High-Resolution Mapping of Crustal Cracking at the Superfast-spreading
EPR at 17-18 S: Preliminary Results From Sojourn II
1998 American Geophysical Union Fall Meeting
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- D.J. Wright
(Dept. of
Geosciences, Oregon State University, Corvallis,
OR 97331-5506)
- R M Haymon, K C Macdonald, S White, J M H O'Neill (All at: Dept. Geological Sciences, UCSB,
Santa Barbara, CA 93106)
- D Scheirer (Geological Sciences, Brown U., Providence, RI 02912)
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We have used the near-bottom imaging capability of Argo II to make
preliminary determinations of the abundance, widths, and spatial
distributions of fissures along a superfast spreading center.
Argo II was used in conjunction with DSL 120 to survey the
narrow axial zone of the southern EPR at 17-18 S. The DSL
120 was towed at an average height of ~75 m above the seafloor
to collect high resolution, 120 kHz sidescan and bathymetry,
while Argo II was towed at an average height of 8-9 m above the
seafloor. Most of our fissure data at this stage come from Argo II
video and Imagenex. Fifteen 45 km-long, axis parallel lines through the
axial zone with line spacings of 10-30 m provided 100% saturation
coverage where the axial zone is less than 100 m wide, down to a
minimum coverage of 45% where the axial zone widens to 400 m. Preliminary
results on a 3rd-order scale from 17 14-40'S show an along-axis
pattern of abundant fissures in the northern half of the survey area
trending to fewer fissures south. Across-axis, fissures are located
primarily within 50 m of the east side of the axis as opposed to the
west. As was found on the EPR at 9-10 the most densely fissured
regions lie within older (Age 2) terrains while the least fissured
areas coincide with where the freshest lavas are found, particularly
just south of the "Spike" region at 17 25'S. Indeed along the 17 S
survey there is a demarcation between older lavas to the north of
the "Spike" region at 17 25'S and younger lavas to the south,
probably as a consequence of the southward propagation of dikes
along-strike away from a shallow, localized site of melt injection
at 17 25'S. The correlation of fissure density with axial lava
age may reveal a similar tendency proposed for 9N of the crust
to accumulate more cracks with time, rather than to widen existing
cracks formed early on. Superimposed on this accumulation of
cracks are spatially localized, relatively recent eruptions
that cover fissures and produce minima in cumulative fissure
width. In these regions average fissure widths and depths
increase due to ridge crest inflation and dike propagation,
resulting from recent magmatic replenishment of the AMC from the
upper mantle, particularly at 17 25-28'S. However, unlike 9N,
preliminary results do not show an increase in cracking towards
the tips of this superfast-spreading segment, suggesting that
cracking on this scale may be driven more by the shallow crustal
injection of magma and propagation of dikes from the "Spike," rather
than by far-field plate stresses.