Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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404 S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 Fig. 7. (A) Representative normalized NRM decay curves. (B) Representative normalized IRM acquisition curves. and the low-temperature recent overprint. The progressive demagnetization of all three components confers a distinctive bSQ shape to the Zijderveld plots (Fig. 5F). IRM experiments do not provide evidence for a distinct magnetic mineralogy associated with the B component. Given that its occurrence is limited to a few sites, the origin of the B component is likely related to an early post-depositional magnetization and not to a late widespread remagnetization event. Delayed magnetization can lead to the record of two successive polarities when a geomagnetic reversal occurs soon after deposition. Some samples with a reversed B component occur in the middle of a long normal polarity magnetozone (Figs. 5F and 8). These are interpreted as recording very short geomagnetic events (cryptochrons) occurring in the middle of larger chrons. The bulk susceptibility of the samples was measured after each thermal demagnetization step using a KLY-2 susceptibility bridge. Most of samples present a similar rapid increase of susceptibility at about 400 8C, which is probably the result of magnetite formation upon heating. No directional changes or remanence intensity peaks correlate with the temperature intervals of increasing susceptibility, indicating that the new forming magnetic minerals are not contributing to the net remanence of the samples. Samples are divided into three classes reflecting the quality of the demagnetization analysis (Fig. 8). The first class contains samples with an ideal demagnetization pattern (Fig. 5A to F). Samples of second class present poorer quality demagnetization curves, but the polarity of the Virtual Geomagnetic Pole (VGP) is clearly identifiable from their ChRM (Fig. 5G,H). The third class includes samples with an unclear demagnetization pattern (Fig. 5I). Results are very good, with 78.5% of the sampled sites represented by at least one first-class sample, 18.1% represented by second-class samples, and only 3.4% only represented by third-class samples (Fig. 8). Normal and reverse mean directions in both sections passed the reversal test with class C [29], indicating that the ChRM was successfully isolated. The mean inclinations of about 358 are significantly lower than that expected from the geocentric axial dipole model at the site (528). Such a discrepancy can be attributed to an inclination error induced upon deposition and early compaction of the sediments, which is often observed in red alluvial sediments carrying a detrital remanence [30,31]. The mean paleomagnetic declinations do not provide evidence for statistically significant vertical axis rotation of the rocks after magnetization. The mean direction of the Changuleh anticline Section is rotated about 78 clockwise, a variation which is within the angular error of the mean (Fig. 6). The ChRM directions were used for calculation of the Virtual Geomagnetic Pole (VGP) latitude at each
S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 405 Fig. 8. Stratigraphic columns, magnetic polarity sequences and plots of stratigraphic position vs. Virtual Geomagnetic Pole Latitude for the three studied sections. Black zones represent normal polarities, while white zones represent reversal polarities. Positions of samples located in Figs. 3 and 4 are indicated. See the text for the sample class characteristics. stratigraphic level. Positive and negative VGP latitudes were interpreted as normal and reverse polarity, respectively, in order to construct a local magnetic polarity stratigraphy for each of the three sections studied (Fig. 8). Most of the magnetozones were determined by two or more consecutive sites, averaging 3.5 sites per magnetozone in the Changuleh anticline and 2.5 in the Zarrinabad syncline, but, respectively, 6 and 7 magnetozones (23% and 39% of the total) were represented by only one site. This indicates that the recovered magnetostratigraphic record may be incomplete, and that the chances of missing chrons of short duration (under 10 5 year) of the GPTS may be significant. However, as we will see below, except for the lowermost part of the series which is problematic, the good match of the polarity stratigraphy with the GPTS suggests a sufficient recovering of true reversal magnetic sequence.
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Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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