bluuebunny/crossref_metadata_2025 huggingface dataset:
Radial and latitudinal variations of the magnetic field strength in the outer heliosphere
Authors: Burlaga L. F. , Ness N. F.
Published: February 2008

This paper analyzes the radial, latitudinal, and temporal variations of the magnetic field in the outer heliosphere, using the observations from Voyagers 1 and 2 and Pioneers 10 and 11. The radial variation of the magnetic field strength measured during 1973‐1989 from 1 to 19 AU is compared with that predicted by Parker's spiral field model. The speed deficit (V ‐ V1)/V1 between 1 and 19 AU is (+0.0027 ± 0.0026)/AU for the Voyager data and (0.0099 ± 0.0024)/AU for the Pioneer 11 data. Thus the Voyager data are consistent with no radial variation of the speed, as assumed in deriving Parker's equations for the spiral field, but the Pioneer data are not. The magnetic flux deficit in the outer heliosphere relative to 1 AU is measured by the quantity D ≡ (A ‐ A1)/A1 versus R. For the Voyager data the 1 AU intercept of the best fit line to these data is (0.029 ± 0.031), which is consistent with zero, as required by the definition of D. For the Pioneer 11 data the corresponding 1 AU intercept is (−0.068 ± 0.044), which implies a flux deficit of (7% ± 4%) at 1 AU, suggesting either a systematic error in the data or statistical uncertainties greater than we estimated. The magnetic flux deficit is given by the slope the best fit line to D versus R. The Voyager data give a slope −(0.0027 ± 0.0036)/AU, which is consistent with zero, indicating no flux deficit in the Voyager data. The Pioneer data give a slope (0.0040 ± 0.0042)/AU, which is also consistent with zero. Since the best fit line to the Voyager observations of D versus R gives a value of D at 1 AU consistent with zero, we may set D(1) = 0 and obtain D(R) = [(0.0001 ± 0.0021)/AU] × (R ‐ 1)(AU), consistent with no flux deficit between 1 and 19 AU from 1977 through 1985. For the years 1986 and 1987 the Voyager data give a nonzero flux deficit equal to (−0.264 ± 0.075)/AU and (−0.171 ± 0.070)/AU, respectively. The Pioneer 11 data show no significant flux deficit during 1986 (+0.19 ± 0.22)/AU. The flux deficit observed by Voyager 2 during these 2 years near solar minimum might be related to the presence of a heliospheric vortex street at this time. The magnetic field strength measured by Voyager 1 at high latitudes is smaller than the magnetic field strength measured by Voyager 2 near the heliographic equator beyond 21 AU. During 1986, when the heliospheric vortex street wrapped field lines in spirals around the axes of the vortices, the source magnetic field strength at 27°N was greater than the source field strength at ∼1°N. During 1988 and 1989, when the vortex street was no longer present, the source field strength at ∼30°N was less than that at ∼3°N to 4°N. The topology of the spiral interplanetary magnetic field is determined by a defect in the heliosphere, the rotating Sun. The language of group theory can describe this and other more complex defects and vector fields in the heliosphere.
A prediction for the onset of cycle 23
Authors: Wilson Robert M.
Published: February 2008

The present sunspot cycle (number 22) is now in decline, having had its onset in September 1986, its primary maximum in July 1989, and a secondary maximum (of lower value) in 1991. Dependent upon whether cycle 22 is a short‐ or long‐period cycle, onset for cycle 23 will occur, respectively, either prior or subsequent to about July 1997. A relation is described, involving the slopes (i.e., the average rates of change in smoothed sunspot number values) as seen during the ascending and descending portions of the sunspot cycle, which appears to clarify, at least for cycle 22, the ambiguity of cycle length. In particular, the relation strongly suggests that cycle 22 is a short‐period cycle and that onset for cycle 23 will come early rather than late, with the most probable date for cycle 23 onset being May–November 1996.
Electron‐acoustic and cyclotron‐sound instabilities driven by field‐aligned hot‐electron streaming
Authors: Mace Richard L. , Hellberg Manfred A.
Published: February 2008

We study the magnetized electron‐acoustic instability in a three‐component plasma comprising cool ions, cool electrons and a hot, drifting electron component. Whereas previous investigations of the electron‐acoustic instability have usually been limited to the regime of strong magnetization, we have considered strong, intermediate, and weak magnetization regimes. In the weak to intermediate regime we identify an electron‐cyclotron‐sound instability which has lower threshold and larger growth rate than the weakly magnetized electron‐acoustic instability for wave propagation angles quasi‐parallel to the ambient magnetic field. The relationship between the cyclotron‐sound and electron‐acoustic instabilities is elucidated. In particular, we observe a “coupling” of the waves in weak magnetic fields, which becomes weaker and finally disappears with progressively larger magnetic field strengths. The relevance of this work to cusp auroral hiss and broadband electrostatic noise in the auroral magnetosphere is discussed.
Phase diagram and elastic properties of Fe 30% Ni alloy by synchrotron radiation
Authors: Huang E. , Bassett W. A. , Weathers M. S.
Published: February 2008

Diffusionless phase transitions of Fe‐Ni alloy with 30 wt % Ni (Fe30Ni) were studied at high temperature and pressure by synchrotron radiation. Several isothermal runs ranging from 25°C to 600°C were carried out at up to 20–60 GPa to determine the phase relationships and elastic properties of the alloy. The ruby fluorescence method was used for pressure measurements at room temperature. The compressibility of the fcc phase of the alloy, determined up to 48 GPa, has a bulk modulus of 160 ± 15 GPa at room temperature assuming K′0 = 4. The average linear thermal expansion coefficient of the fcc phase of Fe30Ni was estimated to be 10.2 ± 1.5 × 10−6 K−1 between 25°C and 600°C. The pressure at high temperature was calculated using the equation of state of the fcc phase assuming that the compressibility of the alloy at high temperature remains the same as at 25°C. The Fe30Ni alloy has both fcc and bcc structures at ambient conditions. However, the bcc phase disappeared (bcc‐out) after pressure was applied to the sample at various temperatures. The bcc‐out transition boundary has a negative slope of −55° ± 5°C/GPa. The fcc phase transformed to a hexagonal close‐packed phase at high pressure (hcp‐in). The slope of the phase boundary of hcp‐in was determined to be 25 ± 5°C/GPa. This result, combined with our previous investigation (Huang et al., 1988), indicates that the slope of the fcc/hcp phase boundary in Fe‐Ni alloys remains constant with an increase of Ni content up to 30 wt %.
An Early Miocene Transition in deformation regime within the Red River Fault Zone, Yunnan, And its significance for Indo‐Asian tectonics
Authors: Harrison T. Mark , Wenji Chen , Leloup P. H. , Ryerson F. J. , Tapponnier Paul
Published: February 2008

Recent work has tended to support the view that lateral extrusion of Indochina along the Red River fault zone was a significant factor in accommodating the convergence of India with Asia. To better understand the nature and timing of this feature, 40Ar/39 Ar age spectrum analyses were undertaken on minerals from the Ailao Shan/Red River metamorphic belt in southern Yunnan, China. Interpreted using diffusion domain theory, these results yield thermal histories in the temperature range 550–150°C. Discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early Miocene. At least in the vicinity of our samples, left‐lateral, strike‐slip ductile deformation appears to have ceased by about 20 Ma. The mylonitic gneisses in the shear zone were subsequently obliquely unroofed, probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief (locally <1 m.y.) but rapid (>100°C/m.y.) cooling episode. This transition may have resulted from a component of northeast‐southwest extension attributable to greater clockwise rotation of Indochina relative to South China after ∼20 Ma than that consistent with pure strike‐slip along the fault. K‐Ar ages on clay minerals from gouge within the normal faults bounding the Ailao Shan and Diancang Shan yield a range of ages between 20 and 185 Ma but show a significant clustering at 20–25 Ma. These results suggest that early Miocene movement on a precursor to the Range Front normal fault in the Ailao Shan caused oblique unroofing of the shear zone. This early Miocene event corresponds in time with a transition from low to very high denudation rates throughout the Himalaya and southern Tibet. This coincidence may reflect that once left‐lateral strike‐slip motion on the Red River fault zone slowed or was terminated, accommodation of much of the continued northward convergence of India occurred on the Main Central Thrust resulting in rapid uplift in the Tethyan Himalaya and Gangdese belt.
Active high‐resolution compressional wave attenuation tomography at Newberry Volcano, Central Cascade Range
Authors: Zucca John J. , Evans John R.
Published: February 2008

In this study, we use the seismic data from a previous study at Newberry to compute its three‐dimensional compressional wave attenuation structure. The data consist of eight explosions recorded on an array of about 120 seismograph stations. The shots are located on two concentric circles centered on the caldera with radii of 39 and 85 km. Attenuation is estimated using spectral ratios to compute δt* at each station for each shot. The data are inverted using a least squares technique. Our results indicate that regions of inferred boiling water (i.e., two‐phase pore fluid) are correlated spatially with regions of high attenuation not corresponding to low velocities. Two regions are of particular interest. One is underneath the Newberry II drill hole, in the central part of the caldera, where results from a flow test suggest that two‐phase fluid conditions existed in the bottom of the hole before the flow test. The other region is beneath the west flank of the volcano, which has been interpreted as an attractive geothermal prospect because of a shallow conductive anomaly. Our results are consistent with the suggestion made in an earlier paper by us that two‐phase geothermal reservoirs can be located within complex three‐dimensional structures by using jointly interpreted compressional wave velocity and attenuation images.
Geometry of the Garlock fault zone based on seismic relection data
Authors: Serpa Laura , Dokka Roy K.
Published: February 2008

Detailed examination of shot gathers from line 3 of the Consortium for Continental Reflection Profiling Mojave experiment indicates a low‐velocity, high‐amplitude seismic event that may originate from a south dipping portion of the Garlock fault zone. Part of this event was previously interpreted as a subhorizontal surface located in the midcrust. Our analysis indicates, however, that the distance to this event varies consistently with the distance of the shot and receivers from the fault zone and that the velocities are too low for seismic waves traveling through predominantly crystalline rocks. These characteristics are more consistent with the presence of a subvertical reflector located near the Garlock fault zone than with the previously inferred subhorizontal reflector beneath the seismic line.
Rheology and deformation mechanisms of an isotropic mica schist
Authors: Shea William T. , Kronenberg Andreas K.
Published: February 2008

We have investigated the transitional, semibrittle deformation of a mica schist (∼75 % biotite) by shortening cylinders cored at 0°, 45°, and 90° to foliation to varying strains, at confining pressures Pc to 500 MPa, constant strain rates ε from 1.5 × 10−7 to 1.6 × 10−4 s−1 and temperatures T from 25° to 400°C. Deformation is concentrated within one or more throughgoing, millimeter‐wide shear zones at all conditions; these localize at low strains (ε < 2%) through the nucleation and coalescence of dense sets of intragranular microkink bands (MKBs). Despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions, the differential stresses required for shear zone development vary little with fabric orientation. Biotite schist undergoes a transition from strain‐softening to steady strength mechanical response at confining pressures in the range 75 to 150 MPa. The pressure sensitivity of strength (characterized by the slope μ of the Mohr envelope) decreases from μ ∼0.5 (at Pc <100 MPa) to μ < 0.1 at pressures greater than 200 MPa, reflecting the increasing contribution of glide and kinking in biotite at higher pressures. However, dilatancy associated with microcracking and void formation along MKB boundaries persists to at least 500 MPa. Within the pressure‐insensitive regime (200 ≤ Pc ≤ 500 MPa), temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law. Samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along T decreasing or ε increasing paths. Path‐dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments. By assuming an exponential relationship between differential stress σd and strain rate ε of the form ε = C exp(ασd) exp (−Q/RT), we fit the data with two end‐member flow laws with a single activation energy Q = 89 kJ/mol, and exponential constants αss = 0.15 ± 0.01 MPa−1 and αws=0.55 ± 0.04 MPa−1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to T and ε, respectively. Application of the results to crustal deformation suggests that mica‐rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range, supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones.
Late Tertiary subsidence history of the southern Levant Margin, eastern Mediterranean Sea, and its implications to the understanding of the Messinian Event
Authors: Tibor Gideon , Ben‐Avraham Zvi , Steckler Michael , Fligelman Haim
Published: February 2008

The late Tertiary subsidence history of the southern Levant continental margin, situated in the southeastern Mediterranean Sea, was quantitatively analyzed. Paleodepth reconstruction across the margin off Ashdod suggests the existence of a deep basin in pre‐Messinian time which resembles the present one. This implies that, the deposition of the evaporites in the study area during the Messinian desiccation of the Mediterranean Sea occurred in a deep basin. The path of the tectonic subsidence of the basement since early Tertiary is generally smooth as expected from the nature of the thermal subsidence. The unconformity beneath the Messinian indicates erosion of 50–200 m at the coastal plain. In the Pliocene, the tectonic subsidence in the coastal plain and shelf area diverts from the expected thermal path and increases from 250 m to 450 m, respectively. In the Quaternary the rate of tectonic subsidence nearly resumed the predicted thermal subsidence. Sedimentation and subsidence rates decrease but are still higher than those of the pre‐Messinian. We suggest that the evolution of the southern Levant margin is most probably influenced by three main causes: (1) the Messinian event in late Miocene, (2) the deposition of large volumes of Nile derived sediments since the Pliocene, and (3) the flexural response of the lithosphere to the load from the Nile delta and/or from the uplift of the Judea Mountains (the western shoulder of the Dead Sea Transform). We interpret the latter to be the cause of the anomalous subsidence of the southern Levant margin during the Pliocene.
Experimental deformation of synthetic magnetite‐bearing calcite sandstones: Effects on remanence, bulk magnetic properties, and magnetic anisotropy
Authors: Jackson Mike , Borradaile Graham , Hudleston Peter , Banerjee Subir
Published: February 2008

We have quantified effects of experimental deformation on the magnetic properties of a set of synthetic “calcite sandstone” samples containing magnetite. The deformation was carried out in a microcomputer‐controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10−5 s−1. Most samples were deformed under dry conditions, but a few were deformed with a pore fluid present; the samples deformed under dry conditions required substantially higher differential stresses. Macroscopically ductile shortening strains of up to 25% produced the following irreversible changes in magnetic properties: (1) increased bulk coercivity, remanence coercivity, and mean anhysteretic remanence susceptibility; (2) decreased mean low‐field susceptibility; (3) decreases in the component of remanence parallel to shortening; (4) smaller decreases for most samples in the component normal to shortening, resulting in a net “rotation” of the remanence away from the shortening axis; (5) larger decreases in the normal component in a few samples, resulting in a net “rotation” of the remanence towards the shortening axis; (6) increased magnetic anisotropy; and (7) increased “deformation” of initial magnetic ellipsoids. A comparison of data for samples deformed under dry and wet conditions (higher and lower differential stresses, respectively) indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain (i.e., rotation and displacement of particles), whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress.
Suppression of dominant topographic overprints in gravity data by adaptive filtering: Southern Wyoming province
Authors: Black Ross A.
Published: February 2008

In areas of the world with a protracted tectonic history, the gravity field may be dominated by anomalies due to individual structural features developed during the latest tectonic event. These individual tectonic features often have a topographic expression. Adaptive filtering techniques can be used to suppress the gravity effects of such features. Iterative space domain techniques are particularly well suited to these types of problems. One such technique, the stochastic gradient algorithm, allows easy updating of filter coefficients at each spatial step through the data. This approach eliminates windowing and spectral estimation problems associated with cross‐spectral admittance schemes. In this study, adaptive filtering of Bouguer gravity and topography in the southern Wyoming Province was successful in suppressing gravity signatures of Laramide structural features. The regional gravity trends due to isostatic effects were significantly absent from the residual anomaly map. A large, continuous, positive gravity anomaly stretching from the southern Wind River Range to the northern end of the Laramie Range was another outstanding feature observed in the residual anomaly. This positive anomaly is about the same magnitude, size and shape as anomalies due to greenstone belts observed in other Archean cratonal areas. Scattered occurrences of greenstone lithologies are known throughout the area of the anomaly, but a continuous feature is absent in outcrop. Due to the unusual structural history of the area, it is possible that such a continuous belt does exist, but was buried at shallow crustal levels by Laramide thrusting, or later collapse of a Laramide uplift due to a change in the regional stress field.
A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic
Authors: Cande Steven C. , Kent Dennis V.
Published: February 2008

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine‐scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Atlantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points plus the zero‐age ridge axis to the composite polarity sequence. The derived spreading history of the South Atlantic shows a regular variation in spreading rate, decreasing in the Late Cretaceous from a high of almost 70 mm/yr (full rate) at around anomaly 33–34 time to a low of about 30 mm/yr by anomaly 27 time in the early Paleocene, increasing to about 55 mm/yr by about anomaly 15 time in the late Eocene, and then gradually decreasing over the Oligocene and the Neogene to the recent rate of about 32 mm/yr. The new time scale has several significant differences from previous time scales. For example, chron C5n is ∼0.5 m.y. older and chrons C9 through C24 are 2–3 m.y. younger than in the chronologies of Berggren et al. (1985b) and Harland et al. (1990). Additional small‐scale anomalies (tiny wiggles) that represent either very short polarity intervals or intensity fluctuations of the dipole field have been identified from several intervals in the Cenozoic including a large number of tiny wiggles between anomalies 24 and 27. Spreading rates on several other ridges, including the Southeast Indian Ridge, the East Pacific Rise, the Pacific‐Antarctic Ridge, the Chile Ridge, the North Pacific, and the Central Atlantic, were analyzed in order to evaluate the accuracy of the new time scale. Globally synchronous variations in spreading rate that were previously observed around anomalies 20, 6C, and in the late Neogene have been eliminated. The new time scale helps to resolve events at the times of major plate reorganizations. For example, anomaly 3A (5.6 Ma) is now seen to be a time of sudden spreading rate changes in the Southeast Indian, Pacific‐Antarctic, and Chile ridges and may correspond to the time of the change in Pacific absolute plate motion proposed by others. Spreading rates in the North Pacific became increasingly irregular in the Oligocene, culminating in a precipitous drop at anomaly 6C time.
Magnetic properties of hematite in lava flows from Iceland: Response to hydrothermal alteration
Authors: Beske‐Diehl Suzanne , Li Huiling
Published: February 2008

The present study investigates changes in the magnetic properties of hematite, more precisely low‐Ti titanohematite, in Icelandic subaerial basalts during low‐temperature metamorphism (<300°C). The results have implications for detection of secondary magnetization directions in basalts and perhaps for seafloor magnetic anomaly modeling. Samples were collected both from outcrops and from the Iceland Research Drilling Project (IRDP) core and represent 3.6 km of crust. The natural remanent magnetization (NRM) unblocked below 590°C during thermal demagnetization was assigned to magnetite and that unblocked above 610°C to hematite. Unblocking temperatures of hematite gradually increased in the laumontite zeolite zone from the original low range of 610°–650°C to 650°–680°C. We attribute this increase to the migration of a small amount of initial Ti out of the hematite lattice. Titanium migration is consistent with microprobe analyses. The intensity of the hematite magnetization correlated with that of magnetite until the top of the laumontite zeolite zone (1.8 km depth). Below this depth, the intensity of the magnetite magnetization decreased and correlation became poor due to alteration of magnetite to nonmagnetic minerals. Both magnetite and hematite appeared to retain their primary directions of magnetization to the bottom of the laumontite zeolite zone even though Ti had migrated out of the lattices. If field‐controlled chemical remanent magnetization is not produced during Ti migration, then seafloor basalts undergoing the same processes may also faithfully retain primary directions of remanence. Below 3.1 km depth in the epidote metamorphic zone, large secondary components in some samples could not be completely removed by thermal demagnetization to 550°C and were attributable to a thermo‐viscous remanent magnetization (thermo‐VRM) in magnetite. The durability of the hematite magnetization suggests that the identification of hematite components in subaerial basalts may help reveal subtle secondary components contained in magnetite directions of subaerial basalts.
Proving the age of a paleomagnetic pole: The case of the Ntonya Ring Structure, Malawi
Authors: Briden J. C. , McClelland E. , Rex D. C.
Published: February 2008

Geological application of paleomagnetic data generally relies on equating radiometric or biostratigraphic age with that of the characteristic remanence. Yet the standard of proof of paleomagnetic age is often unsatisfactory and is extremely difficult to achieve where contact tests or conglomerate tests are unavailable. In this study of the Ntonya igneous ring complex the thermoremanent (TRM) origin of one component of natural remanent magnetization (NRM) is confirmed by paleointensity determination and its carrier is identified mineralogically, and the age of this TRM is estimated by extrapolation from 40Ar/39Ar mineral ages. The characteristic remanence in the Ntonya Ring Structure, which was determined by Briden (1968) as D = 311° I = +43° α95 = 1.9°, pole 28°N 15°W, is shown to be a TRM in single‐domain/pseudo‐single‐domain magnetite, acquired in a field of approximately half the present geomagnetic field strength. The 40Ar/39Ar mineral ages (biotites 471.5 ± 7 Ma; hornblendes 510 ± 7 Ma) and the high precision of the paleomagnetic directions combine in a model of steady cooling (∼ 4.6° m.y. −1) that leads us to the conclusion that the thermoremanence was acquired over approximately 13 m.y., 522 ± 13 m.y. ago. In passing it is shown that the isothermal (IRM) component that dominates the remanence is carried by multidomain magnetite. The study shows how remanence age may be determined with precision in the absence of geological constraints. It makes the Ntonya pole a key point in the African (and Gondwanan) apparent polar wander path, correlating with Middle Cambrian glacial deposits in the Sahara at about 80°S paleolatitude.
Origin of magnetite responsible for remagnetization of Early Paleozoic limestones of New York State
Authors: Suk Dongwoo , Van Der Voo Rob , Peacor Donald R.
Published: February 2008

Scanning electron microscope and Scanning transmission electron microscope observations of thin sections and separates of the Devonian Onondaga and Helderberg limestones and Ordovician Trenton limestone in New York state allow us to identify three types of magnetite: pseudoframboids, nonspherical magnetite, and fine‐grained magnetite. Magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes. Fine‐grained magnetite, consisting of aggregates of one or more rounded single crystals, approximately 2000 Å (200 run) in diameter, could not be observed in thin sections of New York carbonates due to its small size but was found in magnetic extracts. Hysteresis measurements of magnetic extracts verify that fine‐grained magnetite is capable of carrying remanent magnetizations. However, pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain‐sized crystals that also can be carriers of the remagnetization. These data, in combination with observations from other localities, collectively imply that the Alleghenian remagnetization is carried by the fine‐grained magnetite, although pseudoframboidal and nonspherical magnetite may also contribute. Thus the Alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite. Crystallization of magnetite was mediated by fluids, with dissolution and crystallization activated by stress during the Alleghenian Orogeny. The origin of such fluids is unknown, but they may have originated through crustal scale fluid migration. Tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids.
Tectonic corridors in the south Atlantic: Evidence for long‐lived mid‐ocean ridge segmentation
Authors: Kane Kimberlee A. , Hayes Dennis E.
Published: February 2008

The South Atlantic is investigated for evidence of long‐lived (tens of millions of years), tectonic segmentation of the mid‐ocean ridge by examining a variety of geophysical parameters. These parameters include: subsidence rate of the oceanic crust (east flank, west flank, average, asymmetry), zero‐age depth, geoid height decrease with age (geoid rate), and the residual geoid field. The variability in these parameters along‐strike of the mid‐ocean ridge is typically systematic. The ridge can be subdivided into segments (flowline corridors in plan view) within which characteristics are uniform. Eight tectonic corridors have been determined in the area between the Ascension Fracture Zone and the Falkland‐Agulhas Fracture Zone, which serve to define a large scale ridge segmentation on the order of hundreds of kilometers. This segmentation is in addition to the well‐known small‐scale, near‐axis segmentation. Near‐axis geochemical data show boundaries that are generally consistent with the tectonic segmentation. Many of the primary geophysical observations are found to be internally inconsistent with simple thermal conduction models for oceanic crust. Such observations include asymmetric crustal subsidence, large along‐strike variations in subsidence rate and geoid rate, the lack of a systematic correlation between subsidence rate and geoid rate, and the abrupt nature of some of the inferred tectonic boundaries. Thus, the data indicate that in addition to uniform lithospheric cooling, other factors such as variable asthenospheric temperatures are important contributors to the creation and subsequent modification of the oceanic crust and possibly the oceanic lithosphere.
Lithospheric necking and regional isostasy at extensional basins 2. Stress‐induced vertical motions and relative sea level changes
Authors: Kooi Henk , Cloetingh Sierd
Published: February 2008

We investigate long‐wavelength differential vertical motions at extensional basins which are the result of temporal changes in the intraplate stress field in the lithosphere. These stress‐induced vertical motions are intimately coupled to the state of flexure (flexural curvature) of the lithosphere. We compare a previous model of flexure under the sediment load only (FSL model) and a recent more consistent model, which incorporates regional compensation of other vertical loads induced by lithospheric extension as well. These additional loads are controlled by the depth at which the necking of the lithosphere occurs (DON model). Tlie FSL model and models assuming a shallow level of necking (≤15 km) are very similar, they predict tilting of the basin with the hinge point landward of the shelf edge. Compression results in uplift of the landward pan of the basin, which generally corresponds to the coastal plain or inner to middle shelf, and downwarping of the outer shelf and slope. The differential vertical motions correspond to relative sea level cycles which are out of phase in different parts of the basin. Internal architecture of systems tracts can be different from predictions by eustasy. By contrast, deeper levels of necking (>20 km) predict a uniform sense of motion landward of the shelf edge. Type I sequence boundaries can be very easily induced for these conditions, and sequence stratigraphie features are probably hard to distinguish from predictions by eustasy. These results are for a pure elastic plate rheology. Stress‐induced vertical motions for a more realistic depth‐dependent rheology are more complicated because of the additional effect of stress‐induced reduction of flexural rigidity. For these conditions, gradual buildup of tension causes a complete relative sea level cycle. For high tensional stress levels, total vertical motions become rather similar to vertical motions for compression. This may be of importance for the issue of global synchroneity of relative sea level changes.
Oceanic faulting and fault‐controlled subseafloor hydrothermal alteration in the sheeted dike complex of the Josephine Ophiolite
Authors: Alexander Robert J. , Harper Gregory D. , Bowman John R.
Published: February 2008

Based upon detailed mapping (1:10 and 1:100) of a large water‐polished outcrop of the sheeted dike/gabbro transition zone in the Josephine ophiolite of NW California and SW Oregon, the following history of alternating episodes of magmatic, structural, and hydrothermal events has been documented using crosscutting relationships, petrography, geochemistry, and strontium and oxygen isotopic data: (1) crystallization of gabbro and later subvertical mafic dike injection, (2) amphibolite facies metamorphism, (3) extensional faulting and tilting of dikes, (4) continued faulting, tilting, and dike injection associated with retrograde metamorphism at greenschist facies conditions, (5) continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid/rock ratios, and (6) subvertical injection of a highly fractionated dike which truncates all previous features (features 1–5). Trace element geochemistry indicates the highly fractionated dike (feature 6) is genetically related to the other dikes and thus was intruded at or near the paleospreading axis. Hence, all previous events (features 1–5) can be constrained to have occurred at the rift axis, including large‐scale tilting (∼ 50°) of the sheeted dikes and extensional faulting. The [87Sr/86Sr]initial ratios from recrystallized whole rocks and from hornblende, epidote, and prehnite separates from veins display a systematic increase with relative age, from 0.7033 for altered gabbro screens and mafic dikes to 0.7049 for prehnite in the youngest oceanic fault rock. Calculated oxygen isotope fluid compositions for the same suite of samples range from δ18Ofluid = +5 to −1 with time, indicating a change to a seawater‐dominated hydrothermal system with time, consistent with observed increased permeability due to seafloor extensional faulting. The crosscutting relationships, alteration mineral assemblages, and isotopic data suggest (1) an early stage of high‐temperature (≥450°C) hydrothermal alteration with low permeability (i.e., grain‐scale flow), followed by (2) a decrease in temperature (∼350to ≤200°C), and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis. The consistency of these crosscutting relationships at similar pseudo‐stratigraphic levels at different localities in the Josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading (slow‐ to intermediate‐spreading centers), such as the modern Mid‐Atlantic Ridge.
Paleomagnetism of 122 Ma plutons in New England and the Mid‐Cretaceous Paleomagnetic Field in North America: True Polar wander or large‐scale differential mantle motion?
Authors: Van Fossen Mickey C. , Kent Dennis V.
Published: February 2008

A paleomagnetic study of Cretaceous White Mountains plutonic complexes in New Hampshire and Vermont yields high unblocking temperature, dual polarity magnetizations in different types of igneous rocks. The resulting pole position for three plutons (71.9°N, 187.4°E, A95 = 6.9°, age = 122.5 Ma) agrees with previously published mid‐Cretaceous poles for North America, which together give a mid‐Cretaceous standstill reference pole slightly revised from Globerman and Irving [1988] at 71.2°N, 194.1°E (A95 = 3.7°, N = 5 studies). We argue on the basis of the wide geographic distribution of these studies, the variety in tectonic settings and rock types, positive reversal tests, and an overall reversal pattern consistent with geomagnetic polarity time scales, that this mean pole represents the North American mid‐Cretaceous reference field for nominally 36 m.y. (124 to 88 Ma). The standstill pole limits to within ±4°, the motion of the North American plate relative to the Earth's spin axis. During the same mid‐Cretaceous interval, the New England hotspot track (124 Ma Monteregian Hills, 122.5‐Ma Cretaceous White Mountains, and 103‐ to 84‐Ma New England seamounts) requires 11°±4° of north‐poleward motion of North America, in direct conflict with the paleomagnetic standstill. A similar (∼13°) discrepancy is independently demonstrated between the spin axis and the Tristan da Cunha hotspot track on the African plate during the mid‐Cretaceous interval. The hotspot/spin axis discrepancies ended by ∼90 Ma when it is shown that both Atlantic hotspots agree with North American and African dipole paleolatitudes and present‐day locations. Nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis, leaving as possible interpretations true polar wander and large‐scale (but differential) mantle motion. The southerly motion of the mid‐Cretaceous Louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics. The motions of the three widely separated mid‐Cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid‐Cretaceous “superplume.”
The horizontal velocity field in the deforming Aegean Sea region determined from the moment tensors of earthquakes
Authors: Jackson James , Haines John , Holt William
Published: February 2008

We use the spatial distribution of seismic moment tensors of earthquakes in the Aegean region over the time interval 1909–1983 to recover a continuous horizontal velocity field that describes the overall deformation of the lithosphere at large length scales. The calculated velocity field is dominated by two effects: (1) an E‐W right‐lateral shear of the eastern Aegean, related to motion on the North Anatolian fault becoming distributed as it enters the Aegean; and (2) a N‐S extension, probably related to the sinking of the slab in the Hellenic Trench. The southern part of the central Aegean is found to be moving in a SW direction relative to Europe at a rate of about 30 mm/yr (probably a lower bound, with an error of around ± 10 mm/yr) and rotating clockwise. In the seismogenic upper crust this velocity field is accommodated by right‐lateral strike‐slip faults in the eastern Aegean and by normal faults that rotate clockwise in central Greece. A comparison of paleomagnetic declination data with the expected rotation of rigid elongate inclusions in the velocity field shows in most places an agreement in sense and approximate agreement in rate of rotation. Expected rotation rates of line elements are sometimes too low: probably because our derived velocity field is smoothed and unable to match locally high strain rates. There is only one part of the region where line elements are predicted to rotate in either clockwise or counterclockwise directions, depending on their orientation; this is in western Turkey, which, coincident ally, is the only place where paleomagnetic rotations in both directions have been observed. This coincidence in particular suggests to us that the analogy of rigid elongate inclusions in the velocity field, responding to forces on their bases, may be useful in predicting the senses and approximate rates of rotation of crustal blocks in deforming continental regions. The velocity field we obtain preserves the strike directions of the major faults as directions of zero length change, in spite of considerable smoothing. We use this observation to speculate that the interaction between the upper crust and the rest of the lithosphere beneath it may involve an interplay of effects. On one hand the variation of strength with direction in the crust may control the strike directions of faults that form or become reactivated and may also limit the velocity fields that are allowable. On the other hand, the fault bounded blocks may rotate in the velocity field in response to forces on their bases, which would require the velocity field to change with time if the directions of zero length change are fixed to the blocks.
Magnetic susceptibility of the Mont‐Louis andorra ilmenite‐type granite (Pyrenees): A new tool for the petrographic characterization and regional mapping of zoned granite plutons
Authors: Gleizes Gérard , Nédélec Anne , Bouchez Jean‐Luc , Autran Albert , Rochette Pierre
Published: February 2008

We have observed remarkably consistent patterns of concentric zonation in the values of low‐field magnetic susceptibility measurements over the Variscan age Mont‐Louis Andorra granite pluton of the eastern Pyrenees of Andorra, Spain and France. This zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature (as defined by chemical analysis) and the magnetic susceptibility magnitude of the rocks. It reflects the dominantly paramagnetic nature of the granite, carried by Fe‐bearing silicates, and this is demonstrated by the fact that low‐field susceptibilities of representative specimens are almost equal to (1) their high‐field susceptibilities and (2) their calculated susceptibilities using Fe contents, assuming a null ferromagnetic contribution. We conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons.
Blueschist‐facies metamorphism, shear heating, and P‐T‐t paths in subduction shear zones
Authors: Peacock Simon M.
Published: February 2008

The low temperatures recorded by blueschist‐facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15–50 km. In this paper, steady state and transient pressure‐temperature‐time (P‐T‐t) paths followed by subducted material are calculated using analytical expressions presented by Molnar and England (1990), supplemented by two‐dimensional numerical calculations. In the absence of shear heating, calculated steady state P‐T‐t paths are very cold, intersecting 1 GPa (10 kbar) at T < 200°C for a wide range of subduction parameters. For the case of constant shear stress (τ) in the subduction shear zone and typical thermal parameters, steady state P‐T‐t paths intersect the blueschist facies for τ = 10–60 MPa and 0–100 MPa for convergence rates of 10 and 3 cm yr−1, respectively. Blueschist‐facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 MPa. For the case of shear stress increasing linearly with depth, steady state P‐T‐t paths intersect the blueschist facies when τ = 1–9% P and 0–14% P for convergence rates of 10 and 3 cm yr−1, respectively. Subduction zone shear stresses may be considerably lower, and possibly zero, if blueschist terrains represent material metamorphosed during the early stages of subduction (first 5–10 m.y.) while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation. Subduction zone shear stresses of several tens of MPa (at 35 km depth) or a few percent of lithostatic pressure are consistent with a steady state model of Franciscan metamorphism, a progressive underplating model for the formation of the Pelona Schist inverted metamorphic gradient, and surface heat flow measurements in the NE Japan subduction zone.
High‐temperature magnetostriction of magnetite and titanomagnetites
Authors: Moskowitz Bruce M.
Published: February 2008

Magnetostriction of synthetic polycrystalline magnetite and titanium‐rich titanomagnetites has been measured from 25°C to their Curie temperatures. These are the first reported measurements of magnetostriction above room temperature for titanomagnetites. The magnetostriction constants are measured with a recording rotating field magnetostrictometer, which uses a strain gage bridge technique and two rotating SmCo5 permanent magnets. Magnetostriction is recorded continuously as the sample is slowly heated and cooled in a fixed magnetic field. Measurements were made on sintered polycrystalline samples of magnetite, titanomagnetite (Fe3‐xTixO4, x=0.0, 0.4, and 0.6), and an aluminum‐magnesium substituted titanium‐rich titanomagnetite (AMTM60). The thermal dependence of the polycrystalline magnetostriction constant λs(T) varies as (1‐T/Tc)1.3 for titanium‐rich titanomagnetites and as (1‐T/Tc)0.9 for pure magnetite. the polycrystalline magnetostriction constant is well approximated by a power law equation in terms of the saturation magnetization, λs(T)αMs(T) n, where n=2.31 (TM0), 3.47 (TM40), 2.75 (TM60), and 2.80 (AMTM60). The magnetostriction results for AMTM60 are in excellent agreement with previous predictions based on domain observations.
The morphology of lava flows in planetary environments: Predictions from analog experiments
Authors: Griffiths Ross W. , Fink Jonathan H.
Published: February 2008

The rates of surface cooling and lateral spreading are evaluated for lava flows on the surface of Earth, Venus, Mars, the Moon, and Io. Differences between the flow morphologies expected in these environments are then predicted under the assumption that the results of recent laboratory simulations of lavas using wax extruded beneath cold water (Fink and Griffiths, 1990) can be carried over. These experiments involved the spreading of viscous liquid under gravity in the presence of a solidifying surface crust and revealed a set of four distinct surface morphologies. Transitions from one morphology to the next occurred in a well‐defined sequence when the relative rates of surface solidification and lateral spreading were varied. In comparison with subaerial flows on Earth, the surface of lavas solidifies faster on Venus, where the dense atmosphere provides additional convective heat transfer. Lateral flow of lava is much slower under the smaller gravity of the Moon. Hence, for a given extrusion rate and viscosity, solid crust is predicted to form closer to the vent on both the Moon and Venus than on Earth. Equivalently, faster extrusion rates than those on Earth are required on the Moon, Io, and Venus in order to produce a given surface morphology. A comparison of observed structure, flow rates, and estimated viscosities for some well‐documented terrestrial lava flows shows reasonable agreement with the predictions of the model. Extrapolation to extraterrestrial lavas is then illustrated by constraining the effusion rate for a Martian lava flow whose composition is assumed. Spectral information on the chemical composition of Martian flows to be sought by the thermal emission spectrometer on the Mars Observer mission, in conjunction with detailed morphologic observations from the Mars Observer camera, could allow much more accurate estimates of effusion rates for Martian lavas.