RRS Charles Darwin CD99
Cruise summary report
|Ship name (ship code)||RRS Charles Darwin (74AB)|
|Cruise period||1996-03-03 — 1996-04-09|
|Port of departure||Lisbon, Portugal|
|Port of return||Ponta Delgada, Portugal|
|Objectives||Mid-ocean ridge spreading centres are divided into discrete segments about 50 km long, where new lithosphere is created and extended by magmatic (volcanic) and tectonic (faulting) processes.
Our primary objective on this cruise was to quantify the contribution of fault strain to plate separation along a single segment of the Mid-Atlantic Ridge near 29° N, 43° W (Broken Spur segment). This segment was chosen because it has a clearly-defined structure, good existing data coverage extending out to the ridge flanks, and was becoming a focus for international studies.
We intended to use data primarily from TOBI's sidescan, swath bathymetry, and three-component magnetometer to determine the displacements and dips of faults, degree of tilting of fault blocks, spacing of faults, etc. The new swath bathymetry and magnetometer on TOBI had been developed for just this purpose. The SIMRAD multibeam echo-sounder was to be used to make a detailed base map on which to locate TOBI.
We proposed to obtain complete TOBI coverage out to a distance of 35 km from the axis. The survey would extend along strike from the magmatically robust segment centre to the magmatically starved segment end, in order to look for variations in the amount of strain and faulting style over two dimensions.
We also planned to examine the inter-relationship between magmatism and tectonism, using the magnetic data to document the fine structure of the magma emplacement process. The use of three-component magnetics can reveal details of crustal structure (e.g. block rotations) not previously accessible with total-field measurements. The method had been developed for ship-board use by Japanese colleagues, but the use in a deep-towed instrument would be new.
We also intended to study sediment accumulation and mass wasting processes in the median valley walls, using an upgraded chirp profiler to be developed for TOBI, since these processes affect the measurement of strain.
Finally, we were to use the new video imaging tool SHRIMP to ground truth the TOBI data.
|Chief scientist||Roger C Searle (University of Durham, Department of Geological Sciences)|
|Cruise report||(1.43 MB)|
|General||North Atlantic Ocean|
|Specific||Mid Atlantic Ridge|
|Track charts||(0.99 MB)|
|Subsurface temperature and salinity measurements||Quantity: number of deployments = 26|
Description: Deployment of XBT.
|Other physical oceanographic measurements||Quantity: number of deployments = 2|
Description: Velocimeter dips.
|Geology and geophysics|
|Dredge||Quantity: number of hauls = 7|
|Single-beam echosounding||Quantity: number of deployments = 3|
Description: Simrad EM12 deployments
|Single-beam echosounding||Quantity: number of deployments = 1|
Description: Deployment of the echosounder in a sediment search.
|Long/short range side scan sonar||Quantity: number of deployments = 14|
Description: TOBI deployments
|Magnetic measurements||Quantity: number of deployments = 6|
Description: Magnetometer deployments
|Other geological/geophysical measurements||Quantity: number of deployments = 3|
Description: SHRIMP deploments