GEOCHEMISTRY OF MODERN CONTINENTAL BACK-ARC BASINS AND TRIASSIC-JURASSIC COLLISION-INDUCED BACK-ARC SPREADING IN THE CAUCASUS

Back-arc basins behind the main volcanic arc in subduction zones can complicate the tectonic history of ocean basin closure, can fragment continents, and can create new lithosphere via extension and oceanic spreading. Investigating the mechanisms of back-arc basin formation and their temporal evolution is critical for understanding how volcanic arc systems govern the long-term assembly and distribution of the lithosphere. A primary mechanism for initiating back-arc basin formation is thought to be collision of the arc with a buoyant indentor, which induces back-arc spreading along-strike of the collision by forearc rotation and trench retreat.

Between the Black and Caspian Seas, the Caucasus Mountains formed during the Late Cenozoic Arabia-Eurasia continental collision due to closure of a Mesozoic-Cenozoic continental back-arc basin: the Caucasus Basin. However, spatial, temporal, and geochemical patterns of Triassic-Jurassic arc-related magmatism in the Caucasus region have not been fully integrated into a coherent geodynamic model explaining the role of the Caucasus Basin in the evolution of a protracted Phanerozoic convergent margin.

Here, we use two modern continental back-arc basins – the Okinawa Trough and Bransfield Strait – to construct a template linking geochemical signatures with tectonic processes in back-arc settings. We apply this template to the Caucasus and integrate it with available geochronology to understand the formation of the Caucasus Basin and the evolution of the associated arc-back-arc system. Published whole-rock trace-element and Sr-Nd isotopic data from back-arc magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental back-arc basins and that back-arc spreading likely occurred from ~180-160 Ma. Existing U-Pb and ⁴⁰Ar/³⁹Ar geochronology indicates that back-arc spreading was likely induced by along-strike collision of Iran with Laurasia at ~220-210 Ma, initiating trench retreat and southward migration of the Caucasus arc front. This collision-induced, trench-retreat mechanism provides a clear explanation for the Triassic-Jurassic magmatic record in the Caucasus region and may be useful for understanding the evolution of other ancient arc-back-arc systems.