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1.
A Time Frame for Construction of the Kerguelen Plateau and Broken Ridge   总被引：8，自引：10，他引：8  
A key element in achieving Ocean Drilling Program Leg 183 scienceobjectives is determining the age of volcanism at differentlocations across the Kerguelen Plateau and Broken Ridge. Thispaper reports crystallization ages derived from 40Ar–39Arincremental heating experiments, for whole rocks and feldsparsseparated from basement units recovered at Sites 1136, 1137,1138, 1139, 1140, 1141 and 1142. The subaerial environment oferuption at most sites and the generally evolved, high K contentof these lava flows contributed to precise and reproducibleage determinations. Volcanic activity at southern KerguelenPlateau Site 1136 occurred at 118–119 Ma; at Elan BankSite 1137, 107–108 Ma; at central Kerguelen Plateau Site1138, 100–101 Ma; at Skiff Bank Site 1139, 68–69Ma; at northern Kerguelen Plateau Site 1140, 34–35 Ma;and at Broken Ridge Sites 1141 and 1142, 94–95 Ma. Thenew ages allow calculation of melt production rates throughthe  相似文献   

2.
The Naturaliste Plateau is a submarine continental ribbon rifted from the southwest Australian margin during the Early Cretaceous breakup of East Gondwana. It occupied a key position near the juncture of Greater India and the boundary between Australia and Antarctica. However, details of the Early Cretaceous evolution of the plateau are not well known because of limited data. Drilling at Site U1513 during IODP Expedition 369 recovered the first complete Lower Cretaceous succession on the eastern Naturaliste Plateau. The succession includes syn-rift volcanic rocks, Hauterivian to early Aptian volcaniclastic-rich sedimentary rocks, and Albian claystone strata. The 235-m thick volcaniclastic-rich sequence represents the missing post-breakup record in the southwest Australian rifted margin. It spans the transition from syn- to post-rift phase during the final stages of breakup between Greater India and Australia-Antarctica. We report the lithological, petrophysical, geochemical, paleontological, and paleomagnetic characteristics of the sequence, and then synthesize the results to define the Early Cretaceous depositional environment and subsidence history of the Naturaliste Plateau. From the early Hauterivian, weathered volcanic products were eroded and re-deposited locally as a volcaniclastic-rich sequence, with a major contribution from the southern Naturaliste Plateau. The depositional environment evolved from a shelf to upper bathyal condition during the Hauterivian through early Barremian with a decreasing sedimentation rate. This period is defined as a late syn-rift subsidence phase by NW-SE trending extension. After the final breakup with Greater India, the plateau remained at upper bathyal depths with little deposition until the early Aptian. Mid–lower bathyal depths inferred from the Albian claystone strata suggest that the post-rift thermal subsidence commenced during the late Aptian. This two-phase post-rift subsidence reflects the proximity or high temperature of mantle plume, possibly the Kerguelen plume, and its westward migration relative to the southwest Australian rifted margin.  相似文献   

3.
Ocean Drilling Program (ODP) Leg 183 was designed to investigatethe origin and evolution of the large igneous province composedof the Kerguelen Plateau and Broken Ridge. Of the eight sitesdrilled, basalt was recovered from seven, five on the plateauand two on Broken Ridge. We present results from four of thesesites, 1136, 1138, 1141 and 1142. Although this large igneousprovince is interpreted as being derived from the Kerguelenmantle plume, the geochemical characteristics of basalt fromsome parts of the province indicate a role for continental lithosphere.The 118–119 Ma basalt flows recovered in the SouthernKerguelen Plateau (Site 1136) have a more subtle continentalsignature than shown by basalt at Leg 119 Site 738. A continentalsignature is absent in the 100–101 Ma tholeiitic basaltsat Site 1138 in the Central Kerguelen Plateau (CKP); their age-correctedNd–Sr–Pb isotopic values and incompatible elementratios are similar to those estimated for primitive mantle.These flows may represent a major mantle source in the Kerguelenstarting-plume head. The 20 basalt units identified are a productof magma chamber replenishment, fractional crystallization,and resorption of crystallizing phases. The topmost unit, Unit1, is a dacite that evolved from a basalt magma similar to thoserepresented by Units 3–22; unlike the basalts the dacitemagma was probably influenced by continental material. MiddleCretaceous (  相似文献   

4.
Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.  相似文献   

5.
The basaltic basement of the large igneous province formed bythe Kerguelen Plateau and Broken Ridge in the southeastern IndianOcean has been sampled by three Ocean Drilling Program cruises(Legs 119, 120 and 183). Although the Cretaceous parts of thisplateau formed in the embryonic Indian Ocean basin, presumablyby melting associated with the Kerguelen plume, trace elementabundances and isotopic ratios of Sr, Nd and Pb of Cretaceousbasalt from several drill sites indicate that continental lithospherewas involved in their petrogenesis. On the basis of relativedepletions in Nb, Ta and Th, and isotopic characteristics similarto those of EMI ocean island basalt, lavas from Leg 120 Site747 in the Central Kerguelen Plateau contain a component derivedfrom lower continental crust. On the basis of relative abundancesof Sr and Eu and EMI-like Pb isotopic ratios, the source ofbasalt from Leg 120 Site 750 in the northeastern part of theSouthern Kerguelen Plateau also contained a component derivedfrom lower continental crust; in this case, the crustal componentformed as a plagioclase-rich, clinopyroxene-bearing cumulate.Basalts from Leg 120 Site 749 define two distinct isotopic (Sr,Nd and Pb) groups which differ from the isotopic fields forSite 747 and 750 basalts. Among Site 749 lavas, there is subtleevidence for a continental component, broadly similar (i.e.moderate 206Pb/204Pb  相似文献   

6.
Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb/204Pb and isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with εNd(T) = − 13 to −7, (87Sr/86Sr)T=0.7090 to 0.7130 and high 207Pb/204Pb relative to 206Pb/204Pb. In addition, the low-εNd(T) Naturaliste Plateau samples are elevated in SiO2 (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest εNd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb/04Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive εNd, relatively high-206Pb/204Pb plume composition, or a plume source dominated by mantle with εNd of −3 to 0.  相似文献   

7.
The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3 km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5 s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.  相似文献   

8.
Abstract

Subsidence analysis is used here to get a better understanding of the Eo-Cimmerian continental accretion to Eurasia of a block (the Iran Block) of Gondwanian origin. The drift of the block from Gondwana to Eurasia is classically considered as a late Triassic event but the lack of unquestionable age evidence leads to investigate the whole Permian to Jurassic history. Indeed, the subsequent Alpine deformation along the proposed suture that should mark the Eo-Cimmerian collision forbids to characterize the collisional event without ambiguity. Moreover, the Iran block is presently represented by different continental slivers that are disconnected from each other, being in places separated by Cretaceous ophiolites, and it makes unclear if one or several blocks should be taken into account. Subsidence analysis is introduced to solve the problem, in the hope that the sedimentary history in any part of the slivers has registered important crustal events such as breakup and collision and that the few well-preserved stratigraphic sections bear the corresponding subsidence signals. Subsidence analysis is thus applied to geologically disconnected objects in a manner that departs from its traditional use in basin analysis. However, as it introduces quantified data on the behaviour of the crust in response to tectonics, it was shown to be an efficient tool in sorting out the major events amongst various local evidences for crustal unstability. Major results are: – dating the breakup as Early Permian and collision as Middle Triassic; – showing that the accretion of the Iran Block to Eurasia was accompanied by a new breakup that formed a passive margin in Nayband to the Southeast, in contrast to the new active margin that was established along the Abadeh, south-western side; – emphasising the tectonic instability that controlled the continental Jurassic deposits upon the new continent before stabilisation was reached during Late Jurassic-Early Cretaceous times.  相似文献   

9.
During the mid-Cretaceous, extensive magmatism occurred in theIndian Ocean to form volcanic portions of the southern and centralKerguelen Plateau, Elan Bank and Broken Ridge. Basalt was eruptedalso along the rifted margin of eastern India (Rajmahal). Weinvestigated the ages of these Indian basalts using 40Ar/39Arincremental-heating experiments on whole rocks. Our resultsare consistent with the hypothesis that the lava pile of  相似文献   

10.
Subsidence curves from Mesozoic sedimentary basins at the southern Iberian margin (Betic Cordilleras) display pronounced changes in subsidence rates around 85 Ma (chron 34, Late Cretaceous, Santonian to earliest Campanian). The subsidence events correlate with changes in the bulk and clay mineral composition in these basins, as well as with an Eoalpine high-pressure metamorphic event in the western Mediterranean region. The synchroneity with subsidence events observed in basins around the Iberian microplate suggests a causal relationship with the regional plate tectonic setting. We propose that the circum-Iberian subsidence event was largely controlled by the convergence and incipient collision of the Iberian microplate with Africa.  相似文献   

11.
Opening of the Fram Strait gateway: A review of plate tectonic constraints   总被引：1，自引：0，他引：1  
We have revised the regional crustal structure, oceanic age distribution, and conjugate margin segmentation in and around the Lena Trough, the oceanic part of the Fram Strait between the Norwegian–Greenland Sea and the Eurasia Basin (Arctic Ocean). The Lena Trough started to open after Eurasia–Greenland relative plate motions changed from right-lateral shear to oblique divergence at Chron 13 times (33.3 Ma; earliest Oligocene). A new Bouguer gravity map, supported by existing seismic data and aeromagnetic profiles, has been applied to interpret the continent–ocean transition and the influence of Eocene shear structures on the timing of breakup and initial seafloor spreading. Assuming that the onset of deep-water exchange depended on the formation of a narrow, oceanic corridor, the gateway formed during early Miocene times (20–15 Ma). However, if the initial Lena Trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep-water circulation, the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around Chron 5 times (9.8 Ma; Late Miocene). Paleoceanographic changes at ODP Site 909 (northern Hovgård Ridge) are consistent with both hypotheses of gateway formation. We cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the Lena Trough. The gravity, seismic and magnetic observations question the prevailing hypotheses on the Yermak Plateau and the Morris Jesup Rise as Eocene oceanic plateaus and the Hovgård Ridge as a microcontinent.  相似文献   

12.
A bimodal volcanic sequence of 230 m thickness on Skiff Bank,a western salient of the northern Kerguelen Plateau, was drilledduring ODP Leg 183. The sequence comprises three main units:a mafic unit of trachybasalt flows sandwiched between two unitsof trachytic or rhyolitic flows and volcaniclastic rocks. Althoughinterpretation is complicated by moderate to strong alterationof the rocks, their original chemical character can be establishedusing the least mobile major and trace elements (Al, Th, highfield strength elements and rare earth elements). High concentrationsof alkalis and incompatible trace elements indicate that bothmafic and felsic rocks are alkalic. The felsic rocks may havebeen derived by partial melting of mafic rocks, followed byfractionation of feldspar, clinopyroxene, Fe–Ti oxidesand apatite. The mafic and felsic rocks have similar Nd andPb isotopic compositions; 206Pb/204Pb ratios are low (17·5–18·0)but, like the 143Nd/144Nd ratios (0·5125–0·5126),they are comparable with those of basalts from the central andsouthern Kerguelen Plateau (e.g. Sites 747, 749, 750). The Srisotopic system is perturbed by later alteration. There is nochemical or isotopic evidence for a continental crustal component.The bimodal alkalic character and the presence of quartz-phyricrhyolites is interpreted to indicate that the sequence formspart of a shield volcano built upon the volcanic plateau. Theage of 68 Ma, obtained on Site 1139 rocks by Duncan (A timeframe for construction of the Kerguelen Plateau and Broken Ridge,Journal of Petrology 43, 1109–1119, 2002), provides onlya minimum age for the underlying flood volcanic rocks. The highage indicates none the less that Skiff Bank is not the presentlocation of the Kerguelen plume. KEY WORDS: Ocean Drilling Program; Kerguelen Plateau; Skiff Bank  相似文献   

13.
The study of magmatism and tectonic structure of the East Indian or Ninetyeast Ridge (NER) reveals the geochemical similarity of mantle sources for the NER and Kerguelen Plateau melts. Magmas related to the Kerguelen plume were derived from an enriched mantle source, whereas the NER tholeiitic basalts originated from a source contaminated by a depleted material. While, depleted basalt varieties were not found within the NER basalts. It was shown that magmatic rocks forming the NER were generated by high degrees (30%) of partial melting within the ancient Wharton spreading ridge due to the activity of the Kerguelen plume, which was located at this time in the vicinity of the ridge. The most significant impact of the plume on the NER structures was recorded at 70–50 Ma ago.  相似文献   

14.
Heat flow taken between Svalbard and Greenland reveal three thermal provinces:
1. (1) the Molloy Ridge within the Spitsbergen Transform,
2. (2) the Yermak Plateau
3. (3) the northeastern margin of Svalbard (Nordaustlandet).
The Molloy Ridge is a short spreading segment and the average heat flow is much above the Sclater et al. (1971), cooling curve but agrees with values from the Norwegian-Greenland Sea. An additional zone of intrusion identified by heat flow lies to the northwest of the Molloy Ridge. It straddles both the visible fracture zone and part of the Yermak Plateau. A thermal boundary lies between the warm western segment of the Yermak Plateau and the shelf off Nordaustlandet. If the thermal subsidence of the western Yermak Plateau can be traced to the latest heating episode then it is likely that the crust is similar to oceanic in composition and not older than 13 m.y. (approximately 20 m.y. younger than the northeastern segment of the plateau). Plate rotation shows that there was no room for the western segment of the plateau prior to anomaly 7. We postulate that the original transform is associated with the Hornsund Fault zone. In response to deviatoric stress across the oblique ridge-transform system, the Nansen Ridge propagated southwestward aborting the old transform trace, and shifted to its present position.It is suggested that this propagation and migration of the ridge-transform system across a zone of extensional deviatoric stress allowed the massive intrusion of basalt forming the Western Yermak Plateau. The propagation phenomenon coincides with large-scale Tertiary volcanic activity on Svalbard.Readjustment and migration of the oblique transform is still taking place. As the transform-ridge system is liberated from continental constraints, the migration rate will diminish as orthogonality is approached.  相似文献   

15.
通过中国北方５．３０Ｍａ以来含丰富哺乳动物化石的典型剖面的磁性地层研究,建立５．３０Ｍａ以来中国北方陆相哺乳动物群与磁性地层序列对比。中国黄土记录了Ｂｒｕｎｈｅｓ正极性带与Ｍａｔｕｙａｍａ负极性带,黄土年龄约２．５０ＭａＢ．Ｐ．,其中发现有午城动物群、公王岭动物群、阳郭动物群、陈家窝动物群与靖远动物群。三门组记录了Ｍａｔｕｙａｍａ负极性带下段,年龄约１．２０—２．５０Ｍａ,其中发现有三门动物群。静乐剖面的静乐红土对应于Ｇａｕｓｓ正极性带上段,年龄约２．５０—３．００Ｍａ,其中发现有静乐贺丰动物群。游河组记录了Ｇａｕｓｓ正极性带,年龄约２．５０—３．４０Ｍａ,其中发现游河动物群。泥河湾沉积对应于Ｂｒｕｎｈｅｓ正极性带、Ｍａｔｕｙａｍａ负极性带、Ｇａｕｓｓ正极性带,年龄约０．１３—３．４０Ｍａ。其中小长梁剖面记录了Ｂｒｕｎｈｅｓ正极性带与Ｍａｔｕｙａｍａ负极性带上段（含Ｊａｒａｍｉｌｏ亚带）,年龄约０．１３—１．００Ｍａ（其中含化石及石器地层年龄约０．９０—１．００Ｍａ）。榆社盆地麻则沟组记录了Ｇａｕｓｓ正极性带（２．５０—３．４０Ｍａ）,高庄组记录了Ｇｉｌｂｅｒｔ负极性带（３．４０—５．３０Ｍａ）。  相似文献   

16.
ABSTRACT

New analysis of the geophysical data of the ultraslow-spreading Mohns Ridge and its off-axis structure reveals a distinctive asymmetric structure. We calculate residual bathymetry (RB) and residual mantle Bouguer gravity anomaly (RMBA) and decompose the anomalies into symmetric and asymmetric components between the ridge conjugates. The western flank of the Mohns Ridge at crustal age of ~50–15 Ma is characterized by a broad zone of elevated RB and more negative RMBA, which we term the Vesteris Plateau (VP). The VP anomaly has a surface area of ~1.12 × 105 km2 and an excess crust volume of ~2.33 × 105 km3, making it a significant anomaly comparable to other anomalies such as the Bermuda Rise. Extending north of the Kolbeinsey Ridge for more than 500 km, the VP lies above an anomalous upper mantle region of low shear-wave seismic velocity, indicating that the VP might represent the northernmost reach of the Iceland-Jan Mayen mantle anomaly. In addition, the western ridge flank of the Mohns Ridge at crustal age of 6–0 Ma is associated with higher RB and more positive RMBA relative to the eastern conjugate, indicating tectonic uplift and associated exposure of lower crust and upper mantle near the ridge axis.  相似文献   

17.
A thorough examination of geophysical data from the Greenland-Norwegian Sea, Eurasia Basin and southern Labrador Sea shows significant asymmetry of several parameters (basement topography adjusted for sediment loading, free-air gravity anomaly, spreading half-rate and seismicity) with respect to crustal age:
1. (1) Average zero-age depth (0–57 m.y. B.P.), depth of highest rift mountain summits, and depth to magnetic basement (10–30 km from axis of Mohns and Knipovich ridges) is less on the North American plate flanks. The zero-age depth asymmetry is 400–500 m for the Eurasia Basin (0–57 m.y. B.P.) and for Mohns Ridge (57-22 m.y. B.P.), and 150–200 m for younger Mohns Ridge crust (22-0 m.y. B.P.) and for the extinct Aegir Ridge (57-27 m.y. B.P.). There is little or no asymmetry in the Labrador Sea except near the extinct rift valley, where the east flank is 150–300 m shallower. Magnetic depth-to-source computations provide an independent confirmation of basement asymmetry: The belts 10–30 km from the axis of Mohns and Knipovich ridges are 100–150 m shallower on the west flank of these ridges. The shallower ridge flank is topographically rougher, so that average rift mountain summits are 300 m shallower on the west flanks of the Mohns-Knipovich ridges, a larger asymmetry than for average zero-age depth. The amount of topographic asymmetry is greatest near the Mohns-Knipovich bend. Asymmetry appears to be greatest for ridges oriented normal to the spreading direction, and less for oblique spreading.
2. (2) Free-air gravity anomaly asymmetries of +5 to +20 mGal ( + sign indicates west flank is more positive) are associated with topographic asymmetry at least within 10–15 m.y. of the axis of Mohns and Knipovich ridges. Gravity is reduced on the older flanks west of the extinct Mid-Labrador Ridge and east of Mohns Ridge; asymmetric crustal layer thicknesses or densities provide one possible explanation, although deep-seated sources (e.g., mantle convection), unrelated to the crust, cannot be excluded.
3. (3) Spreading half-rate was about 5–15% lower on the North American plate flanks of Mohns Ridge (57-35 m.y.) and in the Eurasia Basin (0–57 m.y.); thus the fast-spreading flank tends to produce deeper, smoother crust. However, topographic asymmetry cannot relate only to spreading-rate asymmetry, since for the young Mohns Ridge crust (<9 m.y. B.P.) faster spreading and higher topography are both associated with the west flank.
4. (4) Mid-plate seismicity is higher on the Eurasia (eastern) flank of Mohns and Knipovich ridge, but this effect may be unrelated to the other three.
The fluid-dynamical model of Stein et al. correctly explains the sense of spreading-rate asymmetry (the North American plate, moving faster over mantle, is growing more slowly). However, the other asymmetries and their causal relationships remain theoretically unexplained.  相似文献   

18.
亚洲夏季风是全球季风系统的重要组成部分,亚洲夏季风的变化对其控制区域自然生态系统的多样性和生态平衡,以及社会经济发展有重要的影响。本文选择位于现代亚洲夏季风边缘区对季风变化响应敏感的湖泊达连海为研究对象,基于陆生植物残体和全有机质的AMS14C定年建立了钻孔顶部24.6 m沉积物的年代框架,利用粒度指标重建了全新世研究区水文变化过程以及亚洲夏季风衰退事件序列。结果显示,沉积物中存在数层砂层,代表了湖泊低水位时期,进而指示了亚洲夏季风衰退事件。这些事件处在11.6~11.3 cal.ka B.P.、10.4~9.5 cal.ka B.P.、6.4~6.0 cal.ka B.P.、4.6~4.4 cal.ka B.P.、3.7~3.4 cal.ka B.P.、3.1~2.9 cal.ka B.P.以及2.0~0.9 cal.ka B.P.,可以发现中晚全新世以来亚洲夏季风衰退事件发生的频率显著增加。进一步与北半球高纬地区与低纬地区的气候突变事件记录对比显示,全新世百年-千年时间尺度上亚洲夏季风强度的变化与低纬ENSO活动存在密切的联系。  相似文献   

19.
Multichannel seismic reflection data from the Southern Kerguelen Plateau show many dipping basement reflectors associated with volcanic flows. These reflectors are quite similar in their shape to seaward-dipping basement reflectors observed along volcanic passive margins. On the Kerguelen Plateau the sources are updip of the basement reflectors, in the presently extinct and eroded volcanoes. We suggest that the same source/reflector geometry may also apply to the seaward-dipping basement reflectors observed along passive margins. We interpret these reflectors to be the result of volcanism on the passive margin which flowed in all directions into the newly created ocean basin at an early spreading stage.  相似文献   

20.
青藏高原东缘处于不均衡状态，自西而东可分为青藏高原弱负均衡重力异常区、龙门山正均衡重力异常区和四川盆地负均衡重力异常区，表明该区的不均衡状态并未导致Airy均衡运动的产生，即龙门山没有均衡下降，而处于不断的隆升状态，显示该地区反均衡运动的构造抬升是导致龙门山隆升的主因。本次采用似三度体重力异常计算方法对该区的正均衡重力异常进行模拟和反演，研究了大尺度地貌分异与均衡重力异常分区之间的相互关系，结果表明，龙门山的下地壳顶面抬升了11．2～12．6km，造成了龙门山的正均衡异常，揭示了构造抬升和剥蚀作用在相似的时间尺度上和空间尺度上控制着龙门山地貌的形成，龙门山的表面隆升是构造隆升和剥蚀作用相叠加的产物。  相似文献   

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