Workshop Report - Ridge 2000 Program

2. Regional Surveys. Regional surveys should encompass
several spreading segments and extend to at least 5 m. y. old
crust. To characterize the physiography, full multibeam
bathymetry and side-scan sonar coverage should be obtained, in
conjunction with the acquisition of ship and airborne gravity
and magnetic measurements (with a nominal spacing of not more
than 5 km). These surveys should be followed or accompanied by
the acquisition of marine seismic data, including multichannel
seismic imaging along the ridge crest and along critical
isochrons on the ridge flanks out to the 5 m. y. isochrons.
Flow-line crossings should be conducted along several transects
within each segment - adjacent to segment boundaries, across the
center of a segment, and in I laces dominated by tectonism or
volcanism. A suite of seismic refraction experiments should be
conducted at several locations to define the deep structure of
the ridge axis and of young oceanic lithosphere. These should
be done at critical isochrons surveyed by reflection profiling.
Microearthquake studies using arrays of on-bottom instruments
should be made to describe the tectonics within the area of the
seismic refraction experiments. The initial morphologic surveys
will be used to guide systematic basement sampling on and
off-axis, in conjunction with near-bottom photography, and other
relevant in situ geophysical measurements.
3. Detailed Local Studies. Detailed studies will be
necessary at both the centers and the edges of spreading segments
and may be separated into two groups: (1) studies that
define the present architecture of the lithosphere and
(2) studies that measure ongoing processes.
The first group of studies requires geological mapping of
the surface of the seafloor as well as investigations of the
crust and mantle at depth. The geological mapping must be
constrained by high resolution bathymetric and side scan sonar
mapping base and include the areal extent of outcropping
basement and surficial units, the locations and orientations of
faults, fissures, ductile deformation features and any movement
indicators on them, and active hydrothermal systems. Direct
outcrop sampling is necessary for geochemical studies and
oriented samples are essential for structural and magnetic
studies. Detailed sampling and mapping of fault scarps could
yield data on the volcanic and possibly plutonic stratigraphy of
the crust. the crack structure and history of subsurface rocks,
the alteration history of the crust, and the deep structure of
hydrothermal systems. Developing a three-dimensional picture of
the lithosphere is an integral part of these studies and it is
likely that the most important data will come from direct
sampling by deep drilling (kilometers depth) and shallower holes
Ieven a few meters). Downhole logging and other experiments are
needed to define the physical properties of rocks at depth.
Correlations between physical properties and geophysical interfaces
mapped from the regional surveys could produce a quantum
leap in interpreting seismic structure. Gravity, magnetic and
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