Weak palaeointensity results over a Pliocene volcanic sequence from Lesser Caucasus (Georgia): transitional record or time averaged field?

Abstract

SUMMARY A palaeointensity study has been carried out on a Pliocene sequence of 20 consecutive lava flows where previous directional results seem to reflect anomalous behaviour of the Earth's magnetic field (EMF), which can be explained by a polarity transition record or non-averaged palaeosecular variation or both. Here, we perform a total of 55 palaeointensity determinations using the original Thellier–Thellier (TT) method and 100 with the IZZI method. We assess the performance of our selection criteria using a set of strict threshold values applied to a set of test data whose TRMs were acquired in known fields. Absolute palaeointensity determinations that passed our selection criteria were obtained on four specimens with the TT method and on 41 specimens with the IZZI method. Application of reliability criteria at a site level yielded palaeointensity results in 8 of 20 studied lava flows. We obtained median values of VADM between 28.9 and 45.6 ZAm2 for the reverse polarity lower Apnia section, while the normal polarity upper section displayed a single value of 54.6 ZAm2. The low palaeointensity values before a transitional direction lava flow and the higher value after it, suggest the common behaviour at the start of a polarity reversal and the recovery after it. However, an isolated record of a stable EMF, where the intensity is lower than the current for the same location (83.7 ZAm2), cannot be discarded. Consequently, this interpretation would support a weak time-averaged field.

An Integrated Paleomagnetic, Multimethod‐Paleointensity, and Radiometric Study on Cretaceous and Paleogene Lavas From the Lesser Caucasus: Geomagnetic and Tectonic Implications

“…Volcanic units in the present study are somewhat thicker than those analyzed by Sánchez‐Moreno et al. (2020), so that cooling time has been estimated to be moderately longer. Our experimental approach leaving samples cool down naturally intends to reduce cooling rate effects.…”

“…Although the difference in cooling time between both cases is one order of magnitude, no differences were observed in the results. Volcanic units in the present study are somewhat thicker than those analyzed by Sánchez-Moreno et al (2020), so that cooling time has been estimated to be moderately longer. Our experimental approach leaving samples cool down naturally intends to reduce cooling rate effects.…”

“…Sánchez‐Moreno et al. (2020) analyzed a basaltic lava flow sequence in Georgia with different Thellier‐type experiments, leaving in one experiment samples cooling naturally and in the other experiment with a fan. Although the difference in cooling time between both cases is one order of magnitude, no differences were observed in the results.…”

“…Due to its location in the Arabia‐Anatolia‐Eurasia collision zone, the Caucasus region is characterized by a complex tectonic history (e.g., Adamia et al., 2011; Rolland, 2017). During the last 3 decades, a number of new paleointensity and paleomagnetic studies have been performed in the Caucasus area employing modern methods and being accessible in international journals (Asanidze et al., 2009; Bazhenov & Burtman, 2002; Bazhenov et al., 1996; Caccavari et al., 2014; Calvo‐Rathert et al., 2008, 2011, 2013, 2015; Camps et al., 1996; Goguitchaichvili & Parès, 2000; Goguitchaichvili et al., 1997, 2000, 2001, 2009, 2016; Meijers et al., 2015; Sánchez‐Moreno et al., 2018a, 2020; Shcherbakova et al., 2007, 2009). Most of these investigations, however, have been carried out on Pliocene and Pleistocene rocks, mainly from the Javakheti Highland.…”

Paleointensity Results From Pliocene Lavas of the Lesser Caucasus Obtained Using the Multispecimen Parallel Differential pTRM Method: A Comparison With Thellier‐Thellier and IZZI Data

“…Paleointensity determinations performed in the previous work of Sánchez-Moreno et al (2020) with the Thellier-Thellier (TT) and IZZI methods (Thellier and Thellier, 1959;Yu et al, 2004) and interpreted with the rather strict set of selection criteria RCRIT (Tauxe et al, 2016) have been reinterpreted in the present study. In order to carry out an analysis of the influence of the selection criteria on the results, a more flexible set of TTP criteria (Leonhardt et al, 2004 modified by;Paterson et al, 2014aPaterson et al, , 2014b has been applied in the reinterpretation.…”