Infiltration of volatile-rich mafic melt in lower crustal peridotites provokes lower crustal earthquakes

Abstract

We document well-preserved extensional shear zones in a rift-related lower-crustal magmatic conduit system, the Reinfjord Ultramafic Complex. Deformation along the shear zones led to seismic events below the seismogenic zone of continental rifts (10–15 kbar and 850–1150 °C), similar to events observed along the East African Rift today. Processes leading to failure of the weakened rocks caused extremely high strain. We mapped pseudotachylytes along a network of millimeter to meter-scale transtensional shear zones associated with gabbronoritic dykes, and a 2 km long low-angle extensional shear zone. Deformation, initiated through priming of the rock by magmatic fluids, exploited subgrains and microfractures in olivine, with CO2-bearing fluids leading to volume expanding reactions (olivine + diopside + CO2 = dolomite + enstatite), enhancing olivine fracturing. Fragmentation of the olivine grains and the addition of weaker phases facilitated strain localization and increased the strain rate by two orders of magnitude through grain size reduction (Sørensen et al., 2019). Sudden failure under transient brittle conditions formed pseudotachylytes. We conclude that failure along shear zones in lower crustal continental rifts is possible without remote stress events, in the presence of pre-existing textural and structural heterogeneities and events of volatile fluid infiltration.