Two-dimensional simulations of surface deformation caused by slab detachment

Detachment of the deeper part of subducted lithosphere causes changes in a subduction zone system which may be observed on the Earth's surface. Constraints on the expected magnitudes of these surface effects can aid in the interpretation of geological observations near convergent plate margins where detachment is expected. In this study, we quantify surface deformation caused by detachment of subducted lithosphere. We determine the range of displacement magnitudes which can be associated with slab detachment using numerical models. The lithospheric plates in our models have an effective elastic thickness, which provides an upper bound for rapid processes, like slab detachment, to the surface deformation of lithosphere with a more realistic rheology. The surface topography which develops during subduction is compared with the topography shortly after detachment is imposed. Subduction with a non-migrating trench system followed by detachment leads to a maximum surface uplift of 2–6 km, while this may be higher for the case of roll-back preceding detachment. In the latter situation, the back-arc basin may experience a phase of compression after detachment. Within the context of our elastic model, the surface uplift resulting from slab detachment is sensitive to the depth of detachment, a change in friction on the subduction fault during detachment and viscous stresses generated by sinking of the detached part of the slab. Overall, surface uplift of these magnitudes is not diagnostic of slab detachment since variations during ongoing subduction may result in similar vertical surface displacements.     

Single-domain and superparamagnetic titanomagnetite with variable Ti content in young ocean-floor basalts: No evidence for rapid alteration

High resolution electron microscope studies have been carried out on ‘zero-age’ (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12°N. Such data lead to characterization of the magnetic minerals, especially those of smaller grain size, which have been hypothesized by Kent and Gee to have undergone grain size-dependent alteration. In addition to larger titanomagnetite grains, abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix. These grains, likely to be single-domain (SD) or superparamagnetic, are associated with apatite, uncommon pyrrhotite and residual glass. The submicrometer titanomagnetite grains have a wide compositional range (0 < x < 0.8), where x is the fraction of ulvöspinel component, whereas the larger, multi-domain (MD)-sized titanomagnetite grains have a narrow composition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely rapid (< 20 ka) alteration of these young basalts.     

A low-β coronal loop model

The low- coronal loop model of Sillen and Kattenberg (1980) is extended to include a surrounding current-free plasma. We calculated the dispersion curves of kink modes by solving the linearized MHD-equations of motion.We found a strong stabilizing influence on the growth rates of kink instabilities due to the surrounding plasma.In loops that are thick, have small current densities and that have a high density and a low magnetic field strength the growth times for kinks become of the order of days.Presently at Caltech, Pasadena, U.S.A.Presently at the FOM-Institute for Plasma Physics, Rijnhuizen, Nieuwegein, The Netherlands.     

Optimized Sample Schemes for Geostatistical Surveys

Sample schemes used in geostatistical surveys must be suitable for both variogram estimation and kriging. Previously schemes have been optimized for one of these steps in isolation. Ordinary kriging generally requires the sampling locations to be evenly dispersed over the region. Variogram estimation requires a more irregular pattern of sampling locations since comparisons must be made between measurements separated by all lags up to and beyond the range of spatial correlation. Previous studies have not considered how to combine these optimized schemes into a single survey and how to decide what proportion of sampling effort should be devoted to variogram estimation and what proportion devoted to kriging An expression for the total error in a geostatistical survey accounting for uncertainty due to both ordinary kriging and variogram uncertainty is derived. In the same manner as the kriging variance, this expression is a function of the variogram but not of the sampled response data. If a particular variogram is assumed the total error in a geostatistical survey may be estimated prior to sampling. We can therefore design an optimal sample scheme for the combined processes of variogram estimation and ordinary kriging by minimizing this expression. The minimization is achieved by spatial simulated annealing. The resulting sample schemes ensure that the region is fairly evenly covered but include some close pairs to analyse the spatial correlation over short distances. The form of these optimal sample schemes is sensitive to the assumed variogram. Therefore a Bayesian approach is adopted where, rather than assuming a single variogram, we minimize the expected total error over a distribution of plausible variograms. This is computationally expensive so a strategy is suggested to reduce the number of computations required     

Architecture and emplacement of the Nebo–Babel gabbronorite-hosted magmatic Ni–Cu–PGE sulphide deposit,West Musgrave,Western Australia

The Nebo–Babel Ni–Cu–platinum-group element (PGE) sulphide deposit in the West Musgrave Block, Western Australia, is the largest nickel sulphide discovery in the last 10 years. The deposit is hosted within a concentrically zoned, olivine-free, tube-like (chonolithic), gabbronorite intrusion associated with the, approximately, 1,078-Ma Giles Complex-layered intrusions in the Warakurna large igneous province. Emplaced into sulphide-free amphibolite facies orthogneiss, the fault-offset Nebo–Babel chonolith extends for 5 km and has a cross-section of 1 × 0.5 km. Igneous mineralogy, fabrics, and textures are well preserved. The lithostratigraphy includes variably textured leucogabbronorites (VLGN) that form an outer shell around mineralised gabbronorite (MGN), with barren gabbronorite (BGN) and oxide–apatite gabbronorite (OAGN) in the middle and lower parts of the chonolith. Mineral and whole-rock geochemistry indicate that the units become progressively evolved in the order: VLGN, MGN, BGN, and OAGN, and that incompatible trace-element concentrations increase downwards within the MGN and BGN. The mineralisation, which is confined to the early, more primitive units (VLGN and MGN), occurs as massive sulphide breccias and stringers and as disseminated gabbronorite-hosted sulphides. The massive sulphides were emplaced late in the intrusive sequence, have different PGE chemistry and Cu tenor to the disseminated sulphides, and have undergone sulphide fractionation. The distribution of disseminated sulphides, which are primary magmatic in origin, is related to chonolith geometry and magma flow regimes, rather than to gravitational settling. Sulfur-bearing country rocks are absent in the Nebo–Babel deposit area, and thus, local crustal S addition was unlikely to have been the major mechanism in achieving sulphide immiscibility. The Nebo–Babel intrusion is part of an originally continuous magma chonolith with multiple and related magma pulses. The parental magma was medium- to low-K tholeiite with 8–9 wt% MgO. The initial magma pulse (VLGN), the most primitive and sulphide saturated, was probably emplaced along a linear weakness in the country rock. After crystallisation of VLGN, marginally more fractionated, sulphide-saturated magma was injected through the thermally insulated core of the conduit, forming the MGN. Successive pulse(s) of more fractionated magma (BGN) were emplaced in the core of the intrusion. After magma flow ceased, closed system crystal fractionation produced consistent mineral and chemical fractionation trends within BGN and OAGN. After crystallisation, the intrusion was overturned and then offset by the Jameson Fault resulting in the apparent ‘reverse’ chemical and mineral trends in Nebo–Babel.