第四纪洞庭盆地构造性质及动力机制探讨

Quaternary Tectonic Characteristics and Dynamic Mechanisms of the Dongting Basin

通过地表观察和第四系钻孔对第四纪洞庭盆地及周缘隆起区的地貌、沉积和断裂构造等进行调查研究,进而厘定洞庭盆地构造活动特征暨构造性质,并探讨其动力机制。研究表明,早更新世-中更新世中期洞庭盆地具断陷性质,具体表现在:①在边界正断裂控制下盆地及其次级凹陷强烈沉降并充填厚度较大的河、湖相沉积;②临澧次级凹陷发育典型封闭性断陷湖盆沉积;③NNE向、NW向、EW向、SN向等多组方向控盆断裂的发育指示区域伸展构造体制。中更新世晚期以来洞庭盆地具坳陷性质,主要表现在:①中更新世晚期洞庭盆地与周缘隆起区发生较大幅度的构造抬升并遭受剥蚀,同时产生构造掀斜与褶皱变形;②晚更新世-全新世洞庭盆地坳陷沉降并接受沉积。分析提出洞庭盆地第四纪构造活动机制:早更新世-中更新世中期盆地的断陷沉降与地幔上隆背景下的深部物质迁出有关;中更新世晚期盆地构造抬升、掀斜和褶皱变形与深部迁出物质的回返以及板块尺度的物质运动和挤压作用有关;晚更新世和全新世盆地坳陷沉降与区域挤压应力下的坳陷或拱坳变形有关,可能同时伴有NE向的深部物质流动与拉张;不同级次和不同规模范围的深部物质迁移运动,造成了第四纪洞庭盆地与其内部次级构造单元之间的叠加关系。

The topography, Quaternary sediment and faults of the Dongting basin and peripheral uplifts were investigated through detailed geological mapping and borehole logging, and the nature of the tectonic characteristics and the dynamic mechanisms of the basin were discussed. The results of this study show that the Dongting basin was a rift basin during Early Pleistocene-middle of Middle Pleistocene, as evidenced by 1 ） the intensive subside and thick alluvial and lacustrine deposits in the Dongting basin and its subordinate depressions formed under the control of the boundary normal faults; 2） the occurrence of the typical closed lake sediments in the Linli depression; 3） the regional extensional tectonic regime as shown by the multi-directional basin control faults. Ever since the late Middle Pleistocene, the Dongting basin evolved to be of depressional, as characterized by 1 ） the uplift and denudation of the Dongting basin and peripheral area as a result of late Middle Pleistocene tectonic tilting and folding of the basin; 2） the subsiding and sedimentation from Late Pleistoncene to Holocene. Regarding the Quaternary tectonic activities and dynamic mechanisms of the Dongting basin, it can be concluded that the rift-related subsiding from Early Pleistocene to middle of Middle Pleistocene might be caused by mantle upwelling, while the Late Pleistocene- Holocene uplift, tilting and folding were possibly related to the regional compression and likely accompanying NE- directed deep matter movement and tectonic extension, and furthermore, the superposition of the Dongting basin and subordinate tectonic units might result from deep matter movements of different grades and different scales.