Abstract
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.
Highlights
The Earth’s magnetic field has often shown wide departures from its usual axial dipole configuration for relatively brief periods of about 3 × 103 years (Gubbins, 1999)
We present here a detailed paleomagnetic, rock-magnetic, and Thellier paleointensity study of an ArAr dated lava flow succession located in southern Georgia
Based on the paleodirectional and rock-magnetic results, 71 samples belonging to 15 lava flows that yielded stable, essentially one-component magnetizations with blocking temperatures compatible with a Ti-poor titanomagnetite phase, low viscosity indices, and nearly reversible k-T curves were pre-selected for Thellier paleointensity experiments
Summary
The Earth’s magnetic field has often shown wide departures from its usual axial dipole configuration for relatively brief periods of about 3 × 103 years (Gubbins, 1999). The original Thoki section studied by Camps et al (1996) is situated at 41◦28.616N latitude and 43◦22.850E, about 1 km S-SE of the village of Thoki and the Tchobareti brook confluent (Fig. 1) This formation consists of two parts, the Lower and Upper Akhalkalaki Sequences, which are clearly separated by an erosion surface. Determination of the viscosity index (Thellier and Thellier, 1944; Prevot et al, 1983) allows estimation of the capacity of a sample to acquire a viscous remanent magnetization and is a useful tool for obtaining information on the paleomagnetic stability of the sample To this end, we positioned samples so that one of their axes was aligned with Earth’s magnetic field and left them for 16 days.
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