Structural inheritance in mountain belts: an Alpine-Apennine perspective

The geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building. In the Alpine-Apennine sector together with other sectors of the Tethyan orogenic system the pre-orogenic crustal template can include arrays of extensional faults. Other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands. While in some areas these inherited features may simply reactivate under inversion, more commonly faults show complex, partial reactivation structures. In volumes of distributed strain, faults may serve to nucleate large-scale buckle folds, for example, along basement-cover interfaces. These different patterns of basement reactivation may reflect spatially varying strength-depth profiles in continental lithosphere that are themselves inherited from spatially-distinct geological histories. Even when not themselves reactivating, basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise
regionally extensive detachment horizons. The 3D form of thrust systems can be strongly compartmentalized by pre-existing cross-faults, such as the oblique lineaments of the Apennines. On a large-scale, the distribution of pre-existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger-scale tectonic processes. In the central Mediterranean the evolution of slab roll-back and the related growth of overlying extensional basins (e.g. Tyrrhenian Sea) may be strongly modulated by the distribution of rift-related weak zones in the adjacent continental crust. The subduction of continental crust will strongly depend on the inherited structure of this crust, specifically the distribution of deep crust of basic composition. This develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening. Investigating all these aspects, from the scale of bulk crustal compositions to the geometry, timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets. The extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self-ordered phenomena modulated by the syntectonic environment remains unclear.