Turbulence measurements performed in the North Atlantic during the James Cook cruise JC087, March 2013

During the cruise the turbulence probe MSS050 was used for microstructure measurements. The profiler is produced by Sea and Sun Technology GmbH in co-operation with ISW Wassermesstechnik. The probe was equipped with 2 velocity microstructure shear sensors, a microstructure temperature sensor, standard CTD sensors for precision measurements, a vibration control sensor, a two component tilt sensor, a fluorescence sensor and surface detection sensor (SD) to indicate the water surface hit at rising measurements. The sampling rate for all sensors is 1024 samples per second, the resolution 16 bit. During the MSS measurements, the ship was moving with speed approx. 0.5-1.0 knots with respect to the water against the wind. In order to take into account the intermittent nature of marine turbulence, repeated MSS measurements were carried out in bursts of typically 10 profiles per station. The measurement interval was approximately 7 (10) min for a profile to 140 (200) dbar. During JC87 cruise 8 series of turbulence measurements were conducted. Fluorescence data was obtained on the 5 of them.

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.