Luminescence Dating Of Paleaoearthquake Events Recorded By Colluvial Wedges On The Ci'ergou Profile Of The Haiyuan Active Fault Zone

The Haiyuan active fault zone located in the northeastern margin of the Qinghai-Tibetan Plateau sub-plate is one of the major strong earthquake belts in the China continent. Many great earthquakes have occurred along the Haiyuan active fault zone evidenced by the history and geologic records, of which a Haiyuan active fault in 1920 is the greatest event in China up to now. A study on geological records of paleaoearthquake events occurred in this active fault zone during the last 50ka not only would be helpful to prediction of the recurrences of the great earthquakes and reducing seismological hazards in this area in future, but also has a significance to understand the latest tectonic activity for the growing northeastern margin of the Qinghai-Tibetan Plateau block. In order to reconstruct a geochronology of the paleaoearthquake events in the active zone during the last 50ka, the Ci'ergou profile, containing several paleaoearthquake records, is selected to investigate luminescence dating of the colluvial wedges and slop-wash sediments in this thesis. This research is focused on two subjects as follows: 1) To find a suitable laboratory technique and protocol of the fine grain luminescence dating for the colluvial wedges and slop-wash deposits in the profile; 2) To reconstruct a geochronological time-scale of the paleaoearthquake events presented in the profile on the basis of luminescence dates obtained from more than twenty samples of the colluvial and slop-wash sediments in the profile, in turn, to provide an available fundament for establishing accuracy recurrence time intervals of the great earthquakes in the active fault zone in the past about 50ka B.P..In the Ci'ergou profile three great earthquake events recorded by the colluvial wedges and faulting scarps, and five cultural remain units, related to ancient human actions after deposition formations, taken place at the upper part of the profile have been recognized based upon the detailed data from field observations and measurements. The 54 sediment samples for luminescence dating were collectedfrom different parts of the three colluvial wedges and slop-washing deposits related to the fault activity, and 12 charcoal debris samples for 14C dating and 4 baked loess samples for luminescence dating from the five cultural remain units was acquired, respectively.Twenty-one of fifty-four colluvial sediment samples and four baked loess samples from the Ci'ergou profile have been dated by the multiple-aliquot poly-mineral fine grain (4-Hum) luminescence method. Two set of the equivalent dose (ED) values for each sample were obtained from the GLSL (Green Light Stimulated Luminescence) measurements, i.e. Post-IR measurements, following the IRSL (Infrared Stimulated Luminescence) measurements on the same set of the fine grain discs with the additive dose, the regeneration and the Australian slide methods respectively. For some of these samples, an equivalent dose (ED) was also determined following the Post-IR on same set of the fine grain discs. The measurements of the IRSL, GLSL (i.e. Post-IR) and TL following the Post-IR for these samples were all carried out in the Daybreak 1100 TL/OSL automatic system. Through analysis of all luminescence data, including ED values, ED plateau and calculated ages, obtained from the IRSL, GLSL (Post-IR) and TL (following Post-IR) measurements for twenty-five samples in the Ci'ergou section, this study achieves the following conclusions:.1. The Post-IR measurements of the fine grain multiple-aliquot may be a more suitable luminescence dating technique and protocol for the colluvial sediments and the baked loess sample. Comparison of the age estimates from the IRSL, Post-IR (GLSL) and TL (following Post-IR) analysis with the 14C dates for the same layer samples shows that the age estimates of the Post-IR are consistent with the 14C dates within two standards deviations (2o). While the age estimates from the Post-IR measurements correspond well to the sequences of sediment strata in the profile.2. According to tests of the t...