bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural framework of the Gulf of Elat (AQABA), Northern Red Sea
Authors: Ben‐Avraham Zvi
Published: April 2008

The Gulf of Elat (Aqaba) occupies the southern part of the Dead Sea rift. The rift is considered to be a plate boundary of the transform type (partially leaky) which connects seafloor spreading in the Red Sea with the Sagros‐Taurus zone of continental collision. The deep water in the Gulf of Elat, up to 1850 m, provides a rare opportunity to examine the process of continental rifting by marine geophysical techniques. The bathymetry alone provides much information about fault patterns in this area. The fragmentation of the once continuous Arabian‐African platform is a complicated process. It shapes the structure of the gulf which has developed through continuing tectonism, primarily consisting of faulting. Recent geophysical and geological studies of the Gulf of Elat including bathymetry, bottom photographs, continuous seismic profiles, seismic refraction, gravity, magnetics, heat flow, and coring provide new information about the shallow and deep crustal structure of this important segment of the world rift system. The shallow structure of the gulf is dominated by three elongated en echelon basins, which strike N20°–25°E. Undulations in the floors of the basins produce several distinct deeps. These basins are interpreted as pullaparts. The new data from the gulf suggest that classical models for the formation of these structures should be modified. Only one of the longitudinal faults of each depression is a strike‐slip fault, while the other is predominantly a normal fault. The situation with the other two sides of the basin which are supposed to be composed of normal faults may also be more complex than previously thought. At least in one case, one such boundary is composed of a strike‐slip fault while the other has no significant fault. Crustal models of the Gulf of Elat based on gravity data indicate that the basins are rather shallow and do not extend into the lower crust. The fill of the basins extends to about 5 km below the seafloor in. the northern and southern basins and less in the central basin. Most significant magnetic anomalies in the Gulf of Elat extend from land into the sea. None of them, however, extends from one coast to another across the gulf. This supports the geological evidence for a shear between the Arabia and Sinai plates. The magnetic field over the southern third of the gulf is rather smooth and markedly different from that of the other parts. The nature of the magnetics, the crustal structure of the western margin, and the heat flow values indicate a thinner crust and different tectonic processes in the southern part of the gulf. Overall, it seems that within the southern Gulf of Elat a transition occurs between crustal spreading that takes place in the Red Sea to a rifting without spreading that takes place along the Dead Sea transform. Spreading activity propagates from south to north. The most active place is the central basin which is being propagated northward into the shallow northern basin.
A mathematical model of compaction‐induced migration
Authors: Rosenblat S.
Published: April 2008

A theoretical study is made of the dynamics of fluids in a horizontal layer of porous material subject to vertical compaction. It is shown that during the compaction process, fluid may be expelled from the layer both vertically and horizontally. When two immiscible fluids are present in the compacting layer, the degree to which each is expelled depends on several factors, most importantly their initial saturations and their viscosity ratio. It is shown that there is a trend toward an equilibrium saturation in the sense that large saturations tend to decrease during compaction, while low saturations tend to increase. Some potential implications for explaining hydrocarbon migration are suggested: in particular, that during compaction it is mainly water which is expelled from a source bed, leaving behind a high hydrocarbon saturation. The hydrocarbon would subsequently migrate independent of water, driven by pressure gradients.
True polar wander and plate‐driving forces
Authors: Davis Dan M. , Solomon Sean C.
Published: April 2008

A net torque on the global lithosphere can be exerted both by “ridge push” and “trench pull” forces. A net ridge push torque is present if there is an asymmetry in the age distribution of seafloor about one or more ocean ridges. A net torque due to trench forces is present if the forces per trench length acting on the subducting and overthrust plates are not equal in magnitude. The combined net torque on the lithosphere contributed by ridge and trench forces must be balanced by an opposing torque, most likely due to basal shear tractions associated with a global rotation of the lithosphere relative to the underlying mantle. Such a rotation should be identifiable as true polar wander. The lack of significant rotation of the lithosphere with respect to the magnetic dipole axis during the Cenozoic, therefore, places a strong constraint on the nature of torques acting on the lithosphere. We suggest that the most likely explanation for negligible true polar wander in the Cenozoic is that the net ridge and trench torques nearly cancel one another. The net ridge and trench torque vectors are in nearly opposite directions and have been so through the Cenozoic, so the condition of near cancellation amounts to a restriction on the magnitude of the force imbalance along trench boundaries. Such a near cancellation of net torques and the resulting negligible true polar wander are likely to be consequences of present plate geometries and need not be representative of earlier eras.
Lateral heterogeneity and azimuthal anisotropy of the upper mantle: Love and Rayleigh waves 100–250 s
Authors: Tanimoto Toshiro , Anderson Don L.
Published: April 2008

The lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring Rayleigh and Love wave phase velocities in the period range 100–250 s. Spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 = m = 10. Slow regions are evident at the East Pacific Rise, northeast Africa, Tibet, Tasman Sea, southwestern North America, and triple junctions in the northern Atlantic and Indian oceans. Fast regions occur in Australia, western Pacific, and the southern Atlantic. These features are also found by a completely different analysis based on the Backus‐Gilbert method. The Backus‐Gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000‐km radius and the errors are about 1% of the phase velocity in the zeroth‐order spherically symmetric earth. Inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors. However, resolution analysis by the Backus‐Gilbert method shows that the current data do not have enough resolution for everywhere on the globe. Only a few regions currently have adequate azimuthal coverage. Thus confirmation of the above correlation requires a more complete data set.
The depths of the deepest deep earthquakes
Authors: Stark Philip B. , Frohlich Cliff
Published: April 2008

The maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics. Although most recent investigations of mantle seismicity and dynamics state that seismicity extends to “about 700 km,” focal depths of 720 km have been reported for large earthquakes, as well as depths greater than 800 km for small events. We have examined focal depths of events in several catalogs and conclude that (1) the deepest reliable focal depths are at about 670–680 km, and events with focal depths beneath 600 km occur in several widely separated geographic regions, (2) observations supporting focal depths exceeding 680 km are usually few, poor, or inconsistent, (3) the reduction of seismic activity beneath about 650 km is quite abrupt. However, we cannot determine whether activity stops completely at 670–680 km, or the maximum size of events decreases gradually beneath 650 km, with magnitudes of 6.5–7.0 occasionally occurring at 650 km, magnitudes of 5.5–6.0 at 680 km, 4.0–4.5 at 695 km, etc. Several models can explain the absence of seismic activity in the lower mantle. Two models are consistent with the abrupt termination of activity at about 650–680 km in widely separated areas: a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle.
Velocity determination from offset vertical seismic profiling data
Authors: Pujol Jose , Burridge Robert , Smithson Scott B.
Published: April 2008

Two methods of computing interval velocities from offset vertical seismic profiles are developed based on a horizontally layered earth model. The first method is a simple iteration procedure by which layer velocities are sequentially computed. For any given layer, the take‐off angle for the ray arriving at the geophone at the bottom of the layer is varied until the velocity computed from travel time differs from the velocity obtained from Snell's law by less than some specified value. This is similar to the “shooting method” in ray tracing. This method requires as many layers as geophone locations. The second method is more general, and the only requirements are that the number of layers be equal to or smaller than the number of geophone locations and that the rays arrive at the geophones from above. This method is based on the iterative solution of a linearized problem obtained by expanding travel times and horizontal distances in Taylor series as functions of velocities and p‐parameters up to first order in dv's and dp's (perturbations in velocities and p‐parameters, respectively) and assuming initial estimates vo's and po's close to the optimum solution. The resulting linear system is solved using a simplified version of the Levenberg‐Marquardt algorithm. Due to the simple structure of the ensuing matrix of derivatives, for each iteration the problem reduces to the solution of an inverse problem for the velocities alone plus the solution of a vector equation for the p‐parameters. This method is fast, and convergence is attained even in the case of poor initial estimates.
A renormalization group approach to the stick‐slip behavior of faults
Authors: Smalley R. F. , Turcotte D. L. , Solla Sara A.
Published: April 2008

We treat a fault as an array of asperities with a prescribed statistical distribution of strengths. When an asperity fails, the stress on the failed asperity is transferred to one or more adjacent asperities. For a linear array the stress is transferred to a single adjacent asperity and for a two‐dimensional array to three adjacent asperities. Using a renormalization group (RG) method, we investigate the properties of a scale invariant hierarchical model for the stochastic growth of fault breaks through induced failure by stress transfer. An extrapolation to arbitrarily large scales shows the existence of a critical applied stress at which the solutions bifurcate. At stresses less than the critical stress, virtually no asperities fail on a large scale, and the fault is locked. Above the critical stress, asperity failure cascades away from the nucleus of failure; we interpret this catastrophic failure as an earthquake and it corresponds to the transition from stick to slip behavior on the fault. Thus the stick‐slip behavior of most faults can be attributed to the distribution of asperities on the fault. We propose our stochastic mechanism as an alternative to the traditional hypotheses for the stick‐slip behavior of faults. A major advantage of our approach is the inclusion of scale invariance. Thus the observed frequency‐magnitude relation for seismicity is a natural consequence of our basic hypothesis.
"Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, Implications for mechanisms of tectonic rotation""
Authors: Wells Ray E. , Coe Robert S.
Published: April 2008

Paleomagnetic and geologic investigations in Eocene volcanic rocks of the southwest Washington Coast Range demonstrate a close relationship between tectonic rotations and the local structural geology. The allochthonous middle Eocene submarine basalt basement of the Crescent Formation consists of several fault‐bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation (20° to perhaps as much as 65°) when compared to the Eocene reference pole for North America. Structural analysis shows the differential rotations postdate middle Eocene folding of the Crescent Formation against the continental margin and predate the unconformably overlying upper Eocene Goble Volcanics, which are rotated about 23° and do not show the same domains of rotation as the underlying Crescent Formation. Post‐Goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults. Major north‐northwest trending faults with several kilometers of dextral displacement form the boundaries of cross‐faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west‐northwest trending sinistral R′ Riedel shears. Thirty‐five to 100% of the observed post‐late Eocene rotations could have occurred by this shear rotation mechanism. Other paleomagnetic study areas in the Coast Range and western Cascades have a similar fault geometry and may also have undergone significant shear rotations. Long‐term northward oblique subduction of the Farallon plate beneath the Coast Range throughout most of the Tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin. Shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large, rigid plates.
The zonal harmonic model of polarity transitions: A test using successive reversals
Authors: Theyer F. , Herrero‐Bervera E. , Hsu V. , Hammond S. R.
Published: April 2008

A recently developed zonal model for the last geomagnetic field reversal, which describes time‐ and latitude‐dependent transitional behavior of intensity and inclination in terms of dominance of low‐order field harmonics, was tested using a latitudinal and chronological succession of transition records. The primary data were derived from a high‐resolution study of five Matuyama to Brunhes deep‐sea cores collected along a 40° meridional band in the north‐central Pacific. The transitions analyzed were the on‐sets and terminations of the Olduvai and Jaramillo events and the Matuyama‐Brunhes boundary. Supplementary data, published by previous workers, included a Jaramillo onset record from the southern Indian Ocean and a transition recorded in the Triassic Chugwater Formation of North America. In a general sense, the measured inclination and intensity records indicate the Jaramillo transitions and the last reversal to be remarkably similar to each other and to differ from the Olduvais. Crosscorrelation of the measured data sets with synthetic zonal harmonic records for the core latitudes indicates that the zonal harmonic model does predict the behavior of Matuyama‐Brunhes reversal in the northern hemisphere. For the older reversals, however, the present model requires substantial modification. This is particularly so in the case of the Jaramillo onset: although two inclination and intensity records of this transition from northern and southern hemisphere sediments strongly resemble each other, the current model produces a synthetic record for one hemisphere that is incompatible with that hemisphere's measured data. Further, to model zonally the Olduvai reversals (for which a wide latitudinal northern hemisphere sampling was available), a substantially different ratio of low‐order multipole components from that of the standard model is required, and the modeled solution was unsatisfactory at very low latitude. A modified zonal harmonic approach, which introduces a small sectoral component, may offer a solution to the incompatibilities uncovered by the present study.
Laboratory simulation of planetesimal collision: 2. Ejecta velocity distribution
Authors: Waza Toshihiko , Matsui Takafumi , Kani Kouki
Published: April 2008

Velocity distributions of fragments produced by low‐velocity impact against rock were obtained from high frame rate photographs. Cylindrical projectiles of mild steel (S15CK) and two kinds of rocks (tuff and basalt) were impacted against spherical rock targets at velocities from 50 to 400 m/s. Target destruction initiates with longitudinal splitting, which is analogous to destructive uniaxial static compression. The ejecta velocity component normal to the incident direction (“lateral component”) differs for impacts into basalt (∼20 m/s) and tuff (∼5 m/s) targets. The lateral component of kinetic energy of the fragments is shown to be controlled by the total strain energy stored before fracturing begins. For equivalent impact energy per unit target mass, ejection velocities of larger fragments from low‐velocity impacts are slightly higher than velocities from high‐velocity impacts; in addition, ejection velocities relative to the impact velocity for low‐velocity impacts are much higher than those for high‐velocity impacts. Therefore, impact velocity and not the imparted energy density is suggested to be an important parameter for classifying these low‐velocity collisional events. The ejecta velocity distribution is also greatly influenced by differences in relative mechanical properties between the projectile and target. Ejecta velocities from rock‐rock collisions are much higher than velocities from steel‐rock collisions even for equivalent impact velocity. This implies that the mechanical properties of planetesimals have played an important role in the early stages of planetary formation.
Introduction to the Special Issue: A Perspective on Magsat Results
Authors: Langel Robert A.
Published: April 2008

It is now over 4 years since the Magsat spacecraft reentered the earth's atmosphere. By almost any measure the project has been a success: launch was within budget and on time; the data acquired exceeded prelaunch quality requirements even though the instrumentation encountered some problems; with the publication of this issue, the number of related scientific papers is near 100.

Magsat project objectives were (1) to obtain accurate vector measurements of the near‐earth geomagnetic field and (2) to use those measurements to identify and model the main (core) and crustal fields. The main field measurements were to be used for the 1980 world charts and for studies of earth's core. The crustal field estimates were to be utilized to aid in modeling large‐scale variations in the geologic and geophysical characteristics of the crust. Although not a primary Magsat objective, studies of fields from external sources were also carried out.
Large‐scale, near‐field magnetic fields from external sources and the corresponding induced internal field
Authors: Langel R. A. , Estes R. H.
Published: April 2008

Magsat data has been analyzed as a function of the Dst index to determine the first degree/order spherical harmonic description of the near‐earth external field and its corresponding induced field. The analysis was done separately for data from dawn and dusk with the following results: dusk external: q10 = 20.3 ‐ 0.68 Dst (nT); dusk internal: g10 = 29987.7 + 0.240q10 (nT); dawn external: q10= 18.62 ‐ 0.63 Dst (nT); dawn internal: g10 = 29992.3 + 0.287q10 (nT), where g10 and q10 are the degree 1, zero‐order, internal and external coefficients, respectively, in a spherical harmonic potential function describing the near‐earth magnetic field. Comparison with POGO data indicates that the constant term relating q10 and Dst has changed about 20 nT between 1970 and 1980, presumably due to an increase in the average ring current intensity. A local time variation of the external field persists even during very quiet magnetic conditions. Both a diurnal and 8‐hour period are present. A crude estimate of sq current in the ±45° geomagnetic latitude range is obtained for 1966–1970. The current strength, located in the ionosphere and induced in the earth, is typical of earlier determinations from surface data, although its maximum is displaced in local time from previous results.
The near‐Earth magnetic field at 1980 determined from Magsat data
Authors: Langel R. A. , Estes R. H.
Published: April 2008

Data from the Magsat spacecraft for November 1979 through April 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth's main magnetic field and its secular variation at epoch 1980.0. The model is called GSFC(12/83). Constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10. The first‐degree external terms and corresponding induced internal terms are given as a function of Dst. Preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current. The final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods: (1) November–December 1979, (2) January–February 1980, and (3) March–April 1980. A correction for the effects of the equatorial electrojet was applied to the dusk data utilized. The solution included calculation of fixed biases, or anomalies, for the observatory data. Although similar in many respects, GSFC(12/83) differs from International Geomagnetic Reference Field 1980 by 3.6 nT in the g10 term and shows a slightly negative B in the northern polar region as well as other differences in secular variation pattern.
Comparison between the recent U.S. composite magnetic anomaly map and Magsat anomaly data
Authors: Schnetzler C. C. , Taylor P. T. , Langel R. A. , Hinze W. J. , Phillips J. D.
Published: April 2008

In order to test the validity of the Magsat anomaly data, the recently published U.S. Geological Survey/Society of Exploration Geophysicists Composite Magnetic Anomaly Map (CMAM) of the conterminous United States was used as a standard, and a comparison was made with these satellite data. The CMAM data set, digitized by the Phoenix Corporation, was resampled and continued upward to 320 km altitude. This upward continued data set was compared with an independently derived Magsat field at the same altitude. Our initial results were unsatisfactory; the CMAM data are dominated by a long‐wavelength surface increasing in amplitude by some 400 nT in a northwesterly direction across the Unites States. This broad trend surface was suppressed by removing a general two‐dimensional polynominal function having nine orders in longitude and five in latitude obtained through least squares fitting. The resulting residual upward continued field was compared with the Magsat data by means of correlation techniques. Correspondence was found between these data sets for many large‐amplitude anomalies, for example, off the East Coast, in Kentucky, Oklahoma, Mississippi embayment, Georgia, Arizona, and Nebraska. Dissimilarities occur over the Rio Grande rift, Minnesota, and Wyoming. The negative anomaly over the Rio Grande rift and the positive anomaly over the midcontinent geophysical anomaly are more pronounced in the CMAM data than in the Magsat results. These latter discrepancies are attributed to the method used to remove external field variations from the satellite data which suppresses north‐south trending structures, i.e., those lying in the orbit direction. CMAM base level problems localized over Minnesota and Wyoming are believed to cause observed discordance between satellite and aeromagnetic data in these regions. This study confirms, with “ground truth,” the validity of these satellite measurements and demonstrates that satellite‐derived anomaly maps can be used to identify base level problems when compiling local or regional aeromagnetic surveys.
Intermediate‐wavelength magnetic anomaly field of the North Pacific and possible source distributions
Authors: LaBrecque J. L. , Cande S. C. , Jarrard R. D.
Published: April 2008

The objective of this study is to investigate the feasibility of extracting intermediate‐wavelength magnetic anomalies of wavelengths between 4000 and 400 km from total field data acquired in marine magnetic surveys. The intention is to develop a second data set which may be used in conjunction with the satellite magnetic field to determine anomaly and source distributions. We apply our technique to the recovery of intermediate‐wavelength total intensity anomalies over the North Pacific from marine survey data. The technique also provides a record of secular variation during the period 1960 to 1980. In areas of high track density the sea surface data can be used to refine the Magsat data processing techniques and also provide further constraints on source models. In areas of reduced sea surface track density the Magsat field provides higher resolution. The Magsat data set also records vector field data. As will be shown, the filtered sea surface data set provides a higher‐resolution field than the Magsat data, due to proximity to lithospheric source bodies. The sea surface data also provide an independent check of the Magsat field, since the two fields should be equivalent after upward continuation of the sea surface field. We notice, however, that for the central Pacific the upward continued sea surface field is approximately 2 times the amplitude of the Magsat field. The spatial correlation of the two anomaly fields is very strong.
An estimation of continental crust magnetization and susceptibility from Magsat data for the conterminous United States
Authors: Schnetzler C. C.
Published: April 2008

The Magsat‐derived magnetic anomaly field over the United States was inverted to magnetization in a seismically determined variable‐thickness lower crust. The results are quite similar to those obtained from a previous inversion of POGO data. The average magnetization is 3.45±1.0 A/m, which gives a bulk magnetic susceptibility of 8.7±2.4×10−2 (SI units) for the lower crustal layer consistent with a mafic composition. These values are also consistent with previous estimates of lower crustal magnetization from aeromagnetic data and with laboratory measurements of mafic rock susceptibilities.
Magsat and POGO magnetic anomalies over the Lord Howe Rise: Evidence against a simple continental crustal structure
Authors: Frey Herbert
Published: April 2008

Magsat average scalar and POGO reduced‐to‐pole magnetic anomaly data both show prominent positive signatures over the Lord Howe Rise. Although generally assumed to be continental in nature, the Lord Howe Rise cannot simply be submerged ordinary continental crust, which would produce a negative anomaly contrast with respect to the surrounding higher susceptibility oceanic crust. Three‐dimensional modeling of the plateau, using the known crustal structure and assuming an induced origin for the satellite elevation anomaly, leads to a model in which the lowest crustal layer in an otherwise “continental” crustal structure has an unusually high susceptibility in the range cgs 0.008–0.010 (SI 0.10–0.13). Replacement or alteration of the lowest layer by a high‐susceptibility rock type may be related to the subsidence of the plateau.
Curie isotherm surfaces inferred from high‐altitude magnetic anomaly data
Authors: Mayhew M. A.
Published: April 2008

Traditional methods of Curie depth estimation involving spectral analysis of low‐level aeromagnetic data require that depth to the bottom of a magnetic crustal layer be large relative to depth to top, so that spectral separation can be achieved. An alternate method for high‐altitude data, where depth to magnetic layer top and bottom are virtually indistinguishable, is described. Apparent magnetization variation in a thin equivalent layer is obtained by inversion of the anomaly data. The information in such a model is essentially the vertical integral of magnetization in the real layer to some resolution limit and within some ambiguity in level. This can be transformed to a model of thickness variation in a layer of constant magnetization, given some minimal constraints. The thickness variations may reflect Curie isotherm undulations in regions of high crustal heat flow; Curie depth models then provide a constraint on crustal thermal models. The method was first applied to a magnetization model for the western United States derived from Magsat data. A Curie surface model was obtained and used to constrain simple thermal models along two crustal sections. Associated surface heat flow is in agreement with observations. The method was then applied to upward continued aircraft data from the Oregon Cascades. Results are in reasonably good agreement with previous results based on spectral analysis and with measured heat flow.
Radar evidence for cratering on Venus
Authors: Burns B. A. , Campbell D. B.
Published: April 2008

To date many craterlike features have been detected in earth‐based radar and Pioneer Venus imagery of the surface of Venus, but whether they are of impact or endogenic origin is still in question. The morphologic and polarization characteristics of these features, as observed from earth‐based radar data obtained at the Arecibo Observatory, provide additional information that may be useful in their identification. This paper examines the extent to which this information can be used, in conjunction with our knowledge of the morphologic and polarization properties of impact craters and volcanic calderas on other terrestrial bodies, to constrain inferences about the origin of the craterlike features on Venus. Results indicate that only two features exhibit unambiguous characteristics that could tentatively distinguish them as volcanic or impact in origin. However, the observed trends imply that any age of the surface derived from observed “crater” densities must represent an upper bound on the actual age.
The deep structure of lunar basins: Implications for basin formation and modification
Authors: Bratt Steven R. , Solomon Sean C. , Head James W. , Thurber Clifford H.
Published: April 2008

We present models for the structure of the crust and upper mantle beneath lunar impact basins from an inversion of gravity and topographic data from the nearside of the moon. All basin models display a thinner crust and an elevated Moho beneath the central basin region compared to surrounding areas, a signature of the processes of basin excavation and mantle uplift during collapse of the transient cavity. There is a general decrease in the magnitude of apparent uplift of mantle material with increasing basin age; we attribute this relation primarily to enhanced rates of ductile flow of crustal material early in lunar history when crustal temperatures were relatively high and the effective elastic lithosphere was thin. The more relaxed topographic and Moho relief associated with older basins on the central nearside may, in particular, be at least partly a consequence of the extensive subsurface heating associated with the formation of the large Procellarum basin. The deep structure of the youngest basins constrains the geometry of the cavity of excavation and the amount of crustal material ejected beyond the basin rim. From the volumes of the topographic basin, of mare basalt fill, and of uplifted mantle material, the volume of crustal material ejected beyond the basin rim for an Orientale‐sized event was of the order of 107 km3. A near‐constant thickness of nonmare crustal material beneath the central regions of young basins of various diameters and preimpact crustal thicknesses suggests that the transient cavity excavated to at least the base of the crust for the largest basins; significant excavation into the mantle may have been impeded by an abrupt increase in strength at the lunar Moho.
Fault type predictions from stress distributions on planetary surfaces: Importance of fault initiation depth
Authors: Golombek Matthew P.
Published: April 2008

Faults on planetary surfaces initiate at depth where stresses due to the weight of overburden are important. Consequently, the prediction of fault type from calculated stresses at depth can yield different faults than those predicted using the surface criteria commonly employed in geophysical models. For elastic plate flexure models of mascon loading on the moon, stresses calculated at the surface predict that the most likely faulting is strike slip (for which there is no direct evidence) at a distance from the basin center where grabens are found. Modifying these plate flexure calculations to account for the weak surface layer of ejecta (megaregolith), 1–3 km‐ thick that overlies the relatively strong lunar lithosphere, results in model stresses in the outer layers of the moon (tens of bars) that are an order of magnitude lower than in the lithosphere (hundreds of bars). Because faults bounding lunar grabens initiate at the mechanical discontinuity between the megaregolith and lithosphere and because the reverse faults likely responsible for wrinkle ridges probably initiate at the basalt‐basin floor contact (0.5–2 km deep), model stresses must be calculated at these depths. Superposing nonisotropic stresses due to the weight of the overburden at the depths of these mechanical discontinuities with plate flexure model stresses correctly predicts maximum stress differences in an interior zone of expected wrinkle ridges surrounded by a zone of concentric normal faults or grabens and effectively eliminates the strike‐slip zone, in agreement with observations. The absence of strike‐slip faults from almost all planetary and satellite surfaces may be explainable if lithospheric deformation model stresses are calculated at important shallow crustal mechanical discontinuities where the maximum compressive stress is vertical, thereby favoring the formation of normal faults. If true, mechanical discontinuities in the shallow crusts of planets and satellites play an extremely important and hitherto unrecognized role in the formation of structures observed on their surfaces.
Acoustically detected hydrocarbon plumes rising from 2‐km depths in Guaymas Basin, Gulf of California
Authors: Merewether Ray , Olsson Mark S. , Lonsdale Peter
Published: April 2008

Plumes extending nearly 1000 m from the 1500–2000 m deep seafloor of Guaymas Basin were detected from below by the 23.5‐kHz inverted echo‐sounder of the Scripps Deep Tow vehicle. Individual sound reflectors (bubbles or drops) rise at approximately 17 cm/s in one plume. The Deep Tow side scan records provide more information on the plumes' structure at the altitude of the vehicle (75 m), where some form multiple side scan targets, one 20 m across. Near‐bottom 4‐kHz profiles show that plumes overlie either young fault traces associated with extensional faulting at the basin's spreading centers or outcrops of tilted beds beside strike‐slip faults. We infer from analysis of the Deep Tow observations, field relationships, and knowledge of the geology of this basin that the plumes are made of light hydrocarbons, perhaps mainly methane, that emanate from seabed seeps. One of the acoustically detected plumes was at a spreading‐axis hydrothermal field, which has many buoyant, acoustically transparent thermal plumes, some of which are rich in dissolved hydrocarbons.
Permeability of unconsolidated sands and porous rocks
Authors: Gangi Anthony F.
Published: April 2008

Krumbein and Monk found an empirical relationship for the permeability of unconsolidated sand as a function of the mean grain diameter (by weight) dw and the standard deviation σϕ (ϕ = −log2 d for d in millimeters). Their expression, for lognormal grain size distributions, is k = 760dw2 exp (−1.31σϕ) for dw in millimeters, σϕ in ϕ units, and k in darcys. Their data have been reanalyzed to express the permeability in terms of the mean grain diameter by number, dN. This representation is a more physical one because the permeability depends upon the number of pores of particular sizes (i.e., upon the number distribution function of pore sizes). It is expected that the pore size distribution function (by number) is more closely related to the grain size distribution function by number than to the grain size distribution function by weight. It was found that equally good fits of all their data are given by k = 665dN2 exp (1.78σϕ3) = 665dw2 exp (−2.88 σϕ2 + 1.78σϕ3) and k = 648dN2 (1 + 2.77σϕ3).It was found that the expression k = 735dN2 gives permeability values that are accurate to 10% for loose, unconsolidated sands (porosity ≅ ≎ 40%±5%) with standard deviation σϕ in the range 0 ≤ σϕ ≤ 0.5. These results may be applied to porous rocks if Berg's semitheoretical/empirical determination of the permeability variation with porosity is used (for porosities in the range 30% ≤ n ≤ 45%). An expression for the theoretical variation of permeability with effective confining pressure is also given.
Spectral and other physicochemical properties of submicron powders of hematite (α‐Fe2O3), maghemite (γ‐Fe2O3), magnetite (Fe3O4), goethite (α‐FeOOH), and lepidocrocite (γ‐FeOOH)
Authors: Morris Richard V. , Lauer Howard V. , Lawson Charles A. , Gibson Everett K. , Nace Georg Ann , Stewart Cheri
Published: April 2008

Spectral and other physicochemical properties were determined for a suite of submicron powders of hematite (α‐Fe2O3), maghemite (γ‐Fe2O3), magnetite (Fe3O4), goethite (α‐FeOOH), and lepidocrocite (γ‐FeOOH). The spectral reflectivity measurements were made between 0.35 and 2.20 μm over the temperature interval between about −110° and 20°C. Other physicochemical properties determined were mean particle diameter, particle shape, chemical composition, crystallographic phase, magnetic properties, and Mössbauer properties. Only the magnetite powders have significant departures from the stoichiometric phase; they are actually cation‐deficient magnetites having down to about 18.0 wt % FeO as compared with 31.0 wt % FeO for stoichiometric magnetite. A structured absorption edge due to crystal field transitions and extending from weak absorption in the near‐IR to intense absorption in the near‐UV is characteristic of the ferric oxides and oxyhydroxides and is responsible for their intense color. Particularly for hematite, the number and position of the spectral features are consistent with significant splitting of the degenerate cubic levels by noncubic components of the crystal field. The position of the crystal‐field band at lowest energy, assigned to the envelope of the components of the split cubic 4T1 level, is near 0.86, 0.91, 0.92, and 0.98 μm at room temperature for hematite, goethite, maghemite, and lepidocrocite, respectively. Comparison with Mössbauer data suggests covalent character increases sequentially through the aforementioned series. The positions of the spectra features are relatively independent of temperature down to about −110°C. The maximum shifts observed were on the order of about 0.02 μm shortward for the ferric oxyhydroxides. Variations in the magnitude of the reflectivity of the hematite powders as a function of mean particle diameter are consistent with scattering theory. The absorption strength of the crystal‐field bands increases with increasing mean particle diameter over the range 0.1–0.8 μm; visually this corresponds to a change in color from orange to deep purple. The position of the split cubic 4T1 band shifts longward by about 0.02 μm with decreasing mean particle diameter over the same range; this trend is consistent with wavelength‐dependent scattering. The cation‐deficient magnetite powders are very strong absorbers throughout the near‐UV, visible and near‐IR; their spectral properties are independent of temperature between about −110 and 20°C.
Length scales for continental deformation in convergent, divergent, and strike‐slip Environments: Analytical and approximate solutions for a thin viscous sheet model
Authors: England Philip , Houseman Gregory , Sonder Leslie
Published: April 2008

The deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere. Exact analytical solutions, for the case of a Newtonian fluid, and approximate solutions, for the case of fluid with power law rheology, show that: When the imposed velocity vector is normal to the boundary (congressional or extensional regime) the deformation field decays away from the boundary with a characteristic length scale 1/3 to 1/10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10, where n is the stress‐strain exponent in the rheology; in contrast, when the imposed velocity vector is parallel to the boundary (transcurrent regime), the length scale of the deformation field is approximately 4 times smaller. In each case these length scales decrease approximately as n−½. The difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust; such forces would modify the ratio of length scales, but not sufficiently to affect this result.
Transient polar motions and the nature of the asthenosphere for short time scales
Authors: Boschi Enzo , Sabadini Roberto , Yuen David A.
Published: April 2008

The role of viscoelastic flows excited by earthquakes in driving short‐term polar motions is investigated analytically by means of a four‐layer model consisting of an elastic lithosphere, a thin asthenosphere, a mean mantle, and an inviscid core. We have employed a linear Maxwell rheology throughout the entire mantle to describe large‐scale geodynamic processes for time spans less than 100 years. The global displacement fields which contribute to the perturbations of the moment of inertia are calculated by solving a two‐point boundary value problem in which the forcing function is a dislocation source. The elements of the perturbated inertia tensor are then employed directly in the Liouville equations for determining the changes of the Chandler wobble and the rate of the earth's rotation as a consequence of large earthquakes. A complete set of solutions to the Liouville equation is used for this problem. This formulation requires the usage of two classes of eigenspectra: one arising from isostatic reequilibration of the mantle due to faulting and the second set involving the gradual readjustment of the spin axis as a result of transient flows in the mantle. We find that the polar motions depend sensitively on the viscosity structure of the asthenosphere and not at all on the underlying mantle. On the basis of the observed polar motion data and the secular changes of the gravitational harmonic coefficient J2, we can rule out a global low‐viscosity zone with short‐term asthenospheric viscosities less than about 5×1018 Pa s and widths greater than 50 km.
Q for teleseismic P waves from central Asia
Authors: Bache Thomas C. , Marshall Peter D. , Bache Lolitia B.
Published: April 2008

A model for the attenuation of high‐frequency (1–8 Hz) P waves is developed for paths from eastern Kazakhstan to arrays in Scotland (EKA), Canada (YKA), India (GBA), and Australia (WRA). The attenuation model includes contributions from both intrinsic absorption and scattering. Distinction between the two is especially important for forward modeling because the dispersion is quite different. However, no satisfactory complex operator for representing the scattering attenuation appears to be available at present. Attenuation consistent with an absorption band model dominates at low frequencies and is strongly dependent on frequency in this band. Attenuation that appears to be due to scattering has an important effect above 2.5 Hz and dominates at high frequency. It can be represented by a constant effective Q−1, or by Q−1 ∝ ƒ. The details of the best fitting model depend on which of these is chosen and on the value of t0* (travel time/Q ratio at long period) which is poorly constrained by these data. The general conclusion is that the intrinsic attenuation can be represented by an absorption band model with 0.04 ≤ τm ≤ 0.1, while 0.5 ≤ t0* ≤ 1.0. Superimposed on this is scattering attenuation, which can be represented by a constant t* of about 0.1 s or by t* = Aƒ, with A about 0.01. Some differences are seen among the four paths, with the greatest attenuation in the 1–3 Hz band seen at YKA and the least at EKA. Above 3 Hz the attenuation effects are essentially identical for all four paths.
Seismic potential for large and great interplate earthquakes along the Chilean and Southern Peruvian Margins of South America: A quantitative reappraisal
Authors: Nishenko Stuart P.
Published: April 2008

The seismic potential of the Chilean and southern Peruvian margins of South America is reevaluated to delineate those areas or segments of the margin that may be expected to experience large or great interplate earthquakes within the next 20 years (1984–2004). Long‐term estimates of seismic potential (or the conditional probability of recurrence within a specified period of time) are based on (1) statistical analysis of historic repeat time data using Weibull distributions and (2) deterministic estimates of recurrence times based on the time‐predictable model of earthquake recurrence. Both methods emphasize the periodic nature of large and great earthquake recurrence, and are compared with estimates of probability based on the assumption of Poisson‐type behavior. The estimates of seismic potential presented in this study are long‐term forecasts only, as the temporal resolution (or standard deviation) of both methods is taken to range from ±15% to ±25% of the average or estimated repeat time. At present, the Valparaiso region of central Chile (32°–35°S) has a high potential or probability of recurrence in the next 20 years. Coseismic uplift data associated with previous shocks in 1822 and 1906 suggest that this area may have already started to rerupture in 1971–1973. Average repeat times also suggest this area is due for a great shock within the next 20 years. Flanking segments of the Chilean margin, Coquimbo‐Illapel (30°–32°S) and Talca‐Concepcion (35°–38°S), presently have poorly constrained but possibly quite high potentials for a series of large or great shocks within the next 20 years. In contrast, the rupture zone of the great 1960 earthquake (37°–46°S) has the lowest potential along the margin and is not expected to rerupture in a great earthquake within the next 100 years. In the north, the seismic potentials of the Mollendo‐Arica (17°–18°S) and Arica‐Antofagasta (18°–24°S) segments (which last ruptured during great earthquakes in 1868 and 1877) are also high, but poorly constrained.
The subsurface structure of Long Valley Caldera, Mono County, California: A preliminary synthesis of gravity, seismic, and drilling information
Authors: Abers Geoffrey
Published: April 2008

Long Valley Caldera is a 0.7 m.y. old volcanic eruptive center associated with an elliptical collapse structure on the eastern margin of the Sierra Nevada. Two‐dimensional models of caldera fill are presented in this study resulting in structures compatible with existing seismic refraction and gravity data. Modifications to previous seismic models include shifting the north caldera rim to the south and decreasing the dip of the west and south boundaries. Densities in the model are not varied independently but are based on empirical velocity‐density relationships and measured densities from surface outcrops and drill cores. These density values are adequate to account for observed gravity anomalies, indicating that the assumed subsurface lithologies are approximately correct. The large gravity anomaly in the shallow eastern caldera is modeled by assuming that the area is underlain by a layer of exceptionally low density.