An excursion to the Bayuda volcanic field of Northern Sudan

A cluster of well-preserved recent volcanoes in the northern Bayuda Desert make up a more or less continuous field some 520 km² in area surrounded by a number of isolated centres of eruption. The volcanoes are numerous but small; up to 400 m in height and 0.35 km² in volume. Most of them are simple composite volcanoes with a pyroclastic cone skirted by a small lava field erupted from the same vent after explosive eruptions had ceased. In a few instances, however, the cone was eviscerated by more violent eruptions, leaving a deep explosion crater. The lavas are all nepheline-normative alkali basalts and contain a variety of xenocrysts and xenoliths from at least three different sources. The distribution of the recent volcanoes was partly controlled by large granitic ring-intrusions of the Basement Complex country rocks. These intrusions belong to the Younger Granite association of late Precambrian or Lower Palaeozoic age and represent a volcanic-intrusive episode widespread in northern Africa. The complexes are composed of cale-alkaline and peralkaline granites and syenites and a related plexus of dyke swarms.     

A comparison of ASCE and FAO-56 reference evapotranspiration for a 15-min time step in humid climate conditions

Many applications in diverse disciplines require estimates of evapotranspiration (ET) at hourly or smaller time steps. The primary objectives of this study were to compare the American Society of Civil Engineers (ASCE) and FAO-56 Penman–Monteith equations for 15-min ET₀ (ET_{0,15-min,ASCE} and ET_0,15-min,FAO) estimations for humid climate conditions and to compare the 24 h sum of ASCE (ET_0,24 h,ASCE) and FAO-56 15-min ET₀ (ET_0,24 h,FAO) with the daily ET₀ (ET_0,d,FAO) computed from the daily FAO-56 equation, which is identical to ASCE daily ET₀ equation. Ten-year, i.e., 1997–2006 continuous 15-min and daily weather data for 11 representative and well-distributed sites throughout Georgia, USA were used. It was evident that during the day, ET_{0,15-min,ASCE} was higher than ET_0,15-min,FAO due to a lower surface resistance parameter value, while at night ET_{0,15-min,ASCE} was lower than ET_0,15-min,FAO due to a higher surface resistance parameter value. The ET_0,15-min,FAO was about 5% less than ET_{0,15-min,ASCE} and ET_0,24 h,FAO was about 5% lower than ET_0,24 h,ASCE. The difference between ET_{0,15-min,ASCE} and ET_0,15-min,FAO during the day and night was highly dependent on wind speed. During the three summer months, i.e., June, July and August, on average, ET_0,24 h,FAO was only 1% higher than ET_0,d,FAO while ET_0,24 h,ASCE was 5% higher than ET_0,d,FAO. For the entire year, ET_0,24 h,FAO was 8% higher than ET_0,d,FAO while ET_0,24 h,ASCE was 13% higher than ET_0,d,FAO. The ET_0,24 h,FAO and ET_0,d,FAO had a better agreement than ET_0,24 h,ASCE and ET_0,d,FAO throughout the year and during the summer months. It is also worth noting that the daily calculations for FAO-56 and ASCE were identical. These results demonstrated that for applications that require 15-min time steps or daily ET₀ for the entire year, the use of ET_0,15-min,FAO and ET_0,24 h,FAO, respectively, will yield more consistent outcomes. The use of ET_0,d,FAO during the summer months can be as accurate as the use of ET_0,24 h,FAO for applications that require daily time steps, such as irrigation scheduling.     

Evaluation of the crustal deformations in the northern region of Lake Nasser (Egypt) derived from 8 years of GPS campaign observations

The proper evaluation of crustal deformations in the Aswan (Egypt) region is crucial due to the existence of one major artificial structure: the Aswan High Dam. This construction induced the creation of one of the major artificial lakes: Lake Nasser, which has a surface area of about 5200 km² with a maximum capacity of 165 km³. The lake is nearly 550 km long (more than 350 km within Egypt and the remainder in Sudan) and 35 km across at its widest point. Great attention has focused on this area after the November 14, 1981 earthquake (M_L = 5.7), with its epicenter southwest of the High Dam.In order to evaluate the present-day kinematics of the region, its relationship with increasing seismicity, and the possible influence of the Aswan High Dam operation, a network of 11 GPS sites was deployed in the area. This network has been reobserved every year since 2000 in campaign style. We present here the results of the analysis of the GPS campaign time-series. These time-series are already long enough to derive robust solutions for the motions of these stations. The computed trends are analyzed within the framework of the geophysical and geological settings of this region. We show that the observed displacements are significant, pointing to a coherent intraplate extensional deformation pattern, where some of the major faults (e.g., dextral strike-slip Kalabsha fault and normal Dabud fault) correspond to gradients of the surface deformation field. We also discuss the possible influence of the water load on the long-term deformation pattern.     

Geochemical and petrographic studies of Ta mineralization in the Nuweibi albite granite complex, Eastern Desert, Egypt

The Nuweibi albite granite is one of 14 known Sn-Ta-Nb bearing granitoids in the Eastern Desert region of Egypt. The granite is a highly leucocratic, albite-rich rock with accessory columbite-tantalite, cassiterite, microlite and ixiolite as well as topaz, garnet and white mica. Ages of 450–600 Ma were obtained from zircons by the ²⁰⁷Pb/²⁰⁶Pb evaporation method. Great uncertainty is caused by the small size and poor quality of the grains, but the precision is sufficient to indicate that the granite is late- or postorogenic with respect to the Panafrican orogeny. The Nuweibi granite is divided into a western and an eastern part by a regional fault. Both parts of the granite are compositionally similar but there are important differences and a clear compositional gap between them, so they are considered separate facies of an intrusive complex. The eastern part of the granite is more highly mineralized, has higher modal albite contents and higher Ta/Nb ratios, both in the whole rock and in the ore minerals. It is suggested that the two parts of the granite evolved from a common source and were emplaced sequentially, the eastern part representing a later, more fractionated magma. Textural evidence strongly suggests that the granite has a magmatic origin overall, but disturbance of geochemical trends at the whole-rock scale and at the scale of zoning profiles in individual grains of columbite-tantalite indicate post-magmatic overprinting. By analogy with other Ta-bearing albite granites, the sodic bulk composition of the Nuweibi granite can be explained by fluorine enrichment in the magma. Fluorine contents in the magma were high enough to stabilize topaz, and muscovites contain 2–4 wt.%. F. However, whole-rock F contents are low. We speculate that the low Ca, Al and P contents of the magma prevented abundant F-bearing minerals to form and led to loss of fluorine to now-eroded roof rocks. Received: 8 November 1995 / Accepted: 10 June 1996     

Seismic velocity estimation from post-critical wide-angle reflections in layered structures

The travel time inversion of wide-angle seismic data is a technique commonly used in the deep seismic sounding. We propose an application of this technique to a smaller scale of a sedimentary layer, where the characteristics of seismic observations changes significantly. Field observations confirmed by synthetic analysis recognize the dominant amplitudes of wide-angle post-critical reflections. A case study is presented in this paper, of a joint interpretation of conventional reflection seismic with reflection imaging, combined with the wide-angle travel time inversion of additional full-spread observations. A joint interpretation results in a precise recognition of the seismic velocity distribution, that is further used for the seismic depth conversion with the uncertainty analysis of the depth of the reflecting horizons. Despite the salt layer in the studied structure this method is able to precisely recognize the seismic velocities of the sub-salt structures.     

Active crustal shortening in NE Syria revealed by deformed terraces of the River Euphrates

The Africa–Arabia plate boundary comprises the Red Sea oceanic spreading centre and the left‐lateral Dead Sea Fault Zone (DSFZ); however, previous work has indicated kinematic inconsistency between its continental and oceanic parts. The Palmyra Fold Belt (PFB) splays ENE from the DSFZ in SW Syria and persists for ～400 km to the River Euphrates, but its significance within the regional pattern of active crustal deformation has hitherto been unclear. We report deformation of Euphrates terraces consistent with Quaternary right‐lateral transpression within the PFB, indicating anticlockwise rotation (estimated as 0.3° Ma^?1 about 36.0°N 39.8°E) of the block between the PFB and the northern DSFZ relative to the Arabian Plate interior. The northern DSFZ is shown to be kinematically consistent with the combination of Euler vectors for the PFB and the Red Sea spreading, resolving the inconsistency previously evident. The SW PFB causes a significant earthquake hazard, previously unrecognized, to the city of Damascus.     

Modelling crack propagation in concrete structures with a two scale approach

A simplified computational technique based on a refined global–local method is applied to the failure analysis of concrete structures. The technique distinguishes the scale of the structure, modelled with large size finite elements, from the scale at which material non‐linearity occurs due to progressive cracking and macro‐crack propagation. The finite element solution is split into two parts: a linear elastic analysis on a coarse mesh over the entire structure and a non‐linear analysis over a small part of the structure where a dense finite element grid is employed. In the non‐linear calculation, a non‐local damage model is implemented. These two computations are coupled with the help of an iterative scheme. The size and location of the region where a non‐linear analysis is performed, is adapted to follow the development of the damage zone. Numerical examples of mode I fracture of concrete specimens with straight and curved cracks are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Permian Calcareous algae from the Khachik Formation at the Ali Bashi Mountains,NW of Iran

Micropaleontological investigations on Permian successions in the NW of Iran based on algae led to the determination of taxa belonging to some families such as Ungdarellaceae (Ungdarella uralica), Gymnocodiaceae (Gymnocodium bellerophontis and Permocalculus sp.), Dasycladales (Mizzia cf. M. yabei), and some microproblematics (Vermiporella nipponica and Pseudovermiporella sodalica). These investigations are mainly focused on the Khachik Formation at the Ali Bashi Mountains. According to the algal community, a lagoon environment is offered for the studied successions.     

Quantitative finite strain and kinematic flow analyses along the Zagros transpression zone, Iran

The Ghouri area in southwest Iran exposes a cross section through the Zagros orogenic belt. The area provides an opportunity to investigate quantitative finite strain (Rs), kinematic vorticity number (W_k), proportions of pure shear and simple shear components, sense of shear indicators, steeply plunging lineations, and other moderate to steeply plunging stretching lineations in a transpressional zone. Based on a classical strain analysis of deformed microfossils with oblate strain ellipsoid shape, the Zagros orogenic belt is classified as a pure-shear dominated zone of transpression, but asymmetry of shear-sense indicators suggests that a significant component of simple shear was involved along the deformation zone boundaries. The long axes of the microfossils and stretched pebbles of a deformed conglomerate were used to indicate the stretching direction in this zone. The stretching lineations have a steep to moderate plunge but a constant strain magnitude. Characteristics of dextral inclined transpressional kinematics in the Zagros continental collision zone were quantified and indicate an estimated k-value < 1, an angle between the maximum horizontal axis of the instantaneous strain ellipsoid and the zone boundary (θ = 32°), asymmetrical dextral shear-sense indicators, and an angle of relative plate motion (α = 25°).