Earthquake generation in the deep crust

Geophysical evidence suggests that the lower crust at depths >15-20 km is seismically active, yielding as many as 20% of global earthquakes with a magnitude exceeding M5. This is difficult to reconcile with the classic models of the deformation behaviour of the continental crust, which predict a weak, aseismic lower crust able to creep viscously to form mylonites.

However, exhumed sections of lower crustal rocks (granulites, amphibolites) commonly expose large volumes of pseudotachylytes (Fig. 1), which are quenched frictional melts produced during seismic slip along faults, i.e., they are fossil earthquakes. One of the best lower crustal section exhumed to the surface is located in Lofoten, and provides a great opportunity to investigate the mechanisms of generation of earthquakes in the deep crust with a combination of field studies, microstructural, petrological, and geochemical analysis.

Deep-seated pseudotachylytes are frequently associated with mylonites, which form as a result of solid-state, viscous creep during the interseismic periods. Thus, associations of mylonites and pseudotachylytes represent a snapshot of the earthquake cycle in the lower crust.

The goal of the project is to understand the mechanisms of earthquake generation in the deep crust and to derive the mechanical properties of deep seismogenic faults throughout the earthquake cycle. These goals will be achieved with a combination of field studies in Lofoten, and of microstructural, petrological and geochemical analysis of samples and thin sections of pseudotachylytes and mylonites.

Figure 1: Example of pseudotachylyte veins and associated white reaction haloes in the granulitic lower crust from Nusfjord, Lofoten

 

Published Sep. 10, 2019 11:02 AM - Last modified Sep. 10, 2019 11:02 AM

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