Concomitant brittle-ductile deformation, fluid-flow and metamorphism during continental subduction : a slow earthquake rock record in the Suretta nappe (Central Alps, Switzerland) ?

Geophysical observations have led to the conclusion that slow earthquakes (EQ) occur in the ductile realm at depths greater than ~20 km, in domains of low Vp and elevated Vp/Vs, consistent with high pore fluid pressure conditions and/or fluctuating fluid conditions. Furthermore, slow EQ are characterized by concomitant viscous aseismic slip and transient frictional slip responsible for tectonic tremors and low frequency earthquakes. Understanding the physics of slow earthquakes can be done by integrating geophysics, rock deformation experiments and numerical models with the observation and characterization of the possible rock record of slow earthquakes.

In this contribution, we contribute to the quest for geological records of slow EQ by adding a new example to the fast growing list of examples. Our approach is based on field observations, petrological and microtextural analysis carried out on exhumed shear zones in late Variscan volcanic rocks from the Suretta nappe (Central Alps, Switzerland). We propose that in continental collision settings, like the Alps, exhumed continental shear zones preserve geological evidence that may be related to paleo-slow earthquakes. We show that burial of continental units is characterized by concomitant frictional and viscous deformation in the ductile realm at temperature conditions above 450°C for a depth range between 18 and 25 km, which resembles those where slow earthquakes are expected. The finite geometry of the shear zone consists of a network of anastomosed mica-rich weak and high strain ductile shear zones of various size (m to km) bounding high strength and low strain domains, resulting in a “mélange” rheology. At a smaller scale, the localization of millimeter to centimeter wide ductile shear zones is controlled by the prior development of a damaged zone that has been detected by imaging volcanic quartz phenocrysts with cathodoluminescence. This damage zone is defined by a domain of high density healed cracks and fluid inclusion planes preserved only in volcanic quartz phenocrysts. These microcracks, follow a riedel-type geometry consistent with the ductile kinematics. The ductile shear zones are commonly crosscut by mono-mineralic quartz veins of a few millimeters in thickness parallel to the shear zone walls and localized in the middle of the shear zone. Quartz veins are characterized by a crack-seal texture with elongated blocky quartz grains perpendicular to the vein-wall interface suggesting that quartz was precipitated from a fluid in a dilatant fracture. These quartz veins are always overprinted by ductile deformation. Ductile deformation is characterized by dynamic recrystallization of the blocky quartz into small new grains formed by bulging recrystallization. When ductile deformation overprinting is high, quartz veins are sheared, isoclinally folded and almost entirely recrystallized into a fine grain aggregate.