祁连山地区河流阶地与第四纪构造隆升

祁连山及邻区河流阶地系列是反映区域构造隆升的重要地貌标志。对祁连山地区东北部的沙沟河、西北部的洪水坝河、东南部的黄河共和段及渭河陇西段、东部的黄河兰州段河流阶地进行了阶地抬升幅度、年代对比研究,分析了祁连山及邻区第四纪构造隆升的特点及其对青藏高原隆升的响应。研究表明,1.6Ma以来祁连山及邻区至少存在5期构造活动,即1.6Ma左右的第1期构造活动;1.2~0.6Ma的第2期构造活动,包括1.2Ma、0.8Ma、0.6Ma的次一期构造活动;0.45~0.25Ma的第3期构造活动;0.2~0.08Ma的第4期构造活动,包括0.15Ma、0.1Ma次一期构造活动;0.08Ma以来的第5期构造活动。祁连山及邻区在第1、2、4、5期构造活动中表现为同步抬升,第3期不同区域抬升时间存在差异。在构造活动强度方面,1.2~0.6Ma(第2期)祁连山东部活动较强,0.45~0.25Ma(第3期)构造活动强度表现为北部强于南部,在北部表现为由西向东不断增强,0.2Ma以来在南、北方向上表现为构造活动强度沿着北东方向减弱,在东、西方向上表现为祁连山西北部的构造活动明显强于东北部。祁连山及邻区东部1.80Ma以来平均抬升速率为0.25mm/a,平均抬升了450m左右,粗略计算2.6Ma以来该区域抬升了600m左右。祁连山及邻区西部河流阶地反映的构造抬升强于东部,据此推断,第四纪以来祁连山及邻区西部抬升或超过600m。

River terrace and Quaternary tectonic uplift in the Qilian Mountain

River terrace series in Qilian Mountain and its adjacent areas is a significant landform symbol reflecting the regional tectonic uplift. In this paper, the authors conducted a comparative study of the amplitude and time of the terrace uplift in the Shagou River in the northeast and the Hongshuiba River terrace in the northwest of the Qilian Mountain, the southeast of the Yellow River in Gonghe and the Weihe River in Longxi as well as the east of the Yellow River in Lanzhou, and analyzed the features of Quaternary tectonic uplifting of the Qilian Mountain and its adjacent areas and its response to Tibetan Plateau uplifting. As shown by the comparative study, there have been at least five phases of tectonic activities in the Qilian Mountain and its adjacent areas since 1.6Ma:the first tectonic activity occurred at 1.6Ma, the second at 1.2~0.6Ma with its second-rate tectonic activities at 1.2Ma, 0.8Ma and 0.6Ma, the third tectonic activity at 0.45~0.25Ma, the fourth tectonic activity at 0.2~0.08Ma with two times of tectonic activities respectively at 0.15Ma and 0.1Ma, and the last tectonic activity since 0.08Ma, respectively. Studies reveal that there existed synchronous uplifting in the first, second, fourth and fifth tectonic activities in the Qilian Mountain and its adjacent areas, whereas the third tectonic activity displayed difference in uplifting time. In combination with the tectonic activity in Qilian orogenic belt, it is held that the strongest activity that occurred in eastern Qilian Mountain was caused by the second tectonic activity. Tectonic activity intensity of the third tectonic activity in 0.45~0.25Ma exhibited the feature that the intensity of the north area was stronger than that of the south area, and the intensity became stronger from west to east on the northern margin of the Qilian Mountain. Since 0.2Ma, the tectonic activity intensity in south and north direction became gradually weak in east-north direction, and the tectonic activity in the west part of the northwest side of the Qilian Mountain is stronger than that in the northeast section. The average amplitude of uplift since 1.80Ma is 0.25mm/a and 450m in the Qilian Mountain. A rough calculation shows that the region has been uplifted by about 600m since 2.60Ma. River terraces show that the tectonic uplift in the west of the Qilian Mountain and its adjacent areas is stronger than that in the east. Therefore, it is considered that the west of the Qilian Mountain and its adjacent areas have been uplifted by 600m since Quaternary.