摘要：
The Qilian Mountains,as a major orogenic belt in the northeastern margin of the Tibetan plateau,is the forefront of the expansion of the plateau to the northeast,where thrusts and folds dominate tectonic deformation.The Baiyang River starts from the inner Qilian Mountains,flowing northward across various structures,and finally into the Jiuxi Basin.This work focused on exhaustive investigations to the terraces on this river to characterize the Late Quaternary tectonic deformation in this region.The results show that (1)these river terraces on the Baiyang River are segmented,of which multiple levels developed at steep terrains and anticlines in the basin.Bounded by the Niutou Mountains,mainly 2-3 and 4-5 levels of terraces formed in the upper and lower reaches,respectively.(2)The longitudinal profiles along the river suggest a vertical motion rate of the Changma fault as(0.32±0.09) mm/a and crustal shortening rate (0.12±0.09) mm/a.There was no vertical activity since the formation of T5 surface(13ka) on the Hanxia-Dahuanggou fault.At the terrace T5(9ka) on the Laojunmiao anticline,fold uplift amounts(6.55±0.5)m and shortening amounts(3.47±0.5) m,yielding uplift and shortening rates (1.23± 0.81) mm/a and (0.67± 0.44) mm/a,respectively.The Baiyang River anticline began to be active about 300ka with uplift and shortening rates(0.21± 0.02) mm/a and (0.14± 0.03) mm/a,respectively since 170ka.(3) In the Qilian Mountains,there were two different deformation characteristics in response to the expansion of the Tibetan plateau.Shear deformation dominates the inner Qilian Mountains,which is manifested as lateral extrusion of blocks.In the northern margin of Qilian Mountains and Jiuxi Basin,the deformation is dominated by compression,expressing crustal shortening and uplift,and the shortening within the basin accounts about half of the total deformation.%祁连山作为青藏高原东北缘的重要造山带,是高原向NE方向扩展的最前缘,逆冲和褶皱作用是青藏高原向N扩展的重要构造变形方式.白杨河发育于祁连山内部,向N汇入前陆区酒西盆地.因此,可以通过白杨河阶地研究祁连山北缘的变形特征.通过对白杨河阶地的详细调查与测量,得到如下认识:1)白杨河阶地具有流域分段性,在地形陡变带及盆地内白杨河背斜区发育多级阶地.以阶地级数来说,以牛头山为界,上游发育2-3级阶地,下游发育4-5级阶地.2)从白杨河阶地纵剖面获得昌马断裂的垂直活动速率为(0.32±0.09) mm/a,地壳缩短速率为(0.12±0.09)mm/a;旱峡-大黄沟断裂T5形成以来(约13ka)没有垂直活动;老君庙背斜区T5阶地(约9ka)褶皱变形隆升量为(6.55±0.5)m,缩短量为(3.47±0.5)m,隆升速率为(1.23±0.81) mm/a,缩短速率为(0.67±0.44) mm/a;白杨河背斜开始活动时期约为300ka BP,其170ka以来的平均隆升速率约(0.21±0.02) mm/a,缩短速率为(0.14±0.03) mm/a;3)北祁连山地区在响应青藏高原向N扩展的过程中表现出2种不同的变形特征:在祁连山内部以剪切变形为主,表现为块体侧向挤出;而在祁连山北缘地形陡变带和酒西盆地内部以挤压变形为主,表现为地壳缩短和隆起,并且盆地内构造缩短变形量占总变形量的50％左右.
