Active fault kinematics and crustal stresses along the Ionian margin of southeastern Sicily

Abstract

Since the late Cretaceous onset of plate convergence between Africa and Europe, the Malta Escarpment has been converted from a Mesozoic passive margin into a mega-hinge fault system with an additional sinistral strike-slip component. The modern tectonic stress regime with NW–SE-directed maximum horizontal stresses has been established since Late Messinian times. Since the Pleistocene, sinistral strike-slip deformation and contemporaneous normal faulting along the Malta Escarpment have induced the opening of oblique trending onshore grabens at the eastern margin of the Hyblean Plateau. In this study, we focus on the kinematics, the controlling state of stress, and the temporal variation of the neotectonic to active strike-slip and normal fault structures. The stress-tensor calculations reveals that the widespread map-scaled to meso-scaled normal fault structures are governed by the long-term extensional state of stress during the Quaternary. This long-term stress tensor is predominantly controlled by gravitational induced stresses due to vertical load ( σ 1= S V) and lateral extension due to the topographic gradient of the Malta Escarpment ( σ 3= S h=NE–SW). In this case, the average tectonic stresses ( σ 2= S H=NW–SE) transmitted by the regional to plate-tectonic stress field are significantly smaller than the gravitational induced stresses. In contrast, the clear localization of conjugate sets of meso-scaled strike-slip fault structures and shear zones without accompanying normal fault structures give strong indications for episodic seismotectonic strike-slip faulting under critical stress conditions. In this state, tectonically induced maximum horizontal stresses are successively increased by ongoing plate convergence from low-level stress magnitudes ( σ 1= S V, σ 2= S H=NW–SE) up to critical stress magnitudes ( σ 1= S H=NW–SE, σ 2= S V), which are significantly larger than gravitational stresses. At the critical state, seismotectonic stress release occurs by active strike-slip faulting, as recently indicated in mid-crustal levels by the moderate 1990 Augusta earthquake, and re-establishes the long-term extensional state of stress. Because the strike-slip faults and shear zones in the study area were formed as surface ruptures, they additionally give indications of episodic large earthquakes in this region with magnitudes greater than 6.0, as reported by several large earthquakes devastating E and SE Sicily in historical times.