The structure and evolution of the Santorini-Amorgos basin: numerical modelling simulation of the interplay between extensional deformation and magmatism

In areas of prolonged extensional deformation, subsequent crustal thinning creates pathways for magma to ascend to shallow crustal levels, leading to a complex interplay between extensional deformation and magmatism with strong feedback. Numerical modeling can provide valuable insights into the involved processes and their temporal evolution.

The Attic-Cycladic Crystalline Complex (ACCC, central Aegean, Greece) formed as a result of prolonged syn- to late-orogenic exhumation, the latter associated with subduction zone retreat in the Neogene to recent. Late-orogenic exhumation was achieved through low-angle crustal-scale detachment systems rooted in the brittle-ductile transition. At present, the central part of the ACCC is generally aseismic, with major active faults and earthquake activity located along the north and, especially, the southern margins. The southern margin also hosts a large part of the modern volcanic arc, which has been established since the Pliocene. At the southeastern margin, the NE-SW Santorini–Amorgos basin represents a complex horst-and-graben structure with sediments exceeding 1000m in thickness. It is characterized by marginal and internal NE-SW oblique-slip normal faults with a significant dextral sense of motion, such as the Santorini–Amorgos Fault Zone, where the > 7 Mw 1956 Amorgos  earthquake occurred. In January-March 2025, swarm-type earthquakes with magnitudes 1≤M≤5.2 were recorded, with hypocenters at depths mainly between 4 km and 15 km, concentrated along a narrow zone extending offshore of Santorini near the Columbo volcanic center towards the NE and south of Amorgos Island.

A plane-strain numerical model was constructed in the NW-SE extension direction to investigate the structural evolution of the region, driven by combined tectonic forces and magmatism. Simulations were performed using a large-strain finite-difference software in two stages. The first stage of the simulation aimed to recreate the basin's structure without interference from magmatic activity. In the second stage, a magmatic chamber was introduced at the weakest point within the obtained structural configuration of the first sequence. The application of this numerical model highlighted key aspects of the interplay between tectonics and magmatism. It successfully simulated the present structural configuration of the Santorini-Amorgos basin and the January-March 2025 swarm-type earthquakes resulting from the initiation and propagation of new cracks, above the brittle-ductile transition.