Analysis of long-term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics     

A. J. Biggin  D. N. Thomas 《Geophysical Journal International》2003,152(2):392-415

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The Effect of Cr2O3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2-Cr2O3 at 1{middle dot}1 GPa     

LIU  XI; O'NEILL  HUGH ST. C. 《Journal of Petrology》2004,45(11):2261-2286

Chromium as Cr³⁺ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al₂O₃in the system and hence the concentration of Al₂O₃ in partialmelts. The effect of Cr₂O₃ on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#_sp). The decrease of Al₂O₃ in the melt with increasing Cr#_spis accompanied by increasing MgO and SiO₂, whereas CaO remainsalmost constant. Consequently, the CaO/Al₂O₃ ratio of the meltincreases with Cr#_sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al₂O₃ ratios. The concentrationof Cr₂O₃ in the melt remains low even at high Cr#_sp, which meansthat the strong effect of Cr₂O₃ on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr₂O₃ nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr₂O₃; CaO/Al₂O₃ of melt; effect of Cr₂O₃  相似文献   

Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc–Ontong Java Plateau convergent zone)     

Eric J. Phinney  Paul Mann  Millard F. Coffin  Thomas H. Shipley 《Tectonophysics》2004,389(3-4):221

Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau–arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an accretionary prism formed during late Neogene (5–0 Ma) convergence between the 33-km-thick crust of the Ontong Java oceanic plateau and the 15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide accretionary prism. We name this prism the “Malaita accretionary prism” or “MAP” after Malaita, the largest and best-studied island exposure of the accretionary prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench.We divide the MAP into four distinct, folded and thrust fault-bounded structural domains interpreted to have formed by diachronous, southeast-to-northwest, and highly oblique entry of the Ontong Java Plateau into a former trench now marked by the Kia–Kaipito–Korigole (KKK) left-lateral strike-slip fault zone along the suture between the Solomon arc and the MAP. The structural style within each of the four structural domains consists of a parallel series of three to four fault propagation folds formed by the seaward propagation of thrust faults roughly parallel to sub-horizontal layering in the upper crystalline part of the OJP. Thrust fault offsets, spacing between thrusts, and the amplitude of related fault propagation folds progressively decrease to the west in the youngest zone of active MAP accretion (Choiseul structural domain). Surficial faulting and folding in the most recently deformed, northwestern domain show active accretion of greater than 1 km of sedimentary rock and 6 km, or about 20%, of the upper crystalline part of the OJP. The eastern MAP (Malaita and Ulawa domains) underwent an earlier, similar style of partial plateau accretion. A pre-late Pliocene age of accretion (3.4 Ma) is constrained by an onshore and offshore major angular unconformity separating Pliocene reefal limestone and conglomerate from folded and faulted pelagic limestone of Cretaceous to Miocene age. The lower 80% of the Ontong Java Plateau crust beneath the MAP thrust decollement appears unfaulted and unfolded and is continuous with a southwestward-dipping subducted slab of presumably denser plateau material beneath most of the MAP, and is traceable to depths >200 km in the mantle beneath the Solomon Islands.  相似文献   

地球磁极倒转与白垩纪超静磁带的非线性分析          下载免费PDF全文

申维 《地球物理学进展》2007,22(1):68-72

地球磁场多次发生南北(正负)磁极位置的变换和白垩纪超静磁带(CNS)的异常现象,这已为大家所公认.但造成这种异常现象的原因,则是迄今未能很好解答的一个难题. 应用非线性理论对地球磁极倒转和白垩纪超静磁带进行了分析, 认为超静磁带事件意味着地球核幔相互作用和外核流体运动可能处于能量最低的状态，地球磁场系统通过不断地与外界交换物质和能量，维持一种空间或时间的有序结构.在121～83Ma期间,无外星撞击地球引起地磁极性倒转,可能是白垩纪超静磁带出现的原因之一.地球磁场极性的随机倒转具有混沌运动的自逆转特性,混沌理论给地磁极性倒转提出了一个简明的动力机制解释.  相似文献   

Flow reversal over a natural pool–riffle sequence: a computational study     

Zhixian Cao  Paul Carling  Rob Oakey 《地球表面变化过程与地形》2003,28(7):689-705

A computational study is presented on the hydraulics of a natural pool–rif?e sequence composed of mixed cobbles, pebbles and sand in the River Lune, northern England. A depth‐averaged two‐dimensional numerical model is employed, calibrated with observed data at the ?eld site. From the computational outputs, the occurrence of longitudinally double peak zones of bed shear stress and velocity is found. In particular, at low discharge there exists a primary peak zone of bed shear stress and velocity at the rif?e tail in line with the local maximum energy slope, in addition to a secondary peak at the pool head. As discharge increases, the primary peak at the rif?e tail at low ?ow moves toward the upstream side of the rif?e along with the maximum energy slope, showing progressive equalization to the surrounding hydraulic pro?les. Concurrently, the secondary peak, due to channel constriction, appears to stand at the pool head, with its value increasing with discharge and approaching or exceeding the primary peak over the rif?e. The existence of ?ow reversal is demonstrated for this speci?c case, which is attributable to channel constriction at the pool head. A dynamic equilibrium model is presented to reconstruct the pool–rif?e morphology. A series of numerical modelling exercises demonstrates that the pool–rif?e morphology is more likely produced by shallow ?ows concentrated with coarse sediments than deep ?ows laden with low concentrations of ?ne sediments. It is concluded that channel constriction can, but may not necessarily, lead to competence reversal, depending on channel geometry, ?ow discharge and sediment properties. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Magnetic reversal frequency and apparent polar wander of the Siberian platform in the earliest Palaeozoic, inferred from the Khorbusuonka river section (northeastern Siberia)     

Yves Gallet  Vladimir Pavlov  Vincent Courtillot 《Geophysical Journal International》2003,154(3):829-840

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Palaeointensity results from the Permo-Carboniferous (Kiaman) reversed superchron: the Great Whin and Midland Valley sills of the northern United Kingdom     

D. N. Thomas  T. C. Rolph  J. Shaw 《Geophysical Journal International》1995,123(3):798-816

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Retrograde processes in migmatites and granulites revisited   总被引：13，自引：1，他引：13  

Michael Brown 《Journal of Metamorphic Geology》2002,20(1):25-40

Many migmatites and granulites preserve evidence of a clockwise P–T evolution involving decompression (decrease in P) while close to the thermal peak. The extent of post‐thermal peak reaction is influenced by several factors, including: (1) the P–T path in relation to invariants in the system and the Clapeyron slopes of the equilibria; (2) the rate of cooling; and (3) the availability of fluid (H₂O‐rich volatile phase or melt) for fluid‐consuming reactions. Reaction may occur between products of a prograde (increasing T) fluid‐generating reaction as the same equilibrium is re‐crossed in the retrograde (decreasing T) sense. In general, reaction reversal or ‘back reaction’ requires the P–T path to approximate isobaric heating and cooling, without significant decompression, and evolved fluid to remain within the equilibration volume. The larger the decompression segment in the P–T evolution, the more chance there is of crossing different reactions along the retrograde segment from those crossed along the prograde segment. For common pelite compositions, we may generalize by considering three pressure regimes separated by the [Spl, Ms, H₂O] invariant in KFMASH (approximately 9 kbar) and the intersection of muscovite breakdown with the H₂O‐rich volatile phase‐saturated solidus (approximately 4 kbar). Reaction reversal cannot occur along P–T paths that traverse around one of these points, but may occur along P–T paths confined to one of the three regimes in between. Additionally, above the solidus, melt segregation and loss potentially change the composition of the equilibration volume; and, the size of the equilibration volume shrinks with decreasing T. Since the proportion of melt to residue in the equilibration volume may change with decreasing size, the composition of the equilibration volume may change throughout the supra‐solidus part of the retrograde segment of the P–T evolution. If melt has been lost from the equilibration volume, reaction reversal may not be possible or may be only partial; indeed, the common preservation of close‐to‐peak mineral assemblages in migmatite and granulite demonstrates that extensive reaction with melt is uncommon, which implies melt isolation or loss prior to crossing potential melt‐consuming reactions. Water dissolved in melt is transported through the crust to be exsolved on crystallization at the solidus appropriate to the intrinsic a(H₂O). This recycled water causes retrogression at subsolidus conditions. Consideration of the evidence for supra‐solidus decompression‐dehydration reactions, and review of microstructures that have proven controversial, such as corona and related microstructures, selvage microstructures and ‘late’ muscovite, leads to the conclusion that there is more than one way for these microstructures to form and reminds us that we should always consider multiple working hypotheses!  相似文献   

Tectonic setting of porphyry Cu–Au, Mo and related mineralization associated with contrasted Neogene magmatism in the Western Sulawesi Arc     

R. Soeria-Atmadja  B. Priadi  T. M. Van Leeuwen  & I. Kavalieris 《Island Arc》1999,8(1):47-55

The Neogene Western Sulawesi Arc, from the south going northwards, can be divided into three magmatic provinces of K alkaline–shoshonitic (AK-SH), high-K calc-alkaline (KCA), and low-K–normal calc-alkaline (TH-CA) affinity, referred to, respectively, as South, Central and North Sulawesi. The origin of this magmatism in terms of subduction and collision processes is contentious. Four widely spaced Cu–Au porphyry, and one Mo porphyry district(s) occur along the Western Sulawesi Arc, with the North Sulawesi province being the most mineralized. This porphyry mineralization is part of a regional belt that extends north into the Philippines and possibly south to the Sunda Arc. In western Sulawesi, common features that define the porphyry belt are obscure because the porphyry districts cannot be simply related, either in terms of their magmatic affinity, nature of basement, or tectonic setting. Nevertheless, it can be suggested that the generation of porphyry Mo systems requires involvement of continental crust in terms of magma source, while Au-rich porphyry systems are independent of the nature of the crust, and are derived from a mantle source.  相似文献   

Geological Evidence for the Presence of the Emperor Event in China     

Ge Tongming  Fan Limin  Liu Jian Guangzhou Marine Geological Survey  Ministry of Geology  Mineral Resources of Chin  Guangzhou 《《地质学报》英文版》1993,67(2):235-245

The presence of the reversals of the geomagnetic field within the Brunhes is indicated by paleomagnetic stuides on Cenozoic volcanic rocks in the Leizhou Peninsula, loess strata and Quaternary marine sediments in the Pearl River Mouth basin in China. These regional reversals occured at 0.58±0.13 Ma, 0.45 Ma and 0.47—0.48 Ma respectively according to the results of K-Ar isotopic dating and climatostratigraphic and biostratigraphic analyses. The reversals equivalent to the Emperor Event is suggested.The reversals revealed in different regions with various lithologic characters in China prove that the Emperor Event is a global reversal event rather than a geomagnetic excursion, and its confirmation will permit a precise definition of the Middle/Late Pleistocene boundary.  相似文献