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دیرینه مغناطیس آتشفشان دماوند در طی نیم میلیون سال گذشته | ||
| فیزیک زمین و فضا | ||
| مقاله 7، دوره 40، شماره 1، اردیبهشت 1393، صفحه 83-93 اصل مقاله (1.13 M) | ||
| شناسه دیجیتال (DOI): 10.22059/jesphys.2014.36698 | ||
| نویسندگان | ||
| محمد حامدپور دارابی* 1؛ علی اکبر بهمنی‌زاده2؛ بهروز اسکوئی3 | ||
| 1استادیار، گروه فیزیک، دانشگاه هرمزگان، ایران | ||
| 2دانشجوی کارشناسی ارشد، گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران، ایران | ||
| 3استادیار، گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران، ایران | ||
| چکیده | ||
| آتشفشان دماوند در طی تاریخ فعالیتهای خود تحت تاثیر فعالیتهای زمین‌ساختی منطقه‌‌ای نیز بوده است. فوران‌‌‌‌های متوالی دماوند در حضور میدان مغناطیسی وقت، این امکان را فراهم آورده است که بتوان از جهت‌‌‌‌‌های مغناطیسی ضبط شده درون گداز‌‌‌های بر هم نهشته ، تاریخچه میدان مغناطیسی زمین را در بازه فعالیتهای دماوند بررسی و همچنین حرکت‌‌‌‌‌های زمین‌ساختی آتشفشان دماوند را با استفاده از جابه‌جایی ظاهری قطب‌های مغناطیسی زمین (Apparent polar wandering) بررسی کرد. نتایج به‌دست آمده از آنالیز‌‌‌های دیرینه مغناطیسی بر روی نمونه‌‌‌‌‌های جمع‌آوری شده از 10 سایت در دامنه آتشفشان دماوند، گویای چرخش مخروط در خلاف جهت عقربه‌‌‌‌‌های ساعت به میزان 7/0 درجه به‌ازای هر 1000 سال است. نمودار دیرینه میل مغناطیسی دو وارونگی مغناطیسی در بازه‌‌‌‌‌های زمانی 7000 سال و 27000 سال را نشان می‌‌دهد که در قسمت آخرین قطبیدگی معمولی (Brunhes) مقیاس زمانی قطبیدگی مغناطیسی بین المللی دورة پلیوستوسن پایانی و دوره هولوسن مشاهده نشده است. | ||
| کلیدواژه‌ها | ||
| آتشفشان دماوند؛ قطب دیرینه مغناطیسی؛ وارونگی قطب‌ها؛ چرخش مخروط دماوند | ||
| عنوان مقاله [English] | ||
| Paleomagnetism of Damavand volcano over the last half million years | ||
| نویسندگان [English] | ||
| Mohammad Hamedpour Darabi1؛ Ali Akbar Bahmanizadeh2؛ Behrooz Oskooi3 | ||
| 1Assistant Professor, Physics Department, Hormozgan University, Iran | ||
| 2M.Sc. Student, Earth Physics Department, Institute of Geophysics, University of Tehran, Iran | ||
| 3Assistant Professor, Earth Physics Department, Institute of Geophysics, University of Tehran, Iran | ||
| چکیده [English] | ||
| Damavand volcano with 5671 m height from sea level which covers 400 square km of area is the highest peak in the Middle East. It is located at 50 km to the east of Capital city of Tehran, Iran. It is in the fumarolic stage and exemplifies ongoing Quaternary volcanic activity. Damavan volcano was influenced by regional tectonics during its history. Allenbach (1966) believed that the volcano originates from a fault that already existed in the sedimentary basin which allowed the magma to rise. Darvishzadeh (1985) believed that the compression movement of Iran plate that influenced the Alborz Mountains which commenced the two curved faults (Vara-rud and Ask) that joined under the cone of the volcano and let the magma rise to the earth's crust. Frequent eruptions during ~ 1.8 Ma of Damavand volcanos make the opportunity to study the Earth’s magnetic field records in its stacked lava to clear out tectonic movement of the cone. Davidson et al., (2004) reported three major phases of volcanic activity during the past 1.8 Ma. They present radiometric age dating (U-TH)/ He, for several samples from different lava deposits around the cone. The youngest deposits of lava are distributed on the western flank of the cone and most of them are Trachytes. On the southern flank of the cone in which lava deposits are of the Trachy-andesites, show the oldest age among the collected samples. We followed Davidson et al., 2004, sampling sites to use their dated ages for our study. We used a water supplied petrol drill to collect 200 oriented samples from 10 sites around the cone, from north-west to the north-east. All samples were thermally demagnetized and results were plotted on orthogonal diagrams and principal component analysis (PCA) was carried out to extract primary magnetic remnant components. Orthogonal diagrams of the sites D1- D10 mostly show two magnetic components of 100-400 ̊ C and 400-600 ̊ C. sites D10, D9, D8 and D6 also show a third hematite bearing component with curie temperature around 675 ̊ C. VRM components exist in very low temperatures with a maximum boundary of 300 ̊ C, they have omitted from the data list during component analysis. Magnetic susceptibility variations with temperature during thermal treatments show a stable mineral composition till 600 ̊ C therefore we have not included the components over 600 ̊ C. Magnetic directions and their palaeomagnetic pole positions were calculated. The calculated α95 for most of the mean directions were low therefore mean directions and consequently pole positions are reliable. Magnetic inclination and declination variations are plotted according to the Davidson et al., 2004, from old to the youngest ages. Our results show two Earth’s magnetic polarity anomalies in 7000 and 27000 years ago, which have not been shown in GPTS for Late Pleistocene and Holocene normal polarity chron (Cande & Kent, 1992). Magnetic inclinations for these two sites show reverse polarity however, magnetic inclination only for 27000 y.a. event have a 180̊ rotation and it has a rotation of 90̊ for the 7000 y.a. event. Therefore a reversal for the 27000 y.a. event and an excursion for the 7000 y.a. event is probable. However we don’t see such record in the previous works therefore these might be the effect of a non-dipole in this area in that time. We have used window method (Besse, & Courtillot, 2002) for averaging paleomagnetic poles in three ranges of age of 7.0-7.2 Ky , 25-65 Ky and 250- 400 Ky. Poles arrangement shows an anti-clockwise rotation of Damavand cone with an average of 7/0 ̊ per 1 Ky. Many faults which are distributed around the cone can be the evidences for such a rotation for the cone. | ||
| کلیدواژه‌ها [English] | ||
| Damavand Volcano, Palaeopole reversals, Cone rotation | ||
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