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A New Scenario for the Origin of the Solar System

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CONFERENCIAS

Allan Boss

A New Scenario for the Origin of the Solar System

José López Ruiz

Procesos de acreción y diferenciación en La Tierra

Juan Pérez-Mercader

    Claves para la búsqueda de vida en el Universo (día 26 de octubre)

Kurt Marti

Mars-Earth connection

Carlos Martín Escorza

Ritmo de caídas de meteoritos en los últimos 2.500 años: ?hay periodicidad?. Los registros de caídas

    de meteoritos de los que se dispone la fecha de ocurrencia del fenómeno no son todo lo abundantes que se desearía para realizar un estudio de frecuencias y posibles periodicidades. Los datos, por el contrario, son escasos y se encuentran muy distribuidos, aunque son mucho más numerosos en épocas recientes que las pasadas, y conforme vamos retrocediendo en el tiempo se dispone de menos referencias. Desde hace

    décadas se conocían sólo las caídas producidas en la parte del planeta que recogían las noticias hsitóricas en los países europeos primero y norteamericanos después, pero su límite no traspasa la Edad Media.

    Recientemente se han añadido a estos los datos obtenidos de los registros de caídas en China, que amplian el tiempo de estos sucesos registrados con la fecha de caída hasta los años cercanos a los 500 a.C. En base a todos estos datos se presentará un análisis general que muestra cual ha sido el ritmo global de caídas y una propuesta de posible periodicidad temporal para todas ellas.

Agustín Fernández-Chicarro

    THE MARS EXPRESS MISSION AND ASTROBIOLOGY

    A.F. Chicarro, European Space Agency, Space Science Department, ESTEC/Code SCI-SB, Postbus 299,

    2200 AG Noordwijk, The Netherlands (email: agustin.chicarro@esa.int).

    The European Space Agency and the scientific community have performed concept and feasibility studies for more than ten years on potential future European missions to the red planet (Marsnet,

    Intermarsnet), focusing on a network of surface stations complemented by an orbiter, a concept which is being implemented by the CNES-led Netlander mission to be launched in 2005. Before that, however, the ESA Mars Express mission includes an orbiter spacecraft and a small lander module named Beagle-2 in remembrance of Darwin‟s ship Beagle. The mission, to be launched in 2003 by a Russian Soyuz rocket, will

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recover some of the lost scientific objectives of both the Russian Mars-96 mission and the ESA Intermarsnet

    study, following the recommendations of the International Mars Exploration Working Group (IMEWG) after the failure of Mars-96, and also the endorsement of ESA's Advisory Bodies that Mars Express be

    included in the Science Programme of the Agency.

    The specific scientific objectives of the Mars Express orbiter are: global high-resolution imaging with 10

    m resolution and imaging of selected areas at 2 m/pixel, global IR mineralogical mapping, global atmospheric circulation study and mapping of the atmospheric composition, sounding of the subsurface structure down to the permafrost, study of the interaction of the atmosphere with the surface and with the interplanetary medium as well as radio science. The goals of the Beagle-2 lander are: geology, geochemistry, meteorology and exobiology of the landing site.

    The scientific payload on the Mars Express orbiter includes a Super/High-Resolution Stereo Colour

    Imager (HRSC), an IR Mineralogical Mapping Spectrometer (OMEGA), a Planetary Fourier Spectrometer (PFS), a Subsurface-Sounding Radar Altimeter (MARSIS), an Energetic Neutral Atoms Analyser (ASPERA), an UV and IR Atmospheric Spectrometer (SPICAM) and a Radio Science Experiment (MaRS). The Beagle-2 lander includes a suite of imaging instruments, organic and inorganic chemical analysis, robotic sampling devices and meteorological sensors (see Table 1).

    The Mars Express mission will address the issue of astrobiology on Mars both directly and indirectly. The majority of instruments on the orbiter will look for indications of favourable conditions to the existence of life, either at present or during the planet‟s past, and in particular for traces of liquid, solid or gaseous water. Therefore, the HRSC camera will take pictures of ancient riverbeds, the OMEGA spectrometer will look for - minerals with OHradicals formed in the presence of water, the MARSIS radar will look for subsurface ice and liquid water, the PFS and SPICAM spectrometers will analyse water vapour in the atmosphere, and finally ASPERA and MaRS will study neutral atom escape from the atmosphere, in particular Ocoming2

    from water and carbonates. The instruments on Beagle-2 will also look for the presence of water in the soil, rocks and the atmosphere, but will also try to find traces of life with direct measurements, such as presence 12of methane (CH) indicative of extanct life and a larger amount of the light C isotope compared to the 413heavier C, which would even indicate the existence of extinct life. Since NASA‟s Viking mission in 1976,

    it is the first time that the exhaustive search for life is so central to a space mission to Mars. Current design estimates allow for an orbiter scientific payload of about 110 kg and 65 kg total lander mass (at launch) compatible with the approved mission scenario. The Beagle-2 lander was selected due to its innovative scientific goals and challenging payload. Beagle-2 will deploy a sophisticated robotic-sampling arm, which could manipulate different types of tools and retrieve samples to be analyzed by the geochemical instruments mounted on the lander platform. One of the tools to be deployed by the arm is a „mole‟ capable of subsurface sampling to reach soil unaffected by solar-UV radiation, another is a corer/grinder to reach the rock under the weathering varnish.

    A Soyuz-Fregat launcher will inject a total of about 1200 kg into Mars transfer orbit in early June 2003, which is the most favorable launch opportunity to Mars in terms of mass in the foreseeable future. The Mars Express orbiter is 3-axis stabilized and will be placed in an elliptical martian orbit (250 10142 km) of

    86.35 degrees inclination and 6.75 hours period, which has been optimized for communications with Beagle-2, the Netlanders, as well as NASA landers or rovers to be launched both in 2003 and 2005. The Beagle-2 lander module will be independently targeted from separate arriving hyperbolic trajectory, enter and descend through the martian atmosphere in about 5 min, and land with an impact velocity <40 m/sec and an error landing ellipse of 100 20 km. A preliminary Beagle-2 landing site has been selected in the Isidis Planitia area (10.6? N, 270? W). The nominal mission lifetime of one martian year (687 days) for the orbiter investigations will be extended by another martian year for lander relay communications and to complete global coverage. The Beagle-2 lander lifetime will be of a few months.

    ESA provides the launcher, the orbiter and the operations, while the Beagle-2 lander is delivered by an UK-led consortium of space organizations. The orbiter instruments are provided by scientific institutions through their own funding. In addition to relaying the data from the Beagle-2 lander to Earth, Mars Express will also service landers and rovers from other agencies during its nominal/extended lifetime. The ground segment includes the ESA station at Perth, Australia, and the mission operations centre at ESOC. The Mars

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Express mission is now in Phase-C/D, with Astrium (formerly Matra Marconi Space) in Toulouse, France,

    as its Prime Contractor and involving a large number of European companies. International collaboration, either through the participation in instrument hardware or through scientific

    data analysis is very much valued to diversify the scope and enhance the scientific return of the mission,

    such as NASA‟s major contribution to the subsurface-sounding radar, and the use of its DSN for increased science data downloading and critical manoeuvres. Also, arriving at Mars at the very end of 2003, Mars

    Express will be followed by the Japanese Nozomi spacecraft a few days later. Both missions are highly complementary in terms of orbits and scientific investigations; Nozomi focusing on the study of the upper atmosphere of Mars as well as the interaction of the solar wind with the ionosphere from a highly elliptic

    equatorial orbit. Close cooperation, including scientific data exchange and analysis, is foreseen by the

    Nozomi and Mars Express teams within a joint ESA-ISAS programme of Mars exploration. For more details on the Mars Express mission and its Beagle 2 lander:

     http://sci.esa.int/marsexpress/ and http://www.beagle2.com/

    Table 1: MARS EXPRESS SCIENTIFIC PAYLOAD

    Acronyms Instruments Principal Countries

    Investigators

Orbiter

    HRSC Super/High-Resolution Stereo G. Neukum D, F, RU, US, FI, I, UK

    Colour Imager

    OMEGA IR Mineralogical J.P. Bibring F, I, RU

    Mapping Spectrometer

    PFS Atmospheric Fourier V. Formisano I, RU, PL, D, F, E, US

    Spectrometer

    MARSIS Subsurface-Sounding G. Picardi I, US, D, CH, UK,DK

    Radar/Altimeter & J. Plaut

    ASPERA Energetic Neutral Atoms R. Lundin & S, D, UK, F, FI, I, US, RU

    Analyzer S. Barabash

    SPICAM UV and IR Atmospheric J.L. Bertaux F, B, RU, US

    Spectrometer

    MaRS Radio Science Experiment M. Paetzold D, F, US, A

    Lander

     Suite of imaging instruments, C. Pillinger & UK, D, F, HK, CH

    BEAGLE-2 organic and inorganic chemical M. Sims

    analysis, robotic sampling devices

    and meteo sensors

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WORKSHOP

    "Astrobiología Amateur y Profesional, presente y futuro" Equipo de Astrobiología Espacio Para Todos

    Jordi Llorca, Antonio Delgado Huertas, Giussepe Pesce, y Daniel Slepikas

    GANIMA. Plataforma aerobótica de investigación estratosférica, definición de lineas de investigación en aplicaciones terrestres, y perspectivas de aplicaciones marcianas".

    Duración estimada: 1 hora.

    Josep María Trigo i Rodríguez (UJI, TMSE), Jordi Llorca Piqué (UB, TMSE), Mark Richard Kidger (IAC, TMSE), Cayetano Santana Gil (TMSE), Antonio Delgado Huertas (OEPT), Ignacio Lirio Barajas (SISC, OEPT), Daniel Osvaldo Slepikas (OEPT), Giuseppe Pesce (OEPT).

PONENCIAS

Francisco Miguel de Sousa Goncalves

    The Planetary Society in Portugal: future activities and cooperation

    Daniel Osvaldo Slepikas (Presidente y fundador de EPT) "Explorando el Espacio desde el Ciberespacio"

Cayetano Santana Gil (TMS)

    Iniciativa privada no lucrativa en la exploración espacial

Vera Assis Fernandes

    Can the Period of Lunar Mare Volcanism Be Extended?

    Recent age determination of lunar meteorite Northwest Africa 032 (NWA032) and of basaltic samples collected by the Luna 24 mission samples have extended the period of lunar volcanism from ~650 Ma to ~ 1000 Ma. NWA032 was extruded at ~2.8 Ga, making it one of the youngest lunar basalts ever analysed. Mare lava extrusions within the Mare Crisium may have occurred for a period of at least ~800 Ma. The continued use of lunar meteorites found on Earth deserts (e.g. Sahara and Antarctica) has the potential of giving new (and possibly improved) insights of the lunar bulk chemistry and ages of volcanism on the Moon as well as the extent of bombardment of the lunar surface.

     Despite the unknown provenance of this type of sample, they have

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shown to be an important complement to the samples collected by the

    Apollo and Luna missions and impose less sampling bias.

    Causes, processes and consequences of tsunamis.

    Studies on the AD 1755 Tsunami (Lisbon).

    Costa P.*, Leroy S.*, Dinis J.**

    *Department of Geography and Earth Sciences, Brunel University,

    Uxbridge, UB8 3PH, Middlesex, United Kingdom

    ** Departamento de Ciencias da Terra, Universidade de Coimbra,

    Largo Marques de Pombal 3000-272 Portugal

    A tsunami is an oceanic gravity wave generated by earthquakes, volcanoes, landslides and meteorite impact. A phenomenon like a tsunami can provoke vast destruction in coastal areas owing to the height of the wave and its speed.

    Historically, Portugal and Spain, being countries intimately related to the sea, have always had concerns with natural hazards in their coastal areas. However the study of tsunamis in Iberia is relatively recent. The most important event, in the last 500 years, for the Iberian Atlantic coast was the AD 1755 tsunami that followed a magnitude 8 earthquake. The historical description and consequences of the AD 1755 tsunami and a brief review of the tsunamis that occurred in the Iberian Peninsula coasts is presented. Although not frequent in the Portuguese and Spanish shores, approximately 40 tsunamis have been registered by historical and field survey data in the last 2000 years in the Atlantic and in the Mediterranean coasts of Iberia. The Gorringe Bank (SW of Cape St. Vincent) has been the source of many earthquakes and tsunamis that struck the Iberian Atlantic Coast. It is probably the most important tsunamigenic area in Europe.

    Due to their huge power, tsunamis can transport marine organisms and sediments far inland. The study of sediments deposited by past tsunamis in lakes and lagoons provides vital information about potential changes either in the coastal morphology or in the environment of coastal areas.

    A series of lakes and coastal lagoons are being studied along the Portuguese coast with the aim of:

    - Discovering and studying the 1755 tsunami sediment layer

    - Understanding the environmental effects of the 1755 tsunami

    - Detecting major environmental changes in that geographical area, since then.

    The sites selected will allow conducting a multiproxy study with a high time resolution. The techniques to be used include magnetic susceptibility, sediment visual description, 210137Pb and Cs dating and a range of sedimentological and palaeoecological proxies with the focus of obtaining well-dated tsunami indicators such as salinity changes, grain size changes, erosive and compaction microstructures. We will also use historical data to collect complementary information about the effects of the tsunami and to reinforce the age-depth model of the sedimentary sequences.

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José Fernando Monteiro

IMPACT STUDIES IN PORTUGAL

     (1)(2)(1)José F. Monteiro , António Ribeiro , José Munhá

     (1)Departamento e Centro de Geologia. Faculdade de Ciências da Universidade de Lisboa, Edifício C-2, Campo Grande, 1700 Lisboa, Portugal. e-mail: jfmontei@fc.ul.pt

     (2) Departamento de Geologia e Lattex. Faculdade de Ciências da Universidade de Lisboa, Edifício C-2, Campo Grande, 1700 Lisboa, Portugal.

    In this work we report the state-of-the-art of our studies in impact geology subjects in Portugal, specially in the following items:

    1 -The Tore Seamount as a possible megaimpact crater in deep ocean and its possible inducement of an ejecta layer near the Cenomanian-Turonian boundary, North of Nazaré and a tsunamic deposit at the same boundary in Nazaré;

    2 - The Guarda Circular Structure as a possible complex deeply eroded impact crater in Central Portugal.

    1. During the TORE-MADERA mission (IFREMER, September-October, 2001, with the TORE MADERA Group) the data gathered allowed the study of the origin and significance of the Tore Seamount. A few dredges revealed the presence of rocks with a texture suggesting impact melt and impact melt breccias. This interpretation must be confirmed by geochemical and isotopic studies in search for the signatures that will characterize the Tore Seamount as an impact structure. The geophysical data (swath bathymetry, bottom imagery, shallow seismic, magnetics and gravimetry) allowed to establish the following: Tore is defined by a central depression attaining 5.5 Km depth, elongated in the NE-SW direction; an eccentric bulge is imaged by thew seismic lines below the sedimentary cover; an external ring with depts ranging from 2 to 4 Km defines an ellongated elipse in the NNE-SSW direction with major and minor axes 120 and 90 Km length respectively. We interpret these facts as the evidence of an oblique impact from SW to NE , at ? 90 MA, that should also be responsible for the focusing of its effects in the shore: the possible ejecta breccia at 10 Km N of Nazaré.

    The ejecta occurs on top of brecciated limestone of Cenomanian age and consists of polymitic breccia that includes several clasts with diagnostic impact features: glass, microscopic spherules and irregular shard like particles. The main mineralogy of the breccia consists of pyrite, hematite, devitrified glass, quartz and calcite; solid hidrocarbons also occur as surface accumulations and impregantions occupying the pore spaces of the breccia.

    Besides these strucutral and textural features, a geochemical anomaly was detected both in the ejecta and in a green mudstone layer (0 to 50 cm thick) occurring on topo of it; the anomalous elemental concentrations are about 5 to 10 times the average continental crustal abundances and include iridium as well as other PGE.

    At Nazaré there is a complete section of Cenomanian-Turonian age. Significantly, a sharp stratigraphic discontinuity is observed at the Cenomanian-Turononian boundary, between highly brecciated (Cenomanian) limestsones and siliciclastic sandstones (Lousões Formation of Turonian age). We propose that the Lousões Formation reflects a series of complex events resulting from tsunamic activity, related to the disruptive impact event at the end of the Cenomanian. Indeed, the sandstones display several characteristics that are typical of tempestite depostis (lamination and parallel laminated sediments, cross bedding, sand with fossil debris and collapsed structures as skin slump blocks).

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    2. In the central part of Portugal, NE of Guarda, not far from the border with Spain, exists a peculiar circular srtucture, with 35 Km diameter, well evident in the topographic and hydrographic pattern. The structure is centered at ? 40º 37‟ N and 7º 6‟ W in the Beira Alta province. The anomalous circular area was originally noted on ERST 1 and ERTS 3 satellite images and, latter, a Landsat mosaic in multispectral scanner colour composition of channels 7, 5, 4 (with spatial resolution of 80 m) of Central Portugal shows clearly the circular structure wich resembles a deeply eroded complex multi-ring impact crater (Grieve, pers. inf.). Geologically, the structure is situated in the Central Iberian Geotectonic Zone of the hercynian orogeny. The country rocks are dominated by different types of alkaline and calc-alcaline hercynian granits and the edge of the structure doesn‟t coincide with the granite intrusions.

    Other less representative formations are present: small gabbroic intrusions from the permo-triassic age, several quartz and basalt-dolerite dykes, very rich in uranium mineralizations. In the central area of the structure we found a breccia probably associated with the doleritic dykes. The breccia also forms dykes with various thicknesses and irregular atitudes crosscuting the biotitic calc-alkaline granite. The breccia contains several fragments of the country rocks: granites, minerals and fragments of the gabbros and dolerites, as well as slates from the pre-ordovician formations. The matrix is dominated by very fine material of different mineralogical composition, sometimes with glassy texture. It was in several fragments of that breccia that we found planar fractures in some quartz grains.

    The granite in the central region of the structure also shows evidence of strong deformation, with monomitic brecciation and traces of decorated planar fractures. No clear PDFs were observed. The quartz grains are intensely fracturated with some rotation and displacement.

PETROGRAPHIC AND CHEMICAL CARACTERIZATION OF

    DIFFERENT CLAST POPULATIONS OF CHAVES HOWARDITE

    José F. Monteiro, José Munhá

    Departamento e Centro de Geologia. Faculdade de Ciências da Universidade de Lisboa, Edifício C-2, Campo Grande, 1700 Lisboa, Portugal. e-mail: jfmontei@fc.ul.pt

    The Chaves meteorite fell on May 3, 1925 in the small village of Vilarelho da Raia, situaded 8 Km North of Chaves town (Portugal) and less than 1 Km with the Spain border. The fall was accompainied by two very strong explosition sounds followed by a noise resembling that of a cannon fire. Three fragments were recovered by the local authorities with a total mass of 2945g. All stones have a shiny dark brownwish-black crust, which is fissured and present areas of striated crust. In the broken surfaces, where the samples are void of fusion crust, light to dark gray interior material is exposed. In addition to a light grey groundmass containing angular fragments varying from white to greenish and sometimes grey pyroxenes, the meteorite shows ophitic texture with white plagioclases, which are up to 9 mm long.

    In thin section Chaves is a complex breccia with a clear regolithic texture. It consists mainly of fragments of single minerals, pyroxene (orthopyroxene and pigeonite) and plagioclase, with several pyroxene fragments up to 2 mm. Both the breccia fragments and the groundmass show the effects of strain, being shottered, and showing undulatory extinction. Dark brown glass and devitrified glass are very common and they are intimately associated with cristaline grains of plagioclase and pyroxene. Turbid glass shows sometimes crystalline needles of pyroxene. Microprobe analysis show that the pyroxenes consists largely of orthopyroxene with lesser amount of pigeonite. Pyroxene have a range composition from WoEnFs to WoEnFs. 2 78 205 37 58.

    The more magnesian pyroxenes have low Ca contents but the more Fe-rich varieties range from WoEnFs to WoEnFs. Feldspars, generally twinned, range in composition from Or24 18 5824 38 38 0.6

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AbAnto OrAbAn with more sodic grains in some of the volcanic clasts. Some silica 5,4 94 0.6 11,4 88

    minerals, quartz and tridymite, were observed in thin sections and, during microprobe work, many small intergrowths between plagioclase and pyroxene were found to contain very high silica. Quartz is always associated with pyroxene, recrystallized pyroxene, glass and magnetite. At least 70% of the opaque minerals is troilite, occuring frequently as fine intergrowths with pyroxene. Chromite, ilmenite and some grains of Fe-Ni metal was encountered. No olivine was found.

    In this work we separated the two great populations of clasts eucrite and diogenite according

    to classic petrographic observations and chemical characterization of the pyroxenes by electron microprobe analysis. The same minerals of both clasts are found in the matrix, showing that this meteorite is a typical howardite, resulting from the mechanical mixture of essentially two fractions of a very differentiated parent body probably the asteroid 4 Vesta. Other type of

    clasts are not excluded in the study of other thin sections. Attending the next NASA Discovery mission to Vesta, the project Dawn and the rarity of this type of meteorites- , future studies of this sample are of great importance.

    THE OURIQUE H4 ORDINARY CHONDRITE

    José F. Monteiro, José Munhá

    Departamento e Centro de Geologia. Faculdade de Ciências da Universidade de Lisboa, Edifício C-2, Campo Grande, 1700 Lisboa, Portugal. e-mail: jfmontei@fc.ul.pt

    In spite of one find and eigth described falls of meteorites in Portugal, we only have six portuguese meteorites available for study. They are: the S. Julião de Moreira IIB octaedrite (find in 1877); the Olivença LL5 chondrite (some fragments fall in portuguese territory); the Chaves howardite (fall in 3 May 1925); the Monte das Fortes L5 Chondrite (fall in 23 August 1950); the Alandroal III A ataxite (fall in 14 November 1968) and the Ourique H4 Chondrite (fall in 28 December 1998). The other tree meteorites, briefly described in the literature are unknown to the scientific community.

The Ourique meteorite was the last known fall of a meteorite in Portugal and it‟s interesting to

    describe some particularities with this fall: it‟s identification as a stone from the sky by a shepherd man; the lack of legislation about the meteorite falls in Portugal; as well as it‟s rapid classification and submission to The Meteoritical Bulletin. Finally we will look to the interesting aspects for future sudies.

    In the 28 December 1998, at preciselly one clock a.m. the people of Aldeia de Palheiros, Ourique, South of Portugal, awake in a fright, by a very strong noise that resembles “the explosion of a petrol station” according to several witnesses. Several eye-witness saw the

    fireball and the first explosion at almost the same time the sky turns white and a small explosion was heard. The fire-brigade and the local police search for signs of an artifical explosition or accident, but most of the people thougth that it was the start of a great thunderstorm. The meteorite was found to days after the fall by a local shepherd who was surprised by the new stones in the rural drift-way and the intense sulfur smell. Suddently he associated the stones with the intense noise two days before, and think that the stones fall from the sky. After that, several people went to the fall place and took severall pieces with them. In the precise point of the fall a small crater (60 x 30 x 15) was formed with an azimute 115º. After the impact, the meteorite was broken in several pieces and the fragments distributed in a conic section with a length of 55 m, where the biggest sample was collect. We estimated a mass of 20-30 Kg for the original rock, but we only recovered 3,5 Kg, including the major fragment (2,6 Kg) that is now at the Museu de História Natural da Universidade de Lisboa.

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    The meteorite shows a grey to greenish color, with the typical black fusion crust, sometimes brownish and with some vitreous lustre. In the hand sample we can see the chondrules, no bigger than 2 mm, the olivines and the metal alloys. It is possible to see the breccia texture with different clasts. The microscopic study shows a texture characteristic of group 4 ordinary chondrites. The opaque minerals are essencialy Fe-Ni and troilite.The chondrules are very frequantly and different types can be found: porphyric encircled by fine-grained rims; granular olivine-pyroxene; radial pyroxene; elipsoidal barred olivine; glassy and polysomatic barred olivine chondrules; and glassy to cryptocristalline chondrules. Some of them are surronded by parcial Fe-Ni rims.

    Preliminary studies by electron microprobe show that olivine presents a molar composition of 18% to 19% of faialite and a molar compostion of 16% of ferossilite. This allow us to classify the Ourique meteorite as H 4 ordinary chondrite. Other studies are under way.

DEXTRAL SHEAR DEFORMATION BELT ON SOUTHERN MARGIN OF EASTERN

    OVDA REGIO, VENUS: GEODYNAMIC IMPLICATIONS.

     (1)(1)(2)I. Romeo, R. Capote & F. Anguita (1) ) Facultad de Ciencias Geológicas, Univ. Complutense de Madrid, 28040 Madrid (España).

    Departamento de Geodinámica. e-mail: iromeobr@geo.ucm.es , capote@geo.ucm.es.

    (2) Facultad de Ciencias Geológicas, Univ. Complutense de Madrid, 28040 Madrid (España).

    Departamento de Petrología y Geoquímica. e-mail: anguita@geo.ucm.es.

    Crustal plateaus are one of the most important element in Venus surface, and the study of this kind of unit is essential for understanding the geological evolution before the global resurfacing event. These regions are flat topped highlands formed of tessera terrain, characterized by superposition of crosscutting structures (1). There are two principal and opposite hypothesis explaining the crustal plateaus origin. One of them advocates a mantle downwelling process as the driving mechanism, producing regional contractional tectonism (2), but some other authors point out that crustal plateau formation is controlled by mantle upwelling (hot spot model) (3). The greatest crustal plateau is Ovda Regio, with three major structures, folds, ribbons and grabens, and a history of contractional and extensional tectonic stages which understanding is highly significant for getting a genetic model. Strike-Slip tectonism has not been described in Ovda Regio, though it has been found in other areas of Aphrodite Terra (4) and Ishtar Terra (5). In this paper we describe a zone with strike-slip faults at Ovda Regio?s southern margin and discuss its relations with regional tectonism there.

    A detailed mapping of southern margin of Ovda Regio, between 78º-86º E longitude and 6º-16º S latitude, based on F-BIDR radar images, has provided significant relative chronological succession of volcanic and tectonic events. We have found a complex deformation domain formed by two different elongated belts, trending N100-110º, and surrounding a central zone characterized by cross structures with a basin and dome structural pattern. The northern internal deformation belt (200 km width) develops a set of en échelon anticlines, with N77 axial trend

    and 20-120 km wavelength, sometimes associated to probable reverse faults. These folds are crosscut by perpendicular extensional structures that we classified as the so called ribbons (6). The geometrical relationships with folds show that ribbons are probably synchronous with the folds or postdate them. There exists a clear obliquity (30º) between the fold axial traces and the Ovda?s margin, implicating a transpression distributed dextral shearing, with the regional shortening axis trending N167º and a N57º extensional axis. This shortening orientation is coherent with both, folds and ribbons and it is significant that it is perpendicular to Ovda Regio?s northern marginal fold belt, which has been interpreted as a collisional belt (2-8). The fold and ribbon geometry and kinematics allow to determine the contribution of these structures

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    to the horizontal dextral regional shearing along the internal belt, that has been calculated in a minimum amount of 5-20 km.

    The general deformation pattern in the external (southern) belt is also coherent with the same regional strain field but in this case different structures accomplish the dextral shear deformation. In this belt there is a deformation zone, 200-260 km wide, defined by three or four fault-like main structures with very straight traces trending subparallel to the plateau margin and more than 600 km long. Detailed geometry show R and P Riedel-like fractures with an anastomosing structure equivalent to the patterns described in brittle strike-slip faults , both in experimental works and in a number of field examples on the Earth. Completing the structural pattern in this external belt there is a set of en échelon ridges with sigmoidal geometry and

    N60-90 dominant direction that can be interpreted as folds with 2-5 km wavelength. This tectonic design represents a shear zone with displacement concentrated along brittle strike-slip faults, and it must be distinguished from the S-C terrains described in Ishtar Terra (5,3) which correspond to a more ductile behavior. Minimum bulk horizontal displacement along this external deformation belt has been estimated from the geometrical relations in almost 12 km. Deformation was developed before and during the global resurfacing event because brittle faults and ridges affect to the volcanic plains.

    The structural observations and probable tectonic evolution in these two transpression belts in southern margin of Ovda Regio significantly agree with a stress field transmitted from the advocated collision in the northern margin (2, 8), whereas contradict in some way the simple upwelling geodynamic model (6).

    th(1) Hansen et al. (1999). Geology Dec. p.1070-1074. (2) King et al. (1998). 29 LPSC: 1209. (3) thPhillips et al. (1998). Science 279:1492-1497. (4) Davis, A.M. & Ghail, T.H.(1999). 30 LPSC: th1330. (5) Hansen (1992). 23 LPSC: 478-479. (6) Ghent et al. (1999). Icarus 139: 116-136. (7) thHansen et al. (1996). Icarus123:296-312. (8) Tuckwell et al. (2002) 33 LPSC: 1566.

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