TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 61 (No. 1), pp. 71-81, 2009

Gilbert-Gauss geomagnetic reversal recorded in Pliocene volcanic sequences from Georgia (Lesser Caucasus): revisited

Avto Goguitchaichvili1, Miguel Angel Cervantes1, Manuel Calvo Rathert2, Pierre Camps3, Jemal Sologashvili4, and Givi Maissuradze5

1Laboratorio Interinstitucional de Magnetismo Natural, Instituto Geofísica-Sede Michoacán,Universidad Nacional Autónoma de México, Morelia, Mexico
2Departamento de Física, Universidad de Burgos, Av. Cantabria, s/n, 09006 Burgos, Spain
3Geoscience Montpellier, Université Montpellier 2, Case 049, 34095 Montpellier, Cedex 5, France
4Tbilisi State University Ivane Djavakhishvili, 3, Tshavtshavadze St., 380064 Tbilisi, Georgia
5Institute of Geology, Alexidze 9, 380009 Tbilisi, Georgia

(Received October 23, 2007; Revised July 23, 2008; Accepted August 2, 2008; Online published January 23, 2009)

Abstract: We carried out a detailed paleomagnetic, rock-magnetic, and Thellier paleointensity study of an ∼3.6 My Pliocene lava flow succession in southern Georgia. An earlier study (Camps et al., 1996) revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. The transitional field was interpreted as an excursion within chron 2Ar or an upper Cochiti-Gilbert reversal. New paleomagnetic data reported here have been obtained from nearby lava successions. In total, about 170 standard paleomagnetic cores belonging to 22 lava flows were collected during the 2005 sample collection campaign. Rock-magnetic experiments showed that the remanence is carried by Ti-poor titanomagnetite in most of the samples. The fraction of grains with multidomain magnetic structure does not seem to be important. Characteristic remanent magnetization was successfully determined on all samples. The direct correlation with the original (Thoki) sequence, previous preliminary measurements of natural remanent magnetization (Sologashvili, 1986), and field observations allowed us to establish a new magnetic stratigraphy. The lower part of section is characterized by intermediate magnetic polarity followed by thick reversely magnetized lavas. The upper sequence, represented by 18 consecutive flows yielded normal magnetic polarity. The mean paleointensity of the intermediate field is drastically reduced with respect to the post-transitional field strength. Based on all available radiometric ages and new paleomagnetic data, it may be speculated that Gilbert-Gauss (R-N) reversal was recorded at the upper part of sequence. Lower intermediate polarity flows possibly represent a form of precursor of this reversal that is similar to the Matuyama-Brunhes geomagnetic transition.
Key words: Paleomagnetism, reversals, excursions, paleointensity, Caucasus.

Corresponding author E-mail: avto@geofisica.unam.mx

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