RRS Charles Darwin CD99

Cruise summary report

Cruise Info. 
Ship name (ship code)RRS Charles Darwin (74AB)
Cruise identifierCD99
Cruise period1996-03-03 — 1996-04-09
Port of departureLisbon, Portugal
Port of returnPonta Delgada, Portugal
ObjectivesMid-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 scientistRoger C Searle (University of Durham, Department of Geological Sciences)
Cruise reportRRS Charles Darwin CD99 cruise report cd99.pdf (1.43 MB) 
Ocean/sea areas 
GeneralNorth Atlantic Ocean
SpecificMid Atlantic Ridge
Track chartsRRS Charles Darwin CD99 cruise track — cd99trk.pdf (0.99 MB) 
Physical oceanography 
Subsurface temperature and salinity measurementsQuantity: number of deployments = 26
Description: Deployment of XBT.
Other physical oceanographic measurementsQuantity: number of deployments = 2
Description: Velocimeter dips.
Geology and geophysics 
DredgeQuantity: number of hauls = 7
Description: Dredges.
Single-beam echosoundingQuantity: number of deployments = 1
Description: Deployment of the echosounder in a sediment search.
Single-beam echosoundingQuantity: number of deployments = 3
Description: Simrad EM12 deployments
Long/short range side scan sonarQuantity: number of deployments = 14
Description: TOBI deployments
Magnetic measurementsQuantity: number of deployments = 6
Description: Magnetometer deployments
Other geological/geophysical measurementsQuantity: number of deployments = 3
Description: SHRIMP deploments