滇西弥渡盆地中晚更新世断陷特征及环境演化

为了解滇西弥渡盆地中晚更新世以来的断陷特征及环境演化,开展了弥渡盆地各级阶地沉积物的热释光测年、沉积相和孢粉的研究工作。通过野外调查发现弥渡盆地西侧至少发育5级阶地,对应于5次间歇断陷阶段,重点调查弥渡盆地西侧的奇家营Ⅰ级阶地、龙华寺Ⅲ级阶地、平安庄Ⅳ级阶地、大树坪Ⅴ级阶地。Ⅰ级、Ⅲ级、Ⅳ级和Ⅴ级阶地热释光测年值分别为 200.48±22.05ka,239.06±26.30ka,320.85±35.29ka和 332.07±36.59ka。年代学分析结果表明: 滇西弥渡盆地的地貌格局形成约33万年来的间歇式断陷运动,而且表现出早期断陷速率快,晚期断陷速率减缓的趋势。从沉积特征分析: 早期沉积特征中出现半深湖沉积物,说明湖盆断陷速率相对较快,反映出该段时期程海断裂带南段的活动性相对较强; 晚期主要以湖泊近岸、浅湖为主,说明湖盆断陷速率相对较慢,反映出该段时期程海断裂带南段的活动性相对较弱。通过阶地堆积物沉积相和孢粉分析研究显示,更新世中晚期弥渡盆地沉积环境主要为山间盆地河湖环境,气候由凉爽湿润→温暖湿润→凉爽湿润转变的特点。弥渡盆地西缘、北缘发育较多洪积扇,洪积扇以线性排列,东缘洪积扇较少,多为单个扇体,说明盆地西、北侧断陷速率相对较快。盆地西侧的洪积扇,由北向南规模逐渐变小,堆积物由粗变细,扇面坡度由大变小,说明北段断陷运动较强。这一现象说明盆地近代具有向北掀斜的趋势。

CHARACTERISTICS OF RIFT AND ENVIRONMENTAL CHANGE DURING MIDDLE AND LATE PLEISTOCENE OF MIDU BASIN IN WESTERN YUNNAN

Midu basin was studied by thermoluminescence dating,sedimentary facies and pollen analysis,to understand the characteristics of rift and environmental Change during Middle and Late Pleistocene in the western Yunnan.According to the field investigation,there are at least five terrace levels presenting in the west side of Midu basin,which correspond to five intermittent rifts.We mainly investigated four terraces in the west side of the Midu basin,namely level Ⅰ-terrace of Qijiaying,level Ⅲ-terrace of Longhuashi,level Ⅳ-terrace of Ping'anzhuang,level Ⅴ-terrace of Dashuping,and the TL dating values of level Ⅰ terrace,level Ⅲ terrace,level Ⅳ terrace,level Ⅴ terrace are 239.06±26.30ka,239.06±26.30ka,320.85±35.29ka,and 332.07±36.59ka,respectively.The chronology analysis indicated that,the landscape of basin and range in the Midu basin western Yunnan was formed by intermittent rift during the last 33ka,and the trend was characterized by a fast rift and then a slow rift.Analysis of sedimentary characteristics showed that the early period the lake basin rift rate was relatively fast because the sedimentary characteristics was presented by the semi-deep lake sediment.Besides,the late sedimentary characteristics,mainly appearing the shore of the shallow lake,showed that lake basin rift rate was relatively slow.It could be explained that the activity of the south section of Chenghai fault zone was weak in that period.The sedimentary facies and pollen analysis showed that,river or lake in the basin among mountains exsited during the middle Late Pleistocene.Moreover,the climate history was from cool and humid,to warm and humid,then to cool and humid again.There are many alluvial fans in the west and north margin of the Midu basin,which are in a linear arrangement.The east margin of the Midu basin appears more than a single fan alluvial body.It meant that the rift rate of the lake basin was relatively fast in the west and north side of the Midu basin. From north to south,the scale of the alluvial fans becomes smaller,and the deposits is from coarse to fine and the fan slope is from big to small in the west side of the basin.We concluded that the rift activity in the northern side of the Midu basin was strong and the trend of northward tilting in the basin has occurred yet.