Styles of deformation in the Umbria-Marche sector of the Apennines fold-and-thrust belt: new insights from balanced geological sections and 3D structural modelling

Understanding how orogenic shortening transfers from the basement to the sedimentary cover is crucial in fold-and-thrust belts. Transfer distance dictates the thrusting style. In cases of 'long-distance rooting' such as the Jura Mts.-Swiss Mollasse Basin-Alps system, the sedimentary cover's shortening leads to a thin-skinned deformation style. Conversely, 'short-distance rooting', as observed in the ramp-dominated sectors of the Southern Alps, results in thick-skinned deformation. However, the distinction of thin- vs. thick-skinned styles of thrusting may be ambiguous. 'Long-distance rooting' may cause thin-skinned deformation at the belt front and thick-skinned deformation where the basement underplates, as seen in the Alpine region. The crucial aspect in fold-and-thrust belt dynamics is whether the basement is extensively underthrust and processed in the orogen's interior ('long-distance rooting'), or if displacement directly transfers from basement thrusts to the sedimentary cover on a local scale ('short-distance rooting'). This fundamental issue is addressed here for the Umbria-Marche zone of the Apennines, which style of thrusting has been the subject of a long-lasting debate. Interpretations proposed in the last decades mostly range from pure thin-skinned to composite models of basement-involved deformation and detachment-dominated thrusting of the sedimentary cover.

We aim to investigate whether the sedimentary cover is more shortened than the basement (i.e., a substantial component of ‘long-distance rooting’ of thrust displacement of the sedimentary cover) or do shortening of basement and cover balance at the scale of the foreland fold-and-thrust belt (i.e., ‘short-distance rooting’ characterizes the Umbria-Marche zone). We integrate the updated 1:50,000 scale geological map (CARG Project) of the Sibillini area (Visso and Ascoli Piceno Sheets) with a 10 m cell-size digital elevation model, the interpretation of vintage seismic lines and gravimetric data. We present a series of new balanced and restored cross-sections, including a crustal section along the trace of available seismic lines covering the entire Apennine and foothills area to the coastline, validated by gravimetric modelling, and thirteen cross-sections used to verify the geometric viability of sedimentary cover structures in the study area. The results of our work suggest coupled deformation of basement and sedimentary cover, which are characterized by similar amounts of shortening (consistent with ‘short-distance rooting’ of thrust displacement). The balanced cross-sections, integrated with a dense grid of (n. 25) additional sections perpendicular to the main structural trends, were used to construct a 3D structural model calibrated by surface geology. This allowed us to reconstruct the main fault surfaces, accounting for the along-strike variability of geological features observed from the map and offering a detailed representation of the geometrical arrangement of key horizons (base-top Calcare Massiccio Fm, top Maiolica Fm, top Scaglia Rossa Fm, top Bisciaro Fm) and their relationships with major faults. Our structural model provides new insights into the architecture, timing of the deformation, and kinematic evolution of the Umbria-Marche sector of the Apennines. This has major implications for a better understanding of deformation style, the role of structural inheritance, and post-thrusting extensional tectonics (which controls the seismotectonic setting of the study area).