What drives extension and seismicity in the central Apennines (Italy)? Insights from 2D seismo-thermo-mechanical modeling

The central Apennines have experienced several destructive normal-faulting earthquakes in the last decade, but fundamental questions about the tectonic mechanisms driving extension persist. Multiple mechanisms have been proposed, including differences in gravitational potential energy (GPE), independent motion of the Adriatic plate, and large-scale uplift following slab detachment. In terms of structure, debates continue about whether the slab has detached and whether the continental Mohos overlap. However, none of these hypotheses have been tested through self-consistent geodynamic modeling. We employ 2D instantaneous seismo-thermo-mechanical models with a visco-elasto-plastic rheology and a strongly slip-rate dependent friction. We systematically explore different lithospheric structures, rheologies and forcings to test these hypotheses and identify the key driving mechanisms of surface deformation and seismicity in the central Apennines. 

Our results confirm that the slab beneath the central Apennines is detached: only a detached slab reproduces normal-faulting earthquakes in the orogen and a gradual increase of horizontal surface velocities up to 3 mm/yr. An attached slab instead produces strong compression and vertical motions inconsistent with observations. The primary driver of extension is Adriatic plate motion, which accounts for approximately two-third of the horizontal surface velocities. The secondary driver is eduction of subducted upper crust, which contributes to approximately one-third of the horizontal surface velocities and facilitates decoupling between the Adriatic and Tyrrhenian plates. On the contrary, differences in GPE arising from topography only have a minor contribution to extension and seismicity. Density differences up to the lithosphere-asthenosphere boundary do play a significant role as it controls upper crust eduction. Lower- and upper crust rheology also control the occurrence and intensity of eduction, thereby affecting plate coupling and seismicity. Additionally, lower crust viscosity of the plate contact area strongly modulates the transfer of deep velocities to the surface, and thereby controls the location of highest surface velocity gradient and seismicity. Hence, our results show that deep structures, rheologies, temperatures and processes have a large control over the location and intensity of crustal seismicity. By refining the geodynamic structure and deciphering the tectonic drivers of seismicity, this study advances the understanding of Apennine geodynamics and seismicity.