Crustal and upper mantle response to lithospheric tear faulting: the Livorno-Sillaro Lineament in the northern Apennines

Lithospheric tear faults are expected to develop in response to along-strike variations in trench retreat. Such structures are observed in the central Mediterranean region, where vertical slab tearing has progressively led to the segmentation of the subduction zone and allowed the development of orogenic curvatures (e.g., the Calabrian Orocline). However, the exact 3D structure of slab tear faults, and their crustal and upper mantle expressions, are still debated. Here we present an analysis of the seismic, structural and morphological features associated with a lithospheric-scale tear fault. We focus on a single tear fault in the northern Apennines, the Livorno-Sillaro lineament, which has previously been described as a traverse fault based on the surface geology. We investigated the 3D lithospheric structure of the Livorno-Sillaro lineament using a range of independent high-resolution data. Our results show that the area of the Livorno-Sillaro lineament is defined by a major crustal and upper mantle discontinuity, which can be recognised along a section parallel to the Apennine belt. The width of the discontinuity is typically 10-20 km. We find that this zone is characterised by an anomalously low density of crustal and intermediate earthquakes, and by an abrupt along-strike variations in the spatial distribution of deep crustal earthquakes. The discontinuity is also reflected by a pronounced (~20 km) step in the Moho depth, and in a complex and incoherent pattern of SKS wave splitting. The upper crustal expressions of the Livorno-Sillaro lineament involve a sharp bend in the orientation of the watershed and an abrupt along-strike change in the denudation rates (as reflected in low-temperature cooling ages). Our results confirm the suggestion that the Livorno-Sillaro lineament is a major lithospheric-scale discontinuity that marks the boundary between domains of contrasting rates of trench retreat (with faster rates southwest of the discontinuity). The discontinuity, rather than acting as a seismic fault, is characterised by a weaker, possibly hotter, aseismic zone. We suggest that similar types of structures may play a crucial role in the development of along-strike segmentation of orogenic belts. Ultimately, further tearing along these types of structures could potentially lead to the development of tear-related magmatism and the formation of slab windows.