A numerical discrete element study on the influence of an embedded viscoelastic-plastic layer at different viscosity values

The major aim of this study was to examine the influence of an embedded viscoelastic-plastic layer at different viscosity values on accretionary wedges at subduction zones. To quantify the effects of the layer viscosity, we analysed the wedge geometry, accretion mode, thrust systems and mass transport pattern. Therefore, we developed a numerical 2D 'sandbox' model utilising the Discrete Element Method. Starting with a simple pure Mohr Coulomb sequence, we added an embedded viscoelastic-plastic layer within the brittle, undeformed 'sediment' package. This layer followed Burger's rheology, which simulates the creep behaviour of natural rocks, such as evaporites. This layer got thrusted and folded during the subduction process. The testing of different bulk viscosity values, from 1 × 10**13 to 1 × 10**14 (Pa s), revealed a certain range where an active detachment evolved within the viscoelastic-plastic layer that decoupled the over- and the underlying brittle strata. This mid-level detachment caused the evolution of a frontally accreted wedge above it and a long underthrusted and subsequently basally accreted sequence beneath it. Both sequences were characterised by specific mass transport patterns depending on the used viscosity value. With decreasing bulk viscosities, thrust systems above this weak mid-level detachment became increasingly symmetrical and the particle uplift was reduced, as would be expected for a salt controlled forearc in nature. Simultaneously, antiformal stacking was favoured over hinterland dipping in the lower brittle layer and overturning of the uplifted material increased. Hence, we validated that the viscosity of an embedded detachment strongly influences the whole wedge mechanics, both the respective lower slope and the upper slope duplex, shown by e.g. the mass transport pattern.

(Table 1) Chemistry of two samples of the red weathered layer at DSDP Hole 55-432A

One argument for the subaerial formation of Nintoku Seamount in the Emperor chain is the occurrence of a red claystone interlayered between two basalt flows in Hole 432A. Detailed study of this material, presented here, defines its nature and composition and strongly indicates its subaerial formation.

Chemical composition of layer silicates from the Sado Ridge, Sea of Japan

Detailed mineralogical investigations of high-Fe layer silicates from loose sediments (glauconite sands) of the Sado Ridge revealed that green aggregates found on submarine rises of the Japan Sea floor have different genesis. It was demonstrated that round dark green grains approximate micas in composition. Primary volcanic rocks presumably have undergone extensive secondary alterations and then were disintegrated. Their disintegration products (protoceladonite) filling pores were redeposited and buried in sediments for a long time. Angular green grains mainly represented by smectite also formed at lower temperatures during disintegration of altered volcanosedimentary rocks. These younger grains had no prolonged exposure. Pseudomorphs of siliceous microplankton consist of both hydromica and smectites. They are presumably authigenic products formed with participation of microorganisms or electrostatic processes (spherical shape), or their combination. The formation mechanism of minerals filling cavities in pyroclastics is not entirely clear.