bluuebunny/crossref_metadata_2025 huggingface dataset:
Fractals and fragmentation
Authors: Turcotte D. L.
Published: February 2008

If the number‐size distribution of objects satisfies the condition N ∼ r−D, then a fractal is defined with a fractal dimension D. In many cases, fragmentation results in a fractal distribution. This is taken as evidence that the fragmentation mechanism is scale invariant. Fragments produced by weathering, explosions, and impacts often satisfy a fractal distribution condition over a wide range of scales. Most correlations for number versus size for meteorites, asteroids, and interstellar grains also satisfy the fractal condition. Fractal behavior implies scale invariance; the renormalization group approach is often applicable to scale invariant processes. Two models are considered for a renormalization group approach to fragmentation; the models yield a fractal behavior but give different values for the fractal dimension. These results indicate that the fractal dimension is a measure of the fragility of the fragmented material.
Sea Beam/Deep‐Tow Investigation of an active oceanic propagating rift system, Galapagos 95.5°W
Authors: Hey R. N. , Kleinrock M. C. , Miller S. P. , Atwater T. M. , Searle R. C.
Published: February 2008

We have tested and corroborated the propagating rift hypothesis with high‐resolution Sea Beam and Deep‐Tow data collected over the Galapagos 95.5°W propagating rift system. The propagating rift is continuously breaking through the Cocos plate at a velocity of about 50 km/m.y. with an azimuth of about 273°, away from the Galapagos hotspot. This process transfers Cocos lithosphere to the Nazca plate, including the preexisting spreading center which fails as it is gradually preempted by the propagating rift. The spreading center azimuth is being changed by about 8° clockwise by this rift propagation. The active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault. Although rift propagation is continuous on a scale of a few kilometers (50,000 years), and lithospheric transferral from one plate to the other occurs mostly continuously, as the preexisting rift fails, it breaks into discrete segments. The geometry of seafloor fabric and tectonic elements such as active and failed rifts, the V‐shaped pattern of pseudofaults, and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted. This quantitative verification provides strong support for the propagating rift hypothesis.
Structural and volcanic expression of a fast slipping Ridge‐Transform‐Ridge‐Plate Boundary: Sea MARC I and photographic surveys at the Clipperton Transform Fault
Authors: Kastens Kim A. , Ryan W. B. F. , Fox P. J.
Published: February 2008

A high‐resolution near‐bottom survey has been conducted of the Clipperton transform fault and adjoining segments of the East Pacific Rise (EPR), using the Sea MARC I side‐looking sonar system and the Lamont‐Doherty Geological Observatory Olympus‐based camera system. The transform fault zone (TFZ) is a narrow, well‐defined belt of transform‐parallel lineaments, which varies along strike from a single, sharp‐edged notch to a complex band of subparallel lineaments up to 1 km wide. The TFZ is set within a 5‐km‐wide band of unusually fine‐grained side scan texture, which could indicate nonbasaltic seafloor and/or pervasively sheared and mass‐wasted basaltic crust The fine‐grained swath is surrounded by constructional volcanic terrain with no hint of strike‐slip motion; this observation puts an upper limit of 5 km on the extent of lateral migration of the TFZ in the last 1.5 m.y. Both ridge transform intersections (RTIs) are dominated by bathymetric highs located on the old plate opposite the spreading center. A mantling of fresh‐looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism; thermal expansion may also play a part. The EPR south of Clipperton has recently experienced extrusion of high effusion rate basalts, burial of faults and fissures by lava flows, and development of vigorous hydrothermal circulation. On the EPR north of Clipperton, the axial zone of faults and fissures tapers toward the transform fault; this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault.
Basin structure on the Norwegian Margin from analysis of digitally recorded sonobuoys
Authors: Eldholm Olav , Mutter John C.
Published: February 2008

During R/V Robert D. Conrad cruise 21 a large number of sonobuoys were recorded in the Vøring, Møre, and adjacent basins at the margin off Norway. The digital sonobuoy data have been transformed into the domain of intercept time tau and ray parameter p and inverted by the tau‐sum method. To obtain better definition of the near‐seafloor velocities, using also the precritical energy, a move‐out correction has been applied to the tau‐p data. Conventional analyses of the x‐t data were also performed. In general, the various types of analyses complement each other and should be used interactively together with forward modeling to obtain the optimal velocity‐depth function. The sonobuoy results have outlined, in a regional sense, the pre‐Cenozoic sedimentary basin at the margin. This basin attains a maximum sediment thickness of 8–10 km of which only the upper half is defined by seismic reflection profiling. At the outer margin the marginal escarpments form major discontinuities in the lateral velocity distribution. The survey has proven that sonobuoy profiles recorded during standard seismic reflection profiling in major marginal basins may provide important new information, particularly about the deeper parts of the basins, that is not directly available from the reflection profiles.
Spectral effects of time‐depth nonlinearities in deep sea sediment records: A demodulation technique for realigning time and depth scales
Authors: Schiffelbein Paul , Dorman LeRoy
Published: February 2008

Carbon‐14 dating and percent CaCO3 in late Pleistocene sediments suggest that deepsea sedimentation rates vary cyclically and that this cyclicity is related to climate. Sedimentation rate variability leads to nonlinearity in the time‐depth mapping function. This nonlinearity can have profound affects on signal spectra, leading to the development of harmonics and intermodulation tones. These distortion effects in the spectra give a direct indication of the degree of nonlinearity, thereby providing a tool for realigning time and depth scales. A tuning technique is developed which assumes a direct link between climate (as measured in δ18O from planktonic foraminifer tests) and sedimentation rates. A criterion of “spectral simplicity,” as quantified in the varimax norm, is used to demodulate the input spectrum. Application of this technique to an equatorial Pacific piston core (ERDC 84) found peak glacial sedimentation rates to be 30% higher than peak interglacial rates, a figure in good agreement with 14C‐based estimates from the same area. This technique is compatible with other time scale tuning techniques such as those using orbital parameters and, in combination with these other techniques, provides a method for fine‐tuning any late Pleistocene record.
Hf, Zr, and REE partition coefficients between ilmenite and liquid: Implications for lunar petrogenesis
Authors: Nakamura Yoichi , Fujimaki Hirokazu , Nakamura Noboru , Tatsumoto Mitsunobu , McKay Gordon A. , Wagstaff Jerry
Published: February 2008

Hf, Zr, and REE partition coefficients between ilmenite and coexisting liquid were determined under near‐lunar conditions. Ilmenite was synthesized at 1125°C under low oxygen fugacity (fo2=10−12.4) from glass of a high‐Ti mare basalt composition, doped with 50–100 μg/g of REE and Hf, and 500–1000 μg/g of Zr. The partition coefficients (D) of ilmenite for Hf and Zr, obtained by isotope dilution analysis, are 0.41 and 0.33, respectively. These values are significantly lower than those of ilmenite (3.0 and 1.8, respectively) from a kimberlite megacryst (Fujimaki et al., 1984a). DREE for the synthesized ilmenite are slightly smaller than those of ilmenite from the kimberlite megacryst, and the DLu is 0.056. These results suggest that ilmenite may have played an important role in the Lu‐Hf evolution of lunar mare basalts. Using newly determined DLu and DHf for ilmenite, the Lu‐Hf evolution or lunar cumulates and the coexisting magma was examined for several different crystallization sequences by a model calculation. The Lu‐Hf variation trend of most high‐Ti mare basalts is well explained by a small degree (less than 5%) of partial melting of the cumulates. However, a higher degree of partial melting is required to explain the variation of very low‐Ti basalts, green glass, and Apollo 12 low‐Ti basalts. Apollo 15 low‐Ti basalts could not be produced by simple partial melting, but may require chromite crystallization as well.
Alkali norite, troctolites, and VHK mare basalts from breccia 14304
Authors: Goodrich Cyrena Anne , Taylor G. Jeffrey , Keil Klaus , Kallemeyn Gregory W. , Warren Paul H.
Published: February 2008

Twelve clasts were separated from breccia 14304 for consortium study: six pristine highlands rocks, two mare basalts, and four nonpristine highlands rocks. The pristine highlands rocks include representatives of the magnesian troctolite‐anorthosite and alkali suites, the two most common subgroups of the Mg suite found at the Apollo 14 site. Two troctolite clasts have olivine (∼Fo 90) and plagioclase (∼An 94) compositions similar to one group of Apollo Htroctolites. One also contains spinel (Mg′66–85). Incompatible element abundances in one are similar to those of 14305 troctolites, although the HREE (heavy rare earth elements) pattern is distinct among Apollo 14 troctolites. A dunite clast (∼Fo 89) may be an unrepresentative piece of a troctolite. Alkali lithologies include an alkali anorthosite and an alkali norite, which is a rock type not previously described. The alkali norite has a pristine igneous texture and contains inverted pigeonite (Mg′ 64), plagioclase (An82), K‐feldspar, ternary feldspar, REE‐rich phosphates, and silica. It resembles alkali gabbronorites from Apollo 14 and 67975 in mineralogy and mineral compositions. Alkali lithologies and phosphate‐bearing magnesian anorthosites from Apollo 14 may have formed from Mg‐rich magmas that assimilated various amounts of material rich in P and REE. This material could be a fractionated derivative of urKREEP. Another pristine clast from 14304 is an Mg‐gabbronorite. The two mare basalt clasts are very high potassium (VHK) basalts. They have 4 mg/g K and K/La ratios of 580 and 700. The parent magmas of VHK basalts could have formed from typical low‐Ti, high‐Al basaltic magmas by assimilation of K‐rich material. This material could also be a fractionated derivative or urKREEP. Nonpristine 14304 clasts include melt‐textured anorthosites and an augite‐rich poikilitic melt rock. The latter is probably polymict, but its major component must be an Mg‐suite gabbro.
Effect of the greenhouse gases (CO2, H2O, SO2) on Martian paleoclimate
Authors: Postawko Susan E. , Kuhn William R.
Published: February 2008

There is general agreement that certain surface features on Mars are indicative of the presence of liquid water at various times in the geologic past. In particular, the valley networks are difficult to explain by a mechanism other than the flow of liquid water. It has been suggested in several studies that a thick CO2 atmosphere on Mars early in its history could have provided a greenhouse warming that would have allowed the flow of water either on the surface or just below the surface. However, we have examined this effect with a detailed radiation model and have found that if reduced solar luminosity early in the history of the solar system is taken into account, even three bars of CO2 will not provide sufficient greenhouse warming. The addition of water vapor and sulfur dioxide (both plausible gases that may have been emitted by Martian volcanoes) to the atmosphere also fail to warm the surface above 273K for reduced solar luminosity conditions. The increase in temperature may be large enough, however, for the formation of these features by brines.
Calibration of the ruby pressure gauge to 800 kbar under quasi‐hydrostatic conditions
Authors: Mao H. K. , Xu J. , Bell P. M.
Published: February 2008

An improved calibration curve of the pressure shift of the ruby R1 emission line was obtained under quasi‐hydrostatic conditions in the diamond‐window, high‐pressure cell to 800 kbar. Argon was the pressure‐transmitting medium. Metallic copper, as a standard, was studied in situ by X ray diffraction. The reference pressure was determined by calibration against known equations of state of the copper sample and by previously obtained data on silver.
Transform seismicity at the intersection of the oceanographer fracture zone and the Mid‐Atlantic Ridge
Authors: Cessaro Robert K. , Hussong Donald M.
Published: February 2008

A seismic array consisting of nine Hawaii Institute of Geophysics (HIG) ocean bottom seismometers (OBSs) was deployed at the eastern intersection of the Oceanographer Fracture Zone (OFZ) and the Mid‐Atlantic Ridge (MAR). The 12‐day experiment was designed to relate low‐magnitude earthquakes to the structure and tectonics of the MAR‐OFZ intersection. An average of 10 locatable events with duration‐based magnitudes between −1.0 and 2.0 were recorded per day. Excellent hypocentral locations of 112 events were obtained. Earthquake locations based on more than eight observations generally show 50% confidence volume constraints within OBS location errors. The earthquake locations cover a broad swath across the corner of the intersection zone. Magnitude‐weighted earthquake location likelihood maps suggest a decline in magnitudes near the intersection bathymetry low. Composite focal plane solutions suggest source mechanisms which indicate that the region is dominated by extensional tectonics. Alternative source solutions indicating translational movement are presented but are inconsistent with apparent bathymetric trends. The transition from the diverging (MAR) to translational (OFZ) plate margin occurs in the context of reduced magma genesis and crustal thinning due to the influence of the adjacent older lithosphere. The region may be described in terms of semirigid plate tectonics accompanying transform valley genesis.
Tilted plutons in the Klamath Mountains of California and Oregon
Authors: Barnes Calvin G. , Rice Jack M. , Gribble Robert F.
Published: February 2008

Geologic and petrologic evidence indicates that the Ashland pluton and the Wooley Creek batholith, two major plutons of the Klamath Mountain province, have been significantly tilted subsequent to emplacement. Late Cretaceous sandstone depositionally overlies the Ashland pluton and dips approximately 25° toward the ENE. Contact metamorphic assemblages in diverse wall rocks surrounding the Wooley Creek batholith suggest at least 30° of tilt toward the WSW. Subsequent erosion has exposed at least 12 km of structural relief in the Wooley Creek batholith. Tilting was accomplished, at least in part, by thrusting of the plutons and their host rocks over low‐density rocks of the western Jurassic belt, followed by regional doming of deeply buried rocks of the underthrust terrane (Condrey Mountain Schist). Diagrammatic reconstruction of the plutons to pretilt positions suggests inward chemical fractionation of the Ashland pluton but upward fractionation of the Wooley Creek batholith and comagmatic Slinkard pluton. The results of this study indicate that rootless plutons in accretionary margins can undergo significant tilting, facilitating petrologic interpretations. In addition, the study suggests that for Klamath plutons, the effects of tilting must be taken into account before meaningful paleomagnetic interpretations can be made.
Open‐ and closed‐system characteristics of a tilted plutonic system, Klamath Mountains, California
Authors: Barnes Calvin G. , Allen Charlotte M. , Saleeby Jason B.
Published: February 2008

The Wooley Creek batholith and Slinkard pluton are adjacent plutons with identical U‐Pb zircon ages of 161 +4/−2 Ma. The plutons dip toward each other, which suggests that they are connected in the subsurface. Geologic and thermochemical evidence shows that the Slinkard pluton is the structurally lowest part of the system and the Wooley Creek batholith the structurally highest, with a total structural relief of at least 15 km. The wall rocks adjacent to the highest level of the Wooley Creek batholith are intruded by mafic to felsic dikes derived from the subjacent pluton. The Slinkard pluton is predominantly quartz diorite but grades from two‐pyroxene diorite to sparse muscovite‐biotite granite and shows little evidence of contamination except near contacts. The Wooley Creek batholith grades upward from two‐pyroxene gabbro to biotite‐hornblende granite and has a central zone in which injected mafic magma was trapped and mixed. Major and trace element data are consistent with closed‐system fractionation of the Slinkard pluton and lower (adjacent) Wooley Creek batholith, but with open‐system differentiation of the upper Wooley Creek batholith and roof‐zone dikes. Andesitic roof‐zone dikes show trace element enrichment similar to the central mixed zone of the Wooley Creek batholith and may have differentiated at that central level of the system.
Structure of the southern Rio Grande Rift from gravity interpretation
Authors: Daggett Paul H. , Keller G. R. , Morgan Paul , Wen Cheng‐Lee
Published: February 2008

Regional Bouguer gravity anomalies in southern New Mexico have been analyzed by two‐dimensional wave number filtering and polynomial trend surface analysis of the observed gravity field. A prominent, regional oval‐shaped positive gravity anomaly was found to be associated with the southern Rio Grande rift. Computer modeling of three regional gravity profiles suggests that this anomaly is due to crustal thinning beneath the southern Rio Grande rift. These models indicate a 25 to 26‐km minimum crustal thickness within the rift and suggest that the rift is underlain by a broad zone of anomalously low‐density upper mantle. The southern terminus of the anomalous zone is approximately 50 km southwest of El Paso, Texas. A thinning of the rifted crust of 2–3 km relative to the adjacent Basin and Range province indicates an extension of ∼9% during the formation of the modern southern Rio Grande rift. This extension estimate is consistent with estimates from other data sources. We interpret the crustal thinning and anomalous mantle to result from magmatic activity related to surface volcanism and high heat flow in this area.
Late Cretaceous to middle Tertiary tectonic history of the northern Rio Grande Rift, New Mexico
Authors: Kelley Shari A. , Duncan Ian J.
Published: February 2008

Apatite fission track ages for samples collected from three mountain ranges on the eastern margin of the Rio Grande rift are used to examine the late Cretaceous to middle Miocene uplift and erosional history of north central New Mexico. The dates indicate that uplift and erosion was in progress in the Sandia Mountains near Albuquerque and in the Taos Range portion of the Sangre de Cristo Mountains near Taos at least 30–35 m.y. ago. Uplift and erosion continued in the Sandia Mountains at a relatively constant rate (81 m/m.y.) until 15 Ma; the rate of uplift and erosion in this area has approximately tripled in the past 15 m.y. (230 m/m.y.). Igneous activity in the Taos Range has largely obscured the early Tertiary uplift and erosional history of this portion of the Sangre de Cristo Mountains. A fission track date from one of the middle Tertiary intrusions in the Taos Range is used to calculate the cooling rate due to uplift and erosion in this area for the past 14 m.y. (210 m/m.y.). The uplift and erosion rates derived from the fission track data for the past 14–15 m.y. are similar to those obtained from other geological evidence. In contrast to the Oligocene to Miocene ages found in the other two areas, the apatite fission track ages from the Santa Fe Range portion of the Sangre de Cristo Mountains near Santa Fe are Late Cretaceous to early Eocene. These dates record the cooling of the area due to uplift and erosion during the Laramide event. The preservation of these older ages indicates that the Santa Fe Range was a low‐lying area during the Oligocene to Miocene, while the surrounding areas (Sandia Mountains and Taos Range) underwent uplift and erosion. Volcanic activity occurred in the vicinity of the two areas of positive relief. Localized crustal extension associated with the volcanism may have contributed, in part, to the uplift of these areas. Using simple, two‐dimensional thermal models, we found that the apparent denudation rates derived from the fission track age data may represent true denudation rates, given certain geologic conditions that may have existed during the uplift and erosion of the Sandia Mountains in the middle Tertiary and the Santa Fe Range in the late Cretaceous.
Evidence of ongoing crustal deformation related to magmatic activity near Socorro, New Mexico
Authors: Larsen Shawn , Reilinger Robert , Brown Larry
Published: February 2008

Recent leveling conducted by the National Geodetic Survey in central New Mexico provides new evidence of ongoing tectonic deformation associated with the Rio Grande rift. A 1980–1981 resurvey over suspected midcrustal and shallow magma bodies near Socorro, New Mexico, indicates continued uplift averaging 0.18 cm/yr between 1951 and 1980. Although this rate is somewhat smaller than that reported previously (0.34 cm/yr between 1912 and 1951), the new observations suggest possible systematic errors in the earlier surveys. Reanalysis of the earlier data gives an average rate of 0.23 cm/yr from 1912 to 1951, consistent not only with the new releveling observations but also with geomorphic evidence which indicates an average uplift rate of 0.18 cm/yr over the past 20,000 years. Thus the rate of uplift may be relatively constant. The new releveling measurements also confirm prominent zones of subsidence flanking the central uplift. Modeling the deformation, together with other geophysical observations, suggests that the movements in the Socorro area are likely associated with the 19‐km‐deep Socorro magma body. Although the details of magma movements are not well constrained, plausible models include flow of magma from the periphery toward the central magma chamber and flow from sources in the lower crust into the midcrust. Horizontal strain calculated for these models is generally consistent with the small strain reported from repeated trilateration surveys in the Socorro area. The Rio Grande rift is clearly an important site for future monitoring of crustal deformation.
Evolution of the Latir volcanic field, Northern New Mexico, and its relation to the Rio Grande Rift, as indicated by potassium‐argon and fission track dating
Authors: Lipman Peter W. , Mehnert Harald H. , Naeser Charles W.
Published: February 2008

Remnants of the Latir volcanic field and cogenetic plutonic rocks are exceptionally exposed along the east margin of the present‐day Rio Grande rift by topographic and structural relief in the Sangre de Cristo Mountains of northern New Mexico. Evolution of the magmatic system associated with the Latir field, which culminated in eruption of a regional ash flow sheet (the Amalia Tuff) and collapse of the Questa caldera 26 m.y. ago, has been documented by 74 new potassium‐argon (K‐Ar) and fission track (F‐T) ages. The bulk of the precaldera volcanism, ash flow eruptions and caldera formation, and initial crystallization of the associated shallow granitic batholith took place between 28 and 25 Ma; economically important molybdenum mineralization is related to smaller granitic intrusions along the south margin of the Questa caldera at about 23 Ma. Interpretation of the radiogenic ages within this relatively restricted time span is complicated by widespread thermal resetting of earlier parts of the igneous sequence by later intrusions. Many samples yielded discordant ages for different mineral phases. Thermal blocking temperatures decrease in the order: K‐Ar sanidine > K‐Ar biotite > F‐T zircon ≫ F‐T apatite. The F‐T results are especially useful indicators of cooling and uplift rates. Upper portions of the subvolcanic batholith, that underlay the Questa caldera, cooled to about 100°C within about a million years of emplacement; uplift of the batholith increases to the south along this segment of the Sangre de Cristo Mountains. Activity in the Latir volcanic field was concurrent with southwest directed extension along the early Rio Grande rift zone in northern New Mexico and southern Colorado. The geometry of this early rifting is compatible with interpretation as back arc extension related to a subduction system dipping gently beneath the cordilleran region of the American plate. The Latir field lies at the southern end of a southward migrating Tertiary magmatic locus, along which much of the southern Rocky Mountains was blanketed by volcanic rocks in Eocene and Oligocene time. This southward migration of igneous activity through the Rocky Mountains may have been a critical precursor that, by heating and weakening the lithosphere, helped localize initial extensional deformation in the eastern Cordilleran region. Ages of rift‐related basaltic volcanism in the Questa‐Latir area span an apparent magma gap 20–10 m.y. ago, inferred by others for more southern sectors of the Rio Grande rift. Rift‐related volcanism was nearly continuous through Miocene and Pliocene time in the northern New Mexico sector of the rift, perhaps because of intersection with the diffuse northeast trending Jemez magmatic zone.
Thermal stress and the spacings of transform faults
Authors: Sandwell David T.
Published: February 2008

Bathymetric charts are used with satellite altimeter profiles to locate major ridge‐transform intersections along five spreading ridges. The ridges are the Mid‐Atlantic Ridge, the East Pacific Rise, the Chile Rise, the Pacific‐Antarctic Rise, and the Southeast Indian Ridge. Analysis of these data show spacings between transform faults W increase linearly with spreading rate ν (W/ν = 6.28 m.y.). This linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate ν. The traction‐free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults. A two‐dimensional thermoelastic solution for the in‐plane stress shows the largest stress component is tensional and parallel to the ridge. Stresses are zero at the ridge and increase as (age)½ to a maximum value at an age of W/4ν. All stress components are small for ages greater than W/ν. When the transform spacing is large compared with the spreading rate (W/ν > 100 m.y.) thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 Ma. The observed maximum ratio of transform spacing to spreading rate (W/ν = 10 m.y.) results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 Ma. Thus transform faults relieve most of the thermal stress. Model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge. Moreover, oceanic intraplate earthquakes rarely occur in lithosphere older than 30 Ma as predicted by the model. These and other geophysical observations confirm Turcotte's hypothesis that transform faults are thermal contraction cracks.
Inverse hydrologic analysis of the distribution and origin of Gulf Coast‐type geopressured zones
Authors: Bethke Craig M.
Published: February 2008

Inverse hydrologic analysis of compaction‐driven groundwater flow provides insights to the distribution and origin of geopressured zones in subsiding sedimentary basins, such as those found in the U.S. Gulf Coast. Occurrences of Gulf Coast‐type geopressures are most frequently attributed to “disequilibrium compaction” caused by slow rates of fluid escape from compacting sediments, “aquathermal pressuring” from thermal expansion of pore fluids, or the subsurface dehydration of smectite. This paper presents an inverse solution to the Lagrangian equation of compaction flow that includes effects of aquathermal pressuring and dehydration reactions. The solution gives the permeability profile required to maintain a lithostatic pressure gradient in a subsiding basin. Comparison of the closed‐form solution with measured permeabilities shows that geopressured zones are likely to form in shaly basins subsiding more than about 1 mm/yr but unlikely to develop in shale‐poor basins or basins subsiding less than 0.1 mm/yr. This result correctly predicts geopressures in the Gulf Coast but suggests that many important sedimentary basins were not significantly overpressured during compaction. Solutions that consider only thermal expansion of pore fluids give required permeabilities about 1.3–1.8 orders of magnitude (factor of 20–60) less than those considering only sediment compaction, indicating that aquathermal pressuring is much less important than disequilibrium compaction in causing geopressures. Solutions accounting for the effects of dehydration reactions show that release of structural water during smectite dehydration can be a significant and perhaps necessary factor in geopressuring. Hydrologic effects of a possible increase in the volume of structural water during dehydration are less significant. The most important contribution of smectite dehydration to development of geopressured zones, however, may be the accompanying reduction of host rock permeability.
Metal‐silicate fractionation in the growing Earth: Energy source for the terrestrial magma ocean
Authors: Sasaki Sho , Nakazawa Kiyoshi
Published: February 2008

Based on the homogeneous accretion model of planets, thermal evolution of the growing earth has been studied. After the surface of the growing earth starts melting owing to the blanketing effect of the primordial atmosphere, gravitational differentiation between metallic and silicate materials takes place in its upper region: the differentiation would give rise to a large amount of gravitational energy release as well as layered structure inside the proto‐earth. A part of released energy would be transported outward by convection. Considering the effects of gravitational fractionation and convection, we have pursued numerically the evolution of the growing earth. Our results show that gravitational differentiation can supply energy large enough to keep the whole silicate layer in the molten state and that half of the released energy is conveyed outward by convective heat transport. In the growing stage, the temperature distribution in the silicate layer always lies between liquidus and solidus temperatures except in an uppermost thin zone. The thermal structure obtained favors a notion of the primordial magma ocean and of chemical differentiation of mantle. The structure of the growing earth which is composed of the central protocore, the metallic layer, and the silicate layer (magma ocean) would become gravitationally unstable and overturn to form a metallic core. Energy released by this overturn would not affect the structure of the upper magma ocean too much. But undifferentiated primordial materials which come from the protocore would add to mantle materials through the overturn and this would help explain the isotopic mantle heterogeneity presently observed.
Permeabilities, interfacial areas and curvatures of partially molten systems: Results of numerical computations of equilibrium microstructures
Authors: von Bargen Nikolaus , Waff Harve S.
Published: February 2008

The general mathematical constraints for thermomechanical equilibrium of partial melts under hydrostatic or nearly hydrostatic stress conditions are well known. However, melt phase geometries under equilibrium conditions have previously been understood only qualitatively. Because of its importance in determining bulk physical and chemical properties of partially molten regions of the crust and the mantle, we have computed the geometries of the liquid phase for a range of melt fractions and dihedral angles. We describe here a theoretical interfacial topology model and its associated calculational algorithms which yield numerically precise equilibrium solid‐liquid interfacial surfaces where the only constraints imposed on the system are constant mean curvature and constant wetting angle in accordance with equilibrium. The calculated geometries are used to determine the permeability of partial melts to fluid flow, interfacial areas, surface energy, minimum channel cross section, surface mean curvature, and maximum trappable melt fraction for melt fractions up to 5% and wetting angles between 20° and 80°. For wetting angles less than 60° the melt is distributed in an interconnected network of channels along the grain edges, and therefore melt mobilization is possible as soon as melt forms. For wetting angles greater than 60° the melt phase will be distributed in isolated pockets at the grain corners if the melt fraction is less than a critical value, whereas an interconnected network of melt channels will be established if the critical melt fraction is exceeded. Wetting angle exerts a first‐order control on the permeability of partially molten systems through its effect on the connectivity of the melt phase. Once a connected melt phase has formed, the permeability is only mildly sensitive to wetting angle.
Distribution of apparent angles on random sections with emphasis on dihedral angle measurements
Authors: Jurewicz Stephen R. , Jurewicz Amy J. G.
Published: February 2008

The distribution of apparent solid‐liquid dihedral angles, normally observed on planar sections of partial melting experiments, is examined in terms of a normal distribution of “true” dihedral angles present in any sample. A theory for predicting the distribution of apparent angles observed on a random planar section of a three‐dimensional aggregate with one true dihedral angle is reviewed and comparisons are made between apparent angle distributions obtained from partial melting experiments and various theoretical distributions. The results of these comparisons suggest that the observed distribution of solid‐liquid dihedral angles in partial melting experiments rarely yield a satisfactory match to the theoretical distribution for a single unique dihedral angle. However, good agreement is generally obtained assuming a normal distribution of true dihedral angles about a mean value. The origin of this normal distribution is discussed both in terms of measuring error and crystalline anisotropy with respect to interfacial energies.
Melting and dissolution kinetics: Application to partial melting and dissolution of xenoliths
Authors: Tsuchiyama Akira
Published: February 2008

Kinetics of melting and dissolution (rates and textures) in magmatic systems is reviewed and applied to partial melting and dissolution of xenoliths. Special attention is paid to melting/dissolution of solid solution crystals, in which characteristic cloudy textures are formed, and diffusion of elements in the crystals plays an important role due to compositional change of the crystals. Based on the above discussion, model calculations are made for xenolith digestion using the haplogabbro system (diopside‐plagioclase). Xenolith digestion can proceed by partial melting of the xenolith itself as well as dissolution. In this case, xenolith size is not important. The digestion rates suggest that many xenoliths could survive only if they were incorporated into a host basaltic magma just prior to eruption. Natural partial melting found in a spinel lherzolite xenolith from Kilbourne Hole, New Mexico, is also examined with additional heating experiments. It is concluded that spongy clinopyroxene in the spinel lherzolite was formed by “partial dissolution,” reaction between clinopyroxene and partial melt, during partial melting at depth for a short run duration (a month or less). Both the model calculations and the heating experiments show that even mafic to ultramafic xenoliths can easily undergo partial melting or react with basaltic melts, and many xenoliths found in basaltic rocks could be incorporated into the basalts just prior to eruptions.
Hydrothermal alteration of a 1 km section through the upper oceanic crust, Deep Sea Drilling Project Hole 504B: Mineralogy, chemistry and evolution of seawater‐basalt interactions
Authors: Alt Jeffrey C. , Honnorez Jose , Laverne Christine , Emmermann Rolf
Published: February 2008

Deep Sea Drilling Project hole 504B penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes. Alteration of the crust at site 504 occurred in four stages, related to the movement of the crust away from the spreading axis: (1) Dikes reacted with seawater (200–>300°C) in the upwelling zone of an axial convection cell at the spreading axis, resulting in the formation of greenschist facies parageneses in veins and host rocks. Mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone, causing a steep temperature gradient at the base of the pillow section. Secondary minerals formed in the lower pillow section from the resultant reducing “mixed” fluids at temperatures of around 100°C. At the same time, the initial effects of “seafloor weathering” began in the upper 320 m of the pillow section at low temperatures (<50°C) and under conditions of open circulation of oxidizing seawater. (2) Following refracturing of the dikes, a second stage of axial upwelling occurred; hydrothermal fluids (200°–380°C) were probably similar to those presently sampled from spreading ridges on the seafloor. Mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz, epidote, and sulfides in veins and formation of a sulfide‐rich stockworklike zone within this transition zone. Alteration of the pillow section proceeded under the prior conditions, with the effects of seafloor weathering extending progressively downward into the crust. (3) Seawater recharge penetrated to depths of at least 1075.5 m subbasement and deposited anhydrite locally in veins. (4) Off‐axis alteration of the dikes was characterized by formation of zeolites, calcite, and prehnite in veins and host rocks from more highly evolved and lower temperature (100°–250°C) fluids. Alteration in the upper pillow section evolved from “seafloor weathering” conditions to more reducing and rock‐dominated, as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Zeolites and calcite were the last phases to form throughout the pillow section. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below. Calculations of seawater‐crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt‐seawater interactions are a sink for seawater Mg and K and a source for Si and Ca.
Crustal structure in Tibet: High seismic velocity in the lower crust
Authors: Chun Kin‐Yip , McEvilly Thomas V.
Published: February 2008

A least squares procedure is used to extract pure path Tibetan group velocities in the period range 8–100 s from fundamental mode Rayleigh and Love waves recorded along 50 mixed paths. The crustal model derived from these dispersion data reveals that the lower half of the very thick (74 ± 10 km) Tibetan crust has a shear velocity similar to those found in the lower crust beneath stable continental and shield regions. A second interesting feature of the model is the presence of a prominent low‐velocity channel at midcrustal depth. The uniqueness of the Tibetan crustal structure places the plateau region in a class by itself among tectonic provinces characterized by high heat flow and/or wide spread recent volcanism, setting stringent constraints on tectonic models of the Tibetan Plateau.
Microcrack porosity and in situ stress in Illinois Borehole UPH 3
Authors: Carlson S. R. , Wang H. F.
Published: February 2008

Differential strain analysis was used to study microcracks in granite core taken from 0.7 to 1.6 km depth from Illinois Deep Hole UPH 3. The majority of the microcracks are believed to have been generated by stress relief upon removal of the core from depth. Volumetric microcrack porosity was found to vary by an order of magnitude. Low values of microcrack porosity were found in highly fractured intervals, where large, open fractures have relaxed a portion of the in situ stress. Plotting microcrack porosity against mean in situ stress reveals a strong positive correlation over a range of 25 MPa. Large, subvertical fractures may locally influence the orientation of stress relief microcracks. The direction of maximum horizontal crack strain was found to be subparallel to the strike of the nearest large fracture in 7 of 8 fine‐grained samples.
Numerical models of the Filchner‐Ronne Ice Shelf: An assessment of reinterpreted ice thickness distributions
Authors: Lange M. A. , MacAyeal D. R.
Published: February 2008

Recent radio echo soundings of the Filchner‐Ronne Ice Shelf by the German Antarctic Expedition, 1983/1984 suggest that previous ice thickness measurements may have misinterpreted an internal radio echo reflecting horizon as the true ice shelf bottom. This, and the analysis of total ice thickness from surface altimetry, suggests that a previously defined thin‐ice region comprising approximately 1/5 of the total ice shelf area may be underlain by a thick layer of possibly saline ice. One possible way to verify the existence of such a layer is by measurement of its influence on the ice shelf flow regime. Here we evaluate this influence by conducting finite element simulations of two alternative ice thickness configurations. We conclude that flow differences are sufficiently large to allow verification of the possible saline basal ice layer provided that surface strain rate measurements are conducted in certain key areas.
Global relationship between oceanic geoid and topography
Authors: Cazenave A. , Dominh K. , Allegre C. J. , Marsh J. G.
Published: February 2008

In order to compare model predictions of upper mantle convection with surface observations, we have computed global residual depth and geoid anomalies on a 5° × 5° grid over all oceans, after removal of lithospheric cooling effects and long‐wavelength geoid components (> 4000 km) related to lower mantle sources. We have derived the transfer function (admittance) of geoid over topography as a function of wavelength λ in the range 1000–4000 km. The observed admittance is positive at all λ and presents a maximum of 2.5–3.5 m/km at λ ≃ 2750 km. Comparing this result with the theoretical admittance computed for the lithospheric cooling plate model, we ascribe most of the admittance peak at λ = 2750 km to errors in the depth‐age and geoid‐age correction. This suggests that a standard age correction applied to all seafloor is not valid. At shorter wavelengths, the admittance remains roughly constant, on the order of 1.5 m/km, and can be explained in terms of sediment loading and crustal thickening effects. Comparing spatially the residual geoid and depth anomaly maps, we note a positive correlation over a few areas coinciding with hot spot swells, volcanic plateaus or seafloor deeps; the geoid versus depth ratio amounts then to ∼ 6–8 m/km in agreement with the values predicted above upwellings and downwellings by models of constant viscosity convection. However, the global admittance analysis leads to much lower values at all wavelengths, suggesting that these areas are too limited in number and in extent to produce any significant signal.
Helium and methane anomalies in domestic well waters in southwestern Saskatchewan, Canada, and their relationship to other dissolved constituents, oil and gas fields, and tectonic patterns
Authors: Dyck Willy , Dunn Colin E.
Published: February 2008

During the summer of 1976 a regional groundwater survey was undertaken in an area of 18,000 km2 in southwestern Saskatchewan. Approximately 940 domestic wells and springs were sampled at a density of 1 per 13 km2, where possible. The samples were analyzed for up to 30 variables, including He, CH4, and many trace and minor elements. The results of the survey are related to regional topographic, geological, and hydrological features. Of particular interest are CH4 and He anomalies associated with known gas and oil reservoirs in the region. Several anomalies of similar magnitude occur away from known reservoirs and hence may point to new ones. The close association of CH4 with He, sample depth and deeper “softer” waters, as indicated by the positive correlations of CH4 with He, Na, CO2−3, HCO−3 and sample depth is postulated to be a good criterion for distinguishing between reservoir CH4 and marsh gas. The coincidence of CH4 and He anomalies with known tectonic features also indicates fracture leakage from depth and the possible existence of oil and gas fields. Regional surveys of domestic well waters can be of use in delineating areas with proven and potential oil and gas reservoirs at depth.
Modeling body wave amplitude fluctuations using the three‐dimensional slowness method
Authors: Burdick L. J. , Salvado C. A.
Published: February 2008

Many types of seismic data indicate that there are substantial three‐dimensional variations in velocity and Q in the lower as well as the upper mantle. A number of studies have established a qualitative link between these variations and fluctuations in body wave amplitudes. Quantitative analyses are rare since they require a method for treating wave propagation in three‐dimensional structures. The three‐dimensional slowness method has been investigated as possible tool for carrying out such calculations. Some elements of the theory behind the method have been clarified in such a way as to make the method easier to apply. Some new developments of the theory have been made as well. It has been shown that there are two possible choices for ray paths in the slowness calculations. It is recommended that ray paths related to generalized rays in the limit of vanishing layer thickness be used rather than the geometric paths in the medium. The slowness integral has been cast in such a form as to have an explicit integration over horizontal takeoff angle as well as vertical takeoff angle (classical ray parameter). The integration over horizontal angle is evaluated in exactly the same way as the integration over the vertical angle. The latter integration has become familiar as the slowness representation of the response of a one‐dimensional medium. It has been shown that the integral over horizontal takeoff azimuth reduces to the convolution integral with 1/t½ for the case of a one‐dimensional medium. We present an alternate geometric interpretation of the slowness method rather than the disk ray interpretation. We consider several different possible applications of the method.