Crustal lineaments, distribution of lower crustal intrusives and structural evolution of the Vøring Margin, NE Atlantic; new insight from wide-angle seismic models     

R. Mjelde  H. Shimamura  T. Kanazawa  S. Kodaira  T. Raum  H. Shiobara   《Tectonophysics》2003,369(3-4):199-218

Five lineaments on the volcanic Vøring Margin, NE Atlantic, have been identified in crustal scale models derived from Ocean Bottom Seismograph (OBS) data. It is suggested that the Vøring Basin can be divided in four compartments bounded by the Jan Mayen Fracture Zone/Lineament, a new lineament defined from this study, the Gleipne Lineament, the Surt Lineament and the Bivrost Lineament. The NW–SE trending Jan Mayen-, Gleipne- and Bivrost lineaments probably represent old zones of weakness controlling the onset of the early Eocene seafloor spreading, whereas the Surt- and New lineaments, rotated ca. 30° symmetrically from the azimuth of the Gleipne Lineament, may represent adjustment features related to the early Cretaceous/early Tertiary rifting. The longest landward extent of a lower crustal high-velocity body, assumed to represent intrusions related to the last phase of rifting, is found between the New Lineament and the Gleipne Lineament, where the body extends across the Helland Hansen Arch. Northeastwards in the Vøring Basin, the landward limit of the body steps gradually seawards, closely related to the interpreted lineaments. Northeast of the Gleipne Lineament, the body terminates close to the Fles Fault Complex, north of the Surt Lineament, it extends across the Nyk High, and northeast of the Bivrost Lineament the intrusions terminate around the Vøring Escarpment. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Icelandic plume, and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement. The increased thickness of the continental crust on the seaward side of the Vøring Escarpment, the upwarping of Moho and thinning of the lower crustal high-velocity layer in the western part of the Vøring Basin, as well as a strong shallowing of the Moho observed in parts of the area between the Jan Mayen Fracture Zone/Lineament and the New Lineament, can be explained by lithospheric delamination models.  相似文献   

Controls on the tectono-magmatic evolution of a volcanic transform margin: the Vøring Transform Margin,NE Atlantic     

Berndt  C.  Mjelde  R.  Planke  S.  Shimamura  H.  Faleide  J.I. 《Marine Geophysical Researches》2001,22(3):133-152

Ocean bottom seismograph (OBS), multichannel seismic and potential field data reveal the structure of the Vøring Transform Margin (VTM). This transform margin is located at the landward extension of the Jan Mayen Fracture Zone along the southern edge of the Vøring Plateau. The margin consists of two distinctive segments. The northwestern segment is characterized by large amounts of volcanic material. The new OBS data reveal a 30–40 km wide and 17 km thick high-velocity body between underplated continental crust to the northeast and normal oceanic crust in the southwest. The southeastern segment of the mar is similar to transform margins elsewhere. It is characterized by a 20–30 km wide transform margin high and a narrow continent-ocean transition. The volcanic sequences along this margin segment are less than 1 km thick. We conclude from the spatial correspondence of decreased volcanism and the location of the fracture zone, that the amount of volcanism was influenced by the tectonic setting. We propose that (1) lateral heat transport from the oceanic lithosphere to the adjacent continental lithosphere decreased the ambient mantle temperature and melt production along the entire transform margin and (2) that right-stepping of the left-lateral shear zone at the northwestern margin segment caused lithospheric thinning and increased volcanism. The investigated data show no evidence that the breakup volcanism influenced the tectonic development of the southeastern VTM.  相似文献   

Potential of submarine-cave sediments and oxygen isotope composition of cavernicolous micro-bivalve as a late Holocene paleoenvironmental record   总被引：2，自引：1，他引：1  

Akihisa Kitamura  Nagisa Yamamoto  Tomoki Kase  Shu-ichi Ohashi  Mayumi Hiramoto  Hitoshi Fukusawa  Tsuyoshi Watanabe  Tomohisa Irino  Hiroshi Kojitani  Michiyo Shimamura  Ikuo Kawakami 《Global and Planetary Change》2007,55(4):301-316

A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (31 m water depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the δ¹⁸O-derived temperature (T_δ18_O) of C. iejimensis suggests that the species grows between April and July; (5) the T_δ18_O of C. iejimensis from cored sediments implies that there were two warmer intervals, at AD 340 ± 40 and AD 1000 ± 40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene.  相似文献   

河南省舞阳县贾湖遗址出土的鲤科鱼类咽齿研究     

中岛经夫  吕鹏  张居中  中岛美智代  模林启介  袁靖 《第四纪研究》2015,35(1)

本文对贾湖遗址出土的鲤科鱼类进行了研究,发现有11种可以确定到属或种的鲤科鱼类.现在生存在中国南方的鲤科鱼类在贾湖遗址的出土,证明当时这个地区的气候比现在要温暖.这些鲤科鱼类是当时居住在贾湖遗址的先民有选择性地进行捕捞的.该研究有助于推进考古遗址出土鱼类遗存的动物考古学研究.  相似文献   

A possible Caledonide arm through the Barents Sea imaged by OBS data     

Asbjrn Johan Breivik  Rolf Mjelde  Paul Grogan  Hideki Shimamura  Yoshio Murai  Yuichi Nishimura  Asako Kuwano 《Tectonophysics》2002,355(1-4)

The assembly of the crystalline basement of the western Barents Sea is related to the Caledonian orogeny during the Silurian. However, the development southeast of Svalbard is not well understood, as conventional seismic reflection data does not provide reliable mapping below the Permian sequence. A wide-angle seismic survey from 1998, conducted with ocean bottom seismometers in the northwestern Barents Sea, provides data that enables the identification and mapping of the depths to crystalline basement and Moho by ray tracing and inversion. The four profiles modeled show pre-Permian basins and highs with a configuration distinct from later Mesozoic structural elements. Several strong reflections from within the crystalline crust indicate an inhomogeneous basement terrain. Refractions from the top of the basement together with reflections from the Moho constrain the basement velocity to increase from 6.3 km s⁻¹ at the top to 6.6 km s⁻¹ at the base of the crust. On two profiles, the Moho deepens locally into root structures, which are associated with high top mantle velocities of 8.5 km s⁻¹. Combined P- and S-wave data indicate a mixed sand/clay/carbonate lithology for the sedimentary section, and a predominantly felsic to intermediate crystalline crust. In general, the top basement and Moho surfaces exhibit poor correlation with the observed gravity field, and the gravity models required high-density bodies in the basement and upper mantle to account for the positive gravity anomalies in the area. Comparisons with the Ural suture zone suggest that the Barents Sea data may be interpreted in terms of a proto-Caledonian subduction zone dipping to the southeast, with a crustal root representing remnant of the continental collision, and high mantle velocities and densities representing eclogitized oceanic crust. High-density bodies within the crystalline crust may be accreted island arc or oceanic terrain. The mapped trend of the suture resembles a previously published model of the Caledonian orogeny. This model postulates a separate branch extending into central parts of the Barents Sea coupled with the northerly trending Svalbard Caledonides, and a microcontinent consisting of Svalbard and northern parts of the Barents Sea independent of Laurentia and Baltica at the time. Later, compressional faulting within the suture zone apparently formed the Sentralbanken High.  相似文献   

Crustal structure between the Knipovich Ridge and the Van Mijenfjorden (Svalbard)     

Ritzmann  Oliver  Jokat  Wilfried  Mjelde  Rolf  Shimamura  Hideki 《Marine Geophysical Researches》2002,23(5-6):379-401

Marine Geophysical Research - The Alfred Wegener Institute of Polar and Marine Research, the University of Bergen and Hokkaido University acquired new seismic refraction data along a transect from...  相似文献   

V_p/V_s ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field     

R. Mjelde  T. Raum  P. Digranes  H. Shimamura  H. Shiobara  S. Kodaira   《Tectonophysics》2003,369(3-4):175-197

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V_p/V_s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V_p/V_s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V_p/V_s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V_p/V_s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V_p/V_s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V_p/V_s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).  相似文献   

Structure of the Jan Mayen microcontinent and implications for its evolution     

Shuichi Kodaira  Rolf Mjelde  Karl Gunnarsson  Hajime Shiobara  & Hideki Shimamura 《Geophysical Journal International》1998,132(2):383-400

  相似文献   

T waves from deep earthquakes generated exactly at the bottom of deep-sea trenches     

Hideki Shimamura  Toshi Asada 《Earth and Planetary Science Letters》1975,27(2):137-142

T waves (seismic water waves), which were generated by deep-focused earthquakes, have been found by an array of sensitive ocean-bottom seismographic observations depolyed on the western Pacific basin. The points of generation of T waves have been exactly located by use of the accurate velocity of water waves which were known from explosions. The positions obtained are at the bottom of deep-sea trenches; however, the positions are slightly (10–35 km) ocean-side of the trench. T waves have been known to be generated by seismic waves which were transmitted from the focus to the trench bottom along the descending lithosphere. The intensity of the observed T waves implies that the Q value along the descending lithosphere is more than 4000. The positions of T-wave generation are consistent with the 8.2- to 8.6-km/s stratified structure of the oceanic lithosphere. T waves from shallow earthquakes beneath the lower continental slope are also clearly observed by bottom seismography.  相似文献   

Temporal and spatial variation of infilling processes in a landslide scar in a steep mountainous region,Japan          下载免费PDF全文

Fumitoshi Imaizumi  Roy C. Sidle  Asako Togari‐Ohta  Makoto Shimamura 《地球表面变化过程与地形》2015,40(5):642-653

The duration of the soil‐depth recovery needed for reoccurrence of shallow colluvial landslides at a given site in humid regions is much longer than the return period of rainfall needed to generate sufficient pore water pressure to initiate a landslide. Knowledge of the rate of change in soil depth in landslide scars is therefore necessary to evaluate return intervals of landslides. Spatial variation in sediment transport at the Kumanodaira landslide scar in central Japan was investigated by field observations. Spatial distribution of the rate of change in soil depth was estimated using sediment transport data and geographic information system (GIS) analysis. Observations revealed that the timing of sediment transport differed for shallow and deep soil layers. Near‐surface sediment transport (mostly dry ravel and some shallow soil creep at depths ≤0·05 m) measured in sediment traps was active in winter and early spring and was affected by freezing–thawing; soil creep of subsoil (i.e. >0·05 m), monitored by strain probes, was active in summer and autumn when precipitation was abundant. Near‐surface sediment flux was estimated by a power law function of slope gradient. Deeper soil creep was more affected by relative location to the landslide scar, which influences soil depth, than by slope gradient. Our study indicated that the rate of soil‐depth recovery is high just below the head scarp of the landslide. Abrupt changes in the longitudinal slope topography immediately above, within and just below the head scarp became smoother with time due to degradation proximate to the landslide head scarp and flanks, as well as aggradation just below the head scarp. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献