Implications of fault reactivation and structural inheritance in the cenozoic tectonic evolution

It has long been realised that the structure of orogenic belts is profoundly influenced by the original architecture and composition of the continental crust that is involved, With the acquisition of seismic data, especially from sedimentary basins in recent years, it is now also accepted that many compressional structures in sedimentary successions lie above earlier basement faults. In many cases the faults themselves have been reactivated under compression. Now in turn many orogenic belts are being re-investigated to examine the specific role of inherited faults and shear zones in the basement, together with the stratigraphic variations they may have engendered in the pre- and syn-orogenic sedimentary cover, in the compressional evolution. In this paper we illustrate these discussions using examples from the Maghrebian-Apennine orogenic system of Italy. The orogenic belt and its foreland have long been known to show prominent thickness and facies variations in the Mesozoic sediments that, for at least some sites, imply crustal fault controls. Yet many structural descriptions of the chain, particularly used for the development of models for the bulk Tyrrhenian-Apennine system, persist in using simple, "thin-skinned" structural styles that pay no heed to the pre-orogenic basin architecture. Using a range of section restoration methods, locally allied to stratigraphic separation diagrams, we draw examples from SE Sicily and its offshore, together with the Apennines of the Latium-Abruzzi and the Umbria-Marche sectors. These approaches reveal that many thrusts have localised on inherited pre-contractional structures, including normal faults that formed during foredeep development. Additionally, cross-faults that compartmentalise the thrust and fold belt at the scale of the chain show evidence of repeated reactivation and have even acted later to channel crustal fluids. These observations strongly suggest basement involvement in the orogen - compressional tectonics was chiefly a "thick-skinned" affair, albeit with some detachment sheets. The values of orogenic contraction implicit in the new models are far less than those deduced from extant "thin-skinned" models and demand rates of shortening that are more in accord with values from other thrust belts world-wide. Consequently it seems to time to re-assess regional-scale tectonic models. Opening up the range of structural geometries beyond those produced by "thin-skinned" models, offers opportunities to reassess the subsurface geometry and structural evolution of individual folds. This may be particularly important for hydrocarbon prospectivity -in predicting and risking models not only of reservoir geometry but also the distribution of fractures and other "sub-seismic" structural damage.