bluuebunny/crossref_metadata_2025 huggingface dataset:
The rupture process of the Great 1979 Colombia Earthquake: Evidence for the asperity model
Authors: Beck Susan L. , Ruff Larry J.
Published: February 2008

We have determined the asperity distribution in the segment of the Ecuador‐Colombia subduction zone that was ruptured by the 1979 (Mw = 8.2) and 1958 (Mw = 7.7) earthquakes. For the 1979 earthquake, we are able to document directivity in the P‐wave source time functions and consequently, have spatially located the moment release. By combining results from both P‐wave and surface wave directivity, we have defined three regions: (1) from 0 to 56 km northeast of the epicenter, with low moment release and a small seismic displacement, (2) the asperity, from 56 to 116 km northeast of the epicenter, with most of the moment release and a displacement of ∼6 m, and (3) a small‐displacement region from 116 to 180–240 km northeast of the epicenter. Thus the 1979 rupture zone is composed of an asperity 60 km in length, bounded by two weak (small displacement) regions. The displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 (the year of the previous great earthquake in this segment). The 1958 earthquake is characterized by the rupture of a small asperity (length scale approximately 25 km) in the epicentral region. The 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event. This detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence.
Gravity and magnetic evidence for a granitic intrusion near Wahmonie Site, Nevada Test Site, Nevada
Authors: Ponce David A.
Published: February 2008

Gravity and magnetic data outline a broad anomaly near Wahmonie Site, Nye County, Nevada. A positive 15‐mGal gravity anomaly with a steep western gradient and a broad magnetic anomaly coincident with the gravity high characterize the area. Two‐dimensional computer models of the gravity data were made using magnetic, seismic, and electric data as independent constraints. The models indicate the presence of a shallow, relatively high density body of 2.65 kg m−3 buried near Wahmonie Site. Aeromagnetic and ground magnetic data also indicate the presence of a large, shallow body. Two smaller local magnetic highs that occur along a magnetic prominence extending northward from the broad anomaly directly correlate to granodiorite outcrops. This indicates that the main anomaly is produced by a large shallow intrusion.
Residual geoid anomalies in Atlantic Ocean Basin: Relationship to mantle plumes
Authors: Bowin Carl , Thompson Geoffrey , Schilling J. G.
Published: February 2008

The relations of geoid data from radar altimeter measurements (GEOS 3) to water depth and geochemical variations along the Mid‐Atlantic Ridge are investigated. Eight geophysical lines across the ridge in the Atlantic Ocean were constructed approximately normal to the local ridge trend. Slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography, which has better symmetry between the two flanks. Subtracting a regional field, based on previous work, produces residual geoid anomalies whose source must be in the earth's outer 600 km. The residual geoid anomalies of the North Atlantic show striking correlation with the axis of the Mid‐Atlantic Ridge, but those in the South Atlantic do not. Instead, major positive residual geoid anomalies appear over the Rio Grande and Walvis ridges. Determination of slope S, residual geoid per million years crustal age, reveals that S increases with greater residual geoid anomaly and with higher (shallower) crestal ridge topography. The slope of 22 cm/m.y. west of the Azores high (near 38°N) suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin, which may normally produce slopes only of about 8 cm/m.y. as found near 28°N away from shallow ridge topography. To account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940°C between the two sites, which appears excessive. Also, the decrease in topographic height with crustal age away from the Azores high is much greater than predicted by theoretical cooling curves. Correlation of residual geoid anomalies with depth, geochemical variations along the length of the Mid‐Atlantic Ridge, and the chemistry of the Rio Grande Rise and Walvis Ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects.
Tidal terms in universal time: Effects of zonal winds and mantle Q
Authors: Merriam J. B.
Published: February 2008

The zonal response coefficient κ is studied, and some improvements in its determination are made. First, the reprocessing of the Bureau International de l'Heure data with the 1980 International Astronomical Union nutation series is found to result in improved estimates of κ. Second, a time series of the angular momentum of the atmosphere is found to have power in the tidal bands, and it is demonstrated that removing the atmospheric influence from the rotation data leads to better estimates of κ. The frequency dependence of κ due to finite dissipation in the earth is computed, and the observations are subsequently shown to limit the allowable models of dissipation. If Q varies with frequency σ, as Q∼σα, then the Mf and Mm terms in UT place an upper bound of about 1/3 on α. Finally, dynamic ocean tide models are studied, and it is concluded that the rotation data cannot distinguish between these and an equilibrium tide.
The α, β, γ phase relations in Fe2SiO4‐Mg2SiO4 and Co2SiO4‐Mg2SiO4: Calculation from thermochemical data and geophysical applications
Authors: Navrotsky Alexandra , Akaogi Masaki
Published: February 2008

On the basis of thermochemical data for olivine (α), modified spinel (β), and spinel (γ) polymorphs of Mg2SiO4, Fe2SiO4, and Co2SiO4, the binary Mg2SiO4‐Fe2SiO4 and Mg2SiO4‐Co2SiO4 phase diagrams are calculated. From Mg/Fe partitioning data between olivine‐garnet, β‐phase‐garnet, olivine‐pyroxene, and β‐phase‐pyroxene, thermochemical parameters for β‐Fe2SiO4 are estimated. The β‐Fe2SiO4 is highly metastable with respect to γ‐Fe2SiO4. The calculated binary Mg2SiO4‐Fe2SiO4 phase diagram shows a narrow field of α+β and a broad field of β+γ, in reasonable agreement with experimental phase relations. The composition range over which β phase solid solution is stable expands with increasing temperature but is generally limited to Mg/(Fe + Mg) > 0.8. In the Mg2SiO4‐Co2SiO4 system, the β phase is calculated to occur only at Mg/(Co + Mg) < 0.05 and Mg/(Co + Mg) > 0.6, with α and γ coexisting at intermediate compositions. Using the above phase relations in Mg2SiO4‐Fe2SiO4 and assuming an adiabatic geotherm, temperatures are calculated to range from about 1573 to 1700 K at depths of 400–520 km for a mantle of Iherzolite composition.
Pole of rotation analysis of present‐day Juan de Fuca Plate motion
Authors: Nishimura Clyde , Wilson Douglas S. , Hey R. N.
Published: February 2008

Convergence rates between the Juan de Fuca and North America plates are calculated by means of their relative, present‐day pole of rotation. Subduction rates, as measured normal to the strike of the Washington‐Oregon trench, are predicted to range from about 40 mm/yr under Washington to 20 mm/yr under Oregon. An integral part of this analysis is the formulation of a numerical inversion scheme which calculates both the pole of rotation between two plates and the associated errors. This method is used to determine the Euler pole for Pacific‐Juan de Fuca. This pole can then be vectorially added to previously published poles for North America‐Pacific and “hot spot”‐Pacific to obtain North America‐Juan de Fuca and “hot spot”‐Juan de Fuca, respectively. The errors associated with these resultant poles are also determined by propagating the errors of the two summed angular velocity vectors. With the assumption that hot‐spots are fixed with respect to a mantle reference frame, if is computed that the average absolute velocity of the Juan de Fuca plate is approximately 15 mm/yr, thereby making it the slowest moving of the oceanic plates.
Transitional paleointensities from Kauai, Hawaii, and geomagnetic reversal models
Authors: Bogue Scott W. , Coe Robert S.
Published: February 2008

Thellier paleointensity results from an R‐N transition zone in Kauai, Hawaii, show that field intensity dropped from 0.431 Oe to 0.101 Oe while the field remained within 30° of the reversed axial dipole direction. A recovery in intensity and the main directional change followed this presumably short period of low field strength. As the reversal neared completion, the field had an intensity of 0.217 Oe while still 40° from the final direction. The relationship of paleointensity to field direction during the early part of the reversal thus differs from that toward the end, a feature that only some reversal models are consistent with. For example, a model in which a standing nondipole component persists through the dipole reversal predicts only symmetric intensity patterns. In contrast, zonal flooding models generate suitably complex field behavior if multiple flooding schemes operate during a single reversal or if the flooding process is itself asymmetric.
Planetocentric versus heliocentric impacts in the Jovian and Saturnian Satellite System
Authors: Horedt G. P. , Neukum G.
Published: February 2008

Readily applicable equations are derived for calculating the impact velocities, collision intervals, and cumulative crater frequencies in a satellite system for planetocentric and heliocentric impactors. Observed cumulative crater frequencies in the Saturnian satellite system and a sometimes observed lack of apex‐antapex asymmetry of crater frequencies favor crater‐producing projectiles orbiting initially in elliptic orbits round the parent planet (planetocentric impactors). On the other hand, a sometimes observed asymmetry of crater size distribution and short collision intervals (<104 years) of planetocentric projectiles favor heliocentric impactors. We conclude that cratering on the Jovian and Saturnian satellites is produced by heliocentric projectiles as well as by captured planetocentric impactors. Small satellites could be crushed several times by major impacts and reaccrete from planetocentric orbits.
Geoid heights over the Louisville Ridge (South Pacific)
Authors: Cazenave Anny , Dominh Kien
Published: February 2008

Geoid heights over the Louisville Ridge seamount chain, deduced from the Seasat satellite altimeter data, have been analyzed and interpreted. Two uncharted seamounts belonging to the chain have been detected. The elastic thickness of the oceanic lithosphere has been determined along the chain. This was done rigorously using the three‐dimensional geoid and bathymetry maps and was also computed approximately using spectral methods on geoid and bathymetry profiles. A slightly increasing elastic thickness from the southeast to the northwest, in the range 15.5–22.5 km has been estimated.
Possible perturbations to hotspot traces and implications for the origin and structure of the Line Islands
Authors: Epp David
Published: February 2008

Perturbations to hotspot signals recorded on the surface of the earth will produce hotspot traces with characteristics more complex than those of simple time‐progressive volcanic chains. Some of these perturbations are a consequence of normal plate tectonics, some are observed interactions of hotspot magmatism with lithospheric structure and processes, and others are hypothetical. The Line Islands volcanic chain has a complex structure and some characteristics that cannot be reconciled with those expected of a simple time‐progressive hotspot trace. Two models for the development of the Line Islands are presented. Model 1 assumes that the Line Islands were formed between 130 and 67 Ma, and model 2 assumes an age span of 90 to about 15 Ma and incorporates the Tuamotu Islands as an extension of the Line Islands. In model 1 most of the complex structure and subsidence of the chain can be explained in terms of perturbations to the hotspot signal. Model 2 gives a good geometric fit to the volcanic chain but fails to explain the anomalous subsidence of the Line Islands.
Thermomechanical evolution of oceanic fracture zones
Authors: Sandwell David T.
Published: February 2008

A fracture zone (FZ) model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere. As the lithosphere cools by conduction, thermal and mechanical boundary layers develop and increase in thickness as (age)1/2. Surface expressions of this development, such as topography and deflection of the vertical (i.e., gravity field), are most apparent along major FZ's because of the sharp age contrast. A simple model, including the effects of lateral heat transport but with no elastic layer, predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the FZ ages. Observations, however, show that both the FZ topography and the deflection of the vertical remain sharp as the FZ evolves. These two observations, as well as the observed asymmetry in deflection of the vertical profiles across the Udintsev, Romanche, and Mendocino FZ's, are explained by including a continuous elastic layer in the model. The asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the FZ. Modeling also shows that the evolution of the FZ topography is extremely sensitive to the initial thermal structure near the ridge‐transform intersection. Model geoid steps and their development with age are used to access techniques for measuring geoid offsets across FZ's. Reasonable step estimation techniques will underestimate the overall step amplitude by up to 50%. This implies that abnormally thin thermal boundary layers, derived from studies of geoid height versus age, are not required by the data.
Determination of stress from slip data: Faults and folds
Authors: Michael Andrew J.
Published: February 2008

A new technique is derived to invert slickenside data for the stress field that caused the faulting episode. This inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes, at the time of rupture, is similar. Study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption. Hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion. Application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes. Comparison of the inversion results with the geometry of the folds shows this application to be successful, greatly increasing the number of data sets that can be used to find the paleostress field.
Magnesian anorthosites and associated troctolites and dunite in Apollo 14 breccias
Authors: Lindstrom Marilyn M. , Knapp Steven A. , Shervais John W. , Taylor Lawrence A.
Published: February 2008

Magnesian anorthosite, a new type of pristine lunar highlands rock, has been found in Apollo 14 breccias. It has primitive (high Ca and Mg) silicate mineral compositions, and high and variable REE concentrations. Variations in REE contents can be accounted for by variations in modal abundance of REE‐rich apatite. Magnesian anorthosites are associated with troctolites and a dunite with very similar mineral compositions and it is suggested that all crystallized from a differentiated troctolitic intrusion. The origin of the REE‐rich apatite is enigmatic. It is unlikely to have crystallized from an igneous liquid in equilibrium with the major minerals in the anorthosite. Possible origins are assimilation of urKREEP or metasomatism by REE‐rich fluids. REE‐rich alkali anorthosites and gabbronorites are also found and are likely to be related to KREEP basalt magmas. Lunar compositional associations are distributed in a regional rather than global manner.
Stratigraphy and geochemistry of the Stone Mountain Core (64001/2)
Authors: Korotev Randy L. , Morris Richard V. , Lauer Howard V.
Published: February 2008

The double drive tube core 64001/2 collected on Stone mountain, station 4, Apollo 16 was expected to contain the best samples of material representative of the Descartes formation. Based on the concentrations of 24 elements in one sample from each of the 68 half‐centimeter dissection intervals of the lower section of the core tube (64001,26.4–60.5 cm depth), no layer of soil enriched in materials thought to be representative Descartes material was found. However, all samples between 26 and 48 cm contain material of mare origin. Concentrations of mare material extending over 2–3 half‐centimeter dissection intervals occur near 31,42, and 47 cm depth. Samples from these three zones contain up to 7%, 12%, and 5%, respectively, mare‐derived material. Cr/Sc ratios indicate that each zone is composed of a different type of mare basalt. Based on FeO and Fe° concentrations and the surface maturity index Is/FeO measured on samples from all 122 dissection intervals of 64001/2, a layer of material slightly more felsic and less mature than typical core material occurs between 14 and 19 cm. The composition of the remaining core material is nearly constant and generally similar to that of other soils from the south and central part of the landing site. All of the soil in the 64001/2 core is mature with respect to surface exposure except the Fe‐poor layer (14–19 cm) and an Fe‐rich layer (42–43 cm), which are submature. Major changes in the stratigraphy of the core as observed in either the maturity or geochemical data occur near 12, 20, and 48 cm. The 64001/2 soils are a complex mixture of Cayley, Descartes, and other materials, including mare basalt.
Applicability of La‐Ce systematics to planetary samples
Authors: Nakamura Noboru , Tatsumoto Mitsunobu , Ludwig Kenneth R.
Published: February 2008

Ce isotopic compositions in several terrestrial and extraterrestrial materials were determined in order to investigate the applicability of using Ce as an isotopic tracer to geological processes. Owing to the low abundances of 138La and 138Ce in nature, our measurements of 138Ce/ 140Ce ratios of natural samples have relatively large (>0.02%) errors, and the variations in Ce‐isotope ratios were barely resolved. A tenuous anticorrelation was observed between εCe and εNd for terrestrial basalts and granites, indicating that with some improvement in analytical techniques the Ce isotopic composition may prove useful as a tracer for geological processes. A very low εCe predicted from the Ce anomaly in the REE pattern of the Khohar L3 chondrite (Nakamura and Masuda, 1973) was not observed in this study, indicating that the Ce anomaly may be caused by terrestrial contamination or alteration. A low εCe estimated for the source of Nakhla is consistent with our previous estimate (Nakamura et al., 1982) of a LREE depleted source.
Petrogenesis of the SNC (shergottites, nakhlites, chassignites) meteorites: Implications for their origin from a large dynamic planet, possibly Mars
Authors: Smith M. R. , Laul J. C. , Ma M. S. , Huston T. , Verkouteren R. M. , Lipschutz M. E. , Schmitt R. A.
Published: February 2008

Abundances of major, minor, and trace elements were determined in the whole rock and mineral separates of the two distinct lithologies present in the Antarctic achondrite Elephant Moraine A (EETA) 79001 via sequential instrumental (INAA) and radiochemical neutron activation analysis (RNAA). Additional chalcophile, siderophile, and volatile trace elements are reported for the shergottites ALHA 77005, Shergotty, and Zagami. The rare earth element (REE) abundances of each of the lithologies of EETA 79001 (A and B) exhibit chondritic normalized patterns similar to ALHA 77005. The close relationship of the Antarctic shergottites indicates that ALHA 77005 is a residual source produced by incongruent melting of a source similar in bulk composition to EETA 79001A and that EETA 79001B and the interstitial phases in EETA 79001A are the melts produced by such melting episodes. Incorporating previous isotopic studies and our chemical data, we suggest that either fractionation of or contamination by feldspar could account for the different shergottite sources. The large ion lithophile (LIL) trace element abundances of the shergottites require variable but extensive degrees of nonmodal melting of isotopically constrained parent sources. The extreme extent of such melting is hypothesized to be represented by ALHA 77005 as a residual composition. Derivation of a pre‐1.3 AE(109yr) fractionation sequence is suggested in which ol, ol + opx, opx + cpx, and opx + cpx + plag sequentially produce residual liquids capable of accounting for the precursor sources for the SNC meteorites. On the basis of chemical, isotopic, and petrologic considerations we conclude that the SNC sources are consistent with their derivation by extensive fractionation of a primitive magma initially produced from a source having chondritic refractory LIL trace element abundances, Fe′(Fe/Fe+Mg atomic)≲0.3, and a mineralogy primarily of ol and pyx. Our model‐dependent deductions suggest that this magma could be derived without plagioclase or garnet left in the residuum. Petrogenetic and age relationships among SNC meteorites suggest a single complex‐provenance on a dynamic planet not unlike the earth. Considering that each of these meteorites possesses chemical, isotopic, and petrologic features consistent with data presently available from Mars, we conclude that a Martian origin is quite probable.
Effect of pressure on infrared‐spectra of ice VII
Authors: Holzapfel W. B. , Seiler B. , Nicol M.
Published: February 2008

The effect of pressure on the infrared spectra of H2O and D2O ice VII has been studied at room temperature and pressures between 2 and 15 GPa with a Fourier transform infrared spectrometer and a diamond anvil high‐pressure cell. Two librational modes (νREu2 and νREu3), one bending mode ν2A2u1), and various overtone bands are well resolved. The stretching modes, ν1 and ν3, are poorly resolved due to overlap with diamond window absorption. Although νREu3 shows a very strong shift to higher frequency with pressure, ν1 and ν3 shift to lower frequencies, and all other lines vary only slightly with pressure. Differences between the spectra of H2O and D2O are discussed.
Accuracy estimate of geoid and ocean topography recovered jointly from satellite altimetry
Authors: Wagner C. A.
Published: February 2008

Joint determination of the geoid and permanent dynamic sea topography is demonstrated by simulation from direct use of satellite altimetry. The separation is feasible because of the disparities between the error spectra of the geoid and its unique perturbative effects as seen on the orbit. The solution for the components of the permanent topography from 14 cm Seasat altimetry promises to yield accurate results down to half wavelengths of about 1000 km. Present discrimination of this surface from disjoint satellite altimeter results appears to be at a scale near 4000 km. The radial effects of uncertainties in the M2 ocean tide on the Seasat orbit and altimeter measurements will probably be only a minor contaminant of the solution.
Mantle upflow tectonics in the Canadian Cordillera
Authors: Gough D. Ian
Published: February 2008

A wide variety of geophysical, geological, and petrological results from the Cordillera of western Canada can be related to the history and present tectonics of the region. Southwest of the Rocky Mountains Belt, which is cratonic, much of the region has been accreted to North America within the last 200 m.y. High heat flux, high electrical conductance, low density, and a thick seismic low‐velocity layer characterize the uppermost mantle beneath the Omineca and Intermontane belts and together present strong evidence of high temperatures and partial melting. The crust in these tectonic provinces is only about 30 km thick, and the lithosphere little or no thicker, as shown both by surface wave dispersion and by lithosphere flexure after glacial unloading. In many characteristics the region strongly resembles the Basin and Range tectonic province in southwestern United States. The present accumulation of partial melt appears to be derived from the mantle, rather than from subducted lithosphere. This is shown by a change of composition from calc‐alkaline volcanics to alkaline olivine basalt about 25 m.y. B.P., a simultaneous change of tectonic style from rightlateral strike slip to normal faulting, the uniform composition of the basalts across the Cordillera, and the uniformly high heat flux across the Omineca and Intermontane Belts. The Basin and Range volcanics changed similarly at the same time, with the appearance of extensional tectonics. Mantle upflow tectonics dominates both regions. The continuing subduction of the Juan de Fuca plate, from 42° to 50°N, does not conflict with a mantle upcurrent in the central Cordillera, because the convergence is slow and the juvenile Juan de Fuca plate thin. North of 49°N, earthquake activity extends no further east than the Coast Plutonic Complex, and focal depths are less than 40 km. The seismicity along the convergent margin has no significant overlap with the area underlain by partial melt. There is no Benioff zone, and the subducted plate appears to lose its identity before it reaches the Intermontane Belt. An upflow in the mantle appears to lie beneath western North America from the Gulf of California to northern latitudes in Canada and to be continuous with an upcurrent beneath the East Pacific Rise of the south Pacific. Some implications for plate dynamics are noted.
Decay of wave fields near an explosive source due to high‐strain, nonlinear attenuation
Authors: Minster J. Bernard , Day Steven M.
Published: February 2008

Laboratory evidence shows that the attenuating properties of geological materials depend nonlinearly on strain amplitude for strains greater than 10−6. Such a dependence is predicted by the amplitudedependent frictional attenuation models of Mavko (1979) and Stewart et al. (1983). At low enough strain these inelastic mechanisms become secondary to anelastic, linear mechanisms and are masked. The nonlinear regime may be encountered in the very near field around a seismic source. We study the case of a spherically symmetric source (explosion) embedded in an attenuating medium and examine the decay of outgoing compressional waves. Comparison with observations from the Cowboy series of explosions in salt shows that it is possible to explain simultaneously the radial decay of peak displacement and peak velocity by either one of two general models: (1) a linear, anelastic model with low Q (∼20) or (2) an amplitude‐dependent Q model in which the anelastic contribution is small in comparison with the inelastic contribution. Laboratory evidence tends to favor the latter. Strictly speaking, neither model preserves the observed Y−⅓ scaling of the wave field with yield of the explosion, but departures from perfect scaling are small in comparison with the scatter of observations. We conclude that near‐source rheology may have an important influence on the effective seismic source associated with far‐field observations.
Introduction to the seafloor mapping section
Authors: Detrick R. S.
Published: February 2008

The widespread use of echo sounders on oceanographic research vessels in the years following World War II revolutionized marine geology by providing a technique for routinely measuring the depth of the seafloor along a ship's track. Within two decades, Precision Depth Recorders (PDRs) had been used to identify the major morphotectonic features in the ocean basins, and compilations of depths measured on thousands of cruise tracks yielded the first reliable global maps of seafloor topography [e.g., Heezen and Tharp, 1960, 1968; Chase et al., 1971]. The existence of these maps played an important role in the development and acceptance of the ideas of seafloor spreading and plate tectonics in the mid and late 1960s.

During the past decade the emphasis in marine geology has shifted away from the reconnaissance geographical surveys of the 1950s and 1960s toward much more detailed studies aimed at investigating the important volcanic, tectonic, and sedimentological processes operating in the ocean basins. With this change has come the need for much higher‐resolution bathymetry maps and new techniques for rapidly surveying large areas of the seafloor. The result has been the development of a suite of new seafloor mapping tools with names like Deep Tow, Sea Beam, Sea MARC I and II, GLORIA, and Seasat. These new tools have provided some remarkable new images of the seafloor and promise to revolutionize our understanding of geological processes in the ocean basins in much the same way that the introduction of the precision depth recorder did 40 years ago.
Three‐dimensional magnetic modeling of a propagating rift, Galapagos 95°30′W
Authors: Miller Stephen P. , Hey R. N.
Published: February 2008

A combined Deep Tow and Seabeam investigation at 95°30′W on the Cocos‐Nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems. The observed 27‐km offset between the axes creates an ideal situation for the application of magnetic methods. The normally magnetized crust of the propagating rift tip penetrates into older crust, which was created when the earth's main field was reversed. As the V‐shaped structure of the rift clearly violates the conventional two‐dimensional assumptions, a full three‐dimensional analysis is required. In addition, the distorting influence of over 1000 m bathymetric relief must be removed from the observed field. Inversions have been performed on the gridded representations of the observed magnetic field and bathymetry, working in the Fourier domain. The result is a gridded rock magnetization distribution. The inversion of the surface data covers a large area, 6000 km2, and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites. In general, the propagating rift process appears to be much more orderly than the dying rift process. The magnetic polarity transition widths are narrower, and the boundaries have fewer undulations than the dying rift, which appears to be quite episodic in behavior. The average propagation rate is 52 mm/yr, compared to the average spreading half‐rate of 29 mm/yr. The locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagating rift requires about 250,000 years. The inversion of the Deep Tow data, near the seafloor, provides a high‐resolution definition of the location of the tip of the propagating rift, at 2°38.1′N, 95°30.0′W.
Bathymetric artifacts in Sea Beam data: How to recognize them and what causes them
Authors: de Moustier Christian , Kleinrock Martin C.
Published: February 2008

Sea Beam multibeam bathymetric data have greatly advanced understanding of the deep seafloor. However, several types of bathymetric artifacts have been identified in Sea Beam's contoured output. Surveys with many overlapping swaths and digital recording on magnetic tape of Sea Beam's 16 acoustic returns made it possible to evaluate actual system performance. The artifacts are not due to the contouring algorithm used. Rather, they result from errors in echo detection and processing. These errors are due to internal factors such as side lobe interference, bottom‐tracking gate malfunctions, or external interference from other sound sources (e.g., 3.5 kHz echo sounders or seismic sound sources). Although many artifacts are obviously spurious and would be disregarded, some (particularly the “omega” effects described in this paper) are more subtle and could mislead the unwary observer. Artifacts observed could be mistaken for volcanic constructs, abyssal hill trends, hydrothermal mounds, slump blocks, or channels and could seriously affect volcanic, tectonic, or sedimentological interpretations. Misinterpretation of these artifacts may result in positioning errors when seafloor bathymetry is used to navigate the ship. Considering these possible geological misinterpretations, a clear understanding of the Sea Beam system's capabilities and limitations is deemed essential.
A Sea Beam investigation of the Clipperton Transform Fault: The morphotectonic expression of a fast slipping transform boundary
Authors: Gallo David G. , Fox Paul J. , Macdonald Ken C.
Published: February 2008

A regional scale bathymetric map of the fast slipping (11.0 cm/yr) Clipperton transform boundary and adjoining East Pacific Rise segments has been constructed using the high‐resolution capabilities of Sea Beam. The transform domain is 85 km long, varies in width from 10 to 20 km, and is defined by an assemblage of transform‐parallel ridges and troughs, the largest of which is a 40‐km‐long median ridge that stands up to 700 m above the surrounding seafloor. These major topographic elements of the transform domain are bisected by a very narrow (<1 km) swath of strongly lineated terrain composed of elongate depressions, narrow lozenge‐shaped ridges, and gullies that integrate spatially to create a narrow zone of disturbed topography. This zone trends 082° and cuts like a knife across the grain, of the Clipperton transform domain, linking one truncated ridge tip with the other. We interpret this swath of disturbed terrain to be the product of strike‐slip tectonism thereby fixing the location of the present‐day plate boundary. The present‐day structural geometry of the Clipperton transform appears relatively simple and characterized by a narrow strike‐slip plate boundary linking two offset ridge axes.
Basin and range large‐scale tectonics: Constraints from gravity and reflection seismology
Authors: Froidevaux Claude
Published: February 2008

Two geophysical informations constrain the mechanical state of the lithosphere in the Basin and Range Province. COCORP reflection seismic structure across Nevada shows a rather smooth reflector at about 30 km depth, which can be interpreted as indicative of a rather flat Moho. On the other hand, the Bouguer map exhibits two marked troughs in the same region, and filtering techniques reveal a predominant 200‐km wavelength of the deep compensating masses of the topography. This paper attributes these deep heterogeneities to lateral thermal variations created by boudinage‐type instabilities of the stretched mantle lithosphere.
Interpretation of satellite elevation magnetic anomalies
Authors: Harrison C. G. A. , Carle H. M. , Hayling K. L.
Published: February 2008

The magnetic field satellite Magsat obtained total field data over almost the whole of the earth. We have studied these data in two different ways. The first way was to use spherical harmonic coefficients. It is often believed that these coefficients can be separated into those originating within the core of the earth and those from the crust at about degree 13 because of the change in slope of the power spectrum. Using coefficients above degree 13 (in two different representations), we have shown that there is little latitudinal variation of the source function and that the continent to ocean source function ratio is almost unity. We suggest that these unlikely observations are in reality due to the possibility that much of the signal carried in coefficients above degree 13 may originate within the core of the earth. If this is true, it is then possible to keep the energy external to the core of the field generated within the core at a constant even though the dipole field has varied in strength by a factor of 3 over the past 104 years. We have also produced models of crustal magnetization using the 2° × 2° average anomalous field and have shown how it is possible to obtain a physically meaningful susceptibility model, in which magnetizations are always positive. This is done by using an annihilator, which is a magnetization distribution within a spherical shell which produces no external field. The use of an annihilator is also appropriate for many problems connected with oceanic basins. Since present‐day oceanic crust is everywhere less than 180 Ma, the average magnetization seen at the altitude of a satellite is either zero for crust which is produced during times of rapid field reversal or positive for crust produced during times of constant normal polarity. Some results from continental and oceanic areas are discussed in the light of applying an annihilator.
Geochronology and petrogenesis of Apollo 14 very high potassium mare basalts
Authors: Shih C.‐Y. , Nyquist L. E. , Bogard D. D. , Bansal B. M. , Wiesmann H. , Johnson P. , Shervais J. W. , Taylor L. A.
Published: February 2008

Rb‐Sr, K‐Ar, and Sm‐Nd isotopic studies were undertaken for two Apollo 14 very high potassium (VHK) mare basaltic clasts from breccias 14305 and 14168. The samples are distinctly more radiogenic than other lunar mare basalts. Rb‐Sr data for the whole rock and mineral separates determine internal isochrons corresponding to essentially identical ages of 3.83 ± 0.08 b.y. and 3.82 ± 0.12 b.y. for samples 14305 and 14168, respectively, for λ(87Rb) = 0.0139 (b.y.)−1. Their corresponding initial 87Sr/86Sr ratios are indistinguishable and are 0.6995 ± 0.0005 and 0.6997 ± 0.0004, respectively. The 39Ar–40Ar age spectra for the two samples are very similar and give identical plateau ages of 3.85 ± 0.02 b.y. for 14305 and 3.85 ± 0.05 b.y. for 14168. The identical Rb‐Sr and K‐Ar isotopic results for the two rocks indicate that they could be derived from the same flow. The Sm‐Nd isotopic data of whole rock and mineral separates for the two VHK basalts define an internal isochron age of 3.94 ± 0.16 b.y. for λ(147Sm) = 0.00654 (b.y.)−1 and an initial 143Nd/144Nd of 0.50673 ± 21. The averaged model age, relative to the chondritic initial 143Nd/144Nd of 0.505893 for the basalts is 4.56 ± 0.04 b.y. The concordance of Rb‐Sr, K‐Ar, and Sm‐Nd ages strongly suggests that VHK basalts crystallized from a melt ∼3.85 b.y, ago. The Rb‐Sr and Sm‐Nd isotopic data indicate that VHK basalts show very large Rb/Sr fractionation but no significant Sm/Nd fractionation at the time of crystallization. The source material for these basalts had a Rb/Sr ratio similar to those of Apollo 14 high‐A1 mare basalts and a nearly chondritic Sm/Nd ratio. Extreme enrichments of Rb/Sr and K/La during the formation of VHK basalts can be adequately explained by an assimilation model that involves basalt/granite interaction. K, Rb‐rich components of granitic wall rocks in highland crust were selectively introduced into ascending hot high‐A1 mare basaltic magma upon contact. A similar crustal assimilation process has been frequently observed in terrestrial, mantle‐derived magma that ascended through the continental crust.
Lunar granulites and their precursor anorthositic norites of the early lunar crust
Authors: Lindstrom Marilyn M. , Lindstrom David J.
Published: February 2008

Lunar granulites are breccias with metamorphic textures. They are not pristine, having been contaminated with meteoritic siderophiles. Nonetheless, they constitute an abundant class of highlands rocks that are important because they are ancient and KREEP‐free. They may represent our best samples of the early lunar crust. The granulites can be divided into ferroan and magnesian groups, and each group can be subdivided on the basis of mineral composition and REE concentrations. Textural and compositional data show that some of the granulites (especially 67215 and 67415) may be derived from distinct anorthositic norite precursors, while some other granulites (79215, 78155) are clearly polymict. Even the polymict granulites probably had anorthositic norites as their dominant precursors, and not anorthosites, norites, or troctolites. The granulites have compositions similar to those of the two lunar meteorites, one of which (Yamato‐791197) is ferroan, the other (ALHA81005) magnesian. These meteorites are soil breccias from an unknown location distant from the Apollo landing sites and contain anorthositic norites as abundant clasts. Compositions of granulites and lunar meteorites more closely resemble the average composition of lunar highlands, as determined by remote sensing, than do those of any other returned lunar samples. The predominance of plutonic anorthositic norite precursors in material having the composition typical of highlands suggests that plutonic anorthositic norites were more abundant in the early lunar crust than is implied by their scarcity among pristine rocks from Apollo missions.
Anorthosite assimilation and the origin of the Mg/Fe‐related bimodality of pristine moon rocks: Support for the magmasphere hypothesis
Authors: Warren Paul H.
Published: February 2008

The lunar magma ocean (or magmasphere) hypothesis, always controversial, has required considerable revision in recent years. The geochemical bimodality of pristine rocks, epitomized by a diagram of Na/(Na + Ca) versus mg′ ratio, has led to proposals that a major fraction of the crust (the Mg‐rich suite) formed as cumulates in numerous intrusions slightly younger than the magmasphere. The precise origin of the bimodality has been elusive, however. One previous suggestion was that Na was lost as a volatile before the ferroan anorthosites crystallized from the magmasphere, whereas the Mg‐rich intrusives retained all of their original Na. However, this model cannot fully explain the bimodality, because the same bimodal pattern is manifested by plotting Eu/Al (a ratio of involatile elements) versus mg′ ratio. Assimilation probably helped to engender the bimodal patterns. Mass/energy balance calculations indicate that large proportions of plagioclase were probably assimilated from the older (magmasphere‐generated) ferroan anorthosite crust by most of the Mg‐rich intrusive melts. The magmasphere, in the absence of assimilation, probably did not yield appreciable plagioclase (the ferroan anorthosite crust) until fractional crystallization of mafic silicates had diminished the melt mg′ ratio to about 0.42. However, assuming identical initial melt composition, an Mg‐rich intrusion assimilating ferroan anorthosite (and perhaps also a small proportion of urKREEP) would have reached plagioclase saturation at much higher mg′, about 0.66. The current version of the magmasphere hypothesis (ferroan anorthosites = magmasphere flotation cumulates; Mg‐rich rocks = products of slightly younger, localized intrusions) appears to be the only plausible mechanism for engendering the Mg/Fe‐related bimodality.
Tidal dissipation in a viscoelastic planet
Authors: Ross Martin , Schubert Gerald
Published: February 2008

The rates of energy dissipation in tidally distorted Kelvin‐Voigt, Maxwell, and standard linear solid (SLS) viscoelastic moon models are calculated for a hypothetical past lunar orbit with semimajor axis 34.2 earth radii and obliquity = 49°. Viscosities of 1014 and 1018 Pa s for the Kelvin‐Voigt and Maxwell rheologies, respectively, are needed to match the dissipation rate calculated using the Q approach with a quality factor Q = 100. The SLS model requires a short time viscosity of 3×1017 Pa s to match the Q = 100 dissipation rate independent of the model's relaxation strength. Since Q = 100 is considered a representative value for the interiors of terrestrial planets, the derived viscosities should characterize planetary materials. Neither the Kelvin‐Voigt nor SLS models simulate the behavior of real planetary materials on long timescales. Therefore the significance of the viscosities inferred for these models is unclear. The Maxwell model behaves realistically on both long and short timescales. The inferred Maxwell viscosity, corresponding to a timescale of days, is several orders of magnitude smaller than the longer timescale (≥ 104 years) viscosity of the earth's mantle.
Dynamic compression and volatile release of carbonates
Authors: Tyburczy James A. , Ahrens Thomas J.
Published: February 2008

Particle velocity profiles upon shock compression and adiabatic release were measured for polycrystalline calcite (Solenhofen limestone) to 12–24 GPa and for porous calcite (Dover chalk, ρo = 1.40 g/cm3, 49% porosity) to between 5 and 11 GPa. The electromagnetic particle velocity gauge method was used. Upon shock compression of Solenhofen limestone, the Hugoniot elastic limit was determined to vary from 0.36 to 0.45 GPa. Transition shocks at between 2.5 and 3.7 GPa, possibly arising from the calcite II‐III transition, were observed. For the Solenhofen limestone, the release paths lie relatively close to the Hugoniot. Evidence for the occurrence of the calcite III‐II transition upon release was observed, but no rarefaction shocks were detected. Initial release wave speeds suggest retention of shear strength up to at least 20 GPa, with a possible loss of shear strength at higher pressures. The measured equation of state is used to predict the fraction of material devolatilized upon adiabatic release as a function of shock pressure. The effect of ambient partial pressure of CO2 on the calculations is demonstrated. P CO2 should be taken into account in models of atmospheric evolution by means of impact‐induced mineral devolatilization. Mass fractions of CO2 released expected on the basis of a continuum model are much lower than determined experimentally. This discrepancy, and radiative characteristics of shocked calcite, indicate that localization of thermal energy (shear banding) occurs under shock compression even though no solid‐solid transitions occur in this pressure range. Release adiabatic data indicate that Dover chalk loses its shear strength when shocked to 10 GPa pressure. At 5 GPa the present data are ambiguous regarding shear strength. For Dover chalk, continuum shock entropy calculations result in a minimum estimate of 90% devolatilization upon complete release from 10 GPa. For calcite, isentropic release paths from calculated continuum Hugoniot temperatures cross into the CaO (solid) + CO2 (vapor) field at improbably low pressures (for example, 10 GPa for a shock pressure of 25 GPa). However, calculated isentropic release paths originating from PT points corresponding to previous color temperature under shock measurements cross into the melt plus vapor field at pressures greater than 0.5 GPa, suggesting that devolatilization is initiated at the shear banding sites.