Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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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