عنوان مقاله [English]
Loess deposits mainly of silt particles with different mineralogical sources have covered about 10 percent of the Earth land area. World-wide studies on loess deposits show that most of the loess deposits accumulated during the Glacial ages while during the inter-glacial phases their accumulation has stopped or dramatically lessened. Loess deposits have recorded the past climatological history within their grains which can be used to distinguish possible climate changes during the Quaternary period. Dating of paleo-proxy records such as loess sediments also have the potential for understanding the chronology of paleo environmental events including tectonic activities. Loess deposits around the world were among the first sediments to be used for luminescence dating particularly within the arid and semi-arid areas.
The development of luminescence dating techniques and our understanding of loess palaeosol sequences have progressed over the past 35 years with both fields of study supporting the development and understanding of the other. Luminescence dating dosimeters are grains of feldspar and quartz, which are both typically abundant in loess, and, because loess is a fine-grained aeolian deposit and its nature implies medium-to-long transport distances, any previous luminescence signal should be reset (or ‘removed’) prior to deposition. Luminescence dating is also an important tool for developing numerical chronologies for loess deposits, providing comparison of different records and the study of correlations between records. Furthermore, the event being dated is the last exposure of the sediments to daylight, which directly relates the luminescence age to the time of deposition of the sediment.
Study and dating of loess deposits from different parts of Iran can help to test this hypothesis. Several investigations on loess deposits from North and North-eastern Iran (including North of Alborz Mountains, Gorgan and Gonbad Kavous) led to limited chronological results. On the other hand, although loess sediments have been distinguished in different parts of central Iran (i.e. Kerman and Yazd provinces) there was not a single study of loess deposits in this area so far. As a result, we decided to attempt a research on loess deposits of central Iran. One of important areas of loess remains in central Iran is Maibod area located in central part of Yazd Province. Maibod is located in the main high-way towards the Persian Gulf region. Loess deposits formed as local ridges accumulated on an extended fault-erosional terrace in southern side of Maibod. Since study and dating of such deposits can clarify parts of tectonic history of Maibod area the outcome from such research is correct can be crucial. Furthermore, investigation on loess deposits of such dating-free location can significantly add to its Quaternary Paleoenvironmental/Paleoclimatological knowledge.
Consequently, we extracted a sample from parts of loess deposits over the Maibod terrace. The extraction point is located on the western side of Maibod City where the loess sediments becoming very hard and they formed wide varieties of yardang to interfluve ridges with a few meters height. Contaminated Quartz grains of 90–180 μm and the post IR single-aliquot regenerative-dose protocol were employed for De determination. Experiments were carried out in Oxford University using a Riso TA-15a automated TL/OSL system (Botter-Jensen, 1997) incorporating an IR laser diode (400mW, 830±5 nm), and a Strontium-90 beta source.
After laboratory works and dating of the sample (at 30000 to 39000 years ago) the accumulation period has been distinguished at around the middle of the last glacial age (Wurm). Considering the major water erosion over both top loess (late Quaternary) and its green marl basement (Neogene) and due to the distinct slope escarpment (terrace-like feature) we can conclude that the accumulation phase was after the last fault activity. In other words, most active seismic event has happened earlier than loess deposition before 3000 to 39000 years ago. Due to the relatively intact feature of nearby ancient Narin castle (3000 years old) we can conclude that the most sever tectonic activities in the area occurred between 3000 to 40000 years ago approximately. Such single conclusion needs to be evaluated with more studies and many OSL dating on loess deposits of central Iran.