Anatomy of earthquakes in the lower continental crust

Francois Renard, University of Oslo

Wednesday, January 20, 2021 at 10:30 AM

Location
Online-only seminar via Microsoft Teams

If most of continental earthquakes occur in the first 15-20 km of the upper crust, lower crust earthquakes exist at depths of 50-60 km in the roots of mountain chains. Such earthquakes are observed, for example, below the Himalayas. Three mechanisms may explain how elastic strain energy is released in the form of earthquakes in the lower crust where rocks are often inferred to be ductile. The first mechanism involves local stress concentrations related to displacements along shear zones. The second mechanism involves the triggering of lower crust earthquakes by strain pulses produced by upper crust earthquakes. The third mechanism involves hydration reaction that may cause embrittlement of the rock. In Norway, outcrops of lower crust have recorded earthquake activity that occurred more than 400 million years ago during the Caledonian orogeny and that allow to test these three mechanisms. Fossil lower crust earthquake are observed because coseismic frictional heating produced numerous pseudotachylytes and fault offsets of up to two meters. Field data at the outcrop scale and images of fault samples in 2D (electron microscopy) and 3D (synchrotron X-ray microtomography) reveal the anatomy of the fault zone. Seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. Data also show that earthquake activity preceded or was coeval with shear zone development and that fluids infiltrated into the earthquake damage zones. These observations indicate that, under mountain chains, the lower continental crust can be initially strong and not as ductile as usually proposed. The damage produced by earthquakes may explain how the rock rheology may evolve from brittle (e.g. granulites) to ductile (e.g. eclogites) through fluid infiltration.

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