Rare Earth Element Geochemistry of the Zigzag - Klondyke Sedimentary Rock Formations: Clues to the Evolution of the Baguio Mineral District (Luzon), Philippines

Abstract. Petrographic analysis of the Oligocene-Miocene Zigzag Formation and Miocene Klondyke Formation of the Baguio Mineral District reveals a transitional arc source for the former and an undissected arc source for the latter. Whole rock geochemistry of these sedimentary rock formations show affiliation of the Zigzag Formation to active continental margin whereas the Klondyke Formation appears to have been derived from an oceanic island arc source. A different aspect of the geology of the district is further gleaned from the whole rock rare earth element geochemistry of these rocks. Samples from the Zigzag Formation are characterized by higher REE concentrations compared to the Klondyke Formation samples. The REE data indicate that the sedimentary rocks of the Klondyke Formation had geochemically-less fractionated igneous lithologies as their progenitor whereas the older Zigzag Formation was derived from a more differentiated source. This new set of information contributes to the understanding of the evolution of this district from a marginal basin to an island arc setting.     

Progression of deformation and sedimentation in the southernmost Andes

The chronology of thrust motion in the Fuegian thin-skinned fold-thrust belt was established using data from the Atlantic coast of Tierra del Fuego. A set of original structural–geological maps showing the distribution of structures, unconformities and synorogenic sequences in the last tip of the Andes reveals the cratonward propagation of thrusts and sedimentary depocenters. A succession of syntectonic angular and progressive unconformities occur in the studied zone: (1) an angular unconformity between Danian and Late Paleocene sequences, (2) a series of progressive and syntectonic angular unconformities developed from the Late Early Eocene to the Late Eocene, and (3) a Lower Miocene syntectonic unconformity. Additional evidence for the time–space location of the thrust-front is provided by the presence of seismically triggered sand intrusions in Late Cretaceous, Late Paleocene and Middle Miocene sequences.The integration of data shows that faulting occurred in three main episodes: San Vicente thrusting, ca. 61–55 Ma, Río Bueno thrusting, ca. 49–34, and Punta Gruesa strike-slip event, ca. 24–16 Ma. San Vicente thrusting represents the onset of thrust propagation onto the foreland craton. The thrust-front endured a major cratonward migration through the Río Bueno thrusting, and remained steady afterward. Punta Gruesa constitutes a strike-slip event, associated with the phase of wrench deformation that influences the southernmost Andes since the Oligocene. Although the overall pattern of faulting was progressively younger cratonward, several episodes of out-of-sequence thrusting and folding occurred.Other features in the southernmost Andes can be linked to these three deformation events to broadly characterize the behavior of the Fuegian orogenic wedge in terms of critical taper models. The Fuegian Andes underwent at least three cycles between subcritical, critical and supercritical stages of behavior in terms of deformation, erosion, and sedimentation.     

Assessment of sidewall erosion in large gullies using multi-temporal DEMs and logistic regression analysis

Although in the last decades gully erosion has been a thriving research field, few studies have specifically addressed the contribution and location of sidewall erosion processes in gullies. In this paper, sidewall erosion in some large gullies in a Mediterranean area (Anoia-Penedès, NE Spain) is mapped and assessed for two time intervals (1975–1995 and 1995–2002), using detailed digital elevation models derived from aerial photographs at a scale of 1:5000 to 1:7000. Logistic regression analysis is applied to compute the probability of occurrence of gully sidewall erosion from terrain variables. The results confirm the complex nature of sidewall processes, whose intensity is most probably related to rainfall characteristics. Prolonged wet soil conditions in the period 1995–2002, together with the large and high-intensity rainfall of an extreme event occurred on 10th June 2000, help to explain the different sediment production rates: 16±0.4 Mg ha⁻¹ year⁻¹ in 1975–1995 and 83±6.3 Mg ha⁻¹ year⁻¹ in 1995–2002. The logistic regression analysis revealed that gully-wall slope angle was the main factor controlling gully sidewall failure. In gully walls with high slope angles, tension crack development is the main process promoting wall collapse. The application of the logistic regression model showed a high overall accuracy (87%) but over 50% of commission and omission errors for the class of interest (sidewall erosion), in agreement with the variance explained by the model.     

Effect of plant growth on Pb and Zn geoaccumulation in 300-year-old mine tailings of Zacatecas,México

Concentrations of Pb and Zn, plant uptake of these metals, the influence of the plants’ growth on the physicochemical properties and metal concentrations in the tailings of an abandoned 300-year-old mine tailing dam in Zacatecas, Mexico were investigated. Tailings were found to be heavily contaminated, with average levels of 2621 ± 53 and 3827 ± 83 mg/kg for Pb and Zn, respectively (maximum concentrations of 8466 ± 116 and 12,475 ± 324 mg/kg, respectively), exceeding international standards. Though physico-chemical conditions (pH, conductivity, redox potential, moisture, organic matter, nitrate, nitrite, ammonium nitrogen, total nitrogen, phosphorus and sulfates) do not favor the development of vegetation, some plants have adapted to these adverse conditions. Moreover, there was a significant reduction of Pb and Zn concentration in the rhizosphere (between 10–78% for Pb and 18–62% for Zn, depending on plant species). Sporobolus airoides showed average biomass concentrations of 173 ± 2 and 313 ± 6 mg/kg, for Pb and Zn, respectively; which implies a risk for mobility and possible incorporation into the food chain. Barcleyanthus salicifolius, Asclepsias linaria and Cortaderia selloana on the other hand, showed average biomass concentrations of 28 ± 3 and 121 ± 5 mg/kg of Pb and Zn, respectively, thus representing a lower biomagnification risk. The effect of these plants to reduce metal concentrations in the rhizosphere, improve physico-chemical conditions in metal polluted substrates, but with limited metal accumulation in biomass, suggests that they can be evaluated for use in stabilizing metal polluted tailings.     

Integrated modelling for water quality management in a eutrophic reservoir in south-eastern Portugal

The Enxoé Reservoir was built in 1998. Since 2000, it has exhibited frequent high chlorophyll-a concentrations, reaching a geometric mean three times higher than the national limit for eutrophication, presenting the reservoir with the highest eutrophic state in Portugal. Toxic algal blooms have also often been observed, which pose serious challenges to water managers, as the reservoir is used for potable water production (25,000 inhabitants). The objective of this study was to implement a reservoir model (CE-QUAL-W2), with inputs from a watershed model (SWAT), in order to represent the actual reservoir state and to test management measures to reduce its trophic level and algal bloom concentration peaks. The integrated model was used to depict the origin of its trophic status. Simulations were also compared to measured data at the reservoir surface (water level, nitrate, orthophosphate, suspended solids, and oxygen) and in water profiles (temperature, oxygen). The model was able to represent stratification and thermocline depths, as well as the actual chlorophyll-a and dissolved oxygen concentrations. The model results showed that internal phosphorus load from deposited sediments was an important factor in fuelling the algal blooms. This process occurs predominantly in summer, when stratification takes place and aeration is reduced, promoting anoxic conditions in the bottom waters. Since the reservoir is relatively shallow (average 5 m), released phosphorus is then easily able to reach the photic zone in most parts of the reservoir, where it is consumed. Different management scenarios were tested, suggesting that a mesotrophic level could barely be reached and maintained simply by reducing the nutrient loads (both external and internal). It is suggested that only an increase in the reservoir dam height could produce a mesotrophic level, averting anoxia by blocking the release of phosphorus from sediments to the photic zone. Future work should focus on a cost–benefit analysis to test the feasibility of each of the proposed scenarios, taking advantage of the integration strategy to assess where in the watershed load reductions would be most effective.     

The Hyperstatic Reaction Method for the Analysis of the Sprayed Concrete Linings Behavior in Tunneling

Hyperstatic Reaction Method considers the interaction between the shotcrete lining and the rock mass modelling half tunnel section by beam elements connected by nodes, which can develop bending moments, axial forces and shear forces. The interaction between ground and support is represented by Winkler type springs in the nodes of the numerical model, in the normal and tangential direction. 1944 analyses were developed considering three different tunnel radius values (2.0, 4.5 and 7.0 m), three different rock types (RMR = 40, RMR = 60 and RMR = 80), three different stress ratios acting on the linings (\(K = 1, \,K = 0.5\) and \(K = 0\)), three different lining thickness values (0.1, 0.2 and 0.3 m) and three values of the average elastic modulus of the shotcrete (6000, 9000 and 12,000 MPa). Besides, also two different stiffness values of the tangential springs at nodes were considered. Results show that the lining thickness and the elastic modulus of the shotcrete have a strong influence on the bending moment values which develop inside. The maximum axial force is not practically influenced by the lining elastic modulus and thickness. Both maximum bending moments and maximum axial forces increase in a linear way with the vertical load value; tunnel radius, \(K\) and RMR values of the rock mass have also a great influence on the maximum bending moment and axial force. Besides, it is possible to note how the absence of shear interaction between the lining and the rock mass causes bending moment values in the lining higher with respect to the case where the stiffness of the tangential springs at nodes equals to half the stiffness of the normal springs. For this reason, the neglectance of the shear interaction between the lining and the tunnel wall leads to an overdesign of the SC lining.     

Slab rollback and microcontinent subduction in the evolution of the Zambales Ophiolite Complex(Philippines):A review

New radiolarian ages show that the island arc-related Acoje block of the Zambales Ophiolite Complex is possibly of Late Jurassic to Early Cretaceous age.Radiometric dating of its plutonic and volcanichypabyssal rocks yielded middle Eocene ages.On the other hand,the paleontological dating of the sedimentary carapace of the transitional mid-ocean ridge-island arc affiliated Coto block of the ophiolite complex,together with isotopic age datings of its dikes and mafic cumulate rocks,also yielded Eocene ages.This offers the possibility that the Zambales Ophiolite Complex could have:(1)evolved from a Mesozoic arc(Acoje block)that split to form a Cenozoic back-arc basin(Coto block),(2)through faulting,structurally juxtaposed a Mesozoic oceanic crust with a younger Cenozoic lithospheric fragment or(3)through the interplay of slab rollback,slab break-off and,at a later time,collision with a microcontinent fragment,caused the formation of an island arc-related ophiolite block(Acoje)that migrated trench-ward resulting into the generation of a back-arc basin(Coto block)with a limited subduction signature.This Meso-Cenozoic ophiolite complex is compared with the other oceanic lithosphere fragments along the western seaboard of the Philippines in the context of their evolution in terms of their recognized environments of generation.     

Intact ash and charred litter reduces susceptibility to rain splash erosion post‐wildfire

This paper describes the changes in soil water repellency and soil hydrological and erosional responses to rainfall at small‐plot scale, arising from a prescribed fire immediately following burning and one year later in a Mediterranean heathland in the area of the Strait of Gibraltar (southern Spain). Very little research has been carried out about the modifications on the ground surface after fire immediately after burning. A prescribed fire was conducted to study short‐term changes of the ground surface immediately and one year following burning. After a prescribed fire, a homogeneous charred litter layer and ash‐bed covered the mineral soil surface. This cover stayed stable on the soil surface during a period of seven days, until strong winds redistributed litter and ashes. The hydrophobicity of the exposed surface (litter and ashes) decreased considerably in relation with the litter layer properties before the fire. Ponding, runoff coefficients and soil loss were determined using simulated rainfall over the litter layer, the ash‐bed and the bare soil. Significant differences were not detected between pre‐ and post‐fire soil loss rates while a charred litter and thick ash layer were present on the ground surface. Runoff and erosion rates increased and time to ponding and runoff decreased when the charred litter and ash layers were artificially removed and the bare soil was exposed. Although wildfires will increase soil erodibility, the trends observed in this study suggest that this increased susceptibility to erosion from rainsplash processes may be limited to some degree while an intact ash and charred litter layer is still present. Copyright © 2009 John Wiley & Sons, Ltd.     

An elasto-visco-plastic model using the finite element method for crustal and lithospheric deformation

A novel numerical model based on solid deformation is presented in this paper. This thermo-mechanical model can simulate the tectonic evolution of crust and (lithospheric and asthenospheric) mantle under different conditions. Our implementation uses the finite element method (FEM) in order to solve the equations. As a Lagrangian approach is employed, remeshing techniques are implemented to avoid distortion problems when a certain deformation threshold is reached. The translation of the state between the old and new mesh is achieved by means of the information stored on Lagrangian particles, which minimizes the diffusion. The model is able to represent elastic, viscous and plastic behaviour inside the studied domain. Three types of creep mechanism (diffusion, dislocation and Peierls) are included. Two different quadrilateral isoparametric elements were implemented and can be employed to perform the calculations. The first one is an element with 4 nodes, selective reduced integration and a stabilization operator to diminish hourglass modes, which reduces the computational time needed. The second one has 8 nodes located in standard positions, uses full integration scheme and has no hourglass modes as it satisfies the Inf-Sup condition. Several test cases with known solutions were run to validate the different aspects of the implementation.