Chris: Core sampling is fun, but the science you can do with those samples is even more fun. In deforming sediments (such as the ones in the Cascadia forearc), the mineral grains start to preferentially align in a direction that reflects the applied strain field. This alignment becomes locked into place as the sediments start to compact and lithify, preserving a record in of how deformation has changed in a particular location over geological timescales. More importantly, a preferred mineral alignment within a sample will lead to it being more easily magnetisable in certain orientations than others – a quantity known as the anisotropy of magnetic susceptibility (warning: scary tensor math may be lurking at that link). Measure this, and you can potentially recover the deformation history.
The anisotropy data from the core section shown yesterday shows that in the youngest sediments from the top 20cm, the mineral alignment is what you’d expect for a locality above a locked subduction thrust: the inferred compression axis is aligned with the direction of plate convergence (the blue line). The same is the case below around 40cm.
In the interval between, however, in the same part of the core as that turbidite horizon, things go a bit haywire. Possible turbidite triggered by an earthquake – and a major deviation in the preserved deformation record? I’d say that’s very interesting.