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

GEOMAGNETISMO Y PALEOMAGNETISMO Geos, Vol. 27, No. 1, Octubre, 2007estimates of fairly good quality. Unfortunately, the systematicpresence of coarse multidomain (MD) titanomagnetite in thetransitional and normal polarity flows did not enable us to obtainreliable paleointensities for none of these flows. The Virtual DipoleMoments (VDM) calculated for the reverse polarity flows vary from3.8 to 6.9x10 22 Am2with an arithmetic mean value of 5.5±0.8x 1022 Am2. Thisvalue is identical to the mean VDM obtained for the 0.3-5 Matime window and thus strengthens the conclusion that the recentgeomagnetic field strength is anomalously high compared to thatolder than 0.3 Ma. 40Ar/39Ar isochron ages were obtained usingthe incremental heating technique on groundmass separates orphenocryst poor whole rock samples. Six of the transitionallymagnetized lavas yielded isochron ages that areindistinguishable from one another at the 95% confidence level.The weighted average age is 862 ± 41 ka. This finding seems tosupport the hypothesis that a geomagnetic excursion has beenrecorded in Tjornes lava flows prior to the Matuyama-Brunhesreversal rather than the actual reversal.GEOPAL-8EARLY CRETACEOUS ABSOLUTEGEOMAGNETIC PALEOINTENSITIESFROM ARAPEY FORMATION (URUGUAY)Cejudo Ruiz Rubén y Gogichaishvili AvtoInstituto de Geofísica, UNAMruben@geofisica.unam.mxAbsolute paleointensity determinations before and during CNSare still scarce and of variable qualities. In this study, we reportnew paleointensity data from Arapey Formation (part of theParaná-Etendeka large igneous province) which have severaladvantages: (1) they are widely distributed in a large volcanicprovince and easy to access; (2) they record faithfully themagnetic field that existed at the time of their eruption; (3) Most ofthem are fresh for isotopic dating and have already yielded reliableK-Ar and Ar-Ar ages. 56 samples from 11 individual flows yieldedacceptable paleointensity estimates. The mean paleointensityvalues per flow are ranging from 47.5 to 21.6 microT and thecorresponding Virtual Dipole Moments (VDMs) are ranging from8.9 to 4.3 (1022 Am2). This correspond to the mean value of 6.8(1.7) x 1022 Am2, which is only slightly lower with respect to thepresent geomagnetic axial dipole.GEOPAL-9PALEOINTENSIDADES OBTENIDOS CONEL METODO DE "MULTI-SPECIMENPARALLEL DIFFERENTIAL PTRM"Böhnel HaraldCentro de Geociencias, UNAMhboehnel@geociencias.unam.mxSe presentará la metodología para obtener paleointensidadesutilizando el método de “multi-specimen parallel differentialpTRM” (Dekkers & Böhnel, 2006). Resultados se han obtenidode rocas volcánicas de México y de Hawaii, con edadescuaternarias y terciarias. En algunos casos se pueden compararlos resultados con otros obtendios aplicando diferentes métodospara paleointensidades.GEOPAL-10PALEOSECULAR VARIATIONS RECORDED BYHOLOCENE SEDIMENTS FROM A MAAR LAKE (LAHOYA SAN NICOLAS) IN GUANAJUATO, MEXICOChaparro Marcos A.E. 1 y 2 , Böhnel Harald 1 ,Molina Garza Roberto S. 1 y Byrne Roger 31 Centro de Geociencias, UNAM2 Instituto de Física Arroyo Seco, Argentina3 University of California, USAmchaparro@geociencias.unam.mxIn this work we report new Paleomagnetic data from a maarlake in central Mexico, La Hoya de San Nicolas, which waspreviously sampled and studied for rock-magnetic, palynologicaland geochemical analyses. The Paleomagnetic investigation wascarried out on three sediment cores (SN-4, SN-5 and SN-6) ofaround 5 m in length. Although other available data have beenobtained from different archives in Mexico, e.g. volcanic rocks,arqueomagnetic material and cave deposits, there is a need ofnew data from continuous records like as lake sediments in orderto describe in detail the Holocene geomagnetic Paleosecularvariation (PSV) in this region.Rock-magnetic results –from studies of magnetic susceptibility,hysteresis cycles, anhysteric and isothermal remanentmagnetization– indicate that the remanent magnetizationof these sediments is controlled by pseudo-single domain(titano)magnetite, which are suitable recorders of thegeomagnetic field. The age-depth model implies averagedeposition rates of about 0.5 mm/yr and a basal age of about10,600 cal. years BP.Magnetic susceptibility and natural remanent magnetizationlogs were used to correlate the Paleomagnetic (i.e. declinationand inclination) data of all sediment cores. As result of applyinga stretching and stacking process, as well as the age-depthconversion, a composite PSV record was obtained. In this PSVcurve, declination and inclination results show an oscillatorybehavior varying in a narrow range, and an interesting swingis observed around the basal ages. Furthermore, a comparisonbetween these results and independent data from lava flow isdone.GEOPAL-11A NEW CATALOGUE OF MESOAMERICANARCHEOMAGNETIC DATASoler Ana María, Gogichaishvili Avto, CaballeroMiranda Cecilia y Urrutia Fucugauchi JaimeInstituto de Geofísica, UNAManesoler@geofisica.unam.mxA new catalogue of archeomagnetic data from Mesoamericacontains all available and updated results reported since early70th plus 27 new archaeomagnetic directions obtained during lastsix years. The most of samples are taken from the burned floorsand walls. In some case, lime plasters are also used. The ageinterval covers from 100 BC to present.78