Hikurangi subduction zone expedition #375 – blog 4

In March 2018, JOIDES Resolution, a large scientific research vessel, headed out to sea to research the Hikurangi subduction zone on expedition #375. This article is the fourth blog from Aliki Weststrate, IODP (International Ocean Discovery Program) Outreach Educator. This is her account of a voyage full of excitement, challenges and science!

It’s week 4, and we’re halfway through expedition #375 to study the Hikurangi subduction zone. Americans call this half-way point ‘hump day’, so there were lots of funny photos of camels with humps pinned up around the ship. Usually we have a dance party to celebrate, but we had one last week, so I think we’re breaking tradition and may have another in week 6. It’s important to balance the long 12-hour shifts with leisure time!

This week, we moved 90 km east of Gisborne to our second site, which has a pragmatic name – U1520. The site is next to the Tūranganui Knoll, which is a 840 m high seamount (an extinct underwater volcano).

The water depth is 3,522 m and is deeper than our first site (U1518) because we are further out from the coastal platform and in a deep ocean basin. We are drilling into sandstone and mudstone rather than into the fault zone as we did before at U1518.

Our mission here

We are collecting sediment cores from up to 1,050 metres below seafloor (mbsf) to study the state and composition of the material here. This material is currently part of the Pacific tectonic plate, and it will eventually (over millions of years) be carried westward as the Pacific plate moves into the trench as if it were on a conveyor belt.

Some of the sediment will be dragged under (subducted into) the Australian plate. The rest will be bulldozed and become part of the accretionary wedge – this is a mass of sedimentary material scraped off the Pacific oceanic crust during subduction and piled up at the edge of the Australian continental crustal plate. You can see it in the coloured map here as the purple, bluish, and green ‘hillsides’ leading up to U1517.

The geological before and after

Understanding what this proto or input sediment is made of and how it behaves before it is dragged into and above the fault zone means we can compare the two sites. We can conduct experiments on this input material by putting it through fluid and pressure tests to mimic what might happen to it as it is transported by the subduction conveyor belt deep into the slow slip source area of the fault zone.

We are also measuring and recording its current state – lithology, fossil types, palaeomagnetic properties, temperature, porosity, density, shear strength and chemistry.

All the experiments we are doing now in the JOIDES Resolution labs we will continue and extend back on land after the voyage. The tests should show us how this input material might behave once it is subjected to the forces of the megathrust zone 20 km west of here. It might also shed light on why there are slow-slip events in this region too.

Our research questions

• Are the rocks and sediments here near the seamount unusual? What are they made of?

• Are they dense and compacted out here in the basin at U1520, and do they release fluid as they are heated and buried on the way into the fault zone and slow slip area?

• How does this incoming sediment react when it comes under increasing friction as it nears the subduction zone?

• Will all of it be subducted, or will some be bulldozed onto the hanging wall above?

What are we seeing here in the cores?

The cores we are bringing up are getting progressively older the deeper we get, and we know this from the type of fossils the palaeontologists are seeing under their microscopes – which is no surprise. What is most noticeable to the untrained eye is how hard and brittle this core is compared to our first site above the fault zone.

At our first site above the fault zone, the core was quite wet and soft, so it was easy for the scientists to get samples out themselves using small cylinders and tubes. Here out at site U1520, the trained technicians are having to use the circular saw to cut samples for the geologists. The cores are almost as hard as cement, with very little fluid in them.