A Studies Of Luminescence Dating Of Paleoearthquakes Events In Active Faults Surrounding The Northern Margin Of Qinghai-Tibet Plateau

The East Kunlun, Altyn and Qilian-Haiyuan fault zones, located on the northern margin of the Qinghai-Tibetan plateau, are of high seismicity with many major earthquakes. It is of importance to study ages of paleoearthquakes in these fault zones for both science and practise. This thesis is focused on determination of ages of paleoearthquakes at several profiles on these fault zones, which are the Xidatan profile and Kusai Lake profile on the East Kunlun, Wudaogou profile at the Yumenzhen NW fault, eastern end of the Altyn fault, and Ciergou profile and Shikaguan profile on the Haiyuan fault. To achieve this purpose, first I analyze the feasibility and reliability of several OSL dating methods (e.g. fine-grain quartz SAR and SMAR protocols) that were suggested in recent years, whether they can be suitable for dating paleoearthquake events. Through comparative studies between several methods and protocols for OSL dating on different types of sediments associated with paleoearthquakes, such as loess and secondary loess, slope deposit, river alluvial, collapse wedge and damped pond, suitable methods and experimental techniques are selected out.Then these methods are used to make a detailed study of luminescence dating and chronology on the events on those profiles mentioned above, yielding the ages of these seismic events.1,Study on luminescence dating methods and experimental techniques for different types of sediments1) Reliability of SAR and SMAR methods for fine-grain quartzThe SAR method has been less used less for fine-grain quartz, and the reliability of SMAR method for non-loess samples has not been tested very well. In this work, the reliability of SAR and SMAR methods is tested on the samples from different environments. The results show that the DE values from SAR and SMAR are consistent in distribution features and statistical analysis. It is found that the distribution of SAR DE values can be used for identification of exposure extent before deposition and bury of samples in the study of the relationship between LN/TN and DE values. The results suggest that the SAR protocol can be replaced by SMAR for dating fine-grain quartz samples. The lower preheat temperature should be applied for dating the sample such as glacifluvial from the East Kunlun fault and salty debris sediments from the Yumenzhen NW fault, so a lower OSL signal recuperation can be accessed.2) Comparison of SAR's ages of fine-grain and coarse quartzConsistent ages with acceptable error ranges measured by SAR protocol for fine-grain and coarse quartz can be estimated if the sample is well-bleached. But a large difference exists between ages from fine-grain and coarse quartz. The results of LEDL05-104 and LEDL05-106(peat) from the Shikaguangou profile at Haiyuan fault zone display a nice similarity between the OSL ages of coarse quartz and 14C dating. A more credible DE value can be estimated by cumulative frequency statistics.3) Polymineral fine-grains IRSL and [Post-IR]OSLFor several samples from different deposition environments in the three regions, at least 200s and more IR bleach time are required to get a plateau region of a [Post-IR]OSL DE value VS IR exposureduration. . But underestimation is found at the re-worked sample of which the DE value is larger ( e.g. >200Gy), and a ~10% of underestimation is implied. The longer IR bleach time should be applied for the sample with large DE values.In general, for dating the sample of sediments related to the fault activity, the optimal luminescence method is SAR for coarse quartz. But the SAR and SMAR methods for fine-grain quartz can be credible when the sample is lack of coarse quartz. IRSL and [Post-IR]OSL signals of fine-grain mixed mineral are affected easily by some factors, so the measured ages have a poorer reliability.2,Chronology of paleoearthquake eventsBased on the above results, a systematic OSL dating for the samples from all profiles is carried out on fine-grain and coarse quartz, by use of SAR and SMAR protocols. The ages of paleoearthquake events recorded on these profiles are estimated.1) The East Kunlun fault zonea: Xidatan profileThere are three paleoearthquake events recorded on the Xidatan profile, of which the ages are : Event I: 0.7±0.4~0.8±0.1ka ;Enent II: 2.0±0.3~0.8±0.1ka ; and Event III: 3.1±0.3~4.6±0.6ka.b: West Kusaihu Lake profileThere is a paleoearthquake events recorded on the west Kusaihu Lake profile, of which the ages is before 3.9±1.1ka.2) Wudaogou profile at Yumen NW fault.The only one event was record on the profile. Its age is estimated 1.4～2ka based on the ages of the new strata and overlying cover that the fault cut through.3) Haiyuan fault zonea: Ciergou profileThe largest collapse wedge III is studied by dating methods in this work. The luminescence ages of the re-worked loess in beds below and above the wedge restrict the paleoearthquake events which occurred 57.9±2.8～26.7±2.1ka before present. The conclusion implies that dating paleoearthquakes by ages of upper and lower strata remains limited.b: Shikaguangou profileThe only one event was record on the profile. Its age is 1.3～1.4ka. The event perhaps occurred at the same time in the mid-segment and west segment of the Haiyuan fault zone.