Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U–Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications

New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean ²⁰⁶Pb/²³⁸U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and  29–26 Ma ⁴⁰Ar/³⁹Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean ²⁰⁶Pb/²³⁸U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation.

We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.     

Monitoring of atmospheric deposition in European forests and an overview on its implication on forest condition

Forest conditions in Europe have been monitored over 20 years jointly by the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) and the European Union (EU). Maps for mean bulk SO₄, NO₃ and NH₄ deposition at around 400 intensive monitoring plots in the years 1999–2001, as well as time trends for the period 1996–2001, are presented. Mean bulk SO₄ deposition at 169 plots mostly located in central Europe decreased from 7.4 to 5.8 kgS ha⁻¹ a⁻¹. Mean NH₄ bulk deposition decreased from 6.2 to 5.3 kgN ha⁻¹ a⁻¹. Nitrate bulk deposition fluctuated around 5 kgN ha⁻¹ a⁻¹. On average, throughfall deposition was considerably higher than bulk deposition. Time trends for mean tree crown defoliation as an overall indicator for forest condition show a peak in the mid 1990s for most of the monitored main tree species and a recent increase for the years 2003 and 2004. Multivariate linear regression analyses show some significant relations between deposition and defoliation. These relations depend on the tree species and site characteristics. Effects of deposition are moderated by the influence of biotic stress factors such as insects and fungi and by abiotic stress factors, such as weather.     

Dating of lower terrace sediments from the Middle Rhine area, Germany

Modern and known-age Pleistocene fluvial sediments were investigated by optical dating of quartz to test the suitability of the approach for dating deposits from the deeply incised Middle Rhine Valley. Samples from modern flood sediments revealed skewed distributions indicating different residual levels of equivalent dose (D_e) within the different aliquots. Nevertheless, a substantial number of aliquots from the modern deposits reflect D_e values close to zero. For the Pleistocene samples, optical ages are in general consistent with age control given by the presence of the Laacher See Tephra and radiocarbon dating. However, some samples overestimate the known age by a few thousand years when using the arithmetic mean. This is apparently explained by including aliquots in the determination of mean D_e where the optical signal was incompletely bleached at deposition. The most difficult issue in this context is identifying a suitable approach that can distinguish between the variability of D_e due to partial bleaching and microdosimetry. However, even when considering these limitations it appears that optical dating will by a quite suitable method to date Pleistocene sediments from such a complex fluvial environment, especially when focusing on a precision scale beyond a few thousand years.     

Imaging Offsets in the Moho: Synthetic Tests using Gaussian Beams with Teleseismic Waves

We carry out a sequence of numerical tests to understand conditions under which rapid changes in crustal thickness can be reliably imaged by teleseismic body waves. Using the finite-difference method over a 2-D grid, we compute synthetic seismograms resulting from a planar P-wavefield incident below the grid. We then image the Moho using a migration scheme based on the Gaussian beam representation of the wavefield. The use of Gaussian beams for the downward propagation of the wavefield is particularly advantageous in certain geologically critical cases such as overthrusting of continental lithosphere, resulting in the juxtaposition of high-velocity mantle material over crustal rocks. In contrast to ray-based methods, Gaussian beam migration requires no special treatment to handle such heterogeneities. Our results suggest that with adequate station spacing and signal-to-noise ratios, offsets of the Moho, on the order of 10 km in height, can be reliably imaged beneath thickened crust at depths of about 50 km. Furthermore, even sharp corners and edges are faithfully imaged when precise values of seismic wave speeds are available. Our tests also demonstrate that flexibility in choices of different types of seismic phases is important, because any single phase has trade-offs in issues such as spatial resolution, array aperture, and amplitude of signals.     

Is pure groundwater safe to drink?: natural "contamination' of groundwater in Norway

The title of this article is designed to provoke. Naturally occurring parameters are, by definition, not contamination. Nevertheless, nature is not necessarily nice, and naturally occurring trace toxins can be every bit as undesirable as their counterparts derived from human pollution.     

Paläogeographische Entwicklungen an mobilen Schollengrenzen im Westhindukusch (Bande Amir,Zentralafghanistan)

In the region of Bande Amir, located on the southern rim of the Tadschik basin, the Mesozoic section starts with detrital and conglomeratic limestones. These marine deposits of Upper Cretaceous age (Cenomanian-Turonian) disconformably cover a truncated complex of Upper Paleozoic rocks (Permian). After an interval of neritic sedimentation (Santonian, Campanian) the Upper Cretaceous sea (Maastrichtian) regressed from the Central Afghanian High. In the Hindukusch mountains the regression is related to crustal movements and regional uplifts, which hinged on the Herat lineament. Intermontane basins, developed on this mobile zone during mid-Tertiary time, gathered continental debris (Neogene conglomerates; Zohak-Formation) and led to the deposition of lacustrine sediments (Ghulghola-Formation). Orogenetic movements along the colliding edges of crustal blocks were rejuvenated in Plio-Pleistocene time, shifting the focus of deposition to the northwest. The basin was finally captured by the Amu Darja drainage system, leading to the development of canyon-like incisions. During warm periods of the Quaternary, fluviatile erosion in Bande Amir was interrupted at least four times by the retention of water behind large dams of travertine.     

Kimberlites: Their relation to mantle hotspots

Available age data support the hypothesis that kimberlite intrusions are formed by mantle hotspots. The hypothesis has been tested by inverting the volcanic traces formed by three hotspots to determine the post-Triassic motions of Africa, South America, and North America relative to these hotspots. Then, using these motions, the kimberlites intruded on these continents within the last 150 m.y. are relocated to their place of origin in the present hotspot reference frame. The result indicates that a majority of the kimberlites formed within 5° of a mantle hotspot. Statistical analysis shows that this kimberlite/hotspot correlation is significant at above the 90% level.     

Sedimentation in Lake Biel,an eutrophic,hard-water lake in northwestern Switzerland

A 2-year study of sedimentation in Lake Biel reveals that three major factors can account for the spatial and temporal patterns of tripton sedimentation. Allochthonous inputs of particulate matter mostly derived from the Aare river during spring snowmelt and other high-water periods, provide about 50% of the carbonate and 50% of the siliceous matter collected in sediment traps. Phytoplankton activity in this eutrophic, hard-water lake accounts for the rest of the carbonate and siliceous matter, the latter being mostly diatom frustules. Sedimentation rates are thus highest during periods of maximum river flooding, maximum phytoplankton activity and following the breakdown of the thermocline, at which time trapped particles settle out. Resuspension of sediment is important in at least one basin. Resolution of carbonate appears to be minor.