Space and Time Variability of Detachment-Versus Ramp-Dominated Thrusting: Insights From the Outer Albanides

Despite their markedly different structural setting, the northern and southern outer Albanides share a common tectonic evolution from detachment-dominated to ramp-dominated, basement-involved thrusting. The former process (mainly Oligocene to Miocene) is essentially related with the occurrence of a thick decollement level represented by Triassic evaporites, while the latter involves basement ramps splaying out from a middle crustal decollement. As this weak crustal layer is inherited from the Mesozoic rifting stage, the original continental margin architecture is interpreted to strongly influence subsequent convergent deformation. The profoundly different nature of the two dominant decollements in the study area controlled the structural style of the fold and thrust belt. The decoupling capacity of the upper decollement is strongly dependent on the thickness of the Triassic evaporites. Where this is significant (>> 1 km; southern outer Albanides), the occurrence of such a thick incompetent layer at the base of competent carbonate units favored the development of break-thrust folds and imbrication of the sedimentary cover. Fold and thrust belt propagation was instead hindered where original stratigraphic variations resulted in a reduced thickness (<= 1 km) of Triassic evaporites. On the other hand, the deeper middle crustal decollement is controlled by basement rheology. Its reactivation during plate convergence was assisted by collision-related thermal weakening of the crust. This process governed late-stage (<5 Ma) crustal-scale tectonic inversion and plays a major role in controlling present-day seismicity.