LOWER AND MIDDLE JURASSIC PALEOMAGNETIC RESULTS FROM THE SOUTH LESSER CAUCASUS AND THE EVOLUTION OF THE MESOZOIC TETHYS OCEAN

Abstract

We studied Lower Jurassic (Pliensbachian-Toarcian) basalts and tuffs and Middle Jurassic (Bajocian) limestones and marls from 9 sites in the southern part of the Lesser Caucasus (ca. 39.3°N, 45.4°E). A component showing rectilinear decay to the origin was reliably isolated from most volcanics. This characteristic component reveals nearly antipodal directions for both polarities and passes the fold test; the conglomerate test performed on lava boulders from the Aalenian basal conglomerates is also positive. We think that a primary remanence (D = 20°, I = 38° a95 = 5°) was isolated from the volcanics. In contrast, Middle Jurassic sediments yielded controversial results, and the only reliably isolated intermediate-temperature component (ITC) does not pass the fold test. The ITC directions show a considerable improvement in data grouping during incremental unfolding, with a maximum at 40% unfolding. We argue, however, that the thus obtained mean direction does not correspond to any paleofield and is most probably an artefact.When compared to reference data for the Eurasian and African plates, the Lower Jurassic mean inclination of 21°N ± 4° agrees well with the latter, thus implying that, in the Early Jurassic, the area studied belonged to Gondwana and was separated by the Tethys ocean from Eurasia, in agreement with paleontological data. We tried to locate the position of the boundary between the Eurasian and Gondwanian realms in the Caucasus region in the Early-Middle Jurassic; unfortunately, the available Jurassic paleomagnetic data from other tectonic units of the region did not reveal any clear pattern.