bluuebunny/crossref_metadata_2025 huggingface dataset:
A neodymium and strontium isotopic study of the Mesozoic calc‐alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California
Authors: DePaolo Donald J.
Published: February 2008

Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by εNd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have εNd = +8.0 to −6.4. In both batholiths, εNd correlates strongly with initial 87Sr/86Sr. Decreasing εNd values are accompanied by increasing 87Sr/86Sr and increasing δ18O; the correlation with δ18O being more pronounced for the Peninsular Ranges. The εNd values show regular geographic variations, as was found previously for initial 87Sr/86Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low εNd values (−11 to −16) and Precambrian model Sm‐Nd ages (1.5 to 1.9 Æ). The country rock εNd values, and those of primitive oceanic island arcs (εNd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O‐Sr isotopic relations and the variation of 147Sm/144Nd with εNd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal‐ and mantle‐derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8‐Æ crust, yielding an average crust formation age of nearly 1 Æ for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with εNd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5‐Æ Sm‐Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm‐Nd isotopic studies for aiding in construction of tectonic models.
Two‐mica granites of northeastern Nevada
Authors: Lee Donald E. , Kistler Ronald W. , Friedman Irving , Van Loenen Richard E.
Published: February 2008

The field settings are described and analytical data are presented for six two‐mica granites from north‐eastern Nevada. High δ18O and 87Sr/86Sr values indicate that all are S‐type granites, derived from continental crust. The major element chemistry and accessory mineral contents of these rocks also are characteristic of S‐type granites. Chemical, X ray, and other data are presented for the micas recovered from these granites. The muscovites are notably high in Fe2O3, FeO, and MgO. Except for one hydrobiotite, each of the biotites has an MgO content near 6.0 weight percent. Two different types of two‐mica granites are recognized in the area of this study. One type is distinguished by the presence of many biotite euhedra within muscovite phenocrysts and by an unusual suite of accessory minerals completely devoid of opaque oxides. This type probably resulted from anatexis of late Precambrian argillites under conditions of relatively low oxygen fugacity, along a line that roughly coincides with the westward disappearance of continental basement. In the other textural type of two‐mica granite the micas are equigranular and there is a greater variety of accessory minerals. The magmatic evolution of this type also appears to reflect the influence of late Precambrian argillites; there may be age differences between the two types of two‐mica granites.
A simple physical model for the terrestrial dynamo
Authors: Olson Peter
Published: February 2008

The strength of the earth's magnetic field results from an equilibration between rates of buoyant energy production and Ohmic dissipation. Changes in magnetic field, in particular the long term changes in dipole moment, provide an indication of changes in core energy sources, and so become critical data for understanding the evolution of both the core and deep mantle. A simple physical model is proposed to establish a connection between dipole moment behavior and production of buoyancy within the core. The model rests on two hypotheses: (1) magnetism is generated by small scale, rotation dominated turbulence consisting of a field of propagating inertial waves, and (2) the turbulence is supported by a flux of buoyancy, thermal or compositional, originating either at the core‐mantle or inner core boundary. The efficiency with which wave kinetic energy is converted to magnetic energy is determined by the wave helicity—the correlation between velocity and vorticity. The wave helicity is non‐zero if there exists a preferred propagation direction. Negative buoyancy generated at the mantle‐core boundary leads to propagation radially inward; positive buoyancy generated at the inner core boundary leads to radially outward propagation. Using parameters appropriate for the earth's core, we find that the inertial wave dynamo dissipates 8 × 1011 W in generating a magnetic field equal to the present terrestrial field. Four energy sources are considered: decay of potassium 40, secular cooling, inner core growth, and differentiation of the core from the mantle. Any of these sources can reasonably support the turbulent dynamo for much of the earth's history.
Parameterized thermal convection in a layered region and the thermal history of the Earth
Authors: McKenzie Dan , Richter Frank M.
Published: February 2008

Parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses. Simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply, and show that layering considerably increases the response time. The same arguments are applied to a planet consisting of spherical shells. They show that the response of the earth's heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface. If it does so, the surface heat flux is only about 50% greater than the heat generation rate. However if upper and lower mantle convection occurs in two separate shells, as seismological and geochemical arguments suggest, the response time is increased. The present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed. The best estimate of the present ratio of the heat loss to heat generation is about two, and this value is consistent with the behavior of an earth model consisting of a two‐layered mantle overlying a core.
The interplay between magnetic fields and convection
Authors: Weiss N. O.
Published: February 2008

The interaction between magnetic fields and convection was first modeled by considering the effect of a solid rotor on an otherwise uniform magnetic field. This approach has subsequently been extended by considering convection in an electrically conducting Boussinesq fluid in the presence of an imposed magnetic field. Nonlinear solutions for two‐dimensional magnetoconvection show both steady and oscillatory behavior.The results of a series of numerical experiments at a fixed Rayleigh number of 105 are presented and contrasted with predictions based on linear theory.
Inelastic neutron scatter iron concentrations of the Moon from orbital gamma ray data
Authors: Davis Philip A. , Bielefeld Michael J.
Published: February 2008

A set of regional Fe values were obtained from the inelastic scatter peak of the Apollo 15 and 16 orbital gamma ray spectra. These Fe (n, n′ γ) values were determined from a reaction that does not depend upon thermal neutrons and should therefore be a test of thermal neutron flux depression. No consistent effects of thermal neutron flux depression are apparent. The majority of the higher energy Fe (n, γ) values are substantiated by the Fe (n, n′ γ) results; differences of more than one sigma between iron (n, n′ γ) values and Fe (n, γ) values are attributed to the difficulty in stripping the variation in natural radioactivity from the inelastic scatter Fe band. The Fe (n, γ) data set is, therefore, shown to be reliable and to reflect the actual Fe variation on the lunar surface.
Pressure effects on bonding in CaO: Comparison with MgO
Authors: Bukowinski M. S. T.
Published: February 2008

The electronic charge distribution and eigenvalues of B1 and B2 CaO were obtained through a set of self‐consistent band structure calculations. The zero pressure electron density is in good agreement with the available X ray determination. The results of the calculations demonstrate that bonding in essentially ionic oxides involves considerable delocalization of the valence electrons. This conclusion is supported by the X ray data and by molecular orbital calculations. A comparison of the electronic structures of MgO and CaO shows that they are consistent with expected trends in ionicity. However, large compressions tend to invert the relationship, with predicted compressions at metallization being lowest for BaO and highest for MgO. Pressure induced changes in the bonding of MgO differ significantly from the corresponding changes in the heavier alkali‐earth oxides because of the presence in the latter of low‐energy d bands. It is suggested that under compression, d bands move down relative to other conduction bands and the valence band. The mechanism that causes this may be quite general and is expected to play a significant role in the optical properties of high pressure transition metal‐bearing minerals. In particular, it is possible that this mechanism is responsible for the pressure‐induced opacity to thermal radiation.
The complete synthesis of seismic crustal phases at regional distances
Authors: Bouchon Michel
Published: February 2008

Using a new method to evaluate Green's functions for elastic layered media, we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust. We choose as source parameters the ones inferred for an earthquake in central France and compare the computed seismograms to the ones recorded at several short‐period stations of the French seismic network. The major features of the records including complexity and duration are well reproduced by the synthetics. Lg waves, which are the prominant phase, are shown to be guided waves made up of SH and SV waves incident on the Moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust. The length of the computed Lg wave coda extends between group velocities of 3.5 and 2.8 km/s and is in good agreement with the data.
The foreshock sequence of the February 4, 1975, Haicheng Earthquake (M = 7.3)
Authors: Jones Lucile M. , Wang Biquan , Xu Shaoxie , Fitch Thomas J.
Published: February 2008

We have examined the locations and radiation patterns of the foreshocks of the February 4, 1975, Haicheng earthquake (M = 7.3). Using arrival times from six local seismic stations, the foreshocks and mainshock were located relative to a master event. The foreshocks occurred in a tight cluster that elongated with time. Before the largest foreshock, the activity was located within a small, approximately equidimensional volume with a diameter of about 2 km. After the largest foreshock, the activity spread northwest and southeast forming a 6‐km‐long, northwest trending zone. First motions and ratios of P to S amplitudes indicate that two different faulting mechanisms occurred during the foreshock sequence. The two radiation patterns can tentatively be correlated with different parts of the zone. The hypocenter of the mainshock was not located on the same fault as that defined by the foreshocks' hypocenters but rather was located 6 km south of and several kilometers shallower than the foreshock cluster. We think this large separation between foreshocks and mainshock in a direction perpendicular both to the plane of rupture of the mainshock and to the trend of the foreshocks might be the result of an en echelon step in the fault that slipped during the mainshock. An analysis of the change in stress due to slip during the foreshocks shows that the increase in shear stress on the mainshock fault caused by the foreshocks is very small and that direct triggering of the mainshock by the foreshocks is unlikely.
Stratigraphy and correlation of the region surrounding the IRDP drill hole 1978, Reydarfjordur, eastern Iceland
Authors: Helgason Johann , Zentilli Marcos
Published: February 2008

The 1.9 km thick sequence intersected by the IRDP 1978 drill hole at Reydarfjordur, eastern Iceland, is correlated with the westward dipping exposed volcanic pile on the basis of extensive field mapping and paleomagnetic studies. The base of the landward extension of sea floor magnetic anomaly 5 (epoch 9) can be located at the top of the Grjota olivine tholeiite formation, intersected at about 1.5 km in the core and exposed at sea level about 9 km east of the drill site. Other exposed marker formations can also be identified in the core. The drill site is located within a relatively dense dyke swarm (10% crustal dilation), previously interpreted as the feeder of the overlying volcanic formations. Field observations, including vertically discontinuous dykes and the lack of lava‐feeder relationships, suggest that the dykes were intruded by lateral injection from the south and are considerably younger than, and thus could not have fed, the exposed lava pile in the vicinity of the drill site. It is proposed that about 10.3 m.y. ago (roughly coinciding with the lower boundary of anomaly 5) the Reydarfjordur volcanic center ceased to be active and volcanic activity shifted about 20 km to the west, where the Breiddalur and Thingmuli volcanic centers later developed. The exposed stratigraphic succession between Thingmuli and Reydarfjordur volcanic centers is composed of lavas deposited in flank areas, relatively removed from a paleovolcanic zone, which now probably lies buried to the west of the IRDP drill site. The lowest 450 m of the core may represent lavas related to activity in the Reydarfjordur volcanic center 13.5 km to the east.
Transport processes on the lunar surface: Comparison of model calculations with radionuclides data
Authors: Langevin Y. , Arnold James R. , Nishiizumi Kunihiko
Published: February 2008

The most useful radionuclides for deciphering the recent history of the lunar surface are 53Mn (t1/2, = 3.7 m. y.) and 26Al (t, 1/2 = 0.72 m. y.) On the time scales of ∼0.1 to 10 m.y. on which these species give information, the lunar surface seems to be disturbed to depths of ∼1 to 50 g/cm2. The most likely agent of such disturbances is meteoritic mixing. Since both large‐scale and small‐scale events are important, the use of Monte Carlo methods was indicated. Experimental evidence is now available for seven lunar cores, representing a significant statistical sample. It thus seems timely to consider the evidence for disturbance recorded in these cores on the time scales of 26Al and 53Mn and to match it against the predictions of a meteoritic mixing model. We show that if secondary processes of meteoritic impacts such as the down‐slope movement of ejecta are taken into account, such a model is consistent with the experimental results (except for the anomalous core 70009), which, however, exhibit a tendency for deeper depth disturbances.
The transition from high temperature creep to fracture in Maryland diabase
Authors: Caristan Y.
Published: February 2008

The transition from high‐temperature creep to brittle fracture in Maryland diabase was investigated as a function of confining pressure and strain rate. Experiments were conducted at 1000°C. Confining pressure was varied to 450 MPa and strain rates from 2×10−3 s−1 to 4×10−6 s−1 At fixed strain rate, the rock strength first increased with pressure, reached a maximum, and then decreased with increasing pressure. Finally, with high pressures, the strength reached an asymptotic value which was the steady state creep strength at that temperature and strain rate. The positive pressure sensitive domain corresponded to brittle behavior, the negative pressure sensitivity domain to a transitional behavior, and the pressure insensitive domain to dislocation creep. The boundary between the last two domains occurred where the strength of the rock was about equal to the confining pressure. Similar variations in strength have been reported in the literature for carbonates and silicates, although not associated with a transitional behavior. Unfaulted specimens deformed in the transitional field showed microcracks and plastically deformed minerals. The boundaries of the transitional domain were extrapolated to geological conditions corresponding to the oceanic lithosphere. The transition depth near ridges is in close agreement with the base of the seismogenic layer (Tapponnier and Francheteau, 1978). However, the old oceanic lithosphere is probably totally brittle over its whole thickness.
A comparison between the tectonic stress measured in situ and stress parameters from induced seismicity at Monticello Reservoir, South Carolina
Authors: Fletcher Jon B.
Published: February 2008

During May and early June 1979, five three‐component digital seismographs were deployed near the Monticello Reservoir, South Carolina, to determine the stress drops of reservoir‐induced earthquakes and compare them with earlier hydrofracture stress measurements of Zoback and Hickman (this issue). Records for 327 events were obtained; 10 of these events, each recorded on four or more stations, were analyzed to determine stress drops. Depths for the 10 events ranged from 0.5 to 1.4 km. Fault plane orientations were calculated from an inversion of the body wave displacements: individual radiation pattern corrections were applied in calculating the moments at each station. Final estimates of seismic moment were obtained by averaging logarithmically the estimates from each component for each event. An arithmetic moment was obtained from the individual estimates of the source radius, and stress drop was then calculated once from the final estimates of seismic moment and source radius for each event. Moments of the 10 digitally recorded earthquakes ranged from 2.5×1017 to 2.7×1018 dyne cm for the P wave data and from 4.6×1017 to 4.6×1018 dyne cm for the S wave data. The stress drops (S wave data only) generally ranged from about 0.2 to 4 bars. Four earthquakes were recorded by a strong motion instrument at the dam site. Magnitudes (M) for these events ranged from 2.8 to 3.0. Moments taken as averages of the values from each component ranged from 1.7 to 3.4×1020 dyne cm, and the stress drops from 13 to 92 bars. Large differences between the corner frequencies of the two components (factors of 2.5 and 4 for two of the events) correspond to a large relative uncertainty for these stress drop values. Nevertheless, tens of bars of stress are apparently released for the largest events in the Monticello sequence but only a few bars are released for the smaller magnitude events. In comparison, the in situ stress data measured in two holes give an estimate of shear stress of about 45 bars at 0.2‐km depth and 30 to 70 bars at 1‐km depth in the northern hole and from 15–20 bars near the surface to almost 70 bars at depth in the southern hole. The variation in shear stress inferred from in situ measurements at the deeper levels indicates an inconsistency between the thrust‐faulting mechanisms of the earthquakes and the state of stress which is more consistent with thrust faulting at 0.2‐km depth and strike slip or normal faulting below. Thus the estimates of shear stress from in situ measurements are of the same order as the stress drops of the M ≃ 2.8–3.0 events but about 1 order of magnitude larger than the stress drops obtained for the smaller events.
Geology of the area of induced seismic activity at Monticello Reservoir, South Carolina
Authors: Secor Donald T. , Peck Lawrence S. , Pitcher David M. , Prowell David C. , Simpson David H. , Smith William A. , Snoke Arthur W.
Published: February 2008

This study provides geological background information necessary for an evaluation of the earthquake hazard in an area of induced seismic activity at Monticello Reservoir, South Carolina. This region contains a thick stratified sequence of Proterozoic Z and Cambrian metasedimentary and metavolcanic rocks. In the early to middle Paleozoic, this sequence was recrystallized and deformed under metamorphic conditions that ranged from greenschist to amphibolite facies and experienced at least two episodes of folding. The region has been intruded by late kinematic to postkinematic granitoid plutons of Silurian and Carboniferous ages and by numerous northwest trending diabase dikes of Late Triassic and Early Jurassic age. The region south of Monticello Reservoir in the Carolina slate belt experienced two episodes of faulting in the late Paleozoic and/or early to middle Mesozoic. The older group of faults trends approximately east, has only small displacements, and is characterized by extensive silicification of the fault zones. The younger group of faults trends approximately north, has experienced dip slip displacements up to 1700 m, and is characterized by carbonate mineralization in the fault zones. Both sets of faults are cut by an undeformed diabase dike of Late Triassic or Early Jurassic age. The induced seismic activity around Monticello Reservoir is occurring in a heterogeneous quartz monzonite pluton of Carboniferous age. Although laterally extensive faults have not been found in the vicinity of the reservoir, the pluton contains large enclaves of country rock and is cut by numerous, diversely oriented small faults and joints. These local inhomogeneities in the pluton, together with an irregular stress field, are interpreted to control the diffuse seismic activity around the reservoir. In view of the apparent absence of lengthy faults, it is unlikely that a large‐magnitude earthquake will occur in response to the stress and pore pressure changes related to the impoundment of Monticello Reservoir.
Experimental measurement of electromagnetic emissions possibly related to earthquakes in Japan
Authors: Gokhberg M. B. , Morgounov V. A. , Yoshino T. , Tomizawa I.
Published: February 2008

Recent observations in the USSR appear to suggest that wide‐band electromagnetic radiation occurs just prior to earthquakes. To apply this phenomenon to earthquake prediction, measurements have been carried out at the Sugadaira Space Radiowave Observatory in Japan under the USSR‐Japan Cooperation Program in 1980. The recorded noise level at 81 kHz is comparatively quiet throughout the day and night. However, about one‐half hour before the main shock of a magnitude 7 earthquake at 0733 UT on March 31, 1980, the instrument recorded an anomalous amplitude increase to 15 dB higher than the normal level. VLF data recorded synoptically at Sugadaira suggest that unusual impulsive radiation at frequencies below 1.5 kHz also occurred shortly before the earthquake. Similar 81‐kHz emissions were observed prior to magnitude 5 and 6 earthquakes on September 25, 1980, and January 28, 1981.
Ophiolites, synthetic seismograms, and oceanic crustal structure: 2. A comparison of synthetic seismograms of the Samail Ophiolite, Oman, and the ROSE refraction data from the East Pacific Rise
Authors: Kempner William C. , Gettrust Joseph F.
Published: February 2008

Synthetic seismograms for the Samail ophiolite, computed using the full reflectivity technique, are used to test the hypothesis that this ophiolite complex can serve as a prototype for young oceanic crust. If so, then this ophiolite complex can be compared with the older (45 m.y.) Bay of Islands (BOI) ophiolite to develop an aging model for oceanic crust. Synthetics for this young emplacement age (5–8 m.y.) ophiolite are compared with ocean bottom hydrophone data from 0.5‐ and 4.5‐m.y.‐age crust obtained during the Rivera Ocean Seismic Experiment. Second‐arrival phases from reflected refractions from one ophiolite velocity‐depth model and data from 4.5‐m.y.‐age crust are in excellent agreement; this agreement suggests that the Samail ophiolite is a good working model for young oceanic crust. Earlier work has shown that the BOI ophiolite is a good prototype for mature oceanic crust. The BOI ophiolite data also suggest that alteration of the lower oceanic crust and upper mantle is limited. We use the petrologic and geochemical properties of the Samail and BOI ophiolites to develop a two‐stage model for crustal aging that directly limits alteration of the lower crust and upper mantle materials.
Paleomagnetic evidence for Post‐Devonian displacement of the Avalon Platform (Newfoundland)
Authors: Kent Dennis V.
Published: February 2008

The possibility that the Avalon Platform, where the Avalonian lithotectonic belt is best developed, was involved in late Paleozoic displacement was tested by paleomagnetic study of red sandstones of the Upper Devonian Terrenceville Formation of eastern Newfoundland. Two magnetization directions were identified by thermal demagnetization analysis of 60 oriented samples from 10 sites: a high blocking temperature, thermally discrete A component of normal and reversed polarity, and an intermediate blocking temperature, thermally distributed B component of reversed polarity. The B component (D = 185.9°, I = −3.3°, a95 = 7.2° for N = 8 sites) is interpreted as a postfolding secondary magnetization and gives a paleomagnetic pole position (latitude = 43.6°N, longitude = 117.1°E) near Early to Late Permian paleopoles for North America. The A component (D = 181.6°,I = 28.0°, a95 = 10.1° for N = 9 sites) is interpreted as the characteristic magnetization possibly dating from near the time of deposition of the Terrence ville Formation. The corresponding paleomagnetic pole position (latitude = 27.4°N, longitude = 123.5°E) falls within a group of Late Devonian‐early Carboniferous paleopoles obtained from the Acadia displaced terrain, encompassing the coastal areas of New England and the Canadian maritimes which form another part of the Avalonian belt. These paleopoles are systematically offset by 15° to 20° in latitude from coeval pole positions obtained from cratonic North America. Thus the Avalonian belt of the northern Appalachians, which is thought to represent a remnant of a Precambrian and early Paleozoic microcontinent on tectonostratigraphic considerations, appears to correspond to a late Paleozoic displaced terrain on the basis of paleomagnetic evidence.
Elastic moduli, thermal expansion, and inferred permeability of two granites to 350°C and 55 megapascals
Authors: Heard H. C. , Page L.
Published: February 2008

The coefficient of thermal linear expansion α, Young's modulus E, and bulk modulus K have been determined for the Westerly and Stripa granites to temperatures T of 350°C and pressures P to 55 MPa. Using conventional triaxial aparatus, displacement measurements were made on three samples from each of three orthogonal directions for both rocks. Comparison of the directional values at any P, T, and those from the nine‐sample population indicated that within our precision, both granites are isotropic in E, K, and α. Both E and K for both rocks decreased with T and increased with P in a nonlinear fashion. From 19° to 350°C, E decreased by as much as a factor of 2 and K decreased by 2 to 3 times, depending on P. From 6 to 55 MPa, E increased by factors of 3 to 6 and K increased by 3 to 5, depending on T. Values for α were neither constant nor a monotonic function of P or T. In both granites over the P range investigated, α typically increased from 6 to 12×10−6 °C−1 at 40°C to 10–15×10−6 °C−1 at 325°C. In both rocks over the T range investigated, increasing P from 6 to 55 MPa generally decreased α by 1–5 10−6 °C−1. Most measurements are consistent with microcracks controlling the thermoelastic response by cracks opening with increasing T and closing with increasing P. Changes in crack porosity ϕ due to bulk compressibility and thermal expansion have been calculated for both granites. Because K and α were nonlinear with P and T, ϕ was inferred to be a complex function of both. Assuming that all cracks affect fluid transport, changes in permeability κ with P and T have also been calculated from κ ∝ ϕ3. These changes have been compared as κ/κ0, where κ0 was the initial value at 0.1 MPa, 19°C. For example, κ/κ0 for Westerly granite was inferred to increase by a factor of 3 from 19° to 300°C at 8 MPa. In Stripa granite at 6 MPa, κ/κ0 decreased ∼25% with T at 19°–100°C, then increased approximately twofold by 350°C.
Structure and origin of the J Anomaly Ridge, western North Atlantic Ocean
Authors: Tucholke Brian E. , Ludwig William J.
Published: February 2008

The J Anomaly Ridge is a structural ridge or step in oceanic basement that extends southwest from the eastern end of the Grand Banks. It lies beneath the J magnetic anomaly at the young end (M‐4 to M‐0) of the M series magnetic anomalies. Its structural counterpart beneath the J anomaly in the eastern Atlantic is the Madeira‐Tore Rise, but this feature has been overprinted by post‐middle Cretaceous deformation and volcanism. In order to study the origin and evolution of the J Anomaly Ridge‐Madeira‐Tore Rise system, we obtained seismic refraction and multichannel reflection profiles across the J Anomaly Ridge near 39°N latitude. The western ridge flank consists of a series of crustal blocks downdropped along west‐dipping normal faults, but the eastern slope to younger crust is gentle and relatively unfaulted. The western flank also is subparallel to seafloor isochrons, becoming younger to the south. Anomalously smooth basement caps the ridge crest, and it locally exhibits internal, eastward‐dipping reflectors similar in configuration to those within subaerially emplaced basalt flows on Iceland. When isostatically corrected for sediment load, the northern part of the J Anomaly Ridge has basement depths about 1400 m shallower than in our study area, and deep sea drilling has shown that the northern ridge was subaerially exposed during the middle Cretaceous. We suggest that most of the system originated under subaerial conditions at the time of late‐stage rifting between the adjacent Grand Banks and Iberia. The excess magma required to form the ridge may have been vented from a mantle plume beneath the Grand Banks‐Iberia rift zone and channelled southward beneath the rift axis of the abutting Mid‐Atlantic Ridge. Resulting edifice‐building volcanism constructed the ridge system between anomalies M‐4 and M‐0, moving southward along the ridge axis at about 50 mm/yr. About M‐0 time, when true drift began between Iberia and the Grand Banks, this southward venting rapidly declined. The results were rapid return of the spreading axis to normal elevations, division of the ridge system into the separate J Anomaly Ridge and Madeira‐Tore Rise, and unusually fast subsidence of at least parts of these ridges to depths that presently are near normal. This proposed origin and evolutionary sequence for the J Anomaly Ridge‐Madeira‐Tore Rise system closely matches events of uplift and unconformity development on the adjacent Grand Banks.
A miocene polarity transition (R‐N) from the Agno Batholith, Luzon
Authors: Williams I. , Fuller M.
Published: February 2008

A geomagnetic reversal record has been obtained from the Agno Batholith (16°15′N, 120°39′E) in Luzon Island, the Philippines. This body is a biotite‐hornblende quartz diorite dated at between 14.5 and 15.0 m.y. by the K‐Ar whole rock method. Hysteresis properties show that the magnetic remanence is dominated by multidomain, low‐titanium titanomagnetite. A stable component of magnetization is isolated upon both thermal and alternating field demagnetization. Normal and reversed polarity zones within the batholith give tightly grouped results which, when compared to other Miocene poles from the Baguio area, allow a tilt correction to be made. The resulting reversed‐to‐normal reversal sequence has a far‐sided VGP path and a decrease in intensity of magnetization of approximately a factor of 5, with a longer duration than the directional change. The VGP path is confined in a longitudinal band 120° from the site and so differs from the characteristic reversed‐to‐normal transition paths in that it is not near‐sided. Intermediate field geometry cannot therefore always be the same for individual reversals. The Agno reversal record is consistent with a quadrupole field and reversal initiated in the northern hemisphere. An octupolar intermediate field would require simultaneous flooding from both polar latitude bands.
Wind streaks in Tharsis and Elysium: Implications for sediment transport by slope winds
Authors: Lee Steven W. , Thomas Peter C. , Veverka Joseph
Published: February 2008

Detailed maps of wind streaks in Tharsis and Elysium have been compiled from Viking Orbiter observations spanning one complete Martian year. The streak pattern is controlled by slope winds on the central volcanoes and on the flanks of the Tharsis bulge, while the global circulation dominates in Elysium. Dust erosion by downslope winds occurs over much of Tharsis and in the vicinity of Elysium Mons; this process is effective even at the low atmospheric pressures found near the summits of the large volcanoes. Erosional streaks are largely absent in Elysium Planitia; net deposition of dust might have occurred during the period of our observations. Surface properties such as slope, thermal inertia, and roughness may influence the efficiency of slope wind production sufficiently to account for the pronounced differences in streak types and patterns present in these two regions.
Chemical weathering and the Viking Biology Experiments on Mars
Authors: Huguenin Robert L.
Published: February 2008

The activity observed by the three Viking lander life detection experiments is reviewed, and the active agents are characterized. It is proposed that the activity in all three experiments may have been caused by a single active agent in the Martian soil, resulting from contemporary chemical weathering of mafic silicates by low‐temperature frost and adsorbed H2O. Earlier laboratory studies indicated that chemisorbed H2O2 may be produced by frost weathering, and it was shown that the active cycle results of the Viking gas exchange (GEx) and labeled release (LR) experiments could be adequately simulated. Those studies were recently supported by LR test standards module (TSM) tests. The TSM tests further supported the prediction that the sterilization and long‐term storage results of the LR and GEx experiments may have been consequences of the frost‐weathering process as well. In addition,those tests now permit the model to be extended to include the activity in the pyrolytic release (PR) experiment. The activity is consistent with the presence of ∼1019 molecules cm−3 of chemisorbed H2O2 in the soil, and its lifetime is estimated to be ∼104–106 years. Decomposition is predicted to result in the production of ∼1013–1015 O2 molecules cm−2 yr−1 per centimeter depth. A comparable loss rate for O2 would be required to maintain the currently observed O2/CO2 abundance ratio of 1.3×10−3 Contemporary surface photooxidation is proposed as a possible candidate loss process. Over geologic time, frost weathering and surface oxidation may have resulted in the loss of ∼1 m to 1 km of ice and produced significant variations in atmospheric and regolith oxidation states. The present‐day loss rates indicate that regolith vapor sources may be replenishing atmospheric H2O at mean annual rates of 10−4–10−1 km3 (equivalent ice) yr−1.
Hydrothermally altered impact melt rock and breccia: Contributions to the soil of Mars
Authors: Allen Carlton C. , Gooding James L. , Keil Klaus
Published: February 2008

Samples of melt rock and breccia from 12 terrestrial impact craters were examined to identify alteration minerals and their conditions of formation. The dominant assemblage in most cases is clay‐silica‐K feldspar‐zeolite, indicating hydrothermal alteration at low pressures and temperatures of 100°–300°C. The clays are predominantly Fe‐chlorites and smectites, in most cases depleted in Al and enriched in Fe and Mg relative to their source rocks. Alteration of impact glass is often complete, while alteration of crystalling melt rock is limited to a few percent of the rock volume. Impact breccia is altered to only a small extent compared to the alteration of glass. Impact‐induced hydrothermal alteration is to be expected at Martian impact sites if significant quantities of ground ice and/or water are present. The alteration products are likely to resemble the clays found at terrestrial craters. Al/Fe fractionation similar to that observed on Earth is strongly suggested by the Viking soil analyses. The quantitative contribution of hydrothermal clays to the soil of Mars is estimated, based on a model which predicts the generation of the equivalent of a 61‐m‐thick blanket of impact melt planet‐wide. The inefficiency of melt production in small craters and the inefficiency of alteration in large craters limit the expected production of clay to a few percent of this figure. Clays produced by impact‐induced hydrothermal alteration may be expected to contribute to the fines on Mars, but this contribution is probably not dominant relative to other suggested sources.
Deformation, gravity, and potential changes due to volcanic loading of the crust
Authors: Rundle John B.
Published: February 2008

A method for calculating deformation, gravity, and potential changes induced by volcanic loads imposed within and on the earth's crust is presented. The method involves application of techniques developed for earthquake loading problems in elastic‐gravitational earth models. By considering the elastic‐gravitational equilibrium of the layered system, the gravitational and displacement response due to intruding masses and to pressurized magma chambers can be computed. For applications involving seamount loading, variations in the height of sea level can be calculated by requiring that seawater lie along an equipotential. In continental loading problems the change in potential can be used to calculate the orthometric height change as distinct from the geometric height change. For some plausible loading geometries, the differences between these two quantities can be significant. Results for various loads and earth structures are shown and discussed. In particular, a magmatic mass intruding into an elastic‐gravitational lithosphere overlying a compressible, gravitating asthenosphere produces distinct gravity anomalies which depend on the depth of intrusion. Applications of this result can be made to problems of isostatic compensation. The intrusion of a magmatic mass, together with the associated pressurization of the magma chamber, produces distinctive surface gravity and deformation in the near field. Gravity gradients, defined as surface gravity divided by uplift, vary with distance from the source. Moreover, orthometric elevations can be significantly (25%) less than geometric elevations. These results have clear application to data from events such as the posteruption deflation of Kilauea volcano.
Paleomagnetism of the Samail Ophiolite, Oman: 1. the sheeted dike complex at Ibra
Authors: Luyendyk Bruce P. , Laws Bruce R. , Day Ron , Collinson Thomas B.
Published: February 2008

We have studied the rock magnetism and paleomagnetism of 200 oriented samples from the middle Cretaceous sheeted dike sequence of the Samail ophiolite, Oman. The samples are from 28 sites spanning a cross‐strike distance of 30 km. Most of the dikes have undergone green‐schist metamorphism. The natural remanent magnetization (NRM) of the samples is about 1 A/m and Qn is also near 1. Thermal and alternating field demagnetization treatment resolves both normal (north) and reversed (south) polarities. About one third of the samples have low median destructive fields (MDF) less than 100 Oe (7.96×103 A turn/m). These low MDF samples have scattered directions and are classified as undetermined polarities (U). Hysteresis measurements show pseudo‐single domain carriers predominate in all samples, although the low MDF samples are suggested to have the largest grains in the samples studied. Thermomagnetic curves show both magnetite and minor maghemite. Thermal demagnetization and microscopic study reveals that hematite is abundant and is the carrier of the R remanence. In addition, several dikes exhibit both N and R polarities. These facts, coupled with the degree of metamorphism, indicate that part of the remanence is secondary and could be due to hydrothermal metamorphism below the seafloor. We suggest that the N polarity is primary and resides in exsolved magnetite, while the R polarity is secondary and resides primarily in metamorphic‐origin hematite. Several short R events observed in the Albian through Cenomanian magnetostratigraphic record are candidates for the R event in the ophiolite if the R polarity originated near the spreading center. Alternately, nonreproducible self‐reversal is another possible origin for the R polarity. Regardless of whether the reversal originated at the spreading center, secondary magnetization in the dikes considerably increases the directional dispersion. Another major factor affecting dispersion is viscous remanent magnetization (VRM) in the larger magnetite grains. This extreme dispersion disallows the Samail ophiolite dike layer as a significant source for marine magnetic anomalies, regardless of the strength of the NRM or the value of Q.

The stable N and R directions yield a paleomagnetic pole which is far sided, suggesting northward movement of the ophiolite. However, the paleomagnetic pole is not as far sided as the contemporaneous Africa pole, which suggests a component of north‐south closure between the ophiolite and Africa prior to northward movement.
Paleomagnetism of the Samail Ophiolite, Oman: 2. The Wadi Kadir gabbro section
Authors: Luyendyk Bruce P. , Day Ron
Published: February 2008

Four stratigraphic (vertical) transects were obtained from the gabbroic and ultramafic section of the Samail ophiolite (middle Cretaceous age). The transect through Wadi Kadir was the most detailed; 65 independent oriented cores were obtained from 22 sites on a 14‐km traverse. From field mapping we infer that this traverse corresponds to a vertical section of 6.0 km, which includes sheeted dikes, plagiogranites, isotropic high‐level gabbro, transitional gabbro, cumulate gabbros, cumulate ultramafics, and ultramafic rocks. There are some noticeable variations in magnetic mineralogy with depth: (1) primary exsolved magnetite is found mainly in the late stage high‐level gabbro, and (2) secondary magnetite is found mainly in the upper 3.5 km of the cumulate gabbros and in the cumulate ultramafics. Most of the gabbros have a stable magnetization direction that is almost horizontal and slightly west of north. The median natural remanent magnetization (NRM) intensity for the cumulate gabbros is 0.93 A/m, The median Qn value is 3.0 with most values falling in the range of 1 to 5. The ultramafic rocks below the gabbroic layer are more strongly magnetized due to higher concentrations of secondary magnetite, and their magnetization is more stable to AF demagnetization than that of the overlying gabbros. It is not clear what fraction of their magnetization was acquired beneath the seafloor because some of the serpentinization is thought to be of subaerial origin. Hysteresis measurements on the gabbros suggest that the magnetic carriers in the gabbros are a few tenths of microns in diameter (pseudo single domain). The magnetic properties of the cumulate gabbro section divide it into an upper intensely magnetized layer (3.5 km thick) and a lower weak layer (2 km thick). This was also found for the Point Sal ophiolite (California). The low intensities below 2 km are in agreement with petrological observations of a minimum in the concentration of secondary magnetite. The upper intense layer can contribute over one half of the amplitude of any marine magnetic anomaly which might have existed when the ophiolite was submarine. The serpentinized rocks also are possible contributors to anomaly amplitude. Consideration of the probable variations in the magmatic and hydrothermal history of the ocean crust and comparison of our results to other studies indicate that it may not be possible to specify a generalized magnetic source model for the plutonic section of the ocean crust.
Constraints on the expansion of Ganymede and the thickness of the lithosphere
Authors: Golombek Matthew P.
Published: February 2008

Grooves and furrows have been interpreted as grabens resulting from extension of Ganymede's lithosphere. Geologically reasonable estimates of the dip (90°–60°) and displacement (less than 1 km to a few hundred meters) of faults bounding grooves on Ganymede allow calculation of the extension and surface area increase (0.04–2%) required for the formation of all grooved terrain. The implied radius increase is thus limited between 0.02 and 1%; 1% planetary expansion is the maximum possible since formation of the grooved terrain began. Similar estimates for faults bounding furrows, the oldest preserved tectonic features on Ganymede, indicate a maximum surface area increase of ∼0.5% for the best developed furrow system, which is located in Galileo Regio. If equivalent furrow systems existed on all of the cratered terrain, then a maximum of∼0.5% increase in surface area of Ganymede is possible after the formation of the lithosphere and prior to the formation of grooved terrain. Faults bounding simple graben originate at a subsurface mechanical discontinuity and propagate upwards, resulting in grabens with similar widths and similar spacings between members of a set. The depth to this mechanical discontinuity for furrows and grooves on Ganymede must give at least a minimum lithosphere thickness, because it is impossible for faults to originate and propagate upwards from a point below the lithosphere. Therefore the minimum lithosphere thickness on Ganymede is 5 km and 9 km (in Marius and Galileo Regiones, respectively) at the time of furrow formation and 4 km at the time of groove formation.