadrupole dynamo families are decoupled most of the time but are coupled across polarity transitions. This suggests a reversal model in which reversals are initiated when the quadrupole family couples critically with the dipole family.
Plate‐driving forces over the Cenozoic Era
Authors: Jurdy Donna M. , Stefanick Michael
Published: February 2008

Plate reconstructions are used to estimate plate torques for the Cenozoic Era, assuming a dynamical balance between active torques (slab‐pull and ridge‐push) and plate drag, as the passive torque. In a published one‐dimensional model for the motion of an oceanic plate the plate drag balances slab‐pull and ridge‐push; expressing each of these forces in terms of age and sub‐duction rate and using values at 15 trenches, coefficients are found which give a least squares fit. Here we have retained a common age factor, which gives a greater weight to older, more rapidly moving plates, and the contribution due to ridge‐push is larger than that found in the previous study, but still much smaller than the slab‐pull force. This one‐dimensional force model is generalized to two‐dimensional plates moving over the Earth's surface, and we obtain a torque balance equation in which slab‐pull and ridge‐push torques are proportional to boundary chord vectors, with the weights depending on powers of subduction velocity at the middle of the chords. For each plate we find the unique angular velocity which satisfies the torque balance requirement, and this balance torque is compared with the plate drag torque obtained from the plate's geometry and (reconstructed) angular velocity. Comparison of the torques is made for two published models for present‐day plate motions, as well as six published reconstructions for time intervals spanning the Cenozoic. For the present the mismatch between the balanced torque and drag torque is about the same as the difference between the two alternative published models. Torques are fairly stable throughout the Cenozoic, with the misfit increasing systematically for earlier reconstructions.
Viscous relaxation of impact crater relief on Venus: Constraints on crustal thickness and thermal gradient
Authors: Grimm Robert E. , Solomon Sean C.
Published: February 2008

Because of the high surface temperature on Venus and the strong temperature dependence of strain rate in silicates, solid‐state creep may be an important mechanism for the reduction of topographic relief on that planet. Venera 15/16 radar altimetric profiles of impact craters 30–140 km in diameter, however, indicate crater depths of several hundred meters, suggesting that substantial relaxation has not occurred for these features. On the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters. From the mean and variance of observed crater depth as a function of crater diameter on Venus and from estimates of the initial depths of fresh impact craters on Venus obtained by scaling from lunar observations, we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level. We develop a general formulation for gravity‐driven flow in a linearly viscous fluid, incorporating the densities and temperature‐dependent effective viscosities of distinct crust and mantle layers. The statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness H and thermal gradient dT/dz. Since dT/dz. can be estimated from global heat loss arguments or thermal models, an upper bound to H may be derived. The preserved relief of the largest craters constrain H to be less than or equal to 10–20 km. Because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation, geological unit, and degree of preservation of morphological detail, extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable. Such an extrapolation, together with isostatic considerations, yields an upper limit to the crustal volume of Venus of 1010 km3. This value is an order of magnitude less than the time‐integrated volume of crust produced on Earth and implies either that the average rate of crustal generation has been much smaller on Venus than on Earth or that some form of crustal recycling has occurred on Venus.
Apparent tectonic rotations, declination anomaly equations, and declination anomaly charts
Authors: Chan Lung S.
Published: February 2008

The conventional method for tilt correction of paleomagnetic data assumes that rock bodies rotate about horizontal axes during deformations. Apparent tectonic rotations, which may produce anomalous declination readings of paleomagnetic directions and other linear elements, can result from the use of such a procedure if the rock bodies deform about inclined axes. The error (declination anomaly) resulting from two apparent tectonic rotations, (1) single rotations about an oblique axis and (2) continuous rotations about a precessing fold axis, can be determined algebraically for given orientations of rotation axis and angles of rotation. The declination anomaly does not depend on the orientation of the paleomagnetic vector or the trend of the rotation axis; rather, it is a function of the plunge of the rotation axis and the angle of rotation. A declination anomaly chart (DAC) is constructed for each of the two processes to determine graphically whether anomalous paleomagnetic data obtained from a study area can result from the process. In the DACs the plunge of the bedding‐pole vector and the declination anomaly resulting from the apparent tectonic rotations are plotted against the plunge of the rotation axis and the angle of rotation. The declination anomaly for an individual site is taken as the deviation of the measured remanent magnetic direction at the site from the reference direction for coeval rocks. Each data point is plotted at the intersection of the declination anomaly curve and the beddingpole curve in the DACs. If the dispersion in remanent magnetic directions over a study area results from one of the apparent tectonic rotations, the data points will lie along a straight line in the corresponding DAC. Potential applications of the DACs include testing the nature of anomalous paleomagnetic directions, determining the deformation style of folds, and restoring structural orientations of tilted blocks.
THE SALTON SEA SCIENTIFIC DRILLING PROJECT
Authors: Elders Wilfred A. , Sass John H.
Published: February 2008

In March 1986 a research borehole, called the “State 2–14,” reached a depth of 3.22 km in the Salton Sea geothermal system of southern California. This was part of the Salton Sea Scientific Drilling Project (SSSDP), the first major (i.e., multimillion dollar) research drilling project in the U.S. Continental Scientific Drilling Program. The principal goals of the project were to investigate the physical and chemical processes of a high‐temperature, high‐salinity, magmatically driven hydrothermal system. The borehole encountered temperatures of up to 355°C and produced metal‐rich, alkali chloride brines containing 25 wt% of total dissolved solids. The rocks penetrated exhibit metamorphism and ore genesis in action. They show a progressive transition from unconsolidated lacustrine and deltaic sediments to hornfelses, with lower amphibolite facies mineralogy, accompanied by pervasive veins containing iron, copper, lead, and zinc ore minerals. The SSSDP included an intensive program of rock and fluid sampling, flow testing, and downhole logging and scientific measurement. The purpose of this paper is to introduce this special section of the Journal of Geophysical Research on the SSSDP, to describe briefly the background of the project and the drilling and testing of the borehole, to summarize the initial scientific results, and to discuss how the lessons learned are applicable to future scientific drilling projects.
Thermal Regime of the State 2–14 Well, Salton Sea Scientific Drilling Project
Authors: Sass J. H. , Priest S. S. , Duda L. E. , Carson C. C. , Hendricks J. D. , Robison L. C.
Published: February 2008

Temperature logs were made repeatedly during breaks in drilling and both during and after flow tests in the Salton Sea Scientific Drilling Project well (State 2–14). The purpose of these logs was to assist in identifying zones of fluid loss or grain and to characterize reservoir temperatures. At the conclusion of the active phase of the project, a series of logs was begun in an attempt to establish the equilibrium temperature profile. Initially, we were able to log to depths below 3 km, but beginning in late May of 1986, it was impossible to log below about 1.8 km owing to casing failure. Our best estimates of formation temperature below 1.8 km are 305° ± 5°C at 1890 m and 355° ± 10°C at 3170 m. For the upper 1.8 km the latest temperature log (October 24, 1986), using a digital “slickline” (heat‐shielded downhole recording) device, was within a few degrees Celsius of equilibrium, as confirmed by a more recent log (July 31, 1987) to a depth of ∼ 1 km. As in most other wells in the Salton Sea geothermal field, there is an impermeable, thermally conductive “cap” on the hydrothermal system; this cap extends to a depth of more than 900 m at the State 2–14 well. Thermal conductivities of 19 samples of drill cuttings from this interval were measured at room temperature. The conductivity values were corrected for in situ porosity as determined from geophysical logs and for the effects of elevated temperature. Thermal gradients decrease from about 250 mK m−1 (same as degrees Celsius per kilometer) in the upper few hundred meters to just below 200 mK m−1 near the base of the conductive cap. Using one interpretation, thermal conductivities increase with depth (mainly because of decreasing porosity), resulting in component heat flows that agree reasonably well with the mean of about 450 m W m−2. This value agrees well with heat flow data from shallow wells within the Salton Sea geothermal field. A second interpretation, in which measured temperature coefficients of quartz‐ and carbonate‐rich rocks are used to correct thermal conductivity, results in lower mean conductivities that are roughly constant with depth and, consequently, systematically decreasing heat flux averaging about 350 mW m−2 below 300 m. This interpretation is consistent with the inference (from fluid inclusion studies) that the rocks in this part of the field were once several tens of degrees Celsius hotter than they are now. The age of this possible disturbance is estimated at a few thousand years.
Formation and evolution of strike‐slip faults, rifts, and basins during the India‐Asia Collision: An experimental approach
Authors: Peltzer Gilles , Tapponnier Paul
Published: February 2008

The processes which have governed the formation and evolution of large Tertiary strike‐slip faults during the penetration of India into eastern Asia are investigated by means of plane strain indentation experiments on layered plasticine models. The steady state deformation of plasticine obeys a power creep flow law
. The stress exponent (n) is between 6 and 9 at 25°C. Uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening, a process observed for strain rates ranging from 3.5×10−5 to 3.6×10−3 s−1. Fault or shear zones form after only 7–10 % bulk strain. Subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine. A series of nine plane strain indentation experiments shows the influence of boundary conditions, as well as that of the internal structure of the plasticine model on the faulting sequence. The ubiquity of strain softening in experimental deformation of a variety of rocks, as well as the widespread occurrence of shear zones in nature suggest that long‐term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain. The deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision‐induced strikeslip faulting in Asia, particularly to total offsets and rates of movements on the faults. The experiments simulate the evolution of the western ends of the strike‐slip faults, which have probably been analogous to trench‐fault‐fault triple junctions. The experiments also illustrate mechanisms for the formation of extensional basins, such as the South China Sea, North China Basin, and Andaman Sea, near active continental margins. The basins, which appear to absorb terminal offsets along major strike‐slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts. The existence of distinct phases of strike‐slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous, non‐steady state deformation of the continental lithosphere.
Morphology and structure of Loihi Seamount based on Seabeam Sonar Mapping
Authors: Fornari Daniel J. , Garcia Michael O. , Tyce Robert C. , Gallo David G.
Published: February 2008

Loihi seamount has been mapped using Seabeam multibeam sonar, and the resulting data have been processed to produce bathymetric maps with a 10‐m contour interval and shaded‐relief perspective images. Analysis of morphological and structural data recorded in the maps and images indicates that constructional volcanism on Loihi has been localized predominantly along its southern and northern rift zones and that these rifts have been active since early in the seamount's history. The distinct asymmetry between Loihi's eastern and western slopes (the eastern slopes are steeper) is attributed to mass wasting of large sections of the volcano's west flank. Two large amphitheater valleys on the seamount's west flank are believed to have resulted from repeated mass‐wasting events triggered by earthquakes associated with an early rift zone whose core now lies buried beneath the block feature present on Loihi's southwest flank. The prominent kink in the present south rift zone of Loihi may represent a fundamental readjustment of along‐rift magma conduits that was caused by gravitational slumping on the southwest flank of the volcano. Lava cones and constructional volcanic ridges are present along the rims of summit pit craters and form a continuous chain of volcanic constructs around the periphery of the summit platform. The concentric arrangement of young eruptive vents pn Loihi's summit may be caused by ring dikes. The morphology and structural interrelationships of pit craters on Loihi's summit suggest that the southeastern pit is the youngest. The formation of a summit platform on Loihi may have been caused by repeated collapse of pit craters and the southward migration of primary magma conduits that underlie the pits. This model predicts a north‐to‐south age progression for volcanism on Loihi's summit.
Experiments and observations bearing on the solubility and diffusivity of carbon in olivine
Authors: Tingle Tracy N. , Green Harry W. , Finnerty Anthony A.
Published: February 2008

The solubility and diffusivity of C in olivine at 0.1, 2.0, and 3.0 GPa have been constrained by annealing single crystals of San Carlos olivine in 14C‐labelled CO2, CO2‐H2O, and graphite at 1200°–1450°C. In nearly all annealed specimens, carbonaceous material was observed at the diffusion interface that consisted of either (1) graphite used as a C source, (2) elemental C (probably graphite) formed by reduction of the C‐O‐H fluid phase present, or (3) carbon‐rich surface films interpreted as silicate dissolved in the fluid phase at high pressure, quenched onto the crystal surface. The significant range of 14C‐beta particles in olivine (and photographic emulsion) coupled with the presence of carbonaceous material at the diffusion interface gives rise to apparent concentration profiles. After removal of the carbonaceous material we retained no unambiguous evidence for C diffusion into olivine. The sensitivity and spatial resolution of the beta track method constrain either the solubility to be less than 30 ppmw (parts per million by weight) C or the diffusivity to be less than 10−12 cm2/s. Observations and geochemical data on natural rocks bearing on the nature and distribution of C in the Earth's upper mantle are discussed in light of these constraints.
A detection of non‐Newtonian gravity
Authors: Romaides Anestis J. , Jekeli Christopher , Lazarewicz Andrew R. , Eckhardt Donald H. , Sands Roger W.
Published: February 2008

An experiment was performed to test Newton's inverse square law of gravitation. The test compared accurately measured gravity values along a 600‐m tower with upward continued gravity estimates calculated from ground measurements using algorithms based on Newtonian potential theory. Gravity measurements with an internal consistency of better than 20 μGal (1 μGal = 10−8m/s2) were made on the tower at intervals of about 90 m using a single LaCoste‐Romberg model G gravimeter. Field measurements of gravity (using the same instrument) and corresponding geodetic positions were performed at 77 points within a radius of 5 km from the tower. These and other gravimetric data within a radius of about 220 km were used to perform the upward continuation using two independent computational techniques. One is least squares collocation, which is based on a statistical interpretation of the local gravity field, and the other determines a local harmonic expansion of the field from the given data. The upward continuation from both methods gave almost identical results and is estimated to have an uncertainty comparable to the in situ measurements. The results of the experiment show a substantial departure from the inverse square law, asymptotically approaching about 500 μGal at the top of the tower, with a standard deviation of better than 40 μGal. This indicates that there is a non‐Newtonian attractive force which falls off rapidly with elevation.
Paleomagnetism of the Oligocene Kalamazoo Tuff: Implications for middle Tertiary extension in east central Nevada
Authors: Hagstrum Jonathan T. , Gans Phillip B.
Published: February 2008

The Oligocene Kalamazoo Tuff (≃35 Ma) was sampled for paleomagnetic analysis across a 100‐km‐wide zone of highly extended crust in east central Nevada to estimate between‐site vertical axis rotations and thus the relative importance of strike‐slip faulting to the mechanism of extension. Subordinate sampling was also undertaken in a younger hornblende dacite intrusion (≃35 Ma) and the overlying tuff of North Creek (≃35 Ma). In general, ash flow tuffs cool and are magnetized within a short period of time relative to geomagnetic secular variation and their eutaxitic structures typically indicate the paleohorizontal. These attributes of ash flow tuffs, in addition to their generally widespread occurrence, make them excellent candidates for combined paleomagnetic and structural studies of regional deformation. However, in this study a number of problems associated with the paleomagnetism of the Kalamazoo Tuff are documented; they include topography‐related variations of the eutaxitic structures, secular variation during slow cooling, postblocking rheomorphic flow, and local remagnetization of the tuff by younger igneous activity. The tilt‐corrected data, with these sources of error reduced or eliminated, exhibit a 28° ± 12° clockwise rotation of the Schell Creek Range relative to the Kern Mountains region. This rotation implies differential extension accommodated by strike‐slip faulting or N‐S shortening. The paleomagnetic results also suggest that large changes in strike of layered units near faults with presumed strike‐slip movement need not be the result of oroclinal bending, but could result from superimposed sets of orthogonal normal faults.
Late Jurassic‐Early Cretaceous evolution of the eastern Indian Ocean adjacent to northwest Australia
Authors: Fullerton Lawrence G. , Sager William W. , Handschumacher David W.
Published: February 2008

Over 9700 km of new aeromagnetic data were acquired off the northwest coast of Australia and combined with existing magnetic data to map magnetic isochrons in the eastern Indian Ocean. The isochrons were used to constrain a tectonic model of the evolution of the seafloor in the Argo, Cuvier, and Gascoyne abyssal plains. A complete set of anomalies, from M26 through M16, was found in the Argo Abyssal Plain, trending generally N70°E. Spreading commenced in the center of the basin at or prior to M26 and propagated outward until at least M24 time. Anomalies M10–MO, recording the separation of Australia and India, were found in the Cuvier and Gascoyne abyssal plains, with a trend of about N30°E. A significant crustal age discontinuity occurs in the vicinity of the Joey Rise where the two lineation sets converge. Because there appears to be no overlap of isochron ages in the two groups, it is not necessary to postulate that a triple junction existed off northwest Australia as has been previously suggested. At M4–M5 time a 10° clockwise change in spreading direction occurred on the Cuvier‐Gascoyne spreading system. This event triggered ridge jumps that transferred two pieces of the Indian plate to the Australian plate. Overlapping spreading on the forming and dying ridges, curved fracture zones and lineations, as well as fanned lineation trends, suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1–2 m.y.