Mysteries of the deep (and bumpy) seafloor

Featured image: Elevation map of a seamount in the central Pacific, shown in a persepctive view. Image courtesy of the NOAA Office of Ocean Exploration and Research (public domain).

Paper: Fluid-rich subducting topography generates anomalous forearc porosity
Authors: Christine Chesley, Samer Naif, Kerry Key, Dan Bassett

Open any geology textbook, and you’re guaranteed to find a cartoon of a subduction zone showing how an incoming oceanic plate dives down beneath another tectonic plate (either continent or ocean) on its way back into Earth’s deep interior. These simple sketches typically show the top of the incoming plate as a smooth, gently curved line meeting and joining another smooth line at the base of the overriding plate – and that’s not exactly wrong, given the enormous scale of a subduction zone compared to the smallness of the drawing. But if you zoom in far enough on oceanic tectonic plates, the seafloor is often rough and bumpy. What happens, then, when rough seafloor heads into a subduction zone?

A new study by Christine Chesley and co-authors uses electrical conductivity to look at the consequences of subducting seamounts, which are small underwater mountains on top of oceanic plates that are formed by volcanic activity. They measured the electrical conductivity of the shallow subsurface across the Hikurangi subduction zone, off the east coast of the North Island of New Zealand.

Since water is highly conductive, porous rocks with water in them light up in electrical conductivity measurements compared to those with low porosity and less water. The measurements made by Chesley and co-authors showed that a seamount on the oceanic plate headed toward the subduction zone was made up of a low-conductivity cap over a highly conductive layer and a low-conductivity center. This layered structure suggests that some porous, water-rich sediments and volcanic rocks are sandwiched between a low-porosity core and a lava flow.

The authors also saw similar conductivity variations in a deeper feature, likely a seamount that subducted a few million years earlier. Above that deeper feature, some spots of high conductivity are interpreted as areas where the upper plate has been broken up in the wake of the subducting seamount and water has migrated upwards into the resulting fractures.

These conductivity measurements are especially exciting for two main reasons. First, geoscientists have long suspected that seamounts plow into the overriding plates in subduction zones like little bulldozers, forming networks of fractures and faults above them, but actually imaging those fractures has proven difficult. The high conductivity spots above the subducted seamount at Hikurangi provide evidence that the seamount has, in fact, done some damage to the base of the overriding plate and created pathways for water within the downgoing plate to percolate upwards. Second, there’s a longstanding debate over whether subducting rough seafloor is likely to make subduction zones more prone to earthquakes because the roughness may get stuck as the plate goes down, building up stress that’s later released in an earthquake. The high conductivity layers imaged within the Hikurangi seamount structures suggest that these seamounts are carrying significantly more water than regular oceanic crust without seamounts, and some of that water might get squeezed out as the seamount subducts. This would make the subduction zone interface more likely to creep gently along, rather than sticking in place until enough stress builds up for a large earthquake.

Seamounts may be small – too small to draw on a simple cartoon of a subduction zone that’s tens of miles wide – but this study suggests these underwater structures could have an outsize impact on subduction zone processes, from water cycling to earthquake hazards.

Mysteries of the deep (and bumpy) seafloor by Hannah Mark is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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