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Ordovician K-bentonites in the upper-plate active margin of Western Gondwana, (Famatina Ranges): Stratigraphic and palaeogeographic significance 总被引：1，自引：0，他引：1
Numerous, thin-bedded, tabular pale-yellowish clay bands are interlayered with black shales in a biostratigraphically constrained Early Ordovician volcano-sedimentary succession at Famatina, western Argentina. This region was part of a fairly continuous upper-plate, convergent volcanic chain that fringed western Gondwana. Mineralogy on both clay and non-clay fractions, whole rock chemistry and field observations on these distinctive event-beds indicate that they originated as relatively coarse fallout tephras, altered first into bentonites and later, through burial metamorphism, into K-bentonites (metabentonites). These tephras were deposited as single crystals and glassy dust or pumiceous fragments in a restricted subtidal environment. The region of Famatina has previously been inferred as the source of abundant distal K-bentonites recorded in the adjacent lower-plate allochthonous Precordillera terrane. However, these K-bentonites within the proximal arc site were unknown and rather unexpected since they are generally better preserved like distal deposits, associated either with central vent plinian–ultraplinian eruptions or with accompanying co-ignimbrite ash clouds. Their chemistry and comparison with those K-bentonites in the Precordillera allow tracing an evolution from volcanic arcs into continental crust. K-bentonites described in this paper are much older than those recorded in the adjacent Precordillera terrane and seem to be associated with a first eruptive period along western Gondwana that has no counterpart in the Argentine Precordillera, suggesting a significant longitudinal separation between these two regions by the Early Ordovician. 相似文献
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Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina 
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下载免费PDF全文 Gilda Collo Federico M. Dávila Wilson Teixeira Julieta C. Nóbile Lucy G. Sant′ Anna Andrew Carter 《Basin Research》2017,29(1):16-40
We present mineralogic, isotopic and thermochronologic analyses on psammopelitic and tuffaceous levels from the Bermejo and Vinchina basins – both foreland depocentres of the Central Andes of Argentina – that define a low‐temperature regime for the crust akin to a slab shallowing and flattening process. The contents of illite in illite/smectite interstratified (I/S) show a progressive illitization into the deeper parts of both basins. The distribution of I/S is compatible with theoretical simulations and predicted heat flow values of ca. 26 mW m?2 in the 8–3.4 Ma interval for the Vinchina Basin and ca. 42 mW m?2 since 9 Ma for the Bermejo Basin. The latter shows heat flow values that are comparable to those reported by magnetotelluric analysis (36–40 mW m?2) in agreement with previously published heat flow calculations along the modern Andean foreland. The Rb–Sr isochrones in psammopelites (<2 μm fractions) show ages between 125 and 165 Ma, whereas the K–Ar ages decrease as the grain size is smaller (136–224 Ma for 1–2 μm, 112–159 Ma for 0.2–1 μm, 76–116 Ma for <0.2 μ and 39.3–42 Ma for <0.1 μm). These ages are significantly older than the sedimentation in the basins (ca. 16 Ma for the Vinchina Basin; U–Pb age), and can be explained by the presence of a significant amount of detrital components, mainly illite, even in the finer fractions. The preservation of detrital ages is consistent with the shallow diagenesis related to a low‐temperature regime, proposed here for the basins. Younger K–Ar ages (21.3–12 Ma) were obtained for a basal tuffaceous level. Clay mineralogy and R0 ordering in the deepest part of the Vinchina Basin, together with the evolution model of I/S with depth, suggest that the burial temperatures would have not exceeded ca. 100°C in agreement with (U–Th)/He analyses performed on apatite extracted from two tuffaceous units. Thermal indicators from both studied basins confirm the existence of a low‐temperature regime during flat subduction. 相似文献
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We investigated time dependent piezomagnetic fields due to volcanic sources embedded in a viscoelastic, homogeneous half-space. Especially in volcanic areas, the presence of inhomogeneous materials and high temperatures produce a lower effective viscosity of the Earth's crust that calls for considering anelastic properties of the medium. Piezomagnetic properties are carried by grains of titano-magnetite, which occupy only a small fraction of ordinary rock volume and are supposed to be elastic, while the non-magnetic surrounding matrix is assumed to be viscoelastic. From all the possible rheological models, we investigated two cases in which the bulk modulus is purely elastic and the shear modulus relaxes as: (i) a Maxwell solid and (ii) a standard linear solid (SLS). We applied the Correspondence Principle to the analytical elastic solutions for pressurized spherical sources and dislocation sources in order to determine the time dependent piezomagnetic fields in a viscoelastic medium. The piezomagnetic field completely vanishes after the relaxation process for a Maxwell rheology, whereas it is found to decrease over time and reach some finite offset value for a SLS rheology. These different behaviours provide helpful hints in understanding the temporal evolution of piezomagnetic anomalies in volcanic regions. 相似文献
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For 5 months before the 2001 Mt. Etna eruption, a progressive gravity decrease was measured along a profile of stations on
the southern slope of the volcano. Between January and July 2001, the amplitude of the change reached 80 μGal, while the wavelength
of the anomaly was of the order of 15 km. Elevation changes observed through GPS measurements during a period encompassing
the 5-month gravity decrease, remained within 4–6 cm over the entire volcano and within 2–4 cm in the zone covered by the
microgravity profile. We review both gravity and elevation changes by a model assuming the formation of new cracks, uniformly
distributed in a rectangular prism. The inversion problem was formulated following a global optimization approach based on
the use of Genetic Algorithms. Although it is possible to explain the observed gravity changes by means of the proposed analytical
formulation, the results show that calculated elevation changes are significantly higher than those observed. Two alternative
hypotheses are proposed to account for this apparent discrepancy: (1) that the assumptions behind the analytical formulation,
used to invert the data, are fallacious at Etna, and thus, numerical models should be utilized; (2) that a second process,
enabling a considerable mass decrease to occur without deformation, acted together with the formation of new cracks in the
source volume. 相似文献