Geochemistry of stream sediments along the western coast of the Gulf of Elat (Aqaba)

One hundred twenty-eight samples of heavy-mineral fractions from sand-size sediments whose mineralogy was previously studied by A. Ayalon, were chosen for this geochemical study. The samples represent the stream sediments along the coastal belt from Elat to Sherm el-Sheikh. This study supplements the geochemical study of A. Zilberfarb and R. Bogoch on the Precambrian terrain of Sinai.The mineral composition of the samples was determined by X-ray diffraction and optical microscopy. In all samples, 14 elements were analyzed: B, Co, Cr, Cu, Ga, Mn, Mo, Nb, Ni, Pb, Sn, Ti, V and Zr. Seven magnetic fractions were separated from twenty of these samples and in these fractions 14 to 20 elements were analyzed, according to the sample. The chemical determinations were mainly by DC emission spectrography.The results were submitted to factor analysis and to association analysis (D. Gill and co-workers). Since the environment is typically arid, there is relatively little chemical weathering. The combined results of both statistical analyses show clearly that three types of source areas can be differentiated: sandstone, metamorphic and granitic terrain. Furthermore, a distinct Mn anomaly was found in the Sherm el-Sheikh area.     

Late Quaternary Paleoceanography of the Gulf of Aqaba (Elat), Red Sea

The quantitative distribution of planktonic foraminifera, pteropods, and coccolithophorids, as well as oxygen-isotope variations were analyzed in four deep-sea cores from the Gulf of Aqaba (Elat) and the northernmost Red Sea. The core record covers about 150,000 yr. Detailed stratigraphic subdivision is facilitated by combining all calcareous plankton groups. Time-stratigraphic correlation and dating beyond the radiocarbon range are possible by comparison of the oxygen-isotope curves. During the glacial maximum salinity rose to more than 50‰, while winter temperature of the upper waters fell by at least 4°C compared to the present. The rise in salinity can be accounted for by sea-strait dynamics and lowering of sea level. The Gulf of Aqaba and the Red Sea were continuously connected through the Straits of Tiran, and there is no indication of desiccation during the glacial maximum.     

Seismic study of pull-apart-induced sedimentation and deformation in the Northern Gulf of Aqaba (Elat)

New multichannel seismic and bathymetric data are presented, which clarify the Plio-Quaternary evolution of the northern Gulf of Aqaba (Elat) and the Dead Sea Transform (DST). The seismic data reveal two main seismic sequences, a lower (pretectonic) and an upper (syntectonic) unit, separated by a prominent unconformity. These units are each linked to a distinct tectonic phase in the history of the DST. Parallel horizons and an undisturbed internal structure point to a tectonic quite time or pure strike-slip without extension or compression during the first (pretectonic) phase. The second (syntectonic) phase, which begins in the early Pliocene, is characterized by a major change in the activity of the DST. The pretectonic sedimentary unit subsided and, consequently, dips southward with a supplementary inclination to the east. The coeval sedimentation of the syntectonic unit is recorded by the divergent reflection pattern and onlap terminations on the unconformity. The apparent fault system seems to be rearranged in the second phase. The stepover of the main strand of the DST from the eastern side of the Elat Deep to the western side of the northern Gulf of Aqaba was mapped in detail for the first time. The very smooth shape of the stepover and the apparent lack of extensional tectonics do not fit with the classical pull-apart basin model for the Elat Deep and point to a decoupling of the crystalline basement from the sedimentary overburden. Comparisons of the new geophysical findings with analog models support this assertion.     

A note on the estimation of sedimentation rates in the Gulf of Elat (= Gulf of Aqaba), by the use of Foraminifera

Abundance fluctuations of Amphistegina in the upper 4–5 cm of a few small cores from the Gulf of Elat look somewhat like rainfall fluctuations in the hinterland. To correlate the two patterns, a sedimentation rate of 2.2–3.3 mm/year must be assumed. A similar rate is obtained from estimates of annual production of Amphistegina.     

Extension-related buttress-like folds,the western side of the Gulf of Suez rift,Egypt

International Journal of Earth Sciences - The wedge-shaped St. Paul block, western side of the Gulf of Suez rift, exposes Late Cretaceous beds that are folded into transverse folds. The block is...     

Seismicity and seismotectonics of the Gulf of Aqaba region

The historical seismicity of the last ten centuries and the instrumental data that occurred in the Gulf of Aqaba region during the period 1982–2008 are evaluated. It is found that 12 historical earthquakes have occurred with average recurrence periods 70–90 and 333–500 years for M?≥?6.0 and 7.0, respectively. Those with M?≤?6.5 appear to be incomplete and require further investigation. More than 98 % of the instrumental data has occurred in the form of swarms and sequences. The first have released about 32 % of the total energy and are most likely related to subsurface volcanic activities. Their epicentral distribution indicates that all regional faults of the gulf area are active in the present, but with clear concentration within the area bound by latitudes 28.2°–29.8° and longitudes 34.4°–35.2°. Regional strike-slip faults of the northern two basins appear to be as twice active as the normal, or more. An appreciable level of seismic hazard is envisaged as the “a” value is 6.0–6.2 while the “b” value shows a temporal variation, mostly in the range 0.8–1.05. More than 95 % of the seismic energy was released from earthquakes shallower than 22 km. This indicates a brittle upper crust and a ductile lower crust and upper mantle. Tectonic movements at depths?>?22 km appear to be aseismic. The epicentral distribution of the five swarms indicates that the lengths of the causative faults varied in the range 45–70 km. The maximum expected magnitude is Mw?=?6.8–7.2. This implies a seismic slip rate of about 0.54–0.8 Cm/year and some 20–30 % of aseismic tectonic movements. This and the sequence nature of the seismicity of this region result in a noticeable hazard reduction. Combining the seismicity data of the Gulf of Aqaba region with other geophysical, geological, tectonic, and environmental data, clearly indicate that the seismicity of this region is as old as the initiation of the gulf itself. No apparent southward or northward migration of activity is observed.     

The roussillon continental margin (Gulf of Lions): Plio-Quaternary paleogeographic interpretation

The paleographic evolution of the Roussillon continental margin during the Plio-Quaternary has been reconstructed on the basis of indirect methods of investigation (continuous seismic reflexion) and of analysis of core samples. Interpretation of the first group is based on the character of the reflectors, their position, morphology, and their correlation with outcrop equivalents. Evolution in this area can be integrated in the more general history of the Gulf of Lions and the western Mediterranean. A remarkable factor is the morphotectonic control played by the Paleozoic substratum until recently. The Quaternary origin of the canyon seems to be characteristic of submarine valleys in this part of the Mediterranean. The different studies confirm the importance of submarine phenomena in the shaping and maintaining their morphology.

The following are defined in the Quaternary complex: alluvial deposits and the fossil channel of the early and mid-Quaternary; the major Tyrrhenian phase of filling; the recent eustatic oscillations. The latter have been defined in the light of analytical data. The oldest sequence reached by drilling for core samples is the transgressive neo-Tyrrhenian. The recent Würm is figured by two regressive phases (Würm III and Würm IV) and one positive interstadial. The Holocene facies enable us to measure the rise of Flandrian sea level at a rate of 10 m per 1,000 years.     

扬子西缘陆内构造转换系统与构造-岩浆-成矿效应

横跨欧亚呈东西延伸的特提斯构造域是全球最重要的有色贵金属成矿带之一,该带向东到达扬子陆块后转向东南,位于转折部位的扬子西缘近年发现系列大型-超大型Cu-Au矿床。以往认为,扬子西缘金沙江—红河大型走滑断裂切穿岩石圈,诱发岩浆上侵,形成富碱斑岩带。本文通过对扬子西缘剑川老君山、鹤庆北衙、姚安老街子—干沟、大姚吁支拉和永仁直苴等多个富碱岩体进行调查和构造应力分析,表明富碱斑岩在沿金沙江—红河断裂两侧分布的同时,也有大量岩体(脉)进入扬子板块内部,沿EW向断裂展布,其成岩成矿年龄为36.87~30.70 Ma,与富碱斑岩带一致,同时岩浆活动表现出由西向东时代变新,碱质增加等自西向东运移的特征;对近EW向岩体(脉)及节理的野外观测和统计分析表明,控岩构造具有东西挤压南北伸展的应力特征,与北西向大型走滑构造配套。由此提出,扬子陆块西缘新生代发育大规模陆内构造转化系统,陆块内部系列富碱岩浆,其形成与深切的NW向大型走滑构造有关,自西而东就位于派生的SN向、EW向次级构造,发育斑岩Cu-Au等成矿作用,成岩成矿与陆内构造转化系统耦合。     

西秦岭北缘断层的多期变形演化及其构造动力学意义

西秦岭北缘断层是青藏高原东北缘新生代盆地与西秦岭地块之间的边界断层,其构造变形的几何学-运动学特征和变形历史等研究对于重建青藏高原东北缘新生代以来的构造变形时空动力学过程,限定新生代盆地构造属性,揭示印度板块-欧亚板块碰撞汇聚的远程构造响应和青藏高原东北缘隆升等重大科学问题具有重要地质约束。本文通过对西秦岭北缘新生代盆地南边界F1断层的断层岩类型及分带、构造要素的几何学-运动学特征等较详细的构造解析,辨认出F1断层6期构造变形：第一期为北西西走向、向北倾斜的韧性-韧脆性伸展正断层作用;第二期为北西西走向、向北陡倾或近直立的高角度逆冲断层作用,指示近南北向挤压缩短作用;第三期为走向近南北、向东或向西陡倾的对冲断层作用,指示了近东西向的挤压缩短作用;第四期为北东向右旋和北西向左旋的走滑共轭断层系统,指示了近东西向的挤压作用;第五期为断层面近直立的北东向左旋、北西向右旋的共轭破裂系统,指示了近南北向挤压作用;第六期为断层面近直立的近东西向左旋和近南北向右旋走滑断层构成了几何学-运动学协调的共轭破裂系统,指示了北东向挤压作用。结合西秦岭北缘渐新世-中新世沉积盆地具有断陷盆地沉积序列特征和上新世具有类磨拉石的冲洪积扇粗砾岩特征以及F1断层多期变形对新生代盆地沉积地层的控制和改造作用分析,认为F1断层第一期韧性-脆韧性伸展正断作用始于渐新世,控制了渐新世-中新世伸展断陷盆地沉积;F1断层第二期高角度逆冲缩短变形使得渐新世-中新世断陷盆地封闭、靠近F1断层的底部砾岩层卷入了挤压逆冲断层作用,断层拖曳使地层产状翘起变陡,这期变形持续到上新世冲洪积扇粗砾岩出现;F1断层第三期与第四期虽然都为近东西向挤压,但第三期为东西向对冲挤出,而第四期为北东向和北西向斜向走滑挤出,其动力学机制是否与青藏高原东北缘西部地壳增厚隆升诱发的中-下地壳向东流动拖曳导致的上地壳东西向挤压缩短尚待证实,由于第三和第四期变形的构造形迹在上新统韩家沟砾岩不存在,因此,这两期变形的时代只能是发生在中新世末期或上新世早期;第五期北东和北西向共轭破裂系统和第六期南北向和东西向共轭破裂系统在渐新统-中新统沉积地层和上新统粗砾岩地层中都存在,其时代无疑是上新世末期或第四纪以来的构造变形,但第五期共轭断层指示的最大主压应力为近南北向,而第六期最大主压应力为北东-南西向,两者夹角约30°,指示两期变形最大主应力方向发生了30°的顺时针旋转,这可能与青藏高原东北缘变形重组过程中块体旋转有关。上述F1断层丰富且复杂的构造变形形迹揭示的断层变形方式和历史演变对于澄清青藏高原东北缘新生代红层盆地构造属性认识上的分歧和高原变形是均匀增厚变形和块体沿断层挤出滑移地壳变形机制的争论等提供了重要的构造依据。     

Geology and tectonic development of the continental margin north of Alaska

The continental margin north of Alaska, as interpreted from seismic reflection profiles, is of the Atlantic type and consists of three sectors of contrasting structure and stratigraphy. The Chukchi sector, on the west, is characterized by the deep late Mesozoic and Tertiary North Chukchi basin and the Chukchi Continental Borderland. The Barrow sector of central northern Alaska is characterized by the Barrow arch and a moderately thick continental terrace build of Albian to Tertiary clastic sediment. The terrace sedimentary prism is underlain by lower Paleozoic metasedimentary rocks. The Barter Island sector of northeastern Alaska and Yukon Territory is inferred to contain a very thick prism of Jurassic, Cretaceous and Tertiary marine and nonmarine clastic sediment. Its structure is dominated by a local deep Tertiary depocenter and two regional structural arches.We postulate that the distinguishing characteristics of the three sectors are inherited from the configuration of the rift that separated arctic Alaska from the Canadian Arctic Archipelago relative to old pre-rift highlands, which were clastic sediment sources. Where the rift lay relatively close to northern Alaska, in the Chukchi and Barter Island sectors, and locally separated Alaska from the old source terranes, thick late Mesozoic and Tertiary sedimentary prisms extend farther south beneath the continental shelf than in the intervening Barrow sector. The boundary between the Chukchi and Barrow sectors is relatively well defined by geophysical data, but the boundary between the Barrow and Barter Island sectors can only be inferred from the distribution and thickness of Jurassic and Cretaceous sedimentary rocks. These boundaries may be extensions of oceanic fracture zones related to the rifting that is postulated to have opened the Canada Basin, probably beginning during the Early Jurassic.     

鄂尔多斯盆地西缘煤田构造演化及其控制因素

鄂尔多斯盆地西缘位于华北陆块和秦祁昆山造山带两个一级大地构造单元之间的过渡带内, 特定的大地构造背景使其具有复杂的构造演化历程及特殊的煤田构造格局。鄂尔多斯盆地西缘由贺兰山逆冲推覆构造系统和六盘山东麓逆冲推覆构造系统组成, 具有"南北分段、东西分带"的特点。为了进一步探讨鄂尔多斯盆地西缘煤田构造格局的形成演化及区域构造控制因素, 本文基于野外地质调查和煤田勘查资料, 恢复了本区自晚古生代以来的沉降抬升史和古构造应力场特征。印支期: 研究区北部最大主压应力方向为北西—南东向, 南部最大主压应力方向为北东—南西向, 燕山期: 北部最大主压应力方向为北西西—南东东向, 南部最大主压应力方向为北东东—南西西向, 喜马拉雅山期: 北部受北西西—南东东向拉张应力, 南部最大主压应力方向为北东—南西向。采用有限元数值模拟, 探讨了鄂尔多斯盆地西缘煤田构造格局的形成与区域构造的演化的关系, 强调北段贺兰山逆冲推覆构造系统的形成与阿拉善地块的向东挤出逃逸密切相关。     

Relocation processing of selected earthquakes in the Gulf of Aqaba region

Earthquakes in the Gulf of Aqaba were compiled from the Jordan Seismological Observatory bulletins, Saudi Arabia (SNSN, King Abdulaziz City for Science and Technology), and Egypt (National Research Institute of Astronomy and Geophysics, Helwan) sources and were relocated after reexamining the P and S arrival times, testing the efficiency of stations used to determine the location of Aqaba events, calculating the average value of V _p/V _s, and choosing the best crustal model for the study area. The earthquake epicenters are distributed along the eastern side of the Gulf of Aqaba.     

扬子板块西缘挖角地区晋宁期岩浆作用及大地构造意义

四川省石棉县挖角地区花岗岩体位于扬子板块西缘与松潘-甘孜地块的结合部位,本文通过详细的野外地质调查,结合岩石学、岩相学、LA-ICP-MS U-Pb年代学和MC-ICP-MS Hf同位素组成研究,厘定了岩体形成时代,探讨了岩浆来源。LA-ICP-MS锆石U-Pb测年结果表明,挖角二长花岗岩形成年代介于852±33Ma和847±44Ma之间,处于晋宁期晚青白口世,与全球Rodinia期泛大陆形成密切相关。锆石原位Lu-Hf同位素组成表明,2个样品的εHf(t)值分别为+3. 9～+9. 3和-8. 0～+6. 5,为典型的壳-幔混合型; Hf同位素的二阶段模式年龄(tDM2)分别为1. 14～1. 49Ga和1. 33～2. 24Ga,均值分别为1. 32Ga和1. 52Ga,表明岩浆源区以中元古代古老地壳基底的部分熔融为主,形成过程中有幔源及古元古代古老地壳物质的贡献。挖角地区晋宁期二长花岗岩体锆石U-Pb年代学和Hf同位素组成,反映了扬子板块西缘晚青白口世岩浆活动期和源区特征,其成因与全球Rodinia泛大陆形成期间岩浆活动和与壳幔混合作用密切相关。本文研究成果,为解释扬子板块西缘的构造演化提供了新的资料证据。     

华北克拉通北缘西段新元古代裂谷的确认与成矿

狼山-渣尔泰山是华北克拉通北缘西段元古宙伸展构造体制下被动陆缘的重要热水喷流成矿带,内产东升庙、炭窑口、霍各乞、甲生盘等大型-超大型矿床,其含矿建造是元古宇渣尔泰山群。该成矿带的主体在狼山山脉主峰地区,产有东升庙、炭窑口、霍各乞三大矿床。自2010年在狼山西南原渣尔泰山群阿古鲁沟组地层中发现新元古代酸性火山岩(锆石年龄816~805 Ma)以来,其东部狼山主峰地区的渣尔泰山群及产在其中的炭窑口、东升庙、霍各乞矿床是否也都可以归于新元古代形成是需要进一步研究的重要科学问题。本文依据的事实有:(1)狼山西南新元古界的碎屑锆石最小年龄为1 155 Ma与碳酸盐岩是方解石大理岩;(2)炭窑口矿区渣尔泰山群顶部刘鸿湾组地层的碎屑锆石年龄的两个年龄峰值分别为1 862~1 762 Ma(最小年龄为1 732 Ma)和2 448 Ma,所含碳酸盐岩全部是白云石大理岩类;(3)狼山北侧狼山群含矿岩组中发现年龄为887 Ma±的新元古代基性火山岩; (4)霍各乞矿床含重晶石与微晶长石的硅质层发育,多与黄铁矿层互层状产出,也有与闪锌矿+磁黄铁矿及方铅矿层互层,激光原位分析得到的黄铁矿、方铅矿、闪锌矿和磁黄铁矿的δ³⁴S为17.60‰~21.97‰,显示喷流成矿的硫同位素组成特征。根据以上事实可以确认:(1)狼山西南含酸性火山岩与方解石大理岩的前寒武纪地层是与狼山南侧炭窑口矿区含白云石大理岩的渣尔泰山群在不同盆地中沉积而成;(2)狼山南侧是中元古代裂谷带,产有以炭窑口、东升庙矿床为代表的中元古代喷流-沉积成矿系统;(3)狼山北侧是新元古代裂谷带,产有以霍各乞矿田为代表的新元古代热水喷流成矿系统。产在新元古代狼山群中的霍各乞矿是介于SEDEX型与VMS型之间、但靠近SEDEX型一侧的热水喷流-沉积矿床。     

Grenville tectonic events and evolution of the Yenisei Ridge at the western margin of the Siberian Craton

Geological, petrologic, geochemical, and isotopic geochronological evidence for Grenville events at the western margin of the Siberian Craton are considered. These events were related to assembly of the Rodinia supercontinent. Multiple manifestations of riftogenic and within-plate magmatism at the final stage of orogenic evolution gave rise to breakdown of Rodinia and the formation of the Paleoasian ocean. The results allowed us to develop a new concept on the Precambrian geological evolution of the Yenisei Ridge and the processes that created its tectonic structure. The chronological sequence of events in the history of the Transangarian Yenisei Ridge is based on geological evidence and isotopic dating of Precambrian complexes variable in geodynamic nature. Four tectonic stages dated at 1.4?1.1, 1.1?0.9, 0.90?0.85, and 0.8?0.6 Ga were controlled by collision and extension recognized from large regional linear crustal structural elements. The evolution of the Transangarian Yenisei Ridge, which lasted for ～650 Ma, corresponds in duration to supercontinental cycles that begin from rifting and breakdown of the predated supercontinent and was completed by orogeny and the formation of a new supercontinent. The regional geodynamic history correlates with the synchronous sequence and similar style of tectonothermal events at the periphery of the large Precambrian Laurentia and Baltica cratons. This is evidenced by paleocontinental reconstructions, which confirm close spatiotemporal links of Siberia with cratons in the northern Atlantic 1400?600 Ma ago and indicate incorporation of the Siberian Craton into the ancient Nuna and Rodinia supercontinents.     

Large earthquakes kill coral reefs at the north-west Gulf of Aqaba

Down‐faulting at the north‐west margins of the Gulf of Aqaba is inferred to have triggered a catastrophic sedimentary event at 2.3 ka that killed the Elat fringing coral reef. Whereas segments of the Holocene reef were perfectly fossilized and preserved beneath a veneer of siliciclastic sediments, other segments were abraded, settled by nomads, and later re‐submerged under 4 m of water. Repeated damage triggered by down‐throwing earthquakes degenerate the fringing reefs of the north‐west end of the gulf. Conversely, on the north‐eastern and southern parts of the gulf, where earthquakes uplift the margins, modern reefs are thriving, attached to uplifted fossil reef terraces. Therefore, coastal subsidence moderates the development of fringing coral reefs during the late Holocene sea‐level stand still.     

The metamorphic sole of the western Tasmanian ophiolite: New insights into the Cambrian tectonic setting of the Gondwana Pacific margin

The Cambrian Ross–Delamerian Orogeny records the first phase of accretional tectonics along the eastern margin of Gondwana following breakup of the supercontinent Rodinia. Western Tasmania represents a key area for understanding the Cambrian tectonic setting of the eastern margin of Gondwana as it is one of the few places where a Tethyan-type ophiolite is preserved and contains the only known exposures of a sub-ophiolitic metamorphic sole associated with the Ross–Delamerian Orogen. This paper presents an integrated study of the field, petrographic, geochemical, and metamorphic characteristics of the metamorphic sole to the western Tasmanian ophiolite. The structurally highest levels of the metamorphic sole consist of granulite–upper amphibolite facies metacumulates and metagabbros. A transition to amphibolite and epidote–amphibolite facies conditions is recorded by metadolerites and metabasalts towards the base of the metamorphic sole. Kinematic indicators in mylonitic amphibolites suggest the metamorphic sole formed in an east-dipping subduction zone located to the east of the Proterozoic continental crust of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. Our new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the eastern margin of Gondwana. The proposed model may help to resolve a controversy related to apparent along-strike variations in subduction zone polarity during the Ross-Delamerian Orogeny and suggests a complex geodynamic setting had developed along the eastern margin of Gondwana by the Middle Cambrian. This study highlights the importance of considering the role of multiple subduction zones in generating metamorphic soles and emplacing ophiolites, which are key events associated with the construction of many orogenic belts worldwide.     

佳木斯地块东缘晚古生代大陆边缘构造演化

佳木斯地块位于中国东北微陆块群的最东缘,其东缘地区晚古生代的岩浆和沉积演变进程为欧亚大陆东缘由被动陆缘向活动陆缘构造环境的转化提供了关键证据。年代学和地球化学研究表明,佳木斯地块东缘中泥盆世黑台组砂岩,形成于被动陆缘的构造环境,黑台组上覆的老秃顶子组流纹岩也形成于被动陆缘的构造环境;晚石炭世珍子山组砂岩,形成于活动陆缘的构造环境;早二叠世的二龙山组安山岩以及相邻地区早二叠世的其它火成岩形成于活动陆缘的构造环境。同时,佳木斯地块东缘泥盆-二叠纪的沉积地层也呈现出由浅海相到陆相地层转化的特征。因此,佳木斯地块东缘由被动陆缘向活动陆缘的转化应该发生在中泥盆世到晚石炭世,而该构造环境的转化也为晚古生代时期蒙古-鄂霍茨克洋向欧亚大陆之下俯冲过程的研究提供了关键信息。