Earthquake-induced transformation of the lower crust.

Abstract

The structural and metamorphic evolution of the lower crust has first order effects on the lithospheric response to plate tectonic processes involved in orogeny, including subsidence of sedimentary basins, stability of deep mountain roots, and extension of high topography regions. Recent research shows that prior to orogeny most of the lower crust is dry, impermeable, and mechanically strong1. During an orogenic event, the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones. In the Bergen Arcs of Western Norway, shear zones initiate as faults generated by lower crustal earthquakes. Seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms. However, regular earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust2. Here, we show that the volume of lower crust affected by such aftershocks is very significant and that fluids driving associated metamorphic and structural transformations of the lower crust follow in the wake of these earthquakes. This provides a novel ‘top-down’ effect on crustal geodynamics and connects processes operating at very different time scales.