Geometrical simulation studies of coastal cliff failures in liassic strata,South Wales,U.K.

Three numerical models of cliff failure have been developed, based on analyses of rockfalls along the 22 km Liassic coastline of the Glamorgan Heritage Coast, U.K. Detailed field investigation of the bucklandi (limestone dominated) and angulata (mudstone dominated) Lias series at four locations confirm the veracity of the models. Translation failure was correctly predicted at the mainly limestone buttressed cliff sites in the bucklandi and at locations where the angulata series formed a high proportion of the cliff mass. Toppling was predicted for vertical and overhanging cliffs with basal undercutting. The factor of safety reduces as the ratio of undercutting depth (d) to tension fracture distance (D) from the cliff face increases. Instability can be triggered by thrust forces generated by wave/tide impact, freeze/thaw and expansion/contraction associated with clay infill. Thrust forces can be modest, ranging from 1·7 to 2·7 MN m², but can exceed the cross-joint strength as weathering proceeds.     

Sr,Nd and Pb isotopic compositions of calc-alkaline and peralkaline silicic volcanics from the D'Entrecasteaux Islands,Papua New Guinea,and their tectonic significance

Summary Sr, Nd and Pb isotope data are presented for a transitional basalt-peralkaline rhyolite suite, and spatially associated calc-alkaline rhyolites from the D'Entrecasteaux Islands, eastern Papua New Guinea. Both suites have a typical convergent margin geochemical signature (i.e. high Zr/Nb, La/Nb, and low Ta/Yb compared with OIB). The transitional basalt-peralkaline rhyolite suite and calc-alkaline rhyolites have a restricted range of²⁰⁶Pb/²⁰⁴Pb (18.522–18.661),²⁰⁷Pb/²⁰⁴Pb (15.543–15.631), and²⁰⁸Pb/²⁰⁴Pb (38.31–38.63) values which overlap the fields of volcanics from Tonga, Fiji, and Pacific MORB and sediments. The transitional basalt-peralkaline rhyolite suite also displays a restricted range of¹⁴³Nd/¹⁴⁴Nd values (0.513053–0.512984), but a much broader range of⁸⁷Sr/⁸⁶Sr (0.703989–0.70585) values. The latter reflects uncertainties in the ages of samples with very high⁸⁷Rb/⁸⁶Sr values and the effects of Sr exchange with seawater. The calc-alkaline rhyolites have consistently lower¹⁴³Nd/¹⁴⁴Nd values (0.512923—0.512867), and a more restricted range of⁸⁷Sr/⁸⁶Sr values (0.703864–0.704028) compared with the transitional basalt-peralkaline rhyolite suite.The isotopic and trace element data are consistent with the interpretation that the calc-alkaline rhyolites were produced by partial melting of a young arc protocrust, whereas associated calc-alkaline basic and intermediate magmas were derived from a depleted mantle source which previously had been modified by subduction along the Trobriand Trough. The transitional basalt-peralkaline rhyolite suite was produced by extensive magmatic differentiation of a parental transitional basalt magma in a relatively shallow magma chamber. The parent magma was produced by partial melting of either a depleted MORB-source mantle or a less-depleted OIB-type source which previously had been modified by subduction processes. The hy- and ol-normative transitional basalt magmas were probably generated in response to lithospheric thinning at somewhat higher pressures than qz-normative calc-alkaline magmas.The close spatial and temporal relationship between the transitional basalt-peralkaline rhyolite suite and the tale-alkaline volcanics reflects the complex and dynamic tectonic setting of eastern Papua and the D'Entrecasteaux Islands. In particular, the change from calc-alkaline to alkaline magmatism appears to have occurred following a change from compressional to extensional tectonics resulting from the westward propogation of the Woodlark spreading ridge into eastern Papua.

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Sr, Nd und Pb Isotopenzusammensetzung saurer kalkalkalischer und peralkalischer Vulkanite der D' Entrecasteaux Inseln, Papua Neuguinea, und ihre tektonische Bedeutung

Zusammenfassung In dieser Arbeit werden Sr, Nd und Pb Isotopendaten transitionaler Basalt-peralkalischer Rhyolithabfolgen und räumlich mit diesen assoziierter kalkalkalischer Rhyolithe der D' Entrecasteaux Inseln im östlichen Papua Neuguinea vorgestellt. Beide Abfolgen zeigen für konvergiernde Plattenränder typische geochemische Signaturen (z.B. hohe Zr/Nb, La/Nb und niedrige Ta/Yb Verhältnisse im Vergleich mit OIB). Die transitionale Basalt-peralkalische Rhyolithabfolge und die kalkalkalischen Rhyolithe zeigen eine geringe Variation in ihren²⁰⁶Pb/²⁰⁴Pb (18.522–18.661),²⁰⁷/²⁰⁴Pb (15.543–15.631) und²⁰⁸Pb/²⁰⁴Pb (38.31–38.63) Verhältnissen, die mit denen von Vulkaniten aus Tonga, Fiji und den pazifischen MOR-Basalten und Sedimenten übereinstimmen. Die transitionale Basalt-peralkalische Rhyolithabfolge zeigt weiters eine geringe Variationsbreite der¹⁴³Nd/¹⁴⁴Nd Verhältnisse (0.513053–0.512984), aber eine größere Streuung der⁸⁷Sr/⁸⁶Sr Verhältnisse (0.703989–0.70585). Letztere reflektiern die Unsicherheiten der Alterseinstufung der Proben mit sehr hohen⁸⁷Rb/⁸⁶Sr Verhältnissen und die Effekte des Sr-Austausches mit Meerwasser. Die⁸⁷Sr/⁸⁶Sr Verhältnisse kalkalkalischer Rhyolithe variieren im Vergleich dazu gerigfügiger (0.703864–0.704028).Isotopen und Spurenelementdaten sind mit der Interpretation, daß die kalkalkalischen Rhyolithe durch partielle Anatexis junger Inselbogen-Protkruste entstanden sind, kompatibel. Die mit ihnen vergesellschafteten basischen bis intermediären klakalkalischen Magmen entstammen einer abgereicherten Mantelquelle, die zuvor durch Subduktionsprozesse entlang des Trobriandgrabens modifiziert wurde. Die transitionale Basalt-peralkalische Rhyolithabfolge entstand durch extensive magmatische Differentiation eines transitionalen basaltischen Stammagmas in einer relativ seicht liegenden Magmenkammer. Das Stammagma wurde durch partielle Aufschmelzung entweder eines abgereicherten MORB-Mantels oder einer weniger stark abgereicherten OIBQuelle, die zuvor durch Subduktionsprozesse modifiziert wurde, gebildet. Die Hyund O1-normativen transitionalen Basaltmagmen wurden wahrscheinlich im Zuge der Ausdünnung von Lithosphäre unter etwas höheren Drucken gebildet als die Quarznormativen kalkalkaischen Magmen.Die enge räumliche und zeitliche Beziehung der transitionalen Basalt-peralkalischen Rhyolithabfolge mit kalkalkalischen Vulkaniten spiegelt die komplexen und dynamischen geotektonischen Vorgänge im östlichen Papua Neuguinea wieder. Der Wechsel von kalkalkalischem zu alkalischem Magmatismus könnte durch die Umstellung von Kompressions- auf Extensionstektonik, die mit der Verlagerung des westlich gelegenen Woodlark Spreadingzenrums in das östliche Papua Neugiunea in Beziehung gebracht wird, erklärt werden.

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With 13 Figures     

Variability of bedload transport and channel morphology in a braided river hydraulic model

This paper investigates variability in bedload transport and channel morphology for 11 replicate experimental runs in an approximately 1:50 braided river model. The experiments, each of 90 h duration, were carried out in a 20 × 3m tilting flume. All the experiments started with the same initial conditions. Bedload transport was measured at 5 min intervals in a collection drum at the exit from the flume. The model showed reasonable hydraulic similarity when compared to prototype rivers. Results show that mean bedload transport rates for the 11 runs vary in the range 0·98 to 1·49gs^?1 (mean + 1·21, coefficient of variation 11 per cent). Within-run transport rates commonly vary from close to zero, to two and occasionally three or four times the mean rate. Within the bedload series, several irregular phases of transport intensity can be observed, but time series analysis of the data show little underlying serial structure (an AR(2) autoregressive model is appropriate). Channel patterns are narrow/braided, are established quickly and remain relatively stable throughout the runs, although channel widths increase between 20 and 103 per cent over the 11 runs. Channel behaviour varies from aggradational to transitional between aggradation and degradation. Time-averaged bedload transport rate is weakly correlated with braiding intensity. In general, these results demonstrate that for a given set of controlling variables, bedload transport and channel morphology can be approximately replicated.     

The SAURON project – I. The panoramic integral-field spectrograph

A new integral-field spectrograph, SAURON, is described. It is based on the TIGER principle, and uses a lenslet array. SAURON has a large field of view and high throughput, and allows simultaneous sky subtraction. Its design is optimized for studies of the stellar kinematics, gas kinematics, and line-strength distributions of nearby early-type galaxies. The instrument design and specifications are described, as well as the extensive analysis software which was developed to obtain fully calibrated spectra, and the associated kinematic and line-strength measurements. A companion paper will report on the first results obtained with SAURON on the William Herschel Telescope.     

Morphology and star formation in nearby low surface brightness galaxies

We present observations ( B, R, K , Hα and H  i ) of six nearby low surface brightness galaxies (LSBGs). They show an astonishing amount of variety; while some systems appear smooth and featureless, others resolve into loose assemblies of gas clouds. We have derived rotation curves, gas surface density profiles and star formation thresholds for three of the galaxies.
The results have been used to test two ideas describing their star formation: one in which star formation depends solely on the H  i gas surface density, and one that depends on differential rotation. We find that a critical H  i surface density criterion in the range  2.6–12.6 × 10²⁰ cm⁻² (2.1–10.1 M_⊙ pc⁻²)  best describes the star-forming ability of these galaxies on local and global scales. A critical gas surface density based on the rotation of the gas is also able to describe the results on a global scale for two of the three galaxies for which we were able to derive rotation curves.     

The heartbeat of the volcano: The discovery of episodic activity at Prometheus on Io

The temporal signature of thermal emission from a volcano is a valuable clue to the processes taking place both at and beneath the surface. The Galileo Near Infrared Mapping Spectrometer (NIMS) observed the volcano Prometheus, on the jovian moon Io, on multiple occasions between 1996 and 2002. The 5 micron (μm) brightness of this volcano shows considerable variation from orbit to orbit. Prometheus exhibits increases in thermal emission that indicate episodic (though non-periodic) effusive activity in a manner akin to the current Pu'u 'O'o-Kupaianaha (afterwards referred to as the Pu'u 'O'o) eruption of Kilauea, Hawai'i. The volume of material erupted during one Prometheus eruption episode (defined as the interval from minimum thermal emission to peak and back to minimum) from 6 November 1996 to 7 May 1997 is estimated to be ∼0.8 km³, with a peak instantaneous volumetric flux (effusion rate) of ∼140 m³ s⁻¹, and an averaged volumetric flux (eruption rate) of ∼49 m³ s⁻¹. These quantities are used to model subsurface structure, magma storage and magma supply mechanisms, and likely magma chamber depth. Prometheus appears to be supplied by magma from a relatively shallow magma chamber, with a roof at a minimum depth of ∼2-3 km and a maximum depth of ∼14 km. This is a much shallower depth range than sources of supply proposed for explosive, possibly ultramafic, eruptions at Pillan and Tvashtar. As Prometheus-type effusive activity is widespread on Io, shallow magma chambers containing magma of basaltic or near-basaltic composition and density may be common. This analysis strengthens the analogy between Prometheus and Pu'u 'O'o, at least in terms of eruption style. Even though the style of eruption appears to be similar (effusive emplacement of thin, insulated, compound pahoehoe flows) the scale of activity at Prometheus greatly exceeds current activity at Pu'u 'O'o in terms of volume erupted, area covered, and magma flux. Whereas the estimated magma chamber at Prometheus dwarfs the Pu'u 'O'o magma chamber, it fits within expectations if the Pu'u 'O'o chamber were scaled for the greater volumetric flux and lower gravity of Io. Recent volumetric eruption rates derived from Galileo data for Prometheus were considerably smaller than the rate that produced the extensive flows formed in the ∼17 years between the Voyager and Galileo missions. These smaller eruption rates, coupled with the fact that flows are not expanding laterally, may mean that the immediate heat source that generates the Prometheus plume is simultaneously running out of available volatiles and the thermal energy that drives mobilization of volatiles. This raises the question of whether the current Prometheus eruption is in its last throes.     

The 1995–2002 Long-Term Monitoring of Comet C/1995 O1 (HALE–BOPP) at Radio Wavelength

The bright comet Hale–Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nançay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH₃OH, H₂CO, H₂S, CS, CH₃CN and HNC) were monitored. CS, H₂S, H₂CO, CH₃CN were detected up to r_h= 3–4 AU from the Sun, while HCN and CH₃OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at r_h= 14 AU. The gas production rates obtained from this programme contain important information on the nature of cometary ices, their thermal properties and sublimation mechanisms.Line shapes allow to measure gas expansion velocities, which, at large heliocentric distances, might be directly connected to the temperature of the nucleus surface. Inferred expansion velocity of the gas varied as r_h ^-0.4 within 7 AU from the Sun, but remained close to 0.4 km s^-1 further away. The CO spectra obtained at large r_hare strongly blueshifted and indicative of an important day-to-night asymmetry in outgassing and expansion velocity. The kinetic temperature of the coma, estimated from the relative intensities of the CH₃OH and CO lines, increased with decreasing r_h, from about 10 K at 7 AU to 110 K around perihelion.