Neogene north American-Caribbean plate boundary across Northern Central America: Offset along the polochic fault

The Polochic fault was a segment of the North American-Caribbean plate boundary across Central America in the Neogene. Its 130 km of left slip was previously determined by matching structures and stratigraphie outcrop patterns of northwest and central Guatemala across the fault. Additional support for the model and the youthfulness of the recorded offset comes from an essentially perfect match of major geomorphic features across the fault. A reconstruction process which eliminates 123 km of left slip brings together rivers and drainage divides that existed before the Polochic became active.With the reconstruction carried across the isthmus on an east-west fault the regional structural geology assumes the coherent pattern of a continuous orogenic belt whose geometry is compatible with the model of collisional tectonics centered on the Motagua “suture zone”. Confined within this belt, narrowed to some 60 km by the reconstruction, lie the major Laramide thrusts, folds and tectonically emplaced serpentinites of Guatemala. Crystalline rocks of Guatemala re-join the Chiapas Massif and Paleozoic sedimentary rocks, exposed in the core of an almost-continuous anticlinorium, extend from southern Chiapas to Lake Izabal.The Polochic does not bend in eastern Guatemala but continues eastward to the Motagua fault where it dies. Westward drift of the northern block resulted in rifting which extended from eastern Guatemala into the Caribbean along the Cayman trough. The Honduras depression may represent an element of a triple junction along with the Polochic and Izabal-Cayman rift.The Polochic continues westward into the Pacific Ocean and offsets the Middle America trench. The Polochic has offset the Miocene volcanic belt of northern Central America, confirming the previous estimate of a Neogene time of movement.About 300 km of relative east-west Neogene displacement has been recorded on the Mid-Cayman rise, only 130 km of which can be accounted for across the Polochic. It is suggested that cumulative extension on north-south faults south of the Motagua fault zone between the trench and the Honduras depression might make up that difference.     

Evaluation of lead movement from the abiotic to biotic at a small-arms firing range

An investigation to characterize the extent and speciation of lead contamination in water, soil, and surrounding biota was conducted at a small-arms firing and skeet range in West Point, New York. Specifically, lead concentrations were examined in sediment, soil, water, plants, fish and invertebrates. There is an elevated concentration of lead in the soil and sediment up to 11,000 g/g and 340 g/g and also evidence of bioconcentration of the lead by the surrounding biota. Earthworms had up to 90% higher concentrations of lead while tadpoles showed 20% higher concentrations compared with their controls. Lead uptake by indigenous plants gave varying results. Two species bioconcentrated lead 20 and 55 times greater than the control plants. These differences were significant (P <0.05 level) when tested by the students t test. Further studies show that the total leachable lead was highest in the invertebrates and vertebrates but not in the plants.     

Retrieval of atmospheric reflectivity profiles in case of long radar pulses

Future space-borne cloud profiling radars (CPR) will employ longer pulse lengths than ground-based systems. As a consequence, reflectivity profiles observed from space will be more strongly convoluted with the radar pulse shape. Here we describe a constrained linear inverse technique that reduces the effects of this convolution. We apply the technique to simulated data, based on actual observations from the ground, and show that significant improvements in the reflectivity profile can be obtained. Average Z errors (dBz) are halved, while cloud boundaries are substantially better retrieved. The results in this paper are relevant to space-borne missions like CloudSat and EarthCARE.     

U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower ²³²Th/²³⁰Th (∼10,000) than those immediately adjacent to the banks (24,000-31,000) and a (²³¹Pa/²³⁰Th) near the production ratio (∼0.1). The change in ²³²Th/²³⁰Th and (²³¹Pa/²³⁰Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of ²³⁰Th and ²³¹Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both ²³²Th/²³⁰Th (∼13,000) and (²³¹Pa/²³⁰Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ²³⁴U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ²³⁴U. Modern marine carbonates are shown to have ²³²Th/²³⁰Th ratios that reflect the local seawater in which they formed.     

Contrasting flow pathways within tropical forest slopes of Ultisol soils

There are very few experimental studies identifying hydrological pathways within rain forest slopes. Such knowledge is, however, necessary to understand why forest disturbance affects rainfall–riverflow response and nutrient migration. This study examines flow pathways within lowland rain forest slopes comprising Udults of the Ultisol soil order. Experimentation was conducted on four SE Asian hillslope units (each 5 × 5 m in plan) in the Bukit Timah catchment (Singapore Island), and in the W8S5 catchment (Sabah, Borneo Island). The flow pathways were identified by artificial tracer experiments. We evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns. The tracer work indicated much faster subsurface flows at Bukit Timah than W8S5 for the storms studied. Some explanation of the greater subsurface waterflows at Bukit Timah in comparison to W8S5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries. Vertical flow of the tracer through the upper 1 m of soil predominated (>90 per cent of percolation) in the Bukit Timah slopes. In some contrast, a major component (approximately 60 per cent) of the tracer percolation was directed laterally within the W8S5 slopes. The flow vectors calculated using the hydrometric methods did, however, grossly under‐estimate the degree of lateral deflection of waterflow generated at W8S5 and to a lesser extent over‐estimated it at Bukit Timah. In part, these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small ‘natural soil pipes’ present within both catchments. In conclusion, the study indicates that marked variations in flow vectors exist within the Udult great group of SE Asian soils and hydrometric calculations may be poor predictors of these dominant pathways. Copyright © 2005 John Wiley & Sons, Ltd.     

Oceanic electromagnetic studies: A review

The review covers the main results of the oceanic electromagnetic studies presented during last 4–5 years. Seafloor electromagnetic observations are dedicated to studies in solid Earth geophysics and oceanography. Thus, a large range of objectives and targets are under investigation. Technological and theoretical advances provide an increase of quantity and quality of the collected seafloor data. The host conductivity models for oceanic crust and mantle are being developed; the possibility of obtaining useful structural constraints in heterogeneous environment are discussed. Significant oceanographic results were derived from electromagnetic field observations; these appeared to be a new powerful method for study of long-term large-scale ocean variability.     

Nitrogen-bearing,aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides

Minerals in eclogites from different localities in the Western Gneiss Region of the Norwegian Caledonides (age 425 Ma) contain a variety of fluid inclusions. The earliest inclusions recognized are contained in undeformed quartz grains, protected by garnet, and consist of H₂O+N₂ (with ). The reconstructed P-V-T-X properties of these fluid inclusions are compatible with peak or early-retrograde metamorphic conditions. Matrix minerals (quartz, garnet, apatite, plagioclase) contain a complex pattern of mostly truly secondary inclusions, dominated by CO₂ and N₂. The textural patterns and P-V-T-X properties of these inclusions are incompatible with the high pressures of the eclogite-forming metamorphic event, but suggest that they were formed during uplift, by a combination of remobilization of preexisting inclusions and influx of external fluids. The fluid introduced at a late stage was dominated by CO₂, and did not contain N₂. The present data agree with theoretical predictions of eclogite fluids from mineral equilibria, and highlight the differences between granulite (CO₂) and eclogite (H₂O+N₂) fluid regimes. The provenance of the nitrogen in the eclogite fluid inclusions represents an important, but unsolved question in the petrology of high-pressure metamorphic rocks.Contribution no. 68 to the Norwegian programme of the International Lithosphere Project     

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Magnetization, susceptibility and Mössbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of theT _2g triplet of 1,120K. Predominant exchange interactions in the iron-rich samples are ferromagnetic withJ=1.2 K, as for other trioctahedral ferrous minerals. A peak in the susceptibility of thuringite occurs atT _m=5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the Mössbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4 Å^?1). A factor favouring this random ferromagnetic ground state over the planar antiferromagnetic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illustrate how long range ferromagnetic order may be destroyed by vortices forming around the Fe³⁺-Fe³⁺ pairs.     

Collisional plateaus

In the fifteen years since the importance of collisional plateaus with thickened continental crust began to be recognized as one of the inevitable consequences of the processes of plate tectonics, rapid progress in their understanding has come from studies of the world's only active terminal collision zones in the Himalayan-Tibetan and Turkish-Iranian plateaus.Ancient collisional plateaus are being recognized throughout the geological record (back to 3.8 Ga) from the occurrence of extensive areas (typically > 500,000 km²) of 8 kbar metamorphism in granulite facies or from the occurrence of extensive areas of higher level minimum-melt composition granite rocks whose isotopic signatures indicate reactivation of existing continental crust rather than addition of new crust from the mantle at the time of collision. Recognition of strike-slip faulting in the ancient collisional plateau areas indicates that “tectonic escape” may have been as important in the past as it is today.Earth may not be the only planet on which collisional plateaus are important. The highlands of Venus (approximately 7% of the surface with elevations over 1.5 km above mean planetary radius) can only exist as a result of crustal thickening, and not as a product of lithospheric thinning. Most of these highlands can be explained by models involving volcanic construction. However, the highest peaks, including Maxwell Montes, the highest mapped area of Venus rising over 10 km above mean planetary radius, require much greater crustal thickening to support them than can reasonably be explained by a volcanic mechanism. Geological features of Maxwell Montes inferred from radar images suggest some analogy between Maxwell Montes and the Tibetan plateau.It is somewhat paradoxical that extensional tectonics are commonly associated with continental collision, and that collision-related rifts may be the only sites where the uppermost layers of a collision-thickened crust are preserved from erosion. Extensional stress fields are generated during continental collision, primarily in areas associated with strike-slip faulting and “tectonic escape”. Additional extensional stresses are gravitationally generated associated with the topography and thickened crust in a collision zone. Tectonically thickened crust is particularly susceptible to rifting as its lithosphere is weak as a result of heating associated with magmatism. This lithosphere is also compositionally weak because of the relatively thick crust, dominated by a weak quartz rheology, and thin mantle lithosphere, dominated by a strong olivine rheology, in comparison with a lithosphere with a more normal crustal thickness. Thus, the common association of rifts and collision zones may be a consequence of both stresses generated during collision and modification of the lithosphere by collision.