Abstract
When sedimentation rates overtake tectonic rates, the detection of ongoing tectonic deformation signatures becomes particularly challenging. The Northern Apennines orogen is one such case where a thick Plio-Pleistocene foredeep sedimentary cover blankets the fold-and-thrust belt, straddling from onshore (Po Plain) to offshore (Adriatic Sea), leading to subtle or null topo-bathymetric expression of the buried structures. The seismic activity historically recorded in the region is moderate; nonetheless, seismic sequences nearing magnitude 6 punctuated the last century, and even some small tsunamis were reported in the coastal locations following the occurrence of offshore earthquakes. In this work, we tackled the problem of assessing the potential activity of buried thrusts by analyzing a rich dataset of 2D seismic reflection profiles and wells in a sector of the Northern Apennines chain located in the near-offshore of the Adriatic Sea. This analysis enabled us to reconstruct the 3D geometry of eleven buried thrusts. We then documented the last 4 Myr slip history of four of such thrusts intersected by two high-quality regional cross-sections that were depth converted and restored. Based on eight stratigraphic horizons with well-constrained age determinations (Zanclean to Middle Pleistocene), we determined the slip and slip rates necessary to recover the observed horizon deformation. The slip rates are presented through probability density functions that consider the uncertainties derived from the horizon ages and the restoration process. Our results show that the thrust activation proceeds from the inner to the outer position in the chain. The slip history reveals an exponential reduction over time, implying decelerating slip-rates spanning three orders of magnitudes (from a few millimeters to a few hundredths of millimeters per year) with a major slip-rate change around 1.5 Ma. In agreement with previous works, these findings confirm the slip rate deceleration as a widespread behavior of the Northern Apennines thrust faults.
Highlights
Buried and blind active faults pose one of the major challenges for understanding seismotectonics in many regions worldwide (Yeats and Lillie, 1991; Stein and Ekström, 1992; Berberian, 1995; Shaw and Shearer, 1999; Walker and Jackson, 2002)
Blind thrusts can be determined by inelastic deformation ahead of fault tip (Roering et al, 1997) that contributes to dissipate the strain in the overlying cover by backthrusting, coupling, and forethrusting (Dunne and Ferrill, 1988)
The seismo-stratigraphic, sedimentologic, and bio-chronostratigraphic characterization of the Plio-Pleistocene sequence reached in the Po Plain and Northern Adriatic Foreland area (Muttoni et al, 2003; Ghielmi et al, 2010, 2013; Amadori et al, 2019, 2020) provides an effective tool to constrain the inception age of thrust faults and their activity age ranges, as well as to estimate displacement and slip rates for the MiddleLate Pleistocene time interval (Figure 4 and Table 1)
Summary
Buried and blind active faults pose one of the major challenges for understanding seismotectonics in many regions worldwide (Yeats and Lillie, 1991; Stein and Ekström, 1992; Berberian, 1995; Shaw and Shearer, 1999; Walker and Jackson, 2002). Analog models show that the upward propagation of thrust faults can be slowed down or even halted by the presence of pre-existing frictional discontinuity (e.g., decollements) above the fault propagating tip (Bonanno et al, 2017) They can be determined by the relatively young inception of fault activity within the current regional stress state or by the interplay between fault activity and sedimentation rates in the cases in which the second one is significantly higher than the first one (Pueyo Anchuela et al, 2016). Blind thrusts are documented as to be the cause of several significant earthquakes and tsunami worldwide (Hauksson et al, 1995; Lettis et al, 1997; Borrero et al, 2001; Hayes et al, 2010; Burrato et al, 2012; McAuliffe et al, 2015) significantly contributing to seismic and tsunami hazard
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