关键词: 阿尔金断裂带, 走滑断层, 断错地貌, 滑动速率

Abstract: The strike-slip rate of the Altyn Tagh Fault zone is a topic full of disputation because of its significance to the comprehension of the formation and evolution of the Qinghai-Tibet Plateau. Identifying the strike-slip rate of the Altyn Tagh Fault zone by geological method and comparing the result with that obtained by other method (GPS Observation) is of great importance to the study of the mechanism of intra-continental deformation. With the help of high-resolution digitalized SPOT images, three typical landform sites of strike-slip fault offset have been identified and investigated. In addition, samples for thermoluminescence (TL) dating were collected and dated to get the ages of different terraces at different sites. The studied area of this paper is the E88°30′～93°05′ segment of the Altyn Tagh Fault, where 3 sites including Ananba gully, Qigequanzi and Yuemakeqi were investigated to get the sinistral strike-slip rates on the middle segment of the Altyn Tagh Fault. At Annanba gully, the Altyn Tagh Fault has two main strands, among which the southern one was studied. By reconstructing the offset terraces, the displacement of the T₃ terrace has been identified as 70±10m and the TL ages of the T₂ terrace has been determined to be 9.36±0.73ka BP, so the strike-slip rate of the south strand of the Altyn Tagh Fault since Holocene can be estimated as 7.5±1.7mm/yr. At Qigequanzi, the Altyn Tagh Fault has four strands, among which the second smallest strand was studied. Here the horizontal displacements of the T₂ and T₃ terraces are 12±2m and 32±4m, respectively, while the TL ages of the T₁, T₂ and T₃ terraces are 5.47±0.42ka BP, 13.86±1.07ka BP and 20.18±1.53ka BP, respectively. The slip rate of this small strand since Holocene calculated from these data is abouT₂.3±0.5mm/yr. Regarding the scale and mode of motion of the four fault strands, the deduced slip rate for the whole fault zone here since Holocene varies from 6.9±1.5mm/yr to 9.2±2.0mm/yr. At Yuemakeqi, the horizontal displacements of the T₂ and T₃ terraces are 30m and 50±10m, respectively, while the ages of the T₁, T₂ and T₃ terraces are 3.46±0.26ka BP, 4.73±0.38ka BP and 7.98±0.61ka BP, respectively, so the strike-slip rate of the Altyn Tagh Fault since 4.73±0.38ka BP can be estimated as 10.6±3.0mm/yr. Basing on the data obtained at the three sites described above, we conclude that the horizontal strike-slip rate on the middle segment of the Altyn Tagh Fault since Holocene is about 7～11mm/yr, which is coincident with the latest GPS (Global Positioning System) result about this region. The slip rate of the Altyn Tagh Fault obtained by this study is much less than 44mm/yr or 25mm/yr that were evaluated previously by some researchers. From this point of view, among the models for the formation and evolution of the Qinghai-Tibet Plateau, the "Crustal thickening" model seems to be more reasonable.

Key words: Altyn Tagh Fault, strike-slip fault, offset landform, slip rate