Evidence of Unusual Geomagnetic Regimes Recorded in Plio‐Pleistocene Volcanic Sequences from the Lesser Caucasus (Southern Georgia)

Abstract

We report a detailed paleomagnetic study on two Plio‐Pleistocene lava flow sequences from the Djavakheti Highland, Lesser Caucasus. The Korkhi sequence is composed of two volcanic successions of distinct age (1.9 and 3.1 Ma), while the Apnia sequence was emplaced between 3.8 and 3.1 Ma according to available radiometric datings. Normal, reverse and intermediate polarities have been determined from both sequences. Mean directions of the normal and reverse polarity groups for each section do not match the expected field direction, but the possibility of tectonic rotations has been dismissed. A composite analysis of paleomagnetic directions, of virtual geomagnetic pole (VGP) scatter and available paleointensity results from a previous study, allow the interpretation of the observed paleomagnetic results. In the Apnia sequence, both a short recording time unable to average paleosecular variation (PSV) and an anomalous Earth magnetic field (EMF) record are responsible for the observed paleomagnetic directions. According to paleomagnetic results and radiometric ages, this sequence most probably records the reverse to normal polarity transition C2Ar to C2An‐3n. The upper Korkhi subsequence yields an anomalous EMF record, reflecting a transitional time interval. Paleomagnetic results and available absolute ages suggest that this subsequence either records transition C2r‐1r to Olduvai or Olduvai to C1r‐2r. The lower Korkhi subsequence registers a normal polarity interval within the Gauss chron, reflecting a stable stage of the EMF.

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

“…The 8 µT value has been chosen taking 15% of the current field in Georgia, as suggested by Sánchez‐Moreno et al. (2018b).…”

“…Here, we use the threshold ε alt < 15% proposed by Sánchez‐Moreno et al. (2018b). Several samples did not provide meaningful results, mostly linked to the presence of two paleomagnetic components.…”

“…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.…”

“…In fact, in Thellier-type experiments (Thellier & Thellier, 1959), which are regarded the most trustworthy because they rely on a robust physical basis, several requirements have to be fulfilled by the analyzed samples in order to deliver reliable paleointensity determinations: (i) remanent magnetization must be of thermal origin, i.e., a thermoremanent magnetization (TRM); (ii) samples must comply with the so-called Thellier laws (Thellier & Thellier, 1959), a requisite which is only rigorously met by noninteracting single-domain (SD) grains (e.g., Dunlop, 2011, and references therein); and (iii) magnetic phases should not experience physical, mineralogical, or chemical alterations during heating, as this can lead to spurious paleointensity estimates. In some cases, apparently reliable determinations may produce erroneous paleointensity results (e.g., Calvo et al, 2002;Draeger et al, 2006;Gribov et al, 2019;Shcherbakov et al, 2019;Smirnov & Tarduno, 2005). As revealed by the paleointensity database PINT2015.05 (Biggin et al, 2009(Biggin et al, , 2010, this has led to a relative scarcity of paleointensity data which in addition are both temporarily and geographically unevenly distributed.…”

Magnetic dating of the Holocene monogenetic Tkarsheti volcano in the Kazbeki region (Great Caucasus)

“…The Caucasus area is characterized by a significant and uninterrupted volcanic activity from the Jurassic to the present day (e.g., Rebaï et al 1993). Since the 1990s, new paleomagnetic and paleointensity studies have been performed in Georgia, applying modern procedures and quality criteria (Calvo-Rathert et al 2015;Sánchez-Moreno et al 2018, and references therein) Nevertheless, high-quality archaeomagnetic data are still scarce. In this study, we tried to date a Holocene monogenetic volcano located in the Kazbeki Volcanic Province by means of a paleomagnetic age determination using the SHA.DIF.14K model for the first time in the Caucasus.…”