Libro de resúmenes [revisión final, 172 páginas] - UGM

Geos, Vol. 27, No. 1, Octubre, 2007GEOMAGNETISMO Y PALEOMAGNETISMOGEOPAL-4GILBERT-GAUSS GEOMAGNETICREVERSAL: NEW DATA AND SUMMARYAngel Cervantes Miguel 1 , GogichaishviliAvto 1 , Calvo Rathert Manuel 2 y Camps Pierre 31 Instituto de Geofísica, UNAM2 Universidad de Burgos, España3 University of Montpellier, Franciaavto@geofisica.unam.mxA detailed paleomagnetic and absolute paleointensity study ofa ~ 3.6 My Pliocene lava flow succession from Caucasus wascarried out. Previous study revealed that several consecutivelava flows record an intermediate polarity direction at thebase of the section followed by a thick reverse polarityzone. The transitional field was interpreted as an excursionwithin chron 2Ar or an upper Cochiti-Gilbert reversal. Newpaleomagnetic data reported in present study are obtained fromnearby lava successions. The direct correlation with originalsequence and field observations allowed to establish a newmagnetic stratigraphy. The lower part of section is characterizedby intermediate magnetic polarity followed by thick reverselymagnetized lavas. The upper sequence, represented by 11consecutive flows yielded normal magnetic polarity. The meanpaleointensity of the intermediate field is 12.8 (2.7) microT (10flows). The reverse polarity paleointensity is higher with a mean27.3 (9.3) microT (22 flows) while normal polarity yielded inaverage 34.2 (6.8) microT. Considering all available radiometricages and new paleomagnetic data it may be speculated thatGilbert-Gauss (R-N) reversal is recorded at the upper part ofsequence.GEOPAL-5FLUCTUATIONS OF EARTH’S MAGNETIC FIELD BETWEEN1300-200 BP AS INFERRED FROM BRAZILIAN POTTERIESHartmann Galvam 1 , Trindade Ricardo 1 ,Gogichaishvili Avto 2 , Morales Juan 2 y Afonso Marisa 11 Universidade de Sao Paulo, Brasil2 Instituto de Geofísica, UNAMavto@geofisica.unam.mxThe evolution of the Earth\'s magnetic field in the 10 to100s of years time-period can provide important informationabout geodynamo variations. Accurate curves for the last 2000years have been developed for Europe and Western Asia. Incontrast, continents in the southern hemisphere count much lessdata; South America contributes with less than 1% of the worldarcheointensity database. In order to derive a South Americanarcheointensity master-curve, we have initiated a systematicpaleointensity survey in well-dated (TL and/or 14C methods)ceramic fragments from southeast and northeast Brazil. Here wereport the first results on pottery collected in 24 Guarani sites withages from 1300 BP up to 200 BP, and colonial historical sites withages from 350 BP to 200 BP.GEOPAL-6PALEOMAGNETIC, ROCK-MAGNETICAND PALEOINTENSITY STUDY FROMVOLCANIC ROCKS OF ISLA SOCORROSparbori Elise 1 , Tauxe Lisa 1 , Urrutia FucugauchiJaime 2 , Gogichaishvili Avto 2 y Bohrson W.A. 31 Scripps Institution of Oceanography, University of California, USA2 Instituto de Geofísica, UNAM3 Central Washington University, USAessbarbori@ucsd.eduWe carried out a joint expedition between the Scripps Institutionof Oceanography and the Universidad Nacional Autonoma Mexicoto Isla Socorro in January of 2005. We collected orientedpaleomagnetic samples from 21 sites from as many as 10different volcanic units. We subjected over 100 specimens to an“IZZI” type (Tauxe and Staudigel, 2004) paleointensity experimentin which in-field and zero-field first steps were alternated todetect high-temperature tails. We also included the standardpTRM checks. Although some specimens worked extremelywell, we observed non-ideal behavior in many specimens. Inan earlier study, Bohrson et al. (1996) proposed a series ofwidespread eruptive events, based on similarities of argon/argondates. Paleointensity from specimens that conform to the strictestacceptance criteria are available from both the (unoriented)original sample collection and our fully oriented (but as yetundated) new collection. Comparison of the two sample setsallows us to tie dates from the dated collection to the new fullyoriented data set.GEOPAL-7MATUYAMA-BRUNHES REVERSAL ONTJORNES PENINSULA (NORTHERN ICELAND)Camps Pierre 1 , Singer Brad 2 , CarvalloClaire 3 , Gogichaishvili Avto 4 y Allen B. 21 Geoscience Montpellier, University of Montpellier, France2 Department of Geology and Geophysics, University of Wisconsin, USA3 Institut de Mineralogie et de Physique de la Matiere,Condensee, Universite Pierre et Marie Curie, France4 Instituto de Geofísica, UNAMpierre.camps@gm.univ-montp2.frWe report paleomagnetic investigations conducted on fouroverlapping stratigraphic sections (57 flows) through the lavapile in Tj¨ornes peninsula (North Iceland). The purpose of thepresent study is to check the existence of intermediate directionspreviously interpreted as belonging to the Matuyama-Brunhesgeomagnetic reversal, completing this record with Thellier-Thellierpaleointensity determinations and 40Ar/39Ar radiometric dating.The directional results corroborate the findings by Kristjansson etal.[1988]: a sequence of reverse, transitional and normal polarityis found on each section. The polarity change is characterizedby a jump from reverse virtual geomagnetic poles (VGPs) to aquasi-cluster of transitional VGPs located over China, followed bya second jump to normal polarity. Reverse polarity flows exhibit amagnetic mineralogy well suited for paleointensity determination.Of the 25 reverse flows, 20 yielded paleointensity77