bluuebunny/crossref_metadata_2025 huggingface dataset:
Micromechanics of pressure‐induced grain crushing in porous rocks
Authors: Zhang Jiaxiang , Wong Teng‐Fong , Davis Daniel M.
Published: February 2008

The hydrostatic compaction behavior of a suite of porous sandstones was investigated at confining pressures up to 600 MPa and constant pore pressures ranging up to 50 MPa. These five sandstones (Boise, Kayenta, St. Peter, Berea, and Weber) were selected because of their wide range of porosity (5–35%) and grain size (60–460 μm). We tested the law of effective stress for the porosity change as a function of pressure. Except for Weber sandstone (which has the lowest porosity and smallest grain size), the hydrostat of each sandstone shows an inflection point corresponding to a critical effective pressure beyond which an accelerated, irrecoverable compaction occurs. Our microstructural observations show that brittle grain crushing initiates at this critical pressure. We also observed distributed cleavage cracking in calcite and intensive kinking in mica. The critical pressures for grain crushing in our sandstones range from 75 to 380 MPa. In general, a sandstone with higher porosity and larger grain size has a critical pressure which is lower than that of a sandstone with lower porosity and smaller grain size. We formulate a Hertzian fracture model to analyze the micromechanics of grain crushing. Assuming that the solid grains have preexisting microcracks with dimensions which scale with grain size, we derive an expression for the critical pressure which depends on the porosity, grain size, and fracture toughness of the solid matrix. The theoretical prediction is in reasonable agreement with our experimental data as well as other data from soil and rock mechanics studies for which the critical pressures range over 3 orders of magnitude.
Paleomagnetism of Middle Miocene volcanic rocks in the Mojave‐Sonora Desert region of western Arizona and southeastern California
Authors: Calderone Gary J. , Butler Robert F. , Acton Gary D.
Published: February 2008

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave‐Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low‐angle normal faults but predate high‐angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time‐distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave‐Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post‐middle Miocene high‐angle normal faulting in this region; (2) there has been no detectable post‐middle Miocene latitudinal transport of the region; and (3) long‐term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations.
Heat flow through the sea bottom around the Yucatan Peninsula
Authors: Khutorskoy M. D. , Fernandez R. , Kononov V. I. , Polyak B. G. , Matveev V. G. , Rot A. A.
Published: February 2008

Heat flow studies were conducted in January–February 1987, off the Atlantic Coast of Mexico on board the R/V Akademik Nikolai Strakhov. Two areas were surveyed, one transecting the Salt Dome Province and the Campeche Canyon, in the Gulf of Mexico, and the other, on the eastern flank of the Yucatan Peninsula. Conductive heat flow through the bottom sediments was determined as the product of vertical temperature gradient and in situ thermal conductivity, measured with a thermal probe using a multithermistor array and real‐time processing capabilities. Forward two‐dimensional modeling allows us to estimate heat flow variations at both sites from local disturbances and to obtain average heat flow values of 51 mW/m2 for the transect within the Gulf of Mexico and 38 and 69 mW/m2 for two basins within the Yucatan area. Sea bottom relief has a predominant effect over other environmental factors in the scatter of heat flow determination in the Gulf of Mexico.
Mariner 9 observations of the south polar cap of Mars: Evidence for residual CO2 frost
Authors: Paige D. A. , Herkenhoff K. E. , Murray B. C.
Published: February 2008

The first spacecraft observations of the south residual polar cap of Mars were obtained by the Mariner 9 orbiter during the Martian southern summer season, 1971–1972. Analyses of Viking orbiter observations obtained 3 Mars years later have shown that residual carbon dioxide frost was present at the south polar cap in 1977. In this study, Mariner 9 infrared interferometer spectrometer spectra and television camera images are used in conjunction with multispectral thermal emission models to constrain the temperatures of dark bare ground and bright frost regions within the south residual cap. The results provide strong evidence that carbon dioxide frost was present throughout the summer season despite the fact that the residual frost deposits observed by Mariner 9 were less extensive than those observed by Viking.
Seasonal recession of Mars' South polar cap in 1986
Authors: James Philip B. , Martin Leonard J. , Henson Jean R. , Birch Peter V.
Published: February 2008

Photographs of Mars obtained during the 1986 opposition of the planet have been used to derive the regression curve for the south polar cap between Ls = 190° and Ls = 255°. The 1986 regression appears to have been unexceptional until after Ls = 230°, when it becomes retarded relative to the normal established by the 1971 and 1977 regressions. A study of the data as a function of the filter used suggests that circumpolar clouds were present in early spring, unlike the case in the 1977 recession.
Order and diversity in the modes of Circum‐Pacific Earthquake recurrence
Authors: Thatcher Wayne
Published: February 2008

Recurrence characteristics of great circum‐Pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture. As noted by others, the pattern of moment release is typically very irregular, with strong concentrations in a few isolated regions of a much larger aftershock zone. Despite this complexity, rupture nucleation is notably systematic, with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release. This generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip. Well‐documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well‐ordered, while source dimensions, slip, and cumulative moment release vary considerably from cycle to cycle. On none of the segments studied do earthquakes consistently recur in nearly identical events. Instead, individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events. Despite these differences the duration of the seismic cycle is approximately uniform. Furthermore, when strain release occurs in a sequence of large earthquakes, these events take place towards the end of the cycle and occur with increasing rupture length and magnitude. These ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults. The ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength. The irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event. Although these mechanistic associations are indirect and tentative, the features of recurrent behavior documented here have implications for long‐term earthquake hazard assessment that are not dependent on the models proposed to explain them. Chief among these are the cycle‐to‐cycle differences among gap‐filling events and the absence of shocks that fill major slip‐deficient regions early in the seismic cycle.
Lithospheric dynamics and tectonic‐stratigraphic evolution of the Ebro Basin
Authors: Zoetemeijer R. , Desegaulx P. , Cloetingh S. , Roure F. , Moretti I.
Published: February 2008

We present analysis of two‐dimensional modeling of the tectonic subsidence in the Ebro Basin along three profiles crossing the Pyrenees, the Ebro Basin, and the Catalan Coastal Range to the east, and the Iberic Cordillera to the west. The deformation of the foreland basin during Tertiary times is simulated using a lithospheric flexure model, incorporating laterally varying elastic thicknesses. The deflection of the plate is the combined result of Pyrenean loading and the loads exerted by the Iberic Cordillera and Catalan Coastal Range. In the eastern part of the Ebro Basin no extra driving force on the Iberian plate is required. The flexural modeling supports the hypothesis that underplating of the Iberian plate under the European plate is not associated with subduction of old oceanic lithosphere. A good fit with the Tertiary basin subsidence data and Bouguer anomalies is obtained for an elastic plate with local weakness zones under the Ebro Basin. The inferred lateral variation in the elastic rigidity is consistent with heat flow measurements in northern Spain and with major Mesozoic extension in the Iberic Cordillera and Valencia Trough.
Diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates: A rock magnetic study
Authors: Jackson Mike
Published: February 2008

Hysteresis measurements, Lowrie‐Fuller tests, Cisowski tests, and low‐temperature demagnetization experiments on samples of the Knox Dolomite, Trenton Limestone, and Onondaga Limestone indicate that the remanence in these remagnetized carbonate units resides principally in single‐domain magnetite, in contrast to previous interpretations. Volumetrically, however, a significant fraction of the magnetite is coarser grained, as previous scanning electron microscopic work on magnetic extracts has shown. These conclusions are based mainly on the characteristic “wasp‐waisted” shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity, both of which clearly indicate a bimodal distribution of coercivities/grain sizes. The magnetization is controlled mainly by cubic magnetocrystalline anisotropy, consistent with the spheroidal particles commonly observed in extracts. Hysteresis trends show that especially the finest particles lack shape anisotropy. In view of these conclusions, the two components of natural remanence in these rocks are interpreted as (1) a late Paleozoic (Kiaman) chemical remanence carried by single‐domain diagenetic magnetite and (2) a Cenozoic thermoviscous overprint residing principally in coarser, pseudosingle domain and multidomain magnetite, also of late Paleozoic diagenetic origin.
Geomagnetic polarity reversals, transition field structure, and convection in the outer core
Authors: Olson Peter , Lee Hagee V.
Published: February 2008

Convection in the outer core is dominated by rotation effects. Theory indicates that convection in a rapidly rotating sphere of conducting fluid produces transversely isotropic magnetohydrodynamic induction, with an axis of symmetry parallel to the rotation vector. We have examined the long‐term behavior of azimuthally averaged core magnetic fields maintained by transversely isotropic induction, using numerical calculations of spherical mean‐field dynamos. The inductive effect of core convection is represented parametrically by a nonlinear, anisotropic α effect with magnitude Rα concentrated near the inner core boundary to simulate convection driven by inner core crystallization. Large values of the magnetic Reynolds number Rα, representing vigorous convection, result in nonreversing and relatively strong dipolar fields. Small but supercritical values of Rα produce relatively weak and oscillatory dipolar fields. Intermediate values of Rα result in a modified square‐wave behavior for the dipole field, consisting of constant polarity epochs with nearly uniform intensity lasting several hundred thousand years, separated by polarity transitions. Duration of polarity epochs is extremely sensitive to Rα. The change from a frequent reversal regime to an infrequent reversal regime (superchron) requires only a small increase in Rα. Transitions in transversely isotropic α2 dynamos are caused by growth of reversed flux zones at high latitudes in each hemisphere, which merge to form a reversed dipolar field. Addition of a large‐scale toroidal shear flow causes reversals to be initiated by growth of reversed flux zones at low latitudes. Virtual geomagnetic pole paths from successive reversals are alternatingly near‐ and far‐sided. Transition fields are predominantly octupolar. The model predicts a positive correlation between polarity epoch duration and dipole field intensity.
Seismicity and tectonics of a continent‐island arc collision zone at the Island of Taiwan
Authors: Hsu Vindell
Published: February 2008

A collision between the Eurasian plate continental margin and a volcanic island arc on the Philippine Sea plate about 4 m.y. ago uplifted the island of Taiwan. Many lines of geological and geophysical evidence suggest that the collision is still active today and is currently propagating southward. Due to plate interaction, geological and tectonic settings have become more complicated since the onset of the Collision. The collision suture is along the Longitudinal Valley (LV) in eastern Taiwan. Seismic studies including earthquake monitoring and seismic refraction lines conducted in 1985 in eastern Taiwan and its offshore area using land seismic stations and ocean bottom seismographs have generated results that better constrain the present‐day plate interaction. Both the crustal velocity structure and earthquake hypocenter distribution suggest that the subduction of the Philippine Sea plate along the Ryukyu trench extends to the east coast of Taiwan near about 23.5°N. Very high seismicity with dominant right‐lateral strike‐slip faulting is found in the back arc region behind the Ryukyu trench. This seismic activity is interpreted as a result of plate convergence and back arc wrenching due to the relative motion between the “Okinawa Platelet” and the Eurasian plate. The region close to the LV in eastern Taiwan showed only shallow and a relatively low level of seismicity during this experiment with higher seismicity to the east of the LV. The lack of intermediate‐depth events suggests that subduction is no longer active along the collision zone and the difference in seismicity between the regions east and west of the LV reflects the difference in strength of the adjacent continental and island arc crusts. To the south of the LV, seismicity continued southward with intermediate‐depth events distributed mainly to the east of Taiwan along a nearly vertical Benioff zone. This supports the argument that convergence and collision are still active south of Taiwan.
Equivalent strike‐slip earthquake cycles in half‐space and lithosphere‐asthenosphere earth models
Authors: Savage J. C.
Published: February 2008

By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike‐slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half‐space (asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half‐space. For the case in which each earthquake ruptures the entire lithosphere (thickness H), the half‐space equivalent slip rate is as follows: Depth interval 0‐H, slip identical to that in lithosphere‐asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2n−1)H to (2n+1)H (n = 1,2,…), slip rate uniform in space and dependent upon time as Fn(t) exp (−t/τa) where Fn is a (n ‐ 1) degree polynomial in t, τa is twice the asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τa, slip rates below 5H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere‐asthenosphere model can be calculated very simply from the half‐space model with time‐dependent slip in the two depth intervals H−3H and 3H−5H, and uniform slip at a rate equal to the secular relative plate velocity below depth 5H. Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half‐space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere‐asthenosphere model would have a 30‐km thick lithosphere and an asthenosphere relaxation time greater than 33 years, but other models are possible.
Spectral properties of land surfaces in the thermal infrared: 1. Laboratory measurements of absolute spectral emissivity signatures
Authors: Nerry F. , Labed J. , Stoll M. P.
Published: February 2008

A laboratory procedure to obtain absolute spectral emissivity signatures of soils in the thermal infrared is developed and discussed. Particular attention is given to correction for environmental radiation and to the estimation of the final accuracy attainable. The study covers the [3.0–5.5] μm and the [8.0–13.5] μm bands with a spectral resolution of 2.5% of λ. The method does not require any a priori assumption on the physical parameters, nor does it require any direct temperature measurement. Absolute emissivity signature ελ can be measured within ±0.6%. Bidirectional reflectivity signatures, ρbλ(0, 0), are obtained as well, against a reflectivity standard: a flame‐sprayed aluminum diffusor. It is spectrally flat and has been calibrated. SiC powder, SiO2 sand, a number of soil samples, montmorillonite, and kaolinite have been measured. Good correlation is found between ελ and ρbλ(0, 0) as to the spectral information involved. An attempt to calculate the form factor Fλ(0) from the emissivity and reflectivity signatures is made. Fλ(0) is related to the bidirectional reflectivity. The accuracy, however, is poor. No spectral information can be inferred from the data. Agricultural soils have very similar signatures in the [8.0–13.5] μm band, dominated by the SiO2 content. Above 10 μm, ελ is rather flat and of the order of 0.97±0.01. Emissivity of clays is very flat and high above 8 μm. In the [3.0–5.5] μm band, emissivity ratios can be large. Characteristic spectral features can be resolved (CaCO3). Thus simultaneous use of both thermal infrared atmospheric windows would certainly improve the interpretation of remotely sensed data.
Magnetization of the oceanic crust beneath the Labrador Sea
Authors: Arkani‐Hamed Jafar
Published: February 2008

The aeromagnetic anomalies of the oceanic crust northeast of Newfoundland, Canada, are strongly skewed, with an anomalous skewness of 33–52°, with respect to the anomalies calculated using the standard sea‐floor spreading model. This paper examines the possibility that the anomalous skewness is on account of an oceanic crust with a thermo‐viscous remanent magnetization, where unlike the standard model the lower crust is magnetic. The thermo‐viscous remanent magnetization of the lower crust beneath the Labrador Sea has dominant contributions to broad‐scale marine magnetic anomalies and to the negative Magsat anomaly of the Sea.
Introduction to Special Section on the Role of Fluids in Sediment Accretion, Deformation, Diagenesis, and Metamorphism in Subduction Zones
Authors: Langseth Marcus G. , Moore J. Casey
Published: February 2008

Accretionary prisms, which develop from materials scraped from the subducting oceanic lithosphere, form within one of the most dynamic structural environments on Earth and constitute critical elements of mountain belts. In the marine setting these large accumulations of sediments grow in a water‐saturated environment; consequently, their tectonic, chemical, and thermal evolution is strongly influenced both by the seawater that is incorporated with the sediment as it is accreted and by water that is rendered from the clays and rocks of the prism as they are subjected to increasing pressure and temperatures (Figure 1). Although accretionary prisms are mechanically similar to subaerial fold‐and‐thrust belts, the latter may develop in consolidated, partially saturated sediments, with less pronounced hydrogeologic effects.
Chemistry, source, and behavior of fluids involved in alpine thrusting of the Central Pyrenees
Authors: Grant N. T. , Banks D. A. , McCaig A. M. , Yardley B. W. D.
Published: February 2008

Quartz veins occupying extensional fractures that formed relatively late in the Alpine thrusting history of the Pyrenees contain abundant inclusions of hypersaline brines. Small amounts of bitumen also occur. A range of brine compositions is apparent from the freezing behavior of fluid inclusions from different samples, with large variations in Na/Ca ratio, but many individual samples contain only one brine composition. All brine types display a bimodal distribution of homogenization temperature, which is interpreted as the result of fluctuation in fluid pressure from a maximum value equal to lithostatic at 5–7 km depth and around 270°C, to a value up to 700 bars lower. This is ascribed to dilatancy pumping. Chemical analyses of brines leached from two samples yields balanced analyses for 14 elements. Both analyses appear to represent sedimentary formation waters and are closely analogous to oil field brines. A basement origin is precluded by the presence of bitumens and the low K/Ca ratios. Differences between the analyses probably reflect different source bed mineralogy, including the presence or absence of dolomite, but Triassic red beds are likely to be a major source. The occurrence of these brines in the fault planes provides constraints on both the sources and movement of the fluids involved in thrusting. Significant infiltration of either surface waters or metamorphic water of dehydration can be ruled out because of the high salinities (typically, 20–25 wt % dissolved salts). Hence although permeability was locally high due to fracturing around the faults, fluids were pumped into these fractures from sedimentary formations adjacent to the fault. Individual fluid pathways were short‐lived relative to the duration of the deformation event, so that different fluids were pumped into the faults at different times in response to local fault propagation.
Chemistry of waters sampled from oceanic basement boreholes, 1979–1988
Authors: Mottl Michael J. , Gieskes Joris M.
Published: February 2008

Waters have been sampled from the basement section of four Deep Sea Drilling Project holes (395A, 396B, 418A, and 504B) since 1979, in an attempt to determine the composition of oceanic basement formation waters. Samples have been obtained by two methods: active, in which water is drawn under negative pressure from the interstices of the wall rocks, and passive, in which it is collected from the open borehole. The most successful sampling has been done on re‐entering the holes months to years after they were last disturbed by drilling and/or pumping of surface seawater downhole. Most of the samples show large changes in composition from seawater that are consistent with alteration of basalts at the in situ temperatures of 10° to 146°C. The samples are typically depleted in Mg2+, K+, Na+, sulfate, and 87Sr, and enriched in Ca2+ and Si. However, all of the samples are contaminated to some extent, mainly by seawater which is either introduced during drilling or sampling, flows downhole due to underpressure in the upper basement, or mixes downhole due to convective overturn in the hole itself. The latter process is apparently common and dilutes any formation water which may enter the hole via diffusion and horizontal convection. In hole 504B, the tritium content of the waters indicates that 24 to 35% of the surface seawater which originally filled the hole is still present after periods of 470 to 1233 days. The more general problem of what has displaced the surface seawater and produced the large chemical changes observed in the borehole waters, whether exchange with formation water or simply reaction of seawater with the wall rocks of the hole, remains unsolved. As a result, no component of true basement formation water has been identified unambiguously in any of the holes. This problem may be solved by (1) compartmentalizing a hole with a series of plugs to limit downhole mixing or (2) using a straddle packer to draw a series of samples from the interstices of the wall rocks of a hole, either shortly after drilling or later following re‐entry. In either case, an array of chemical tracers will have to be measured, either in situ or in the laboratory, to infer the nature of the waters obtained.
Earth's magnetic field in the Seventeenth Century
Authors: Hutcheson Kenneth A. , Gubbins David
Published: February 2008

Catalogs of old magnetic measurements, contemporary publications, and ship's logs have been researched to compile a data set of geomagnetic observations of declination and inclination in the interval 1601 A.D. to 1694 A.D. This search produced 2766 measurements of declination, with quite good global coverage, and 37 of inclination. All inclinations were in the northern hemisphere and most in Europe. A literature search revealed many archeomagnetic and paleomagnetic measurements of field components for the seventeenth century: 40 of these were selected and used in the modeling procedure (intensity measurements were not found sufficiently reliable to allow any conclusions to be made about the overall strength of the geomagnetic field and declinations were too few to be of use). This data set was analyzed to produce a field model for the seventeenth century; the model was denoted epoch 1647 A.D. although no corrections were made for secular variation. The model exhibits many of the stationary features of the recent geomagnetic field and indicates absence of secular variation from the Pacific and a westward drift rate of about 0.25° per year in areas of the core‐mantle boundary beneath the south Atlantic and Indian oceans between 1647 A.D. and 1715 A.D., the date of Bloxham's (1986) earliest field model. This is likely to remain the earliest field model because only three inclination measurements have been found in the sixteenth century; the data set will be used in the future to form a time‐dependent model for the historical period.
New global vector magnetic anomaly maps derived from Magsat data
Authors: Cohen Yves , Achache Jose
Published: February 2008

New global maps of the vector magnetic anomaly field of Earth are derived from Magsat data. Improvement on earlier published vector and scalar maps is achieved with the following procedure: (1) The noise level of vector measurements associated with the discontinuities of the in‐flight sensor attitude determination is reduced by selecting the data. (2) A monthly model of the main field is computed to account for secular variation, using only quiet day data. (3) The time dependence of the ring current effect is taken into account using hourly values of the Dst index based on ground obseivations. (4) Sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity. These indices provide a better discrimination of quiet data than the global Kp index usually used for that purpose. (5) A time‐ and longitude‐dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes. Anomaly maps of X and Z components are obtained using selected measurements within the reduced altitude range: 350–425 km. These maps are compared with earlier Magsat anomaly maps, and their respective resolutions are discussed. These maps evidence large anomalies of lithospheric origin within the oceanic domain. In particular, the Z map displays a characteristic pattern of long‐wavelength anomalies in these areas. It is first shown that some of these anomalies, particularly in the North Atlantic and Indian oceans, may be correlated with the age of the lithosphere. Then, a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced. This suggests that long‐wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography.
The electrical conductivity distribution beneath Vancouver Island: A region of active plate subduction
Authors: Kurtz Ronald D. , DeLaurier Jon M. , Gupta Jagdish C.
Published: February 2008

Magnetotelluric (MT) and geomagnetic depth sounding (GDS) data were recorded at 25 locations across Vancouver Island, Canada, over the subducting Juan de Fuca plate. The GDS data respond to a number of local conductive features at frequencies above 10 Hz while at longer periods, the in‐phase and quadrature induction arrows are colinear and are aligned approximately normal to the continental shelf with a maximum amplitude response near the southwest coast. Interpretation of the MT data show the presence of a conducting zone at depths greater than 20 km beneath Vancouver Island and correlating with a strong seismic reflector. Extensive testing with two dimensional models shows the MT and GDS data are consistent with a proposed geoelectrical structure containing a conducting layer extending from the Pacific Ocean, dipping beneath Vancouver Island, and in electrical contact with the mainland conductive region. The layer has a horizontal conductance of 200 S that may decrease to 100 S beneath the northeast side of Vancouver Island. Related geophysical studies suggest the conductive zone is above the actual thrust surface of the Juan de Fuca plate. The conducting layer is most likely the result of cracks and pores filled with saline fluids which are supplied by water subducted with the oceanic crust and by dehydration reactions. The presence of fluids has significant implications for thrust earthquakes and for metamorphic reactions that occur in subduction zones. The conductance of the upper mantle (between 90 and 390 km) is best modelled at approximately 1500 S.
Heterogeneities in the thickness of the elastic lithosphere of Mars: Constraints on heat flow and internal dynamics
Authors: Solomon Sean C. , Head James W.
Published: February 2008

Derived values of the thickness of the effective elastic lithosphere on Mars are converted to estimates of lithospheric thermal gradients and surface heat flow by finding the thickness of the elastic‐plastic plate having the same bending moment and curvature, subject to assumed strain rates and temperature‐dependent flow laws for crustal and mantle material. Local thermal gradients and heat flow values so estimated were 10–14 K km−1 and 25–35 mW m−2, respectively, at the time of formation of flexurally induced graben surrounding the Tharsis Montes and Alba Patera, while gradients and heat flow values of less than 5–6 K km−1 and 17–24 mW m−2, respectively, characterized the lithosphere beneath the Isidis mascon and Olympus Mons at the time of emplacement of these loads. On the basis of the mean global thickness of the elastic lithosphere inferred to support the Tharsis rise and estimates of mantle heat production obtained from SNC meteorites, it is suggested that the present average global heat flux on Mars is in the range 15–25 mW m−2. Approximately 3–5% of this heat flux during the Amazonian epoch has been contributed by excess conducted heat in the central regions of major volcanic provinces. Most likely, this excess heat flux has been delivered to the base of the lithosphere by mantle plumes. The fractional mantle heat transport contributed by plumes during the last 2 b.y. on Mars is therefore similar to that at present on Earth.
Extracting shear wave polarizations from different source orientations: Synthetic modelling
Authors: Igel Heiner , Crampin Stuart
Published: February 2008

Shear wave splitting, diagnostic of some form of seismic anisotropy, has been identified in the past, principally by examining polarization diagrams and by rotating the axes of recordings to separate the shear wave polarizations. We introduce a new technique for identifying polarizations of shear waves when the data has been recorded with more than one source orientation. The technique, suitable for vertical seismic profiles (VSPs) or reflection data, averages over an entire shear wavetrain, eliminating source related effects while retaining the anisotropy related polarizations. In shear wave reflection experiments and shear wave VSPs the distribution of displacement directions depends on the elastic properties of the medium (anisotropy), the source orientation, and the effects of scattering. We linearly combine seismograms from different source polarizations in order to simulate seismograms for any source polarization. This allows us to show the distribution of displacement directions as a function of source polarization in a high‐relief display. The diagrams show characteristic features in the presence of anisotropy which could help to determine the polarizations of the two split shear waves in difficult circumstances. The techniques are tested with synthetic seismograms.
The magnetic structure of Axial Seamount, Juan de Fuca Ridge
Authors: Tivey Maurice A. , Johnson H. Paul
Published: February 2008

Axial Seamount is a large, volcanically active seamount located in the eastern Pacific on the central Juan de Fuca (JDF) ridge at 46°N, 130′W. Sea surface magnetic anomaly data show that Axial lies completely within crust formed during the Brunhes normal polarity epoch. The edifice, however, does not produce a simple positive magnetic anomaly related to the excess mass above mean seafloor. Several pronounced negative magnetic anomalies are associated with the seamount, including both a circular‐shaped low located west of the summit and an elongate low (trending NW) on the southeast flank of the volcano. These two anomalies form part of a NW‐SE magnetic trough which extends across the seamount edifice oblique to the general JDF ridge trend of N20°E. A third negative anomaly is observed on the northern flank of the seamount associated with North Helium Basin, a deep elongate basin which indents the northeast flank of the seamount. Both inversion of sea surface magnetics data and forward modelling of topography, constrained by magnetization values of dredged samples, show that bathymetric effects can explain the Helium Basin anomaly, but topography alone cannot produce the low magnetization values which extend across the seamount summit. Magnetization of crustal rocks does not vary spatially over the seamount, so that negative anomalies at the summit can be explained by variations in the effective thickness of the magnetic source layer. A simple forward model of uniform magnetization shows that the anomaly source layer would have to thin by 50% (to less than 700 m thick) in order to generate the observed magnetic response. A thinned source layer is consistent with the presence of either a substantial magma chamber at depth beneath the summit of Axial or a zone of substantial crustal alteration. The shape of the anomaly implies the trend of either of these sources must be oriented obliquely to the general ridge trend but parallel to the direction of the absolute plate motion.
A seafloor and sea surface gravity survey of Axial Volcano
Authors: Hildebrand John A. , Stevenson J. Mark , Hammer Philip T. C. , Zumberge Mark A. , Parker Robert L. , Fox Christopher G. , Meis Philip J.
Published: February 2008

Seafloor and sea surface gravity measurements are used to model the internal density structure of Axial Volcano. Seafloor measurements made at 53 sites within and adjacent to the Axial Volcano summit caldera provide constraints on the fine‐scale density structure. Shipboard gravity measurements made along 540 km of track line above Axial Volcano and adjacent portions of the Juan de Fuca ridge provide constraints on the density over a broader region and on the isostatic compensation. The seafloor gravity anomalies give an average density of 2.7 g cm−3 for the uppermost portion of Axial Volcano, The sea surface gravity anomalies yield a local compensation parameter of 23%, significantly less than expected for a volcanic edifice built on zero age lithosphere. Three‐dimensional ideal body models of the seafloor gravity measurements suggest that low‐density material, with a density contrast of at least 0.15 g cm−3, may be located underneath the summit caldera. The data are consistent with low‐density material at shallow depths near the southern portion of the caldera, dipping downward to the north. The correlation of shallow low‐density material and surface expressions of recent volcanic activity (fresh lavas and high‐temperature hydrothermal venting) suggests a zone of highly porous crust. Seminorm minimization modeling of the surface gravity measurements also suggest a low‐density region under the central portion of Axial Volcano. The presence of low‐density material beneath Axial caldera suggests a partially molten magma chamber at depth.
Geochemistry of hydrothermal fluids from Axial Seamount hydrothermal emissions study vent field, Juan de Fuca Ridge: Subseafloor boiling and subsequent fluid‐rock interaction
Authors: Butterfield David A. , Massoth Gary J. , McDuff Russell E. , Lupton John E. , Lilley Marvin D.
Published: February 2008

Hydrothermal fluids collected from the ASHES vent field in 1986, 1987, and 1988 exhibit a very wide range of chemical composition over a small area (∼60 m in diameter). Compositions range from a 300°C, gas‐enriched (285 mmol/kg CO2), low‐chlorinity (∼33% of seawater) fluid to a 328°C, relatively gas‐depleted (50 mmol/kg CO2), high‐chlorinity (∼116% of seawater) fluid. The entire range of measured compositions at ASHES is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust, followed by partial segregation of the vapor and brine phases. Other mechanisms proposed to produce chlorinity variations in hydrothermal fluids (precipitation/dissolution of a chloride‐bearing mineral or crustal hydration) cannot produce the covariation of chlorinity and gas content observed at ASHES. There is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation, in which gases are partitioned according to Henry's law and all salt remains in the liquid phase. Significant enrichments in silica, lithium and boron in the low‐chlorinity fluids over levels predicted by the model are attributed to fluid‐rock interaction in the upflow zone. Depletions in iron and calcium suggest that these elements have been removed by iron‐sulfide and anhydrite precipitation at some time in the history of the low‐chlorinity fluids. The distribution of low‐ and high‐chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability. The relatively shallow depth of the seafloor (1540 m) and the observed chemistry of ASHES fluids are consistent with phase separation in the sub‐critical or near‐critical region.
The Fourth International Conference on Mars
Authors: Jakosky Bruce M.
Published: February 2008

This dedicated issue of the Journal of Geophysical Research contains papers dealing with all aspects of Martian science. The impetus for the issue was the Fourth International Conference on Mars, held in Tucson on January 10–13, 1990. The third conference had been held 7 years earlier (see Journal of Geophysical Research, 87(B12), 1982); in the intervening time period, significant new analysis of the existing data has taken place, and further robotic exploration of Mars has been both planned and carried out. The papers contained in this issue reflect the mature nature of our understanding of the Martian environment, and they contain new results that significantly affect our view of the planet.
A new survey of multiring impact basins on Mars
Authors: Schultz Richard A. , Frey Herbert V.
Published: February 2008

Multiring impact basins have profoundly influenced the geologic evolution of Mars. We compile and summarize the evidence for Martian impact basins and suggest eight new examples. Multiring basins on Mars define three morphologic subclasses with increasing basin size. Basins having diameters 300 < D < 1850 km are morphologically comparable to the classic lunar Orientale basin. Argyre type basins (1850 < D < 3600 km) are characterized by a rugged annulus and concentric grabens. The largest, Chryse type basins (D > 3600 km) have extremely shallow topographic profiles and numerous concentric structures expressed as scarps, massifs, and channels. Radial and concentric structures analogous to those associated with Orientale are not apparent for basins of Argyre size or larger. These variations in basin morphology and structure may be associated with mechanical interactions between basin‐forming impacts, relatively thin, weak lithosphere, and, for the largest impacts, spherical target geometry. Multiring basins are recognized on all parts of Mars, including Tharsis, Elysium, and the northern lowlands. Much of the subsequent resurfacing of cratered terrain such as Lunae Planum ridged plains is associated spatially with multiring basins. Nucleation of long‐lived volcanic complexes in Tharsis and Elysium was probably aided by early impact basins. The planetary terrain dichotomy was probably established during the period of heavy meteoritic bombardment, and subsequent processes in the northern plains region were not sufficiently vigorous to destroy or completely obscure the underlying multiring basin fabric. The revised population of multiring basins is consistent with the size frequency distribution of craters < 500 km in diameter on Mars. Basins having diameters ∼ 500 < D < 1850 km are characterized by a cumulative frequency slope of −0.75±0.16, whereas the larger basins define a slope of −1.7±0.45. This change in slope occurs over the same diameter range as the significant change from Orientale type basin morphology at smaller diameters to Argyre and Chryse type basin morphology. The population of craters and basins on Mars confirms that the relative crater densities on the terrestrial planets due to heavy bombardment are comparable, perhaps suggesting a common source for the impacting objects.
Origin and evolution of valleys on Martian volcanoes
Authors: Gulick Virginia C. , Baker Victor R.
Published: February 2008

Morphological analyses of six Martian volcanoes, Ceraunius Tholus, Hecates Tholus, Alba Patera, Hadriaca Patera, Apollinaris Patera, and Tyrrhena Patera, indicate that fluvial processes were the dominant influence in the initiation and subsequent development of many dissecting valleys. Lava processes and possibly volcanic density flows were also important as valley‐forming processes. Fluvial valleys are especially well developed on Alba Patera, Ceraunius Tholus, and Hecates Tholus. These valleys are inset into the surrounding landscape. They formed in regions of subdued lava flow morphology, contain tributaries, and tend to widen slightly in the downstream direction. Lava channels on Alba Patera are located on the crest of lava flows and have a discontinuous, irregular surface morphology, and distributary patterns. These channels sometimes narrow toward their termini. Possible volcanic density flow channels are located on the northern flank of Ceraunius Tholus. Valleys dissecting Apollinaris Patera, Hadriaca Patera, and Tyrrhena Patera appear to have a complex evolution, probably a mixed fluvial and lava origin. They are inset into a subdued (possibly mantled) surface, lack tributaries, and either have fairly constant widths or widen slightly downvalley. Valleys surrounding the caldera of Apollinaris appear to have formed by fluvial and possibly by volcanic density flow processes, while those on the Apollinaris fan structure may have a mixed lava and fluvial origin. Valleys on Tyrrhena have broad flat floors and theater heads, which have been extensively enlarged, probably by sapping. Hadriaca valleys have a trough‐shaped form like those on the fan structure on Apollinaris but are more enlarged and have theater heads. Fluvial valleys on the Martian volcanoes generally followed a two‐stage evolutionary sequence similar to that of the Hawaiian volcano valleys, including (1) initial formation by surface runoff and (2) subsequent enlargement by groundwater sapping. Because basalt is highly permeable, valleys on Hawaii Island formed only in regions where lava flows were mantled by less permeable volcanic ash. Similarly, fluvial valleys on the Martian volcanoes, particularly those on Alba Patera, formed in regions where lava flow morphology is subdued, probably by ash mantling. Thus the presence or absence of this mantling may explain why fluvial valleys are present on some Martian volcanoes but not on others of the same age. The formation of fluvial or fluvial‐modified valleys on the older volcanoes Ceraunius, Hecates, Tyrrhena, Apollinaris, and Hadriaca implies that surface and near‐surface water activity was widespread during late Noachian to early Hesperian (period of heavy bombardment). However, the presence of anomalously young fluvial valleys on Alba Patera indicates that fluvial valley activity did not entirely cease near the end of late heavy bombardment but rather persisted in isolated regions until mid to late Amazonian.