焉耆盆地北缘和静逆断裂-褶皱带古地震破裂方式和时间序列

古地震研究是构造地质基础研究工作之一,获得较为精细的古地震结果有利于提高对断层构造变形的样式、强度以及时间的认识。焉耆盆地是南天山东段的山间盆地,现今的构造应力场特征以挤压兼有走滑为主。盆地南北缘断裂均为全新世活动断裂,南缘开都河断裂以走滑运动为主。盆地北缘断裂向盆内扩展的新生和静逆断裂-褶皱带以逆冲运动为主,且具备发生7级以上大地震的能力。因此,对于焉耆盆地北缘和静逆断裂-褶皱带的古地震破裂方式和发生时间的研究具有重要意义。调查发现,其中的哈尔莫敦背斜南翼主逆断裂以30°左右向盆内逆冲,在河漫滩和T1阶地上形成了3排断层陡坎。在3条断层陡坎上开挖的5个探槽中,通过标志地层建立的时间序列可以确定6次古地震事件的先后关系。利用14C和光释光(OSL)测年手段获得了探槽中相关地层和坎前堆积物的沉积时代,利用逐次限定法得到了各次古地震事件的发生时间和全新世以来2ka左右的古地震复发间隔。结果显示F1断层在所有的古地震事件中都发生了破裂,F2断层只在事件E时产生了破裂,F3断层只在事件D和事件E中发生过破裂。从古地震事件上分析,事件D是一次3条断层同时破裂的事件,事件E是一次F1和F32条断层同时破裂的事件,其他事件都只在F1断层上破裂。和静逆断裂的古地震破裂同时存在必然性和不确定性。

THE RUPTURE MODEL AND TIME SERIES OF PALEOEARTHQUAKES OF THE HEJING THRUST FAULT-FOLD BELT IN THE NORTH MARGIN OF THE YANQI BASIN

Paleoearthquake study is a basic research that can be favorable for the understanding of deformation pattern, intensity and time scale of the fault structure. The Yanqi Basin is an intermountain basin located in the eastern part of southern Tianshan Mountains. The present-day tectonic stress field of the basin is dominated by compression with strike-slip component. Both the north and the south marginal faults are Holocene active faults. The Kaiduhe Fault on the southern margin is a strike-slip fault. The Hejing Fault on the northern margin is a neogenic thrust-fold belt dominated by thrust faulting and extending towards the basin. The Hejing thrust fault has the potential to generate M7 earthquake in the future. Therefore, it is important to study the rupture model and time series of paleoearthquakes of the Hejing thrust fault-fold belt. According to the surveys, the major thrust fault at the south limb of the Haermodun anticline thrusts inwards the basin, with a dip angle of 30°, producing three paleoseismic scarps in T₁ terrace and washland. We determine the time series of six paleoseismic events by the cut-cover relationship of marker stratums in five trenches, date the age of stratums and colluvial wedges by ¹⁴C and OSL dating method, and obtain the recurrence intervals of the paleoseismic events using progressive constraining method. The rupture models of the Hejing fault are summarized. The fault F₁ ruptured in every paleo seismic event, but fault F₂ only ruptured in event E, F₃ only ruptured in event D and E. In other words, Event D ruptured the three faults simultaneously, Event E ruptured two faults, and the other events only ruptured fault F₁. There exist both certainty and uncertainty in the rupturing of Hejing thrust fault in the paleo-earthquakes.