ديرينه مغناطيس آتشفشان دماوند در طي نيم ميليون سال گذشته

Paleomagnetism of Damavand volcano over the last half million years

آتشفشان دماوند در طي تاريخ فعاليت‌‌‌‌‌‌هاي خود تحت‌ تاثير فعاليت‌‌‌‌‌‌هاي زمين‌ساختي منطقه‌‌اي نيز بوده است. فوران‌‌‌‌هاي متوالي دماوند در حضور ميدان مغناطيسي وقت، اين امكان را فراهم آورده است كه بتوان از جهت‌‌‌‌‌هاي مغناطيسي ضبط شده درون گداز‌‌‌هاي بر هم نهشته ، تاريخچه ميدان مغناطيسي زمين را در بازه فعاليت‌‌‌‌‌هاي دماوند بررسي و همچنين حركت‌‌‌‌‌هاي زمين‌ساختي آتشفشان دماوند را با استفاده از جابه‌جايي ظاهري قطب‌هاي مغناطيسي زمين (Apparent polar wandering) بررسي كرد. نتايج به‌دست آمده از آناليز‌‌‌هاي ديرينه مغناطيسي بر روي نمونه‌‌‌‌‌هاي جمع‌آوري شده از 10 سايت در دامنه آتشفشان دماوند، گوياي چرخش مخروط در خلاف جهت عقربه‌‌‌‌‌هاي ساعت به ميزان 7/0 درجه به‌ازاي هر 1000 سال است. نمودار ديرينه ميل مغناطيسي دو وارونگي مغناطيسي در بازه‌‌‌‌‌هاي زماني 7000 سال و 27000 سال را نشان مي‌‌دهد كه در قسمت آخرين قطبيدگي معمولي (Brunhes) مقياس زماني قطبيدگي مغناطيسي بين المللي دوره پليوستوسن پاياني و دوره هولوسن مشاهده نشده است.

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 earthʹs 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 ?95 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 180? rotation and it has a rotation of 90? 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.