1 هیئت علمی موسسه ژئوفیزیک
2 موسسه ژئوفیزیک دانشگاه تهران
عنوان مقاله [English]
Luminescence dating is now an important element in the suite of Quaternary geochronological methods. The major area of growth in the past two decades has been its application to Quaternary sediments, where the method provides an absolute age for the last exposure of the constituent grains to daylight.
Such deposits are generally composed of wind-blown sediment grains that have been exposed to sufficient sunlight to zero the optically stimulated luminescence (OSL) signal during the most recent sediment transport event. In such instances, the measured equivalent dose (D_(e" " )) may closely approximate the true burial dose (D_(b" " )). By contrast, water-transported sediment grains are frequently exposed to insufficient sunlight to fully erase the OSL signal, owing to the reduced efficiency of bleaching beneath a cover of water (among other factors).
Duller (2008) differentiated between two types of partially bleached sediments: 'type A' where all the grains are partially bleached to the same extent (i.e. homogeneously bleached); and 'type B' where different grains have been exposed to sunlight of differing intensity and/or duration causing varying amounts of residual trapped charge to remain in the grains (i.e. heterogeneously bleached).
This circumstantial result shows high variation in the range D_(e" " ) distribution. Understanding the dose distribution from a sample is essential for obtaining the appropriate burial dose (D_(b" " )). To allow one to assess the shape of a sample’s dose distribution, a sufficient number of D_(e" " ) values must be obtained.
The single aliquot regenerative dose (SAR) procedure for feldspar was used by Duller (1991) and further developed by Murray and Wintle (2000) describing how D_(e" " ) values were calculated. Using the SAR protocol, each aliquot provides an independent estimate of D_(e" " ) and by taking measurements on many separate aliquots the distribution of D_(e" " ) within a sample can be assessed.
To deal with D_(e" " ) various possibilities, several informal approaches and parametric statistical models have been used to estimate the D_(b" " ) of interest, of which some were developed originally for fission track analysis (Galbraith and Laslett,1993).
The most commonly used models for OSL dating have been adopted from fission track analysis and are described
in detail by Galbraith (2005). The Central Age Model (CAM) is appropriate for sediments which have been well bleached and the Minimum Age Model (MAM) assumes that only part of the sample were bleached at deposition, and that the remaining grains were bleached to differing degrees. The value appropriate for calculating the age of the sample is defined by the population of grains at the lower end of the distribution.
The Eastern Mosha Fault (EMF) and the North Tehran Fault (NTF) are two major active faults of the southern central Alborz mountains, located in proximity of Tehran (population ~15 million). The Ira trench site is located at the linkage zone between the North Tehran Fault and Eastern Mosha Fault. This trench contains different kind of sediments including alluvial, and colluvial sediments. Therefore, CAM and MAM are suitable methods to be employed for dating samples collected from the Ira site. Dating these samples is useful to calculate kinematics on the two faults in their linkage zone (Ghasemi et al,. 2014). The ages of samples from Ira trench can also be used for seismic hazard analysis for this heavily populated major city. As a case study the samples collected from this trench was employed and new ages were determined.