The article “On the Mechanical Role of Smectite in Subduction Zones” by Peter Vrolijk discusses hydrated phyllosilicate minerals called smectites. The author pays special attention to smectite in the decollement zones, mineralogical reasons for instability of smectites and their distribution in oceanic sediments, evolution of sedimentary sequence, and implications of subduction zones.
The author defines smectites as “main component of sediments entering a subduction zone”. (p. 703) Generally, smectites are found in hemipelagic sediments and they are made of authogenetic component.Strata being rich with smectites are structurally weak and highly porous.
Decollement zones are favorable for creating smectite-rich horizons. The author claims that sediment consolidation process can’t take place without smectite-rich subduction zones. Smectites are minerals with water between individual phyllosilicate layers and, therefore, their crystal structure is rather weak because of interlayer water. Thorough the process of deformation water remains in sediment as it is responsible for smectite weakness. The presence of smectites depends on the influx of detrital clay.Vrolijk claims that Coulomb wedge theory should be used in assessing geometry of accretionary prisms.
Nevertheless, this theory is challenged by the fact that coefficient of basal sliding friction is far from coefficient of friction is smectites. If the coefficient is closer, than the theory will be more realistic and will assist in estimation of fluid pressures in decollement. Diagenetic and metamorphic history suggests that smectites are likely to be transformed into illite as it is inevitable part of consolidation process.The onset of seismisity is related to strengthening of minerals and materials during the process of smectite metamorphism. The article contributes development of new technologies in geophysics as the author provides new theories how to estimate fluid pressures and to identify presence of smectite.
The article contributes to understanding geophysical processes as it describes thoroughly structure of smectite, mineralogical reasons for its instability and implications of subduction zones.
