共查询到20条相似文献,搜索用时 31 毫秒
1.
Bo A. Sjöberg 《Studia Geophysica et Geodaetica》1993,37(4):415-422
Summary The Maastrichtian and Danian limestone sequence known from Limhamn (southern Sweden) contains ferromagnetic minerals dispersed in a diamagnetic matrix. The initial magnetic susceptibility measured does, on the whole, agree with the iron amounts registered by X-ray. The aluminium contents indicate that the magnetic minerals are of terrigenous origin. 相似文献
2.
B. A. Sjberg 《Physics of the Earth and Planetary Interiors》1992,70(3-4):187-193
A 290 m sequence of Maastrichtian and Danian limestones from Limhamn, southern Sweden, has been investigated magnetically. The observed mean natural remanent magnetization is 24 μ A m−1. The remanent magnetization is carried by pseudo-single-domain or single-domain magnetite particles, and was observed by various rock magnetic methods. Though influenced by diamagnetism, the anisotropy was determined. Paleomagnetically, polarity changes could be determined in several samples in spite of the low remanences measured. The results are in agreement with the magnetostratigraphy based upon ocean floor anomalies. 相似文献
3.
Jaume Dinarès-Turell Juan Ignacio Baceta Xabier Orue-Etxebarria Stefano Lorito 《Earth and Planetary Science Letters》2003,216(4):483-500
The magnetostratigraphy of a 54-m-long section above the Cretaceous-Tertiary boundary at the sea-cliff section of Zumaia in the Basque basin (northern Spain) has been established. The section encompasses the entire Danian and the lower part of the Selandian stages as indicated by calcareous plankton biostratigraphy. The studied interval consists of (hemi)pelagic limestone-marl alternations in the form of couplets and bundles, which range from centimetre/decimetre to metre scale respectively and a few thin-bedded calcareous turbidites. The magnetostratigraphy, based on samples from about 200 stratigraphic levels, allows the identification of six reversal boundaries from chron C29r to C26r at a bed level. The spatial (or temporal) evolution of periodicities from a lithologically coded series is studied with the continuous wavelet transform technique. A preliminary age model based on the standard CK95 GPTS indicates that the basic lithologic carbonate-marl couplet corresponds to the 19-23-kyr precession cycle (21-31-cm cycle in the depth domain) and that a bundle cycle (usually groups of four to six basic couplets) with global periodicity centred at 1.22 m corresponds to the ∼110-kyr eccentricity cycle. We have tuned the bundle cycles to the Va03_R7 eccentricity orbital solution [Astrophys. J. 592 (2003) 620-630] following an initial match of a node of the ∼2.4-Ma eccentricity modulatory cycle in the target time series to particularly carbonate-rich bundles from the upper part of the Zumaia section that displays significant power of a 4.4-m-period cycle corresponding to the ∼404-kyr eccentricity cycle. Consistency between lithologic patterns and characteristics in the eccentricity target is reasonably met although the ∼404-kyr eccentricity cycle is not persistent throughout. The tuning, however, appears robust as it brings the age of the K/T boundary at ∼65.8 Ma. It is argued that a sea-level signal (tectonically driven?) is superimposed on the climatic forcing at the Milankovitch band masking the full expression of the low-frequency astronomical periods. We provide a cycle-tuned duration for all intervening Early Palaeocene polarity chrons and estimate relative ages for bioevents. The cycle-tuned chronology indicates that the CK95 GPTS overestimates the duration of chrons C28 and C27 by 20 and 26% respectively. Our data may prove useful to better constrain Early Palaeocene biostratigraphy of calcareous plankton and in the redefinition of the boundary between the Danian and Selandian stages. 相似文献
4.
This paper introduces how the ratioR of the characteristic stiffness of rock samplevs. the stiffness of testing machines would influence the rupturing process and the acoustic emission (AE) on the part of the
tested rock samples. Result of the experiment shows: WhenR>0.20, the rock sample would rupture abruptly; whereas whenR<0.20, the rock samples would rupture slowly. When the samples rupture abruptly, the time-dependent variation of the AE rate
takes such a pattern:peak value—stable low values—rises to the maximum value (concentration)—drops back to the minimum value
(quiescence)—(rises again)—ruptures. Moreover, smallerR-value tends to be associated with longer quiescence and vice versa. WhenR>1.50, no pre-failure quiescence is detected. When the rock samples rupture slowly, the variation pattern of the AE rate (after
the stress has increased to more than 50% of the rupturing stress) is as the following:stable low (or high) values—rises (or
drops) to its maximum (or minimum) values and then continues for some time—ruptures.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 223–233, 1991.
This study is supported by the Chinese Joint Seismological Science Foundation. 相似文献
5.
Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis
of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial
and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis
measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex
magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in
composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most
sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about
72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α95=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history
of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of
the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing40Ar/39Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed
Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of
the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr. 相似文献
6.
E. L. Afraimovich I. K. Edemsky S. V. Voeykov Yu. V. Yasukevich I. V. Zhivetiev 《Geomagnetism and Aeronomy》2010,50(1):79-95
The morphology of medium-scale traveling wave packets is for the first time presented based on the total electron content
(TEC), measured at the global network of GPS receivers (up to 1500 stations) during the long period (from 1998 to 2007) and
at the GPS/GEONET dense Japan network (1220 stations) in 2008—2009. In the time domain, these packets are chains of narrowband
TEC variations (trains) with a duration of about 1—2 h, a total duration of up to 6 h, and a variation period of 10—30 min.
In the winter Northern Hemisphere, traveling wave packets are observed mostly 3 h after the passage of the morning solar terminator.
In the equinox they appear after the passage of the solar terminator without a pronounced delay or advance. In summer traveling
wave packets are registered 1.5—2 h before the appearance of the evening solar terminator at the observation point when the
solar terminator passes in the magnetically conjugate region. The spatial structure of traveling wave packets is characterized
by a high degree of anisotropy and coherence at a distance larger than ten wavelengths (the wavelength is 100—300 km). A high
quality of the oscillatory system and synchronization with the appearance of the solar terminator at the observation point
and in the magnetically conjugate region indicate that the generation of traveling wave packets by the solar terminator is
of the MHD nature. Our results for the first time experimentally confirm the hypothesis that the solar terminator generates
ion sound waves, proposed by Huba et al. [2000b]. 相似文献
7.
Under normal temperature, the creep experiments with complete samples of Gabbro and Marble rocks are made under uniaxial compression.
It is found that at the instantaneous creeping stage, AE activities increase with loading; at the steady creeping stage, large
AE signals may appear at lower background of AE action, andm—value which shows the relationship between AE amplitude and frequency keeps stable on the low value or decreases; at the
accelerate creeping stage, AE activities increase andm—value decreases quickly or decreases again after recovery. These experimental results are related to the quality of the samples.
In this paper, AE activities during three stages of creeping process are connected with the seismic patterns (for exampleb—value, foreshocks). Finally, it is pointed out the possibility that the foreshock—mainshock—aftershock earthquake sequence
has been formed by the mechanism of creep fracture of crustal rocks.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 104–112, 1991.
This subject is sponsored by the Chinese Joint Seismological Science Foundation. 相似文献
8.
Jacek Grabowski János Haas Emö Márton Andrzej Pszczółkowski 《Studia Geophysica et Geodaetica》2010,54(1):1-26
Jurassic-Cretaceous sediments of Transdanubian Range in Northern Hungary mostly retain their primary magnetizations and are
suitable for detailed bio- and magnetostratigraphic studies. The Lókút section, 13 m in thickness, is localized in the central
part of the Transdanubian Range. It contains the Jurassic/Cretaceous boundary in pelagic carbonate facies. Although the colour
of the rocks changes from reddish-pinkish in the bottom to almost white at the top of the section, magnetite was identified
as a magnetic carrier without evidence of hematite. Integrated bio- and magnetostratigraphical investigations resulted in
construction of chronostratigraphical scheme. The section, embraces magnetozones from M21r to M18r, of upper Lower Tithonian
(Parastomiosphaera malmica Zone) to Lower Berriasian age (Calpionella alpina Subzone). Sedimentation rate of pelagic limestones
increased from 1–3 m/My during Tithonian to 5–7 m/My during Berriasian. The sedimentation rate and its changes up the section
are comparable to those from the Jurassic/Cretaceous boundary sections of Trento plateau (Southern Alps, Italy) — sedimentary
environments of Trento plateau and central Transdanubian Range in that time might be similar. Sedimentation rate within Umbrian
Apennine basins and Križna unit in the Western Tatra Mts. seems significantly higher. Analysis of rock magnetic parameters
reveals that detrital input was much lower into the Lókút section than into Križna basin in the Tatra Mts. (Zliechov trough).
Increase of sedimentation rate occurs in both sections in the Upper Tithonian — Lower Berriasian. It coincides with the onset
of calpionellid limestone facies and is related to increased productivity of calcareous micro- and nannoplankton. Detailed
correlation of both sections basing on rock magnetic parameters and susceptibility changes is, however, not possible. They
are dependent mostly on the local sedimentary conditions (Bakony Mts. — deep water plateau; Križna unit — deep water trough)
and correlation with any “global” paleoenvironmental (climatic, eustatic) trends is not straightforward. 相似文献
9.
A mathematical model is presented describing the vertical concentration gradient of a chemical species in the sediments of
a lake as a function of external input variation and of characteristic parameters (mass sedimentation rate or sinking velocity,
distribution factor between dissolved and particulate phase, hydraulic loading). Delayed response of external input variations
on sedimentation due to finite species residence times is taken into account. The model is tested for Pb, Cd, Cu, and Zn in
Greifensee (Switzerland) where—based on sediment data—an increase of external input of these metals by a factor 2.5 (Pb, Cd,
Cu) and 4 (Zn) is found for the last 35 years.
相似文献
10.
E. I. Suetnova 《Izvestiya Physics of the Solid Earth》2010,46(6):529-537
The problem of compaction of depositing sediments is studied for different geophysical characteristics of the process of sedimentation.
A model is proposed for describing the compaction of sediments with growing thickness. The model is based on the modern concepts
of the viscoelastic rheology of accumulating sediments; and, in terms of mathematics, it is a system of nonlinear differential
equations in partial derivatives, which describe the processes of compaction and filtration with an increasing thickness of
sediments. I have studied numerically the dependence of the regimes of compaction on the formulated characteristic criteria
of similarity in the system of equations. Fluid-dynamic criterion, which characterizes the ratio of the rate of sedimentation
to the rate of the filtration of a fluid (the rate of fluidization), and the rheological criterion of compaction, which characterizes
the proportion between the viscous and elastic strains on the time scale of the process of compaction, i.e., the ratio of
the two time scales—the time scale of the elastic strains’ relaxation and the time scale of the viscous compaction. The complex
modeling of the mechanics and hydrodynamics of the interrelated processes of compaction and movement to the surface of the
saturating fluids with an increase in the thickness of sediments reproduces the mechanism of the formation of different regimes
of compaction, including those, which lead to an anomalously high pore pressure and to significant, in some circumstances
step-like rise in the pressure of the saturating fluid, frequently observed at depths of 2–3 km. 相似文献
11.
Bo A. Sjöberg 《Studia Geophysica et Geodaetica》1992,36(1):42-50
Summary The magnetic fabric of the Limhamn limestones is characterized by a low initial magnetic susceptibility. Of 68 specimens with a positive susceptibility, 10 show a distinct anisotropy. In nine of these, the axes of minimum susceptibility are practically normal to the magnetic foliation plane. The observed anisotropy pattern has some resemblance to a magnetic fabric acquired by deposition under influence of currents. A post-depositional origin due to gravitational compaction seems, however, more probable.Presented at 2nd conference on New Trends in Geomagnetrsm, Castle of Bechyn, Czechoslovakia, September 24–29, 1990. 相似文献
12.
The fast onset of a substorm—a substorm “explosion”—is usually associated with the moment of stability loss of the magnetoplasma
equilibrium in the geomagnetic tail. The origination of such a process either from the near-Earth part of the plasma sheet
or from its remote part, which is highly stretched into the tail, is now being studied theoretically and verified experimentally
(at the present time, in the THEMIS project). In the first case, the resulting disturbance must have the form of a ballooning
mode; in the second case, of tearing perturbation. However, in both cases, this stability loss, i.e., a quick breakdown in
the balance, replacing the slow quasi-static evolution of configuration, can only occur as a nonlinear process. Taking into
account the specific properties of the configuration and possible disturbances in it, we indicate why such a process cannot
be the previously proposed “substorm detonation.” It is shown that a suitable mathematical model is a nonlinear dynamical
bifurcation occurring on a small time scale, with a delay relative to the moment of passing the marginally stable state. 相似文献
13.
Summary We have studied 454 oriented samples from seven loess outcrops in the Czech Republic for comparison and correlation of the
magnetic properties with those of the loess profiles in China and Central Asia. Three sections at Sedlec (Prague), Zeměchy
and Dolní Věstonice cover the time span of the last glacial-interglacial cycle. Loess from the middle Pleistocene period including
pedocomplexes V and VI was studied at Sedlešovice (Znojmo) and Karlštejn. Loess of uncertain — possibly lower to middle Pleistocene
age — was sampled at Sedlec near Mikulov. At Červeny Kopec (Red Hill, Brno) we investigated the oldest loess near the Matuyama/Brunhes
(M/B) boundary. The characteristic remanence (ChRM) determined after magnetic clearning is of normal polarity throughout all
sections except at Červeny Kopec. There, two polarity changes from reversed to normal were found within two sections initially
thought to represent two successive stratigraphic intervals. However, the marked similarities between lithological, susceptibility,
declination and inclination profiles lead us to suggest that the two polarity changes are in fact one and the same. The two
sections are therefore laterally equivalent and both have recorded the M/B field reversal. The susceptibility variations were
used to mutually correlate the Czech sections and to construct a composite profile across the present day loess outcrops in
the Czech Republic. They also serve as a paleoclimate proxy which can be compared and correlated with the paleoclimatic records
observed in the Chinese and Central Asian loess. The origin of the low field susceptibility variations was studied by measuring
the frequency dependence of susceptibility which revealed that the enhancement within the pedocomplexes is controlled by the
amount of fine-grained ferromagnetic minerals present. 相似文献
14.
Relationship between coral growth rate and sea surface temperature in the northern part of South China Sea during the past 100 a 总被引:4,自引:0,他引:4
Baofu Nie Tegu Chen Meitao Liang Youqiang Wang Jinliang Zhong Yuanzhi Zhu 《中国科学D辑(英文版)》1997,40(2):173-182
To research the relationship between coral growth rate and sea surface temperatnre (Tss), 5 cores of livingPorites lutea were collected from the Xisha Islands and the southern Hainan Island waters and measured. The results of the study show that
there is an obviously positive correlation between the coral growth rates and theTss records from the northern part of South China Sea. The annual growth rates of the five samples ofPorites lutea during the past 100 a are in the range of 7—15 mm/a, and their mean value is 11 mm/a. The correlation coefficients between
the coral growth rates and the Tss records from the waters during 1961—1993 are in the range of 0.77—0.89. As a result, a thermometer of the coral growth rate
is established. A hindcasting Tss, in the waters from 1993 to 1961 has been obtained with an error of about 0.12—0.17°C. Based upon the calculated result,
the rising rate of Tss in the northern part of South China Sea during the past 100 a is 0.20°C. which is higher than that of the air temperature
in China (0.09°C/100 a), but lower than that of the global temperature and that of Tss in the tropical western Pacific Ocean.
Project supported by the National Nutural Science Foundation of China and the Multidesciplinary Oceanographic Expedition Team
of Chinese Academy of Sciences to the Nansha Islands. 相似文献
15.
In this paper, we attempt to use satellite gravity data and a new inversion method to study the lateral density anomaly distribution
in the mantle.
First, density difference Δρ(τ,θ,φ) is expanded in terms of a three—dimensional orthogonal function system, the coefficients of the expansion are to be determined.
Then, a set of observation equations is established from the relationship between density anomaly and disturbing geopotential.
In the equations the unknown vector contains the coefficients of density anomaly expansion, the observational vector is obtained
by computing geopotential perturbations using the potential coefficients of GEM10B, and a filtering process is done for the
observational values by properly selecting the harmonic degrees of geopotentical. Finally, the lateral density variations
in the lower mantle (670 km toCM boundary) are investigated. In this case, the degrees of disturbing geopotential are selected as 2–11, the truncated degrees
of density anomaly expansion are taken asL=6 andK=4, and the damping least squares method is used to solve the observation equations.
The resulting model shows the high level of density perturbations at 670 km discontinuity and core — mantle boundary, a high
— density zone circumscribing the Pacific and a lower—density region under the center of Pacific. These features are in agreement
with the three—dimensional seismic velocity variation features by Dziewonski (1984). In the Antarctic region and some parts
of Atlantic and Indian Ocean, however, the resulting density anomalies are negatively correlated with the seismic velocity
anomalies, the cause resulting in these phenomena is preliminarily analysed in this paper.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 53–65, 1991.
The principle and method represented in this paper can also be suitable to study the lateral density anomaly distribution
in the earth’s crust and the upper mantle. 相似文献
16.
Metamorphic characteristics and geotectonic implications of the high-pressure granulites from Namjagbarwa, eastern Tibet 总被引:20,自引:0,他引:20
A large area of high-pressure garnet-kyanite granulite is exhumed in the Namjagbarwa area, which provides a window for observing
the deep crust rocks and structures of the Tibetan Plateau. Three mineral assemblages can have been distinguished in the garnet-kyanite
HP granulites by petrography, i.e. M1. Mus+Bi+P1+Q, M2. Gt+Ky +perphite/antiperphite+Rt+Q, M3. Gt+Sill+Cord+Sp+Ilm ± Opx. Metamorphic conditions of the peak granulite assemblages (M2) formatted by thickening of crusts, with available isotopic ages of 45–69 Ma, are at 1.4—1.8 Gpa and 750—850°. Their retrograde assemblages overprinted by decompressure during the uplift, with available isotopic ages of 18—23 Ma, were
formed at 0.60—0.70 Gpa, 621—726°. The thermobarometric evaluation, petrogenetic grid and corresponding isotopic ages indicate
a clockwise isothermal decompression metamorphic path. The HP granulite metamorphic history indicates that the collision of
the Indian Plate with the Eurasian Plate had begun at 70 Ma, far earlier than the widely accepted 45 Ma.
Project supported by the National Natural Science Foundation of China (Grant No. 49732100), the National Key Project for Basic
Research, and the Chinese Academy of Sciences Project for Tibetan Research Project (GrantNos. KZ951-A1-204, KZ95T-06). 相似文献
17.
Physicochemical and microbiological characteristics of the bathing waters in Varna’s Black Sea coastal area were investigated
during year 2007 at 23 monitoring stations. Most of the determined physicochemical parameters—pH, mineral oils, surface active
substances, phenols, dissolved oxygen, nutrients and microbiological parameters—“Total coliforms”, “Faecal coliforms”, “Faecal
streptococci” were in compliance with the guidline’ limits and exhibited good water quality. Ammonium and phosphate’s pollution
above the limits was determined at the South beach, Officers beach and Central beach situated in Varna’s central bathing zone.
For the period of 13.08 to 24.10.2007—70% of the South beach samples analyzed for NH4+ exceeded the limits 60 times and the concentrations of PO43− exceeded the limits 17.5 times. Some deviations from the guidline’ limits regarding the microbiology were exhibited at the
same beaches. A conclusion is made that the area of study is not yet seriously threatened, in spite of the rapid recreation
during the last years. 相似文献
18.
Geochemical research on C—O and Sr—Nd isotopes of mantle-derived rocks from Shandong Province,China 总被引:1,自引:0,他引:1
This paper presents systematic studies on the C—O and Sr—Nd isotopic compositions for Cretaceous Badou carbonatites, Fangcheng
basalts, and Jiaodong lamprophyres and Paleozoic Mengyin kimberlites in Shandong Province, China. Paleozoic kimberlites have
normal and uniform C—O isotopic compositions with δ13C and δ18O in the range of −4.8‰—−7.6‰ and +9.9‰—+13.2‰, respectively. However, Cretaceous three different types of mantlederived rocks
have quite different C—O isotopic compositions, indicating that the mantle sources are probably partially contaminated with
organic carbon-bearing crustal materials. These Cretaceous rocks show uniform and EMII-like Sr—Nd isotopic compositions and
also indicate that the mantle sources were affected by recycled crustal materials. Comparative studies of C—O and Sr—Nd isotopes
reveal that the lithospheric mantle beneath the eastern North China Craton had different isotope characteristics in the Paleozoic,
the early Cretaceous, and the Tertiary time. This demonstrates that the lithospheric mantle beneath the region underwent at
least twice reconstructions since the Paleozoic. Available data imply that the first reconstruction mainly happened during
the Triassic-Jurassic time with gradual changes and the second in the Cretaceous with abrupt changes. Results also show that
the early Cretaceous (especially at 120-130 Ma) was perhaps the key period leading to the dramatic change of the Mesozoic
geodynamics on the eastern North China Craton. 相似文献
19.
G. V. Vergasova E. S. Kazimirovskii N. M. Polekh J. Xiong L. Liu 《Geomagnetism and Aeronomy》2011,51(6):762-773
Results of studies of the wave structure of the critical frequencies of the ionospheric F2 layer with periods of planetary waves for two Asian stations—Irkutsk and Wuhan (China)—are presented. Estimates of the appearance
frequency, amplitudes, and the lifetime of oscillations with periods typical of planetary waves (2–25 days) are obtained.
It is shown that these characteristics depend on the season and place of observation. The appearance of joint periodicities
in the critical frequencies at both stations, as well as in the planetary index of geomagnetic activity Ap, is noted. 相似文献
20.
Age of Seychelles–India break-up 总被引:1,自引:0,他引:1
J.S. Collier V. Sansom O. Ishizuka R.N. Taylor T.A. Minshull R.B. Whitmarsh 《Earth and Planetary Science Letters》2008,272(1-2):264-277
Many continental flood basalt provinces are spatially and temporally linked with continental break-up. Establishing the relative timing of the two events is a key step in determining their causal relationship. Here we investigate the example of the Deccan Traps and the separation of India and the Seychelles. Whilst there has been a growing consensus as to the age of the main phase of the Deccan emplacement (65.5 ± 1 Ma, chron 29r), the age of the rifting has remained unclear. We resolve this issue through detailed seafloor magnetic anomaly modeling (supported by wide-angle and reflection seismic results) of the north Seychelles and conjugate Laxmi Ridge/Gop Rift margins, and geochemistry and 40Ar/39Ar geochronology of rocks from the north Seychelles margin. We show that syn-rift volcanics offshore the Seychelles Islands in the form of seaward-dipping reflectors were most likely erupted during chron 28n, and the first organized seafloor spreading at the Carlsberg Ridge also initiated during this chron at 63.4 Ma. The severing of the Seychelles occurred by a south-eastward ridge propagation that was completed by the start of chron 27n (~ 62 Ma). A brief, pre-28r phase of seafloor spreading occurred in the Gop Rift, possibly as early as 31r–32n (~ 71 Ma). Initial extension at the margin therefore preceded or was contemporaneous with the Deccan emplacement, and separation of the Seychelles was achieved less than 3.5 Ma afterwards. This is the shortest time interval between flood basalt emplacement and break-up yet reported for any continental flood basalt-rifted margin pair. A contributing factor to the apparently short interval in the Deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere. However, we conclude that external plate-boundary forces, rather than the impact of a mantle plume, were largely responsible for the rifting of the Seychelles from India. 相似文献