388 research outputs found
Radiometric age constraints for glacial terminations IX and VII
Buried sedimentary aggradational sections deposited between 800 ka and 600 ka in the
Tiber River coastal alluvial plain have been studied using borecores from around Rome. 40Ar/39Ar
ages on sanidine and/or leucite from intercalated tephra layers and paleomagnetic investigation of
clay sections provide geochronological constraints on the timing of aggradation of two of these
alluvial sections, and demonstrate that they were deposited in response to eustatic sea level rise
caused by glacial terminations IX and VII. 40Ar/39Ar age data indicate ages of 802 ± 8 ka and 649
± 3 ka for glacial terminations IX, and VII, respectively, providing a rare test, beyond the range
of U-series dating for corals and speleothems (~500 ka), of the astronomically calibrated
timescale developed for oxygen isotope records from deep sea cores
Assessing the volcanic hazard for Rome. 40Ar/39Ar and In-SAR constraints on the most recent eruptive activity and present-day uplift at Colli Albani Volcanic District
We present new 40Ar/39Ar data which allow us to refine the recurrence time for the most recent eruptive activity occurred at Colli Albani Volcanic District (CAVD) and constrain its geographic area. Time elapsed since the last eruption (36 kyr) overruns the recurrence time (31 kyr) in the last 100 kyr. New interferometric synthetic aperture radar data, covering the years 1993–2010, reveal ongoing inflation with maximum uplift rates (>2 mm/yr) in the area hosting the most recent (<200 ka) vents, suggesting that the observed uplift might be caused by magma injection within the youngest plumbing system. Finally, we frame the present deformation within the structural pattern of the area of Rome, characterized by 50 m of regional uplift since 200 ka and by geologic evidence for a recent (<2000 years) switch of the local stress-field, highlighting that the precursors of a new phase of volcanic activity are likely occurring at the CAVD
40Ar/39Ar and 14C geochronology of the Albano maar deposits: Implications for 2 defining the age and eruptive style of the most recent explosive activity at Colli 3 Albani Volcanic District, Central Italy
New 40Ar/39Ar and 14C ages have been found for the Albano multiple maar pyroclastic units and underlying 25
paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near 26
Rome, Italy, consisting of seven eruptions (Albano 1 27
^
=
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oldest). Both dating methodologies have been applied
on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two 28
current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The 40Ar/39Ar 29
ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, 30
which are indistinguishable from the previously determined 40Ar/39Ar ages of the proximal Albano units 1, 2, 31
5 and 7, thus confirming their stratigraphic correspondence. 32
Twenty-one 14C ages of the paleosols beneath Albano units 3, 5, 6 and 7 were found for samples collected 33
from 13 proximal and distal sections, some of which were the same sections sampled for 40Ar/39Ar 34
measurements. The 14C ages were found to be stratigraphically inconsistent and highly scattered, and were 35
systematically younger than the 40Ar/39Ar ages, ranging 36
^
from 35 ka
^
to 3 ka. Considering the significant
consistence of the 40Ar/39Ar chronological framework, we interpret the scattered and contradictory 14C ages 37
to be the result of a variable contamination of the paleosols by younger organic carbon deriving from the 38
superficial soil horizons. 39
These results suggest that multiple isotopic systems anchored to a robust stratigraphic framework may need 40
to be employed to determine accurately the geochronology of the CAVD as well as other volcanic districts. 4
Logarithmic rate dependence in deforming granular materials
Rate-independence for stresses within a granular material is a basic tenet of
many models for slow dense granular flows. By contrast, logarithmic rate
dependence of stresses is found in solid-on-solid friction, in geological
settings, and elsewhere. In this work, we show that logarithmic rate-dependence
occurs in granular materials for plastic (irreversible) deformations that occur
during shearing but not for elastic (reversible) deformations, such as those
that occur under moderate repetitive compression. Increasing the shearing rate,
\Omega, leads to an increase in the stress and the stress fluctuations that at
least qualitatively resemble what occurs due to an increase in the density.
Increases in \Omega also lead to qualitative changes in the distributions of
stress build-up and relaxation events. If shearing is stopped at t=0, stress
relaxations occur with \sigma(t)/ \sigma(t=0) \simeq A \log(t/t_0). This
collective relaxation of the stress network over logarithmically long times
provides a mechanism for rate-dependent strengthening.Comment: 4 pages, 5 figures. RevTeX
Crustal fault reactivation facilitating lithospheric folding/buckling in the central Indian Ocean
Ionic and electronic properties of the topological insulator BiTeSe investigated using -detected nuclear magnetic relaxation and resonance of Li
We report measurements on the high temperature ionic and low temperature
electronic properties of the 3D topological insulator BiTeSe using
ion-implanted Li -detected nuclear magnetic relaxation and
resonance. With implantation energies in the range 5-28 keV, the probes
penetrate beyond the expected range of the topological surface state, but are
still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice
relaxation measurements reveal isolated Li diffusion with an
activation energy eV and attempt frequency s for atomic site-to-site hopping. At lower
temperature, we find a linear Korringa-like relaxation mechanism with a field
dependent slope and intercept, which is accompanied by an anomalous field
dependence to the resonance shift. We suggest that these may be related to a
strong contribution from orbital currents or the magnetic freezeout of charge
carriers in this heavily compensated semiconductor, but that conventional
theories are unable to account for the extent of the field dependence.
Conventional NMR of the stable host nuclei may help elucidate their origin.Comment: 17 pages, 12 figures, submitted to Phys. Rev.
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