Seismic attenuation,normal moveout stretch,and low‐frequency shadows underlying bottom simulating reflector events

In many cases, the seismic response of bottom‐simulating reflectors is characterised by low frequencies called “low‐frequency shadow”. Generally, this phenomenon is interpreted as attenuation due to partial saturation with free gas. Actually, this frequency loss may have multiple causes, with a normal moveout stretch as a possible candidate. To analyse this phenomenon, we compute synthetic seismograms by assuming a lossy bottom‐simulating layer, with varying quality factor and thickness, bounded by the upper hydrate‐brine/gas‐brine and lower gas‐brine/brine interfaces. First, we estimate the shift of the centroid frequency of the power spectrum as a function of the travelled distance of the seismic pulse. Then, we perform one‐dimensional numerical experiments to quantify the loss of frequency of the seismic event below the bottom‐simulating reflector as a function of the quality factor of the bottom‐simulating layer and its thickness (due to wave interference). Then, we compute shot gathers to obtain the stacked section, with and without the normal moveout stretch correction and with and without the presence of wave attenuation in the bottom‐simulating layer. The results indicate that the low‐frequency shadow due to the normal moveout stretch is stronger than that due to attenuation and may constitute a false indicator of the presence of gas. In fact, often, the low‐frequency shadow overlies events with higher frequencies, in contradiction with the physics of wave propagation. This is particularly evident when the low‐frequency shadow is so extensive that the presence of high frequencies below cannot be justified by the acquisition geometry.     

Rb/Sr ages of rocks from the Chilean metamorphic basement

Eleven whole rock Rb/Sr age determinations from the Chilean metamorphic basement — formerly considered as Precambrian — define two limiting reference isochrons of 342 and 273 my. Analized rocks are phyllites and schists of sedimentary origin with mineralogical assemblages mainly corresponding to the greenschist facies.Two metamorphic series characterize the Chilean metamorphic basement: an eastern low P/T series, and a western high P/T series. Four of the analyzed samples belong to the former and seven to the latter.The obtained age is interpreted as the age of a main metamorphic episode of the basement. This episode would have taken place in the Upper Paleozoic.     

Late Paleozoic K/Ar ages of blueschists from Pichilemu,Central Chile

Two crossite concentrates and one blueschist whole rock were analyzed by the K/Ar method. These samples belong to the high/intermediate pressure Western Series of the Chilean metamorphic basement and, in this area, are intruded by a small monzonite body.Ages obtained were 211 m.y. and 329 m.y. for the mineral concentrates and 211 m.y. for the whole rock. Discussion based on crystal size as a factor for retention of⁴⁰Ar during localized re-heating of the metamorphic rocks due to the monzonitic intrusion leads to the acceptance of 329 m.y. as the minimum age of crossite crystallization. This age agrees with the whole rock Rb/Sr limiting reference isochrons (273–342 m.y.) previously obtained for the metamorphic basement of Central Chile which did not include samples of the present area.This age provides the first evidence of a Paleozoic blueschist assemblage in the eastern Pacific border and would suggest the existence of a Late Paleozoic subduction zone along the western margin of South America.     

Ash layers in deep-sea sediments as tracers of arc volcanic activity: Japan and Central America as case studies

Abstract Arc volcanic activity on opposite sides of the Pacific Ocean (Japan and Central America) has been investigated by examining the number of volcanic ash layers recorded in Neogene and Quaternary deep-sea sediments. The data suggest that ash layers counted in deep-sea sediments may provide a reliable record of arc volcanism. The study is based on a quantitative analysis of arc volcanic activity using cores collected on DSDP (Deep-Sea Drilling Project) and ODP (Ocean Drilling Program) legs. Five distinct parameters which might affect ash distribution in marine sediments were reviewed: nature of the eruption, wind influence, settling conditions, diagenesis, and plate motion. Of these five, past atmospheric circulation was the most significant. The main constraint on the analysis is that temporal scattering of ash is not directly related to wind pattern variations. Results of this analysis are correlated with dating of terrestrial volcanic sequences. Although marine tephra records for individual regions reveal minor differences in the episodes of volcanic activity, a general correlation exists between activity of arc volcanism in Japan and in Central America. Two important pulses of arc volcanism occurred during Middle Miocene times (18–13 Ma) and Plio-Quaternary times (5–0 Ma). These episodes of intense volcanism are separated by a well recorded quiescent period during Late Miocene times. These correlating episodes of the volcanic record indicate a direct link between arc volcanism and the global tectonic evolution of the Pacific ocean margins.     

Experimental analysis of groundwater flow through a landslide slip surface using natural and artificial water chemical tracers

Artificial and natural tracer tests combined with high accurate electronic distancemeter measurements are conducted on a small landslide with a well known slip surface geometry. Outflow yields and chemical contents are monitored for all the experiment duration and they analyzed to estimate the slip surface hydraulic parameters. The main result is that the slip surface acts as a drain for groundwater flows that evacuates interstitial pressures in the slope and brings the sliding mass to be more stable one. Copyright © 2007 John Wiley & Sons, Ltd.     

Rb—Sr and Sm—Nd geochronology of lower Proterozoic granite—greenstone terrains in French Guiana,South America

Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis.The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratio (I_Nd) of 0.51002±9 (2 σ), corresponding to ?_Nd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio (I_Sr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths.The positive ε_Nd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high ε_Nd(T) value for these rocks excludes any significant crustal contamination during magma genesis.The French Guianese orthogneisses yield a low I_Sr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition.The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic.     

The oceanic substratum of Northern Luzon: Evidence from xenoliths within Monglo adakite (the Philippines)

Abstract   A 8.65 Ma adakitic intrusive sheet exposed near Monglo village in the Baguio District of Northern Luzon contains a suite of ultramafic and mafic xenoliths including in order of abundance: spinel dunites showing typical mantle-related textures, mineral and bulk rock compositions, and serpentinites derived from them; amphibole-rich gabbros displaying incompatible element patterns similar to those of flat or moderately enriched back-arc basin basalt magmas; and amphibolites derived from metabasalts and/or metagabbros of identical affinity. A single quartz diorite xenolith carrying a similar subduction-related geochemical signature has also been sampled. One amphibolite xenolith provided a whole-rock K–Ar age of 115.6 Ma (Barremian). We attribute the origin of this suite to the sampling by ascending adakitic magmas of a Lower Cretaceous ophiolitic complex located at a depth within the 30–35 km thick Luzon crust. It could represent an equivalent of the Isabela-Aurora and Pugo-Lepanto ophiolitic massifs exposed in Northern Luzon.     

Calc-alkaline and alkaline miocene and calc-alkaline recent volcanism in the Southernmost Patagonian Cordillera, Chile

In the southernmost Patagonian Cordillera (south of Lat. 54°45′) previously unrecorded calc-alkaline dacites and andesites, and adjacent alkali-basalts, were dated at 21 and 18 Ma (K-Ar), respectively. In the same area post-glacial low-K calc-alkaline andesites, identified at Isla Cook, represent the southernmost Recent volcanism recorded so far in South America, about 400 km south of Monte Burney, previously known as the southernmost Recent volcano. The association of contemporaneous calc-alkaline and alkali volcanics suggests a subduction-related environment for the Miocene volcanism, and the Recent calc-alkaline volcanics indicate that currently there is a small component of subduction of the Antarctic Plate beneath the South American part of the Scotia Plate.     

Extreme deuterium enrichment of organic radicals in the Orgueil meteorite: Revisiting the interstellar interpretation?

The insoluble organic matter of the carbonaceous meteorites contains radicals having a polyaromatic structure and a heterogeneous distribution. By using Hyperfine Sublevel Correlation spectroscopy (HYSCORE) in pulsed Electron Paramagnetic Resonance (pulsed-EPR), whereby nuclear frequencies of magnetic nuclei and their hyperfine interaction with electron spin of radicals are detected with high resolution, the radicals are shown to be considerably enriched in deuterium in the Orgueil meteorite, with a D/H ratio of 1.5 ± 0.5 × 10⁻². These radicals hold 3.6 ± 1.2 × 10⁻³ H relative to total organic H.Analysis of hydrogen and deuterium hyperfine interactions indicates that the deuterium atoms are localized in the benzylic position, on aliphatic carbons bonded to aromatic radical moieties. This type of C-H bond exhibits one of the smallest bond energy, reinforcing the recent finding that the lower the C-H bond energy the higher the deuterium-enrichment (Remusat L., Palhol F., Robert F., Derenne S. and France-Lanord C. (2006) Enrichment of deuterium in insoluble organic matter from primitive meteorites: a solar system origin? Earth Planet. Sci. Lett.243, 15-25). Such a behavior is difficult to reconcile with the usual interpretation according to which high D/H ratios represent survivals of interstellar grains. More likely, the deuterium-enrichment process took place after the formation of organic grains whose initial isotopic compositions was close to the protosolar D/H ratio. These grains were possibly loaded at the surface of the protosolar disk where they exposed to the intense solar UV irradiation, triggering an isotopic exchange with deuterium-rich highly reactive ions.     

Q-band esr studies of fossil teeth: Consequences for esr dating

At least 14 axial-symmetry ESR lines in Plio-Quaternary enamel fibres were observed by Q-band technique. Some lines follow a law e.g. the main anisotropic A-line whose g = 2.0018 and g = 1.9977 are close to g_eff = 2.0020 and g_eff = 1.9975 respectively of the isotropic doublets B- and E-lines. The other lines are blocked below 220K e.g. one isotropic hyperfine (g = 2.0033, a = 21.6 Gauss) M-septet and one isotropic singlet (g = 2.0007) D-line. By heating, the M-septet intensity increases up to 250°C and is lost at 280°C; B- and E-lines decrease while a new isotropic (g = 2.0056) C-line appears. At 300°C, only the two C- and A-lines usually remain and hold up to 400°C.The natural dose rate of fossil enamel is calculated from U-, Th-, K-content and disequilibrium measured both in enamel and dentine, bones and sediments in contact, by methods of low-background γ-spectrometry, isotopic dilution and fission-tracks mapping.ESR dating of quaternary Proboscidae, Bovidae, Equidae and Cervidae has been attempted by a new comparative method: the A-line intensity, normalized for the same dose rate and weight, of various fossil dental enamels varies quasi-linearly with the age of the geological sites from which they are issued.