In-situ physical properties of submarine slides along the Lesser Antilles Arc derived from rock physics models

Submarine slides are ubiquitous along the flanks of volcanic islands and continental margins. They alter seafloor morphology, transport huge sediment volumes, and sometimes generate tsunamis. Constraining in-situ sediment physical properties, and in particular, pore fluid pressure in submarine slide debris offers insight into slope failure processes. Unfortunately, in-situ measurements of physical properties are difficult to acquire and often require specialized tools or long-term sub-seafloor hydrogeological observatories. Here, using data collected from the Lesser Antilles Volcanic Arc during IODP Expedition 340, we demonstrate that rock physics models (e.g. Dvorkin et al., 1999; Mavko et al., 2009) applied to shipboard physical properties measurements provide a valid approach for estimating in-situ P-wave, S-wave, and Poisson's ratio values for slide debris. The rock physics approach presented here is especially valuable at depths less than 80 m below the seafloor where shallow slides often exist but open-hole well logging is limited. Seismic velocities, and in particular, Poisson's ratio values obtained using the rock physics model provide insight into subsurface pore-pressure in submarine slide complexes along the Lesser Antilles Arc. Near the volcanic arc, submarine slide debris has anomalously high P-wave and S-wave velocities and low Poisson's ratios, atypical of shallowly buried marine sediments, implying over-compaction and perhaps rapid dewatering. In the slide apron away from the arc, however, slide debris generally has high porosity, low seismic velocity and anomalously high Poisson's ratio values. The inferences obtained using rock physics models are consistent with numerical models and analog laboratory experiments of debris flows that infer normal dewatering, compaction, and erosion in the run-out area of submarine slides but higher porosity and elevated fluid pressure in submarine debris flow aprons. Analysis of rock physics model results shows that Poisson's ratio typically drops sharply at the base of slides. We hypothesize that the basal boundaries of submarine slides consist of stronger, dewatered material overlain by weaker, less-compacted sediment. The analysis implies significant variability in sediment strength and permeability with depth in deep-water basins where slide debris accumulates. At IODP sites 1398 and 1399 in the Grenada Basin we observe high Poisson's ratios and anomalously high porosity in sediments older than 100ky. Our analysis suggests the permeability of some shallowly buried (<200 m below sea-level) slide debris exceeds 10^-17 m^2 at these sites; this in-part explains why elevated pressures exist in the basin. The analysis implies near-lithostatic pressure at some locations beneath the Grenada basin and perhaps in other similar settings where distal mass wasting acts as the dominant sediment source.