STEP faults and lithosphere dynamics in the Mediterranean

Abstract

The locus of continual active tearing at the lateral termination of a subduction zone is dubbed a STEP, and the surface trace left in the wake of a propagating STEP is called the STEP fault. STEP faults have played a major role in the (geologically) fast reorganization of plate boundaries in the Mediterranean, where present-day surface deformation is distributed. This thesis focuses on the effect of passive margins on the propagation of STEPs and lithosphere dynamics in the Mediterranean, studied through physical computer models. Many STEP settings observed on Earth today display a geometry in which the trench is oriented approximately perpendicularly to the STEP fault. Model geometries include a passive margin that is oriented at some angle with respect to the trench. Passive margins steer STEPs in case of favorable orientation with respect to the strike of the trench. In other cases, STEPs will continue to propagate in their original direction, that is, straight into an oceanic basin or a passive margin. Models show that the orthogonal setup of STEP fault and subduction trench is one that STEP systems will evolve towards. Subsequently, Nijholt tailors model setups to resemble the geometry of the African continental margin in the south-central Mediterranean: the domain of the Calabrian subduction zone. Model predictions indicate that the SW STEP of the Calabrian trench propagated eastward along the African (Sicilian) passive margin and then propagated into the Ionian basin. The geodynamic context of deformation in the Ionian basin, offshore Calabria and Sicily, constitutes two factors: STEP fault activity and a wide, lithospheric, dextral shear corridor. Nijholt then focuses on present-day kinematic observations in this same area through an interplay of known tectonic forces. The geodetically measured velocity field, seismicity and sense of slip on regional faults in the south-central Mediterranean can be reproduced by force-based models. These show that the regional imprint of tectonic forces is driven by Africa-Eurasia plate convergence, lateral variations in gravitational potential energy and slab pull. The magnitude of the resistance to fault slip on regional fault zones turns out to be variable. Whereas high resistance on faults that hosted major historical earthquakes confirms the high probability of future natural disasters, the Calabrian subduction interface is unlikely to host a future, major earthquake. In the westernmost Mediterranean, the formation of the Gibraltar arc was also dominated by subduction and STEP activity. However, subduction activity has faded and present-day deformation is distributed. Lastly, Nijholt employs a grid search methodology and finds that it is possible to constrain the rheology, resistance to slip on major fault systems, and slab pull and trench suction forces for the Gibraltar arc domain in light of the available kinematic observations. The observed surface velocity field, seismicity and sense of slip on regional faults in the Gibraltar arc appear to result mainly from Africa-Europe plate convergence and lateral variations in gravitational potential energy. Slab pull from the Gibraltar slab is very likely transmitted poorly into the surface plate.