Crustal and upper mantle responses to lithospheric segmentation in the northern Apennines

Abstract

Lithospheric tear faults are expected to develop in response to along-strike variations in the rates of slab rollback. However, the exact geometry of such structures and their crustal and upper mantle expressions are still debated. We present an analysis of seismic, structural, and morphological features that possibly represent the expression of lithospheric segmentation in the northern Apennines. Geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern Apennines, characterized by a change in the spatial distribution of intermediate-depth seismicity, along-strike variations in the pattern of crustal seismicity, and a bend in the Moho topography. The near-surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern Apennines in the proximity of the Livorno-Sillaro Lineament. We interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern Apennines, marking the boundary between domains that underwent contrasting styles of lithospheric deformation, which are either associated with different rates of slab rollback or a transition from underplating to retreat. We suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries, allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures. Ultimately, further tearing along such structures could potentially lead to the occurrence of tear-related magmatism and the formation of slab windows.

Crustal structure and deformation across a mature slab tear zone: the case of southern Tyrrhenian subduction (Italy)

“…However, the width of the deformation zone depends on many parameters (e.g., crustal geotherm, mineralogy, and water fugacity), and our value should be considered as a soft indication. In any case, our estimate of this parameter is also in agreement with the extension of a deformation zone found in the northern Apennines [Rosenbaum and Piana Agostinetti, 2015;Piana Agostinetti, 2015]. The detected anisotropy has to be regarded as a key feature generated by the opening of the vertical slab tear.…”

Seismic evidence for tearing in the subducting Indian slab beneath the Andaman arc

“…As a consequence of such rollback, the subducted slabs experience deformation in terms of bending and/or tearing. This creates a discontinuity in the geometry of the subducting slab along the trench [ Spence , ] and usually occurs in the case of a retreating lithospheric segment [ Rosenbaum and Piana Agostinetti , ]. The important factors, which control slab rollback, are the buoyancy of the lithosphere and the contrasting physical properties with respect to the ambient asthenosphere [ Garfunkel et al , ].…”

“…Tearing of oceanic plates has been documented in many subduction zones of the world [ Isacks et al , ; Forsyth , ; Chatelain et al , ], based on the observations or interpretations of the abrupt break in seismicity [ Reyners et al , ] or the sudden change in the velocity anomalies evidenced in tomographic models [ Lucente et al , ; Benoit et al , ; Giacomuzzi et al, ]. Examples include the South Sandwich trench [ Barker , ], Tonga trench [ Millen and Hamburger , ], Lesser Antilles [ Clark et al , ], Gibraltar arc [ Meighan et al , ], and north Apennines [ Rosenbaum and Piana Agostinetti , ].…”

The life cycle of subcontinental peridotites: From rifted continental margins to mountains via subduction processes

“…1D) (Nirta et al, 2007). These transverse tectonic lineaments appear to have been longlived lithospheric discontinuities: in the Late Cretaceous, they accommodated the early Apennine convergence by transpression (Nirta et al, 2007); in the Neogene, they controlled the lateral non-cylindricity of the thrust system as well as the segmentation of the sedimentary basins in the Tyrrhenian side of the northern Apennines (Fazzini and Gelmini, 1984;Rosenbaum and Agostinetti, 2015). Like the Alfeo-Etna fault system of the Calabrian arc, these lineaments have been interpreted as Mesozoic structures of the passive continental margin that were reactivated during subduction rollback of the Adriatic-Ionian lithosphere (Royden et al, 1987).…”