Sustainable densification of the deep crust

Abstract

The densification of the lower crust in collision and subduction zones plays a key role in shaping the Earth by modifying the buoyancy forces acting at convergent boundaries. It takes place through mineralogical reactions, which are kinetically favored by the presence of fluids. Earthquakes may generate faults serving as fluid pathways, but the influence of reactions on the generation of seismicity at depth is still poorly constrained. Here we present new petrological data and numerical models to show that in the presence of fluids, densification reactions can occur very fast, on the order of weeks, and consume fluids injected during an earthquake, which leads to porosity formation and fluid pressure drop by several hundreds of megapascals. This generates a mechanically highly unstable system subject to collapse and further seismic-wave emission during aftershocks. This mechanism creates new pathways for subsequently arriving fluids, and thus provides a route for self-sustained densification of the lower crust.

Spatio-Temporal Evolution of Intermediate-Depth Seismicity Beneath the Himalayas: Implications for Metamorphism and Tectonics

“…ongoing seismic deformation with high-quality locations, that is only a small snapshot with respect to typical dehydration periods (∼1,000 years) compared to calm metamorphic times (Malvoisin et al, 2020). Therefore, potential future directions should include longer and denser seismic networks in the region that can help provide more robust locations along with modeling studies of rock mechanics.…”

“…Understanding whether metamorphic reactions play a role in intermediate-depth seismicity in the Himalayas is of particular importance as these earthquakes serve as the primary modern analogue for interpreting petrological observations of the former Caledonides orogen mid-crust, now exposed at the surface in Norway. Starting with the seminal paper of Austrheim (1987) describing fluid migration and eclogitization, there has been nearly 35 years of research involving mineralogical, petrological, rock physics, mechanical, seismic, deformation and other, complementary studies on this topic (e.g., Petley-Ragan et al, 2019;Zertani et al, 2019;Malvoisin et al, 2020;Zertani et al, 2020). The occurrence of present-day intermediatedepth seismic events in the Himalayas that can potentially help explain the petrological observations made at the surface in Norway remains an exciting prospect.…”

Buoyancy versus shear forces in building orogenic wedges

“…However, the preservation of high-grade metamorphic rocks at the near-surface environment indicates that the crustal metamorphic rocks are not always at thermodynamic equilibrium as is assumed here. The rate of metamorphic re-equilibration is strongly affected by temperature and by the availability of fluids (Rubie, 1986;Austrheim, 1987;Malvoisin et al, 2020). Coupling mineralscale phase equilibria modelling to large-scale geodynamic models remains challenging.…”

Nanoscale earthquake records preserved in plagioclase microfractures from the lower continental crust

“…Plagioclase responds by microfracturing and fragmentation followed by fluidand stress-induced recrystallization (Mukai et al, 2014;Petley-Ragan et al, 2018;Soda and Okudaira, 2018). Grain size reduction by fracturing and subsequent nucleation and recrystallization localizes strain in the lower crust, defining a transition from brittle to crystal-plastic deformation mechanisms with the potential to develop into shear zones (Svahnberg and Piazolo, 2010;Menegon et al, 2013;Okudaira et al, 2016;Marti et al, 2017). Thus, recrystallization and subsequent shear may overprint any microstructural record of the high-intensity stress conditions created by an earthquake.…”