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Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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problems <strong>of</strong> this area close to the Iran–Iraq Border,<br />

some <strong>of</strong> the sampling intervals were up to 30 m.<br />

Overall, 46 sites were sampled in the Zarrinabad<br />

syncline, 94 on the Changuleh anticline, and 9 in the<br />

Changuleh syncline with two cores per site.<br />

3.2. Paleomagnetic analysis<br />

S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 403<br />

Paleomagnetic samples were analysed at the Laboratory<br />

<strong>of</strong> Paleomagnetism <strong>of</strong> the CSIC-University <strong>of</strong><br />

Barcelona at the Institute <strong>of</strong> Earth Sciences bJaume<br />

AlmeraQ in Barcelona (Spain). The Natural Remanent<br />

Magnetization (NRM) was measured in a three axes<br />

superconducting rock magnetometer (2G Enterprises)<br />

and stepwise thermal demagnetization was applied to<br />

all samples up to complete removal <strong>of</strong> the NRM. This<br />

allowed us to isolate the different paleomagnetic<br />

components and to interpret the demagnetization data<br />

from the vector endpoint diagrams (Fig. 5) [28].<br />

Stereographic projections <strong>of</strong> stable components are<br />

shown in Fig. 6. A limited number <strong>of</strong> representative<br />

samples were demagnetised using a tumbling AF<br />

demagnetizer (Fig. 5C). These same samples were<br />

then subjected to stepwise IRM acquisition in order to<br />

estimate the remanence carrying mineralogy (Fig. 7A).<br />

The average NRM intensity was <strong>of</strong> the order <strong>of</strong> 10 3<br />

A/m, ranging from 10 4 to 0.14 A/m. In most <strong>of</strong> the<br />

samples, thermal treatment revealed the presence <strong>of</strong> a<br />

low-temperature component, which parallels the<br />

present north-directed ambient field. This recent over-<br />

print was removed after moderate heating to 250–350<br />

8C (Fig. 5B,D,E,F) or by applying a low alternating<br />

field <strong>of</strong> 5 mT (Fig. 5C). A stable characteristic<br />

remanent magnetization (ChRM) showing either a<br />

normal (Figs. 5A,D and 6) or reverse polarity (Figs.<br />

5B,C,E and 6) was commonly observed, representing<br />

more than 50% <strong>of</strong> the initial NRM.<br />

Demagnetization <strong>of</strong> the ChRM revealed a linear<br />

trend towards the origin with maximum unblocking<br />

temperatures typical <strong>of</strong> hematite, ranging from 630 to<br />

650 8C (Figs. 5A,D and 7A). Some samples, however,<br />

showed a significant decay <strong>of</strong> the remanence from 500<br />

to 600 8C (Figs. 5B and 7A), which suggests the<br />

additional occurrence <strong>of</strong> magnetite. This is supported<br />

by the low coercivity <strong>of</strong> the ChRM component<br />

revealed by AF demagnetization <strong>of</strong> some samples<br />

(Fig. 5C). This is also in agreement with IRM<br />

acquisition experiments, which <strong>of</strong>ten reveal steep<br />

acquisition curves at fields lower than 0.1 T (Fig.<br />

7B). In many samples, most <strong>of</strong> the IRM is acquired at<br />

relatively low fields (b0.3 T), but remain unsaturated<br />

at fields up to 1.0 T (Fig. 7B). All the observations are<br />

in agreement with a magnetic mineralogy consisting<br />

<strong>of</strong> a mixture <strong>of</strong> both magnetite and hematites in<br />

varying proportions.<br />

A small number <strong>of</strong> samples exhibited a complex<br />

demagnetization trend (Fig. 5F), with the presence <strong>of</strong><br />

an intermediate temperature component (B component).<br />

The B component is easily identifiable when<br />

recording a magnetization opposed to both the ChRM<br />

Fig. 6. Equal-area stereographic projections <strong>of</strong> characteristic directions (normal and reversed polarities). Mean directions with 95% confidence<br />

limit are displayed. N = number <strong>of</strong> directions; dec=declination; inc=inclination; k = precision parameter; a95 =confidence limit.

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