Opening and closure of a marginal Southern California lagoon inlet

Over the past 50 yr, direct observations of the inlet status (open or closed) of San Dieguito Lagoon, a typical southern California lagoon located in Del Mar, California, have shown that river flooding is the major natural determinant of inlet conditions on time scales longer than a few years. River flooding is strongly dependent on rainfall in the San Dieguito River watershed and on the influences of two water storage reservoirs in the area. Rainfall fluctuates on yearly and longer time scales and undergoes cycles of wet and dry periods. Over short time periods, ranging from a few months to several years, inlet status is primarily determined by the available tidal prism and littoral sand transport. Recognition of these factors is crucial in order to correctly evaluate the probability that a small lagoon will remain open naturally. A probability approach is essential because the variables controlling inlet conditions are random in nature. The results of our study show that the inlet will remain open naturally 34% of the time. The tendency to remain open is vastly smaller during years of dry weather (12%) versus times of above-average rainfall (66%).     

The role of the Spitsbergen shear zone in determining morphology,segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km² of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge. This revised version was published online in November 2006 with corrections to the Cover Date.     

Exploring the future of rural–urban connections in sub-Saharan Africa: modelling urban expansion and its impact on food production in the Addis Ababa region

Abstract

The built-up area of Addis Ababa and its surrounding towns is expanding into the peri-urban region leading to high losses of farmland, directly influencing the food production for the urban population. This paper investigates the patterns of settlement growth in the region surrounding Addis Ababa and their impact on peri-urban agriculture using an urban spatial scenario design model. The effects of two population density scenarios are explored within the framework of a proposed master plan. The model output was used to estimate areas of different suitability levels that would be lost to the modelled settlement expansion. The settlement area in 2038 would represent 29% of the case study’s total area in the low-density scenario but only 19% in the high-density scenario. Compared to the low-density scenario, the high-density scenario would only require a third of the agricultural land transformed into settlement areas. Settlement development would contribute to higher losses of land suitable for cultivating important export products, high nutritional value and import-substituting products. The scenario approach can support sustainable regional planning for settlement expansion that conserves valuable farmland in the peri-urban area and contributes to building capacity for strategic planning of the city regions of sub-Saharan Africa.     

The sedimentary record of the Issyk Kul basin,Kyrgyzstan: climatic and tectonic inferences

A broad array of new provenance and stable isotope data are presented from two magnetostratigraphically dated sections in the south‐eastern Issyk Kul basin of the Central Kyrgyz Tien Shan. The results presented here are discussed and interpreted for two plausible magnetostratigraphic age models. A combination of zircon U‐Pb provenance, paleocurrent and conglomerate clast count analyses is used to determine sediment provenance. This analysis reveals that the first coarse‐grained, syn‐tectonic sediments (Dzhety Oguz formation) were sourced from the nearby Terskey Range, supporting previous thermochronology‐based estimates of a ca. 25–20 Ma onset of deformation in the range. Climate variations are inferred using carbonate stable isotope (δ¹⁸O and δ¹³C) data from 53 samples collected in the two sections and are compared with the oxygen isotope compositions of modern water from 128 samples. Two key features are identified in the stable isotope data set derived from the sediments: (1) isotope values, in particular δ¹³C, decrease between ca. 26.0 and 23.6 or 25.6 and 21.0 Ma, and (2) the scatter of δ¹⁸O values increased significantly after ca. 22.6 or 16.9 Ma. The first feature is interpreted to reflect progressively wetter conditions. Because this feature slightly post‐dates the onset of deformation in the Terskey Range, we suggest that it has been caused by orographically enhanced precipitation, implying that surface uplift accompanied late Cenozoic deformation and rock uplift in the Terskey Range. The increased scatter could reflect variable moisture source or availability caused by global climate change following the onset of Miocene glaciations at ca. 22.6 Ma, or enhanced evaporation during the Mid‐Miocene climatic optimum at ca. 17–15 Ma.     

Experimental Investigation of Frictional Behavior Between Cohesive Soils and Solid Materials Using Direct Shear Apparatus

Soil shearing resistance is very important while designing various structures which have direct contact with soil, for example, sheet piles, piles, retaining walls, reinforced earth structures and shallow foundations. Even though designers use empirical values for their design, it is very important to obtain more accurate values for soil–solid materials shearing resistance. In this work, laboratory tests have been carried out to investigate the effect of roughness interface and texture models on friction angle between cohesive soils and steel, as well as abrasive paper material, using direct shear tests. All tests were carried out under consolidated drained shear conditions. The behavior at the soils–solid interface was found to vary according to surface roughness. It also seems that the type of material (steel or paper of abrasive) used does not have a major influence on the shear strength. As far as roughness is concerned, friction behavior is likely to be generally classified into three failure modes, namely full sliding at the interface, shear failure within the soil, and a mixed behavior where interface sliding and shear deformation of the soil specimen proceed simultaneously. However, for the second mode, the shear strength at the interface soil-rough solid materials steel was found to be lower than the shear strength of the soil, for a soil that is classified as high plasticity clay. Furthermore, it was found that the interfacial shear strength is independent of the texture surface for a given roughness.     

Paleoproterozoic bimodal post-collisional magmatism in the southwestern Amazonian Craton,Mato Grosso,Brazil: Geochemistry and isotopic evidence

This paper discusses geological and geochemical aspects of a Paleoproterozoic volcano-plutonic association that crops out in southwestern Amazonian Craton, Mato Grosso, Brazil. The study area was divided into undeformed and deformed domains, based on structural and geochronology studies. The undeformed domain is composed mainly of felsic explosive and effusive flows. Inter-layered mafic flows of basalts and sedimentary rocks are also present. The deformed domain is mainly composed of titanite and hornblende-bearing monzogranite to syenogranite and biotite monzogranite, while granodiorite is less common. U–Pb single zircon analyses yielded ages of 1.8–1.75 Ga in granites and felsic volcanic rocks for both domains. Basalts from the undeformed domain are phaneritic, fine-grained, and are often hydrothermally altered. They show tholeiitic affinity and are LREE enriched. Their trace element composition resembles those of within-plate associations. The ε_Nd (t = 1.75 Ga) for all these rocks are positive, ranging from 0.12 to 1.49, which reflect a juvenile source. The felsic volcanism comprises subalkaline rocks with high K contents and is divided into two groups: crystal enriched ignimbrites and effusive rhyolites. REE patterns of effusive rocks show negative-Eu anomalies and are smooth in the ignimbrites. Trace element patterns similar to those of the effusive rocks and ignimbrites are found in magmatic rocks derived from sources affected by subduction-related metasomatism. Hornblende and biotite granites occur in the deformed felsic plutonic domain. These rocks show evidence of low-temperature metamorphism and deformation, and in some places, of hydrothermal alteration. The LREE/Nb (or Ta) ratios of these rocks are consistent with those observed in granites of post-collisional settings. The ε_Nd (t = 1.75 Ga) values are slightly negative on average, with few positive values (?1.41 to +2.96). These data are interpreted as indicative of a magmatism produced during a post-collisional event from mixed sources: a metasomatised mantle and a Paleoproterozoic continental crust. An intracontinental shearing with age of 1.7–1.66 Ga created the Teles Pires–Juruena lineament which partially controlled this magmatism.     

A microseismic experiment in Abu Dhabi,United Arab Emirates: implications for carbonate reservoir monitoring

A microseismic experiment utilizing a single downhole array of eight 3-component receivers was conducted in an offshore oilfield in the emirate of Abu Dhabi in the United Arab Emirates. The Lower Cretaceous Thamama Group is the major carbonate reservoir in the field producing from six zones. Microseismic data was acquired in conjunction with gas injections that took place in the Thamama IV A and IV B reservoirs during 9 days of acquisition. The aim of the experiment was to monitor the microseismic activity arising in the carbonate reservoirs as a result of gas injection. A total of 103 microseismic events were detected from two of the receivers. For the majority of these events, there were no well-defined P wave arrivals, probably due to the presence of strong background noise and the very weak microseismic signals. The results from this experiment indicate that the detected events are probably related to the microseismic activities caused by the gas injection within the Thamama IV B reservoir. Therefore, downhole microseismic monitoring methods can potentially provide valuable information about the fracture systems within the carbonate reservoirs of Abu Dhabi's oilfields.     

Stable isotope evidences of jarosite–barite mineralization in the Rangan rhyolitic dome NE Isfahan,Iran

The Rangan area is part of Cenozoic magmatic belt of central Iran. Eocene volcanic flows and pyroclastic rocks are intruded by a Neogene rhyolitic dome along the major Qom–Zefreh fault. The dome is pervasively hydrothermally altered. The main mineral assemblage is jarosite+barite+pyrite+quartz+sericite. This assemblage indicates acid sulphate or advanced argillic alteration. Sulfur and oxygen isotope data (δ³⁴S & δ¹⁸O (SO₄)) obtained from jarosite and barite indicate a mixing episode during the evolution of hydrothermal system and reflect the overlapping of two distinct sources of acid sulphate alteration in Rangan, i.e., a magmatic–hydrothermal fluid reacting with steam-heated meteoric water. Considering the position of brittle–ductile transition and major fault movements, jarosite and barite seemingly precipitated from rapid injection of magmatic–hydrothermal fluids into the upper portions of a steam-heated environment.     

Alexandria-Nile Delta coast,Egypt: update and future projection of relative sea-level rise

Previous studies have indicated that the Nile River deltaic plain is vulnerable to a number of aspects, including beach erosion, inundation, and relatively high rates of land subsidence. This issue motivates an update and analysis of new tide-gauge records, from which relative sea-level changes can be obtained. Estimated rates from five tide gauges are variable in terms of magnitude and temporal trend of rising sea level. Analysis of historical records obtained from tide gauges at Alexandria, Rosetta, Burullus, Damietta, and Port Said show a continuous rise in mean sea level fluctuating between 1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria harbor, while the higher one at the Rosetta promontory. These uneven spatial and temporal trends of the estimated relative sea-level rise (RSLR) are interpreted with reference to local geological factors. In particular, Holocene sediment thickness, subsidence rate and tectonism are correlated with the estimated rates of relative sea-level change. From the relatively weak correlation between them, we presume that tectonic setting and earthquakes, both recent and historical ones, contribute more to accelerated RSLR than that of dewatering and compression/dewatering of Holocene mud underlying the Nile Delta plain. As a result, large areas of the coastal plain have been subsided, but some sectors have been uplifted in response to tectonic activities of thick underlying older strata. Projection of averaged sea-level rise trend reveals that not all the coastal plain of the Nile Delta and Alexandria is vulnerable to accelerated sea-level rise at the same level due to wide variability of the land topography, that includes low-lying areas, high-elevated coastal ridges and sand dunes, accretionary beaches, and artificially protective structures. Interaction of all aspects (tectonic regime, topography, geomorphology, erosion rate, and RSLR rate) permitted to define risk areas much vulnerable to impacts of sea incursion due to accelerated sea-level rise.     

Isotopic studies on detrital zircons of Silurian�CDevonian siliciclastic sequences from Argentinean North Patagonia and Sierra de la Ventana regions: comparative provenance

The Silurian?CDevonian siliciclastic sedimentary units known as Sierra Grande Formation and the upper part of the Ventana Group crop out in the eastern area of the North Patagonian Massif and in the Ventania system, toward the Atlantic border of Argentina. Both sequences show similar stratigraphical characteristics and were deposited in a shallow marine platform paleoenvironment. Previous contributions have provided evidence of an allochthonous Patagonia terrane that amalgamate to Gondwana during the Permian?CTriassic. However, other lines of research support a crustal continuity southward, where the Pampean and Famatinian events extend into the northern Patagonia. In either case, the detrital input to the Eo?CMesopaleozoic basins generated along the passive margin tectonic setting should reflect the sedimentary sources. In this contribution, new age data on the sedimentary provenance of these units is provided by U?CPb and Lu?CHf isotopic studies on detrital zircons, using LA-ICP-MS and SHRIMP methodologies. The main sedimentary sources of detrital zircons for both regions are of Cambrian?COrdovician and Neoproterozoic age, while a secondary mode is Mesoproterozoic. Zircons from older cratonic sources (Mesoarchean?CPaleoproterozoic ages) are scarcely recorded. The sample from the upper section of the Devonian Lolén Formation (Ventana Group) shows an important change in the sedimentary provenance, with a main mode of Mesoproterozoic detrital zircons. Detrital source areas considering the orogenic cycles known for southwest South America (Famatinian, Pampean?CBrasiliano, Mesoproterozoic?C??Grenvillian?? and Paleoproterozoic?C??Transamazonian??) are proposed.