High stress amplifications in seismogenic continental lower crust from field and microstructural indicators

Abstract:

Increasing recognition of the ability of the continental lower crust to deform via seismic rupture has brought with it particular consideration of how to obtain the high stresses required to nucleate rupture at such depth. Within an exhumed section of anhydrous lower crustal anorthosites in Lofoten, Norway, excellent exposure of a shear zone network displaying mixed brittle-ductile behaviour captures evidence of transiently amplified stresses related to seismic failure. Pseudotachylyte-bearing faults, indicative of seismic ruptures, form networks cutting low-strain blocks between localized viscous shear zones. The pseudotachylyte-bearing faults formed concurrently with viscous creep at lower crustal depths. Applying typical seismic scaling laws to displacement and length measurements for these faults leads to stress drop estimates that range in excess of 1 GPa, consistent with the failure strength of intact anorthosite at lower crustal depths, but far in excess of stress drops observed in the shallower seismogenic zone. We propose that high stresses were locally amplified within the low- strain blocks by the activity of the bounding shear zones. Microstructural investigation of the faults and their damage zones reveals a sequence of deformation spatially related to faulting, including low-temperature plasticity and fragmentation in pyroxenes. These observations support transient and localized amplification of stresses, at times likely > 1 GPa, before and during rupture, although the exact timing relative to nucleation and propagation sometimes remains difficult to constrain. Together, these combined field and microstructural observations provide a valuable record of lower crustal earthquake nucleation plus the resulting mechanisms of seismic damage, and invite further comparison particularly with numerical and experimental studies.