The aseismic-seismic transition and fluid regime along subduction plate boundaries and a fossil example from the Northern Apennines of Italy

This paper reviews observations and theories for the aseismic-seismic transition in the megathrust between the incoming and overriding plates at a subduction zone. The temperature of the aseismic-seismic transition appears to be quite similar at erosive and accretionary margins, despite large differences between them in the lithology of the seismogenic subduction channel that composes the ‘megathrust’ plate interface. This fact, and the recent laboratory demonstration that both smectite and illite are velocity-strengthening in creep, suggests that the oft-postulated change in mechanical behavior of the megathrust due to a smectite-illite clay mineral transformation at ~150°C is not the cause of the onset in seismogenesis at these temperature conditions within the subduction channel. Field observations from fossil megathrust zones suggest that a temperaturedependent change in the availability of in-situ fluid is likely to play a key role in the onset of seismogenesis. Perhaps the causal link is to the smectite-illite transformation and other metamorphic dewatering reactions that liberate water at ~150°C, under conditions where these reactions are an important local source of hydrous fluids. Field studies of fossil megathrusts support the hypothesis that fluids ‘control’ seismogenesis, and indicate that there are large fluid pressure variations during the seismic cycle. In the fossil erosive megathrust system preserved in the Apennines, two décollements are simultaneously active at the roof and base of the subduction channel. The uppermost (non-seismogenic) portion of the megathrust even appears to alternate between tensional and compressional modes of failure during the seismic cycle along the deeper portions of the megathrust.