Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong Lower Crust

Abstract

The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite, ± garnet ± K‐feldspar. Amphibole‐plagioclase geothermobarometry and thermodynamic modeling indicate that pristine and mylonitized pseudotachylytes formed at 650–750°C and 0.7–0.8 GPa. Thermodynamic modeling indicates that a limited amount of H2O infiltration (0.20–0.40 wt. %) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites.

Pseudotachylyte as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia)

“…The metamorphic conditions of these sections correspond to depths well below the usual brittle-ductile transition zone for crustal rocks (< 15 km) and thus the assumed lower limit for earthquake nucleation. Sibson (1980) reported mutually overprinting pseudotachylytes and mylonites from the Outer Hebrides Thrust (NW Scotland) and similar observations were made by Moecher and Steltenpohl (2009) and Menegon et al (2017) in the Lofoten region (N Norway), by Hobbs et al (1986) in the Redbank Shear Zone (Arunta Block, Central Australia), and by in the Woodroffe Thrust (Central Australia). Mutual overprinting has been interpreted to reflect the generation of pseudotachylytes and mylonitization under the same conditions (Altenberger et al, 2011(Altenberger et al, , 2013Clarke and Norman, 1993;Moecher and Steltenpohl, 2011;Pennacchioni and Cesare, 1997;Pittarello et al, 2012;Ueda et al, 2008;White, 1996White, , 2004White, , 2012.…”

“…Modified after Evins et al (2010). Brune, 1972;Sibson, 1975), can be locally abundant in exposures of lower crust (Altenberger et al, 2011(Altenberger et al, , 2013Austrheim and Boundy, 1994;Clarke and Norman, 1993;Steltenpohl, 2009, 2011;Pittarello et al, 2012;Orlandini et al, 2013;Menegon et al, 2017). The metamorphic conditions of these sections correspond to depths well below the usual brittle-ductile transition zone for crustal rocks (< 15 km) and thus the assumed lower limit for earthquake nucleation.…”

Weak and slow, strong and fast: How shear zones evolve in a dry continental crust (Musgrave Ranges, Central Australia)

“…In the Musgrave Ranges, this fluid driven mechanism cannot explain the formation of pseudotachylytes as the rocks are dry, and there is no evidence for enhanced fluid activity such as coeval veins or hydrous mineral growth. Under dry lower crustal conditions, rocks may be strong (Menegon et al, 2011, Yardley & Valley, 1997) and this could explain initial fracturing (Menegon et al, 2017), but the stress fluctuations interpreted here cannot be explained by a simple model with persistently strong lower crust. Influx of water-rich fluid along such precursor fractures, causing hydration, associated mineral reactions, and changes in the crystal plastic deformation mechanism(s), has been established as a mean to localize shearing and cause local and bulk weakening of initially dry lower crust (Menegon et al, 2017).…”

“…Under dry lower crustal conditions, rocks may be strong (Menegon et al, 2011, Yardley & Valley, 1997) and this could explain initial fracturing (Menegon et al, 2017), but the stress fluctuations interpreted here cannot be explained by a simple model with persistently strong lower crust. Influx of water-rich fluid along such precursor fractures, causing hydration, associated mineral reactions, and changes in the crystal plastic deformation mechanism(s), has been established as a mean to localize shearing and cause local and bulk weakening of initially dry lower crust (Menegon et al, 2017). However, this is not appropriate in the current study area, where the mineral assemblage in the shear zones is little changed and there is no evidence for alteration halos or other signs of fluid introduction.…”

“…The role of precursor fractures in shear zone nucleation and strain localization has been widely recognized in mid-crustal environments (Guermani and Pennacchioni, 1998;Mancktelow and Pennacchioni, 2005;Pennacchioni et al, 2006;Pennacchioni and Mancktelow, 2007;Pennacchioni and Zucchi, 2013;Schrank et al, 2008;Sibson, 1980) and to a more limited extent in the lower crust (Austrheim et al, 1996;Menegon et al, 2017;Steltenpohl et al, 2006). An interplay between "transient discontinuities", marked by pseudotachylyte formation, and ongoing ductile shear interpreted to occur under middle to lower crustal conditions, has also been described in a limited number of previous studies (Hobbs et al, 1986;Menegon et al, 2017;Sibson, 1980;White 1996White , 2004White , 2012.…”