I asked Professor Roger Searle if he would write a few words and draw a few diagrams to explain exactly what we are going to study at the Mid Atlantic Ridge. Roger is very keen for the wider public understand the science (indeed I believe it is Roger’s idea to have a teacher at sea). If you have any questions after reading the blog please do ask. Roger is most happy to answer any questions. Over the course of the next few weeks I will be telling you more about how the OBS equipment, autosub and seismic surveys work. Tomorrow I’ll tell you all about my day with the OBS team. For those you following the blog closely you’ll be pleased to know that Autosub’s test today went very well and she is ‘fit for work’.
So over to Roger:
What do we hope to learn?
About 10 years ago, scientists discovered that there is a new mode of seafloor spreading operating in some 50% of the Mid-Atlantic Ridge, and we wish to understand this important process. Whereas the ‘classical’ mode of spreading is symmetric (see diagram), this new mode is highly asymmetric.
In the symmetric mode, as the plate separate, ductile asthenosphere wells up to fill the gap, and partially melts (about 10%). The melt rises and solidifies to form the gabbro plutons, doleritic dykes and basaltic lavas of the crust. The melt may accumulate in one or more small mid-crustal magma chambers on the way up. Once the crust has formed, the stresses pulling the plates apart produce normal faults, with displacements of ~100m, which add some 5-10% of strain. The structures of both plates are very similar, as are their accretion or spreading rates; new material is added equally to both sides.
The newly discovered asymmetric mode is quite different. We think this mechanism is triggered when for some reason less melt is delivered from the mantle, so a thinner crust is formed, and some of the normal faults may reach as far as the mantle. If seawater can percolate down them to reach peridotite at <~500ºC, it will react to produce the mineral serpentine, and the rock serpentinite. Serpentine is very weak, so will lubricate the fault, making it easy to continue slipping. Such faults may penetrate right through the lithosphere (detachment fault), and accumulate displacements of tens of kilometres over millions of years (see diagram). The hanging wall plate (right-hand one in the diagram) receives less melt than normal and so is thinner. The footwall plate (on the left) is formed by pulling mantle material (peridotite) up. However, in some places sampling suggests there may be significant amounts of gabbro intruded into it. If all of the plate separation is taken up by slip on the detachment fault, then no new material will be added to the hanging wall plate, and spreading will be 100% asymmetric.
Although we think we understand the broad outlines of the asymmetric spreading process, many of the details are unknown, including:
- Do detachment faults really steepen at depth?
- Are detachments isolated or are they connected at depth?
- Is new material really added to the plates asymmetrically?
- What are the different rock types in the hanging wall and footwall plates, and how are they distributed?
These are the questions we intend to address on this expedition.
We are carrying out several experiments. One, already underway, has detected thousands of small earthquakes, which we hope will help determine the positions and shapes of active faults. On the current expedition, we will do two further seismic experiments. One will use Ocean Bottom Seismographs to record acoustic pulses set off at the sea surface. This will show us the varying velocity of sound in the lithosphere, and help us deduce the types and structures of the rocks. Another method will record similar acoustic pulses but reflected within the lithosphere and returned almost vertically to an array of towed hydrophones, to show the structure in a vertical section.
With Autosub, we will measure in fine detail variations in the Earth’s magnetic field, and thus infer the spreading rate in each plate, and compare this between areas of symmetric and asymmetric spreading. Autosub will also measure very high resolution topography (shape of the seafloor), showing details of volcanoes and faults, and it will measure quantities such as the cloudiness of the seawater, which may show the presence of hydrothermal plumes.
Thanks very much Roger,