Abstract
Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle–ductile transition and seismogenesis. In a subduction environment, switching in deformation mode and mechanisms and scale of localization may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). However, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in some cases for the localization of deformation and shear zone development, thus representing a case in which switching deformation mechanisms and scale and style of localization (deformation mode) interact and relate to each other. This contribution analyses an example of a millimetre-scale shear zone localized by brittle precursor formed within a host granitic protomylonite. The studied structures, developed in ambient pressure–temperature (P–T) conditions of low-grade blueschist facies (temperature T of ca. 300 °C and pressure P ≥ 0. 70 GPa) during involvement of Corsican continental crust in the Alpine subduction. We used a multidisciplinary approach by combining detailed microstructural and petrographic analyses, crystallographic preferred orientation by electron backscatter diffraction (EBSD), and palaeopiezometric studies on a selected sample to support an evolutionary model and deformation path for subducted continental crust. We infer that the studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations, may be considered as a small-scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow-slip structures. Our case study represents, therefore, a fossil example of association of fault structures related to stick-slip strain accommodation during subduction of continental crust.
Highlights
The study of deformation fabric of fault rocks has been crucial for the development of a general conceptual model for crustal-scale fault zones (Sibson, 1977, 1983; Scholz, 1988, 2002; Handy et al, 2007; Cooper et al 2010; Platt and Behr, 2011)
The increasing PT conditions determine the transition from a seismogenic frictional regime, dominated by pressure-sensitive deformation and involving cataclasis and frictional sliding, to a viscous regime (Rutter, 1986; Schmid and Handy, 1991; Montési and Hirth, 2003; Handy and Brun, 2004), where dominantly aseismic, mainly crystal-plastic and continuous shearing is localized within mylonitic shear zones
On the basis of overprinting relationships we may infer that the oldest deformation fabric of our analysed sample is represented by the subvertical protomylonite foliation (Fig. 2), which shows microstructural features described in the text
Summary
The study of deformation fabric of fault rocks has been crucial for the development of a general conceptual model for crustal-scale fault zones (Sibson, 1977, 1983; Scholz, 1988, 2002; Handy et al, 2007; Cooper et al 2010; Platt and Behr, 2011). A complex transitional behaviour involving mixed continuous and discontinuous, distributed vs localized, and cyclic switching in deformation mechanisms over large variations in strain rates is inferred at the transition between frictional and viscous domains, a depth interval in the crust which contains the typical hypocentres and rupture depths of large earthquakes in continental crust (Sibson, 1983; Kohlstedt et al, 1995; Scholz, 2002; Handy and Brun, 2004; Pennacchioni et al, 2006) In subduction settings, this transition zone is located between 10 and 35 km depth depending on slab dip and thermal structure (i.e. between temperatures of 150 and 350–450 ◦C) and along the subduction interface is recognized as the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremors and slow-slip events (Rogers and Dragert, 2003; Liu and Rice, 2007; Hacker et al, 2003; Vannucchi et al, 2008; Meneghini et al, 2010; Angiboust et al, 2014, 2015; Andersen et al, 2014; Hayman and Lavier, 2014; Fagereng et al, 2014). We show that the brittle–viscous transition preserved in the Popolasca granitoids of northern Corsica can be explained by transient high fluid pressures triggering brittle deformation in an otherwise viscous regime, and discuss the related implications for fault-slip behaviours in subduction zones
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