共查询到20条相似文献，搜索用时 987 毫秒
1.
Compressional wave attenuations and velocities have been measured as a function of confining pressure in ophiolite samples representing a cross-section of the oceanic crust and uppermost mantle. Data are presented for basalts, diabase dikes, gabbros and a suite of serpentinites and peridotites showing a range of serpentization. An ultrasonic pulse-echo spectral ratio technique was used to determine the attenuations to confining pressures of 500 MPa. From this data a Q profile for the oceanic crust and upper mantle is presented. Q is found to moderately increase with depth through the pillow basalts of the upper oceanic crust. The sheeted dike rocks of Layer 2C show an increase in Q with depth due to progressive metamorphism (from greenschist to amphibolite facies). Q drops abruptly from Layer 2C to Layer 3, though it is not clear why the gabbros have such low Q's. The crust-mantle boundary is a Q discontinuity; however, the Q contrast between Layer 3 and the upper mantle could be altered by upper mantle serpentinization, interlayered gabbros and peridotites at the boundary, or serpentinized peridotite diapirs intruding the gabbroic section. Q varies significantly with the percentage of serpentinization in the ultramafic samples, with the largest changes in Q being at the extremes of zero and full serpentinization. Q is sensitive to the overburden pressure for all of the samples. 相似文献
2.
Nazarova Katherine A. Wasilewski Peter J. Dick Henry J.B. 《Marine Geophysical Researches》2000,21(5):475-488
Magnetic properties and bulk densities of 27 serpentinized harzburgite samples from the Islas Orcadas Fracture Zone, located in the vicinity of the Bouvet Triple Junction, have been measured and analyzed. Polished sections were examined using reflected light and scanning electron microscopy to characterize the size and geometric arrangement of opaque minerals. The relationship between the saturation magnetization (IS) and remanent coercive force (HR) is considered in terms of the amount of ferrimagnetic material and maghemitization. A suite of continental serpentinites from Canada is offered as contrast, to consider the role of weathering and maghemitization. Magnetite in the Islas Orcadas serpentinites is variably maghemitized, whereas continental serpentinites do not appear to contain maghemitized oxides. We verify this with optical microscopy, thermomagnetic analyses and cryogenic temperature cycling of saturation remanence. Maghemitization serves to reduce initial magnetic susceptibility, and introduce error in the use of IS to evaluate the magnetic mode of magnetite. The presence of maghemite and the existence of a three dimensional vein network for magnetite geometry would suggest that magnetic hysteresis parameters can not reliably indicate grain size. Magnetic hysteresis ratios fall in a restricted range regardless of coercivity. The apparent grain size configured in a three dimensional vein network plus maghemitization might be responsible for this observation. Maghemitization does not affect thermal magnetic stability and enhances the geophysical importance of remanence in serpentinites. Paleomagnetic data suggest that important information about the geologic circumstances for oceanic rock serpentinization is embodied in the paleomagnetic records. This observation may be very important for generation of long wavelength aeromagnetic and possibly even satellite magnetic anomalies. 相似文献
3.
4.
Petromagnetic studies of a representative spinel peridotite collection from a number of localities along slowly spreading mid-ocean ridges were performed. It was established that oceanic peridotites usually contain two or more generations of ferrimagnetics formed in assemblages with metamorphic silicates at different temperatures. The crystallization of different magnetite (or magnetite-rich spinel) generations in the rocks is manifested by the appearance of inflection points in the thermomagnetic curves. The particular petrographic features of the metamorphism observed in the peridotites correlate well with the temperatures of the inflection points in the thermomagnetic curves. Generally, the serpentinization of primary minerals in oceanic peridotites starts at 210–225°C (green mesh serpentines and bastites) and finishes at 135–190°C (colorless plate serpentines, hourglass-textured serpentines, and colorless homoaxial bastites). Crystallization of ferrimagnetic grains associated with talc, chlorite, and amphibole in the course of medium-grade metamorphism, which precedes the serpentinization, was observed in the major part of the collection examined and occurred at temperatures higher than 370°C. Seprentinization of metamorphic minerals (predominantly talc) with the formation of antigorite and crystallization of ferrimagnetic grains in oceanic peridotites occurred at 250–350°C, previous to the serpentinization of primary silicates. Thus, the petromagnetic thermometry of metamorphic events in oceanic peridotites may provide significant progress in the understanding of the geological process accompanying the formation of the oceanic lithosphere in slowly spreading ocean ridges. 相似文献
5.
We present magnetic field data collected over the Mid-Atlantic Ridge in the vicinity of the Atlantis Fracture Zone and extending out to 10 Ma-old lithosphere. We calculated a magnetization distribution which accounts for the observed magnetic field by performing a three-dimensional inversion in the presence of bathymetry. Our results show the well-developed pattern of magnetic reversals over our study area. We observe a sharp decay in magnetization from the axis out to older lithosphere and we attribute this decay to progressive low temperature oxidation of basalt. In crust which is 10 Ma, we observe an abrupt increase in magnetic field intensity which could be due to an increase in the intensity of magnetization or thickness of the magnetic source layer. We demonstrate that because the reversal epoch was of unusually long duration, a two-layer model comprised of a shallow extrusive layer and a deeper intrusive layer with sloping polarity boundaries can account for the increase in the amplitude of anomaly 5. South of the Atlantis Fracture Zone, high magnetization is correlated with bathymethic troughts at segment end points and lower magnetization is associated with bathymetric highs at segment midpoints. This pattern can be explained by a relative thinning of the magnetic source layer toward the midpoint of the segment. Thickening of the source layer at segment endpoints due to alteration of lower oceanic crust could also cause this pattern. Because we do not observe this pattern north of the fracture zone, we suggest it is a result of the nature of crustal formation process where mantle upwelling is focused. South of the fracture zone, reversals along discontinuity traces only continue to crust 2 Ma old. In crust >2 Ma, we observe bands of high, positive magnetization along discontinuity traces. We suggest that within the discontinuity traces, a high, induced component of magnetization is produced by serpentinized lower crust/upper mantle and this masks the contribution of basalts to the magnetic anomaly signal. 相似文献
6.
Serpentinites, which contain up to 13 wt% of water, are important reservoirs for chemical recycling in subduction zones. In the past two decades, forearc mantle serpentinites were identified in different locations around the world. Here, we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount, which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench. Nearly all the collected samples are extensively hydrated, and olivine grains in ultramafic rocks are replaced by serpentine minerals, with only one sample preserving remaining trace of orthopyroxene. Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich(～42 wt%), but have low contents in Al_2O_3, CaO, rare earth and high field strength elements, which is consistent with the overall depleted character of their mantle protoliths. Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25% partial melting of the presumed fertile mantle source, which is generally lower than those of peridotites from Torishima Forearc Seamount, Conical Seamount and South Chamorro Seamount(mostly25%). All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements(Li, Sr, Pb and U). In details, Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb, and display remarkably high abundances of U. These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid, without or with little contribution from slab-derived fluids. The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge. 相似文献
7.
Mjelde Rolf Kasahara Junzo Shimamura Hideki Kamimura Aya Kanazawa Toshihiko Kodaira Shuichi Raum Thomas Shiobara Hajime 《Marine Geophysical Researches》2002,23(2):169-183
On the Vøring volcanic passive margin offshore mid-Norway, NE Atlantic, a lower crustal body with P-wave velocities in the range of 7.1–7.7 km/s has been mapped by twenty two-dimensional Ocean Bottom Seismograph (OBS) profiles. The main aim of the present paper is to evaluate to what extent the lower crust is consistent with magmatic intrusions or serpentinized peridotite. The relatively low V
p/V
s ratios of 1.75–1.78 modelled for the lower crust under the continental part of the Vøring Plateau are consistent with mafic intrusions mixed with blocks of stretched continental crust, but not with the presence of partially serpentinized peridotites. The lower crustal high-velocity body is restricted to the area of the Late Cretaceous/Early Tertiary rift that lead to continental break-up in Early Eocene. The same model can explain the observations in the northern Vøring Basin, but in the central and southern Vøring Basin the seismic velocities do not preclude a model involving serpentinized peridotite in addition to intrusions and continental remnants. On the west Iberia non-volcanic margin a similar layer is interpreted as serpentinized peridotite. The existence of Moho reflections, the observation of S-wave anisotropy but absence of P-wave anisotropy, uncertainties regarding supply of water to allow for significant serpentinization and very low stretching factors compared with the west Iberia Margin, are among factors that argue against the presence of serpentinized peridotite in the Vøring Basin. 相似文献
8.
Crustal structure of the ultra-slow spreading Knipovich Ridge,North Atlantic,along a presumed amagmatic portion of oceanic crustal formation 总被引：4，自引：4，他引：0
Aleksandre Kandilarov Rolf Mjelde Kyoko Okino Yoshio Murai 《Marine Geophysical Researches》2008,29(2):109-134
The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the
autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction
of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure
and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built
and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated.
The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has
low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks
seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative
of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in
thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year
from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading
(5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro
and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of
serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers. 相似文献
9.
The purpose of our work was to obtain the most possible detailed information about the composition, concentration, and structural
features of the magnetic minerals contained in the rock to reveal the differences in the magnetic properties of the peridotites
under various circumstances of the mantle magmatism and different conditions of metamorphism. To do this, we examined and
analyzed the magnetic and petrographic characteristics of four collections of oceanic and alpinotype spinel peridotites. The
main object for comparing the magnetic characteristics was the Gorringe ridge, which lies in the eastern part of the Atlantic
Ocean. The peridotite samples from the Gorringe ridge differ from the other collections in many magnetic parameters: I
n
, χ, Q, I
rs
/I
s
, H
c
, H
cr
, and H
m
. The principal question of our work was to clarify the nature of the Earth’s crust where the Gorringe ridge formed. This
subject was studied many times in the literature, but the researchers did not reach a common opinion. In accordance with our
data, the spinel peridotites from the Gorringe ridge represent a subcontinental lithosphere mantle of the Iberian continental
margin. During the metamorphism, the formation of magnetite occurred in the peridotites of the Gorringe ridge in several stages
and had a regressive character. Our investigations explain the results of the analysis of the anomalous magnetic field over
the Gorringe ridge, which is characterized by sharp roughness and high intensity of the local signchanging anomalies. 相似文献
10.
国际大洋钻探计划自１９８３年以来，已成功地实施了６６个航次的深海钻探作业，获取钻探岩芯逾１０００００ｍ。在关于大陆边缘的构造演化，海洋地壳的形成与发展，地壳流体循环和壳幔相互作用海洋沉积结构和古海洋学，地球大气圈、水圈、冰圈、生物圈的长期变化等众多研究领域取得了一系列重大科研成果。 相似文献
11.
Geochemical and magnetic properties of basalts dredged along a propagating spreading centre (central part of the North Fiji Basin) have been analysed. Two phenomena appear to be important in controlling variations of natural remanent magnetisation (NRM): 1) an extensive magma differentiation to Fe-Ti basalts increases with the intensity of NRM; and both increase towards the propagator tip, 2) low temperature oxidation (maghemitisation) seems to have already occurred along the ridge axis for the zero age basalts. This paper shows that despite the apparent lack of correlation between NRM and the maghemitisation process (masked by the effect of extensive magma differentiation), the latter involves a change of the domain state of magnetic carriers, from pseudo single-domain to single-domain. We separated the NRM into 4 partial NRMs (pNRM) depending on the coercivity of grains. This analysis showed that the contribution to the remanence of grains with high coercivity increases with maghemitisation, whereas the contribution of grains with weak coercivity decreases.Despite the relatively high degree of maghemitisation, the variations of natural remanent magnetisation intensity principally reflect the variations of magmatic processes. This joint work on magnetic and magmatic properties of basalts collected on the NS-propagating spreading centre from the North Fiji Basin reinforces the magnetic telechemistry hypothesis of Vogt (1979). 相似文献
12.
Paleomagnetic properties of sediment cores were examined to reconstruct paleodepositional conditions in the Korea Deep Ocean Study (KODOS) area, located in the northeastern equatorial Pacific. The studied KODOS sediments have a stable remanent magnetization with both normal and reversed polarities, which are well correlated with the geomagnetic polarity timescale for the late Pliocene and Pleistocene. Average sedimentation rates are 1.56 and 0.88 mm/kiloyear for the Pleistocene and late Pliocene, respectively. Clay mineralogy and scanning electron microscope analyses of the sediments indicate that terrestrial material was transported to the deep-sea floor during these times. The variations of sedimentation rates with age may be explained by the onset of the northern hemisphere glaciation and subsequent climatic deterioration during the Pliocene and Pleistocene. For the Pleistocene, an increasing sedimentation rate implies that input of terrestrial materials was high, and also a high input of biogenic materials was detected as a result of increased primary production in the surface water. The down-core variations in paleomagnetic and rock-magnetic properties of the KODOS sediments were affected by dissolution processes in an oxic depositional regime. As shown by magnetic intensity and hysteresis parameters, the high natural remanent magnetization (NRM) stability in the upper, yellowish brown layers indicates that the magnetic carrier was in pseudo-single domain states. In the lower, dark brown sediments, only coarse magnetic grains survived dissolution and the NRM was carried by more abundant, multi-domain grains of low magnetic stability. The down-core variation of magnetic properties suggests that the KODOS sediments were subjected to dissolution processes resulting in a loss of the more stable components of the magnetic fraction with increasing core depth. 相似文献
13.
G. Leitchenkov J. Guseva V. Gandyukhin G. Grikurov Y. Kristoffersen M. Sand A. Golynsky N. Aleshkova 《Marine Geophysical Researches》2008,29(2):135-158
About 16,000 km of multichannel seismic (MCS), gravity and magnetic data and 28 sonobuoys were acquired in the Riiser-Larsen
Sea Basin and across the Gunnerus and Astrid Ridges, to study their crustal structure. The study area has contrasting basement
morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the Riiser-Larsen Sea shelf, 26–28 km
under the Gunnerus Ridge, 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-km-wide block with
increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted
as associated with emplacement of mafic intrusions into the continental margin of the southern Riiser-Larsen Sea. In addition
to previously mapped seafloor spreading magnetic anomalies in the western Riiser-Larsen Sea, a linear succession from M2 to
M16 is identified in the eastern Riiser-Larsen Sea. In the southwestern Riiser-Larsen Sea, a symmetric succession from M24B
to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It
is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The seismic stratigraphy
model of the Riiser-Larsen Sea includes five reflecting horizons that bound six seismic units. Ages of seismic units are determined
from onlap geometry to magnetically dated oceanic basement and from tracing horizons to other parts of the southern Indian
Ocean. The seaward edge of stretched and attenuated continental crust in the southern Riiser-Larsen Sea and the landward edge
of unequivocal oceanic crust are mapped based on structural and geophysical characteristics. In the eastern Riiser-Larsen
Sea the boundary between oceanic and stretched continental crust is better defined and is interpreted as a strike-slip fault
lying along a sheared margin. 相似文献
14.
Giant Megamullion in the Parece Vela Backarc Basin 总被引：1，自引：0，他引：1
Ohara Yasuhiko Yoshida Tsuyoshi Kato Yukihiro Kasuga Shigeru 《Marine Geophysical Researches》2001,22(1):47-61
We present results of high-resolution bathymetric studies of the extinct intermediate-spreading Parece Vela Basin in the northwestern Pacific, where we have identified an extremely large mullion structure, here termed a giant megamullion. We find that the giant megamullion is nearly an order of magnitude larger than the similar structures in the slow-spreading Mid-Atlantic Ridge (`megamullions'). The giant megamullion has slightly elevated mantle Bouguer anomaly, and yields serpentinized peridotites and gabbros, suggesting that they are exposing oceanic crust and upper mantle. An off-axis rugged `chaotic terrain' was also identified in the Parece Vela Basin. The terrain consists of isolated and elevated blocks capped by corrugated axis-normal lineations, and associated deeps. We thus interpret it as analogues to the Mid-Atlantic Ridge megamullions. We propose that amagmatic tectonics producing the giant megamullion and the chaotic terrain occupied a significant part in crustal construction in the Parece Vela Basin evolution. 相似文献
15.
The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot. 相似文献
16.
The Ocean Drilling Program (ODP) Leg 184 recovered a continuous sedimentary sequence since Miocene at Site 1148 in the northern South China Sea (SCS), which provided a unique record for better understanding the tectonic, environmental and climatic evolution of the SCS. In this study, we report a detailed paleomagnetic study on 881 discrete samples from the upper 460 m cores of Hole 1148A through stepwise alternating field demagnetization (AF) or thermal demagnetization (TD). Rock magnetic results from isothermal remanent magnetization (IRM) acquisition, temperature dependence magnetic susceptibility (κ-T) and hysteresis loop parameters of representative samples indicate that the main magnetic mineral is low coercivity pseudo-single domain magnetite. The characteristic remanent magnetization (ChRM) with moderate downward or upward inclinations was identified after removing a low-stability drilling-induced remanence with steep downward inclinations. A new 23 Myr magnetostratigraphic time framework was established by correlating the magnetic polarity sequence to the Geomagnetic Polarity Time Scale 2012 (GPTS 2012; Ogg, 2012) in combination with biostratigraphic data. The mean sediment accumulation rates for lithostratigraphic Units V and IV, Unit III and lower Unit II, upper Unit II, Unit IB and Unit IA are calculated as ∼20.8 m/Myr, ∼15–17 m/Myr, 24.18 m/Myr, 17.07 m/Myr and 49.01 m/Myr, respectively. 相似文献
17.
18.
Most of the hydrocarbon fields in the oceanic and thinned suboceanic crust are associated with deep-seated faults. This fact
might be caused by methane formation under the serpentinization of the upper mantle rocks and its transfer to the sedimentary
cover with hydrotherms. The northern part of the Indian Ocean deformation zone may be of considerable promise for the formation
of deposits of such genesis. This is favored by the occurrence of the long-living hydrothermal activity over the deep-seated
faults and by quite the thick sedimentary cover (over 1 km). In the course of cruise 32 of the R/V Akademik Kurchatov, a buried anticline formation with an acoustic anomaly of the bright-spot type was discovered 450 km southwards from Sri
Lanka Island. This may point to the presence of a hydrocarbon accumulation in the sedimentary cover. 相似文献
19.
海洋核杂岩 总被引：1，自引：0，他引：1
为了解释洋壳中大量铲形正断层及垂直洋中脊的大量线理(如大西洋中脊的巨型窗棱构造)等现象,通过与大陆上变质核杂岩对比,近来提出了一种新的海底构造类型———海洋核杂岩。在洋底深地震剖面上核杂岩结构形态可以分为3部分,其中第1部分为层1和层2,以脆性变形为特征;第2部分为脆-韧性过渡层,拆离带发育其中,由白色结壳式碳酸盐岩和强烈蛇纹石化的橄榄岩或玄武岩、超镁铁质糜棱岩、糜棱状辉长岩等组成。拆离面之上为未变质的薄层海洋沉积层,其下为热洋幔的退变质岩石组成;第3部分为核部,以塑性变形为特征,常被超基性岩体(尤其是辉长岩侵入体)底辟侵入。与大陆变质核杂岩相比,海洋核杂岩具有明显的独特性。海洋核杂岩的拆离断层同样有数十千米的位移量,因而,可能导致出现海底磁条带的局部错位现象,使得洋壳磁条带的平面结构复杂化。 相似文献
20.
D.Gopala Rao 《Marine Geology》1984,61(1):103-110
Total magnetic intensity and bathymetric surveys were carried out in the offshore area of Ratnagiri on the western continental shelf of India and an isomagnetic anomalies map at a contour interval of 50 nT was prepared which reveals N-S trends of magnetic anomalies. Two-dimensional model and spectral studies of these anomalies were carried out, and subsurface models of the geology in the area have been derived from anomalies at a number of places. The results suggest that the anomalies occur over a magnetic crystalline basement at a depth of 1–1.2 km which is similar (in magnetization) to onshore basalts of northwest India. These anomalies are believed to be an expression of a considerable thickness (around 1.7 km) of basalt, underlain by sediments. Identification of these basalts in offshore areas along the northwestern continental shelf of India would support (1) the idea that the onshore Deccan basalts of western India and the rhyolitic tuffs at the Laccadive ridge system (DSDP Site 210) are related to the same volcanic events, and (2) subsequent downfaulting of onshore Deccan basalts into the Arabian Sea and submergence below the Tertiary sediments. 相似文献