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1.
The anisotropy of magnetic susceptibility (AMS) of lava flows is an innovative method which has been proved to be directly related to the shear history of lava. One of the advantages of this method is that it can be used in the absence of other morphological features commonly employed to study the mechanism of emplacement of lava flows. This feature of the AMS method makes it very attractive to gain insight into the mechanism of emplacement of massive, relatively featureless, long lava flows such as those forming flood basalt provinces. In this work, we report the results of the measurement of AMS as a function of vertical position within the Birkett lava flow, one of the Columbia River Basalt Group flows. The observed variation of AMS allows us to identify at least 16 discrete events of lava injection and to estimate the thickness of individual injection events. The AMS-estimated thickness of each injection event (in the range of 0.5-4.0 m) coincides with the range inferred for injected lava pulses in modern Hawaiian lava flows. Thus, the evidence provided by the AMS method supports the notion that at least some flood basalt lava flows were emplaced by the same mechanism as many present-day inflated pahoehoe flows. Regarding the orientation of the principal susceptibilities, in the central part of the flow they define a preferred orientation along an E-W trend, whereas in the outer parts of the flow they have a NNE-SSW trend. This difference in the orientation of the principal susceptibilities is interpreted as the result of a change of flow direction of the lava as emplacement progressed. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-002-0203-8. 相似文献
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Ramacciotti Carlos D. Casquet César Baldo Edgardo G. Galindo Carmen Pankhurst Robert J. Verdecchia Sebastián O. Rapela Carlos W. Fanning Mark 《International Journal of Earth Sciences》2018,107(7):2605-2625
International Journal of Earth Sciences - The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to... 相似文献
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Edgardo Cañón-Tapia 《Pure and Applied Geophysics》1994,142(2):365-382
Twenty-eight parameters used to characterize measurements of the anisotropy of magnetic susceptibility are compared theoretically in this work by introducing the concept of the field of susceptibility tensors, which allows the representation of parameters as families of lines in a plane. It is demonstrated that the foliation and lineation parameters are but a special case of the shape parameters, implying that the resolution of these two rock fabric elements using AMS measurements alone is more an artifact of the numerical range of definition of some parameters than a quantification of two physically independent features. Also, it is shown that parameters presumably of the same type do not necessarily yield equivalent interpretation of results in a qualitative sense, and therefore, caution should be strongly exercised when parameters are to be selected. Paramters quantifying the degree of anisotropy are, in general, equivalent to each other because of the very small departure observed in natural rocks from the isotropic case. However, a final consideration of the possible ability to differentiate rock types and a convenient range of values allowing expression of the degree of anisotropy in a well-defined percentage are pointed out as the main factors to be considered before selecting one parameter within this class. 相似文献
4.
C. Casquet C.W. Rapel R.J. Pankhurst E.G. Baldo C. Galindo C.M. Fanning J.A. Dahlquist J. Saavedra 《地学前缘(英文版)》2012,3(2):137-145
The role played by Paleoproterozoic cratons in southern South America from the Mesoproterozoic to the Early Cambrian is reconsidered here.This period involved protracted continental amalgamation that led to formation of the supercontinent Rodinia.followed by Neoproterozoic continental break-up,with the consequent opening of Clymene and Iapetus oceans,and finally continental re-assembly as Gondwana through complex oblique collisions in the Late Neoproterozoic to Early Cambrian.The evidence for this is based mainly on a combination of precise U-Pb SHRMP dating and radiogenic isotope data for igneous and metamorphic rocks from a large area extending from the Rio de la Plata craton in the east to the Argentine Precordillera in the west and as far north as Arequipa in Peru.Our interpretation of the paleogeographical and geodynamic evolution invokes a hypothetical Paleoproterozoic block(MARA) embracing basement ultimately older than 1.7 Ga in the Western Sierras Pampeanas(Argentina),the Arequipa block(Peru),the Rio Apa block(Brazil),and probably also the Paraguaia block(Bolivia). 相似文献
5.
Giovanni Carraro René A. Méndez Edgardo Costa 《Monthly notices of the Royal Astronomical Society》2005,356(2):647-653
We report on VI charge-coupled device photometry of two fields centred in the region of the open clusters NGC 6404 and 6583 down to V = 22.0 . So far these clusters have never been studied, and we provide for the first time estimates of their fundamental parameters, namely, radial extent, age, distance and reddening. We find that the radius of NGC 6404 is 2.0 arcmin, as previously proposed, while the radius of NGC 6583 is 1.0 arcmin, significantly lower than previous estimates. Both clusters turn out to be of intermediate age (0.5–1.0 Gyr old), and located inside the solar ring, at a Galactocentric distance of about 6.5 kpc. These results make these objects very interesting targets for spectroscopic follow-up to measure their metallicity. In fact, they might allow us to enlarge by more than 1 kpc the baseline of the radial abundance gradient in the Galactic disc towards the Galactic Centre direction. This baseline is currently rather narrow especially for clusters of this age. 相似文献
6.
C. Casquet R.J. Pankhurst C. Galindo C. Rapela C.M. Fanning E. Baldo J. Dahlquist J.M. Gonzlez Casado F. Colombo 《Precambrian Research》2008,165(3-4):205-220
A deformed ca. 570 Ma syenite–carbonatite body is reported from a Grenville-age (1.0–1.2 Ga) terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. This is the first recognition of such a rock assemblage in the basement of the Central Andes. The two main lithologies are coarse-grained syenite (often nepheline-bearing) and enclave-rich fine-grained foliated biotite–calcite carbonatite. Samples of carbonatite and syenite yield an imprecise whole rock Rb–Sr isochron age of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029); SHRIMP U–Pb spot analysis of syenite zircons shows a total range of 206Pb–238U ages between 433 and 612 Ma, with a prominent peak at 560–580 Ma defined by homogeneous zircon areas. Textural interpretation of the zircon data, combined with the constraint of the Rb–Sr data suggest that the carbonatite complex formed at ca. 570 Ma. Further disturbance of the U–Pb system took place at 525 ± 7 Ma (Pampean orogeny) and at ca. 430–440 Ma (Famatinian orogeny) and it is concluded that the Western Sierras Pampeanas basement was joined to Gondwana during both events. Highly unradiogenic 87Sr/86Sr values in calcites (0.70275–0.70305) provide a close estimate for the initial Sr isotope composition of the carbonatite magma. Sm–Nd data yield Nd570 values of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean from depleted mantle with a component from Meso/Neo-proterozoic lower continental crust. 相似文献
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8.
C.W. Rapela R.J. Pankhurst C. Casquet E. Baldo C. Galindo C.M. Fanning J.M. Dahlquist 《Journal of South American Earth Sciences》2010,29(1):105-127
New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullún), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ullún. The oldest rocks are the Andino-type granitic orthogneisses of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma. The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites (1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The emplacement of these mantle-derived magmas during an extensional episode produced a widespread thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and back-arc complexes dominated the Sierra de Pie de Palo–Ullún sector, that was fully developed ca. 1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at ～1165 Ma was roughly coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are registered in the Pie de Palo–Ullún sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent. 相似文献
9.
Magnetic properties of two apparently unrelated lava suites (one tholeiitic and the other alkalic) coexisting in the central parts of the Peninsula of Baja California, Mexico, were measured in this study. Macroscopic indicators and measurements of anisotropy of magnetic susceptibility (AMS) were combined to infer the flow direction on those lavas. These measurements were used to investigate the likelihood of the existence of an abnormally long tholeiitic lava flow in this part of the Peninsula. The obtained results indicate that the tholeiitic flows in the region constitute regular length flows, and are likely to have been issued through vents located within the area of study. Additionally, comparison of the magnetic signature of tholeiitic and alkalic lavas reveals a systematic difference in some of the magnetic parameters (bulk susceptibility and Curie temperatures). In particular, it is proposed that the narrow range of Curie temperatures characteristic of each lava type can be used as a reliable proxy for the identification of each lava type in the region. All of these findings show that the alkalic and tholeiitic lavas of the region are more closely related to each other than previously suspected.Editorial responsibility: R. Cioni 相似文献
10.
The Rio Negro has responded significantly in the Late Pleistocene and Holocene to lagged environmental changes largely associated with activity during the last glacial in the Amazon basin. On the basis of geological structure, the Rio Negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles. No deposits of the Upper Pleniglacial were recognized in the field. The oldest recognizable Late Pleistocene alluvial unit is the Upper Terrace of Middle Pleniglacial age (ca. 65–25 ka) (reach I), tentatively correlated with the oldest terrace identified on the left bank of reach III. At that time, the river was mainly an aggradational bed load system carrying abundant quartz sand, a product of more seasonal conditions in the upper catchment. The late glacial (14–10 ka) is represented by a lower finer-grained terrace along the upper basin (reach I), which was recognized in the Tiquié, Curicuriarí, and Vaupes rivers. At that time, the river carried abundant suspended load as a response to climatic changes associated with deglaciation.Since about 14 ka, the river has behaved as a progradational system, infilling in downstream series a sequence of structurally controlled sedimentary basins or ‘compartments,’ creating alluvial floodplains and associated anabranching channel systems. Reach II was the first to be filled, then reach III, both accumulating mainly sand. Fine deposits increase downstream in reach III and become predominant in some anabranch islands of the distal reach. The lowermost reaches of the Negro (V and VI) have been greatly affected by a rising base level and associated backwater effect from aggradation of the Amazon during late glacial and recent times. Reach V has acted almost entirely as a fine sediment trap. The remarkable Anavilhanas archipelago is the product of Holocene deposition in the upper part of this sedimentary basin; however, suspended sediment load declined about 1.5 ka, prior to the lower part of this basin becoming infilled.The progradational behavior of the Rio Negro, filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones, illustrates how a large river system responses to profound changes in Late Quaternary base level and sediment supply. The most stable equilibrium conditions have been achieved in the Holocene in reaches IIb and IIIa, where an anabranching channel and erosional–relictual island system relatively efficiently convey water and sediment downstream. Reaches IIIb and V never achieved equilibrium conditions during the Holocene, characterised as they are today with incomplete floodplains and open water. 相似文献