Large paleoearthquakes and Holocene faulting in the Southeastern Gorny Altai: implications for ongoing crustal shortening in Central Asia

Shrinking of intermontane basins and expansion of their flanking ranges by reverse faulting and backthrusting in two counter-dipping systems is a typical mechanism of crustal shortening and mountain building in Central Asia. This mechanism is realized along the Kurai Fault Zone (southeastern Gorny Altai). Motions on two reverse fault systems maintained thrusting of the Kurai Range and the Kubadru Uplift on the Kurai Basin sediments and caused the growth of a foreberg before the mountain front. Forberg separates narrow Aktash Basin from the Kurai Basin. The paleoearthquakes were generated by reverse faults that delineate the foreberg. Analysis of the <i>QuickBird</i> satellite images, drone imagery, trenching, archaeoseismological research, radiocarbon dating, dendrochronology, and previous results show that eleven large (<i>М</i>_W = 6.5–7.6) paleoearthquakes left traces as surface ruptures along the Kurai Fault Zone: twice before 7.5 ka BP, three events between 7.5 and 5.9 ka BP (7.0, 6.3, and 5.9–5.8 ka BP), one from 5.8 to 4.6 ka BP, four more at 4.6, 3.2, 1.5, and 1.3–1.2 ka BP, and the ultimate earthquake no older than 1450–1650 AD. The time difference between large earthquakes was from 200 to 1700 years. Surface faulting occurred mainly along the northern border of the foreberg where fault scarps are progressively younger northward and the Cenozoic sediments of the Aktash Basin thus become involved into uplift. GPR data to a depth of 12 m confirm the complex structure and slip geometry of the observed surface ruptures. The fault scarps are located &lt; 1 km from the planned route of the gas pipeline from Russia to China, and the potential seismic hazard has to be taken into account in its design and construction.

在中亚的地壳缩短与造山作用中，双倾向相反构造体系内的逆冲断层作用与反冲断层作用引发山间盆地收缩、侧翼山脉扩张，是典型的地壳缩短与造山机制。该机制沿库赖断裂带（Kurai Fault Zone，戈尔诺阿尔泰（Gorny Altai）东南部）展布。两套逆冲断层系统的活动使得库赖山脉（Kurai Range）与库巴德鲁隆起（Kubadru Uplift）逆冲推覆于库赖盆地沉积物之上，并在山前带形成了前缘隆起（foreberg）。该前缘隆起将狭窄的阿克塔什盆地（Aktash Basin）与库赖盆地分隔开来。划定该前缘隆起边界的逆冲断层曾引发古地震。对快鸟卫星（QuickBird）影像、无人机影像、探槽开挖、古地震考古研究、碳十四测年、树木年代学以及前人研究成果的综合分析表明，沿库赖断裂带共有11次矩震级（M_W）为6.5~7.6的大型古地震留下了地表破裂痕迹：7.5 ka BP之前发生2次，7.5~5.9 ka BP之间发生3次（对应年代分别为7.0、6.3及5.9~5.8 ka BP），5.8~4.6 ka BP之间发生1次，4.6、3.2、1.5及1.3~1.2 ka BP各发生1次，最后一次地震的年代不早于公元1450~1650年。大型古地震的复发间隔为200~1700年。地表断层活动主要发育于前缘隆起的北缘，该处断层崖的形成年代向北逐渐变新，阿克塔什盆地的新生代沉积物因此被卷入隆升过程。深度达12米的探地雷达（Ground Penetrating Radar，简称GPR）数据证实了观测到的地表破裂的复杂构造特征与滑动几何形态。该断层崖距规划中的中俄天然气管道线路不足1公里，其潜在地震危险性必须在该管道的设计与施工中予以充分考量。