Crustal structure and origin of the Cape Verde Rise

The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m^− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.     

Oceanic tholeiites on the Cape Verde Islands: petrochemical and geochemical evidence

The mineralogy, major element composition, REE patterns and Sc, Co, Hf, Cr, Sr, and Cu contents of nine pillow lavas from the pre-Cenozoic basement complex of Maio reveal the presence of oceanic tholeiites on the Cape Verde archipelago.These uplifted tholeiites constitute the oldest rocks exposed on the islands and are related to an Early Cretaceous (or still somewhat older?) oceanic ridge crest. It is suggested that the formation of east-west trending fracture zones in the Atlantic Ocean induced the alkalic character of the Cape Verde magmatism from the middle of the Cretaceous up to historical times.     

A propagating fracture model versus a hot spot origin for the Canary islands

The evidence for a hot spot origin for the Canary Islands is reviewed, and it is found that the geochronological and geophysical data do not support this model. The propagating fracture hypothesis explains in a more convincing way the location and chronology of the Archipelago, whose three main periods of activity — corresponding to basal complexes, table-land basalts and recent emissions — are suggested to be related in time and genetically to the three main tensional phases in the nearby High Atlas Mountains.     

Mantle plume heterogeneity versus shallow-level interactions: A case study,the São Nicolau Island,Cape Verde archipelago

We present new Sr–Nd–Pb isotopic data, as well as major- and trace-element concentrations, for 19 basaltic samples from São Nicolau Island, Cape Verde archipelago. Fine-scale study of the island argues in favor of mixing between four endmembers to explain isotopic variations of collected samples: 1) a radiogenic endmember (⁸⁷Sr/⁸⁶Sr ~ 0.7034; ¹⁴³Nd/¹⁴⁴Nd ~ 0.51285; ²⁰⁶Pb/²⁰⁴Pb ~ 20.0; ²⁰⁷Pb/²⁰⁴Pb ~ 15.65; ²⁰⁸Pb/²⁰⁴Pb ~ 39.8) representative of the Cape Verde plume deep source; 2) an unradiogenic endmember having isotopic compositions resembling those of Atlantic MORB dredged at the same latitude; 3) a low Sr–high Nd and Pb endmember identified as the Jurassic MORB basement of the archipelago; and 4) São Vicente-like Cape Verde carbonatites. Compositional and isotopic results show that most of the measured variations can be related to mixing of plume-derived melts with shallow-level reservoirs. Therefore the source heterogeneity of the Cape Verde plume is much smaller than the one sampled in basaltic samples. This observation illustrates how caution is required when interpreting global OIB data in terms of mantle topology without filtering from the contribution of shallow-level reservoirs.     

Flow in an aquifer charged with hot water from a fault zone

Many geothermal anomalies are intersected by vertical fault zones (narrow zones of fractured material with large effective permeability). These conduits are probably responsible for much of the upwelling of hot water from depth. This paper considers a shallow aquifer intersected by a vertical fault. The fluid flow in the aquifer is numerically modeled as a two-dimensional problem. It is observed that the temperature distribution in the aquifer is governed primarily by lateral flow of hot water supplied from the intersecting vertical fault and only secondarily by conduction. The numerical results also provide a possible explanation for the local temperature maxima and inversions occasionally observed in borehole measurements. The present model is an alternative to that based on mushroom-shaped isotherm distributions found in high Rayleigh number large-scale circulation cell calculations.     

TopSTO: a 115-line code for topology optimization of structures under stationary stochastic dynamic loading

The use of topology optimization in structural design under dynamic excitation is becoming more prevalent in the literature. While many such applications utilize frequency or time domain formulations, relatively few consider stochastic dynamic excitations. This paper presents an efficient and compact code called Top STO for structural topology optimization considering stationary stochastic dynamic loading using a method derived from random vibration theory. The theory, described in conjunction w...     

Late Cretaceous-Early Cenozoic volcanism of Southern Mongolia: A trace of the South Khangai mantle hot spot

The Gobi-Tien Shan volcanic area (in Southern Mongolia) is part of the South Khangai volcanic region (SKVR). The formation of its lava fields was related to three stages of volcanic activity: the Late Cretaceous (88–71 Myr), Paleocene-Early Eocene (62–47 Myr), and Early Oligocene (37–30 Myr). Volcanic occurrences of different age are represented by trachybasalt, trachyandesitobasalt, basanite, and melanephelinite with similar geochemical characteristics, which are also close to the geochemical characteristics of OIB basalt. The isotope composition (Sr, Nd) of the rocks indicates that the magma sources were formed as a result of mixing of a moderately depleted PREMA mantle and an EM-I mantle enriched in neodymium.The patterns of migration of volcanic centers of different ages over the area of interest have been studied. The earliest (Late Cretaceous) volcanic occurrences were concentrated mainly in the south of the area, the Paleocene-Early Eocene eruptions took place at the center of the area, and the Early Oligocene volcanism occurred in the northern area. The observed migration of the volcanic activity centers is related to lithospheric plate motions relative to a localized source of hot mantle (the South Khangai mantle hot spot), which controlled volcanic activity within SKVR. In the lithospheric structure of this region, local asthenospheric high, reaching a depth of ～50 km, correspond to this hot spot.     

Upper mantle structure in the ocean-continent transition zone: Kamchatka

We consider results from modeling the crustal and upper mantle velocity structure in Kamchatka by seismic tomography and compare these with gravity data and present-day tectonics. We found a well-pronounced (in the physical fields) vertical and lateral variation for the upper mantle and found that it is controlled by fault tectonics. Not only are individual lithosphere blocks moving along faults, but also parts of the Benioff zone. The East Kamchatka volcanic belt (EKVB) is confined to the asthenospheric layer (the asthenosphere lens) at a depth of 70–80 km; this lens is 10–20 km thick and seismic velocity in it is lower by 2–4%. The top of the asthenosphere lens has the shape of a dome uplift beneath the Klyuchevskoi group of volcanoes and its thickness is appreciably greater; overall, the upper mantle in this region is appreciably stratified. A low-velocity heterogeneity (asthenolith) at least 100 km thick has been identified beneath the Central Kamchatka depression; we have determined its extent in the upper mantle and how it is related to the EKVB heterogeneities. Gravity data suggest the development of a rift structure under the Sredinnyi Range volcanic belt. The Benioff zone was found to exhibit velocity inhomogeneity; the anomalous zones that have been identified within it are related to asthenosphere inhomogeneities in the continental and oceanic blocks of the mantle.     

Estimating sediment accumulation rates in Manila Bay, a marine pollution hot spot in the Seas of East Asia

The GEF/UNDP/IMO/PEMSEA project identifies Manila Bay as among the marine pollution hot spots in the Seas of East Asia. ²¹⁰Pb dating of its sediment can provide a historical perspective of its pollution loading. However, the validity of ²¹⁰Pb dating in a complex dynamic coastal system of Manila Bay may come into question. Land-based sediment input can be high and physical and biological processes can possibly disturb the sediment layers. In this report, the ²¹⁰Pb profiles of sediment cores from different parts of the bay are presented. The linear sedimentation rates are shown to be higher in the recent past and are also variable across the bay. The largest change in sedimentation rate, coincided with the occurrence of a volcanic eruption in 1991 and is shown by applying a variant of the CIC model in sedimentation rate calculations. The data suggest that ²¹⁰Pb dating can be useful in estimating relative magnitudes of sedimentation rates, even in a complex dynamic coastal system like Manila Bay.     

龙门山断裂带地壳精细结构与汶川地震发震机理

利用2001年1月至2008年6月四川固定地震台网和临时地震台站记录到的大量P波到时资料,反演了龙门山断裂带及周边地区的地壳精细三维P波速度模型. 结果表明,汶川主震以北和以南地区的结构存在较大差异,以北地区的龙门山断裂带具有很强地壳不均匀性,这与该区发生了大量汶川地震的余震相一致. 这些结果有意义地改进了前人对龙门山断裂带仅为不同块体过渡带的认识. 汶川主震震源区下方存在有明显低波速异常体,表明流体可能存在于龙门山断裂带内. 这些流体可能直接影响汶川大震的形成. 本文的成像结果为下地壳流沿龙门山断裂带上浸提供了可靠的地震学证据.     

The capture efficiency map: the capture zone under time-varying flow

The capture zone or contributing area of a ground water extraction well can be defined as that portion of the aquifer from which the well draws its water. Accurate delineation of capture zones is important in many ground water remediation applications and in the definition of wellhead protection areas. Their mathematical delineation is often simplified by using quasi-steady-state models based on time-weighted average pumping rates and background hydraulic gradients. We present a new semianalytic approach for the definition of capture zones under transient-flow conditions. We then use this approach to evaluate the effects of time variations in the direction of the background hydraulic gradient on capture. Results are presented in the form of capture efficiency maps (CEMs). Although the area contributing to a given well is found to generally expand relative to the steady-state average capture zone when the gradient direction varies, the zone of 100% capture may expand or contract depending on site-specific conditions. We illustrate our CEM approach by applying it to the design of a plume containment system.     

A longitudinal seismic reflection profile of the Reykjanes Ridge: Part II — Implications for the mantle hot spot hypothesis

A longitudinal seismic reflection profile of the Reykjanes Ridge, together with earthquake seismicity patterns, is interpreted in terms of the mantle plume hypothesis. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing.The 57°N transition from fractured to unfractured basement occurs about 900 km southwest of the postulated Iceland mantle plume. The fractured province exhibits higher seismicity and rougher basement, on transverse profiles, than does the unfractured province. A similar transition to rougher, more seismic ridge crest also occurs 900 km northeast of Iceland. We propose that flowage of hot, basalt-rich asthenosphere away from the Iceland hot spot keeps the axial lithosphere hot, thin, sparsely fractured, and relatively aseismic out to 900 km from the plume. Similar effects are evident in the vicinity of some other plumes located near spreading axes. Some plumes also exhibit a greater number of earthquakes at some distance from the spreading axis — possibly a reflection of non-axial igneous activity or fracturing due to local, plume-generated stresses.The regional basement slope along the longitudinal profile is about 8 × 10^?4. If this slope represents a balance between viscous and gravity forces in the flow, a viscosity of the order 10¹⁹ poises can be estimated from the Poiseuille equation.A peculiarly flat, opaque reflector was discovered near the Reykjanes axis, about 300 km southwest of Iceland. Several hypotheses are advanced to account for such reflectors by the exceptional volcanic activity associated with high plume discharge.     

Contour scaling of a sandstone by salt weathering under simulated hot desert conditions

A laboratory study has been used to investigate relationships between salts and contour scaling—a weathering feature commonly observed on rock surfaces in salt-rich environments. Surface disaggregation and essentially surface-parallel cracks were produced in sandstone blocks using 10 per cent solutions of sodium sulphate and magnesium sulphate applied daily to single exposed surfaces for sixty days. A control block soaked once in saturated magnesium sulphate and subsequently wetted daily with distilled water showed extensive surface disaggregation, but no cracking. Both surface disaggregation and subsurface cracking were associated with relative concentrations of microcrystalline salt. A tentative model of contour scaling is proposed, which involves linking together potential cracks by salt-induced fracturing of intervening, crack-stopping grains. Further control blocks treated respectively with 10 per cent and saturated sodium chloride showed no evidence of subsurface cracking and only limited surface disaggregation of the ‘saturated’ block.     

Cosmogenic 3He exposure dating of the Quaternary basalts from Fogo,Cape Verdes: Implications for rift zone and magmatic reorganisation

We have used cosmogenic ³He to date pre- and post-collapse lava flows from southwestern Fogo, Cape Verdes, in order to date rift zone magmatic reorganisation following the lateral collapse of the flank of the Monte Amarelo volcano. The post-collapse flows have exposure ages ranging from 62 to 11 ka. The analysis of multiple flow tops on each lava flows, often at different elevations, provides an internal check for age consistency and the exposures ages conform with stratigraphic level. The exposure ages suggest that volcanic activity along the western branch of the triple-armed rift zone was more or less continuous from before 62 ka to approximately 11 ka. The absence of magmatic activity for the last 11 kyr reflects a structural reconfiguration of the volcano and may be related to renewed flank instability. This volcanic hiatus is similar in duration to that observed in the Canary Islands. Replicate ³He exposure ages of a pre-collapse flow (123.0 ± 5.2 ka) brackets the time of the Monte Amarelo collapse between 62 ka and 123 ka. Reproducible cosmogenic ³He exposure ages of less than 123 ka from flows away from major erosion features demonstrates that the technique is a viable alternative to the radiocarbon, K/Ar and ⁴⁰Ar/³⁹Ar chronometers for dating recent volcanism in arid climate zones.     

On the non-reducibility of non-stationary correlation functions to stationary ones under a class of mean-operator transformations

Some special functional equations involving means and related to a problem of reducibility of some classes of correlation functions are considered. We show some characterizations of the reducibility problem under several choices of the mean operators and different weak regularity assumptions imposed on the involving functions. We find that mean-generated correlation functions are completely irreducible, in the sense that, for this broad class of correlation functions, there does not exist a non-trivial solution associated to the Perrin–Senoussi problem.     

Badlands as a hot spot of petrogenic contribution to riverine particulate organic carbon to the Gulf of Lion (NW Mediterranean Sea)

Determining the riverine carbon fluxes to oceans is critical for an improved understanding of C budgets and biogeochemical cycles (C, O) over a broad range of spatial and time scales. Among the particulate organic carbon (POC) involved in these fluxes, those yielded by sedimentary rocks (petrogenic POC: pPOC) remain somewhat uncertain as to their source on continental surfaces. Based on time series from long‐term observatories, we refine the POC and sediments flux of the Rhône River, one of the major tributaries to the Mediterranean Sea. Radiocarbon measurements on a set of riverine samples and forward modelling were used to (i) determine a modelled pPOC content and pPOC/POC ratio for each sample set, (ii) assess pPOC flux delivered to the NW Mediterranean Sea, and (iii) estimate the badlands contribution from the Durance catchment to both the pPOC and to sediment discharges. The weighted pPOC flux contributes up to 26% of the POC flux (145 Gg yr^‐1) discharged into the Mediterranean Sea, whereas the weighted pPOC content reaches 0.31 wt%. Despite their low contributive surface area (0.2%), badlands provide, respectively, 12, 3.5 and 14% of the pPOC, POC and sediment fluxes to the Rhône River. Consequently, such rocks can be considered as a major source of pPOC and sediments for the NW Mediterranean Sea and potentially for oceans. We suggest that river‐dominated ocean margins, such as the Rhône River, with badlands in their catchment could export a significant amount of pPOC to the oceans. Copyright © 2018 John Wiley & Sons, Ltd.     

Wavelet‐based non‐stationary response analysis of a friction base‐isolated structure

A wavelet‐based stochastic formulation has been presented in this paper for the seismic analysis of a base‐isolated structural system which is modelled as a two‐degree‐of‐freedom (2‐DOF) system. The ground motion has been modelled as a non‐stationary process (both in amplitude and frequency) by using modified Littlewood–Paley basis wavelets. The proposed formulation is based on replacing the non‐linear system by an equivalent linear system with time‐dependent damping properties. The expressions of the instantaneous damping and the power spectral density function (PSDF) of the superstructure response have been obtained in terms of the functionals of input wavelet coefficients. The proposed formulation has been validated by simulating a ground motion process. The effect of the frequency non‐stationarity on the non‐linear response has also been studied in detail, and it has been clearly shown how ignoring the frequency non‐stationarity in the ground motion leads to inaccurate non‐linear response calculations. Copyright © 2000 John Wiley & Sons, Ltd.     

Root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate

The root‐zone moisture replenishment mechanisms are key unknowns required to understand soil hydrological processes and water sources used by plants. Temporal patterns of root‐zone moisture replenishment reflect wetting events that contribute to plant growth and survival and to catchment water yield. In this study, stable oxygen and hydrogen isotopes of twigs and throughfall were continuously monitored to characterize the seasonal variations of the root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate in South Australia. The two studied hillslopes (the north‐facing slope [NFS] and the south‐facing slope [SFS]) had different environmental conditions with opposite aspects. The twig and throughfall samples were collected every ~20 days over 1 year on both hillslopes. The root‐zone moisture replenishment, defined as percentage of newly replenished root‐zone moisture as a complement to antecedent moisture for plant use, calculated by an isotope balance model, was about zero (±25% for the NFS and ± 15% for the SFS) at the end of the wet season (October), increased to almost 100% (±26% for the NFS and ± 29% for the SFS) after the dry season (April and May), then decreased close to zero (±24% for the NFS and ± 28% for the SFS) in the middle of the following wet season (August). This seasonal pattern of root‐zone moisture replenishment suggests that the very first rainfall events of the wet season were significant for soil moisture replenishment and supported the plants over wet and subsequent dry seasons, and that NFS completed replenishment over a longer time than SFS in the wet season and depleted the root zone moisture quicker in the dry season. The stable oxygen isotope composition of the intraevent samples and twigs further confirms that rain water in the late wet season contributed little to root‐zone moisture. This study highlights the significant role of the very first rain events in the early wet season for ecosystem and provides insights to understanding ecohydrological separation, catchment water yield, and vegetation response to climate changes.     

Gas tracer transport through a heterogeneous fracture zone under two phase flow conditions: Model development and parameter sensitivity

Large amounts of gas can result from anaerobic corrosion of metals and from chemical and biological degradation of organic substances in underground repositories for radioactive waste. Gas generation may lead to the formation of a gas phase bubble and to the migration of radioactive gaseous species. Transport occurs in, at least, in two forms: (1) gas bubble, migration is controlled by advection, dispersion and diffusion in the gas phase, and (2) within water pockets, the dissolved species migrate mainly by diffusion. We consider a two-dimensional system representing an isolated heterogeneous fractured zone. A dipole gas flow field is generated and gas tracers are injected. The delay in the breakthrough curves is studied. A simple method is used to solve the gas species transport equations in multiphase conditions. This method is based on a formal analogy between the equations of gas transport in a two phase system and the equations of solute tracer transport in water saturated systems. We perform a sensitivity analysis to quantify the relevance of the various transport mechanisms. We find that gas tracer migration is very sensitive to gas tracer solubility, which affects gas tracer transport of both mobile and immobile zones, and shows high sensitivity to diffusion in the gas phase, to heterogeneity and to gas pressure, but the largest sensitivity was observed with respect to injection borehole properties, i.e. borehole volume and water filled fraction.