An analysis of the spatial distribution of the seismicity of the southern Tyrrhenian Sea is presented with the aim of putting constraints on the geometry of the complex Benioff zone of this area by relocating the earthquakes that occurred in the period 1985–1990. The data set used in this analysis is based on information from ING, ISC, and NEIS databases.
In order to ensure the quality of epi/hypocentral locations, we based our selection of earthquakes on both the available number of P- and S-readings (at least eight P- and two S-pickings) and the epicentral distances of the stations reporting the event (< 5°). Such a selection enabled us to obtain a catalogue of 450 well-located events with focal depths of up to 500 km.
The seismogenetic volumes estimated from the distribution of hypocentres of the whole seismicity analysed were interpreted according to a recent geodynamic model proposed for this area. This has led us to the hypothesis of an active NW-SE-oriented subduction of the lithosphere generated and stressed by the opening of the Tyrrhenian Basin. 相似文献
Establishing the age and crustal nature of exotic terranes and their underlying basements helps to determine their paleogeographic origin and tectonic histories. We present U–Pb ages of zircons and Sm–Nd whole rock isotopic data for volcanic and plutonic rocks of the Carolina terrane, one of several peri-Gondwanan terranes that were accreted to the margins of the circum-Atlantic continents during the Paleozoic. Volcanism in this subduction-related arc culminated in the eruption of the Morrow Mountain rhyolite, at ca. 540 Ma; thus, magmatism in the Carolina terrane ceased at the beginning of the Cambrian. The presence of inherited zircons and non-juvenile depleted mantle model ages of Carolina slate belt rocks favor a basement that is, at least in part, composed of evolved continental crust. Ages of inherited xenocrystic zircons cluster at ca. 1000, 2100 and 2500 Ma. These ages, in addition to volcanism at ca. 618–540 Ma, correlate best with well-known tectonic events in present-day northern South America. Specifically, the Orinoquian-Sunsas, the Trans-Amazonian and the Central Amazonian orogenic zones are likely candidates for potential basement correlatives to the Carolina terrane. Sm–Nd isotopic signatures vary significantly, but permit assimilation of Orinoquian age (1000 Ma) crust by magmas derived from the depleted mantle in a subduction (arc-related) setting. Our findings are also consistent with proposed correlations between the Carolina terrane and Avalonia which is likewise believed to have formed along the northern margin of present-day South America. 相似文献
Terrestrial ecosystem disturbances inferred from the fossil hemlock pollen decline (ca. 4,800 BP) and recovery (ca. 3,800 BP) affected van Nostrand Lake, including the diatom communities. Ecological models suggest the lake responded by eutrophying, reflecting higher nutrient influx resulting from increased erosion. A decline in lake productivity followed as the forest vegetation recovered and erosion slowed. Lastly, as the forest switched from early and middle successional species to mature species, lake productivity increased as erosion increased nutrient inflow, especially phosphorus. However, this eutrophication response was delayed or buffered, perhaps due to wetland development surrounding the lake. The lake never fully returned to its initial, pre-hemlock decline state, but oscillated between more eutrophic and more mesotrophic or oligotrophic stages, possibly reflecting other disturbances in the catchment and climatic change. 相似文献
We developed an inference model to infer dissolved organic carbon (DOC) in lakewater from lake sediments using visible-near-infrared
spectroscopy (VNIRS). The inference model used surface sediment samples collected from 160 Arctic Canada lakes, covering broad
latitudinal (60–83°N), longitudinal (71–138°W) and environmental gradients, with a DOC range of 0.6–39.6 mg L−1. The model was applied to Holocene lake sediment cores from Sweden and Canada and our inferences are compared to results
from previous multiproxy paleolimnological investigations at these two sites. The inferred Swedish and Canadian DOC profiles
are compared, respectively, to inferences from a Swedish-based VNIRS-total organic carbon (TOC) model and a Canadian-based
diatom-inferred (Di-DOC) model from the same sediment records. The 5-component Partial Least Squares (PLS) model yields a
cross-validated (CV) RCV2 R_{CV}^{2} = 0.61 and a root mean squared error of prediction (RMSEPCV) = 4.4 mg L−1 (11% of DOC gradient). The trends inferred for the two lakes were remarkably similar to the VNIRS-TOC and the Di-DOC inferred
profiles and consistent with the other paleolimnological proxies, although absolute values differed. Differences in the calibration
set gradients and lack of analogous VNIRS signatures in the modern datasets may explain this discrepancy. Our results corroborate
previous geographically independent studies on the potential of using VNIRS to reconstruct past trends in lakewater DOC concentrations
rapidly. 相似文献
From the observed light curves of six eclipsing binaries with solar-type activity and from the analysis of their period variations some problems in the interpretation of the observations came out: Are these binaries similar to classical RS CVn stars? Is the stellar surface spot coverage the unique interpretation of the observed light curves? Are these systems really linked with W UMa's? In a further study some models will be tempted to try to solve these problems. 相似文献