Vergence: an evaluation

Fold asymmetry is an independent concept distinct from facing. Fold vergence is a direction relating to the sense of asymmetry of folds. The complementary concept of cleavage vergence is defined allowing asymmetry to be used to locate major fold traces where no minor folds exist. Cleavage vergence is a valuable tool in complex structural terrains. Both concepts can be used singly or in combination to identify refolding. Vergence can define structural way-up where sedimentological or stratigraphic way-up cannot be recognised.     

Rare earth element geochemistry of potassic lavas from the Birunga and Toro-Ankole regions of Uganda,Africa

Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas.     

Progressive deformation and reorientation of fold axes in a ductile mylonite zone: the woodroffe thrust

The structural geometry of a mylonite zone (the Woodroffe thrust) and the country rock in its immediate vicinity is described. Mylonitic schistosity formed axial planar to folds in country rock foliation and contains a mineral elongation lineation which is constant in orientation. However, the fold axes (and associated intersection lineation) spread in orientation within the mylonitic schistosity but with a strong maximum parallel to the mineral elongation lineation. It is demonstrated that the fold axes formed initially at approximately 90° to mineral elongation but rotated with increase in strain towards it. Where this phenomenon was homogeneous on a macroscopic scale, rotation of large blocks of country rock across zones of mylonitization accompanied reorientation of fold axes within the mylonite.The controversy of progressive simple versus progressive pure shear for mylonite zones is discussed in the light of recent fabric and other evidence. It is concluded that the inhomogeneous forms of both progressive pure shear and progressive simple shear played a part and that the former dominated initially but gradually gave way to the latter until brittle rupture with large simple-shear displacements on a zone lubricated by the formation of pseudotachylite, brought granulite over amphibolite facies rocks.     

Growth defects in metamorphic biotite

Metamorphic biotites examined by transmission electron microscopy contain planar defects on the (001) plane, superlattices, twins and a microstructure causing streaking of k≠3n rows. Analysis of the fringe contrast shows that the fault vectors associated with the planar defects are either R ₁=±1/3 [010], R ₂=±1/6 [310] or R ₃=±1/6 [3 \(\bar 1\) 0]. Structural considerations indicate that a stacking fault R ₁, R ₂ or R ₃ is most likely to exist in the octahedral layer rather than the potassium layer. The result of such a fault on a unit layer of mica is effectively to rotate it through ±120° about c* (equivalent to the common mica twin law). These stacking faults can provide the mechanism for producing the ±120° rotations associated with the common mica polytypes. Furthermore, many of the observed microstructures can be generated by these stacking faults.     

New clues to the impact broadening mystery in radio recombination lines

Problems where impact broadened radio recombination lines appeared narrower than predicted first showed up ∼40 years ago at frequencies below ∼3 GHz. But it was soon found that the observations could be explained by throwing out the uniform density models and replacing them with variable density ones. However, this problem re-appeared recently when a mysterious line narrowing above quantum numbers of (n,Δn)=(202,8) was reported from sensitive observations of Orion and W51 near 6 GHz. Here it is demonstrated that the narrowing is unlikely to be caused by the data processing technique and therefore must be source related. It is further demonstrated that the observed line narrowing can be tied to one of the fundamental properties of radio recombination lines; namely the fact that the spacing of adjacent n-transitions increases with frequency. The line narrowing is observed to begin when the n-transition density, D _n , exceeds ∼11.6 transitions per GHz. This may imply that it is somehow related either to a previously overlooked effect in the impact broadening process, or to some unknown parallel process, that is tied to the separation between adjacent n-transitions. Based on these results it can be concluded, as has also been concluded in several theoretical investigations, that the observed line narrowing is not tied to a fixed range of either n or Δn.     

THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology

We present measurements of the altitude and eastward velocity component of mesospheric clouds in 35 imaging sequences acquired by the Mars Odyssey (ODY) spacecraft’s Thermal Emission Imaging System visible imaging subsystem (THEMIS-VIS). We measure altitude by using the parallax drift of high-altitude features, and the velocity by exploiting the time delay in the THEMIS-VIS imaging sequence.We observe two distinct classes of mesospheric clouds: equatorial mesospheric clouds observed between 0° and 180° L_s; and northern mid-latitude clouds observed only in twilight in the 200–300° L_s period. The equatorial mesospheric clouds are quite rare in the THEMIS-VIS data set. We have detected them in only five imaging sequences, out of a total of 2048 multi-band equatorial imaging sequences. All five fall between 20° south and 0° latitude, and between 260° and 295° east longitude. The mid-latitude mesospheric clouds are apparently much more common; for these we find 30 examples out of 210 northern winter mid-latitude twilight imaging sequences. The observed mid-latitude clouds are found, with only one exception, in the Acidalia region, but this is quite likely an artifact of the pattern of THEMIS-VIS image targeting. Comparing our THEMIS-VIS images with daily global maps generated from Mars Orbiter Camera Wide Angle (MOC-WA) images, we find some evidence that some mid-latitude mesospheric cloud features correspond to cloud features commonly observed by MOC-WA. Comparing the velocity of our mesospheric clouds with a GCM, we find good agreement for the northern mid-latitude class, but also find that the GCM fails to match the strong easterly winds measured for the equatorial clouds.Applying a simple radiative transfer model to some of the equatorial mesospheric clouds, we find good model fits in two different imaging sequences. By using the observed radiance contrast between cloud and cloud-free regions at multiple visible-band wavelengths, these fits simultaneously constrain the optical depths and particles sizes of the clouds. The particle sizes are constrained primarily by the relative contrasts at the available wavelengths, and are found to be quite different in the two imaging sequences: r_eff = 0.1 μm and r_eff = 1.5 μm. The optical depths (constrained by the absolute contrasts) are substantial: 0.22 and 0.5, respectively. These optical depths imply a mass density that greatly exceeds the saturated mass density of water vapor at mesospheric temperatures, and so the aerosol particles are probably composed mainly of CO₂ ice. Our simple radiative transfer model is not applicable to twilight, when the mid-latitude mesospheric clouds were observed, and so we leave the properties of these clouds as a question for further work.     

Observed and modelled tidal currents in the New Zealand region

Tidal currents derived from current meter measurements are compared with the output from a barotropic tidal model of the New Zealand region. For the semi‐diurnal constituents there was very good agreement for the M₂ tide and good agreement for the S₂ tide. For the diurnal constituents (K_l, O_l) it was found that as the amplitude of the constituents decreased so did both the model/observation agreement and the accuracy of the observed tidal ellipse parameters. Consequently it was not possible to decide whether differences arose through shortcomings in the model or in the data. However, the overall performance of the model as a prognostic tool for ocean tidal current simulation appears to be good.     

El Niño and decadal effects on sea‐level variability in northern New Zealand: A wavelet analysis

Sea‐level data from two sites in northern New Zealand, along with the Southern Oscillation Index (SOI), are analysed for interannual and decadal variability using wavelets. The analysis shows, using statistically significant wavelet power, there is a significant relationship between mean sea level (MSL) and SOI. However, the relationship is highly variable, both in magnitude and in the range of time‐scales over which it occurs. This non‐stationarity necessitates the use of techniques such as wavelets for analysis. An interdecadal response in MSL around northern New Zealand has been isolated, with shifts occurring in 1950 and the late 1970s. This behaviour in MSL appears to coincide with shifts in the Pacific Decadal Oscillation, thought previously to be largely centred in the North Pacific. A strong correlation between SOI and sea surface temperature (SST) is also demonstrated. This relationship appears to be stable in magnitude (a large change in SOI produces a large change in SST) and to occur over the same range of time‐scales. More SST and MSL data are required for other parts of New Zealand to determine whether these findings apply elsewhere.     

Rift kinematics preserved in deep-time erosional landscape below the northern North Sea

Our understanding of continental rifting is, in large parts, derived from the stratigraphic record. This record is, however, incomplete as it does not often capture the geomorphic and erosional signal of rifting. New 3D seismic reflection data reveal a Late Permian-Early Triassic landscape incised into the pre-rift basement of the northern North Sea. This landscape, which covers at least 542 km², preserves a drainage system bound by two major tectonic faults. A quantitative geomorphic analysis of the drainage system reveals 68 catchments, with channel steepness and knickpoint analysis of catchment-hosted palaeo-rivers showing that the landscape preserved a >2 Myr long period of transient tectonics. We interpret that this landscape records a punctuated uplift of the footwall of a major rift-related normal fault (Vette Fault) at the onset of rifting. The landscape was preserved by a combination of relatively rapid subsidence in the hangingwall of a younger fault (Øygarden Fault) and burial by post-incision sediments. As such, we show how and why erosional landscapes are preserved in the stratigraphic record, and how they can help us understand the tectono-stratigraphic evolution of ancient continental rifts.     

Arsenic within the secondary environment resulting from geogenic inputs,Harlech Dome,United Kingdom

There has been a growing awareness of As in the environment due to both anthropogenic and geogenic loading. This study quantifies the presence of As in river water, river sediment and groundwater within the Harlech Dome region of the UK, an area underlain by Cambro–Ordovician aged mineralization. Published data regarding As levels in UK groundwater are relatively rare compared to elsewhere. Arsenic levels in groundwater (0.1–18 µg l^?1) and river sediment (1.5–142 mg kg^?1) exceed environmental quality guidelines and indicate the release of As into the secondary environment from predominantly geogenic sources. Modelling of dissolved As speciation using ORP–pH diagrams predicts the presence of As in both arsenate (As⁵⁺) and arsenous acid (As³⁺) species, which possess differing solubilities and mobilities. Calculation of enrichment factors (EFs) for As in river sediments, using Al as a reference element, indicates highest EFs, and therefore As release, occurs in streams draining the Coed-y-Brenin region in the south-east of the Harlech Dome. This area is underlain by an as yet unexploited porphyry copper deposit. Data indicate that the presence of potentially harmful elements within the environment may occur with or without the presence of major anthropogenic activity.