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261.
Recent seismic field work has revealed high lower-crustal velocities under Ninetyeast Ridge, Indian Ocean, indicating the presence of crustal underplating ( Grevemeyer et al . 2000 ). We used results from Ocean Drilling Program (ODP) drill cores and cross-spectral analysis of gravity and bathymetric data to study the impact of the underplating body on the subsidence history and the mode of isostatic compensation along Ninetyeast Ridge. Compared with the adjacent Indian basin, the subsidence of Ninetyeast Ridge is profoundly anomalous. Within the first few millions of years after crustal emplacement the ridge subsided rapidly. Thereafter, however, subsidence slowed down significantly. The most reliable model of isostasy suggests loading of a thin elastic plate on and beneath the seafloor. Isostatic compensation of subsurface loading occurs at a depth of about 25 km, which is in reasonably good agreement with seismic constraints. Subsurface loading is inherently associated with buoyant forces acting on the lithosphere. The low subsidence may therefore be the superposition of cooling of the lithosphere and uplift due to buoyant material added at the base of the crust. A model including prolonged crustal growth in the form of subcrustal plutonism may account for all observations. 相似文献
262.
The Raobazhai ultramafic body of the North Dabie Complex is re-interpreted as a mantle-derived peridotitic slice enclosed in, and isofacially metamorphosed with, surrounding granulite-to-amphibolite facies gneisses. The ultramafic sheet consists mainly of metaharzburgite, but includes subunits of metadunite and mylonitic lherzolite. The rocks contain spinel but neither garnet nor plagioclase. However, in the mylonitic lherzolite, fine-grained intergrowths of spinel, orthopyroxene and clinopyroxene outline domains resembling the habit of garnet in two dimensions; broad-beam microprobe analyses imply pseudomorphs after a pyropic garnet precursor. The mineral assemblage of the metadunite and metaharzburgite is: olivine (Fo92 )+orthopyroxene (En92 )+tremolitic-to-magnesiohornblende+Mg–Al-chromite, indicating amphibolite facies recrystallization. The mineral assemblage of the mylonitic lherzolite is: olivine (Fo90 )+orthopyroxene (En90 )+clinopyroxene+Cr-bearing spinel+pargasitic amphibole, indicative of granulite-to-amphibolite facies metamorphism. Phase equilibria and geothermometric estimations show that the Raobazhai meta-ultramafics have undergone at least three stages of recrystallization: (I) 950–990 °C, (II) 750–860 °C, and (III) 670–720 °C, assuming equilibrium in the spinel peridotite stability field ( c. 6–15 kbar), although an early, high-pressure stage (≥18 kbar) is probable, based on the inferred garnet pseudomorphs. Petrochemical and geothermobarometric data suggest that the ultramafic slice represents a fragment of the mantle wedge, tectonically incorporated into subducted continental crust and re-equilibrated at granulite-to-amphibolite facies conditions while being exhumed to shallow levels. 相似文献
263.
GOCE gravitational gradients along the orbit 总被引:6,自引:3,他引:3
Johannes Bouman Sophie Fiorot Martin Fuchs Thomas Gruber Ernst Schrama Christian Tscherning Martin Veicherts Pieter Visser 《Journal of Geodesy》2011,85(11):791-805
GOCE is ESA’s gravity field mission and the first satellite ever that measures gravitational gradients in space, that is,
the second spatial derivatives of the Earth’s gravitational potential. The goal is to determine the Earth’s mean gravitational
field with unprecedented accuracy at spatial resolutions down to 100 km. GOCE carries a gravity gradiometer that allows deriving
the gravitational gradients with very high precision to achieve this goal. There are two types of GOCE Level 2 gravitational
gradients (GGs) along the orbit: the gravitational gradients in the gradiometer reference frame (GRF) and the gravitational
gradients in the local north oriented frame (LNOF) derived from the GGs in the GRF by point-wise rotation. Because the V
XX
, V
YY
, V
ZZ
and V
XZ
are much more accurate than V
XY
and V
YZ
, and because the error of the accurate GGs increases for low frequencies, the rotation requires that part of the measured
GG signal is replaced by model signal. However, the actual quality of the gradients in GRF and LNOF needs to be assessed.
We analysed the outliers in the GGs, validated the GGs in the GRF using independent gravity field information and compared
their assessed error with the requirements. In addition, we compared the GGs in the LNOF with state-of-the-art global gravity
field models and determined the model contribution to the rotated GGs. We found that the percentage of detected outliers is
below 0.1% for all GGs, and external gravity data confirm that the GG scale factors do not differ from one down to the 10−3 level. Furthermore, we found that the error of V
XX
and V
YY
is approximately at the level of the requirement on the gravitational gradient trace, whereas the V
ZZ
error is a factor of 2–3 above the requirement for higher frequencies. We show that the model contribution in the rotated
GGs is 2–35% dependent on the gravitational gradient. Finally, we found that GOCE gravitational gradients and gradients derived
from EIGEN-5C and EGM2008 are consistent over the oceans, but that over the continents the consistency may be less, especially
in areas with poor terrestrial gravity data. All in all, our analyses show that the quality of the GOCE gravitational gradients
is good and that with this type of data valuable new gravity field information is obtained. 相似文献
264.
K. Vijaya Kumar W. G. Ernst C. Leelanandam J. L. Wooden M. J. Grove 《Contributions to Mineralogy and Petrology》2011,162(4):867-888
In a geochemical and geochronological investigation of Archean and Proterozoic magmatism in the Nellore Schist Belt, we conducted
SHRIMP U–Pb analyses of zircons from two cospatial granitic bodies at Guramkonda and Vendodu. The former is a Ba- and Sr-rich
hornblende-bearing tonalite, whereas the latter is a Rb-, Zr-, Pb-, Th-, U-, and REE-rich biotite-bearing leucogranite. The
Guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt, whereas the Vendodu leucogranite contains
residual/partially melted plagioclase grains. Both rock types contain two generations of zircon: tonalite contains a group
of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt
(quartz + feldspar matrix), and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase
as well as a group of zircons with visible cores mantled by later rim growth. Cathodoluminescence images also clearly document
two distinctly textured varieties of zircon: the tonalite contains a population characterized by narrowly spaced uninterrupted
oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark (U-rich) and light
(U-poor) regions; the leucogranite contains U-rich zoned zircons and U-poor zircon cores mantled by U-rich rims. The REE chemistry
of zircon cores from the Vendodu leucogranite is very similar to the REE of zoned zircons from the Guramkonda tonalite. Zircon
ages from both plutons exhibit bimodal distributions in U–Pb concordia diagrams. The tonalite defines an age of 2,521 Ma ± 5 Ma
for zoned magmatic zircons and 2,485 Ma ± 5 Ma for unzoned newly
precipitated zircons, whereas the leucogranite has an age of 2,518 Ma ± 5 Ma for U-poor zircon cores (relics of the tonalite pluton) and
2,483 Ma ± 3 Ma for U-rich zoned magmatic zircons. The trace element geochemistry of the ~2,520 Ma zircons is distinctly different
from the ~2,485 Ma zircons, irrespective of the host rock. Our textural, CL image, and SHRIMP U–Pb analyses document the origin
of the leucogranite by partial melting of the tonalite. High alkalis (Na2O + K2O), Rb, Nb, HREE, FeOt/MgO and low Ca, Al, Ba, Sr, and large negative Eu anomalies characterize the leucogranite as a thermal minimum melt, whereas
the very low K and Rb of the tonalite attests to its residual nature. We suggest that the leucogranite formed by high-T (900–950°C),
moderate-pressure (<10 kbar) dehydration partial melting of the tonalite under reducing conditions. The calculated source
compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have
been produced in a subduction-zone environment. The leucogranite probably formed in a post-collisional realm immediately after
accretion of the tonalitic crust. 相似文献
265.
Karen Fontijn Gerald G. J. Ernst Costanza Bonadonna Marlina A. Elburg Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2011,73(9):1353-1368
The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit
from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall
deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H
T of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 108 kg/s. Analytical calculations result in H
T values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 108 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of
thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0 × 1012 kg), whereas analytical derivation yields an erupted mass of 1.1 × 1012 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption.
The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity,
a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic
density current deposits. 相似文献
266.
267.
Ultrahigh-pressure metamorphism and decompressional P-T paths of eclogites and country rocks from Weihai, eastern China 总被引:6,自引:0,他引:6
Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula. 相似文献
268.
269.
We describe results of an experiment in which the Hamburg Large-Scale Geostrophic Ocean General Circulation Model was driven by a spatially correlated white-noise freshwater flux superimposed on the climatological fluxes. In addition to the red-noise character of the oceanic response, the model exhibits pronounced variability in a frequency band around 320 years. The centers of action of this oscillation are the Southern Ocean and the Atlantic.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil. 相似文献
270.
Much has been learned about Large Igneous Provinces (LIPs) and their database greatly expanded since their first formal categorization in the early 1990s. This progress provides an opportunity to review the key characteristics that distinguish LIP events from other melting events of the upper mantle, and to reassess and revise how we define LIPs. A precise definition is important to correctly recognize those LIP events with regional to global effects, and to aid in refining petrogenetic models of the origin of LIPs. We revise the definition of LIPs as follows: “Large Igneous Provinces are magmatic provinces with areal extents > 0.1 Mkm2, igneous volumes > 0.1 Mkm3 and maximum lifespans of ~ 50 Myr that have intraplate tectonic settings or geochemical affinities, and are characterised by igneous pulse(s) of short duration (~ 1–5 Myr), during which a large proportion (> 75%) of the total igneous volume has been emplaced.” They are dominantly mafic, but also can have significant ultramafic and silicic components, and some are dominated by silicic magmatism. In this revision, seamounts, seamount groups, submarine ridges and anomalous seafloor crust are no longer considered as LIPs. Although many of these are spatially-related features post-dating a LIP event, they are constructed by long-lived melting anomalies in the mantle at lower emplacement rates, and contrast with the more transient, high magma emplacement rate characteristics of the LIP event. Many LIPs emplaced in both continental and oceanic realms, are split and rifted apart by new ridge spreading centres, which reinforce the link with mid-ocean ridges as a post-LIP event. Three new types of igneous provinces are now included in the LIP inventory, to accommodate the recognition of a greater diversity of igneous compositions, and preserved expressions of LIP events since the Archean: 1) giant diabase/dolerite continental dyke swarm, sill and mafic–ultramafic intrusion-dominated provinces; 2) Silicic LIPs; and 3) tholeiite–komatiite associations, which may be Archean examples of LIPs. A revised global distribution of LIPs for the Phanerozoic is presented. Establishing the full extent of LIPs requires well-constrained plate reconstructions, and at present, plate reconstructions for the Precambrian are poorly known. However, the possibility of reconstructing the LIP record back to and into the Archean and using this expanded LIP record to better constrain the origins and effects of LIPs is an exciting frontier, and our revised definition is a contribution to that effort. 相似文献