Evolution of the Greater Caucasus Basement and Formation of the Main Caucasus Thrust, Georgia

Along the northern margin of the Arabia‐Eurasia collision zone in the western Greater Caucasus, the Main Caucasus Thrust (MCT) juxtaposes Paleozoic crystalline basement to the north against Mesozoic metasedimentary and volcaniclastic rocks to the south. The MCT is commonly assumed to be the trace of an active plate‐boundary scale structure that accommodates Arabia‐Eurasia convergence, but field data supporting this interpretation are equivocal. Here we investigate the deformation history of the rocks juxtaposed across the MCT in Georgia using field observations, microstructural analysis, U‐Pb and 40Ar/39Ar geochronology, and 40Ar/39Ar and (U‐Th)/He thermochronology. Zircon U‐Pb analyses show that Greater Caucasus crystalline rocks formed in the Early Paleozoic on the margin of Gondwana. Low‐pressure/temperature amphibolite‐facies metamorphism of these metasedimentary rocks and associated plutonism likely took place during Carboniferous accretion onto the Laurussian margin, as indicated by igneous and metamorphic zircon U‐Pb ages of ~330–310 Ma. 40Ar/39Ar ages of ~190–135 Ma from muscovite in a greenschist‐facies shear zone indicate that the MCT likely developed during Mesozoic inversion and/or rifting of the Caucasus Basin. A Mesozoic 40Ar/39Ar biotite age with release spectra indicating partial resetting and Cenozoic (<40 Ma) apatite and zircon (U‐Th)/He ages imply at least ~5–8 km of Greater Caucasus basement exhumation since ~10 Ma in response to Arabia‐Eurasia collision. Cenozoic reactivation of the MCT may have accommodated a fraction of this exhumation. However, Cenozoic zircon (U‐Th)/He ages in both the hanging wall and footwall of the MCT require partitioning a substantial component of this deformation onto structures to the south.Plain Language SummaryCollisions between continents cause deformation of the Earth’s crust and the uplift of large mountain ranges like the Himalayas. Large faults often form to accommodate this deformation and may help bring rocks once buried at great depths up to the surface of the Earth. The Greater Caucasus Mountains form the northernmost part of a zone of deformation due to the ongoing collision between the Arabian and Eurasian continents. The Main Caucasus Thrust (MCT) is a fault juxtaposing old igneous and metamorphic (crystalline) rocks against younger rocks that has often been assumed to be a major means of accommodating Arabia‐Eurasia collision. This study examines the history of rocks along the MCT with a combination of field work, study of microscopic deformation in rocks, and dating of rock formation and cooling. The crystalline rocks were added to the margins of present‐day Eurasia about 330–310 million years ago, and the MCT first formed about 190–135 million years ago. The MCT is likely at most one of many structures accommodating present‐day Arabia‐Eurasia collision.Key PointsAmphibolite‐facies metamorphism and plutonism in the Greater Caucasus basement took place ~330–310 MaThe Main Caucasus Thrust formed as a greenschist‐facies shear zone during Caucasus Basin inversion and/or rifting (~190–135 Ma)The Main Caucasus Thrust may have helped facilitate a portion of at least 5–8 km of basement exhumation during Arabia‐Eurasia collisionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/1/tect21292-sup-0002-2019TC005828-ts01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/2/tect21292-sup-0006-2019TC005828-ts05.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/3/tect21292_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/4/tect21292-sup-0003-2019TC005828-ts02.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/5/tect21292-sup-0005-2019TC005828-ts04.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/6/tect21292.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154626/7/tect21292-sup-0004-2019TC005828-ts03.pd

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

We report paleointensity results obtained with the multispecimen method (MSP) over the Pliocene sequence of Apnia (Georgia) which records a polarity reversal. Paleointensity determinations with the multispecimen technique were performed on 12 flows with the original (MSP-DB) and the domain-state corrected (MSP-DSC) protocol. Eight MSP-DSC determinations passed the proposed quality criteria. To obtain highly reliable data through the agreement between intensity values from different methods, MSP results were combined with paleointensities from a previous study with Thellier-type methods and especially strict selection criteria (RCRIT) on same flows (Sánchez-Moreno et al., 2020). Application of this multimethod procedure resulted in three new paleointensities including both MSP and Thellier-type results and an additional one obtained with two different Thellier-type methods, yielding one paleointensity of 36.9 µT in the normal-polarity, and three paleointensities between 19.2 and 24.1 µT in the reverse-polarity section. Additionally, Thellier-type data have been reinterpreted in this study with more flexible criteria (TTP) and the results combined with the MSP data. As a result, four flows yield paleointensities including MSP and Thellier-type determinations and seven include paleointensities obtained with two different Thellier-type methods. Results range from 37.2 and 44.3 μT in the normal-polarity and from 12.5 to 24.6 μT in the reverse-polarity section. Comparison of results from the four flows yielding multimethod determinations applying RCRIT criteria with those from the same flows under TTP criteria yields no significant difference in paleointensity values and their experimental uncertainty. Thus, application of a multimethod approach supports the possibility of using TTP criteria.Project PID2019-105796/10.13039/501100011033 (Agencia Estatal de Investigación, Spain), project BU066U16 (Junta de Castilla y León, Spain) and pre-doctoral grant BES-2013-064060 (MINECO, Spain). MCR acknowledges funding from the Fulbright Commission and the Spanish Ministry of Science, Innovation and Universities for a research stay at Hawaii University at Manoa. AG is grateful to the financial support given by DGAPA-PAPIIT IN101717. At Montpellier laboratory, the FUReMAG rapid furnace construction was supported by the French National Agency for Research (ANR-12-BS06-0015)

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

The radiocarbon technique is widely used to date Late Pleistocene and Holocene lava flows. The significant difference with palaeomagnetic methods is that the 14C dating is performed on the organic matter carbonized by the rock formation or the paleosols found within or below the lava flow. On the contrary, the archaeomagnetic dating allows to date the moment when the lava is cooling down below the Curie temperatures. In the present study, we use the paleomagnetic dating to constrain the age of the Tkarsheti monogenetic volcano located within the Kazbeki Volcanic Province (Great Caucasus). A series of rock-magnetic experiments including the measurement of hysteresis curves, isothermal remanence, back-field and continuous thermomagnetic curves were applied. These experiments indicated that Pseudo-Single-Domain Ti-poor titanomagnetite is responsible for remanence. A characteristic remanent magnetization was obtained for all twenty analyzed samples yielding a stable single magnetization component observed upon both thermal and alternating field treatments. Comparison of the mean directions obtained (Inc = 48.6º, Dec = 6.4º, A95 = 4.0° and K = 67) with the SCHA.DIF.14k model yielded two main time intervals (4740–4650 or 4427– 4188 BC) as the best age estimate of the Lesser Tkarsheti lava flow. These results suggest an earlier age (between approximately 200 and 700 years) for this monogenetic lava flow than expected from the estimated age provided by a former 14C dating obtained in 1973 on woody remains. This first attempt to use the archaeomagnetic technique in the Caucasus indicates that the SCHA.DIF.14k geomagnetic model may be successfully used for dating purposes in the region.Projects BU0066U16 and BU235P18 (Junta de Castilla y Leon, Spain) and the European Regional Development Fund (ERDF). AG is grateful for financial support of CONACyT 252149 and UNAM-PAPIIT project 101717. MC-R and AC acknowledge the financial support given by the Junta de Castilla y León (project BU235P18) and the European Regional Development Fund (ERD)

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

Sixteen rhyolitic and dacitic Cretaceous and Paleocene-Eocene lavas from the Lesser Caucasus have been subjected to paleomagnetic and multimethod paleointensity experiments to analyze the variations of the Earth's magnetic field. Paleointensity experiments were performed with two methods. Thellier-type experiments with the IZZI method on 65 specimens (nine flows) yielded 15 successful determinations and experiments with the multispecimen method on 14 samples (seven flows) yielded two successful determinations. The joint analysis of the results obtained with both methods produced a mean FuK = (19.9 ± 3.7) µT for upper Cretaceous and FPg = (20.7 ± 3.3) µT for Paleogene sites. Low virtual axial dipole moments for the Cretaceous (3.4 × 1022 Am2) and Paleogene (3.5 × 1022 Am2) samples support the idea of a lower average dipole moment during periods of stable polarity of the Earth magnetic field. Mean flow paleomagnetic directions did not match expected upper Cretaceous to Paleogene directions calculated from the European Apparent Polar Wander Path. While inclination results roughly agreed with expected values, a group of sites showed nearly North-South paleodeclinations (D = 1.1° ± 14.2°), and another group displayed eastward deviated paleodeclinations (D = 72.9° ± 26.6°). These results suggest the occurrence of nearly vertical-axis rotations, probably as a result of continental collision since Oligocene. In addition to paleomagnetic and palaeointensity analyses, new K-Ar absolute age determinations have been performed on three of the studied sites, yielding Late Cretaceous ages (78.7 ± 1.7, 79.7 ± 1.6, and 83.4 ± 1.8 Ma (2σ)).Project PID2019-105796GB-100/AEI/10.13039/501100011033 (Agencia Estatal de Investigación, Spain). M. Calvo-Rathert acknowledges funding from the Fulbright Commission and the Spanish Ministry of Science, Innovation, and Universities for a research stay at Hawaii University at Manoa. A. Goguitchaichvili acknowledges financial support from UNAM-PAPIIT no. IN101920. N. García-Redondo acknowledges financial support from Junta de Castilla y León and the European Research Development Fund (ERDF). EHB acknowledges financial support for laboratory maintenance and measurements to SOEST-HIGP and National Science Foundation grants. These is SOEST 11143 and HIGP 2420 contribution

Constitutive Overexpression of Muscarinic Receptors Leads to Vagal Hyperreactivity

BACKGROUND: Alterations in muscarinic receptor expression and acetylcholinesterase (AchE) activity have been observed in tissues from Sudden Infant Death Syndrome (SIDS). Vagal overactivity has been proposed as a possible cause of SIDS as well as of vasovagal syncopes. The aim of the present study was to seek whether muscarinic receptor overexpression may be the underlying mechanism of vagal hyperreactivity. Rabbits with marked vagal pauses following injection of phenylephrine were selected and crossed to obtain a vagal hyperreactive strain. The density of cardiac muscarinic receptors and acetylcholinesterase (AchE) gene expression were assessed. Blood markers of the observed cardiac abnormalities were also sought. METHODOLOGY/PRINCIPAL FINDINGS: Cardiac muscarinic M(2) and M(3) receptors were overexpressed in hyperreactive rabbits compared to control animals (2.3-fold and 2.5-fold, respectively) and the severity of the phenylephrine-induced bradycardia was correlated with their densities. A similar overexpression of M(2) receptors was observed in peripheral mononuclear white blood cells, suggesting that cardiac M(2) receptor expression can be inferred with high confidence from measurements in blood cells. Sequencing of the coding fragment of the M(2) receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression. Significant increases in AchE expression and activity were also assessed (AchE mRNA amplification ratio of 3.6 versus normal rabbits). This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression. Alterations in M(2) receptor and AchE expression occurred between the 5th and the 7th week of age, a critical period also characterized by a higher mortality rate of hyperreactive rabbits (52% in H rabbits versus 13% in normal rabbits) and preceeded the appearance of functional disorders. CONCLUSIONS/SIGNIFICANCE: The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it. Since similar vagal disorders were observed recently by us in SIDS, muscarinic receptor overexpression could become a marker of risk of vasovagal syncopes and SIDS