Ash Layers: The Controlling Factor On Translational Sliding Offshore Central America?

The erosive convergent margin of Central America is dominated by the fast subduction of the rough Pacific plate. Off Costa Rica the morphology of the subducted oceanic plate is characterized by numerous seamounts and the Cocos Ridge. Off Nicaragua fewer seamounts and bend ing-related faults dominate the morphology. In both areas seamount subduction with resulting slope uplift or subduction erosion at the base of the upper plate causes oversteepening and frequent slope failure. Our investigation focuses on translational slides off Nicaragua (Skempton ratio < 0,15) Three submarine translational slides of different scales were investigated by gravity coring during METEOR M66 expedition in 2005. On board sediment physical properties investigations revealed ash layers situated on top of older and overconsolidated sections of clayey material beneath a much younger and much less dense clay section on top of it. The jump in density and consolidation with depth led to the assumption that this sucession represents a slide event. As the ash layer also showed lower shear strengths and higher porosities, it was interpreted as the failure plane. Sediment cores show that numerous ash layers are intercalated with marine clays on an average of every 30cm with macroscopic thickness of some mm to more than 5cm. Recovered ash layers differ in two ways from pelagic sediments. Firstly they have higher contents of silt and sand causing higher intrinsic permeabilities. Secondly they consist of disc shaped glass shards which causes higher consolidation rates. This is proved by our first laboratory shear box tests, where ash matter compacted with much higher values than spherical grain shaped reference material of the same grain size. Both factors together could cause a peak pore pressure if ashes compacted rapidly, for instance in a seismic event like the earthquake 1992 off Nicaragua. This would effectively reduce the shear strength between the ash particles and facilitate translational failure. To test this hypothesis and to analyse the relation between pore water pressures and shear strengths under drained conditions, we have modified a shear box, to simultaneously measure pore water pressure and shear strength. We will present field observations from cruise M66 as well as first results from laboratory deformation experiments, supporting our model.