青海鄂拉山断裂带晚第四纪构造活动及其所反映的青藏高原东北缘的变形机制

鄂拉山断裂带是分隔青海乌兰盆地 (柴达木盆地的一部分 )与茶卡—共和盆地的一条重要边界断裂 ,长约 2 0 7km ,由 6条规模较大的主要以右阶或左阶次级断裂段羽列而成 ,阶距约 1～ 3.5km。该断裂右旋走滑的起始时代为第四纪初期 ,约在 1.8～ 3.8MaB .P .期间 ,大的地质体累积断错约 9～12km。断裂新活动形成了一系列山脊、冲沟和阶地等的右旋断错及断层崖、断层陡坎等。晚更新世晚期以来 ,鄂拉山断裂带的平均水平滑动速率为 (4 .1± 0 .9)mm/a ,垂直滑动速率为 (0 .15± 0 .1)mm/a。鄂拉山地区的构造变形受区域NE向构造应力作用下的剪切压扁与鄂拉山断裂的右旋剪切和挤压的共同影响 ,共和—茶卡盆地和乌兰盆地均属于走滑挤压型盆地。青藏高原东北缘地区在区域性北东向挤压的作用之下 ,应变被分解为沿北西西向断裂的左旋走滑和沿北北西向断裂的右旋走滑运动 ,形成一对共轭的剪切断裂。鄂拉山断裂及其他北北西走向断裂的发展演化和变形机制表明青藏高原东北缘向东的挤出和逃逸是非常有限的。

The tectonic activity and deformation features during the Late Quaternary of Elashan Mountain active fault zone in Qinghai Province and its implication for the deformation of the northeastern margins of the Qinghai-Tibet Plateau

The Elashan Mountain active fault zone is an important boundary between the Qaidam basin in the west and the Chaka-Gonghe basin in the east in Qinghai-Tibet Plateau. This fault is about 207 km long and mainly consists of six discontinuous sub-parallel strands with either right-step or left-step echelon separating each apart. The step distance is about 1～3.5 km. The initiation of right-lateral strike slip motion is in about 1.8～3.8 Ma, i. e. the Early Quaternary. The offsets of the large geological body, e. g. Paleozoic geological contacts and Mesozoic granites, are about 9～12 km. The Late Quaternary tectonic activity is characterized by both right-lateral strike-slip and NEE-direction crustal shortening. The average right-lateral strike-slip rate of the Elashan Mountain active fault zone is about 4.1±(0.9) mm/a and the vertical slip rate is 0.15±0.1 mm/a since the Late Pleistocene. The deformation of the Elashan Mountain area, under the action of NE direction regional structural stress and right-lateral shear and compression, formed the strike-slip compressive basins: the Wulan basin and Chaka-Gonghe basin that developed on the east and west side of the Elashan Mountain active fault zone, respectively. It appears that the NE Qinghai-Tibet Plateau has subjected to NE-direction compression resulting in both conjugate shear and compression. Thus, the NNW-trending fault had right-lateral displacement, and the NWW-trending faults had left-lateral offset as well as compression. This pattern of deformation suggests that NE-direction crustal shortening may accommodate large amounts of regional deformation, and that the eastward extrusion appears to be insignificant.