Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan connection

The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental mountain building at different stages of development. The region is structurally dominated by several major E—W left-lateral strike-slip fault systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be traced along the surface for over 350 km. The eastern two-thirds of the fault system ruptured during a major earthquake (M = 8.3) in 1957, whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the length of the fault system. Detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross-sectional geometries, with thrust faults rooting into oblique-slip and strike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, will lead to growth and coalescence of the restraining bends into a continuous sublinear range, possibly obscuring the original strike-slip fault system; this may be a common mountain building process.

The largely unknown Gobi-Tien Shan fault system is a major left-lateral strike-slip fault system (1200 km + long) that links the southern ranges of the Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Tien Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Quaternary activity. The total displacement is unknown, but small parallel splays have apparent offsets of 20 + km, suggesting that the main fault zone has experienced significantly more displacement. Field investigations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fault system. The spatial relationship between topography and active faults in the Barkol Tagh and Bogda Shan strongly suggests that these ranges are large, coalescing, restraining bends that have accommodated the fault's left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien Shan formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system is one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low, E–W trending corridor between the northern edge of the Tibetan Plateau and the Hangay Dome, presumably in response to the Indo-Eurasian collision 2500 km to the south.     

Tectonic model of Egypt based on magnetic analysis

The main target of this work was to study the dynamics of the Earth’s crust for Egypt based on the magnetic survey. High-resolution land magnetic data were analyzed, combined with the results of GPS and seismic stress analyses. The constructed tectonic map shows that the N35°-N45°W trend of the structure (related to the Red Sea and Gulf of Suez tectonics) predominates along the Gulf of Suez, Red Sea, covering wide parts of the study area. The N45°-N65°E tectonic trend (related to the Syrian Arc tectonics), prevailing in the northern part of Egypt, is of the second rank. The Aqaba (N15°-25°E) and E-W trends prevail in the northern part and along the transition zone of stable/unstable shelves. The depth to the basement rocks ranges from the surface along the Red Sea and southern parts of Egypt to more than 4 km below sea level at the northern part of the study area.     

Deformation of the Earth’s crust in the Northern Tien Shan according to the earthquake focal data and satellite geodesy

For evaluating the deformations of the Earth’s crust in the Northern Tien Shan, we calculated the mode and intensity of the seismotectonic deformations (STD) for this region. The input for these calculations were the catalog data on the focal mechanisms of earthquakes, obtained by wave inversion of the signals recorded at the Kyrgyz seismic network (KNET) for the period 1994–2006. In the construction of STD maps, a modern approach to the classification of seismotectonic deformations was applied. This approach distinguishes eleven typical patterns of deformation. The areal distributions of the Lode-Nadai coefficient, as well as of the vertical component and the aspect angle of the deformed state were obtained. At the same time, based on the GPS measurements in the Northern Tien Shan during 1994–2006, the rates of dilatation and shear deformation of the Earth’s crust were estimated. A comparison between the directions of strain axes derived from the GPS data and from the earthquake focal data is carried out.     

蒙古中南部地区噪声层析成像

依托中蒙国际合作项目"远东地区地磁场、重力场及深部构造观测与模型研究",我们首次获取了蒙古中南部地区密集的地震台阵观测资料.本研究收集了69套宽频带台站2011年8月至2013年7月间的垂直向连续记录,利用噪声互相关方法计算了台站间的经验格林函数,并采用基于连续小波变换的频时分析方法,提取了1478条周期6~30s的瑞雷波的相速度频散曲线.利用DitmarYanovskaya方法,我们构建了研究区6~30s瑞雷波的高分辨率(0.5°×0.5°)相速度分布图.结果表明,蒙古中南部地区的地壳上地幔速度结构存在横向非均匀性,但并不显著(相对变化~±2%).短周期(如6s)的相速度分布与地表地质构造具有明显的相关性,具体来说,北部山盆显示为高速,南部盆地、戈壁滩均显示为低速;随着周期的增大(如15s,20s),地形的控制作用相对减弱.较长周期(30s)的相速度分布图上,南部的高速区进一步扩大,与南部地壳厚度较薄有关.蒙古主构造线南北两侧相速度分布有明显差异,暗示它不仅是地表地形和构造的分界线,而且还是地壳结构的分界线.从6~30s中戈壁一直显示为低速,可能与该区新生代火山活动有关;研究区北部杭爱山—肯特盆地一直显示为相对的高速,与该区具有较老、稳定的地层有关.     

Active intracontinental transpressional mountain building in the Mongolian Altai: Defining a new class of orogen

Mountain ranges that are actively forming around the western and northern perimeter of the Indo-Eurasia collisional deformational field, such as the Mongolian Altai, comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens. Late Cenozoic uplift and mountain building in the Mongolian Altai is dominated by regional-scale dextral strike-slip faults that link with thrust and oblique-slip faults within a 300-km-wide deforming belt sandwiched between the more rigid Junggar Basin block and Hangay Precambrian craton. Dominant orogenic elements in the Mongolian Altai include double restraining bends, terminal restraining bends, partial restraining bends, single thrust ridges, thrust ridges linked by strike-slip faults, and triangular block uplifts in areas of conjugate strike-slip faults. The overall pattern is similar to a regional strike-slip duplex array; however, the significant amount of contractional and oblique-slip displacement within the range and large number of historical oblique-slip seismic events renders the term “transpressional duplex” more accurate. Intramontane and range flanking basins can be classified as ramp basins, half-ramp basins, open-sided thrust basins, pull-apart basins, and strike-slip basins. Neither a classic fold-and-thrust orogenic wedge geometry nor a bounding foredeep exists. The manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown; however, crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late Cenozoic regional epeirogenic uplift in the Russian Altai and Sayan regions.     

Comparison of seismotectonic peculiarities of Altai Mountains and Mongolian Altai

The paleoseismogeological studies within the two representative segments of fold systems in the Altai mobile belt (Altai Mountains and Mongolian Altai) have been carried out. These studies revealed the primary seismodislocations (seismic ruptures) of both the ancient historic and prehistoric strongest earthquakes. Based on amplitudes of single displacements (reverse fault and normal fault motions were 0.5–1.6 m) and with the significant strike slip component (more than 1.5 m) taken into account, the magnitude of ancient events in the Altai Mountains was determined at approximately 7.5. The recurrence period is 1400 years on average for the M = 7.0 events and 2100 years for the M = 7.5 ones. In the western Mongolian Altai, reverse fault motions were up to 2 m, corresponding to an earthquake magnitude of about 8.0 (an analog with the 1931 Fuyun earthquake). The recurrence period for the strongest earthquakes in Mongolian Altai was longer than that for the Altai Mountains, about 3000 years. This can be explained by higher magnitude values for western Mongolian Altai. It also can be concluded that the seismic regimes of the Altai Mountains and Mongolian Altai remained almost unchanged during the entire Holocene.     

基于GPS应变与震源机制解应力反演喜马拉雅构造带现今地壳形变特征

喜马拉雅构造带及其临近区域是印度板块与欧亚大陆板块挤压碰撞的前缘地带.本文利用GPS实测速度场与震源机制解数据分别计算了研究区域现今地壳岩石圈表面的GPS应变场及岩石圈内部的主应力分布,研究了印度板块持续挤压作用下板块边界带地壳岩石圈现今地壳形变的空间分布特征.结果显示,南北向的剧烈挤压变形与东西向的拉伸变形是现今青藏高原南缘地壳岩石圈的主要变形特征.其中南北向的地壳挤压变形主要集中在主前缘冲断带与雅鲁藏布江缝合带之间.东西方向上,南北走向的亚东—谷露断裂是区域地壳东西向伸展变形的重要分界断裂.75°E是研究区域地壳形变的另一条显著不连续边界,其西侧地壳主压应变强度低、方向弥散且最大主压应力方向一致性较差,而东侧地壳主压应变方向与主压应力方向以及地壳水平运动速度场方向均具有较好的一致性.布格重力异常的小波多尺度辨析结果显示该分界带与循喜马拉雅西构造结楔入欧亚大陆的印度板块密切相关.     

新疆天山地区壳幔S波速度结构特征及变形分析

天山地区地质构造复杂,地震活动频繁,其壳幔变形和深部结构一直受到学者们的高度关注.然而,由于天山地区地震台站资料较少,致使壳幔变形研究结果与解释存在诸多争议.本研究利用在天山地区（40°N-46°N,78°E-92°E）新布设的11个流动宽频带地震台站和该地区39个固定台站的观测资料,采用接收函数与面波联合反演方法,获得了研究区地壳厚度及壳幔S波速度结构.反演结果显示天山地区（41.5°N-44°N,78°E-88°E）平均地壳厚度为56 km,塔里木盆地（40°N-41.5°N,79°E-90°E）、准噶尔盆地（44°N-46°N,82°E-90°E）和吐鲁番盆地（42°N-43°N,88°E-90°E）具有较厚的沉积层,地壳平均厚度为43 km、53 km和46 km,整体表现为天山厚、盆地相对较薄的特征;在研究区南天山的最高峰（42°N,80.5°E）及北天山的最高峰（43.5°N,86°E）附近,中下地壳存在较厚的低速层,我们认为在强烈挤压作用下低速、低强度的中下地壳强烈变形可能是导致该区域快速隆升的主要原因.在研究区中部,位于塔里木盆地与准噶尔盆地之间的天山地区,中下地壳及上地幔均存在低速层,且盆地莫霍面向天山倾斜明显.结合前人的研究成果推测,在南北向构造挤压应力作用下,塔里木盆地与准噶尔盆地发生了向天山造山带方向的双向壳幔层间插入俯冲.在研究区东部,塔里木盆地东北缘与天山东部接触带的地壳内没有明显的低速层,推测应处在早期挤压变形状态,该区域的壳幔边界为缓变的速度梯度带,可能与上地幔热物质侵入或渗透有关.     

Crustal structure of northeastern margin of the Tibetan Plateau by receiver function inversion

Using seismic data of about one year recorded by 18 broadband stations of ASCENT project,we obtained 2547 receiver functions in the northeastern Tibetan Plateau.The Moho depths under 14 stations were calculated by applying the H-κ domain search algorithm.The Moho depths under the stations with lower signal-noise ratio(SNR) were estimated by the time delay of the PS conversion.Results show that the Moho depth varies in a range of ~40–60 km.The Moho near the Haiyuan fault is vague,and its depth is larger than those on its two sides.In the Qinling-Qilian Block,the Moho becomes shallower gradually from west to east.To the east of 105°E,the average depth of the Moho is 45 km,whereas the west is 50 km or even deeper.Combining our results with surface wave research,we suggest a boundary between the Qinling and the Qilian Mountains at around 105°E.S wave velocities beneath 15 stations have been obtained through a linear inversion by using Crust2.0 as an initial model,and the crustal thickness that was derived by H-κ domain search algorithm was also taken into account.The results are very similar to the results of previous active source studies.The resulting figure indicates that low velocity layers developed in the middle and lower crust beneath the transition zone of the Tibet Block and western Qinling,which may be related to regional faults and deep earth dynamics.The velocity of the middle and lower crust increases from the Songpan Block to the northeastern margin of Tibetan Plateau.Based on the velocity of the crust,the distribution of the low velocity zone and the composition of the curst(Poisson's ratio),we infer that the crust thickening results from the crust shortening along the direction of compression.     

Breakup of Australia and Antarctica estimated as mid-Cretaceous (95 ± 5 Ma) from magnetic and seismic data at the continental margin

A positive magnetic anomaly marks the seaward edge of the magnetic quiet zone along the southern margin of Australia eastward between 114° and 131°E and along the conjugate Antarctic margin between 105° and 132°E. This anomaly was originally interpreted as the oldest seafloor-spreading anomaly—A22, revised by Cande and Mutter to A34—in the Southeast Indian Ocean, but is better modelled as the edge effect at the continent-ocean boundary (COB) constrained by seismic data. Continental crust abuts the oceanic sequence of normal and reversed spreading blocks, truncated within the Cretaceous normal interval at an extrapolated age of 96 Ma, rounded to 95 ± 5 Ma to take into account the uncertainty of the initial spreading rate and of the location of the COB. The occurrence of the anomaly on both margins defines this as the age of breakup. Farther east between 131° and 139°E on the Australian margin, the COB anomaly is modelled as due to the same kind of effect but with successively younger ages of truncation to 49 Ma, interpreted as indicating the most recent ridge-crest jumping to the Australian COB. The magnetic data from the conjugate sector of Antarctica, albeit scanty, are consistent with this interpretation.The 95 ± 5 Ma age of breakup coincides with that of the breakup unconformity in southern Australia, expressed by a short mid-Cretaceous lacuna in the Otway Basin between faulted Early Cretaceous rift-valley sediments of the Otway Group and the overlying Late Cretaceous Sherbrook Group, and by an unconformity of similar age in the Great Australian Bight Basin.     

中国东部海域岩石圈结构面波层析成像

本文通过面波层析成像得到了中国东部海域及邻近地区的地壳上地幔S波速度图像,给出了主要构造单元的区划及其结构特征,并讨论了速度结构与现今构造活动及构造演化历史的关系.研究区内中下地壳的平均速度与地震活动存在比较显著的关系,强震基本都发生在低速区内或高低速过渡区.太行山以东地壳内存在几条北西向低速带,其中张家口—渤海地震带下方的低速带最为显著.东部海域划分成北黄海、南黄海、东海、和冲绳海槽等4个构造块体.北黄海具有较薄较高速的岩石圈,与南华北盆地类似,推测是中生代特提斯洋向北俯冲造成岩石圈减薄的遗迹.北华北地区具有低速的地壳和较厚的岩石圈,岩石圈地幔速度偏低且上下比较均匀,可能反映中生代沿北方缝合带持续碰撞作用的特点.南黄海具有相对较厚的岩石圈,较多地保存了下扬子克拉通的特征.在下扬子与华北地块的拼合过程中,洋壳俯冲可能是北黄海和苏皖地区上地幔低速特征的成因.在125°E以东的朝鲜半岛地区未发现这一拼合过程的遗迹.有可能整个朝鲜半岛都是华北地块的一部分;但也有可能是太平洋俯冲和日本海张开的作用完全改造了朝鲜半岛的岩石圈上地幔,抹去了以往构造运动的痕迹.东海地区的地壳厚度,特别是岩石圈厚度向冲绳海槽方向减小,反映出菲律宾海板块俯冲在弧后广大地区都有影响.冲绳海槽地区可见俯冲的菲律宾海板片以及板片上方显著低速的地壳和上地幔,为冲绳海槽的弧后扩张机制提供了证据.     

Geochemical characterization of major elements in desert sediments and implications for the Chinese loess source

Mineral dust released from the desert is one of the important components of atmospheric aerosols. Arid and semi-arid deserts, sandy lands in northern China and their adjacent Gobi Desert lands in northern China and neighboring Mongolia(hereinafter referred to as Gobi) are potential sources of mineral dust in Asia. However, there is currently a lack of systematic studies on the characteristics of major elements in the potential mineral dust source area. This study investigates the major elements of 310 surficial sand samples in the stabilized and semi-stabilized dune fields from 12 deserts/sandy land and Gobi in northern China and southern Mongolian Gobi and compiles published data. We identify four regions with distinct geochemical characteristics:(1) Taklimakan, Kumtag and Qaidam deserts in western China;(2) Badain Jaran, Tengger, Hobq, and Mu Us deserts in the central and western regions of northern China;(3) Hulun Buir, Onqin Daga and Horqin sandy lands in northeast China; and(4) Gobi and Gurbantunggut deserts. The spatial distributions of the SiO_2 and CaO contents in Chinese deserts are highly variable. The average content of SiO_2 generally reflects an increasing trend from west to east, while the average content of CaO shows a decreasing trend from west to east. We demonstrate that the spatial variation of major elements is likely controlled by two key scenarios: the composition of source rocks and the mineral maturity caused by the supply of fresh materials. The SiO_2/(Al_2O_3+K_2O+Na_2O) ratio of desert sediments is relatively lower in western China and may be caused by high ferricmagnesia and high carbonate minerals; this ratio is relatively higher in the northeast sandy lands and may be linked to a lack of fresh material supply and the presence of high K-feldspar minerals in source rocks. The deserts can be further distinguished by ternary diagrams with SiO_2/10-CaO-Al_2O_3,(K_2O+Na_2O)-CaO-Fe_2O_3 and CaO-Na_2O-K_2O. The comparison of major elements between desert sediments and loess suggests that the western and/or central deserts in China may be the potential provenances of loess on the Loess Plateau.     

Syn-extensional intra-plate trachydacite-rhyolitic dome volcanism of the Mesa Central, southern Sierra Madre Occidental volcanic province, Mexico

Oligocene dome complexes of trachydacitic to rhyolitic composition are common in the southern portion of the Mesa Central physiographic province, which forms part of the southern Basin and Range extensional province as well as of the southern Sierra Madre Occidental volcanic province. Generally, dome complexes occur aligned with regional fault systems, mostly associated with the southern Basin and Range province, and thus suggesting that faults controlled the felsic magmas that formed these domes. Two distribution patterns are evident, one aligned NE–SW and another aligned NNE. The set of domes were emplaced at 33–28 Ma. Emplacement of domes occurred in three continuous phases starting with those of trachydacite affinity at 33–32 Ma, to trachydacite–rhyolitic at 32–31 Ma, and finally to those with rhyolitic composition at 31–28 Ma. Felsic magmas that originated the domes were apparently generated by partial melting at the base of the continental crust. Contrary to previous hypothesis, our evidence suggest that these magmas in these particular areas of the Mesa Central were not accumulated in large magma reservoirs emplaced at shallow levels in the crust, but crossed the continental crust directly. Since continental crust in this region is relatively thin (30–33 km), we propose that an intense extensional episode favored the direct ascension of these magmas through the brittle crust, with little interaction with the country rock during ascent to the surface, to end up forming aligned dome chains or complexes. Geochemical data favors this model, as the felsic rocks show no depletions in Nb and Th but instead relatively enrichment in these elements. REE show flat or concave up patterns, suggesting that the magmas involved enriched (fertile), metasomatized lithospheric fluids that generated partial melting at the base of the continental crust. Based upon these data, we infer an intra-plate tectonic setting for these rocks.     

西藏中南部格仁错断裂张剪性质及其区域动力学意义

喀拉昆仑-嘉黎断裂带(KJFZ)是青藏高原中南部一条规模宏大断裂带,因其是青藏高原向东运移的南边界,其构造展布和活动性质与高原隆升、侧向挤出和东西向伸展等科学问题关系密切,也是研究高原变形机制和地球动力学重要场所.本文选取该断裂带中部NW向格仁错断裂(GRCF),对断裂沿线进行较详细地质地貌调查,对冲沟位错和断层陡坎等地貌单元进行测量,并进行探槽开挖,结合天然剖面揭示了断裂产状,详细研究断裂活动性质,发现断裂除前人认为的右旋走滑性质外,还具有明显的张性正断性质,断裂向北陡倾,且北盘相对南盘下降,全新世以来右旋走滑速率和正断速率分别为2.98 mm/a和0.2~0.5 mm/a之间.前人在共轭的北东向断裂研究中,也发现除左旋走滑性质外,同样具有正断分量,表明现今高原中南部上地壳除传统认为处东西向张性应力场环境外,南北向也具有明显张性特征.而受印度板块向北东向挤压作用,高原岩石圈总体应处于挤压应力状态.因此,推测现今该区域上下地壳处于应力解耦状态,并尝试建立区域张剪性地壳动力学模型:在南北向挤压应力场的作用下,中下地壳缩短凸起隆升,导致上地壳在各方向都表现为不同程度的张性特征.     

青藏高原岩石圈三维电性结构

本文报道通过综合大地电磁调查数据研究青藏高原岩石圈三维电阻率模型的初步成果.大地电磁法调查区域已经覆盖了高原大部分面积,为全区三维电阻率成像研究打下了可靠的基础.对多个测区大地电磁数据进行精细的同化处理和反演成像,取得了青藏高原可靠的岩石圈三维电阻率结构图像.成像的区域为28°N—35°N,80°E—104°E.三维反演计算时采用的网格尺寸为20km×20km,垂直方向不等间距剖分为26层.结果表明,青藏高原现今岩石圈电阻率扰动主要反映印度克拉通对亚欧大陆板块俯冲引起的热流体运动和大陆碰撞和拆离产生的构造.在岩石圈地幔,察隅地块、喜马拉雅地块和拉萨地块东部联成统一的高电阻率地块,它们反映了向北东俯冲的印度克拉通.雅鲁藏布江、班公—怒江和金沙江缝合带都有明显的低电阻率异常,表明岩石圈深处有热流体活动.雅鲁藏布江、班公—怒江和金沙江缝合带都有明显的低电阻率异常,也表明它们的岩石圈还有流体活动.青藏高原东部的低阻区沿100°E向地幔下方扩大,反映了金沙江断裂带有切穿岩石圈的趋势.地幔电阻率平面扰动的模式显示,青藏高原东西部的地体碰撞拼合形式和方向是不同的.在青藏高原西部,羌塘、拉萨和喜马拉雅等地体从北到南碰撞拼合.在青藏高原东部,羌塘—拉萨、察隅、印支、雅安和扬子等地体多方向拆离拼合,在地壳造成不正交的拆离带和压扭构造系.从高阻-低阻区的分布看,东部的地体拼合有地幔的根源,今后还会进一步发展.察隅地块岩石圈对青藏高原东部的楔入,使其北部和东部地块的岩石圈发生拆离撕裂,也造成热流体上涌的低电阻率异常.     

The vP Velocity Structures of Sub-blocks in the North China Craton from Active Source Waveform Modeling and Tectonic Implications

We present the 1-D crustal velocity structure of the major tectonic blocks of the North China Craton(NCC) along 36°N based on synthetic seismogram modeling of long-range wide-angle reflection/refraction data. This profile extends from southwest Yan'an of central Shaanxi Province of China(109.47°E), across the southern Trans-North China Orogen(TNCO), the southwestern part of the North China Plain(NCP), the Luxi Uplift(LU) and the Sulu Orogen(SLO), ending at Qingdao City of Shandong Province, the eastern margin of China(120.12°E) along 36°N. We utilized reflectivity synthetic seismogram modeling of the active source data to develop 1-D velocity structures of the sub-blocks of the NCC. Our final model shows that the NCC crust varies remarkably among the tectonic units with different velocity structure features. Higher lower crustal velocity and Moho depth ～42 km is a major feature of the crust beneath southern Ordos Blockt. The TNCO which is composed of Lyuliangshan Mountains(LM), Shanxi Graben(SXG) and Taihangshan Mountains(TM) shows dominant trans-orogenic features. The NCP shows a dominant thickening of sediments, sharp crust thinning with Moho depth ～32 km and significant lower average velocity. The SLO and the LU shows a stratified crust, higher average velocity and crust thinning with Moho depth of ～35 km. Our model shows the coincidence between the deep structure and the surface geology among all the tectonic sub-blocks of the NCC.     

The characteristics of the shear-wave attenuation field in the Altai lithosphere and their relationship to seismicity

The short-period S-wave attenuation field has been mapped for the lithosphere of the Altai and adjacent areas in Mongolia and southern Siberia. A total of approximately 500 earthquake seismograms were used. These were recorded at the Makanchi and Ulan-Bator stations at distances of ~300–1900 km. It is shown that the attenuation of shear waves is much stronger in the west of the area of study compared with the east. A V-shaped band of high attenuation in the upper mantle has been identified in the west of the area where the epicenters of the magnitude 6.9 1990 Zaisan and the magnitude 7.3 2003 Chuya earthquakes were situated; a similar band extends northwestward to the west of Lake Ubsu Nur. The attenuation is comparatively low in the rupture zones of large (М ≥ 7.0) pre-1970 earthquakes. It was concluded that fluid-charged zones are formed in the lithosphere prior to large earthquakes in the Altai, as well as in other areas of Central Asia. Following large seismic events, the fluids were rising into the crust from the upper mantle during a few tens of years, thus reducing the attenuation of Sn waves. We have identified zones of high attenuation with no significant earthquakes being recorded there during historical time. It is our opinion that earthquake precursory processes may be occurring in these zones.     

基于噪声面波直接反演法研究天山中段地壳S波速度结构

天山造山带是新生代以来复活隆升的陆内造山带,强烈的地震活动性使得理解和认识天山造山带深部结构及盆山耦合关系尤为重要。文章中使用天山造山带及邻区(40°～49°N,79°～93°E)85个台站2017—2019年的背景噪声资料,结合背景噪声互相关方法获得了6～52 s瑞利波相速度频散曲线,利用基于射线追踪的面波直接反演法对天山中段地壳三维S波速度结构及盆山耦合关系进行研究。结果显示：地壳浅层S波速度分布与构造单元中沉积层厚度相关,塔里木盆地北缘、准噶尔盆地南缘表现为低速,天山造山带表现为高速;到了中下地壳,天山造山带下方存在被高速异常包裹的低速体;莫霍面附近,天山造山带表现出相对低速;准噶尔盆地南缘和天山造山带的地壳厚度分别在45～50 km、50～62 km之间,沿南北向,天山造山带莫霍面呈现较为宽缓的形态;在82°～86.5°E之间,塔里木盆地和准噶尔盆地向天山下方双向俯冲,86.5°～88°E之间,准噶尔盆地向天山南向俯冲,由西向东,不同盆山耦合关系揭示了新生代以来天山中段不同区域构造运动差异,为进一步探讨造山动力过程提供参考。     

Fault tectonic analysis of Kii peninsula,Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

The southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.     

Dynamic processes in the system of interacting geospheres at the Earth’s crust-atmosphere boundary

Results of various instrumental observations of geodynamic processes in the Earth’s crust and geophysical fields at the crust-atmosphere boundary in the Oka area of the Nelidovo-Ryazan tectonic structure and adjacent fractures and in the zone of the Gornyi Altai earthquake of September 27, 2003, are presented. The correlations between the geophysical fields are determined from the results of processing and analysis of microseismic vibrations, the emanation field of natural radon, and variations in the electric field in ground and the magnetic field in the surface atmospheric layer. Tidal deformations of the crust were considered as external effects enhancing the interactions between the geospheres. It is noted that tectonic faults determining the block structure of the crust are recognizable as anomalous variations in the geophysical fields and their high cross-correlation.