Conceptual, analogue and numerical models of crustal deformation generally assume distributed flow of a relatively weak and viscous middle to lower crust but field exposures of exhumed crustal sections commonly show strong strain localization on shear zones and fractures. These natural observations suggest that the interplay between fracture and flow may be important in localizing and accommodating deformation throughout the whole crust and not just within the “seismogenic zone” limited to the upper 10-15 km of the crust. Fracturing provides discrete discontinuities and damaged zones that are exploited by ductile shear. It promotes fluid infiltration and alteration that may either weaken or harden the rock, which in both cases can lead to localization of ductile deformation. Strongly localized zones in (relatively “dry”) middle to lower crust or high pressure (subducted) units are often associated with significant volumes of pseudotachylyte. The origin of pseudotachylyte is still controversial: it has usually been taken to represent quenched, locally molten rock caused by rapid heating during seismic frictional sliding and thus to be a marker for paleo-earthquakes. Their existence in granulite or eclogite facies rocks is thus used as direct evidence for major earthquakes in the deep crust. However, the origin of pseudotachylyte remains controversial and the controversy has recently been revived with the reintroduction of models involving thermal runaway in strongly localized crystal-plastic shear zones and direct amorphization during strongly localized shearing and grain size reduction, without the necessary transition through a melt phase. All these models require strongly localized shear and high stress, which also has direct implications for potentially non-lithostatic pressures in the lower crust. We invite contributions from field studies, microstructural observations, laboratory studies, and dynamic modelling dealing with this broad topic of the interaction between brittle and ductile deformation in the middle to lower crust, the mechanisms of intermediate to deep earthquakes, and pseudotachylytes as potential markers of deep paleo-seismicity.
Invited speakers:
Stefan Schmalholz, University of Lausanne, Switzerland
Claudia Trepmann, Ludwig Maximilian University, Munich, Germany