Neogene patterns of relative plate motion for Africa-Europe: some implications for recent central Mediterranean tectonics     

S. Mazzoli  Mark Helman 《International Journal of Earth Sciences》1994,83(2):464-468

A detailed relative motion picture for the Neogene Africa-Europe plate kinematics is presented. The kinematic reconstruction was carried out using the finite difference solution between the rotation parameters determined for Anomalies 7 to 2 in the Africa-North America-Europe plate motion circuit. The analysis shows a motion of Africa with respect to Europe which is NNE directed during Late Oligocene to Burdigalian times, becoming NNW trending from the Langhian to the early Tortonian; from upper Tortonian times onward, the motion changes to a clear north-west directed convergence. Major Late Neogene tectonic features of the central Mediterranean region can, to a large extent, be explained within the context of the reconstructed major plate motions. Late Tortonian to Recent Africa-Europe slip vectors are compatible with a variety of geological phenomenoa such as north-west directed subduction beneath Calabria, south-east translation of Calabria and extension in the Tyrrhenian Sea, north-west trending slip vectors from thrust earthquakes between Gibraltar and Sicily, and dextral strike-slip across the North African margin.  相似文献   

Low-background gamma counting: applications for210Pb dating of sediments   总被引：1，自引：0，他引：1  

Claire L. Schelske  Arthur Peplow  Mark Brenner  Craig N. Spencer 《Journal of Paleolimnology》1994,10(2):115-128

Sediment cores from three lakes were dated with²¹⁰Pb using a constant rate of supply (CRS) model. We used low-background gamma counting to measure naturally occurring levels of²¹⁰Pb,²²⁶Ra, and¹³⁷Cs in sediment samples because sample preparation is simple and non-destructive,²²⁶Ra activity provides a direct measure of supported²¹⁰Pb activity for each sample analyzed, and¹³⁷Cs activity may provide an independent age marker for the 1962–1963 peak in atmospheric fallout of this radionuclide. In one core supported²¹⁰Pb activity was estimated equally well from²²⁶Ra activity of each sampling interval or from the mean total²¹⁰Pb activity of constant activity samples at depth. Supported²¹⁰Pb activity was constant with depth in this core. In a short freeze core, determining²²⁶Ra activity of every sample proved advantageous in estimating supported²¹⁰Pb activity because supported²¹⁰Pb activity could be estimated from²¹⁰Pb measurements only at the deepest sampling interval. Supported²¹⁰Pb activity estimated from²²⁶Ra activity also yielded more precise estimates of highly variable sedimentation rates. In the third core²²⁶Ra activity exceeded²¹⁰Pb activity at the top of the core and varied 20 fold with depth. This high input of²²⁶Ra in disequilibrium with²¹⁰Pb is attributed to recent erosion of radium-bearing materials in the drainage basin. These data invalidate the assumption that supported²¹⁰Pb activity is constant in sediment cores and can be estimated from the mean total²¹⁰Pb activity at depths where²¹⁰Pb activity is constant. We recommend using gamma counting or another independent assay of²²⁶Ra to validate the assumption of constant supported²¹⁰Pb activity in sediment cores if there is reason to expect that²²⁶Ra activity varies with depth.This is the fourth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr. P.G. Appleby is guest editing this series.  相似文献   

Land Use Evolution in Hill Regions of the Dominican Republic*     

Mark O. Zweifler  Michael A. Gold  Robert N. Thomas 《The Professional geographer》1994,46(1):39-53

This study combines data from a social survey and an air photo time series to gain both a temporal and spatial understanding of Dominican hill land use evolution. Research results indicate that disparities in present land use and productivity between two hill land study sites stem from their different stages of development along a common land use continuum. The authors emphasize the importance of understanding land use history when identifying key constraints to long-term sustainability of hill land farming systems.  相似文献   

The source of Central American lavas: inferences from geochemical inverse modeling     

Mark D. Feigenson  Michael J. Carr 《Contributions to Mineralogy and Petrology》1993,113(2):226-235

The rate earth element chemistry of a large suite of samples from the Central American volcanic front has been determined to understand their petrogenesis. Different segments of the volcanic front are compared on the basis of their calculated source rare earth patterns as deduced from inverse modeling. The results yield a range in the extent of light rare earth enrichment of the source, as well as in source mineralogy. Moderateto-strong LREE enrichment and high modal garnet contents are observed for the sources of both Guatemala and central Costa Rica, whereas slight LREE depletion and little or no garnet occurs in the Nicaraguan source region. Although distinct source regions beneath each Central American segment are permitted by the modeling, it is more likely that the mantle wedge is broadly homogeneous but locally heterogeneous. Small volume, relatively enriched, garnet-bearing veins surrounded by a matrix of isotopically depleted mantle periodotite could exist throughout the mantle wedge. Apparently distinct sources occur due to the variation in partial melting beneath the different segments, controlled by the amount of subduction-generated flux per unit wedge volume, which in turn is a function of the dip of the subducted lithosphere.  相似文献   

Further ${}^{40}\text{A}\text{r}{}^{39}\text{Ar}$ evidence for the multi-collisional heating of the Kirin chondrite     

T.Mark Harrison  Wang Sungshan 《Geochimica et cosmochimica acta》1981,45(12):2513-2517

Results of an ${}^{40}\text{Ar}{}^{39}\text{Ar}$Ar age spectrum obtained on a sample of the Kirin chondrite (K-5-13) show a similar character to previous published analyses of Kirin samples K-1 and K-2. The K-5-13 age spectrum shows clear evidence of having been substantially outgassed during a presumed collisional event about 0.5 Ga, ago, in good agreement with the estimate obtained from K-2, The differing amounts of ⁴⁰Ar loss registered by K-2 and K-5-13 during the 0.5 Ga event of about 60 and 50%, respectively, allows calculation of their vertical separation in the parent body at about 10cm.  相似文献   

Stable carbon isotope ratio variations of organic matter in Orca Basin sediments     

Mark A Northam  David J Curry  Richard S Scalan  Patrick L Parker 《Geochimica et cosmochimica acta》1981,45(2):257-260

Orca Basin is a highly reducing basin on the slope of the Gulf of Mexico. Stable carbon isotope ratios and total organic carbon percentages were determined for two cores within the basin and one control core outside the basin. The results show that the organic carbon content of the basin cores is consistently 2–3 times greater than that of the control core. The Pleistocene-Holocene boundary, indicated by a break in the δ¹³C depth profile, occurs at a greater sediment depth in the basin cores than in the control core. A small sampling interval has made it possible to detect an unexplained fine structure in the δ¹³C profile not previously observed.  相似文献   

Petrology and geochemistry of the island of Sarigan in the Mariana arc; calc-alkaline volcanism in an oceanic setting     

Arend Meijer  Mark Reagan 《Contributions to Mineralogy and Petrology》1981,77(4):337-354

Isotopic studies of rocks from oceanic island arcs such as the Marianas indicate that little, if any, recycling of continental material (e.g. oceanic sediments) occurs in these arcs. Because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs, an important question is whether the andesites of continental arcs are produced by a fundamentally different (more complex?) mechanism than the lavas of oceanic arcs. An excellent opportunity for study of this question is provided by the island of Sarigan, in the Mariana active arc, on which calc-alkaline andesites (including hornblende-bearing types) are exposed along with more mafic lavas. Available isotope data suggest the Sarigan lavas (including the andesites) were derived from mantle material with little or no involvement of continental components. Ratios of incompatible elements suggest that most of the Sarigan lavas were derived from similar source materials. Absolute abundances of incompatible elements vary irregularly within the eruptive sequence and indicate at least 5 distinct magma batches are represented on Sarigan. Major element data obtained on the lavas and mineral phases in them, combined with modal mineral abundances, suggest that the calc-alkaline nature of the volcanic rocks on Sarigan results from the fractional crystallization of titanomagnetite in combination with other anhydrous phases. Amphibole, although present in some samples, is mainly a late-crystallizing phase and did not produce the calc-alkline characteristics of these lavas. Gabbroic samples found in the volcanic sequence have mineralogc and geochemical characteristics that would be expected of residual solids produced during fractional crystallization of the Sarigan lavas. When combined, data on the lavas and the gabbros suggest the following crystallization sequence: olivine — plagioclase — clinopyroxene — titanomagnetite — orthopyroxene±hornblende, biotite and accessory phases. These results lead to the conclusion that calc-alkaline magmas can be generated directly from mantle sources.  相似文献   

Predictive Depositional Model for Glacial Aquifers     

Mark T. Stewart 《Ground water》1981,19(2):133-137

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Investigations of an intrusive contact,northwest Nelson,New Zealand—I. Thermal,chronological and isotopic constraints     

T.Mark Harrison  Ian McDougall 《Geochimica et cosmochimica acta》1980,44(12):1985-2003

In the Nelson area of New Zealand, intrusion of the Separation Point Batholith 114 Ma ago caused thermal metamorphism in the adjacent Devonian Rameka Gabbro that allowed radiogenic ⁴⁰Ar to diffuse partially out of hornblendes in the gabbro.Comparison of mineral ages in the batholith (obtained by the K-Ar, ${}^{40}\text{Ar}{}^{39}\text{Ar}$, Rb-Sr, U-Pb and fission track methods) with closure temperature estimates for the various isotopic systems has allowed thermal histories of several samples to be resolved. These curves indicate that, following emplacement, the batholith cooled at an exponentially decreasing rate reaching 100°C by about 75 Ma ago.Oxygen isotope measurements on minerals from the gabbro and granodiorite give calculated equilibrium water compositions of +7 and +9 of δ¹⁸O_SMOW respectively. Measured porosities from samples of the gabbro yield a model permeability of 3 × 10^?17 m¹². These results confirm that heat transfer from the batholith to the gabbro was predominantly by conduction. A numerical heat flow model has been developed for the region, and shows that samples in the gabbro near the contact reached ~600°C. whereas 5 km away from the contact, the samples were only raised ~ 50°C above the ambient temperature.Excess ⁴⁰Ar was identified as causing a significant scatter in the K-Ar ages of hornblendes from the gabbro. Though obscured by this effect, these data suggest a crystallization age of the Rameka Gabbro of 367 Ma, which is consistent with geological evidence.  相似文献   

¹H and ¹³C NMR of marine humic acids     

Patrick G. Hatcher  Robert Rowan  Mark A. Mattingly 《Organic Geochemistry》1980,2(2):77-85

¹³C and ¹H NMR spectra were obtained for humic acids isolated from marine sediments. NMR shows great promise in identifying structural components of humic acids as some new and interesting structural features are identified. Aliphatic structures were found to constitute a much larger fraction of humic acids than previously thought, and they appeared to be highly branched. Although the aromatic content of terrestrial humic acid was found to be lower than expected, the aromaticity appears to be a specific discriminator of terrestrial/aquatic source types. A humic acid isolated from an anoxic algal sapropel was found to be composed predominantly of polyuronic acids and different than other aquatic sedimentary humic substances.  相似文献