Strength evolution of the lower crust

The deformation behaviour and the strength of the lower continental crust dictate the elevation, topography, and internal dynamics of continental orogens. Yet, there are fundamentally different models for the strength of the lower crust at the high temperatures and pressures (T > 600° C, P > 7 kbars) typical of lower crustal deformation. The widespread presence of strongly deformed, lower crustal gneisses on Earth’s surface has led to the assumption that the lower crust is invariably weak and capable of flowing at high temperatures.

On the contrary, many exhumed lower crustal sections contain evidence of earthquakes that nucleated in the lower crust at depths > 30 km. This indicates that the lower crust, in such locations, was mechanically strong, brittle and rigid, and incapable of flowing.

The Bergen Arcs in western Norway is the ideal natural laboratory to investigate under which conditions (temperature, pressure, fluids, deformation mechanisms) the lower crust can be mechanically weak and able to flow, or mechanically strong and seismogenic. The Bergen Arcs expose strongly deformed granulitic gneisses, which are locally cut by lower crustal seismogenic faults (Fig. 1). This indicates that the gneisses have experienced, in the lower crust, the transition from mechanically weak to mechanically strong during their geological history.

The goal of the project is to determine the metamorphic and deformation conditions of weak vs strong lower crust and to identify the cut-off temperature of lower crustal seismicity. These goals will be achieved with a combination of field studies in the Bergen Arcs, and of microstructural, petrological and geochemical analysis of samples and thin sections of variably deformed metamorphic rocks and fault rocks/shear zones.

Image may contain: Soil, Wall, Geology, Line, Tree.
Figure 1 – Example of strongly deformed granulitic gneiss cut by a seismic fault in Holsnøy (Bergen Arcs). Click here for a bigger picture.

 

Tags: crustal evolution, crustal deformation, earthquakes
Published Aug. 31, 2020 11:57 AM - Last modified Aug. 31, 2020 11:57 AM

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