Agroclimatological suitability mapping for dryland maize production in Lesotho

The climatic potential of maize under dryland farming in Lesotho, southern Africa, was investigated using five suitability indices comprising probability of accumulating heat units of greater than 1,500 growing degree days, probability of a frost-free growing season, probability of seasonal rainfall of more than 500 mm, probability of drought during the flowering to grain-filling stages and the slope of an area. A geographic information system layer was prepared for each of these parameters and the layers overlaid using different weights for each of the climatic suitability indices to obtain an agroclimatic maize suitability map for Lesotho. This analysis yielded different suitability classes. This variability points to prevalence of climatic constraints that need to be acknowledged when attempting to identify management strategies that can optimize the rain-fed maize production in climatically variable environments.     

Brittle deformation in the inner NW Alps: from early orogen-parallel extrusion to late orogen-perpendicular collapse

Internal parts of the Alps have undergone widespread extensional deformation in the course of their Neogene exhumation history. Palaeostress inversion methods are used to map the prevailing stress fields and their evolution through time. Here we present new data from 100 sites with a total of about 2000 faults/striae couples, covering a large portion of the inner north‐western Alps. Palaeostress tensors are mostly extensional, although one‐third of them are transcurrent. The dominant direction of minimum horizontal stress axes (σ3) is in an orogen‐parallel (N30° to N70°) orientation around the bend of the north‐west alpine arc. A comparison between this older (Neogene, post‐metamorphic) stress field with the current stress and strain field determined from seismotectonics and geodesy indicates a change in deformation mode from early orogen‐parallel extrusion to a late and ongoing orogen‐perpendicular spreading.     

Oil families and their source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea

Thirty-one crude oils and 15 source rocks were selected for molecular geochemical and isotopic analyses in order to establish the genetic relationships between discovered oils and petroleum source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea. Three groups of oils were recognized. Group I oils are only found in the upper section of the Liusagang Formation, with a moderate abundance of C₃₀ 4-methylsteranes, low oleanane contents and lighter δ¹³C values, showing a close relation to the shale occurring in the upper section of the Liusagang Formation. Group II is represented by the majority of the discoveries and is distributed in multi-sets of reservoirs having different ages. The oils are characterized by a high abundance of C₃₀ 4-methylsteranes, low to moderate abundance of oleanane and heavy δ¹³C values, and shows a good correlation with the lacustrine shale and oil shale in the middle section of the Liusagang Formation. Group III oils occurred in the lower section of the Liusagang Formation. The oils have a lower concentration of C₃₀ 4-methylsteranes, relatively high abundance of oleananes and their δ¹³C values are intermediate. Oils of this group correlated well with the shallow lake-delta mudstone of the lower section of Liusagang Formation. These oil-source genetic relationships suggest a strong source facies control on the geographic distribution of oil groups within the Weixinan Sub-basin. The geochemical data indicate shale in the middle section of the Liusagang Formation has an excellent oil generation potential and the lower and upper sections contain dark shale and mudstone with good to fair oil potential. Future exploration or assessment of petroleum potential of the sub-basin could be improved by considering the proposed genetic relationship between the oil types and source rocks, as well as their distribution.     

Stable isotope,petrological, and fluid inclusion studies of minor mineral deposits from the McArthur Basin: Implications for the genesis of some sediment‐hosted base metal mineralization from the Northern Territory

The geology, stable isotopes and fluid inclusions from mineralized and unmineralized Middle Proterozoic sequences of the McArthur Basin, Northern Territory, have been studied at Eastern Creek, Bulman Mines, Beetle Springs, and other localities in the McArthur Basin where disseminated sulphides in unmineralized black shales were available from drill core. At Eastern Creek, galena and minor chalcopyrite (δ³⁴S+3.6 to +11.2%o) occur in an evaporitic sedimentary sequence. Barite (δ³⁴S+18.4 to +24.7%o) also occurs, and saline brines are trapped along healed fractures in the barite. Pressure‐corrected trapping temperatures in the barite (95–138°C), and in vein dolomite (158–168°C) agree with temperature estimates from the degree of maturation of the sedimentary organic matter. The δ¹⁸O and δ¹³C_Co2 values of the mineralizing fluid were calculated to be +3.5 to +4.5%o and ‐2.7%o, respectively. Sedimentary dolomite has restricted δ¹³C and δ¹⁸O ranges, within the reported ranges for non‐mineralized Middle Proterozoic dolomite. An ore formation model developed for Eastern Creek, in which a basinal fluid at about 200°C carrying base metals and sulphide was released from underlying sediments during local fault movement, may be applicable to a number of other deposits. The mineralization deposited from these fluids occurs only below the pre‐Roper Group unconformity, implying that it may be older than the basal Roper Group. The δ³⁴S values of iron sulphides in fine grained black dolostones (not associated with mineral deposits) from the McArthur Basin were assessed in the light of the values found for sulphides in modern organic‐rich sedimentary environments. The data so obtained suggest that the considerable concentration of iron sulphide in the mineral deposits formed, at least in part, from heated basinal waters and that disseminated iron sulphides remote from mineralization also formed from a similar source.     

Co‐seismic deformation and post‐glacial slip rate along the Magallanes‐Fagnano fault,Tierra Del Fuego,Argentina

Across the extreme south of Patagonia, the Magallanes‐Fagnano Fault (MFF) accommodates the left‐lateral relative motion between South America and Scotia plates. In this paper, we present an updated view of the geometry of the eastern portion of the MFF outcropping in Tierra del Fuego. We subdivide the MFF in eight segments on the basis of their deformation styles, using field mapping and interpretation of high‐resolution imagery. We quantify coseismic ruptures of the strongest recorded 1949, M_w7.5 earthquake, and determine its eastern termination. We recognize several co‐seismic offsets in man‐made features showing a sinistral shift up to 6.5 m, greater than previously estimated. Using ¹⁰Be cosmogenic nuclides depth profiles, we date a cumulated offset in post‐glacial morphologies and estimate the long‐term slip rate of the eastern MFF. We quantify a 6.4 ± 0.9 mm/a left‐lateral fault slip rate, which overlaps geodetic velocity and suggests stable fault behaviour since Pleistocene.     

Sediment-charged flash floods on Montserrat: The influence of synchronous tephra fall and varying extent of vegetation damage

On 20th May 2006 the Soufrière Hills Volcano on the Caribbean island of Montserrat experienced a large dome collapse and intense rainfall generated flash floods. The floods had very high loads of volcanic debris derived both from this and previous eruptions and can thus be classified as lahars. The floods reached unusually high water levels and caused substantial geomorphic change in the Belham Valley. Detailed rainfall and geomorphological data, coupled with the precise timing of events and yewitness accounts have facilitated an assessment of the relative importance of rainfall volume and intensity, older volcanic debris, pre- and syn-flood tephra fall and the extent of pre-flood vegetation damage for the behavior of this and subsequent sediment-laden floods in this setting. The change in runoff behavior was controlled by preexisting vegetation damage and synchronous tephra fall and this was critically important in controlling the impact of these flash floods. Although rainfall intensity and volume have some control on flood occurrence they are not the critical control on flash flood impact on the geomorphology in the Belham Valley. A significant conclusion of this study is that the extreme nature of the flash floods was not caused by extreme rainfall (as is commonly believed to be the primary cause of flash floods) but rather it was the result of changed runoff behaviour caused by the widespread syn-flood tephra deposition and importantly the widespread vegetation damage by volcanic-associated acid rain in the preceding weeks.     

A Genetic Algorithm Variational Approach to Data Assimilation and Application to Volcanic Emissions

Variational data assimilation methods optimize the match between an observed and a predicted field. These methods normally require information on error variances of both the analysis and the observations, which are sometimes difficult to obtain for transport and dispersion problems. Here, the variational problem is set up as a minimization problem that directly minimizes the root mean squared error of the difference between the observations and the prediction. In the context of atmospheric transport and dispersion, the solution of this optimization problem requires a robust technique. A genetic algorithm (GA) is used here for that solution, forming the GA-Variational (GA-Var) technique. The philosophy and formulation of the technique is described here. An advantage of the technique includes that it does not require observation or analysis error covariances nor information about any variables that are not directly assimilated. It can be employed in the context of either a forward assimilation problem or used to retrieve unknown source or meteorological information by solving the inverse problem. The details of the method are reviewed. As an example application, GA-Var is demonstrated for predicting the plume from a volcanic eruption. First the technique is employed to retrieve the unknown emission rate and the steering winds of the volcanic plume. Then that information is assimilated into a forward prediction of its transport and dispersion. Concentration data are derived from satellite data to determine the observed ash concentrations. A case study is made of the March 2009 eruption of Mount Redoubt in Alaska. The GA-Var technique is able to determine a wind speed and direction that matches the observations well and a reasonable emission rate.     

Detecting mass substructure in galaxy clusters: an aperture mass statistic for gravitational flexion

Gravitational flexion has been introduced as a technique by which one can map out and study substructure in clusters of galaxies. Previous analyses involving flexion have measured the individual galaxy–galaxy flexion signal, or used either parametric techniques or a Kaiser, Squires and Broadhurst (KSB)-type inversion to reconstruct the mass distribution in Abell 1689. In this paper, we present an aperture mass statistic for flexion, and apply it to the lensed images of background galaxies obtained by ray-tracing simulations through a simple analytic mass distribution and through a galaxy cluster from the Millennium Simulation. We show that this method is effective at detecting and accurately tracing structure within clusters of galaxies on subarcminute scales with high signal to noise even using a moderate background source number density and image resolution. In addition, the method provides much more information about both the overall shape and the small-scale structure of a cluster of galaxies than can be achieved through a weak lensing mass reconstruction using gravitational shear data. Lastly, we discuss how the zero-points of the aperture mass might be used to infer the masses of structures identified using this method.