Refined Extraction Of The Tectonic Signature From Late Cenozoic Lacustrine Sedimentas

Lacustrine sediments have the advantages of relatively stable sedimentary environment,large amount of information,good continuity and high resolution,compared with other terrestrial sediments.It can not only record the environmental evolution on a long-term scale in geological history,but also record climate changes and earthquake activity on a short-term scale and even human history.The change of clastic particles in lacustrine sediments can not only restore the evolution of the lake itself,but also reveal the changes of regional climate,and more importantly,it also records a large amount of information of local and regional tectonic activities.In this thesis,we selected the lacustrine sediments of Sikouzi in Guyuan,Ningxia,and Lake Ebinur in Xinjiang as the research objects and analyzed the major and trace elements and high-resolution grain-size and magnetic susceptibility,the purpose of this study is to extract the tectonic and climatic signals from the lacustrine sedimentary records,and to further reveal the climate change and tectonic activities during the Mid-Miocene Climate Transition in the Sikouzi area and since the late Pleistocene in the Ebinur Lake region.The fluctuating characteristics of major elements in Sikouzi lacustrine sediments and typical loess-paleosol are similar,and both fluctuate within a narrow range.In addition,there is a good exponential linear correlation between them,and they are similar to the result of the homogenization of the average content of elements in the upper crust continental.The lacustrine sediments of Sikouzi are mainly fine-grained sediments,and the content of fine-grained sediments below 125μm is close to 96%of the total;the frequency curve shape is similar to that of typical loess-paleosol and red clay;The content of samples with mode grain-size less than 125μm content is close to95%of all samples,and the content of samples with a mode grain-size below 60μm is close to 89%;The C-M distribution of most Sikouzi lacustrine sediments is parallel to the C=M baseline;The Y value of more than 75%of the Sikouzi lacustrine sediments is less than Y=-2.7411.All these may mean that the lacustrine sediments of Sikouzi are not only aeolian origin,but may have a similar dust source area with loess-paleosol.Moisture evolution in Central Asia including Northwest China shows less similarity with its surroundings and attracts a growing number of studies.In this study,a well-dated thick lacustrine sequence is chosen in Northwest China and detailed geochemical analysis is conducted during the Middle Miocene Climate Transition（MMCT,14-11 Ma）.The multi-proxy records（Na₂O/Al₂O₃,CIA,Rb/Sr）revealed that chemical weathering was the strongest during 11.85-11 Ma,the coldest period in 14-11Ma as evidenced by the global deep-sea oxygen isotope records.Accordingly,we conclude that global climate cooled during MMCT and reached the coldest during11.85-11 Ma.Thus,the westerly circulation became the strongest during this period,which brought more water vapor to Northwest China and the chemical weathering was significantly improved.On the other hand,the significant decrease in temperature led to the marked weakening of evapotranspiration,and thus the effective humidity was relatively increased.Both aspects contribute greatly to the significant enhancement of chemical weathering in study area.The high-resolution grain size,magnetic susceptibility,L^*,a^*,proxy index of chemical weathering intensity and oxygen isotope record system of Sikouzi lacustrine sediments reveal that during 14-13.3 Ma,there have three tectonic activities（Ⅰ1:13.95-13.91 Ma,Ⅰ2:13.82-13.78 Ma,Ⅰ3:13.41-13.36 Ma）.The extremely low chemical weathering intensity at 11.85 Ma and the increasing correlation between the magnetic susceptibility（SUS）and EM3 at 11.85-11.54 Ma may mean that the study area was affected by tectonic activities during this period.The 13.3-11Ma,which is mainly affected by climate,can be further divided into three types and seven sub-stages.They include Warm Period Ⅱ1:13.3-13 Ma,cooling period Ⅱ2:13-12.65 Ma,Ⅲ1:12.65-12.32Ma and Warming Period Ⅲ2:12.32-11.85 Ma,Ⅳ2:11.54-11.4 Ma,Ⅳ3:11.4-11Ma.In addition,during the period of 13.3-11.85 Ma,the maximum mode grain-size of the ultrafine component was about 11Φ（0.4-0.5μm）,which can be used as a new index to indicate a relatively warm climate.From the lacustrine sedimentary records of Lake Ebinur,11 tectonic activity stages were extracted,from bottom to top they were 20.8-20.7 ka,17.3-17.1 ka,～13.9 ka,～13.4 ka,9.9 ka,8.6 ka,7.8 ka,7.4 ka,4.3 ka,3.17-3.25 ka,2.5 ka.In addition to this,there are the cooling events around～16.2 ka,and the tectonic and climatic interaction stages from 13.3-11 ka.