Middle miocene deduction and late miocene beginning of collision registered in the hengchun peninsula: Geodynamic implications for the evolution of Taiwan

Taiwan is located in the axis of the Manila Trench. It results from an oblique collision between the northernmost part of the Luzon arc and the Chinese passive margin. This active collision follows the subduction of the Oligocene-Miocene oceanic crust of the South China Sea along the Manila Trench. The tectonized Chinese margin emerged in the Hengchun peninsula (South Taiwan). Gentle folds which are delineated by the Quaternary reefal limestones demonstrate Recent deformations. These folds deformed a thick detrital sequence of Miocene age (Ssuchung Chi series) which was previously strongly folded and thrust westward (axis NS-N20) upon the Renting mélange of Latest Miocene age. These main deformations, sealed by the Middle Pliocene, are the evidence for the onset of collision in this part of Taiwan at the end of the Miocene. Because of its obliquity, the collision started already in the northern part of Taiwan during the Late Miocene (6-7-8 Ma ?).The Ssuchung Chi series, a sequence of proximal turbidites, has contained, since the Middle Miocene (NN 6~13 Ma), fragments of an Oligocene to Lower Miocene oceanic crust. This ophiolitic material is very similar to the East Taiwan Ophiolite of the Coastal Range. It originated most probably from a slice of South China Sea crust obducted in Middle Miocene times (13–14 Ma) upon the Chinese margin (North of the Hengchun peninsula). This obduction occurred 7 to 8 Ma before the beginning of collision. These results make it possible to propose an evolutionary model for Taiwan from the Oligocene to the Recent, with the different phases of a collision between a volcanic arc and a passive margin.     

The stratospheric quasi-biennial oscillation observed in the semidiurnal ground pressure data

Ground pressure observations made at Macao (22^○N, 113^○E) from 1953 to 1991 are analyzed and compared with the stratospheric quasi-biennial oscillation (QBO) data obtained during the same interval. The periods of the two phenomena and their time evolution are found to be close to each other. Furthermore, the time series of the stratospheric winds and the S2(p) QBO signature are highly correlated, thus confirming earlier analysis. On this basis, pressure measurements obtained at Batavia (now Djakarta: 6^○S, 107^○E) from 1870 to 1944 are used to trace back the QBO phenomenon before the advent of systematic stratospheric balloon measurements. The inferred period, which varies between 25 and 32 months, suggests that the QBO has been present in the atmosphere at least since 1870.     

Detailed gravity study of the offshore structure of Piton de la Fournaise volcano,Reunion Island

We present an interpretation of gravity data acquired in 1984 by the French R/V Jean Charcot on the submarine part of the eastern flank of Piton de la Fournaise volcano. We comment on the Bouguer anomaly map and give a quantitative interpretation of three gravity profiles. The main results are that a gravity high over Grand Brûlé, the lower subaerial part of the eastern flank, does not extend far offshore and that an anomalous topographic feature, discovered in 1982 on the submarine eastern flank, is characterized by a large negative anomaly. We propose three hypotheses to explain the origin of this anomaly, i. e., it marks the site of a new volcano, or it is a consequence of lateral volcanism from a volcano older than Piton de la Fournaise, or more probably, it represents a great landslide deposit.     

A new method for sampling and analyzing volcanic sublimates — application to Merapi volcano, Java

A new method for the sampling of sublimates from high-temperature volcanic gases has been used at Merapi volcano, Java, in 1978. The sublimates were collected on the inner walls of silica tubes introduced into fumarolic vents. Volcanic gases were allowed to move freely through the tubes and as they cooled, a fraction of the volatile components condensed on the inner walls of the tubes along the temperature gradient. The sublimates were then analyzed by a combination of light microscopy, scanning electron microscopy, electron microprobe and X-ray diffraction.Six successive zones of different compositions and mineralogical associations have been identified along the covered range of temperatures (900° to around 400°C). From the high to the low temperatures, these zones are composed of: (1) cristobalite, magnetite, hercynite; (2) molybdenite; (3) acmite; (4) halite, sylvite; (5) sphalerite, pyrite; and (6) galena. Equilibrium calculations show that these crystalline phases are stable for pS₂, pC₁, and pO₂, values typical of magma-buffered gases that have not been contaminated by atmospheric oxygen.The deposits observed in the tubes may be useful in aiding the understanding of the mechanisms acting during the cooling of the gaseous phase on its way to the surface and before its emission into the atmosphere.     

Authigenic smectite on diatom frustules in Bolivian saline lakes

Observation under the electron microscope of diatom frustules from Bolivian Altiplano saline lakes shows that many of these are coated with particles occurring as tiny sheets. The frustules can be found to be almost completely replaced by these sheets. Isolated sheet aggregates seem to have resulted from completely transformed frustules. Section observations of altered frustules bear out that the sheets have grown from biogenic silica through replacement. Selected area diffraction, dark field observation, microdiffraction, and elemental microanalysis show that the particles on the diatom frustules consist of a poorly crystallized MG-smectite.The unambiguous localisation of this authigenesis allows us to reconstruct its hydrochemical and sedimentological environment. Observation of the most recent lake sediments has pointed out that at least two main conditions are required for this authigenesis at 5°C: saturation with respect to amorphous silica, and a pH above 8.2. Variations in the Mg concentration have no significant effect.     

Fracturation tertiaire et évolution des contraintes en Bavière orientale: le Jura franconien et la forêt bavaroise (R.F.A.)

Résumé L'analyse géométrique et cinématique de la fracturation dans le Jura franconien et la bordure occidentale du Massif bohémien met en évidence les mécanismes de la tectonique cassante d'âge teriaire, essentiellement ceux des grands accidents SE-NW (lignes de Pfahl et de Franconie) qui ont présenté successivement des jeux en coulissement dextre, en faille normale et en faille inverse. Cinq états de contrainte ont été caractérisés et comparés à ceux définis antérieurement dans le Jura souabe, en liaison avec la cinématique des plaques Europe et Afrique: compression N-S, distension E-W, compression NE-SW puis E-W, distension N-S.

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The analysis of both the geometric and the kinematic cenozoic faulting in the Franconian Jura and in western Bohemia enables to characterize the mechanisms of cenozoic tectonics. More especially dextral strike-slip, normal fault and reverse fault have successively occurred along large SE-NW fault line, as the Pfahl and the Franconian fault systems. Five main stress patterns have thus been identified and then compared with those previously described in Swabian Jura. They are related to the kinematic of Europe and Africa plates, i. e.: (1) N-S compression, (2) E-W distension, (3) SW-NE compression, (4) E-W compression, (5) N-S distension.

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Zusammenfassung Durch die geometrische und kinematische Analyse der Brüche wird die tertiäre Bruchtektonik (tectonique cassante) im Fränkischen Jura und am westlichen Rand des Böhmischen Massifs belegt. Es handelt sich im wesentlichen um die großen SE-NW orientierten Störungen der Linie von Pfahl und der Fränkischen Linie, die nacheinander als dextrale Blattverschiebungen, als Normalverwerfungen und als inverse Verwerfungen wirkten. Fünf Spannungszustände können charakterisiert und mit jenen, die bereits im Schwäbischen Jura definiert wurden, verglichen werden. Sie spiegeln die verschiedenen Stadien der kinematischen Entwicklung der europäischen und afrikanischen Platte wider: Kompression N-S, Zerrung E-W, Kompression NE-SW, sodann E-W, Zerrung N-S.

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North Atlantic Oscillation sensitivity to the El Niño/Southern Oscillation polarity in a large-ensemble simulation

A large ensemble modeling experiment with the Melbourne University General Circulation Model is presented. Thirty 17-year-long independent simulations were performed. All integrations were forced by the same observed sea surface temperatures, obtained from the Atmospheric Model Intercomparison Project II. The simulations were analyzed to assess the sensitivity of the North Atlantic Oscillation (NAO) to the El Niño/Southern Oscillation (ENSO) polarity. The results show signals of the ENSO phases both in the mean strength of the NAO as well as in its internal variability. During the cold ENSO phase, the probability density function of the NAO index presents a small but positive mean value, whereas it is negative during the warm ENSO phase. Also, the NAO variability associated with each ENSO phase shows a different behavior: during the warm phase the probability density function of the NAO index presents a larger variance and suggests a bimodality, whereas no bimodality is suggested in the cold phase.     

The establishment of the atmospheric emission inventories of the ESCOMPTE program

Within the frame of the ESCOMPTE program, a spatial emission inventory and an emission database aimed at tropospheric photochemistry intercomparison modeling has been developed under the scientific supervision of the LPCA with the help of the regional coordination of Air Quality network AIRMARAIX. This inventory has been established for all categories of sources (stationary, mobile and biogenic sources) over a domain of 19,600 km² centered on the cities of Marseilles–Aix-en-Provence in the southeastern part of France with a spatial resolution of 1 km². A yearly inventory for 1999 has been established, and hourly emission inventories for 23 days of June and July 2000 and 2001, corresponding to the intensive measurement periods, have been produced. The 104 chemical species in the inventory have been selected to be relevant with respect to photochemistry modeling according to available data. The entire list of species in the inventory numbers 216 which will allow other future applications of this database. This database is presently the most detailed and complete regional emission database in France. In addition, the database structure and the emission calculation modules have been designed to ensure a better sustainability and upgradeability, being provided with appropriate maintenance software. The general organization and method is summarized and the results obtained for both yearly and hourly emissions are detailed and discussed. Some comparisons have been performed with the existing results in this region to ensure the congruency of the results. This leads to confirm the relevance and the consistency of the ESCOMPTE emission inventory.     

High-Frequency Hydroacoustic Monitoring in an Underground Iron Mine

The monitoring of the stability of old mines constitutes an important research objective for our institution, BRGM. The study reported here shows the contribution of high-frequency (>30 kHz) acoustic emissions to the detection of the damage within a rock mass, during an experiment within a pilot site of an old flooded iron mine. The experiment consisted of recording all the hydroacoustic events in a broad frequency band (between 30 Hz and 180 kHz), during 18 months. The monitoring network has been calibrated by a triggered block fall that made it possible to highlight a relationship between the occurrence of high-frequency/low-frequency hydroacoustic emissions and rock falls. The events recorded have been associated with the micro-failure of the rock mass near the roof, prior to the detachment of the blocks. This monitoring showed important high-frequency hydroacoustic activity, which may be associated with mechanical instabilities generated by the evolution of water pressure during the experiment. In conclusion, the high-frequency hydroacoustic activity appears to be a good indicator of instability and, therefore, this new technique constitutes a promising tool for monitoring abandoned underground cavities.