HORIZONTAL MOVEMENT CHARACTERISTICS OF THE ACTIVE SANWEISHAN FAULT AND ITS MECHANISM: CONSTRAINTS ON THE GROWTH OF THE NORTHERN QINGHAI-TIBETAN PLATEAU

The northern margin of the Qinghai-Tibet Plateau is currently the leading edge of uplift and expansion of the plateau. Over the years, a lot of research has been carried out on the deformation and evolution of the northeastern margin of the Qinghai-Tibet Plateau, and many ideas have been put forward, but there are also many disputes. The Altyn Tagh Fault constitutes the northern boundary of the Qinghai-Tibet Plateau, and there are two active faults on the north side of the Altyn Tagh Fault, named Sanweishan Fault with NEE strike and Nanjieshan Fault with EW strike. Especially, studies on the geometric and kinematic parameters of Sanweishan Fault since the Late Quaternary, which is nearly parallel with the Altyn Tagn Fault, are of great significance for understanding the deformation transfer and distribution in the northwestward extension of the Qinghai-Tibet Plateau. Therefore, interpretation of the fault landforms and statistical analysis of the horizontal displacement on the Sanweishan Fault and its newly discovered western extension are carried out in this paper. We believe that the Sanweishan Fault is an important branch of the eastern section of the Altyn Tagh fault zone. It is located at the front edge of the northwestern Qinghai-Tibet Plateau and is a left-lateral strike-slip and thrust active fault. Based on the interpretation of satellite imagery and microgeomorphology field investigation of Sanweishan main fault and its western segments, it's been found that the Sanweishan main fault constitutes the contact boundary between the Sanweishan Mountain and the alluvial fans. In the bedrock interior and on the north side of the Mogao Grottoes, there are also some branch faults distributed nearly parallel to the main fault. The main fault is about 150km long, striking 65°, mainly dipping SE with dip angles from 50° to 70°. The main fault can be divided into three segments in the spatial geometric distribution:the western segment(Xizhuigou-Dongshuigou, I), which is about 35km long, the middle segment(Dongshuigou-Shigongkouzi, Ⅱ), about 65km long, and the east segment(Shigongkouzi-Shuangta, Ⅲ), about 50km long. The above three segments are arranged in the left or right stepovers. In the west of Mingshashan, it's been found that the fault scarps are distributed near Danghe Reservoir and Yangguan Town in the west of Minshashan Mountain, and we thought those scarps are the westward extension of the main Sanweishan Fault. Along the main fault and its western extension, the different levels of water system(including gullies and rills)and ridges have been offset synchronously, forming a series of fault micro-geomorphology. The scale of the offset water system is proportional to the horizontal displacement. The frequency statistical analysis of the horizontal displacement shows that the displacement has obvious grouping characteristics, which are divided into 6 groups, and the corresponding peaks are 3.4m, 6.7m, 11.4m, 15m, 22m and 26m, respectively. Among them, 3.4m represents the coseismic displacement of the latest ancient earthquake event, and the larger displacement peak represents the accumulation of coseismic displacements of multi-paleoearthquake events. This kind of displacement characterized by approximately equal interval increase indicates that the Sanweishan Fault has experienced multiple characteristic earthquakes since the Late Quaternary and has the possibility of occurrence of earthquakes greater than magnitude 7. The distribution of displacement and structural transformation of the end of the fault indicate that Sanweishan Fault is an "Altyn Tagh Fault"in its infancy. The activities of Sanweishan Fault and its accompanying mountain uplift are the result of the transpression of the northern margin of the Qinghai-Tibet Plateau, representing one of the growth patterns of the northern margin of the plateau.     

Strymon and Strymonikos Gulf basins (Northern Greece): Implications on their formation and evolution from faulting

In Central and Eastern Macedonia of Northern Greece large NW–SE trending basins filled up mainly with terrestrial sediments developed during the Neogene over the Alpine basement rocks. Among them, the Strymon basin was established along the NNW–SSE trending Strouma/Strymon Lineament which formed over the tectonic boundary of the Serbomacedonian and Rhodope massifs, both representing the hinterland of the Hellenic orogen. The present study suggests that the Strymon basin was not formed as a syn-detachment basin over the Strymon Valley Detachment Fault, considered to have caused exhumation of the Rhodope massif metamorphic complex. Instead, transpressional s.l. tectonics dominated the region in the Late Oligocene-Early Miocene and it progressively changed into a wrench tectonics under which the Strymon basin has been initiated in the Middle Miocene. The basin continued to develop further under a short-lived NW–SE extension in the Middle-Late Miocene. The whole deformation is attributed to the late-stage collisional processes between the Apulia and Eurasia plates. The prevalent NE–SW extension has been constrained later on in the Late Miocene and Pliocene times activating both low-angle and high-angle NW–SE trending faults and causing the regional tilting towards the SW of the mountain fault blocks (i.e., mountain chains). From Quaternary onwards, the Strymon basin has been separated from the Strymonikos Gulf basin due to an N–S extension that mainly activates E–W striking normal faults.     

宗务隆山南缘断裂构造地貌特征与晚第四纪滑动速率

宗务隆山南缘断裂位于柴达木盆地东北缘,是祁连山南缘与柴达木盆地的边界逆断裂,对其晚第四纪活动性进行研究对于理解祁连山地区应变分配模式以及该地区断裂向柴达木盆地内部的挤压扩展过程具有重要意义。文中通过遥感影像解译和野外地质调查,结合GPS地形剖面测量以及宇宙成因核素与光释光定年,对宗务隆山南缘断裂拜京图和蓄集乡等段落开展了详细的研究。综合分析拜京图和蓄集乡地区不同期次洪积扇的垂直位错以及相应地貌面的年龄,得到宗务隆山南缘断裂晚第四纪以来的平均垂直滑动速率为(0. 41±0. 05) mm/a,水平缩短速率为0. 47～0. 80mm/a,约占祁连山地区地壳缩短速率的10%。这些结果有助于进一步理解祁连山地区的应变分配模式以及柴达木盆地北缘地区的构造变形机制。     

Postmagmatic cooling and late Cenozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41°−42°15′S

Zircon and apatite fission track (FT) thermochronology was applied to investigate the history of cooling and denudation of the Southern Andes between 41° and 42°15′S in relation to the late Cenozoic activity of the Liquiñe-Ofqui fault zone (LOFZ) and the northward migration of the Chile Triple Junction (CTJ). Fifty-six zircon and 51 apatite FT ages, plus 37 apatite confined track-length distributions were obtained mainly from plutonic rocks of the North Patagonian Batholith (NPB) in the main Andean Cordillera. Apatite FT ages and track lengths indicate a stage of rapid cooling at ∼5--3 Ma along both sides of the LOFZ, whereas older Miocene ages with monotonous cooling histories were obtained further away from the fault. Zircon FT ages range from Cretaceous to Pliocene, with marked differences observed along and across the LOFZ. Three different types of temperature-time histories characterise the post-magmatic cooling of the NPB in the region: deep intrusions with moderate and steady cooling rates, intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling, and rapidly cooled and exhumed shallow intrusions, the latter with younger ages towards the fault zone. The most prominent denudation episode along the LOFZ is late Miocene to Pliocene, coeval with plate tectonic reconstructions for the arrival and subduction of the Chile Rise beneath the Taitao Peninsula.     

TEXTUAL RESEARCH OF 1568 M7 GAOLING EARTHQUAKE IN SHAANXI AND ANALYSIS OF ITS SEISMOGENIC STRUCTURE

Study of historical earthquake is one of the important methods to understand the seismic activities and analyze the seismogenic faults. On the May 25^th, 1568 AD, a destructive earthquake occurred to the northeast of the present-day city of Xi'an, Shaanxi Province. Because this earthquake happened shortly after the 1556 M8 earthquake and was regarded as an aftershock, it has received little attention in previous studies. Previous earthquake catalogue agreed in assigning a magnitude 6 3/4 to this earthquake but had different epicentral locations and seismic intensity, and the seismogenic structure remains ambiguous. Based on textual research of historical earthquake and field investigation, the Jingyang County, Gaoling County, and Xianning County, were the worst hit area by the earthquake, and the areas, including Yongle Town, Gaozhuang Town at southeastern Jingyang County to Gaoling County and its southeastern present-day Jijia and Zhangbu, should be the mesoseismal area of this earthquake. The epicenter intensity of this earthquake is Ⅸ⁺(9~10 degrees), and the magnitude is estimated to be 7. The isoseismal lines were drawn to exhibit the various intensities of the areas damaged during the event, with its major axis directed NWW. Intensities reached Ⅸ⁺ in the zone extending west-northwest parallel to the Weinan-Jingyang Fault. This fault, characterized by a normal fault that developed during the Cenozoic extensional history of the Weihe Basin, dipping to the north at an angle of 60°~80°, is one part of the southern boundary faults in Weihe graben. There are geomorphological and geological evidences of recent activity of the fault during (180±30)a BP to (1 600±30)a BP. At T₁-T₂ fluvial terraces on the north bank of Weihe River, the scarps were faulted during Ming Dynasty, and sandy soil liquefaction, dense structural tensional fissures and faulted strata are noted in stratigraphic profiles and trenches. Thus, we suggest that this fault can reliably be regarded as being active during Holocene, and re-name the earthquake as the Shaanxi Gaoling earthquake.     

2022年1月8日青海门源MS6.9地震震前变形特征分析

2022年1月8日1时45分27秒青海海北藏族自治州门源县发生 M_S6.9地震,基于大地测量资料详细分析震源区的构造运动、应变演化以及深部变形特征,对于发震机理及震后危险性分析具有重要的意义。本文利用1991—2016、2017—2020期中国大陆地区GNSS速度场,分析了青藏高原东北缘各断裂带的运动学特征、门源地震震中和周边区域的地壳应变及其动态演化特征;结合剖面投影和非线性拟合算法,定量计算了托莱山、冷龙岭断裂的滑动速率和闭锁深度,得到以下认识:①青藏高原东北缘不同断裂带的运动学特征差异较大,整体以地壳缩短运动为主,局部区域伴随旋转运动; ②震中位于面膨胀率和最大剪应变率高值区的边缘,与前人关于强震地点的认识基本一致; ③区域应变参数的时空演化过程显示,震中附近应变特征整体变化不大,表明断层可能处在孕震晚期阶段; ④托莱山断裂带具有较高的滑动速率和闭锁深度,结合库仑应力的研究结果认为,该断裂未来一段时间的地震危险性仍值得关注。     

四川汶川地震断裂带科学钻探2号孔(WFSD-2)岩性特征和断裂带的结构

以WFSD-2钻孔岩心为研究对象,通过详细的岩心编录和岩石学、构造地质学等研究,识别出该钻孔岩心具有6段岩性,从上向下依次为彭灌杂岩(0~599.31m)、三叠系须家河组二段(599.31~1211.49m)、彭灌杂岩(1211.49~1679.51m)、三叠系须家河组三段(1679.51~1715.48m)、彭灌杂岩(1715.48~2081.47m)、三叠系须家河组四段(2081.47~2283.56m)。彭灌杂岩主要以花岗岩和火山岩为主,三叠系须家河组沉积岩以砂岩、粉砂岩、泥岩、页岩、煤层(线)和砾岩为主。3套彭灌杂岩与三叠系须家河组沉积岩重复出现,时代较老的岩性段逆冲覆盖在新的地层之上,表明龙门山构造带由一系列逆冲岩片叠置而成。岩心中断裂岩较为发育,主要为断层角砾岩、碎裂岩和断层泥,反映出脆性变形作用的特点。通过对断裂岩的统计分析,厘定了20余条产状不同、规模不等的次级断裂带,断裂带宽度和断裂密度峰值显示FZ600、 FZ720、 FZ782、 FZ817、 FZ922、 FZ951、FZ1449、 FZ1681、FZ2082为主要断裂带,其中FZ1681系规模最大的一条断裂。依据断裂岩的组合特征可以将岩心中断裂带的结构以断层泥为核部划分为两大类：对称型断裂带和不对称型断裂带。根据地表破裂带、WFSD-1钻孔岩心中主滑移带位置的几何关系、岩性分层等因素,可推断汶川地震主滑移带应位于FZ1134、FZ1449或FZ1681之中,同时也暗示该地区经常发生类似汶川地震的大地震活动。研究表明,龙门山地区经历了强烈的构造缩短和快速隆升作用,暗示龙门山地区构造活动非常强烈。     

大柳树坝址岩体松动类型与变形模式研究

通过对黄河黑山峡大柳树坝址区岩体野外实地调查和室内平确资料的分析,将岩体松动的类型划分为:断层控制松动型、节理裂隙控制松动型、顺层滑移松动型、卸荷松动型、倾倒松动型,并概化其松动模式,为此类特殊岩体的利用和改良提供科学的依据。     

中国川西地区鲜水河断裂和则木河断裂几何学、运动学特征及地震活动性对比研究

鲜水河断裂与则木河断裂在几何学特征、运动学特征和地震活动性方面既有明显的相似之处，又有着重要的差别。由于这两条断裂带都位于川滇菱形块体的北东边界，同属川西巨型左旋走滑断裂带的组成部分，因此在断裂的几何格局、活动方式和地震活动等方面有许多相似之处。然而，在菱形块体自北西向南东方向运动的过程中，由于其东部受到四川地块的阻挡使得块体边界的位移呈现由北西向南东递减的趋势，进而造成了两条断裂带在地震活动性方面的差异。根据详细的野外调查和已有成果，我们认为，断层的活动方式、滑动速率以及变形和变位的调整方式等运动学特征决定了两条断裂带在地震活动性方面的特征，而这些又与断裂带的几何学特征及与周围断裂的组合方式密切相关。通过对两条断裂带的对比研究，可以使我们对每条断裂有更好的理解和深入的认识。     

垂直和倾斜断层围陷波的谱元法数值模拟及其波场特征对比

Fault zone trapped waves( F ZTWs) m ainly travel along the fractured fault zone( F Z)which is of low velocity and high attenuation. FZTWs often carry significant information about a fault  s internal structure,so it is important to understand their wave field characteristics for FZ structure inversion. Most previous simulations are based on vertical faults,while in this paper we implement the FZTW simulations on vertical or inclined faults and compare their wave fields in both time and frequency domains. The results show that the existence of fault zone and inclined angle of fault can significantly influence the features of waves near faults. In amplitude,a fault zone can generate a larger amplitude of waves. The velocity contrast between two walls of fault may lead to amplification of amplitudes in the low velocity fault wall. In frequency,a fault zone tends to influence the waves in the low frequency range. In a pattern of particle polarization of FZTWs,it tends to be single direction for vertical faults but fork to multiple directions for inclined faults,which might provide a new way to study the fault zone with FZTWs. These conclusions may be valuable for FZ structure inversion,and will enhance the knowledge on near-fault strong ground motions.