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

GEOMAGNETISMO Y PALEOMAGNETISMO Geos, Vol. 27, No. 1, Octubre, 2007GEOPAL-1PALEOMAGNETISMO DE LAS OFIOLITASY ROCAS VOLCÁNICAS DE CUBANORORIENTAL: UN ESTUDIO PILOTOBatista Rodríguez José Alberto 1 , Alva Valdivia Luis 2 ,Blanco Moreno Jesús Antonio y Urrutia Fucugauchi Jaime 21 Instituto Superior Minero Metalúrgico de Moa, Cuba2 Instituto de Geofísica, UNAMjabatistar@yahoo.comSe realizó un estudio paleomagnético piloto de las ofiolitasy rocas volcánicas de Cuba nororiental (19 sitios, con 135núcleos orientados) de edad Jurásico Superior-Eoceno paradeterminar sus propiedades magnéticas e inferir aspectosrelacionados con la historia tectónica de esta región de Cuba.La desmagnetización por campos alternos resultó ser másefectiva que la desmagnetización por temperatura, con laexcepción de algunos sitios de gabros, que presentan una o doscomponentes de magnetización con temperatura de desbloqueomedia. Se definió la magnetización remanente característica en102 muestras. Los experimentos magnéticos indican que losportadores magnéticos son magnetita pura o titanomagnetitas ypirrotina en un estado dominio pseudo-simple. Aparecen fuertesmagnetizaciones de origen viscoso, que se eliminan entre 10-30mT y 500-550oC, las mismas se destruyen completamenteentre 35-80 mT o 575oC, indicando magnetita como el portadorprimario de la remanencia. Se infieren fases ferrimagnéticascon diferentes coercitividades a partir del comportamientode tipo ‘pot-bellied’ y ‘wasp-waisted’ detectados durante losexperimentos de histéresis. De forma general las rocasanalizadas poseen medias y altas magnetización de saturacióny coercitividades. En 14 sitios se determinaron direccionespaleomagnéticas bien definidas y estables, con direcciones NEy NW de la magnetización remanente característica, las queson confiables para propósitos tectónicos y geológicos. Laspaleodirecciones medias de la región investigada es I = 32.65o,D = 15.25o, k = 3.9 y Alfa95 = 9.14, con el polo paleomagnéticoen Plat= 75.33o y Plong= 173.3o.GEOPAL-2PALEOMAGNETISM AND TECTONICSALONG WESTERN-CENTRAL PART OFTHE TRANS MEXICAN VOLCANIC BELTRosas Elguera José 1 , Gogichaishvili Avto 2 ,Alva Valdivia Luis 2 y Urrutia Fucugauchi Jaime 21 Universidad de Guadalajara2 Instituto de Geofísica, UNAMjrosaselguera@yahoo.comThe Trans-Mexican Volcanic Belt (TMVB), one of the largestcontinental volcanic arcs built on the North America plate,spans about 1000 km and crosses central Mexico from thePacific Ocean to the Gulf of Mexico. The initial stage of theTMVB is marked by widespread Miocene basaltic volcanism,emplaced from the Nayarit state, in the west, to the longitudeof Mexico City. This volcanism is characterized by plateau-likestructures resulting from the shield volcanoes and fissure lavaflows, which have an estimated aggregate volume rangingbetween 3200 and 6800 km3. The western-central Mexico hasbeen affected by right-lateral transtension within the westernTMVB but previous paleomagnetic studies indicate some 15-20°anticlockwise tectonic rotations for the Rio Grande de Santiagocanyon and surrounding areas, in accordance with a Mioceneleft-lateral transtensional tectonic regime. We present a summaryof paleomagnetic and rock-magnetic studies of that Miocenevolcanic succession from the TMVB. The analisis consist of 114consecutive basaltic lava flows (more than 550 oriented samples)from four localities: Tepic, Guadalajara, Los Altos, and Queretarowhich span from 11 to 7.5 Ma.GEOPAL-3NEW CONTRIBUTIONS TO THE TIME-AVERAGEDFIELD GLOBAL DATABASE: PALEOMAGNETISMOF AR-AR DATED VOLCANICS FROMTHE TRANS-MEXICAN VOLCANIC BELTPetronille Marie 1 y Gogichaishvili Avto 21 Institut de Physique de Globe de Paris, France2 Instituto de Geofísica, UNAMavto@geofisica.unam.mxIn this study, we report a detailed rock-magnetic andpaleomagnetic investigation of lava flows associated toTrans-Mexican Volcanic Belt (TMVB)- one of the largestcontinental volcanic arcs of the North American plate spanningabout 1000 km from the Pacific to the Gulf of Mexico.380 oriented samples coming from 62 independent coolingunits were collected. All these sites were recently datedby means 40Ar-39Ar systematics and span from 5.7 Mato 2 ka.. We studied in details three volcanic fields: 1)the Ceboruco-San Pedro, 2) Tequila at the western partof TMVB and 3) Alto de Lucero- Chiconquiapo-Palma Solaareas. Rock-magnetic experiments which included continuoussusceptibility and hysteresis measurements point to simplemagnetic mineralogy. In most of cases, the remanence is carriedby Ti-poor titanomagnetite of pseudo-single-domain magneticstructure. The characteristic paleodirections are successfullyisolated for 56 units. The mean paleodirection obtained inthis study, discarding intermediate polarity sites, is I = 35.8º,D = 0.9 º, k = 56, #95 = 5.4º. These directions arepractically undistinguishable from the expected Mio-Pliocenepaleodirections, as derived from reference poles for the NorthAmerican polar wander curve and in agreement with previouslyreported directions from nearby lavas of Trans-Mexican VolcanicBelt. This suggests that no major tectonic deformation occurredin studied area. The paleosecular variation is estimated troughthe study of the scatter of virtual geomagnetic poles giving SF= 16.1 with SU = 20.3 and SL = 12.3 (upper and lower limitsrespectively). These values are consistent with the value predictedby the latitude-dependent variation models for the last 5 Ma. Theinteresting feature of the paleomagnetic record obtained here is anoccurrence of intermediate magnetic polarity for two consecutivelavas dated as 2.04 ± 0.04 and 1.97 ± 0.04 Ma respectively, whichmay correspond to the worldwide observable Reunion event.Two independent lava flows dated as 362±13 and 354±5 karespectively, yield transitional paleodirections as well, probablycorresponding to the Levantine excursion.76