Cretaceous Geodynamics and Magmatism in the Balkans: Contributions from Numerical Modelling

Cretaceous geodynamics in the present-day Balkan Peninsula has long remained unconstrained, with the central issue being the evolution of the final stages of the Vardar branch of the Neotethys. The onset of the intra-oceanic subduction is well constrained by the Middle Jurassic metamorphic soles. The same is true for the timing of latest ophiolite obduction events dated to Latest Jurassic – Earliest Cretaceous. Nonetheless, it remained unclear whether the emplacement of the Vardar zone ophiolites also marked the final closure of the last Tethyan realm in the Balkans. The main arguments for the persistence of the ocean throughout Cretaceous draw from the existence of the basalts in the Sava-Vardar suture zone (SVZ), and the 90-75 Ma old Timok magmatic complex (TMC); the former were initially considered parts of Upper Cretaceous ophiolites, whereas the latter is part of the wider Apuseni-Banat-Timok-Sredniegorie (ABTS) belt, known for its subduction-related geochemistry and metallogeny. In light of these considerations, many authors envisioned an open ocean with an active subduction in the post-obduction stages, spanning most of the Cretaceous, with the Oman case being the closest present-day analogue. However, recent studies have shown that SVZ basalts are not ophiolites, and that the existence of an open oceanic basin in the Cretaceous times is not supported by geological evidence, leaving the geochemical signature of the TMC magmatic products as the sole argument for the subduction until the end of the Cretaceous.

In this contribution, we present our recent 2D and 3D numerical modelling results aimed at investigating the geodynamic context for the Upper Cretaceous magmatism in the TMC and SVZ, by analyzing the scenario in which Vardar Neotethys had closed in the uppermost Jurassic, roughly contemporaneously with the emplacement of its ophiolites. To this end we use numerical forward modelling techniques to solve the continuity, Stokes and energy conservation equations, utilized in I2VIS and I3VIS codes for 2D and 3D, respectively. We develop a 2D model of intra-oceanic subduction. The model reproduces the closure of Vardar Neotethys alongside the obduction of Vardar ophiolites. We investigate the subsequent post-obduction slab dynamics. We simulate slab detachment at ca. 400 km depth, followed by subsequent rebound of its still attached (shallower) part. The hydrated mantle of the subducted slab undergoes delayed partial melting providing the geochemically adequate source for the TMC “subduction-like” magmatism. Regarding the Upper Cretaceous SVZ basalts and particularly taking into account that at least some of them are found intruding Cretaceous pull-apart basins, we adopt the interpretation that these magmatic bodies are products of post-collisional transtension tectonics. We present 3D model results showing how strike-slip motions can reactivate the suture and how inherited lithospheric-scale weak structures control the timing of partial melting as well as the volume and spatial distribution of the generated magma.