Geometry, segmentation pattern and displacement variations along a major Apennine thrust zone, central Italy

Hundreds of kilometres-long, arc-shaped, continuous thrust faults commonly imply very large displacements by detachment-dominated thrusting. Therefore, their occurrence has a large impact on fold–thrust belt structure. One of the major thrust faults of the central-northern Apennines, the Umbria–Marche–Sabina Thrust Zone, is traditionally believed to represent one such feature, characterised by displacements of several tens of kilometres. However, detailed studies of this structure revealed that it is actually composed of a series of partially overlapping fault segments, rather than consisting of a single, continuous thrust. Cross-section balancing and restoration, carried out by the integration of surface geological data with available sub-surface information, points out relatively limited amounts of thrust displacement (<10 km). Displacement–distance profiles show moderate displacement gradients well compatible with those reported for coherent thrust sheets. They also suggest that individual thrust segments were originally isolated and then grew by lateral propagation, leading to overlap and variable fault interaction within relay zones. The relationship between maximum displacement and fault trace length tends to follow a power-law distribution, as it commonly occurs for fault populations. However, a better correlation could be obtained by further segmentation of the northernmost (blind) fault in an area of no seismic data, thus confirming that fault scaling relationships may be useful for pointing out possible problems with the structural interpretation of poorly constrained areas and for exploring viable alternative solutions.