Plio-Pleistocene age of high-potassium volcanism in the northwestern part of the hellenic arc

Summary Two newK/Ar dates firmly establish that the high potassium volcanic rocks of Thessaly are of late Pliocene and Pleistocene age, considerably younger than the geochemically similar rocks of the Dodecanese.

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Das plio-pleistozäne Alter des kalireichen Vulkanismus im Nordwest-Teil des hellenischen Bogens

Zusammenfassung Zwei neue Altersbestimmungen nach derK/Ar-Methode zeigen, daß die kalireichen Vulkanite Thessaliens spät-pliozänen und pleistozänen Alters sind und damit beträchtlich jünger als die geochemisch ähnlichen Gesteine des Dodekanes.

     

Plate tectonics and volcanism in the gibraltar arc

Late Tertiary and Quaternary volcanism of southeastern Spain can be fitted in a platetectonics model, taking into account the post-Paleozoic evolution of the stable and semimobile Iberian areas and the new orogenic belts bordering the Mediterranean between Africa and the Iberian Peninsula.The occurrence and distribution of calc-alkaline and potassic volcanism suggest an oceanic crust sinking downwards from the Iberian plate. This active margin is causally related to the convergence and collision of Iberia and Africa during Late Cretaceous—Early Miocene time span.A pre-collision distensive phase is inferred from the stratigraphie and tectonic record between the Triassic and Late Cretaceous, while since the Late Miocene another distensive phase is related to the actualistic features.     

River profiles along the Himalayan arc as indicators of active tectonics

Longitudinal profiles along sixteen major transverse Himalayan rivers add important constraints to models of active continental subduction and its evolution. These profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases. The base of the zone of increased gradients correlates with (1) the topographic front between the Lesser and High Himalayas, (2) the narrow belt of intermediate-magnitude thrust earthquakes, (3) the Main Central Thrust zone (MCT). These features define a small circle in the central portion of the Himalayan arc. These correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising High Himalayas and the Lesser Himalayas. No sharp increases in gradient are observed near the Main Boundary Thrust (MBT), except on a few rivers, such as the Jhelum or Kundar, where the MBT lies close to both the MCT and the seismic belt. Thus, it is unlikely that the MBT is a major tectonic boundary. The diversion of river courses along the MBT and around anticlines in the Sub Himalayas has probably been caused by aggradation near the rosion-deposition boundary, upstream of uplifts in the Mahabharat range and Sub Himalayas.A parallel is drawn between the Himalayas and New Guinea based on the hypothesis that continent-arc collision, of the type occurring in northern Australia, preceded continent-continent collision in the Himalayas. The present sedimentary/tectonic phase in New Guinea resembles the Subathu (Paleocene-Eocene) phase in the Himalayas. Incipient counterparts of the major Himalayan structures, including the MCT and the MBT, are recognized in New Guinea. The drainage patterns in the Himalayas and in New Guinea bear a similar relation to major structures. This suggests that (1) the tectonic evolution of the Himalayas has been rather uniform since early stages of collision, and (2) the Himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the High Himalayas.     

Examples of geomorphologic and geological hazards in Algeria

We present three geomorphologic and geological phenomena that have occurred in Algeria in recent years: (i) the Bab El Oued mudflow on 11 November 2001, which claimed several hundred lives, (ii) a soil collapse induced by sand liquefaction triggered by the Boumerdes earthquake (M _w = 6.8) on 21 May 2003, and (iii) landslides that are threatening Constantine city, for which a hazard map is presented using a qualitative approach. We briefly describe and analyze these natural disasters, and in the first two cases propose the application of geophysical techniques such as ambient noise recordings and electrical imagery to help evaluate their extent and potential threat. Finally a landslide hazard map of Constantine is proposed.     

The role of transcurrent intra-arc tectonics in the configuration of a volcanic arc

Field investigations show that Plio-Quaternary Mexican and Quaternary Ecuadorian volcanic arcs are coeval with intra-arc transcurrent or transtensional tectonics with motions along faults parallel or subparallel to the arc. This relationship can also be observed in the Quaternary Kamchatka arc. These arcs have an anomalously wide distribution of volcanic centres which cannot be explained simply by a low dip in the subducting lithosphere. Comparison with the intra-arc tectonics and geometry of other Quaternary arcs reveals that strike-slip fault zones, parallel to the arc, favour magma rising in a broad surficial zone and may channel magma feeding mainly huge stratovolcanoes.     

New data on tectonics of Mendeleev Ridge and adjacent geological structures

The comprehensive analysis of potential field data and recent seismic data revealed two systems of fractures bounding horsts and grabens in terms of the Mendeleev Ridge. The northern part of the ridge is marked by development of pull-apart structures indicating the former existence of oblique extension settings. The area between Mendeleev and Alpha ridges is occupied by a wide NW?SE–extending sinistral strike-slip zone. It is concluded that these ridges are of continental origin representing former parts of Arctida (Hyperborea) in the pre-Cretaceous time. The ridges were separated and their crust significantly altered during Cretaceous tectono-magmatic activation in the region.     

Trigger effects in the evolution of geological structures as applied to regional tectonics

In the course of reconstructing the geodynamic evolution of certain regions and structures, it is pointed out that the formation algorithm, which operates for a long time, abruptly changes unaffected by any obvious factor: a thermal event, collision of lithospheric plates, etc. It is suggested that the sharp change in the geological evolution algorithm is related to accumulation of the energy potential, the relaxation of which is initiated by a trigger mechanism that leads to realization of the stored energy in a new form. Several geodynamic situations, the origin and development of which are related to the trigger mechanism, are considered in the paper: morphostructural subdivision of paleobasins, interaction of deformation and metamorphism, vertical accretion of a granitic-metamorphic layer, and development of horizontal protrusions (plate flows) in the Earth’s lithosphere.     

The relationship between the deep structure and quaternary tectonics of the pamir and Tien-Shan

The Quaternary period in the Pamir and Tien-Shan is marked by a sharp increase in the intensity of uplift which followed a short tectonically calm period in the late Pliocene and the early Pleistocene. Rates of the upward movement increased from the early Pleistocene to Holocene. The structural pattern of the region was formed throughout the whole neotectonic stage but the present shape is mainly the result of the Quaternary movements. A number of large uplifts and relative depressions can be identified in the Tien-Shan and Pamir. Only some of the greatest depressions were subject to absolute downwarping in their central parts during the Quaternary time. Numerous new data on the thickness of the earth's crust of Central Asia obtained in the past few years have allowed construction of a new relief map of the Mohorovičić discontinuity in the Tien-Shan and Pamir and of the discontinuity in relationship to the character of the Quaternary tectonic movements. As a result, correspondences relating to the development of the earth's crust were established and a suggestion was made as to the current rejuvenation of the structural pattern, with both an anti-Tien-Shan and a meridional direction dominating in the Pamir. The comparison of the development of the earth's crust as a whole with the lateral heterogeneities of the upper mantle confirmed the assumption about the current rejuvenation. At the same time it made evident once again that there are significant differences in the deep processes taking place in the Pamir and Tien-Shan. Combining analyses of both the subsurface and the surface development in this part of Asia provided a better view of the characteristics of the deep fault zones. This has allowed the authors to outline the latent ruptures and establish individual blocks of the earth's crust that show different structure and development.     

Recent observations of meteotsunamis on the Finnish coast

We present four case studies of exceptional wave events of meteorological origin, observed on the Finnish coast in the summers of 2010 and 2011. Eyewitnesses report unusually rapid and strong sea-level variations (up to 1 m in 5–15 min) and strong oscillating currents during these events. High-resolution sea-level measurements confirm the eyewitness observations, but the oscillations recorded by tide gauges mostly have a considerably smaller amplitude. The oscillations coincide with sudden jumps in surface air pressure at coastal observation stations, related to the passage of squall lines or gust fronts. These fronts propagate above the sea at a resonant speed, allowing efficient energy transfer between the atmospheric disturbance and the sea wave that it generates. Thus, we interpret the observed sea-level oscillations as small meteotsunamis, long tsunami-like waves generated by meteorological processes and resonance effects.     

1:25万地质填图进一步揭开了青藏高原大地构造的神秘面纱

125万地质填图进一步揭开了青藏高原地区大地构造的奥秘阿尔金山是昆仑、祁连-秦岭造山系的一部分;阿尔金断裂确是一条大型转换断层;木孜塔格-玛沁缝合带和金沙江缝合带均是华力西缝合带;松潘甘孜三叠系沉积盆地是劳亚大陆南部边缘的浊积岩盆地;冈底斯带曾经历了重要的印支造山运动;不存在从古生代延续到三叠纪的大洋盆地,即不存在所谓古特提斯或永久特提斯;古生代时期,在青藏高原地区亦不存在具古生物、古地理分隔意义的大洋盆地,当时,包括中朝、扬子、塔里木以及青藏高原地区在内的中国大部分均位于古亚洲洋主洋盆--中亚-蒙古带之南,属冈瓦纳大陆结构复杂的北部边缘;雅鲁藏布江和班公湖-怒江带是特提斯洋中的孪生姊妹,它们均是从三叠纪起就发展成大洋裂谷带的;以雅鲁藏布江带为主洋盆带的特提斯洋,从三叠纪晚期开始消减,经历了印支、燕山、喜马拉雅3个阶段脉动式板块汇聚造山过程.     

Recent tectonics in the northern part of the Knipovich Ridge,Atlantic ocean

The walls of the Knipovich Ridge are complicated by normal and reverse faults revealed by a high-frequency profilograph. The map of their spatial distribution shows that the faults are grouped into domains a few tens of kilometers in size and are a result of superposition of several inequivalent geodynamic factors: the shear zone oriented parallel to the Hornsunn Fault and superposed on the typical dynamics of the midocean ridge with offsets along transform fracture zones and rifting along short segments of the Mid-Atlantic Ridge (MAR). According to the anomalous magnetic field, the Knipovich Ridge as a segment of the MAR has formed since the Oligocene including several segments with normal direction of spreading separated by a multitransform system of fracture zones. In the Quaternary, the boundary of plate interaction along the tension crack has been straightened to form the contemporary Knipovich Ridge, which crosses the previously existing magmatic spreading substrate and sedimentary cover at an angle of about 45° relative to the direction of accretion. The sedimentary cover along the walls of the Knipovich is Paleogene in age and has subsided into the rift valley to a depth of 500–1000 m along the normal faults.     

Recent plate tectonics of the Arctic Basin and of northeastern Asia

The recent tectonics of the Arctic Basin and northeastern Asia are considered as a result of interaction between three lithospheric plates: North-America, Eurasia and Spitsbergen. Seismic zones (coinciding in the Norway-Greenland basin with the Kolbeinsey, Mohns and Knipovich ridges, and in the Arctic Ocean with the Gakkel Ridge) clearly mark the boundaries between them. In southernmost Svalbard (Spitsbergen), the secondary seismic belt deviates from the major seismic zone. This belt continues into the seismic zone of the Franz Josef Land and then merges into the seismic zone of the Gakkel Ridge at 70°–90°E. The smaller Spitsbergen plate is located between the major seismic zone and its secondary branch.Within northeastern Asia, earthquake epicenters with magnitude over 4.5 are concentrated within a 300-km wide belt crossing the Eurasian continent over a distance of 3000 km from the Lena estuary to the Komandorskye Islands. A single seismic belt crosses the northern sections of the Verkhoyansky Ridge and runs along the Chersky Ridge to the Kolymo-Okhotsk Divide.To compute the poles of relative rotation of the Eurasian, North-American and Spitsbergen plates we use 23 new determinations of focal-mechanism solutions for earthquakes, and 38 azimuths of slip vectors obtained by matching of symmetric mountain pairs on both sides of the Knipovich and Gakkel ridges; we also use 14 azimuths of strike-slip faults within the Chersky Ridge determined by satellite images. The following parameters of plate displacement were obtained: Eurasia/North America: 62.2°N, 140.2°E (from the Knipovich Ridge section south of the triple junction); 61.9°N, 143.1°E (from fault strikes in the Chersky Ridge); 60.42°N, 141.56°C (from the Knipovich section and from fault strikes in the Chersky Ridge); 59.48°N, 140.83°E, α = 1.89 · 10⁻⁷ deg/year (from the Knipovich section, from fault strikes in the Chersky Ridge and from the Gakkel Ridge section east of the triple junction). The rate was calculated by fitting the 2′ magnetic lineations within the Gakkel Ridge).North-America/Spitsbergen: 70.96°N, 121.18°E, α = −2.7 · 10⁻⁷ deg/year from the Knipovich Ridge section north of the triple junction, from earthquakes in the Spitsbergen fracture zone and from the Gakkel Ridge section west of the triple junction). Eurasia/Spitsbergen: 70.7°N, 25.49°E, α = −0.99 · 10⁻⁷ deg/year (from closure of vector triangles).     

Collision tectonics in the late Precambrian Eastern Ghats Mobile Belt: mesoscopic to satellite-scale structural observations

Regional mapping of a section across the Eastern Ghats Mobile Belt (EGMB) north of the Godavari graben in Eastern Peninsular India by using Landsat Thematic Mapper data enables recognition of a number of shear zones, lineaments, and structural domes and basins. A conspicuous megashear occurs at the western boundary of the granulite facies rocks of the EGMB adjacent to the Archean granite-greenstone craton. The confinement of a suite of alkaline igneous rocks to this shear zone is a notable feature. The strike extensions of this shear belt extend through to the Elchuru alkaline complex, Prakasam District, Andhra Pradesh, and the syenite plutons of Koraput district, Orissa. The contrasting lithologies, metamorphism and structural history on either side of the shear zone suggests that it might be a Precambrian suture zone. The mesoscopic structural features in the EGMB include prominent foliation with moderate to steep dips, folds, faults/shears, S-C fabrics, pinch and swell structures and other linear fabric elements. These observations favour the consideration of drastic crustal shortening and thickening and a complex deformational sequence. The major rock units in this part of EGMB comprise garnetiferous sillimanite gneisses, quartzites and calc-granulites forming the khondalitic suite of rocks and a wide variety of charnockitic rocks. The contact of the two rock units is generally sheared and often migmatised. The structural fabric suggests two major tectonic events: an essentially horizontal tectonic regime resulting in thrust systems and associated structures, subsequently followed by strike-slip tectonics characterized by high shear strains. Features such as westward-verging thrusts, large-scale recumbent folds, major shear zones, structural domes and basins, indications of tectonic crustal shortening, extensive calc-alkali magmatism and widespread migmatization in the region are attributed to collisional processes during Proterozoic times. The spatial disposition of the EGMB and its linkage with the distribution of similar rock units during the late Precambrian time in a global tectonic scenario are discussed.     

青藏高原冈底斯岩浆弧的分带性及其地质意义

冈底斯岩浆弧中段岩浆岩明显可分为相对集中又有所套叠的南北2个岩带.公穷松多-郎那花岗岩带位置略偏南,被晚期旋回林子宗群火山岩不整合覆盖.岗在-青都花岗岩带位置略偏北,带内各岩体均侵入于林子宗群火山岩.公穷松多岩带(早期旋回)从晚侏罗世开始,至晚白垩世早期结束;青都岩带(晚期旋回)从晚白垩世晚期开始,至始新世结束.2个岩带岩石分别从俯冲型产物开始,至后碰撞型产物结束,为2个较完整的岩浆旋回,暗示雅鲁藏布江构造带构造演化历程的复杂性.     

江苏东部沿海五大地学景观

赣榆秦山岛“神路”、连云港云台山海蚀洞、北北西向淤泥质断层海岸线、辐射状沙洲和东台谅港“两分水”以及海门东灶蛎蚜山牡蛎礁是江苏东部沿海五大地学景观,对它们的研究和开发具有重要的理论价值和实际意义。