From soft sediment deformation to fluid assisted faulting in the shallow part of a subduction megathrust analogue: the Sestola Vidiciatico tectonic Unit (Northern Apennines, Italy)

Abstract

The Sestola Vidiciatico tectonic Unit (SVU) accommodated the early Miocene convergence between the subducting Adriatic plate and the overriding Ligurian prism, and has been interpreted as a field analogue for the shallow portion of subduction megathrusts. The SVU incorporated sediments shortly after their deposition and was active down to burial depth corresponding to temperatures around 150 °C. Here, we describe the internal architecture of the basal thrust fault of the SVU through a multi-scale structural analysis and investigate the evolution of the deformation mechanisms with increasing burial depth. At shallow depth, the thrust developed in poorly lithified sediments which deformed by particulate flow. With increasing depth and lithification of sediments, deformation was accommodated in a meter scale, heterogeneous fault zone, including multiple strands of crack-and-seal shear veins, associated with minor distributed shearing in clay-rich domains and pressure solution. In the last stage, slip localized along a sharp, 20 cm thick shear vein, deactivating the fault zone towards the footwall. The widespread formation of crack-and-seal shear veins since the first stages of lithification indicates that failure along the thrust occurred at high fluid pressure and low differential stress already at shallow depth. Progressive shear localization occurs in the last phases of deformation, at temperatures typical of the transition to the seismogenic zone in active megathrusts

Does subduction of mass transport deposits (MTDs) control seismic behavior of shallow–level megathrusts at convergent margins?

“…3G). These features indicate that sediments had relatively homogeneous shear strength, and that they were well lithified, with volume reduction up to 50% (e.g., Vannucchi et al, 2012) The first appearance of brittle behavior in lithified components of subducted sediments was probably recorded at shallow conditions, around 80 °C (Fisher and Byrne, 1987;Dielforder et al, 2016;Mittempergher et al, 2017). This inference implies that even if the MTDs are initially relatively stiffer and less prone to deformation, their low permeability, isotropic nature and small volume reduction may slow down the lithification process with respect to host sediments, and may allow vast amounts of fluids to reach deeper down at subduction plate interfaces, strongly influencing strain partitioning both inside and along the boundaries of MTDs.…”

Development of an Intrawedge Tectonic Mélange by Out‐of‐Sequence Thrusting, Buttressing, and Intraformational Rheological Contrast, Mt. Massico Ridge, Apennines, Italy

“…They can develop within forearcs at very shallow depths (a few hundred meters) along the basal décollement at the toe of accretionary wedges (i.e., subduction interface mélanges, e.g., Cloos & Shreve, ; Fagereng & Cooper, ; von Huene et al, ; Meneghini et al, ; J. C. Moore, ; J. C. Moore et al, ; Ujiie, ; Vannucchi et al, ; Xia & Platt, ). Their deformation can continue at depths exceeding 10 km along the subduction interface, even reaching metamorphic conditions along the subduction channel (e.g., Cowan, ; Fagereng & Cooper, ; Kim et al, ; Kusky et al, ; Mittempergher et al, ; Vannucchi et al, ; Zhang et al, 2015). In addition, tectonic mélanges can develop at the base of thrust sheets within wedge interiors (i.e., intrawedge mélanges, e.g., Festa, Pini, Dilek, & Codegone, ; Festa, Pini, Dilek, Codegone, Vezzani, et al, ; Morley et al, ; Vannucchi & Bettelli, ).…”

Distributed deformation along the subduction plate interface: The role of tectonic mélanges

“…Quartz-filled cracks in sandstone lenses, perpendicular to foliation, and veins parallel to the foliation ( Figure 10 to Figure 12) are therefore kinematically consistent and were formed contemporaneously in spite of their different orientation. Foliation-parallel veins have also been reported in the Franciscan Complex in the USA, in the Kodiak Complex in Alaska, in the Chrystalls Beach Complex in New Zealand, and in the Internal Ligurian Units in Italy (Fagereng and Harris, 2014;Fagereng et al, 2011;Mittempergher et al, 2018). In these examples as well as in Shimanto, foliation-parallel veins are interpreted as extensional shear veins and record increments of deformation in crack -seal textures (Fagereng and Harris, 2014;Fagereng et al, 2011;Fisher and Byrne, 1990;Brantley, 1992, 2014;Fisher et al, 1995).…”

“…Conditions of formation of such veins span a large temperature range. They can be found at low temperatures (~150°C) in the Mugi mélange lower and "colder" thrust sheets , in the relatively shallow subduction channel of the Apennines (Mittempergher et al, 2018;, in unlithified to semi -lithified rocks from the Internal Ligurian Unit and during the very shallow (40-70°C) stage of burial of Infra-Helvetic Flysch units of the Alps (Dielforder et al, 2015). Nevertheless, veins were also formed…”