Abstract
This study presents the preliminary results of the upper Olduvai transition record analyzed from the Wulochi sedimentary sequence of the Coastal Range, eastern Taiwan. Variations of both the declination and paleolatitudes of VGP reveal an antisymmetric characteristic about the mid-point of the transition. However, variation of the VGP latitudes shows that the first half record is a three stage'stop and go' pattern similar to that of the same event reported from Po river, Italy, by Tric et al. (1990) while the second half fluctuated more. The later part might also indicate that secular variation of the earth's magnetic field was still dominant during this transition. For the longitudes of the VGP path, they are found to be largely confined to a belt from meridian 200° to 250° centered around 230° which is about 40° west of the American continents. This result is dissimilar to many other paths which passed through them. However, the Coastal Range has undergone a clockwise rotation of about 30° during Plio-Pleistocene (Lee, 1989; Lee et al., 1990; 1991); the discrepancy might result from this clockwise rotation event. After the correction of the rotation angle, it is found that the VGP path is much closer to the American continents. In addition, VGP path seems to show afar-field phenomenon and magnetite is identified to be the major magnetic mineral contained in our samples.
Highlights
Polarity reversal of the geomagnetic field is one of the most remarkable phenomena of our planet
The typical demagnetization curves of samples (Figure 2) indicate that after a few low temperature thermal treatment steps, the secondary components of natural remanent magnetization (NRM) could be cleaned. Both of these results suggest that the characteristic directions of NRM of the studied samples could be determined by applying the linear regression method on several demagnetized steps before 450°C
The distributions of paleo-declinations, inclinations and paleolatitudes of samples as a function of stratigraphical height of the studied transition record are shown in Figure 6, and Figure 7 shows the correspondent virtual geomagnetic poles (VGP) path plotted on the geographical coordinates
Summary
Polarity reversal of the geomagnetic field is one of the most remarkable phenomena of our planet. Paleomagnetic records of the polarity transition contribute valuable information about the behavior of the geomagnetic field during a reversal and may be capable of placing constraints on possible reversal mechanisms. Transitional records have become more available and allow the development of transitional field models (Hillhouse and Cox, 1976; Hoffman, 1977; Hoffman and Fuller, 1978; Fuller et al, 1979; Williams and Fuller, 1981; Hoffman, 1982). Models with axisymmetric fields were proposed by Hoffman (1977) and Hoffman and Fuller (1978), which predicted a dependence mainly upon the site latitude for transitional records. Evidence for a non-axisymmetric component led to the 'flooding' model proposed by Hoffman (1979, 1981) and Williams and Fuller (1981)
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