A damage model for the continuum rheology of the upper continental crust

In orogenic zones it is often considered appropriate to use a continuum rheology to model the deformation of the upper continental crust. In this paper we derive an applicable rheology utilizing fiber-bundle and continuum-damage models. We show that the results are identical and yield a power relation between stress σ and strain rate of the form σ= ^ρ−1. We constrain the applicable values of ρ utilizing Omori's law for the decay of aftershocks and conclude that ρ−1 is in the range of 5–15. With this strong nonlinear viscous rheology the behavior of the deforming upper crust approaches that of a perfect-plastic material.     

Evolutionary connections of biological kingdoms based on protein and nucleic acid sequence evidence

Evolutionary trees are derived based on nucleic acid and protein sequences and the methods of numerical taxonomy. From 5S ribosomal RNA sequences, molecules that participate in protein synthesis, the order of branching of many prokaryote species and the nuclear and organellar components of eukaryote species are inferred. All oxygen-evolving forms, including the chloroplasts and blue-green algae, are to be found on a single branch; the mitochondria are found with the Rhodospirillaceae, nonsulfur photosynthetic bacteria; and the eukaryote cytoplasmic components are found with the archae-bacteria, including Thermoplasma and Sulfolobus. This tree clearly supports the symbiotic origin of the eukaryote organelles. There is much redundant information available from c-type cytochromes, cytochromes c′, 4Fe-4S ferredoxins and 5.8S rRNA molecules. A gene duplication of ferredoxin, probably predating all of the species divergences, permitted us to infer an ancestral molecule that can be placed on the tree to give the base. The heterotrophic anaerobes have a sequence most resembling the ancestral sequence, suggesting that the ancestor may have been a heterotroph. Ten prokaryote species occur on both the c-type and the c′-type cytochrome trees. There is only one inconsistency in the topologies of the two trees, which may reflect a gene transfer among prokaryotes confirming that, for these genes of fundamental importance, gene transfer followed by acceptance is rare. The eukaryote trees derived from 5S and 5.8S rRNA suggest that the earliest eukaryotes were unicellular and that the multicellular forms of plants, animals, fungi and slime molds arose independently.     

Sensitivity of cool-temperate forests and their fossil pollen record to rapid temperature change

Simulations of cool-temperate forest growth in response to climatic change using the JABOWA computer model show that a decrease of 600 growing degree-days (equivalent to a 2°C decrease in mean annual temperature) causes red spruce (Picea rubens) to replace sugar maple (Acer saccharum) as the dominant tree. These changes are delayed 100–200 yr after the climatic cooling, producing gradual forest changes in response to abrupt temperature changes, and reducing the amplitude of response to brief climatic events. Soils and disturbances affect the speed and magnitude of forest response. The delayed responses are caused by the difference in sensitivity of adult trees and younger stages. The length of the delay depends on the life history characteristics of the dominant species. Delayed responses imply that fossil pollen deposits, even if they faithfully record the abundances of trees in forests, may not be able to resolve climatic changes within 100–200 yr, or to record very brief climatic events. This explains why pollen deposits do not as yet show responses to climatic changes during the past 100 yr. Only the Little Ice Age, which lasted several centuries, caused sufficient forest change to be recorded in fossil pollen, and only at certain sites.     

An objective technique for determining end-member compositions and for partitioning sediments according to their sources

A modification of Q-mode factor analysis provides an objective technique for end-member characterization of geologic mixtures. Factor analysis has not often been used to determine the actual composition of end-member sources in geologic mixtures, because transformations of the original data variables during the analysis result in negative factor scores for some variables and negative “concentrations” of some variables in the end-members. We use a non-orthogonal rotation of end-member vectors toward the mean vector to bring the end-members into the positive vector space.This technique is demonstrated on a large data set of elemental analyses of bulk sediment surface samples from the Nazca Plate in the southeast Pacific. These data were chosen because they had been studied in detail previously. Our technique identified five geologically reasonable end-members which represent detritus, siliceous biogenic sediment, hydrothermal sediment, hydrogenous or authigenic sediment, and a refractory residue left after the dissolution of biogenic debris. The number, composition, and distribution of the end-members are similar to those derived from previous extensive partitioning studies of the sediments.     

Water flow and the vertical distribution of meiofauna: A flume experiment

Laboratory experiments were conducted in a recirculating saltwater flume to determine if flow influences the vertical distribution of meiofauna within fine cohesive sediments. Replicate flume boxcores collected from a nonvegetated mudflat were exposed to either a no flow or a flow experimental treatment. After 3 h in the flume, the boxcores were sampled by taking smaller sediment cores and sectioning these cores vertically in 2-mm intervals. There was no statistical difference in the number of copepods (adults, copepodites, and nauplii) in the top 2mm of sediment in flow vs. no flow treatments. The number of nematodes and foraminiferans within the top 2 mm of sediment was significantly lower in the flow treatments than in the no flow treatments. Downward migrations may decrease the susceptibility of these fauna to passive erosion by tidal currents.     

An investigation of the relationship between uniaxial compressive strength and degradation for selected rock types

The relationship between uniaxial compressive strength and degradation was investigated for selected rock types, by using regression analyses to determine whether degradation was a useful predictor of compressive strength. In addition, the effects of aggregate particle size, number of hammer blows during the degradation test, engineering index properties, petrographic characteristics, and water saturation on the compressive strength-degradation relationship were evaluated. The results show that strong inverse relationships exist between compressive strength and degradation (measured on a 9.5-4.75-mm size aggregate) for sandstones and igneous/metamorphic rocks, but that no significant relationship exists for limestones/dolomites. The results also indicate a strong positive correlation between degradation and L.A. abrasion loss and can be used to establish a limit of allowable degradation for practical applications. Engineering index properties do not significantly affect the relationship between compressive strength and degradation but petrographic characteristics are important in explaining the strength and degradation behavior of the rocks studied. Water saturation decreases compressive strength and increases degradation to varying degrees.     

Stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western Canadian shield

New U–Pb geochronology for an extensive exposure of high-pressure granulites in the East Lake Athabasca region of the western Canadian shield is consistent with a history characterized by 2.55 Ga stabilization of cratonic lithosphere, 650 million years of lower crustal residence and cratonic stability, and 1.9 Ga reactivation of the craton during lithospheric attenuation and asthenospheric upwelling. High precision single-grain and fragment zircon data define distinctive discordia arrays between 2.55 and 1.9 Ga. U–Pb ion microprobe spot analyses yield a similar range of U–Pb dates with no obvious correlation between date and cathodoluminescence zonation. We attribute the complex U–Pb zircon systematics to growth of the primary populations during a 2.55 Ga high-pressure granulite facies event (~1.3 GPa, 850°C) recorded by the dominant mineral assemblage of the mafic granulite gneisses, with subsequent zircon recrystallization and minor secondary zircon growth during a second high-pressure granulite facies event (1.0 GPa, ~800°C) at 1.9 Ga. The occurrence of two discrete granulite facies metamorphic events in the lower crust, separated by an interval of 650 million years that included isobaric cooling for at least some of this time, suggests that the rocks resided at lower crustal depths until 1.9 Ga. We infer that this phase of lower crustal residence and little tectonic activity is coincident with an extended period of cratonic stability. Detailed structural and thermochronological datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years. Extended lower crustal residence would logically be the history inferred for lower crust in most cratonic regions, but the unusual aspect of the history in the East Lake Athabasca region is the subsequent lithospheric reactivation that initiated transport of the lower crust to the surface. We suggest that a weakened strength profile related to the 1.9 Ga heating left the lithosphere susceptible to far-field tectonic stresses from bounding orogens that drove the lower crustal exhumation. An ultimate return to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1.7 Ga.     

Emerging flux in active regions

We have compared the rates at which flux emerges in active and quiet solar regions within the sunspot belts. The emerging flux regions (EFRs) were identified by the appearance of arch filament structures in H. All EFRs in high-resolution films of active regions made at Big Bear in 1978 were counted. The comparable rate of flux emergence in quiet regions was obtained from SGD data and independently from EFRs detected outside the active region perimeter on the same films. The rate of flux emergence is 10 times higher in active regions than in quiet regions. A sample of all active regions in 31 days of 1983 gave a ratio of 7.5. We discuss possible mechanisms which might funnel new magnetic flux to regions of strong magnetic field.     

Forests in Limbo: Assessing Costa Rica's Forest and Land Reform Policies

In this article, we consider the stalled implementation of two state policies among recipients of land reform in Costa Rica: the transfer of authority over forests on state agrarian reform lands over to the Ministry of the Environment, and the Rural Development Institute’s requirement to issue land title to the recipients of its land reform projects. To date, neither of these policies is even partially implemented, resulting in both forests and marginalized landholders existing in a state of dual tenure limbo. In this article we draw on document analysis, interviews with policymakers, and interviews with recipients of land reform to determine why these policies have not been implemented. In addition, we assess the potential consequences for forest conservation that this situation has produced by mapping the extent and characteristics of forests that occupy this state of uncertainty.