James R. Cochran, Lamont-Doherty Earth Observatory
Fremantle-Fremantle
9 Dec 1994-22 Jan 1995
We are conducting a two year geophysical study of the ridge crest and flanks
of the Southeast Indian Ridge (SEIR) between 95oE and 108oE. This region
includes the transition zone from an axial bathymetric high to an axial rift
valley, located somewhere between 101oE and 105oE, and areas on either side of
the transition where the two styles of axial tectonics are well developed. The
overall objective of this work is to gain insight into the transition from an axial
rift valley morphology characteristic of slow spreading ridges to an axial bathymetric
high characteristic of fast spreading ridges. Specifically we wish to determine the manner
in which this transition occurs, what factors control the transition and what its effects
are on axial tectonics, segmentation, ridge axis isostasy, the form of the axial intrusion zone,
and the formation and characteristics of abyssal hills.
We believe for a number of reasons that the region near 100oE on the SEIR is the best
area for a study of the transition in axial morphology at intermediate spreading rates.
Both "slow spreading" and "fast spreading" type ridges are well developed in adjacent areas
at the same spreading rate and at nearly the same zero-age depth. The region is well
removed (approx. 1000 km) from hot (or cold) spots and the transition in morphology does
not mark an abrupt transition between a hot spot and not-hot spot regime, as is the case
on the Galapagos spreading center. Even if there is a mantle flow from the Amsterdam or
Kerguelen hot spots toward the AAD, the gradual and consistent slope of the zero-age depth
along the ridge axis allows us to conclude that variations in mantle temperature and other
properties should also be gradual and consistent across the survey region. It is thus an
ideal region to examine what triggers the change in axial tectonics and morphology, how the
transition occurs and what are its consequences. Finally, from a logistics point-of-view, the
survey area is within a reasonable transit distance (4.5 days) from Fremantle, Australia.
Survey Plan - Our study is built around a 38 day field program which includes SEA BEAM 2000
multibeam bathymetric and side-scan sonar data, as well as gravity and magnetics measurements.
Examination of Eltanin seismic reflection lines across this portion of the SEIR shows negligible
sediment thickness in the areas near the ridge where our work is concentrated. We thus do not
believe that the expense for seismic reflection profiling can be justified.
SEA BEAM 2000 is a 121 beam swath mapping system with a beam width of approx. 2o per beam. The
total width of the swath is 120o, so that an area 3.4 times the water depth is mapped. The nominal
swath width is thus 10-11 km at water depths of 2900-3300 m which characterize most of our field area.
SEA BEAM 2000 also collects backscatter data which can be processed to produce amplitude and textural
data similar to that produced by side-scan systems such as SeaMARC II. The side-scan has a swath width
comparable to the bathymetric swath and a 1024 pixel across-track resolution, so that in the water depths
in which we will be working, backscatter data is sampled at 10 m intervals in the across track direction.
There are three main elements to the field program. The first is an along axis survey of the ridge crest
between transforms located at 95oE and 108oE. The second element is detailed surveys of three regions,
one with a well developed axial high morphology, one with a well developed rift valley morphology and one
centered on the transition between an axial high and a rift valley. The third element of the field
program is a series of nine long swath-bathymetry, side-scan sonar and geophysical lines extending
to a distance of 200 km from the axis on both flanks.