Remanent Magnetization Vector Research Articles

Gravity and magnetic models of the Midcontinent Rift in eastern Lake Superior

Gravity and magnetic models of the Midcontinent Rift (MCR) in eastern Lake Superior supplement recent structural and stratigraphic interpretations based on the seismic reflection method. An algorithm developed to accommodate spatially varying direction and magnitude of magnetization within a magnetic source is used in both forward and inverse modeling procedures. Structural attitudes of rift-filling basalts derived from seismic reflection sections are used to rotate the Keweenawan remanent magnetization vectors in the direction of deformation. An iterative linear inversion routine calculates magnitudes of induced and remanent magnetizations, as well as normal and reversed polarity basalt flow distributions. The results indicate that the Koenigsberger ratios of these basalts generally range from 1 to 3, which is in agreement with values obtained from rock property measurements. The models also suggest that the greater volume of the Keweenawan basalt section in eastern Lake Superior is reversely polarized and that remanent magnetizations persist to depths of up to 20 km. Our results, supplemented by isotopic and paleomagnetic data, suggest that the vast majority of the basalts predate 1097 ± 1 Ma. A prominent positive magnetic anomaly and a corresponding gravity low strike west across the trend of the rift from the vicinity of Michipicoten Island. These anomalies may reflect a relatively strongly magnetized, felsic igneous body of late-middle to upper Keweenawan in age. Forward gravity models suggest clastic sedimentary rocks up to several kilometers thick overlay the volcanic rocks in localized depressions. Deep crustal seismic data used to constrain gravity models provide evidence of anomalously dense lower crust beneath the MCR.

Angle-dependent magnetic-relaxation studies in single-crystal YBa2Cu4O8.

The relaxation behavior of the remanent magnetization vector ${\mathbf{M}}_{\mathit{R}}$ in single-crystal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ was studied as a function of the orientation angle between the c axis and the external field by measuring the time dependence of the components of ${\mathbf{M}}_{\mathit{R}}$ parallel and perpendicular to the c axis. At low temperatures (T/${\mathit{T}}_{\mathit{c}}$0.55) a rotation of ${\mathbf{M}}_{\mathit{R}}$ towards the c axis is observed for all orientation angles. For T/${\mathit{T}}_{\mathit{c}}$>0.55 and the initial external field not parallel to the c axis ${\mathbf{M}}_{\mathit{R}}$ turns towards the ab plane both in time and with temperature. It is suggested that the unusual relaxation behavior may be associated with a dimensional crossover from 2D to 3D superconducting behavior.

Three‐axis static alternating field demagnetization of rocks and the identification of natural remanent magnetization, gyroremanent magnetization, and anisotropy

A theoretical basis is given for two empirically derived methods of three‐axis alternating field (AF) demagnetization of static samples used initially by Zijderveld and Dunlop to remove “spurious” components of magnetization produced by the AF. It is shown theoretically that both methods of treating the experimental results to reveal the natural remanent magnetization (NRM) vector are precisely those which algebraically eliminate gyroremanent magnetization (GRM). By implication, the “spurious” remanences found by these and other workers are almost certainly of gyromagnetic origin and not remanences produced as a result of shortcomings in instrumentation. It is further demonstrated that by suitably treating the data obtained by either three‐axis method, GRM can be identified and used to provide information about the anisotropy of the rock.

Paleomagnetic evidence for repeated glacial lake missoula floods from sediments of the Sanpoil River Valley, Northeastern Washington

Recent explanations of widespread rhythmically layered sediments in eastern Washington as the result of repeated great floods from glacial Lake Missoula implicitly suggest a paleomagnetic test for validity. If each conjectural flood layer is separated by years or decades, as hypothesized, a sequence of several such flood beds should record measurable secular variation in geomagnetic field direction. In the Sanpoil River valley where the rhythmite sequences are thought to have been deposited in glacial Lake Columbia, the paleomagnetic test consists of measuring remanent magnetization (RM) directions for thick, upwardly fining beds inferred to be sediments deposited by the influx of flood waters from glacial Lake Missoula into glacial Lake Columbia. Laboratory measurements of samples from three widely spaced sections along the Sanpoil River yield RM vectors with erratic inclinations, apparently affected by varying contributions of inclination error and (or) compaction shallowing, but with declinations that generally differ statistically from one flood to the next and that show the same west-to-east trend at all three locations. The rates of declination change inferred from these data are consistent with modern rates, thus providing the first geophysical evidence supporting the timing in the tens-of-floods theory.

Computation of the palaeointensity of the geomagnetic field

Abstract A procedure for statistical analysis of data obtained by the Thellier palaeointensity method is presented. The procedure comprises multicomponent analysis of natural remanent magnetization (NRM) vectors, the development of a statistical model of the NRM-TRM (thermoremanent magnetization) diagram, computation of the regression line and of the confidence interval for its slope by use of the one-way analysis of variance. The criterion for testing the differences between results obtained from a set of samples is derived and the problem of accumulation of results is discussed. The usefulness of the method is demonstrated on a few samples of Tertiary porcellanites from northern Bohemia.

Estimation of depth and magnetization of magnetic sources from magnetic data: The linearized continuous inverse problem for 21/2D structures

The estimation of the depth to the top and bottom of a magnetic source from magnetic data defines a nonlinear inverse problem, while the evaluation of the distribution of magnetization determines a linear inverse problem. In this paper, these interpretation problems are resolved in the continuous case of 21/2D magnetized bodies with lateral magnetization variations. A formulation of the magnetic problem accounting for different directions of remanent and total magnetization vectors and including a more general definition of apparent susceptibility is presented. Differences between 2D and 21/2D formulations are stressed, as regards the anomaly amplitude, shape and zero-level.

Shape analysis of aeromagnetic anomalies in the southern Italian region for the evaluation of crustal block rotations

An extensive analysis of aeromagnetic data from the Italian region is presented, regarding the evaluation of the azimuthal direction of the total-magnetization vector. A method based on the Zietz and Andreasen (1967) correlation between anomaly shape and direction of the total magnetization vector was used. A discussion related to possible traps in using this method is presented, with the aid of several tests on synthetic anomalies. The analysis points out a number of anomalies revealing abnormal directions, either SW-NE or SE-NW, which were used to divide the investigated area into five regions with different trends. Finally, assuming elevated Koenigsberger ratios for the source-rocks, the declinations of the remanent magnetization vector were judged very similar to those estimated for the total-magnetization vector. Thus, by comparing these values with the normal declination of the area, rotation values of the crustal blocks related to the investigated anomalies were proposed. Finally, these were compared with the available geological and palaeomagnetic information and significant correspondences were found.

Effects of tectonic deformation on the remanent magnetization of rocks

Paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector. Deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis. However, the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known. Anisotropy of magnetic susceptibility (AMS) provides a quantitative basis for strain determination. The principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid. A linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types. If the correlation method used is valid, AMS observations can be used to establish the magnitudes and directions of the principal strains. Strain modifies bedding planes and realigns the grains that carry magnetic remanence. Compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain. The effects of strain on bedding can be corrected using passive marker theory. Attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive. During deformation linear and planar elements deflect away from the axis of maximum shortening. The passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter. Tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions. The passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix.

Magnetic anisotropy of rocks and its application in geology and geophysics

Magnetic anisotropy in sedimentary rocks is controlled by the processes of deposition and compaction, in volcanic rocks by the lava flow and in metamorphic and plutonic rocks by ductile deformation and mimetic crystallization. In massive ore it is due to processes associated with emplacement and consolidation of an ore body as well as to ductile deformation. Hence, it can be used as a tool of structural analysis for almost all rock types. Morcover, it can influence considerably the orientation of the remanent magnetization vector as well as the configuration of a magnetic anomaly over a magnetized body. For these reasons it should be investigated in palaeomagnetism and applied geophysics as well.

Paleomagnetic evidence for tectonic rotation of the Belchertown Pluton, west central Massachusetts

A paleomagnetic study of the Belchertown pluton, a syntectonic Devonian intrusion in the Bronson Hill anticlinorium, Massachusetts, indicates the presence in 23 samples from eight sites in primary quartz monzodiorites of a strong, stable, and consistently oriented remanent magnetization vector (D = 154.9°, I = −13.4°, K = 20.8° and α95 = 12.4°) with a high Q value (7.0). The remanence is inferred to be of thermal and/or thermochemical origin acquired during the original crystallization and cooling of the pluton. Recrystallized equivalents of the primary rocks, which occur as a surrounding border zone, yield weak and inconsistent results (four samples from two sites) due to the absence of opaque minerals. The mean Belchertown vector is discordant with respect to the mean North American Devonian vector (D = 170° and I = +32°) in such a direction as to suggest that the entire pluton has undergone some 30°–60° of rotation to the northwest about a NE-SW axis. The high Q value of the primary rocks and their isolation from magnetic country rocks by the surrounding non-magnetic border zone allow computer modeling of the three-dimensional shape of the pluton using the measured remanent vector and the total field aeromagnetic anomaly. The resulting model is consistent with the proposed amount and direction of tectonic rotation of a once roughly vertically symmetrical funnel-shaped intrusion.

Determination of the paleomagnetic field intensity by the dispersion of the remanent magnetization vector of the sediment rocks

It is shown that the intensity of the initial paleomagnetic field can be determined by the empirical distribution of the vector of the sediment rock magnetization.

Paleomagnetism and the alpine tectonics of Evrasia III

Oriented samples of basalt, basaltic tuff and limestone from the central Lebanon Mountains and the Tartous coastal area were studied with regard to their paleomagnetic characteristics. The age of the Lebanese samples varies from Upper Jurassic to Lower Cretaceous. The Syrian basalts are from Pliocene age. The initial remanent magnetization vectors were rather scattered, but after cleaning with a.c. fields up to 3,000 Oe they concentrated on the following average directions: Upper Jurassic : D = 93°, I = 10°; Lower Cretaceous : D = 314°, I = 9°; Pliocene : D = 170°, I = −35°. The Jurassic and the Pliocene direction are considered to be reversed. These results show a great discrepancy with pole positions already known for Eurasia and for Africa. The deviations may be explained in terms of local tectonic movements or continental drift. The local tectonic disturbances, however, are small.

A paleomagnetic investigation of some of the Precambrian igneous rocks of southeast Missouri

Measurements of the magnetic susceptibility and the intensity and direction of remanent magnetization were made on 325 2.5-cm cylindrical cores prepared from 189 samples of the felsites, granites, and diabases exposed in the Ironton, Fredericktown, and Coldwater quadrangles, Missouri. These measurements show, first, that the directions of remanent magnetization, although quite variable, are significantly different from the direction of the present geomagnetic field in southeast Missouri and, second, that partial demagnetization in an ac field of 300 to 700 oe greatly improves the initial scatter of the remanent magnetization vector by diminishing the anomalous components of viscous and isothermal remanent magnetization shown by many samples. The paleomagnetic data from five rock units indicate a Precambrian geomagnetic pole in the vicinity of 150°W and 5°N.

The magnetism of some Permian red sandstones from northwestern Turkey

A series of eighteen oriented samples collected from the Permian of the Amasra region (Black Sea coast) revealed after progressive demagnetization a characteristic remanent magnetization vector with a declination of 290° and an inclination of −14 3 4 ° , as opposed to the declination of 210° and inclination of −15 1 2 ° calculated for the same region on the basis of the mean Permian pole position as determined from samples obtained from stable parts of the European continent. Such deviations are a common feature of material collected from the neighbourhood of the alpine orogenic belt; in this instance the discrepancy is tentatively explained in terms of regional strike-slip faulting.

Magnetic Behavior in the Transition Region of a Hematite Single Crystal

The magnetic behavior in the transition region of a hematite single crystal from Elba has been investigated carefully. The remanence-temperature relationship and the thermal hysteresis effect (i.e., the remanenceis a double-valued function of temperature) have been obtained for each of three mutually perpendicular directions of the crystal in the temperature range between 488° and 77°K. A graphical representation of the ferromagnetic state has been constructed to show the direction, the magnitude, and the turning of the remanent magnetization vector with the temperature in the transition region. Another experiment, which consists of measuring the remanence along different directions in the plane containing the [111] direction,and the projection of the remanent magnetization vector in the (111) plane for three different temperatures,also has been carried out. These experimental data show that the direction and the magnitude of the remanent magnetization vector changes continuously between the [111] direction and the (111) plane when the temperature changes through the transition region.