M. Fuller

Statistical assessment of the preferred longitudinal bands for recent geomagnetic reversal records

Records of recent polarity reversals recorded in sediments show a preponderance of transitional VGPs situated in the longitudinal sector over the Americas and, to a lesser extent, over its antipode. A series of tests based on the methods of circular statistics show that this distribution is not consistent with the statistical fluctuations of a random probability distribution. The longitudinal bias in the distribution, therefore, requires a physical explanation.

A possible source for the Martian crustal magnetic field

Abstract The crustal magnetic field of Mars differs markedly from Earth’s as the Martian features are an order of magnitude stronger and are not distributed over the whole globe as on Earth, but are concentrated in a band in the southern highlands. Unless the Martian dynamo gave a surface field more than an order of magnitude stronger than the current geodynamo gives on Earth, which seems unlikely, some special process, which has not affected the terrestrial crust, operated on Mars to give the observed distribution of anomalies and to magnetize its ancient crust so efficiently. We suggest that water reacting with ancient Martian atmospheric carbon dioxide could give rise to fluids that dissolve igneous rocks in the crust and precipitate iron-rich carbonates, as observed in Martian meteorites. In the southern highlands, thermal decomposition of such iron-rich carbonates during metamorphism could give rise to plentiful single-domain magnetite and generate a potent source for the Martian crustal field. The lack of anomalies in the northern plains may result from higher water–rock ratios that prevented the formation or decomposition of iron-rich carbonates or the survival of single-domain magnetite.

Paleomagnetic records of secular variation from Lake Michigan sediment cores

Abstract Continuous measurements of the natural remanent magnetization of six cores from central Lake Michigan have been made, using a cryogenic magnetometer designed to permit the passage of long cores through its sensing region. The output of each of the three-component sensors is deconvolved with the appropriate system response function and combined to yield continuous records of inclination, declination and intensity with a resolution of approximately 6 cm. The paleomagnetic record extends to about 13,500 years B.P. and includes two features which may be excursions of the earths field. Other features may be correlated between different cores, and the magnetic sequence is, in general, consistent with the stratigraphic sequence. There is an indication of a repeated sequence of field changes, which is somewhat reminiscent of the solar field cycle.

A new quality check for absolute palaeointensities of the Earth magnetic field

Absolute palaeointensity determinations of recent lavas from Mt. Etna have been carried out using the Koenigsberger-Thellier-Thellier (KTT) method but with two different data presentations. The first, which is the standard method, compares the natural remanent magnetization (NRM) demagnetized and the thermoremanent magnetization (TRM) acquired in successive temperature intervals cumulatively. The second compares the NRM and TRM in the successive intervals in an incremental manner. The latter method appears to distinguish aberrant behaviour better than the classical method. Such aberrant behaviour can be related to changes in hysteresis ratio parameters, saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force, which are reflected by displacements in the Day et al. (1977) plot. These changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling. The changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations.

Magnetic behavior of magnetite and rocks stressed to failure — Relation to earthquake prediction

Polycrystalline magnetite and rock samples have been subjected to uniaxial isothermal compression to failure in an attempt to see whether any magnetic expression of dilatancy could be observed. The change in magnetic susceptibility, remanent moment and total moment were recorded continuously as a function of increasing compression. Changes in the slopes of the magnetization-stress curves were observed for stresses corresponding to the onset of dilatancy, and anomalous effects were seen at the approach to failure. Marked directional changes of the magnetization vector were frequently present. Samples carrying weak field thermo-remanent magnetization stressed in zero field showed anomalous increases in intensity of magnetization. In contrast samples carrying saturation isothermal remanent magnetization were demagnetized on the application of stress in zero field. Single-domain material showed virtually no change due to compression. For multidomain material, magnetic observations permitted prediction of failure in the laboratory. This study suggests the eventual possibility of earthquake prediction using magnetic precursors, particularly with three-component measurements, and it indicates that multidomain materials are better transducers of the seismomagnetic effect than single-domain materials.

Sedimentary records of reversal transitions ‐ Magnetization smoothing artefact or geomagnetic field behaviour?

Sedimentary records of reversals for the past 12 million years exhibit preferred longitudes of their VGP paths. This observation may be related to long term aspects of core-mantle dynamics. However, it has been suggested that sedimentary VGP paths could be an artefact due to smoothing which mixes non antipodal pre- and post-transitional directions. We show here that smoothing over unrealistically long time scales is required to generate intermediate directions of magnetization which are purely mixtures of pre- and post-transitionals directions in sediments whose remanence is primarily depositional (DRM) or post-depositional remanence (PDRM). Sedimentary records of transitions showing VGP paths confined to the planes of immediately pre- and post-transitional VGPs have however been observed. These can be explained by non-axisymmetric bias in the non-reversing field [Gubbins and Bloxham, 1987; Constable, 1992], which plays a role in reversals. Therefore, provided that magnetization directions are properly cleaned to give single component directions, the preferential longitudes of reversal VGP paths will be due to field behaviour and not to an artefact of the magnetization process. If, however, mixtures of primary magnetization acquired at the time of the formation of the rock and secondary magnetization acquired later are measured, then artefacts will be generated.

Paleomagnetism of the Ocotillo Badlands, southern California, and implications for slip transfer through an antidilational fault jog

Abstract The right-lateral Coyote Creek fault is the southernmost segment of the San Jacinto fault zone in California. At the Ocotillo Badlands, an anticlinorium of locally updomed material is exposed at a step-over between echelon segments of the fault. Here, uplift and deformation most probably result from multiple slip increments transferred across the antidilational fault jog, under the assumption that material away from the jog region deforms by rigid body translation. Within the 2 km wide fault jog, contraction has occurred by folding of the Quaternary sedimentary strata about east-west trending hinge lines. Structural analysis of this deformation shows that folding has accommodated ∼ 800 m of fault slip transferred through the antidilational jog. By comparison, total right slip on the Coyote Creek fault is 2.5 km, measured 25 km to the northwest at Coyote Ridge. A magnetostratigraphic study of the deformed strata was undertaken to determine the longevity of this fault discontinuity at the Ocotillo Badlands. The eroded core of the updomed material at the Ocotillo Badlands exposes a 325 m thick sequence of sedimentary rock, made up by the lacustrine Borrego Formation (∼ 200 m) and the overlying conglomeratic Ocotillo Formation (∼ 125 m). Stepwise thermal demagnetization of samples from the exposed strata suggests that the formation boundary also marks a magnetic polarity reversal, of reversed field to normal field as one travels upwards through the composite section. The inferred primary magnetization is probably produced by a detrital remanent magnetization (DRM) and is often overprinted with a weak, viscous present field component. Comparison with magnetostratigraphy of the Borrego Badlands 10 km to the northwest indicates that the reversal sampled is the onset of the Jaramillo event (0.97 Ma). The presence of a 20° angular unconformity within the upper portion of the exposed stratigraphy shows that deformation within the Ocotillo Badlands began during deposition of the Ocotillo Formation, shortly after the time of the field reversal. If total slip on the Coyote Creek fault at the Ocotillo Badlands is greater than 800 m, it appears that the fault jog has been a transitory feature within the fault zone, with slip alternately bypassing it or being transferred across it. Such switchyard behavior of slip transfer through the fault jog has important implications for understanding structural controls on earthquake rupture.

Preferred Bands of Longitude for Geomagnetic Reversal VGP Paths: Implications for Reversal Mechanisms

Different analysis of new records of recent polarity reversals and of many records published in the last years show a remarkable confinement of the transitional VGP’s to the longitudinal sector over the Americas and, to a lesser extent, over its antipode. This structure has persisted in time for at least 11 million years. The two longitudinal sectors defined by the VGP’s are of particular significance in reconstructions of the present day geomagnetic field and of fluid motions in the surface layers of the core. They are also found to coincide with anomalously fast regions in the mantle and at the core-mantle boundary documented by seismic tomography. Therefore the observed VGP paths link features of the core fluid motion to the temperature in the mantle, fluid motions in the core being driven by temperature patterns at the core mantle boundary during reversals.