elongated marginal highs. These domed-shaped axial highs are particularly prevalent south of latitude 38°10
0 S at 39°35
0 S and 39°45
0 S (dredge station SO63DS)
where detailed studies were conducted. The northern segment (north of 38°10
0 S, at
37°50
0 S on the ridge) is of particular interest since it represents the area where the
hotspot of the Foundation Volcanic Chain meets the spreading PAR axis. Deeptowed camera stations (OFOS and SCAMPI), dredging and TVG (television-grab)
stations were conducted and revealed the presence of numerous active and inactive
hydrothermal fields along the northern (north of 38°20
0 N) ridge segment. Recently
erupted lava flow was detected with the deep towed camera station (OFOS) in a
small graben located at the top of a dome shaped feature near dredge station 86DS
(37°39
0 S) made by the R.V. SONNE in 1995 (Leg 100). This recent volcanism was
revealed when comparing previous (N.O. L’ATALANTE) deep towed television
(SCAMPI # 2) coverage carried out in February 1997 in the same graben, which
showed hydrothermal deposits. Returning to the same spot during R.V. SONNE’s
Leg 157 in 2001, we could not find the same features. The hydrothermal material
was buried by newly erupted silicic lava. Silicic lavas (andesite and dacite)
associated with sulfide deposits were recovered from the axial grabens as well as
from other topographic highs forming at least four volcanic domes along the
various segments. This was a major discovery because silicic domed shaped
features, although common in subaerial silicic volcanic events, were never seen to
occur on the seafloor at depths of more than 2,000 m. The silica-rich lava consisting of andesite and dacite occurred at 37°39
0 S (SO100-86DS), at 37°50
0 S, at
38°05
0 S (station SO100-91DS), near 39°35
0 S and near 39°45
0 S. These silicic lavas
are associated with basaltic tubes and pillow lavas, which have covered the lower
slopes of the ‘‘domes’’ (see Chap. 5). The silicic ‘‘domes’’ are narrow, about 3 km
long and 1 km wide, and are gouged by small summit depressions or grabens that
are a few tens of meters deep, at depths of 2,100–2,200 m. Silicic lavas are highly
viscous and they give rise to explosive volcanism.
Further to the south of latitude 39°35
0 S, the ridge segment shows a smooth
topography with an almost constant depth of about 2,150 m and a continued and
gentle slope deepening southwards, reaching about 2,300 m depth near 39°56
0 S
(Fig. 7.33). This segment-end near 40°S consists of small discontinuities (DEVAL)
marked by a depression (2,300 m) between the two incipient overlapping segments.
Thus, in the vicinities of this DEVAL tip near 39°56
0 S, the general pattern of the
axial orientation is changing more to a northerly (005°N) direction. At least four
Non-Transform-Offsets (NTO) propagating ridge segments are observed along this
nearly N–S oriented ridge segment. The segment-ends of the NTO occur at 39°56
0 S,
40°14
0 S, 40°37
0 S, 40°50
0 S and 41°13
0 and show an increase of intervening depths
from 2,300 m to 2,600 m between the segment-ends southwards, before reaching
the southern overlapping spreading center (OSC) near 41°13
0 S. The segments
migrate southwards and show a general deepening of the topography. Also, from
the nature of the collected samples (degree of rock alteration and manganese
hydroxide coating), it was inferred that the volcanic activity of the northern ridge
segment had decreased concomitantly southwards towards the OSC. Thus the
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7 Oceanic Spreading Ridges and Sea Floor Creation
0 S at 39°35
0 S and 39°45
0 S (dredge station SO63DS)
where detailed studies were conducted. The northern segment (north of 38°10
0 S, at
37°50
0 S on the ridge) is of particular interest since it represents the area where the
hotspot of the Foundation Volcanic Chain meets the spreading PAR axis. Deeptowed camera stations (OFOS and SCAMPI), dredging and TVG (television-grab)
stations were conducted and revealed the presence of numerous active and inactive
hydrothermal fields along the northern (north of 38°20
0 N) ridge segment. Recently
erupted lava flow was detected with the deep towed camera station (OFOS) in a
small graben located at the top of a dome shaped feature near dredge station 86DS
(37°39
0 S) made by the R.V. SONNE in 1995 (Leg 100). This recent volcanism was
revealed when comparing previous (N.O. L’ATALANTE) deep towed television
(SCAMPI # 2) coverage carried out in February 1997 in the same graben, which
showed hydrothermal deposits. Returning to the same spot during R.V. SONNE’s
Leg 157 in 2001, we could not find the same features. The hydrothermal material
was buried by newly erupted silicic lava. Silicic lavas (andesite and dacite)
associated with sulfide deposits were recovered from the axial grabens as well as
from other topographic highs forming at least four volcanic domes along the
various segments. This was a major discovery because silicic domed shaped
features, although common in subaerial silicic volcanic events, were never seen to
occur on the seafloor at depths of more than 2,000 m. The silica-rich lava consisting of andesite and dacite occurred at 37°39
0 S (SO100-86DS), at 37°50
0 S, at
38°05
0 S (station SO100-91DS), near 39°35
0 S and near 39°45
0 S. These silicic lavas
are associated with basaltic tubes and pillow lavas, which have covered the lower
slopes of the ‘‘domes’’ (see Chap. 5). The silicic ‘‘domes’’ are narrow, about 3 km
long and 1 km wide, and are gouged by small summit depressions or grabens that
are a few tens of meters deep, at depths of 2,100–2,200 m. Silicic lavas are highly
viscous and they give rise to explosive volcanism.
Further to the south of latitude 39°35
0 S, the ridge segment shows a smooth
topography with an almost constant depth of about 2,150 m and a continued and
gentle slope deepening southwards, reaching about 2,300 m depth near 39°56
0 S
(Fig. 7.33). This segment-end near 40°S consists of small discontinuities (DEVAL)
marked by a depression (2,300 m) between the two incipient overlapping segments.
Thus, in the vicinities of this DEVAL tip near 39°56
0 S, the general pattern of the
axial orientation is changing more to a northerly (005°N) direction. At least four
Non-Transform-Offsets (NTO) propagating ridge segments are observed along this
nearly N–S oriented ridge segment. The segment-ends of the NTO occur at 39°56
0 S,
40°14
0 S, 40°37
0 S, 40°50
0 S and 41°13
0 and show an increase of intervening depths
from 2,300 m to 2,600 m between the segment-ends southwards, before reaching
the southern overlapping spreading center (OSC) near 41°13
0 S. The segments
migrate southwards and show a general deepening of the topography. Also, from
the nature of the collected samples (degree of rock alteration and manganese
hydroxide coating), it was inferred that the volcanic activity of the northern ridge
segment had decreased concomitantly southwards towards the OSC. Thus the
248
7 Oceanic Spreading Ridges and Sea Floor Creation