蒙古国南戈壁Harshatewula地区二长花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

对蒙古国南戈壁Harshatewula地区二长花岗岩进行LA-ICP-MS锆石U-Pb精确定年。结果表明,该岩体LA-ICP-MS锆石U-Pb年龄为326.4±1.1Ma,成岩时代为早石炭世。本次所测岩体年龄是迄今为止该区获得的较为可靠的直接测年数据,为北山构造带、哈尔力克-大南湖岛弧带与蒙古戈壁-天山古生代岛弧岩浆岩带为一条古生代岛弧岩浆岩带的认识提供了年龄依据,对该区构造地质演化研究具有重要意义。     

Lithospheric stress in Mongolia,from earthquake source data

Lithospheric stress in Mongolia has been studied using mechanisms of 84 M_(LH)≥ 4 earthquakes that occurred in the 20 th century and instrumental seismic moments of 17,375 M_(LH)≥2.5 events recorded between 1970 and 2000.The M_(LH)≥ 3.5 earthquakes mostly have strike-slip mechanisms in southern and central Mongolia,with frequent reverse-slip motions in the west and normal slip in the north,especially,in the area of Lake Hovsgol.The principal stresses are,respectively,S_HS_vS_h in the center and in the south;high horizontal compression with S_HS_hS_v in the west;and a heterogeneous stress pattern with S_vS_HS_h in the north.According to seismic moments of M_(LH)=2.5 events,oblique slip generally predominates over the territory,at S_v≈S_HS_h,while frequent strike slip motions in the west record high horizontal compression(S_HS_vS_h).Earthquake mechanisms show the principal horizontal compression S_H to be directed W-E in the east,NE-SW in the central and Gobi-Altay regions,and approximately N-S in the west of Mongolia.The patterns of principal lithospheric stresses in the territory of Mongolia have undergone three events of dramatic change for a few recent decades,and these events were synchronous with three similar events in the Baikal rift system(BRS):in the latest 1960 s,latest 1970 s to earliest 1980 s,and in the latest 1980 s to earliest 1990 s.The seismicity of Mongolia has been controlled by superposition of variable stresses associated with rifting activity pulses in the neighbor BRS on the background of quasi-stationary super-regional compression.     

Variations in radon activity in the crustal fault zones: Spatial characteristics

The data of the profile gas emanation survey conducted on three spatial scales in separate regions of the Mongolia-Baikal seismic belt are generalized to establish the regularities of the spatially heterogeneous distribution of soil radon activity above the active faults in the Earth’s crust. It is shown that the shapes, sizes, and contrast of the near-fault radon anomalies are complicated by erosion and weathering; however, the critical role in their formation is played by the structural-geological controls, which determine the internal structure and recent activity of the fault zones. As a consequence, the cross-fault shape of the studied radon anomalies is vitally controlled by four structural situations, which correspond to the combinations of the structural type of the fault (localized/distributed) and the presence/absence of the fine filler material in the zone controlled by the fault. The cross-fault dimension of the emanation anomaly is commensurate or slightly larger than the width of the fault zone comprising all the fractures and joints associated with the formation of the main fault, which, due to the low permeability of the tectonites, is in most cases marked by the lowest concentration of soil radon. The contrast of the emanation anomalies, which we suggest to estimate in terms of a relative parameter K _Q , gravitates to certain levels of this parameter. This provides the basis for distinguishing five groups of the fault zones with low (K _Q ≤ 2), moderate (2 < K _Q ≤ 3), increased (3 < K _Q ≤ 5), high (5 < K _Q ≤ 10), and ultrahigh (K _Q > 10) radon activity. The previous studies show that for increasing the efficiency of the emanation survey in the fault zones, it is advisable to set up long profiles, reduce the measurement step in the vicinities of the main faults, specify the threshold of identifying the anomalies at the arithmetic mean level over the profile, and use the relative parameter K _Q for comparing and estimating the faults in terms of the intensity of their radon activity.     

Diagnostics of the stress state of the lithosphere in Mongolia based on seismic source data

The obtained results indicate that earthquakes of Mongolia form under the conditions of an alternating field of regional stresses, which is related to pulsed activations in the BRS and superimposed on the quasi-stationary field of global stresses of the Earth.     

蒙古中南部地区面波相速度层析成像

本文采用小波变换频时分析技术提取了1893条蒙古中南部地区双台间基阶Rayleigh波相速度频散曲线,通过对提取到的相速度频散进行二维反演,首次重构了蒙古高原中南部10~80 s周期内分辨率为0.5°×0.5°的相速度分布图.结果表明,短周期相速度频散(10~20 s)明显受地表地形控制,表现为杭爱—肯特山盆地为相对高速异常,而南部的戈壁带表现为相对低速异常;在30 s到60 s周期内,南部的戈壁带和北部的杭爱—肯特山盆地都显示出稳定的高速异常,而中部戈壁带则表现为低速异常.该低速异常区与新生代火山岩出露位置的一致性,暗示该低速异常可能与新生代火山活动有关.穿过蒙古高原中部的两条主要断裂带与研究区内的地震波速度分布具有很好的一致性,可能暗示两条断裂带一直延伸到整个岩石圈.此外,蒙古中南部地区的壳幔速度低于全球平均大陆值,且其相速度频散曲线与遭受破坏的克拉通,尤其大陆裂谷很相似,可能暗示着研究区具有薄的、活跃的岩石圈构造.     

On a more precise assessment of seismic hazards for the city of Ulaanbaatar,Mongolia

The recent geological, geophysical, and paleoseismic research of the Hustai, Emeelt, and Gunjiin active seismogenic faults in the vicinity of Ulaanbaatar provided evidence of prehistoric seismic events there. In particular, dislocations of two fault-forming paleo-earthquakes were revealed in the Gunjiin tectonic zone. The shocks occurred in the Late Pleistocene-Early Holocene. The presence of ruptures, initially identified by morphological characteristics, was proven by trenching studies and materials of geophysical profiles. The magnitudes of the ancient earthquakes are estimated to be 6.5–7.0. The obtained results provide grounds for reassessment of the possible level of seismic hazards for the territory of Ulaanbaatar to at least 8.     

蒙古西北部Tolbo湖沉积物粒度敏感组分记录的全新世风沙活动

作为全球主要的粉尘源区之一,亚洲中部干旱区全新世环境变化备受关注,但目前仍缺乏连续的风成沉积记录。本文利用蒙古西北部Tolbo湖岩芯（TB19-A,332 cm）沉积物,在AMS14C定年和粒度分析基础上,采用对数正态分布函数拟合及粒级-标准偏差相结合的方法分离出沉积物中风成组分以重建区域风沙活动历史。结果显示,Tolbo湖沉积物中值粒径为15～84μm和大于84μm的组分分别是区域尘暴和局地风沙活动的良好指标。蒙古西北部地区末次冰消期晚期（13.75 11.6 cal ka BP）,Tolbo湖流域内有较强的局地风沙活动,可能与YD寒冷事件有关。全新世以来,流域内局地风沙活动与蒙古西北部区域尘暴活动的变化具有一致性,具体表现为：早中全新世风沙活动总体较弱,其中9.7 8.6 cal ka BP和7 6 cal ka BP为区域尘暴较强时段;5 cal ka BP之后局地风沙活动与区域尘暴强度显著增加,直至0.7 cal ka BP两者均维持在较高水平;0.3 cal ka BP以来区域尘暴活动再次增强。蒙古西北部与中国北方公海、博斯腾湖记录的尘暴历史具有一致的变化趋势,反映了与西伯利...     

Relationship between Radon and the Tectonic Activity of Faults in Central Mongolia

Doklady Earth Sciences - The relationship between radon and the tectonic activity of faults for two geodynamically active regions of Central Mongolia is shown with account for the influence of...     

中蒙边界中东段地层划分与对比

中国与蒙古交界地区,早前寒武系—新生界分布广泛,地层划分系统差别较大。根据近年来内蒙古地层年代划分研究成果,初步对中蒙边界中东段蒙古地层岩石、化石进行了整理与划分,并以中国出露的地层为参照,进行中蒙边界中东段地层划分与对比。结果表明,中蒙边界中东段地层具有连续性,可以统一划分对比。