摘要:

Norman D. Watkins' career in geophysics spanned some of the most exciting years hi the history of the field. He was born in England at Sunbury‐on‐Thames on February 15, 1934. He took a B.Sc. from the University of London hi geology with mathematics and physics. His postgraduate work started in 1957, and by 1964 he had completed an M.Sc. in exploration geophysics at Birmingham, an M.Sc. in geophysics at Alberta, and a Ph.D. at London, also in geophys

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03507

年代:1980

数据来源: WILEY

摘要:

Several different models for the secular variation of the earth's magnetic field have been proposed. None of the proposed models fits well the observational evidence about the secular variation, which calls for an increase in the nondipole field contribution toward virtual geomagnetic pole (VPG) scatter of a factor of 2 on going from equator to pole, a symmetric distribution of VGP's at all latitudes (and hence a nonsymmetric distribution of directions), and a drifting portion of the non‐dipole field significantly greater than the standing portion. A new model for secular variation is introduced which fits the observed data better than the others. The secular variation data from over 1000 lava flows from Iceland are discussed, and it is shown that the low‐latitude VGP's do not fit in with a Fisher distribution. Instead, the whole distribution of VGP's is well fit by a combination of about 0.9 of a Fisher distribution and about 0.1 of a random distribution. The low‐latitude poles also come from lavas which have a lower than normal intensity of magnetization. These low‐latitude poles are caused either by excursions of the earth's magnetic field or during times when the earth's magnetic field is undergoing a reversal. By looking at the statistics of when these low latitude poles occur in the section, it is possible to infer that either there are many undetected reversals or there is a high chance (about 40%) that when the dipole field decreases in intensity (before a possible reversal), it can increase in intensity with the same polarity that it origina

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03511

年代:1980

数据来源: WILEY

摘要:

A paleomagnetic study has been made of the red dacite and pillow basalt of the Duffer Formation of the Pilbara craton, Australia. The dacite is currently the oldest dated rock unit in Australia, having a zircon U‐Pb age of 3452±16 Ma. Vector analysis of alternating field and thermal demagnetization results yield three components of magnetization. The high‐temperature, high‐coercivity hematite (H) component in the red dacite appears to be postfolding in age, as does a low‐temperature, low‐coercivity (L) component observed in the pillow basalt. However, an intermediate‐temperature, intermediate‐coercivity magnetite (M) component observed in both the pillow basalt (five sites) and the red dacite (six sites) is clearly prefolding in age and thus older than 3.0 Ga. This M component appears to be the oldest magnetization yet recorded in paleomagnetic studies and yields a paleomagnetic pole, probably original, at 43.9°N, 86.3°E (A95= 7.2°). A tentative Archean apparent polar wander path for Australia suggests that the L and H components are also ancient magnetizations, probably 3.0 and 2.6 Ga old, respectively. The primary M component establishes the existence of the geomagnetic field at

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03523

年代:1980

数据来源: WILEY

摘要:

All available paleomagnetic data for the last 130 m.y. meeting certain selection criteria are drawn from standard compilations and returned to precontinental drift site locations and orientations using spreading poles based on sea floor magnetic anomalies. The resulting data set is divided into six time periods, and global patterns of inclination and declination anomalies are used to identify important nondipole components. Spherical harmonic analysis is then performed to evaluate the corresponding coefficients. An axially symmetric quadrupole component (g20) is found to be important for at least the last 100 m.y., and an axially symmetric octupole component (g30) for the last 50 m.y. Possible correlations are noted between the variation with time of the reversal frequency of the geomagnetic field, compiled by A. Cox, the polarity bias published by Irving and Pullaiah, and our nondiple field magnitude. Both current explanations for the nondipole components seen in paleomagnetic data, the offset dipole model and Cox's zonal nondipole model, are incompatible with the large values of g30found in this study. However, as a possible alternative, Cox's model can be generalized to accomodate the new information.

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03529

年代:1980

数据来源: WILEY

摘要:

Paleomagnetic studies indicate that the present and historical nondipolar fields are typical in some respects to the nondipolar component of the paleofield generally. We apply this principle to the investigation of the global distribution of the auroral zones during epochs of reduced dipole field strength. The poleward borders of the auroral zones map out along field lines to a distance where the dipolar term dominates the nondipolar terms even for reductions to 10% of the present dipole strength. Thus the dipole scaling relations that describe the interaction with the solar wind can be used to map out the contour of the polar borders that occur when the dipole strength is reduced while the nondipolar terms are held fixed. The results show that the nondipolar terms produce elongated auroral zones that extend to appreciably lower latitudes than are reached when the nondipolar terms are ignored.

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03549

年代:1980

数据来源: WILEY

摘要:

The magnetic properties of six ophiolite complexes from around the world, ranging in age from Jurassic to Miocene, are presented. An emphasis is placed in our study on the petrologic and isotopic data from these ophiolite complexes in order to determine first whether the rock samples presently available represent the pristine ocean crust or whether they have been altered subaerially since their formation. Five of the ophiolites are found to be acceptable, and the conclusion is overwhelmingly in favor of a marine magnetic source layer that includes not only the pillow lavas but also the underlying dikes and gabbro. At the moment, however, our observations do not suggest that the magnetic contributions of the basaltic dikes should be overlooked in favor of gabbro. A second important conclusion is that nearly pure magnetite could indeed be a magnetic carrier which contributes to marine magnetic anomalies. It only awaits discovery by deeper ocean crustal penetration by future Deep Sea Drilling Project legs.

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03557

年代:1980

数据来源: WILEY

摘要:

Magnetic modeling studies have been conducted on the newly named Watkins seamount in the central Pacific. The results, while preliminary, give a paleomagnetic pole position of 69.8°N and 13.3°E. The pole position lies near pole positions of other Cretaceous seamounts from the central Pacific, indicating that the seamount may be Cretaceous in age. The paleoinclination suggests that the seamount formed south of the magnetic equator, a hypothesis consistent with overall Pacific plate motion since the Late Cretaceou

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03567

年代:1980

数据来源: WILEY

摘要:

A large number of sedimentary rock formations on the Colorado Plateau and in the Wyoming foreland have been investigated paleomagnetically in an attempt to determine the geomagnetic polarity during the Middle and the early Late Jurassic, a period frequently postulated to have been of constant normal polarity. A survey of published paleomagnetic litature shows this postulate to be without grounds. A wide range of sedimentary lithologies, both red and nonred, were investigated. Most formations displayed a complex, multivectorial magnetization, even to very high stability ranges. However, a strikingly large amount of reversed polarity was observed and relatively little normal Jurassic polarity. Because of the widespread geographic data base, the multitude of lithologies, and the amount of time represented, it is tentatively concluded that the data at least partially reflect the character of the geomagnetic field during the Jurassic. Weak intensities in all formations, and the instability, may support the marine magnetic anomaly hypothesis of weak geomagnetic field intensity for this time. These data do not support the hypothesis of a long period of normal polarity during the Jurassic. They in turn, suggest that the Jurassic field might have frequently been of reversed polarity.

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03572

年代:1980

数据来源: WILEY

摘要:

Paleomagnetic data are reported from kimberlite occurrences of three distinct age groups: (1) National (one pipe), D = 221°, I = −51°, k = 41, α95= 7.2°, age ∼1140 m.y., (2) Swartruggens (three fissures), mean D = 293°, I = −46°, k = 22, α95= 26,9°, age = ∼150 m.y., and (3) six bodies in and adjacent to Lesotho, D = 350°, I = −69°, K = 66, α95= 8.3° with age spread of ∼90 + 5 m.y. Analysis of these and previously published data suggests that most individual kimberlite bodies give acceptable paleomagnetic (as distinct from virtual geomagnetic) poles. This leads to a revised apparent polar wander curve for Africa in upper Mesozoic time and suggests that the time of kimberlite emplacement does coincide with intervals of rapid plate motion, although this cannot be the primary c

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03587

年代:1980

数据来源: WILEY

摘要:

Pelagic carbonate rocks possess many suitable characteristics for paleomagnetic and magnetostratigraphic studies. Paleomagnetic results are summarized for seven lengthy sections of pelagic limestones and marls from Umbria and the southern Alps in Italy. Differences in apparent polar wander paths from these two regions are interpreted in terms of tectonic rotation of allochthonous Umbria. The magnetic stratigraphies of the paleontologically dated sections are independent of their tectonic differences and are combined to form a continuous record of geomagnetic polarity for the Barremian through Maastrichtian stages of the Cretaceous. All but one of the reversals in these sections are confirmed by duplication in at least one other section. Additional Cretaceous reversals have been reported in other land sections and in DSDP (Deep Sea Drilling Project) and IPOD (International Program of Ocean Drilling) cores. Some of these reversals are not defined well magnetically, and confirmation of others is clouded by imprecise paleontological dates. If real, they are probably of short duration. The confirmed reversal sequence correlates well with the Cretaceous oceanic magnetic anomaly sequence. The ages of certain key anomalies are altered: Late Cretaceous anomalies 29–34 are younger, and Early Cretaceous anomaliesM0 andM1 are older than previously thought. The longer duration of the Cretaceous magnetic quiet interval of normal polarity results in a reduction of corresponding sea floor spreading rates to about 70% of earlier values, but they are still appreciably higher than during formation of the precedingMsequence anomalie

ISSN:0148-0227    

DOI:10.1029/JB085iB07p03597

年代:1980

数据来源: WILEY