A microearthquake study of the Mygdonian graben (northern Greece)

During March and April 1984, a temporary network of 29 portable stations was operated in the region of the Mygdonian graben near Thessaloniki (northern Greece), where a destructive earthquake (M_s = 6.5) had occurred in the Summer of 1978. During a period of six weeks we recorded 540 earthquakes with magnitudes ranging from −0.2 to 3.0. From this set of data, 254 events are selected which according to us have a precision in epicenter and depth better than 1.5 km. A total of 54 single-event focal mechanisms have been determined.The seismicity and focal mechanisms show a rather complex pattern. There are no clear individual faults, but the E-W and NW-SE striking zones show N-S extension. Zones striking NNE-SSW show dextral strike-slip motion but NW-SE zones with sinistral strike-slip are also observed.In the center of the graben where the 1978 earthquake was located, we observe several thrust mechanisms distributed in two groups showing either NW-SE or E-W compression; these earthquakes seem to be located 2 km above the earthquakes showing normal mechanisms.The mean direction of the T-axes, found from the focal mechanisms, trends N15° and dips sub-horizontal.We propose a model for the formation and evolution of a complex graben system comprising several stages. In the initial stage the deformation occurs along pre-existing NW-SE or NNE-SSW faults, with normal or strike-slip movements. In the second stage, a new, E-W trending group of normal faults is formed over the ancient fault network. These new faults have a direction perpendicular to the mean T-axis and accommodate better the actual state of stress. At this stage the initial faults adjust to the deformation produced by the E-W trending new faults, and may constitute geometric barriers to the evolution of the new normal faults.     

Late Cenozoic uplift along the northern Dead Sea transform in Lebanon and Syria

Evidence of long-term, late Cenozoic uplift, as well as strike-slip faulting, is revealed by topographic and geological features along the northern 500 km of the Dead Sea fault system (DSFS)—the transform boundary between the Arabian and African plates in the eastern Mediterranean region. Macro-geomorphic features are studied using a new, high-resolution (20 m pixel) digital elevation model (DEM) produced by radar interferometry (InSAR). This DEM provides a spatially continuous view of topography at an unprecedented resolution along this continental transform from 32.5° to 38° N. This section of the left-lateral transform can be subdivided into a 200 km long Lebanese restraining bend (mostly in Lebanon), and the section to the north (northwest Syria). Spatial variations in Cenozoic bedrock uplift are inferred through mapping of topographic residuals from the DEM. Additionally, high altitude, low-relief surfaces are mapped and classified in the Mount Lebanon and Anti Lebanon ranges that also provide references for assessing net uplift. These results demonstrate an asymmetric distribution of post-Miocene uplift between the Mt. Lebanon and Anti Lebanon ranges. Antecedent drainages also imply that a major episode of uplift in the Palmyride fold belt post-dates the uplift of the Anti Lebanon region. North of the restraining bend, the Late Miocene surface is preserved beneath spatially extensive lava flows. Hilltop remnants of this paleosurface demonstrate Pliocene-Quaternary uplift and tilting of the Syrian Coastal Range, adjacent to the DSFS north of the restraining bend. This late Cenozoic uplift is contemporaneous with strike-slip along the DSFS. Geometrical relationships between folds and strike-slip features suggest that regional strain partitioning may accommodate a convergent component of motion between the Arabian and African plates. This interpretation is consistent with regional plate tectonic models that predict 10–25° of obliquity between the relative plate motion and the strike of the DSFS north of the restraining bend. We suggest that this convergent component of plate motion is responsible for uplift along and adjacent to the DSFS in the Syrian Coastal Range, as well as within the Lebanese restraining bend.     

Texture and tectonic attribute of Cenozoic basin basement in the northern South China Sea

Based on the drilling data,the geological characteristics of the coast in South China,and the interpretation of the long seismic profiles covering the Pearl River Mouth Basin and southeastern Hainan Basin,the basin basement in the northern South China Sea is divided into four structural layers,namely,Pre-Sinian crystalline basement,Sinian-lower Paleozoic,upper Paleozoic,and Mesozoic structural layers.This paper discusses the distribution range and law and reveals the tectonic attribute of each structural layer.The Pre-Sinian crystalline basement is distributed in the northern South China Sea,which is linked to the Pre-Sinian crystalline basement of the Cathaysian Block and together they constitute a larger-scale continental block—the Cathaysian-northern South China Sea continental block.The Sinian-lower Paleozoic structural layer is distributed in the northern South China Sea,which is the natural extension of the Caledonian fold belt in South China to the sea area.The sediments are derived from southern East China Sea-Taiwan,Zhongsha-Xisha islands and Yunkai ancient uplifts,and some small basement uplifts.The Caledonian fold belt in the northern South China Sea is linked with that in South China and they constitute the wider fold belt.The upper Paleozoic structural layer is unevenly distributed in the northern South China.In the basement of Beibu Gulf Basin and southwestern Taiwan Basin,the structural layer is composed of the stable epicontinental sea deposit.The distribution areas in the Pearl River Mouth Basin and the southeastern Hainan Basin belong to ancient uplifts in the late Paleozoic,lacking the upper Paleozoic structural layers.The stratigraphic distribution and sedimentary environment in Middle-Late Jurassic to Cretaceous are characteristic of differentiation in the east and the west.The marine,paralic deposit is well developed in the basin basement of southwestern Taiwan but the volcanic activity is not obvious.The marine and paralic facies deposit is distributed in the eastern Pearl River Mouth Basin basement and the volcanic activity is stronger.The continental facies volcano-sediment in the Early Cretaceous is distributed in the basement of the western Pearl River Mouth Basin and Southeastern Hainan Basin.The Upper Cretaceous red continental facies clastic rocks are distributed in the Beibu Gulf Basin and Yinggehai Basin.The NE direction granitic volcanic-intrusive complex,volcano-sedimentary basin,fold and fault in Mesozoic basement have the similar temporal and spatial distribution,geological feature,and tectonic attribute with the coastal land in South China,and they belong to the same magma-deposition-tectonic system,which demonstrates that the late Mesozoic structural layer was formed in the background of active continental margin.Based on the analysis of basement structure and the study on tectonic attribute,the paleogeographic map of the basin basement in different periods in the northern South China Sea is compiled.     

藏北地区钻孔大地热流值测量

本文利用藏北地区三口天然气水合物钻孔测温数据,在分析样品热导率测试结果基础上,计算了藏北地区的热流值.对于样品热导率值,首先根据样品孔隙度对实验室测试结果进行了饱水校正,计算热流时采用的是对应井段的岩石热导率饱水校正值的厚度加权平均值.地温梯度以三口钻孔48 h的测温数据为基础,回归三口井的地温梯度,计算时去除了浅部受地表温度和冻土带对温度影响的数值.A钻孔地温梯度分为200~438 m和438~882 m两段回归,分段热流的加权平均值作为钻孔热流值,计算结果为42.7 mW·m^-2;B钻孔和C钻孔回归地温梯度时未分段,热流计算结果分别为58.3 mW·m^-2、70 mW·m^-2.综合分析认为,岩石圈断裂、地幔上涌、碰撞造山过程中的剪切生热等因素可能造成了班公湖—怒江缝合带以南热流值较高,而北部羌塘地块热流值相对较低.

     

辽河盆地东部凹陷新生代伸展-走滑构造作用及断裂构造特征

基于最新的三维地震资料处理与地震剖面解释、地震相干切片分析和平衡剖面恢复等方法,对辽河盆地东部凹陷所发育的断裂几何形态、盆地演化过程和走滑构造平面特征进行研究,并结合区域板块构造活动背景,分析其对郯庐断裂带新生代时期活动的响应.结果表明：辽河盆地东部凹陷为伸展和走滑两期构造变形叠加的产物,是具有"下断上坳"双层结构的裂谷型盆地.盆地演化过程经历了强烈断陷期（Es3）、区域隆升期（Es2）、断坳转化期（初始走滑期）（Es1）、坳陷沉降期（强烈走滑期）（Ed）和构造反转期（Ng-现今）5个演化阶段.研究区主要发育正断层、逆断层、走滑正断层和走滑逆断层4种断层类型,经伸展间歇期和后期区域挤压作用,发育两期正反转构造.盆地经历的走滑运动过程可细化为初始走滑（Es1）,强烈走滑（Ed）和衰减走滑（Ng）3个阶段.     

Magnetostratigraphy of the late Cenozoic Laojunmiao anticline in the northern Qilian Mountains and its implications for the northern Tibetan Plateau uplift

The uplift process of the Qinghai-Tibetan Plateau holds the key to understand the dynamic mechanisms of continental crust shortening and mountain-building and to test the relationship between the Tibetan uplift and tectonic-climatic coupling and environmental im-pacts[1―4].However,there are still many debates in the process and mechanism of how the Tibetan Plateau uplifted to the present configuration.Among various approaches to solve these key questions,dating of the Cenozoic stratigraphy …     

Evolution of the Upper Cenozoic Magmatic Arc and plate boundary in northern New Zealand

The Upper Cenozoic Magmatic Arc in northern New Zealand was initiated when the Indian-Pacific plate boundary first spread through the North Island approximately 20 m.y. ago. Six geographically separated magmatic arcs are recognized in succession. The first (20-15 m.y.) was sited over a basement depression; lavas were basic to intermediate and largely submarine; mineralization was minor. Subsequent arcs were sited over basement horst and characterized by sub-aerial intermediate to acid magmas. After prolonged andesitic/dacitic activity (18-6 m.y.) with minor mineralization, prolific rhyolite/ignimbrite eruption began at about 6 m.y., with abundant mineralization. Behind-arc activity produced localized basalt fields in the north, and geographically restricted high-potash andesites in the south.The first four arcs in the series are aligned at about 70° to the active Tonga-Kermadec-Taupo arc. The migration and rotation of the older New Zealand arcs are ascribed to four processes taking place at the plate boundary. These are: (1) anti-clockwise bending of the crust of western North Island, obliquely to the movement of the underlying lithosphere of the Indian plate, beginning at about 3 m.y., accompanying (2) dextral transcurrent displacement of 230 km with respect to eastern North Island; taking place mostly from 3 to 0 m.y.; (3) steepening of the Benioff zone from an initial 18° dip at 20 m.y. to the present 55° to 60°; and (4) fracturing of the west-dipping lithospheric slab to give two parallel, low-potash andesitic arcs between 18 and 15 m.y.Eastern North Island is deduced to have been “floating” while Pacific plate lithosphere passed beneath it throughout the Upper Cenozoic; accordingly it is designated the Hawkes Bay Crustal Microplate.There is good agreement between major tectonic events in the South Pacific deduced by Molnar et al. from magnetic anomaly studies and major tectonic events on land. A tentative history of the Southwest Pacific is proposed for the last 40 m.y.     

Thermochronological constraints on two pulses of Cenozoic high-K magmatism in eastern Tibet

Cenozoic high-K magmatism was vigorously activated in eastern Tibet and controlled by the Early Tertiary pull-apart basins induced by strike-slip faults or Late Tertiary-Quaternary rift basins. These small plutons, dykes and volcanic rocks spatially appea…     

Late Cenozoic magnetic polarity stratigraphy in the Jiudong Basin,northern Qilian Mountain

Late Cenozoic sediments in the Hexi Corridor, foreland depression of the Qilian Mountain preserved reliable records on the evolution of the Northern Tibetan Plateau. Detailed magnetic polarity dating on a 1150 m section at Wenshushan anticline in the Jiudong Basin, west of Hexi Corridor finds that the ages of the Getanggou Formation, Niugetao Formation and Yumen Conglomerate are >11-8.6 Ma, 8.6-4.5 Ma and 4.5-0.9 Ma respectively. Accompanying sedimentary analysis on the same section suggests that the northern Tibetan Plateau might begin gradual uplift since 8.6-7.6 Ma, earlier than the northeastern Tibetan Plateau but does not suppose that the plateau has reached its maximum elevation at that time. The commencement of the Yumen Conglomerate indicates the intensive tectonic uplift since about 4.5 Ma.

     

Cenozoic volcanism and lithospheric tectonic evolution in Qiangtang area, northern Qinghai-Tibet Plateau

Accompanying with the shortening,thickening and uplifting of the lithosphere,a series of Cenozoic potassic volcanic rock zones are developed in the northern Qinghai-Tibet Plateau.From south to north,the volcanic rocks can be divided into three volcanicrock belts:Qiangtang-Nangqian volcanic belt,Middle Kunlun-Hoh Xil volcanic belt and Western Kunlun-Eastern Kunlun volcanic belt[1].Spatiotemporal evolu-tion of the volcanism and the origins of magmas con-strains on the pulsing uplifting and …     

南海北部新生界地层的远偏移距折射波速度特征

南海北部为张裂大陆边缘,在新生代时期经历了复杂的地质演化,具有良好油气勘探前景.本文首次利用南海北部某区域长排列地震数据的远偏移距折射波速度计算方法获得南海北部地层层速度,并进行了分析,此外,还提出了非初至折射波的概念.本方法在共偏移距剖面上识别、拾取折射波层位,得到折射波走时的横向连续变化数据,利用不同共偏移距同一折射波层位的到达时差计算该折射层位的速度结构.该方法在南海北部陆坡某区域地震资料的应用,揭示了新生界地层层速度的整体结构,反映了新生代不同时代地层的速度变化特征,为进一步利用折射层速度资料探讨南海新生代地质演化提供了新的视角.研究表明,与常规初至折射波法相比,远偏移距折射波能够提供更多的地质信息;该方法的初步应用能够对远偏移距折射波的研究提供进一步的视角.     

青藏高原东缘新生代隆升剥露与断裂活动的低温热年代学约束

青藏高原东缘作为高原生长的东边界,其新生代以来隆升剥露与扩展模式备受关注.高原内部平缓的地貌和边界构造带不显著的缩短变形被认为是下地壳流作用的重要证据.然而近年来,越来越多的低温热年代学研究结果表明,中-晚新生代以来跨不同断裂带存在显著的差异性隆升剥露,指示了断裂体系在青藏高原东缘构造变形与演化中的重要作用.本文系统收集区域内现有不同封闭温度体系的低温热年代学数据,综合分析结果表明青藏高原东缘隆升剥露及生长扩展与整个高原抬升具有准同步性.最为广泛和显著的剥露主要发生在~30 Ma以来,且高原东缘的最大侵蚀量区受控于断裂活动,快速侵蚀带的空间分布与鲜水河断裂带相一致.在区域尺度上,现有数据所揭示的剥露事件启动、持续时间及速率的显著差异性揭示了断层活动对青藏高原东缘地表剥露过程的控制作用.本文提出青藏高原向东扩展是多阶段、非均匀过程,新生代以来不同断裂带在青藏高原向东扩展过程中起到了至关重要的作用,不支持"下地壳流假说"强调的"东缘上地壳变形不显著"的认识.     

Geodynamic characteristics of tectonic extension in the northern margin of South China Sea

Based on the geothermal and gravitation methods, this paper investigated the rheological and thermal structure of the lithosphere under the northern margin of South China Sea. The result shows that the temperature of the upper crust is 150–300°C lower than that of the lower crust, and the viscous coefficient of the upper crust is 2–3 orders of magnitude larger than that of the lower crust. It reveals that the upper crust is characterized by brittle deformation while the lower crust by ductile deformation. A channel of lower-viscosity should be formed between the upper and lower crust when the lithosphere is scattered and spreads out toward ocean from northwest to southeast along the northern margin of South China Sea. And, a brittle deformation takes place in the upper part of the lithosphere while a ductile deformation takes place in the lower part of the lithosphere due to different viscous coefficients and temperature. The layered deformation leads the faulted blocks to rotate along the faulting and the marginal grabens to appear in the northern margin of South China Sea in Cenozoic tectonic expansion.     

青藏高原东缘新生代隆升剥露与断裂活动的低温热年代学约束

青藏高原东缘作为高原生长的东边界,其新生代以来隆升剥露与扩展模式备受关注.高原内部平缓的地貌和边界构造带不显著的缩短变形被认为是下地壳流作用的重要证据.然而近年来,越来越多的低温热年代学研究结果表明,中-晚新生代以来跨不同断裂带存在显著的差异性隆升剥露,指示了断裂体系在青藏高原东缘构造变形与演化中的重要作用.本文系统收集区域内现有不同封闭温度体系的低温热年代学数据,综合分析结果表明青藏高原东缘隆升剥露及生长扩展与整个高原抬升具有准同步性.最为广泛和显著的剥露主要发生在~30 Ma以来,且高原东缘的最大侵蚀量区受控于断裂活动,快速侵蚀带的空间分布与鲜水河断裂带相一致.在区域尺度上,现有数据所揭示的剥露事件启动、持续时间及速率的显著差异性揭示了断层活动对青藏高原东缘地表剥露过程的控制作用.本文提出青藏高原向东扩展是多阶段、非均匀过程,新生代以来不同断裂带在青藏高原向东扩展过程中起到了至关重要的作用,不支持"下地壳流假说"强调的"东缘上地壳变形不显著"的认识.     

The blanketing effect of sediments in basins formed by extension: a numerical model. Application to the Gulf of Lion and Viking graben

A numerical finite difference model is employed to reconstruct the time/temperature history of sediments in basins formed by extension, in which crustal thinning by stretching as well as the effects associated with sedimentation and compaction are taken into account. Two extreme cases of basins were investigated to identify the scale of this mechanism: the Gulf of Lion illustrates a case of a young basin, in which the average sedimentation rate is high (620 m/Ma), and the Viking graben in the North Sea offers a case of a basin formed in the Triassic, but where average sedimentation rate is much less (37 m/Ma). In the former, the sediments absorb 30% of the surface heat flow, while in the latter the effect is only 10%.     

Cenozoic stratigraphy and sedimentation history of the northern Philippine Sea based on multichannel seismic reflection data

Abstract To clarify the regional distribution and characteristics of the sedimentary deposits in the northern part of the Philippine Sea, multichannel seismic reflection surveys of 26 864 km in total length were performed. The seismic reflection data were interpreted and correlated with available Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) data and a general stratigraphic framework of the area was established. The sedimentary deposits in this area were divided into five layers; Units I, II, III, IV and V in ascending order. Their approximate geological ages are the Early Eocene, Middle to Late Eocene, Oligocene, Miocene and Pliocene‐Pleistocene, respectively. Seismic records were classified into three seismic facies, Facies A, B and C, on the basis of their characteristics. They were judged to represent pelagic and hemipelagic sediments of non‐volcanic origin, fine pyroclastic sediments and coarse pyroclastic or volcanic sediments, respectively, by comparing them with lithological data in the DSDP/ODP holes. From the thickness and facies distributions of these sediments, a sedimentary history in the area was reconstructed as follows. The oldest sediments in the study area, Unit I, interfinger with some parts of the Daito Ridge (acoustic basement) in the Minami Daito Basin. The geological age of the unit is estimated by microfossils in the sediment and supports the idea that this part of the Daito Ridge is composed of the Early Eocene oceanic basalt. Later, a fair amount of sediments were deposited in the Minami Daito Basin in the Middle to Late Eocene age. A large volume of volcanic materials was supplied from the Paleo‐Kyushu‐Palau Ridge in the Kita Daito Basin in the Eocene and Oligocene ages. The eastern part of the Shikoku and Parece Vela basins is characterized by volcanic sediments supplied from the Nishi Shichito and West Mariana Ridges in the Miocene age. However, pelagic and hemipelagic sediments prevail in the northern part of the Shikoku Basin in the Miocene or later. In short, the area of principal sedimentation has generally shifted from west to east through geological time, reflecting the evolution of the island arc systems with the same trend in the northern Philippine Sea.     

柴北缘骆驼泉剖面新生代地层磁组构特征及其构造意义

青藏高原东北部新生代构造演化对理解高原隆升和变形模式具有重要意义, 而目前对于该地区挤压应力方向转变过程仍存在很大争议.本文对柴北缘逆冲带北西部骆驼泉剖面新生代地层开展系统磁组构(本文特指磁化率各项异性)研究, 以揭示该地区挤压应力方向的转变特征.系统岩石磁学结果表明, 骆驼泉剖面新生代样品中主要磁性矿物是顺磁性组分和赤铁矿以及少量磁铁矿.通过对磁组构特征分析及其与古水流方向对比表明, 骆驼泉剖面新生代地层磁组构主要为初始变形磁组构, 可用于指示沉积成岩时期的挤压应力方向.磁组构结果揭示, 骆驼泉地区挤压应力方向在上干柴沟组下部沉积时期为NNE-SSW向, 而上干柴沟组上部和油砂山组沉积时期转变为NE-SW向.结合柴北缘逆冲带已有磁组构结果指出, 该地区早期N-S向或NNE-SSW向挤压应力可能与印度—欧亚板块早新生代以来近N-S向碰撞挤压过程有关, 指示印度—欧亚碰撞的挤压应力自下干柴沟组下部沉积时期就已传播至高原东北部地区; 而后期NE-SW向挤压应力方向与该地区现今GPS揭示的上地壳运动方向一致, 可能与该时段高原东北部巨型走滑断裂构造体系(尤其是阿尔金断裂)有关.此外, 柴北缘逆冲带新生代挤压应力方向转变在其北西部起始于上干柴沟组下部沉积时期, 而南东部起始于上油砂山组下部沉积时期, 与地震反射剖面揭示的断裂活动等地质证据共同揭示柴北缘逆冲带新生代的构造活动自靠近阿尔金断裂的北西部向南东部传播和扩展.综合分析青藏高原东北部地区挤压应力方向转变和其他地质证据发现, 挤压应力方向转变显示出自柴北缘逆冲带北西部向东、西和南向扩展特征, 与阿尔金断裂在上干柴沟组下部-上油砂山组下部沉积时期剪切应力集中于断裂本身, 而上油砂山组下部沉积以来开始散布于高原东北部内部地区的两阶段走滑活动相关.

     

南海北部琼东南盆地地层结构与地壳伸展特征

琼东南盆地发育于前新生代基底之上,作为南海被动大陆边缘一部分,记录了南海北部裂陷盆地结构及其演化.利用最新钻井、反射地震、重力等资料,分析新生代盖层和前新生代基底地壳结构,建立盆地地层结构模型,然后计算全盆地地壳伸展变化特征.结果表明:新生代地层序列的盆地充填由西向东逐渐减薄,古近纪、新近纪以及第四纪期间(45 Ma~现今)最后沉积中心呈现逐渐向西或西南迁移趋势.下地壳局部表现为地震速度偏高(厚度2~4 km,vP>7.0 km·s-1,水平延伸范围约为40~70 km).重震联合模拟显示这里存在密度偏高特征,推测存在可能与张裂晚期和扩张早期岩浆物质底侵或混合到伸展程度较低的大陆地壳有关.计算获得的前新生代基底地壳厚度由在弱展区域陆架区约25 km,在减薄最大区域中央坳陷为3 km.伸展系数(β)最高值大于6.0出现在中央坳陷,低值小于2.0在坳陷南北两侧,说明地壳在盆地中央拉伸比较剧烈.     

Late Cenozoic deformation subsequence in northeastern margin of Tibet --Detrital AFT records from Linxia Basin

Two events of Tibet uplifting are revealed by detrital apatite fission track (AFT) age data from Linxia Basin. They occurred at about 14 and 5.4-8.0 MaBP respectively. We interpret the first one to be related to the uplifting of the northern Tibet, which might have resulted from convectively removing the thickened lower lithosphere. The second one is a result of Laji Mountain uplifting. Numerous studies of the Tibetan Plateau suggest that the onset time of the deformation in the northeastern margin of Tibetan Plateau and the time of Tibet attaining to its present elevation is about 8 MaBP. They are approximately coincident with the uplift of Lajishan Mountain. It suggests that the northeastern margin of Tibet propagated northeastwardly to its present site in about 8 MaBP for accommodating the sustained convergence between India-Eurasia plate and for keeping its high elevation. The active block pattern dominating the strong earthquake distribution of Chinese continent probably formed at about 8.0-5.4 MaBP.     

大西洋中北部双频微地动特征

本文对大西洋中北部两侧五个地震台站2015年记录到的地震数据进行处理,计算噪声功率谱密度和概率密度函数,并通过极化分析对双频微地动不同周期的主导源区方位角分布进行了分析.研究结果显示:大西洋中北部台站双频微地动发生显著分裂,各台站的峰值周期各不同,且来自相同方向和不同方向的双频微地动都有可能产生双频微地动分裂;大西洋中...