THE LAHRAULI UREILITE

A meteorite which fell at Lahrauli, district Basti, U.P. (India) in 1955 has been examined for mineralogical, chemical and cosmogenic characteristics. It contains pigeonite Wo_7.7Fs₁₈ (with Cr₂O₃ = 1.2% and Al₂O₃ = 0.4%), olivine Fo₇₉ (with CaO = 0.3% and Cr₂O₃ = 0.7%) and diamond. The basic similarities of this meteorite to Goalpara, Dyalpur, Havero and Novo Urei indicate that it is a ureilite. Cosmic ray tracks have been measured in the olivine and pyroxene grains. Track density of 2.3 × 10⁶ per cm² in olivines and VVH/VH ratio of 10^?3 is similar to that observed in other meteorites.     

Non-parametric Seismic Hazard in Mines

— Seismic hazard analysis methods in mines are reviewed for the purpose of selecting the best technique. To achieve this goal, the most often-used hazard analysis procedure, which is based on the classical frequency-magnitude Gutenberg-Richter relation, as well as alternative procedures are investigated.¶Since the maximum regional seismic event magnitude m _max is of paramount importance in seismic hazard analysis, this work provides a generic formula for the evaluation of this important parameter. The formula is capable of generating solutions in different forms, depending on the assumptions of the model of the magnitude distribution and/or the available information regarding past seismicity. It includes the cases (i) in which seismic event magnitudes are distributed according to the truncated frequency-magnitude Gutenberg-Richter relation, and (ii) in which no specific model of the magnitude distribution is assumed.¶Both synthetic, Monte-Carlo simulated seismic event catalogues, and actual data from the copper mine in Poland and gold mine in South Africa, are used to demonstrate the discussed hazard analysis techniques.¶Our studies show that the non-parametric technique, which is independent of the assumed model of the distribution of magnitude, provides an appropriate tool for seismic hazard assessment in mines where the magnitude distribution can be very complex.     

Two populations of carbonate in ALH84001: geochemical evidence for discrimination and genesis

We present major and trace-element, oxygen isotope, textural, and structural data for carbonates and related phases in the SNC meteorite ALH84001. These data document the existence of at least two distinct carbonate populations: one composed of finely zoned, chemically and isotopically heterogeneous concretions of magnesio-siderite with distinct white magnesite rims, and a second composed of relatively homogeneous, isotopically and compositionally simple domains of ankeritic carbonate and intimately intergrown glass and fine-grained pyroxene. We suggest on the basis of textural evidence and geochemical systematics that the first population consists of low-temperature aqueous precipitates, and the second is produced by shock melting of the first. Values of δ¹⁸O and Sr/Ca ratios are correlated with one another in magnesio-siderite concretions; the trend formed by these data is consistent with the predicted relationship for inorganic precipitation of carbonate from a solution of constant composition between temperatures of ∼190°C (for concretion cores) to 20°C (for magnesite-rich concretion rims). Given the assumptions inherent in this temperature estimate, the aqueous fluid parental to carbonate concretions is constrained to have a δ¹⁸O of −5‰ VSMOW (significantly mass fractionated compared with expected juvenile martian volatiles) and minor-element abundances broadly similar to terrestrial seawater.     

Combining stochastic facies and fractal models for representing natural heterogeneity

Sedimentary deposits are often characterized by various distinct facies, with facies structure relating to the depositional and post-depositional environments. Permeability (k) varies within each facies, and mean values in one facies may be several orders of magnitude larger or smaller than those in another facies. Empirical probability density functions (PDFs) of log(k) increments from multi-facies structures often exhibit properties well modeled by the Levy PDF, which appears unrealistic physically. It is probable that the statistical properties of log(k) variations within a facies are very different from those between facies. Thus, it may not make sense to perform a single statistical analysis on permeability values taken from a mix of distinct facies. As an alternative, we employed an indicator simulation approach to generate large-scale facies distributions, and a mono-fractal model, fractional Brownian motion (fBm), to generate the log(k) increments within facies. Analyses show that the simulated log(k) distributions for the entire multi-facies domain produce apparent non-Gaussian log(k) increment distributions similar to those observed in field measurements. An important implication is that Levy-like behavior is not real in a statistical sense and that rigorous statistical measures of the log(k) increments will have to be extracted from within each individual facies. Electronic Publication     

Do reduction spots predate finite strain? A magnetic diagnosis of Cambrian slates in North Wales

The purple Cambrian slates of North Wales contain green-colored, irregular patches drawn out along cleavage and the more familiar ellipsoidal reduction spots that are parallel to cleavage. Because parallelism of pre-strain ellipsoids could only be produced by extremely high strain, we reject the hypothesis that these are strain markers. Here, we use magnetic methods to resolve this argument. The magnetic analysis reveals that the magnetic fabric of purple slate differs from the green variety of the slate only in the oxidation state, and indicates that the reduction process postdates cleavage. This suggests that the shape of reduction spots reflects the anisotropy of diffusion during reduction, not finite strain and alignment of an initially ellipsoidal object in the slates.     

North Chilean forearc tectonics and cenozoic plate kinematics

The continental forearc of northern Chile has been subjected to contemporaneous extension and compression. Here, cross-sections constructed across the forearc are presented which show that since initial shortening, deformation of the forearc has occurred in two tectonically distinct areas. These inner and outer forearc areas are separated by the strain discontinuity of the Atacama fault system and the tectonically neutral Central Depression.

The outer forearc, the Coastal Cordillera, exhibits extensional tectonics, with large (up to 300 m) normal fault scarps preserved. These faults cut the earlier thrusts responsible for the elevation of Jurassic rocks at the coast above their regional elevation. The normal faults have been re-activated, displacing Quaternary salt deposits in the Salar Grande. This re-activation of the basement faults is probably due to the subduction of anomalously thick oceanic crust, producing an isostatic imbalance in the outer forearc. In the inner forearc, cross-sections through the Sierra del Medio and Cordillera de Domeyko show that structures of the Pre-Cordillera are best explained by a thick-skinned thrust system, with localized thin-skinned tectonics controlled by evaporite detachment horizons.

Current forearc deformation features indicate a strong degree of correlation between subduction zone geometry and forearc tectonics. The timing of Cenozoic tectonism also fits well with established plate motion parameters, and the spatial and temporal variation in the state of stress of the forearc shows a close relationship throughout the Cenozoic to the plate kinematics and morphology of the subducting Nazca plate.