Paleomagnetic Inclination Research Articles

Paleomagnetism of Early Tertiary Alaska Peninsula Rocks and Implications for Docking of Peninsular Terrane: ABSTRACT

In order to refine the tectonic history of the peninsular terrane, Alaska, 22 sites (averaging 10 samples/site) in Paleogene Tertiary volcanic and sedimentary formations were sampled in the vicinity of Chignik, on the Pacific side of the Alaska Peninsula. Ten of the sites were drilled in the early Oligocene Meshik volcanics, ranging from andesite to basalt, and the other twelve sites were drilled in the late Eocene Tolstoi Formation sediments. Nine of the volcanic sites yielded stable R and/or N characteristic magnetization. Virtually no fine-grained, interbedded sediments occur with the Meshik volcanics at the sample sites, thus making reliable paleohorizontal determinations difficult. Although flow attitudes were tentatively used, it became rapidly apparent that problem of initial dip were insurmountable. As a result, all volcanic sites were considered unreliable for determining a meaningful paleomagnetic inclination. Upon thermal demagnetization, five of the sedimentary sites were judged stable. The mode of the paleomagnetic direction was calculated, D/I = 349.8/75.3 (s95 = 8.5), indicating no significant rotation. Uncertainties in structural corrections, however, may render only the inclination meaningful, which, from McFadden statistics, yields I = 75.9, ^agr95 = 7.9, corresponding to a paleolatitude of 63.3°. This paleolatitude agrees with the expected value for the North American craton at 40 m.y. B.P., implying that the peninsular terrane had docked by at least that time. End_of_Article - Last_Page 682------------

Paleolatitudes determined from paleomagnetic data from vertical cores

Most of the paleomagnetic data available from oceanic plates consist of measurements of inclination but not declination from azimuthally unoriented vertical cores. Similarly, the basic paleomagnetic data to be analyzed from crustal blocks that have rotated by varying amounts about vertical axes are a set of paleoinclinations. A new method is proposed for determining the paleocolatitude of a sampling site and the associated confidence limits. Paleomagnetic inclinations are first converted to paleocolatitudes and averaged to produce a mean colatitude. After showing that this mean is biased, we derive a paleocolatitude correction to remove the bias, using a model for geomagnetic secular variation based on the latitude dependence of secular variation during the past 5 million years. After testing the validity of the model by applying it to paleomagnetic data from Quaternary lavas for which both inclination and declination are known, we apply the method to inclination‐only paleomagnetic data from Suiko Seamount, the most complete data set from a drilling site. The method is then applied to paleomagnetic data from all Deep Sea Drilling Project sites for the Pacific plate to obtain for each site an unbiased mean paleocolatitude and its confidence limits. These paleocolatitudes and confidence limits are then used as input for a least squares analysis to obtain Early and Late Cretaceous poles for the Pacific plate and 95% confidence limits for these poles. It is shown that the scatter of individual site paleocolatitudes about the best fit pole is consistent with the confidence limits at each site as determined by the new method of analysis.

Paleomagnetism of the Paleocene Ghost Rocks Formation, Prince William Terrane, Alaska

The mean paleomagnetic inclination of Paleocene volcanics from two regions on Kodiak Island in the Ghost Rocks Formation suggests that the Prince William terrane originated at mid latitudes, about 25° south of its ‘expected’ Alaskan latitude in Paleocene time. The remanent magnetization of these rocks passes both the fold and reversal tests and is well constrained in age. The mean declinations of the two regions, however, differ by approximately 120°, suggesting they have rotated with respect to each other, perhaps during emplacement. We suggest that, despite the lack of evidence for a major Tertiary suture zone between the Prince William terrane and central Alaska, the Prince William and perhaps adjacent terranes may have lain substantially south of their present position in the Early Tertiary.

Geomagnetic paleointensities from radiocarbon‐dated lava flows on Hawaii and the question of the Pacific nondipole low

Radiocarbon ages have been published for nine basaltic lava flows on the island of Hawaii; the ages range from 2600 to somewhat older than 17,900 years B.P. By using the Thelliers' method in vacuum, geomagnetic paleointensity values were obtained from eight of the lavas; the ninth proved unsuitable. The paleointensities for the four youngest flows (2600–4600 years B.P.) yield virtual dipole moments (VDM's) that are 20% greater to more than twice the worldwide values for those times obtained by V. Bucha from archeomagnetic data. The dispersion of virtual geomagnetic poles for the eight lavas is 15.5°, appreciably larger than the average for older lava flows on Hawaii. These results contrast with the historic magnetic field in the region of Hawaii, in which both secular variation and nondipole components are very low. At about 10,000 years B.P. the measured VDM is not very different from the long‐term worldwide average but differs considerably from a smooth extrapolation of Bucha's average curve. At about 18,000 years B.P. the measured VDM is very low and is associated with an unusually shallow paleomagnetic inclination for the latitude of Hawaii. These new paleointensity and paleodirectional data strongly suggest that sizable nondipole geomagnetic fields have existed in the vicinity of Hawaii at various times during the Holocene epoch and perhaps earlier.

Shallow inclinations of remanent magnetism in Deep-Sea Drilling Project igneous cores: Geomagnetic field behavior or postemplacement effects?

Paleomagnetic results from abyssal igneous cores collected during the Deep-Sea Drilling Project (DSDP) are compared to reference data sets from lava sequences for the period between 13 and 2 m.y. ago in Iceland. Statistical modeling shows that the paleomagnetic data from Iceland can be modeled approximately by a virtual geomagnetic pole distribution which is 84% Fisherian and 16% random. Transforming this result into values of inclination which can be expected at other latitudes provides a measurement of the probability that negative inclinations can occur during times of normal polarity in the northern hemisphere. It appears that inclination of remanent magnetism from holes within 30° of the paleoequator cannot provide reliable measurements of polarity. Comparison of paleomagnetic inclinations in basalt from DSDP leg 37 with our model shows that factors other than the geomagnetic field behavior almost certainly were the cause of the measured inclinations. Postemplacement tectonic rotation around an axis parallel to the ridge crest is therefore inferred. Data from leg 46 contain too few measurements for a reliable comparison to be made.

Magnetic properties of the oceanic crust: Considerations from the results of DSDP leg 34

Paleomagnetic samples of basalt and sediment from four Deep-Sea Drilling Project sites in the Nazca plate were studied in order to detect any possible paleolatitude changes and to evaluate the contribution of different portions of the igneous basement to the overlying magnetic anomalies. Penetration of the basement basalts yielded both coarse-grained massive flow units and fine-grained pillow basalts. The massive flow samples had titanomagnetite grains that were relatively unoxidized, while all the pillow basalts had been extensively subjected to low-temperature oxidation. Average intensity of magnetic remanence in the massive flow units (99.4×10−4 emu/cm3) was approximately an order of magnitude higher than that found in the pillow basalts (10.3×10−4 emu/cm3). All coarse-grained massive flow samples had large components of soft magnetization that was probably due to a viscous remanent magnetization (VRM) acquired both in situ and during the drilling process. In some cases this VRM component dominated the initial thermal remanence and could be responsible for the inability of some areas of the oceanic crust to record observable magnetic anomaly patterns. Large deviations in stable inclination from that expected from an axial centered dipole field were recorded by both basalt and sediment samples. Tightly grouped and anomalously high inclinations in the basalt samples from the younger sites indicate that the upper part of the oceanic igneous crust was formed in less than 102 yr, and this formation may be episodic. Paleomagnetic inclinations from the sediments, averaged over roughly 106 yr for each site, indicated no latitudinal change for the Nazca plate between 15 and 40 m.y. B.P. and are consistent with a southward paleolatitude movement of 3°–5° during the last 15 m.y.

Paleomagnetic evidence of northward movement of the Pacific plate in deep-sea cores from the Central Pacific Basin

Seven deep-sea sediment cores recovered in the central equatorial Pacific collectively span a magneto- and biostratigraphically determined age interval ranging from about 0.1 to 21 m.y. B.P. Measured values of paleomagnetic inclination and their systematic variation with depth in these cores denote relative motion between the central Pacific lithosphere and the magnetic field of the earth. Assuming that the position of the earth's dipole field remained essentially parallel to the present spin axis during the interval, the data provide evidence of a marked decrease in the northward rate of plate motion from about 11 cm/yr to about 6 cm/yr at approximately 12 m.y. B.P. This date of change of motion as well as the northward direction and overall average rate of about 8 cm/yr throughout the last 21 m.y., agree reasonably well with results of other studies of the tectonic history of the Pacific plate and ridge system. More significantly, however, these preliminary results demonstrate the usefulness of the paleomagnetic record in deep-sea sediment cores spanning sufficiently long intervals of time as an aid in reconstructing plate motions.

Sea-floor dynamic processes as the possible cause of correlations between paleoclimatic and paleomagnetic indices in deep-sea sedimentary cores

Recent publications showing correlations between the variation of intensity of magnetization (J) and paleomagnetic inclination I on the one hand, and diverse paleoclimatic indices on the other, have been interpreted to represent direct cause and effect relationships between climate and geomagnetism. It is proposed that several sea-floor dynamic processes, which are climatically variable, are at least equally feasible causes of the observed correlations. Thus, for variation in J, it is a simple matter to invoke variations in such factors as the magnetic particle size fraction below 50 microns, carbonate content, or laminar flow layer thickness, all of which can change with climate. For variation in I, climatically related variation of the velocity gradient in the laminar flow layer can be employed to explain the correlation. Prior to accepting the suggested geomagnetic control of climate as real, it is recommended that simple measurements of the temporal and geographic variation in fabric and composition of the deep-sea sediments involved should be carried out, as a check for the effect of bottom dynamic processes.

Paleomagnetism of Midway Atoll lavas and northward movement of the Pacific plate

Two deep drill holes through the reef limestones of Midway Atoll penetrated 120 m and 19 m of basaltic lavas that were dated by the K Ar method at 18 my. Inclinations of natural remanent magnetization have been measured in 173 specimens cut from 57 core samples from 13 of the lava flows. The mean paleomagnetic inclination is27.6° ± 6.8°, corresponding to a paleolatitude of14.7° ± 4.2°. The present latitude of Midway is 28°, suggesting a northward component of motion of the Pacific plate of approximately 13° or 1400 km in the last 18 my. The paleolatitude of Midway is thus not significantly different from the present latitude (19°) of the active volcanic island of Hawaii. The paleomagnetic data from the Midway basalts thus support the hypothesis of Wilson and Morgan that volcanic heat sources are fixed with respect to the Earth's mantle below the asthenosphere and their apparent migration with time is due to plate motion.