摘要:

We consider the problem of constructing a time‐dependent map of the magnetic field at the core‐mantle boundary. We use almost all the available data from the last 300 years to produce two maps, one for the period 1690–1840 and the other for 1840–1990. We represent the spatial dependency of the field using spherical harmonics, the time dependency using a cubic B‐spline basis, and seek the smoothest solutions compatible with the observations. Particular attention must be paid to the effects of the crustal field in the data. We argue that for observations from permanent magnetic observatories the most efficient strategy is to use first‐differences of annual means; for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field. The resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models, demonstrating that very simple models fit the data. The resulting time‐dependent field map exhibits much of the same structure in the field and its secular variation identified in ea

ISSN:0148-0227    

DOI:10.1029/92JB01591

年代:1992

数据来源: WILEY

摘要:

The secular variation of the Earth's magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle. Adequate observations to map the magnetic field at the core‐mantle boundary extend back over three centuries, providing a model of the secular variation at the core‐mantle boundary. Here we consider how best to analyze this time‐dependent part of the field. We propose that the first step should be to seek the steady core flow that best fits the field maps, isolating what we call the steady secular variation, the part of the secular variation explained by steady core flow. To calculate steady core flow over long time periods, we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability. We perform this procedure for the periods 1840–1990 and 1690–1840 and find that well over 90% of the variance of the time‐depedent field can be explained by simple steady core flow. The core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals. Although we can explain a large part of the signal with these flows, none of them provides an adequate explanation of the secular variation. In particular, the fit to the original observations from which the field models were derived is inadequate. We examine the residual secular variation, the part of the signal that remains. We argue that it is dominated by the effects of nonsteady flow and magnetic diffusion. We do not find any compelling evidence for torsional oscillations as the dominant ingredient of this nonsteady part of the secula

ISSN:0148-0227    

DOI:10.1029/92JB01469

年代:1992

数据来源: WILEY

摘要:

Observations of the Earth's nutations are used to explore the possible presence of a conducting layer at the base of the mantle. The existence of such a layer is suggested by recent experiments on iron and silicate mixtures at high pressures and temperatures (Knittle and Jeanloz, 1986, 1989), which indicate that iron may react with silicate minerals to produce metallic compounds. Nutations provide an ideal tool for investigating the existence of such a layer because they are sensitive to the electrical properties within a thin magnetic boundary layer along the core‐mantle boundary (CMB). Evidence in favor of a conducting layer is found in the effects of ohmic dissipation which cause the amplitude of the Earth's nutation to be out‐of‐phase with tidal forcings. Although out‐of‐phase components in the Earth's nutations are conventionally attributed to the dissipative influences of ocean tides and mantle anelasticity, these effects appear incapable of explaining nutation amplitudes determined from very long baseline interferometry (VLBI) observations. The additional effects of ohmic dissipation, enhanced by the presence of a thin conducting layer at the CMB, are sufficient to reconcile theory and observation. The largest mismatch between observations and theory, taking account of ocean tides and mantle anelasticity, is a discrepancy of 0.39 ± 0.04 milliarcseconds in the out‐of‐phase component of the retrograde annual nutation. This discrepancy can be eliminated if the lowermost 200 m of the mantle has a conductivity of 5 × 105S m−1, and the magnetic energy in the spherical harmonic components of the field at the CMB in degreesl>12 is approximately 4 times greater than the energy inferred in degreesl≤ 12 from surface observations. Unfortunately, the VLBI observations do not uniquely resolve the mantle conductivity and magnetic energy, so that trade‐offs exist between these two parameters. Nevertheless, a high mantle conductivity is favored on the grounds that the alternative, involving high levels of magnetic energy, yields an excessive amount of ohmic heating. Other interpretations of the VLBI discrepancy are also discussed. One such alternative involves viscous dissipation at the CMB, requiring a kinematic visc

ISSN:0148-0227    

DOI:10.1029/92JB00977

年代:1992

数据来源: WILEY

摘要:

Anisotropy of anhysteretic remanence (AAR), scanning electron microscopy (SEM), and X ray pole figure goniometry studies of clay‐rich sediments were conducted to delineate the interactions between magnetite and clay particles which cause inclination shallowing during compaction. These studies used synthetic sediments composed of kaolinite or illite and two grain sizes of magnetite, 0.45 μm and 2–3 μm. Natural marine sediments which contained 40–50% clay were also used. The sediments were compacted by pressures as high as 0.157 MPa. The sediments' porosity decrease suggests that the compaction experiments model burial depths up to 400–500 m. The main result of this study is that inclination shallowing and void ratio versus pressure curves show two points where behavior changes. A rapid decrease in inclination and void ratio occurs up to a pressure of 0.02 MPa. This is accompanied by a magnetic intensity decrease of 20–30%. Compaction at pressures between 0.02 MPa and 0.05 MPa causes a further decrease in these parameters at moderate rates, while compaction at pressures above 0.05 MPa causes very little change. Percent magnetic anisotropy and X ray pole figure intensity ratio versus pressure curves show only one point where behavior changes at 0.05 MPa with a rapid increase at lower pressures and little change at higher pressures. A model to explain this behavior, based on the SEM observations as well as the magnetic anisotropy, pole figure, and inclination data, suggests that compaction up to 0.02 MPa causes a decrease in pore volume with little reorientation of clay particles. Some magnetic particles are firmly attached to clay particles, whereas other loosely attached particles are either randomized or subvertical ones are preferentially disturbed causing inclination shallowing and an intensity decrease. At pressures between 0.02 MPa and 0.05 MPa, magnetite particles have become firmly attached to clay particles and start to follow the reorientation of clay particles as the clay fabric develops. At 0.05 MPa there is a major change in the clay microstructure with a horizontal fabric becoming evident. Compaction at higher pressures causes little further volume loss and clay particle reorientation and hence little additional inclination shallowing. The randomization/disturbance process suggested by this model causes slightly more than half of the inclination shallowing observed in the samples studied, whereas magnetite‐clay attachment causes the remainder of the inclina

ISSN:0148-0227    

DOI:10.1029/92JB01589

年代:1992

数据来源: WILEY

摘要:

The collision and accretion of the Alexander terrane profoundly influenced the geologic history of Alaska and western Canada; however, the terrane's displacement history is only poorly constrained by sparse paleomagnetic studies. We studied the paleomagnetism of the Hound Island Volcanics in order to evaluate the location of the Alexander terrane in Late Triassic time. We collected 618 samples at 102 sites in and near the Keku Strait, Alaska, from the Late Triassic Hound Island Volcanics, the Permian Pybus Formation, and 23‐Ma gabbroic intrusions. We found three components of magnetization in the Hound Island Volcanics. The high‐temperature component (component A) resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area. We think it is primary, or acquired soon after eruption of the lavas, principally because the directions pass a fold test. The paleolatitude indicated by this component (19.2° ± 10.3°) is similar to those determined for various portions of Wrangellia, consistent with the geologic interpretation that the Alexander terrane was with the Wrangellia terrane in Late Triassic time. We found two overprint directions in the Hound Island Volcanics. Component B was acquired 23 m.y. ago due to intrusion of gabbroic dikes and sills. This interpretation is indicated by the similarity of upper‐hemisphere directions in the Hound Island Volcanics to those in the gabbro. Component C, found in both the Hound Island Volcanics and the Permian Pybus Formation, is oriented northeast and down, fails a regional fold test, and was acquired after regional deformation around 90 to 100 Ma. This overprint direction yields a paleolatitude similar to, but slightly higher than, slightly older rocks from the Coast Plutonic Complex, suggesting that the Alexander terrane was displaced 17° in early Late Cretaceous time. The occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the Hound Island Volcanics (Hillhouse and Grommé, 1980). Finally, great‐circle analysis of the paleomagnetic data from the Pybus Formation suggests the Alexander terrane may have been in the northern hemisphere in

ISSN:0148-0227    

DOI:10.1029/92JB01361

年代:1992

数据来源: WILEY

摘要:

Samples of Tertiary gabbro from 24 sites in the Keku Strait, Alaska, help constrain the displacement history of the Alexander terrane. Step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant40Ar/39Ar age of 23.1 ± 1.7 Ma. The characteristic magnetization resides in magnetite, is easily isolated by thermal and alternating field demagnetization, and has both normal and reversed polarities. The mean paleomagnetic pole, with no structural correction, is latitude 87.1°N, longitude 141.6°E,A95= 10.1°, with N = 20 sites. This pole indicates insignificant tectonic displacement (0.5° ± 8.2° southward) and rotation (0.6° ± 15.2° counterclockwise). We therefore conclude that any northward displacement or vertical axis rotation of the Alexander terrane, and/or tilting in the vicinity of the Keku Strait must have occurred bef

ISSN:0148-0227    

DOI:10.1029/92JB01360

年代:1992

数据来源: WILEY

摘要:

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 fromGloberman 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 gav

ISSN:0148-0227    

DOI:10.1029/92JB01466

年代:1992

数据来源: WILEY

摘要:

A high‐precision, high‐resolution geologic map explicitly documents relationships between tectonic features and large steep‐sided, sulfide‐sulfate‐silica deposits in the vigorously venting Endeavour hydrothermal field near the northern end of the Juan de Fuca Ridge. Water depth in the vent field varies from 2220 to 2200 m. Location of the most massive sulfide structures appears to be controlled by intersections of ridge‐parallel normal faults and other fracture‐fissure sets that trend oblique to, and perpendicular to the overall structural fabric of the axial valley. The fractured basaltic substrate is primarily composed of well‐weathered pillow and lobate flows. As presently mapped, the field is about 200 by 400 m on a side and contains at least 15 large (>1000 m3) sulfide edifices and many tens of smaller, commonly inactive, sulfide structures. The larger sulfide structures are also the most vigorously venting features in the field; they are commonly more than 30 m in diameter and up to 20 m in height. Actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller. Maximum venting temperatures of 375°C are associated with the smaller structures in the southeastern portion of the field; highest‐temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20°–30°C lower. Hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture‐controlled black smoker activity near the top of the structures. A different type of high temperature venting takes place from the upper sides of the structures in the form of “overflow” from fully exposed, quiescent pools of buoyant 350°C vent water trapped beneath overhanging sulfide‐sulfate‐silica ledges, or flanges. These flanges are attached to the upper, outer walls of the large sulfide edifices. Two types of diffuse venting in the Endeavour field include a lower temperature 8°–15°C output through colonies of large tubeworms and 25°–50°C vent fluid that seems to percolate through the tops of overhanging flanges. The large size and steep‐walled nature of the these structures evidently results from sustained venting in a “mature” hydrothermal system, coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris an

ISSN:0148-0227    

DOI:10.1029/92JB00174

年代:1992

数据来源: WILEY

摘要:

Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot‐spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow‐spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow‐spreading ridges to the conductive heat loss that enables the mantle to cool as it rises ben

ISSN:0148-0227    

DOI:10.1029/92JB01749

年代:1992

数据来源: WILEY

摘要:

To estimate the effects of temperature‐ and pressure‐dependent viscosity on passive flow beneath midocean spreading centers, we have developed a numerical approach combining a finite element method to solve for the three‐dimensional flow and a finite difference solution for temperatures. This numerical method is applied to study an idealized spreading center consisting of a 100‐km transform fault offsetting two ridge segments. Viscosities are given according to the temperature and pressure dependence of thermally activated creep for Newtonian and non‐Newtonian rheology. Melt production rates are calculated by using an adiabatic melt relation and our flow and temperature fields. The density variation of mantle due to thermal expansion is converted into gravity anomaly. The results are compared with those of a model with uniform viscosity. The thickening of the high‐viscosity layer causes mantle upwelling in the variable‐viscosity model to be stronger but narrower. Faster upwelling reduces heat loss to the surface during ascent, leading to greater melt production. Under the same boundary conditions, the variable‐viscosity model creates thicker crust and the predicted thickness is more nearly independent of spreading rate. The zone of melt production is narrower and the rate of melt production decreases more abruptly near the transform fault. The gravity anomaly caused by the mantle thermal structure for the variable‐viscosity model is similar to that of the constant‐viscosity model with a maximum differenc

ISSN:0148-0227    

DOI:10.1029/92JB01467

年代:1992

数据来源: WILEY