Normal faulting interaction revealed by out-of-phase Quaternary uplift-rate changes implied by studying deformed marine terraces

We have mapped and refined the chronology of raised and tectonically deformed Middle-Upper Pleistocene marine terraces in the Messina Strait, southern Italy, within the upper plate affected by crustal extension above the Ionian Subduction Zone. We have mapped up to thirteen palaeoshorelines which identify Middle-Upper Pleistocene sea-level highstands. Our interpretation reveals the chronology and geometry of deformation since ~500 ka for the Reggio Calabria Fault, the Armo Fault and the Messina-Taormina Fault. We show that the spatial patterns of uplift vary both along the strike of these normal faults and through time, and, given the across strike arrangement of the faults, also reveal how the contribution of each fault to the regional strain-rate developed through time. For example, uplift-rates mapped within the footwalls and hangingwalls of the investigated active faults were not constant through the Upper Pleistocene, with a marked change in the location of strain accumulation at ~50 ka. Conversion of uplift rates into fault throw-rates suggests that the three faults has similar throw-rates prior to ~50 ka (in the range 0.77–0.96 mm/yr), with the Armo and Reggio Calabria faults then switching to lower rates (0.32 mm/yr and 0.33 mm/yr respectively), whilst the Messina-Taormina Fault accelerated to 2.34 mm/yr. The rate of regional extension, which has been approximated by summing the implied heave rates across the three faults, was constant through time despite this re-organisation of local strain accumulation at ~50 ka. We explain these out-of-phase fault throw-rate changes during the constant-rate regional extension conditions as due to interactions between these upper plate normal faults. We discuss how fault throw-rates changing through time may affect a long-term seismic hazard assessment within active normal fault systems.