bluuebunny/crossref_metadata_2025 huggingface dataset:
On the thermal history, thermal state, and related tectonism of a moon of fission origin
Authors: Binder Alan B. , Lange Manfred A.
Published: February 2008

The thermal history of an initially totally molten moon of fission origin properly accounts for (1) the mare basalt epoch, in terms of its duration, the depth of the source region, and degrees of partial melting which produced the magmas; (2) the present‐day heat flow of 17–18 ergs cm−2 s−1; and (3) the current high temperatures of the lower mantle as deduced from magnetic and seismic data. The model moon has a radius decrease of 5.4 km (3.1 times;10−3R) during lunar history. This value is within the rather poorly defined limits for the maximum change of the lunar radius of 10−3–10−2 R. The tectonic response to this 5.4‐km radius decrease and the associated thermoelastic stresses, stresses which have been dominantly tensional throughout lunar history, might be found in the larger‐scale normal faults, grabens, and tensional fractures of the lunar grid or lineament system. However, the majority of the thermoelastic stresses produced by the cooling of the moon have been dissipated via aseismic creep in the upper parts of the lunar mantle, not via faulting activity. A lower limit of 1024 P for the viscosity of the mantle of the moon (at subsolidus temperatures) is suggested, based on the apparent absence of solid state convection in the moon at any time during its history. This value is 103 times larger than that for the terrestrial mantle, a difference which might be due to the anhydrous nature of the lunar mantle versus the ‘wet’ mantle of the earth. The calculated energy derived from the thermoelastic stresses in the type A moonquake zone is orders of magnitude smaller than the available tidal energy. Hence the thermoelastic stresses are not an important energy source for the tidal moonquakes. The thermoelastic stresses can easily supply the energy for the high‐frequency teleseismic (HFT) moonquakes. The relative rarity of HFT's is explained by the long times (108–109 years) needed to accumulate the energy required to initiate faulting in the predicted source regions. These regions are in the uppermost mantle (depths between 80 and 200 km), where tensional quakes can occur, and at 10‐km depths in the crust, where compressional quakes can occur. The consistency between our thermal history model results and the corresponding characteristics now known for the moon add further support for the fission model for the origin of the moon.
A review of magnetic stratigraphy investigations in Cretaceous pelagic carbonate rocks
Authors: Lowrie W. , Channell J. E. T. , Alvarez W.
Published: February 2008

Pelagic carbonate rocks possess many suitable characteristics for paleomagnetic and magnetostratigraphic studies. Paleomagnetic results are summarized for seven lengthy sections of pelagic limestones and marls from Umbria and the southern Alps in Italy. Differences in apparent polar wander paths from these two regions are interpreted in terms of tectonic rotation of allochthonous Umbria. The magnetic stratigraphies of the paleontologically dated sections are independent of their tectonic differences and are combined to form a continuous record of geomagnetic polarity for the Barremian through Maastrichtian stages of the Cretaceous. All but one of the reversals in these sections are confirmed by duplication in at least one other section. Additional Cretaceous reversals have been reported in other land sections and in DSDP (Deep Sea Drilling Project) and IPOD (International Program of Ocean Drilling) cores. Some of these reversals are not defined well magnetically, and confirmation of others is clouded by imprecise paleontological dates. If real, they are probably of short duration. The confirmed reversal sequence correlates well with the Cretaceous oceanic magnetic anomaly sequence. The ages of certain key anomalies are altered: Late Cretaceous anomalies 29–34 are younger, and Early Cretaceous anomalies M0 and M1 are older than previously thought. The longer duration of the Cretaceous magnetic quiet interval of normal polarity results in a reduction of corresponding sea floor spreading rates to about 70% of earlier values, but they are still appreciably higher than during formation of the preceding M sequence anomalies.
The crustal structure of the kane fracture zone from seismic refraction studies
Authors: Detrick Robert S. , Purdy G. M.
Published: February 2008

A detailed seismic refraction experiment was carried out across the Kane Fracture Zone near 24°N, 44°W using explosive and air gun sound sources and eight ocean bottom hydrophone receivers. The shooting lines and receivers formed a ‘T’ configuration across the fracture zone, with two receivers located about 50 km apart in the fracture zone trough and the remaining six receivers positioned 25–30 km apart on either side of the fracture zone. The crustal thicknesses and velocities observed at the receivers located north and south of the Kane Fracture Zone fall within the range of those typically observed for normal oceanic crust. There is no convincing evidence for significantly different crustal thicknesses or upper mantle velocities on either side of the fracture zone despite a 10‐m.y. age difference. Anomalously thin crust is present beneath the Kane Fracture Zone trough with total crustal thicknesses of only 2–3 km, about half the thickness of normal oceanic crust. This crust is also characterized seismically by low compressional wave velocities (∼4.0 km/s) at shallow depths and the absence of a normal layer 3 refractor. This anomalous crust extends over a width of at least 10 km. Dense, high‐velocity mantle type material may also exist at shallow depths beneath the adjacent Kane Fracture Zone ridge. Results from other geological and geophysical studies of fracture zones suggest that this type of crustal structure may by typical of many Atlantic fracture zones. We propose that the anomalously thin crust found within these fracture zones is a primary feature caused by the accretion of a thinner volcanic and plutonic layer within the fracture zone. This anomalous crust, which probably is restricted to a zone no wider than a typical transform fault valley (∼10 km) in most cases, is inferred to consist of a few hundred meters of extrusive basalts and dikes overlying about 2 km of gabbro and metagabbro, possibly interbedded with ultramafics. This anomalously thin crustal section may be extensively fractured and brecciated at shallow levels by faulting in the active transform domain. A relatively narrow zone of thin crust within fracture zones can explain a number of geological and geophysical characteristics of fracture zones including the depth of the transform fault valley and the exposure of deep crustal and upper mantle rocks in the walls of fracture zones.
A stochastic fault model: 1. Static case
Authors: Andrews D. J.
Published: February 2008

The number‐size distribution of earthquakes requires that irregularities exist on a fault at all length scales. The assumption of self‐similar irregularity is used to formulate a stochastic description of the faulting process. A random irregularity is termed self similar if it remains statistically similar upon a change of length scale. Self‐similar geometric irregularity of a fault surface is represented in this model by stress and friction functions that fluctuate self similarly on a plane. If the set of rupture areas of all earthquakes on the brittle portion of a fault plane is assumed to be self similar, then the number of ruptures with area greater than A is proportional to 1/A. If stress drop is independent of earthquake size, then the number of earthquakes with moment greater than M0 is proportional to M0−⅔. The size of an earthquake is determined by spatial fluctuation of the initial stress and sliding friction functions. The spectrum of the stress function is related to both the average stress drop as a function of earthquake size and the number‐moment distribution. A model of the slip and stress change functions of an earthquake is constructed in the Fourier transform domain. While the stress function becomes smoother in an earthquake at the length scale of the rupture, it becomes rougher at shorter length scales to prepare the fault for future smaller earthquakes. Seismicity is a cascade of stored elastic energy from longer to shorter wavelengths.
Diffusion in the rock matrix: An important factor in radionuclide retardation?
Authors: Neretnieks Ivars
Published: February 2008

This paper discusses migration of radionuclides in the bedrock surrounding a repository. Currently available models use either a surface reaction or a bulk reaction concept to describe the retardation of migrating nuclides. The first model assumes that the nuclide reacts only with the surface of the fissures. This implies that the rock matrix is not utilized as a sink. The other model implies that the whole bulk of the rock is accessible to the nuclides. The paper analyzes the accessibility of the rock matrix to the radio‐nuclides. The transport mechanisms are shown to be flow of water and nuclides in the fissures and transport of nuclides from the water in the fissures into water in the microfissures of the rock by pore diffusion. The diffusion of the nuclides into the rock matrix and their sorption onto the surfaces of the microfissures are the main mechanisms retarding migration from a repository. The diffusivity of the nuclide may be as important as its sorption equilibrium constant. Diffusivities in the pores and microfissures in such dense rocks as granite under confining pressure of hundreds of bars can be expected to be 6–20% of the diffusivity in water. These data are obtained from electrical resistivity measurements of saltwater‐filled granites. Porosity of such granites varies from 0.4 to 0.9%. The apparent diffusivities in the granites will then vary between 0.25 · 10−12/Kdρp and 10 · 10−12/Kdρp m2/s, where Kdρp is the volume equilibrium constant. This varies from the porosity of the rock for nonsorbing species to up to and over 104. For a 100‐year contact time a nonsorbing nuclide can be expected to penetrate tens of meters of the rock matrix and a strongly sorbing nuclide with Kdρp larger than 104 will penetrate a few millimeters. The diffusion into the rock matrix can enhance the retardation by many orders of magnitude as compared to retardation by surface reaction in fissures only. The retardation may, on the other hand, be many orders of magnitude smaller than the maximum value that could be obtained if all the rock matrix were accessible. This depends very much on the fissure widths and spacings.
Effect of volatiles on attenuation (Q−1) and velocity in sedimentary rocks
Authors: Clark V. A. , Tittmann B. R. , Spencer T. W.
Published: February 2008

Small amounts of water have previously been found to have a dramatic effect on the value of the seismic dissipation factor Q−1. This suggests the importance of controlling the environment in which Q−1 measurements are made. Since the effect of moisture is so large, studying the variation in Q−1 with partial pressure of water and other volatiles can provide important clues for understanding the attenuation mechanisms in ‘room dry’ rocks. Thus measurements have been made of both Q−1 and velocity for several limestones (all >99% CaCO3) and sandstones. The Q−1 of both limestones and sandstones is affected by the presence of water vapor, but at any given partial pressure of water (PH2o) the Q−1 of the limestones is lower than the Q−1 of the sandstones. Velocities of the sandstones decreased by as much as 30% as the relative humidity was changed from 0 to 80%. Velocities of limestones of comparable porosity decreased only 2% over the same range. Simultaneously with the Q−1 and velocity measurements, adsorption isotherms (∼23°C) were measured on a companion sample. Generally, limestones adsorbed less water at a given partial pressure than the sandstones. The amount of water adsorbed, at most 2–3 mg/g,does not increase the density enough to account for the decrease in velocity, particularly in the sand‐stones. The Brunauer‐Emmett‐Teller equation was used to calculate surface areas from the adsorption isotherms. Surface areas ranged from 0.5 to 3.4 m2/g. Limestones had lower surface areas than the sandstones. The larger surface areas in the sandstones can probably be attributed to varying amounts of clays.A plot of Q−1versus mass adsorbed shows that most of the increase in dissipation is associated with the adsorption of the first two monolayers of water. A fixed amount of water causes a greater change in the Q−1 of the sandstones than of the limestones. One sandstone was measured by using anhydrous benzene as the adsorbate. In contrast to the action of the water, Q−1 changed very little with increased benzene pressure for small P/P0, even though significant amounts of benzene were adsorbed. For P/P0 greater than 0.7, there were significant increases in Q−1. It appears that there are two mechanisms of attenuation in ‘dry’ rocks: one is observed at low P/P0 and is associated with surface effects that are strongly dependent on the nature of the adsorbate, and the other is observed at high P/P0 and is associated with fluid flow in fine pores.
Paleozoic paleomagnetism and northward drift of the Alexander Terrane, southeastern Alaska
Authors: van Der Voo Rob , Jones Meridee , Gromme C. Sherman , Eberlein G. Donald , Churkin Michael
Published: February 2008

Paleozoic limestone, graywacke, sandstone, milestone, red beds and volcanic rocks of the Alexander terrane, southeastern Alaska, have yielded six paleomagnetic pole positions after thermal and alternating‐field demagnetization. These poles are from sample groups of late Middle Ordovician, Late Ordovician, Devonian, Late Devonian, and early and late Carboniferous age. To test various tectonic models for the structural development of this part of western North America, the paleomagnetic results are compared to those for the North American craton. It is found that the observed inclination and declination values deviate significantly from the values predicted for the present‐day position of the Alexander terrane (55.5N, 133.5W). Better matching can be obtained for a paleoposition of the terrane at about 40N, 120W, in the present position of western Nevada and northeastern California. In addition, an in situ 25° clockwise rotation of the terrane is required to restore it to its original position.
Tectonic stress: Models and magnitudes
Authors: Solomon Sean C. , Richardson Randall M. , Bergman Eric A.
Published: February 2008

The combination of plate tectonic models of intraplate stress and observations of the directions of principal midplate stresses provides two types of constraints on the magnitude of deviatoric stress in the lithosphere. First, the fit of global tectonic stress models to observations can constrain stress magnitudes if the magnitude of one of the component driving forces can be independently estimated. Second, if in the vicinity of a calculable local source of stress the observations of stress orientations indicate comparable control by the local and regional stress field, then the magnitude of the regional field may be estimated. Global intraplate stress models have been calculated using a finite element technique for a wide variety of possible driving force combinations. The best fitting global stress models include ridge pushing forces as an essential element and have deviatoric stress magnitudes comparable to the horizontal compressive stress exerted by ridge elevation, estimated on independent grounds to be 200–300 bar. Regional stresses in the Indian plate are shown on the basis of midplate earthquake mechanisms to be larger than the ∼100‐bar additional compressive stress exerted by the isostatically compensated topography of the Ninetyeast Ridge. In the vicinity of Hawaii, however, plate bending stresses of perhaps 1‐kbar or greater magnitude may dominate regional stress. An average level of several hundred bars for the magnitude of typical intraplate deviatoric stresses is consistent with these various constraints.
Frictional heating, fluid pressure, and the resistance to fault motion
Authors: Lachenbruch Arthur H.
Published: February 2008

Expansion of pore fluid caused by frictional heating might have an important effect on the factional resistance and temperature during an earthquake and a controlling influence on the physics of the earthquake process. When confined water is heated, the pressure increases rapidly (≳10 bars/°C). As Sibson (1973) has pointed out, this could cause a sharp reduction of effective normal stress and dynamic friction on the fault surface. Whether or not this transient stress reduction occurs depends upon the tandem operation of several processes, any of which can break the chain that links frictional heat to frictional stress: the friction must cause an appreciable temperature rise (imposing conditions on the width of the shear zone and rate of conductive transport); the temperature rise must cause an appreciable fluid pressure rise (imposing conditions on the rate of pore dilatation or hydrofracturing, and the rate of Darcian transport); the fluid pressure rise must cause an appreciable reduction of friction (requiring the presence of a continuous fluid phase). Each process depends upon event duration, particle velocity, and the initial value of dynamic friction. With the present uncertainty in the controlling parameters (principally permeability, width of the shear zone, initial stress, and factors controlling transient hydrofracture and pore dilatation) a wide variety of fault behavior is possible. Limits to fault behavior for various ranges of the controlling parameters can be estimated from the governing equations, however, and results can be summarized graphically. If the effective stress law applies and pore dilatation is unimportant, dynamic friction would drop from an initial value of 1 kbar to ∼100 bars when shear strain reached 10 for most earthquakes if the permeability were less than 0.1 μdarcy; the maximum temperature rise would be only ∼150°C irrespective of final strain. If the permeability were ≳100 mdarcies, however, friction would be unaffected by faulting and temperatures could approach melting for shear strains ∼20. For permeabilities ∼1 mdarcy, friction could be reduced appreciably during large earthquakes, but during small ones it could not. Combined with thermal effects, dilatational strain of a few percent of pore volume could lead to virtually frictionless faulting or increasing frictional resistance, dependeing upon its sign; unstable propagation of hydrofractures (after fluid pressure exceeded the least principal stress) could cause a sudden increase in fault friction. Strengthening due to cooling and Darcian flow at the conclusion of an earthquake could occur in seconds or weeks depending upon event duration, transport parameters, and shear zone width; it might influence the redistribution of stress by aftershocks.
Limits on lithospheric stress imposed by laboratory experiments
Authors: Brace W. F. , Kohlstedt D. L.
Published: February 2008

Laboratory measurements of rock strength provide limiting values of lithospheric stress, provided that one effective principal stress is known. Fracture strengths are too variable to be useful; however, rocks at shallow depth are probably fractured so that frictional strength may apply. A single linear friction law, termed Byerlee's law, holds for all materials except clays, to pressures of more than 1 GPa, to temperatures of 500°C, and over a wide range of strain rates. Byerlee's law, converted to maximum or minimum stress, is a good upper or lower bound to observed in situ stresses to 5 km, for pore pressure hydrostatic or subhydrostatic. Byerlee's law combined with the quartz or olivine flow law provides a maximum stress profile to about 25 or 50 km, respectively. For a temperature gradient of 15°K/km, stress will be close to zero at the surface and at 25 km (quartz) or 50 km (olivine) and reaches a maximum of 600 MPa (quartz) or 1100 MPa (olivine) for hydrostatic pore pressure. Some new permeability studies of crystalline rocks suggest that pore pressure will be low in the absence of a thick argillaceous cover.
Dislocations and nonelastic processes in the mantle
Authors: Minster J. Bernard , Anderson Don L.
Published: February 2008

Dislocations in solids contribute to anelastic attenuation, relaxation of the shear modulus, transient creep, and steady state flow. These properties of the mantle may therefore be related. The glide and climb of dislocations appear to have the appropriate time constants to explain seismic wave attenuation and mantle viscosity, respectively. The dislocation density of the mantle depends on the ambient stress. The characteristic time scales of dislocation relaxation depend on dislocation length and temperature. These time scales for the mantle can be inferred from seismic wave attenuation and postglacial rebound, thereby potentially yielding information about dislocation density, stress, and temperature. The thickness of the ‘rheological’ lithosphere depends on stress and duration of load as well as age. Kilobar level stresses can be supported in the lithosphere for times greater than 106 years. The relaxation time decreases rapidly with temperature. The asthenosphere can therefore only support small stresses on time scales of geological interest.
Seamount loading and stress in the ocean lithosphere
Authors: Lambeck Kurt , Nakiboglu S. M.
Published: February 2008

One of the principal arguments for large stress differences in the lithosphere comes from the modeling of the deflection of the ocean lithosphere under seamount loads. Most published models indicate maximum stress differences of 2–3 kbar per kilometer of deflection, and maximum values approach 10 kbar. Geophysical support for the elastic plate theory comes mainly from gravity, which requires a broad negative anomaly on the flanks of the actual seamount, although these anomalies are not very sensitive to the density structure of the crust or to the density of the sediment fill‐in or the extent of this fill‐in. The stress in the plate, however, is sensitive to these parameters, and in consequence, gravity is not a reliable indicator of the stress state. The lithospheric flexure model has been examined in some detail to evaluate this stress dependence on crustal densities and rheology. The approximations inherent in the linear theory have also been investigated. The main conclusions are that the stress differences can be significantly reduced (1) by adopting a lower density for the sediment fill‐in than the usual 2.8 g cm−3, (2) by introducing a depth‐dependent nonelastic rheology, and (3) by introducing large‐deflection theory for the larger loads. For large loads, such as Oahu Island discussed by Watts (1978), the maximum stress under the load can be reduced to about 2 kbar or less near the upper surface of the plate, and the maximum stress differences σrr ‐ σzz need not exceed 1 kbar. The introduction of the nonelastic rheology results in a stress, temperature, and load duration dependence of the ‘apparent’ flexural rigidity; large loads, loads of long duration, or loads on a high‐temperature lithosphere all result in a thinner apparent plate thickness than small loads, loads of short duration, or loads on a cool lithosphere, all other factors being equal.
Global gravity field to degree and order 30 from Geos 3 satellite altimetry and other data
Authors: Gaposchkin E. M.
Published: February 2008

A description of the earth's gravity field in spherical harmonics through degree and order 30 is obtained by combining satellite‐to‐sea‐surface altimetry data from Geos 3, terrestrial gravity measurements, and satellite perturbation analysis. Reference orbits and a tide model are used with the altimetry data to calculate sea‐surface heights, and these heights are assumed to be the geoid. The altimeter‐determined geoid heights are averaged for each 1°×1° ocean‐surface area, with an average root‐mean‐square uncertainty of approximately 2 m. The 25,295 1°×1° estimates are then treated by autocovariance analysis to obtain 550‐km×550‐km block geoid undulations. A set of 37,415 1°×1° surface‐gravity estimates is treated in the same way to obtain block gravity anomalies. The combined sets of blocks provide an estimate of the geopotential in each of 1635 out of 1654 possible surface elements. The two sets of block anomalies then give a set of normal equations for the spherical‐harmonics coefficients and are combined with the normal equations obtained from laser data on ten close‐earth satellites. Also obtained is the earth's semimajor axis: αe = 6,378,138.23 ± 1.3 m.
Nature and origin of hematite in the Moenkopi Formation (Triassic), Colorado Plateau: A contribution to the origin of magnetism in red beds
Authors: Walker Theodore R. , Larson Edwin E. , Hoblitt Richard P.
Published: February 2008

Petrographic studies show that hematite is present in the Moenkopi Formation in at least five and possibly six forms: (1) microcrystalline hematite, (2) crystals of specular hematite, (3) polycrystalline and monocrystalline grains, (4) grains of partly hematitized ilmenite, (5) grains composed of primary ilmenite‐hematite intergrowths, and (6) ultrafine pigment. The microcrystalline hematite and crystals of specular hematite are unequivocally authigenic. They form cement in interstitial and secondary voids, and they have replaced detrital iron‐bearing silicate minerals. Furthermore, microcrystalline hematite is superimposed on other authigenic cementing minerals such as potassium feldspar, calcite, dolomite, and quartz, and in some cases it has replaced authigenic pyrite. In addition, both microcrystalline and specularite crystals are common daughter products of intrastratally altered biotite grains. Thermodynamic considerations coupled with studies of hematite‐magnetite relationships in modern sediments indicate that most of the hematite in the polycrystalline grains, and probably the monocrystalline grains as well, was formed authigenically by post‐depositional replacement of detrital grains of magnetite. The ilmenite probably has similarly altered in situ to hematite. The only hematite of unquestionable detrital origin in the red beds is the hematite in the ilmenite‐hematite intergrowths (‘tiger striped’ grains) and that in monocrystalline detrital grains containing rutile exsolution platelets, both of which are products of high‐temperature processes. With the exception of the ultrafine pigment, each of the above forms is coarser grained than the superparamagnetic threshold for hematite, and therefore each contributes components of remanent magnetism to the rocks. Inasmuch as most of the hematite varieties represent authigenic products of intrastratal alterations that require considerable geologic time, we conclude that the red bed remanence is largely chemical remanent magnetization (CRM) acquired over long time intervals. The pigment in the Moenkopi red beds consists partly of authigenic ultrafine red iron oxide and partly of translucent microcrystalline hematite. The ultrafine red iron oxide may or may not be hematite, but even if it is, the grain size probably lies below the paramagnetic threshold for hematite. Much of the pigment, therefore, may not contribute greatly to the remanent magnetism in the rocks.
On the applicability of thermal conduction models to mid‐plate volcanism: Comments on a paper by Gass et al.
Authors: Vogt P. R.
Published: February 2008

The proposition that mid‐plate (‘hot spot‘) magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate. Although hot spots perhaps tend to ‘avoid’ cratonic areas, the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle. Plate thickness or speed is of lesser importance. The high level of African mid‐plate volcanism since 25 m.y. B.P. appears to be part of a global pattern controlled by events in the mantle rather than a result of the Africa plate coming to rest over sublithospheric thermal sources about 40 m.y. B.P.
Relationship of the 1979 Southern California Radon Anomaly to a possible regional strain event
Authors: Shapiro M. H. , Melvin J. D. , Tombrello T. A. , Mendenhall M. H. , Larson P. B. , Whitcomb J. H.
Published: February 2008

During the second half of 1979, anomalously high emanation of radon was recorded at two stations of the automated radon‐thoron monitoring network operated by the W. K. Kellogg Radiation Laboratory of the California Institute of Technology. The two stations exhibiting major anomalies, Kresge and Dalton Canyon, are located approximately 30 km apart on the frontal fault system of the Transverse Ranges of southern California. At Kresge the anomaly began on June 21, 1979, and continued through December 1979. At Dalton Canyon the anomaly started about 3 weeks later and also continued through December 1979. At both sites the anomalous levels of radon decreased (but did not return entirely to normal values) shortly before October 15, 1979. During the week of October 15, 1979, a 6.6‐M earthquake occurred about 290 km to the southeast of the two stations, and later in that week, earthquakes of magnitude 4.2 and 4,1 occurred at Malibu and Lytle Creek. The latter two events were within 60 km of the monitors. A radon‐thoron monitor at Lytle Creek recorded no long‐term anomaly but did record a sharp spikelike decrease in the radon level on October 13, 1979. Coincident with our observations of anomalous radon levels, other investigators have reported anomalies or suspected anomalies in several other geodetic, geophysical, and geochemical signals from the same general region. The rapid temporal development of several of the anomalies together with the large area over which they were observed suggests that a large‐scale strain event took place which may have been responsible both for the widespread anomalies and for the seismicity that occurred in the region subsequent to the onset of the anomalies.
Detection of tectonomagnetic events using multichannel predictive filtering
Authors: Davis P. M. , Jackson D. D. , Searls C. A. , McPherron R. L.
Published: February 2008

High‐precision geomagnetic measurement depends on eliminating variations of ionospheric and magnetospheric origin. The commonly used technique of simply taking differences between total field magnetometers is only partially successful. Our method involves finding the multichannel Wiener filters which predict the field variation at a given total field magnetometer of an array from the fields of the remaining magnetometers and a three‐component magnetometer. The difference is then taken between the total field and the predicted field, leaving a cleaned total field. Filter lengths and number and choice of input channels are determined using methods of statistical parametric model fitting. The resulting filters which are defined on short noise free record sections are found to be effective over the remainder of the record. Detailed analysis of data from the University of California, Los Angeles, array in southern California, for which we have the best vector data, shows that for optimal predictive cleaning it is essential to use vector components and that only the two components orthogonal to the total field direction are necessary. This means that the field‐aligned component may be omitted from a vector station, which simplifies the instrumentation considerably. Analysis of the standard deviation of the residuals after cleaning (0.1 nT for hourly averages) shows that it is 8 times that expected from digitization noise alone. The most probable explanation for this is that measurement noise is about 3 times the digitization interval (0.25 nT) and that higher‐precision measurements are required for further improvement. This conclusion has been confirmed by tests on the instruments at close spacing. Application of the method to data taken in central California and Hawaii reveals tectonomagnetic effects which are otherwise hidden in noise. Wiener filters are especially suitable for real‐time analysis, which is an important factor in earthquake prediction.
The effect of stress on radon emanation from rock
Authors: Holub R. F. , Brady B. T.
Published: February 2008

A uranium‐bearing granitic rock of known radon emanation was deformed to failure under uniaxial stress. Radon emanation and microcrack activity were recorded continuously during the test. Radon concentration in the closed system was measured by an alpha detector. A decrease in emanation was observed during the initial loading of the sample and is interpreted as associated with the closure of pre‐existing cracks within the rock. At a uniaxial stress equal to approximately one‐half the ultimate strength, a temporary increase (50%) in the radon emanation was observed that correlated with an increase in microcracking within the sample. When the stress was removed for several hours, the original emanation was restored. However, once the specimen failed, a temporary emanation increase of 120% was observed. The emanation then decreased to a level nearly 5% greater than the prefailure specimen emanation.
Metamorphism at the base of the Samail Ophiolite, southeastern Oman Mountains
Authors: Ghent Edward D. , Stout Mavis Z.
Published: February 2008

Metamorphic rocks, showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies, occur beneath the Samail ophiolite. The metamorphic protolith was a section of marine basalts, Mn‐rich cherts, and argillites. Garnet‐clinopyroxene amphibolites occur within 2 m of the contact, hornblende‐clinopyroxene assemblages occur within 80 m of the contact, and lower amphibolite facies rocks persist to 130 m from the contact. The higher‐grade rocks show polyphase deformation, cataclastic fabrics, and partial retrogression to greenschist facies assemblages; however, lower‐grade phyllites show evidence of only one episode of metamorphic crystallization. Unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation. The distribution coefficient KD = FeO/MgO gar/FeO/MgO cpx ranges from 4.4 to 5.3, suggesting a T range of 755°– 865°C at 200 MPa. Low jadeite content in clinopyroxene and low glaucophane content of Ca amphiboles are consistent with moderate pressures of crystallization. The metamorphic sheet is inferred to be of composite origin. The highest‐grade rocks were tectonically transported the greatest distance, and the lower‐grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks. Residual heat from the ophiolite is the dominant heat source for the metamorphism, and this could have been augmented by a limited amount of frictional heat generated during thrusting. These thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the Samail ophiolite.
Sm‐Nd, Rb‐Sr, and 18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail Ophiolite
Authors: McCulloch Malcolm T. , Gregory Robert T. , Wasserburg G. J. , Taylor Hugh P.
Published: February 2008

The Sm‐Nd, Rb‐Sr, and 18O/16O isotopic systems have been used to distinguish between the effects of seafloor hydrothermal alteration and primary magmatic isotopic variations. The Sm‐Nd isotopic system is essentially unaffected by seawater alteration, while the Rb‐Sr and 18O/16O systems are sensitive to hydrothermal interactions with seawater. Sm‐Nd mineral isochrons from the cumulate gabbros of the Samail ophiolite have an initial 143Nd/144Nd ratio of εNd = 7.8 ± 0.3, which clearly substantiates the oceanic affinity of this complex. The initial 143Nd/144Nd ratios for the harzburgite, plagiogranite, sheeted diabase dikes, and basalt units have a limited range in εNd of from 7.5 to 8.6, indicating that all the lithologies have distinctive oceanic affinities, although there is also some evidence for small isotopic heterogeneities in the magma reservoirs. The Sm‐Nd mineral isochrons give crystallization ages of 128 ± 20 m.y. and 150 ± 40 m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh, which is approximately 300 km NW of Ibra. These crystallization ages are interpreted as the time of formation of the oceanic crust. The 87Sr/86Sr initial ratios on the same rocks have an extremely large range of from 0.7030 to 0.7065 and the δ18O values vary from 2.6 to 12.7. These large variations clearly demonstrate hydrothermal interaction of oceanic crust with seawater.
Permeability of granite in a temperature gradient
Authors: Morrow C. , Lockner D. , Moore D. , Byerlee J.
Published: February 2008

Changes in permeability of granite were measured as water flowed through samples in a temperature gradient. The experimental conditions simulated those around the ‘very deep hole concept’ nuclear waste repository. Temperature was maintained between 200° and 310°C, in a borehole of a cylindrical sample. Confining pressures of 30 and 60 MPa, with corresponding pore pressures of 10 and 20 MPa, simulated depths of burial of approximately 1.2 and 2.4 km, respectively. A small pore pressure gradient enabled distilled water to flow from the borehole (high temperature) to the outside of the sample (low temperature). Tests were run for intact samples with initial permeabilities of several hundred nanodarcies and for samples containing throughgoing fractures, with initial permeabilities of about 1 millidarcy. In all cases, permeability decreased between 1 and 2 orders of magnitude at a rate that increased with higher temperatures. At 200°C, permeability dropped by an order of magnitude over a 1‐month period, whereas at 310°C, permeability dropped sharply within a few days to 5% of the initial value. The dissolution of quartz and feldspar and redeposition of these minerals within cracks at lower temperature was found to be the major cause of reduction of permeability. If similar processes occurred near a deep hole nuclear waste repository site in granite, then migration of radionuclides away from the site might be suppressed, even if the rock surrounding the waste contained fractures.
Bidirectional reflectance spectroscopy: 1. Theory
Authors: Hapke Bruce
Published: February 2008

An approximate analytic solution to the radiative transfer equation describing the scattering of light from particulate surfaces is derived. Multiple scattering and mutual shadowing are taken into account. Analytic expressions for the following quantities are found: bidirectional reflectance, radiance factor, radiance coefficient, normal, hemispherical, Bond, and physical albedos, integral phase function, phase integral, and limb‐darkening profile. Scattering functions for mixtures can be calculated, as well as corrections for comparing experimental laboratory transmission or reflection spectra with observational planetary spectra. An expression for the scattering efficiency of an irregular particle large compared with the wavelength is derived. For closely spaced, nonopaque particles this efficiency is approximated by (1 + αDe)−l, where α is the true absorption coefficient and De is an effective particle diameter of the order of twice the mean particle size. For monomineralic surfaces it is shown that α = ( 1 − w)/wDe, where w is the single‐scattering albedo and can be determined from reflectance measurements of a powder, so that α may be calculated from reflectance. This theory should be useful for interpretations of reflectance spectroscopy of laboratory surfaces and photometry of solar system objects. From photometric observations of a body the following may be estimated: average single‐scattering albedo, average particle phase function, average macroscopic slope, and porosity.
The spectral reflectance of water‐mineral mixtures at low temperatures
Authors: Clark Roger N.
Published: February 2008

Laboratory reflectance spectra in the 0.325–2.5μm region of bound water, water‐mineral mixtures, mineral grains on frost, and frost on minerals are presented. The materials used in this study are montmorillonite, kaolinite, beryl, Mauna Kea red cinder, and black charcoal. It is found that the wavelengths of bound water and bound OH absorptions do not shift appreciably with temperature and can be detected when large amounts of free water ice are present. The decrease in the visible reflectance seen in many planetary reflectance spectra containing strong water ice absorptions can be explained by water‐mineral mixtures, mineral grains on frost, or frost on mineral grains. Mineral grains on frost are detectable in very small quantities (fractional areal coverage ≲0.005) depending on the mineral reflectance features, while it takes a thick layer of frost (≳1 mm) to mask a mineral below 1.4 μm, again depending on the mineral reflectance. Frost on a very dark surface (albedo about 6%) is easily seen; however, a dark mineral mixed with water could completely mask the water absorptions (shortward of 2.5 μm).
Martian ages
Authors: Neukum G. , Hiller K.
Published: February 2008

The subjects of this paper are a discussion of the methodology of relative age determination by impact crater statistics, a comparison of currently proposed Martian impact chronologies for the determination of absolute ages from crater frequencies, a report on our work of dating Martian volcanoes and erosional features by impact crater statistics, and an attempt to understand the main features of Martian history through a synthesis of our crater frequency data and those published by other authors. Two cratering chronology models are presented and used for inference of absolute ages from crater frequency data: model I, with nearly equal Martian and lunar cratering rates around (ca.) 4‐ to 10‐km crater sizes, and model II, equivalent to model I for ages > 3.5 · 109 years but with a factor of 2 higher Martian cratering rate at ages <3 · 109 years. Those model cratering chronologies are applied to the data. The interpretation of all crater frequency data available and tractable by our methodology leads to a global Martian geological history that is characterized essentially by two epochs of activity. The division between the two epochs is measured at a cumulative crater frequency value for 1‐km craters (crater retention age) of N(1) = 8 · 10−4 (km−2) corresponding to an absolute age of ca. 3 · 109 years (applying model I cratering chronology) and of ca. 1.5 · 109 years (applying model II cratering chronology). In the ancient epoch all major events like emplacement of the plains lavas, the piling up of most volcanic constructs, and large‐scale erosion of channels and mensae (highland/northern lowland boundary) have taken place. During the younger epoch, only the big Tharsis shield volcanoes were active, and some minor erosion took place. This means that Mars is not a youthful planet but an ancient one with respect to most of its surface features.
Passive margins: A model of formation
Authors: Le Pichon Xavier , Sibuet Jean‐Claude
Published: February 2008

The stretching model of McKenzie is applied to the formation of passive continental margins, assuming local isostatic equilibrium. We present the quantitative implications of the model; we then discuss its fit to the IPOD data on the Armorican and Galicia continental margins of the northeast Atlantic. The amount of brittle stretching observed in the upper 8 km of the prestretched continental crust reaches a maximum value of about 3. This large amount of thinning is comparable to the thinning of the whole continental crust observed by seismic refraction measurements and required by the model for the whole lithosphere. This agreement suggests that the simple stretching model is a good first approximation to the actual physical process of formation of the margin. It is thus possible to compute simply the thermal evolution of the margin and to discuss its petrological consequences. It is also possible to obtain a quantitative reconstruction of the edge of the continent prior to breakup. Finally, the large slope of the base of the lithosphere during the formation of the margin results in a force similar but opposite to the ‘ridge‐push’ force acting on accreting plate boundaries.
The P velocity within the Tonga Benioff Zone determined from traced rays and observations
Authors: Huppert Lawrence N. , Frohlich Cliff
Published: February 2008

P waves with travel time residuals between 0 s and −12 s are observed at regional stations in Samoa (AFI) and Raoul Island (RAO) for 39 earthquakes in Tonga with focal depths between 70 km and 300 km. These anomalously large residuals apparently are produced because seismic phases travel along the strike of the Tonga Benioff zone within the high‐velocity subducted lithosphere for up to 1200 km before arriving at AFI and RAO. To eliminate erroneous residuals caused by poor event locations, we selected 11 stations and reread the available P times at these stations for the 39 events. These arrivals and (pP‐P) intervals were used to relocate the events using a variant of the joint hypocenter determination method. Then the pattern of residuals at AFI and RAO (not used in the relocation) was compared to the pattern of residuals expected for various models of the subducted lithosphere, as determined by ray tracing. The observed pattern of residuals at AFI is consistent with the ray‐traced models if some of the first arrivals are produced by rays traveling directly along the strike of the subducting lithosphere, and if others are produced by rays which reflect once off the upper surface of the subducting lithosphere before arriving at AFI. The observed residuals can be explained by a model where the P velocity in the subducted lithosphere is 8% higher than the velocity in the Herrin model. The residuals are fit even better by a layered slab model in which the seismic velocity is about 6% higher than the Herrin velocity at the upper surface of the subducted lithosphere and about 9% higher at the bottom of the slab. These velocity contrasts could be produced if the temperature in the slab was 700°C cooler than the surrounding mantle, and if there were no partially melted material within the slab. The existence of these anomalously large residuals suggests that the high velocity region in the upper 300 km of the mantle beneath Tonga must be fairly continuous over distances of 1000 km and more.
Seismicity at intersections of spreading centers and transform faults
Authors: Rowlett Hugh
Published: February 2008

Ocean‐bottom seismographs were used in a microearthquake monitoring experiment at the eastern junction of the Oceanographer transform with the mid‐Atlantic ridge at 35°N. Microearthquake activity at the junction occurred over a broad area (>7 km). These microearthquakes ‘cut across’ the corner between the transform and median valley and are associated with fault scarps that form the inner walls on the west and north sides of the median and transform valleys. At intersections of other major fracture zones (>100‐km offset) and slow to moderate spreading centers microearthquake activity is also diffuse and cuts across the corner between the spreading center and transform fault. The narrow zone of decoupling (∼1 km) observed between spreading center and transform boundaries by detailed geological studies at the Tamayo/East Pacific Rise and Vema/mid‐Atlantic Ridge intersections suggest that the diffuse seismicity (20 to 30 km in width) does not reflect a diffuse plate boundary at the transition from rift to transform valley. Instead, the faulting probably reflects internal deformation of the corner by secondary faults off of the plate boundary.