CAN SUBDUCTION OF SEDIMENTARY MÉLANGES CONTROL THE MECHANICS OF SHALLOW MEGATHRUST?

The occurrence of giant earthquakes both in settings characterized by sediment-flooded trenches (e.g., Sumatra, Central-South Chile, Alaska/Aleutians) and sediment-poor ones (e.g., Kamchatka, Northern Chile, Northern Peru, Northern Japan), suggests that the internal architecture and the mechanical properties of subducted material (e.g., composition, friction and strength properties, permeability, stiffness, fracture toughness, erosive or accretionary kind of margins), as well as the thickness of the subduction plate interface (or subduction channel), represent effective factors in influencing the seismic behavior. On the basis of field observations, we propose that the incorporation of medium (10-1000 km²) to giant (≥1000 km²) heterogeneous mud-rich sedimentary mélanges (mass transport deposits) within the subduction thrust interface may play a significant role in controlling the seismic behavior, strongly modifying its internal architecture and the mechanical and physical properties of subducted material. Depending of the dimension and distribution of subducted sedimentary mélanges with respect to the thickness of the subduction thrust interface, we document that the their heterogeneous block-in-matrix fabric and composition may significantly smooth and diminish the effectiveness of seismic ruptures trough the redistribution of high-pressured fluids and strain, favoring an overall trend that resembles the characteristics expected for low-frequency earthquakes widely detected along the circum-Pacific subduction zones. Considering the difficulty to seismically observe the internal architecture of modern subduction thrust interfaces and to distinguish sedimentary mélanges overprinted by tectonics (i.e., polygenetic mélanges) from tectonic ones, direct field observation of exhumed analogues shear zones is a key factor to better understand the role played by subduction of sedimentary mélanges in controlling seismic ruptures.