Refining rates of active crustal deformation in the upper plate of subduction zones, implied by geological and geodetic data: The E-dipping West Crati Fault, southern Italy.
Description
We investigate crustal deformation within the upper plate of the Ionian Subduction Zone (ISZ) at different time scales by (i) refining geodetic rates of crustal extension from continuous Global Navigation Satellite System (GNSS) measurements and (ii) mapping sequence of Late Quaternary raised marine terraces tectonically deformed by the West Crati normal fault, in northern Ca-labria. This region experienced damaging earthquakes in 1184 (M 6.75) and 1854 (M 6.3), possi-bly on the E-dipping West Crati fault (WCF) which, however, is not unanimously considered to be a seismogenic source. We report geodetic measurements of extension and strain rates across-strike the E-dipping WCF and throughout the northern Calabria obtained by using veloci-ties from 18 permanent GNSS stations with series length longer than 4.5 years. These results sug-gest that crustal extension may be seismically accommodated in this region by a few normal faults. Furthermore, by applying a synchronous correlation approach we refine the chronology of understudied tectonically-deformed palaeoshorelines mapped on the footwall and along the strike of the WCF, facilitating calculation of the associated fault-controlled uplift rates. Raised Late Quaternary palaeoshorelines are preserved on the footwall of the WCF indicating that “re-gional” uplift, likely related to the deformation associated either with the subduction or mantle upwelling processes, is affected by local footwall uplift. We show that GIS-based elevations of Late Quaternary palaeoshorelines, as well as temporally constant uplift rates, vary along the strike of the WCF, implying normal faulting activity through time. This suggests that (i) the fault slip-rate governing seismic hazard has also been constant over the Late Quaternary, over multiple earthquake cycles and (ii) our geodetically-derived fault throw-rate for the WCF is likely a more than reasonable value to be used over longer time scales for an improved seismic hazard assess-ment. Overall, we emphasize the importance of mapping crustal deformation within the upper plate above subduction zones to avoid unreliable interpretations relating to the mechanism controlling regional uplift.