southern segment moving away from the influence of the Foundation Volcanic
Chain hotspot could become a ‘‘normal spreading center’’.
The last area surveyed during Leg 157 of the R.V. SONNE was the southern
OSC region near 41°35
0 S, which shows an overlap basin (depression) of about
2,800 m separating the two ridge segment propagators (Fig. 7.33). In this region,
the eastern and the western ends of the two major spreading ridge segments north
and south of the 41°35
0 S OSC are en echelon and curving towards each other with
small offsets less than 2 km wide. A tall seamount about 2,210 m in height rises
above the seafloor at 2,420 m deep, and it is about 200 m shallower than the
surrounding topography of the ridge propagator’s tips. The seamount is asymmetrical in shape. Since the base on the eastern (2,600 m) flank propagator is
shallower than the one to the west (2,820 m), it is believed that magma was
channeled from the eastern end (tip) of the OSC propagator to construct the
seamount.
Interaction between off-axis hotspot and spreading ridge
magmatism
As mentioned in another chapter (see Chap. 9), upwelling of magmatic flows
could be either transported laterally at the lithosphere-asthenosphere boundary or
come close to the sea floor underneath spreading ridge segments as well between
diverging plates giving rise to hotspot types of activity.
Two main hypotheses are proposed to explain the volcanic activities giving rise
to the off-axis volcanoes in the vicinities and on the axis of the PAR:
1. The first hypothesis implies that two separate mantle upwelling zones occur:
one deep seated mantle plume underneath the lithosphere ([70 km depths)
giving rise to the hotspot volcanoes, and a second shallower upwelling magmatic zone within the lithosphere (\70 km depth) giving rise to spreading ridge
lava. In this case, the mixing between the two types of magma will give a
hybrid composition during subcrustal magma channeling. This could explain
the compositional variation of the lava erupted such as the alkali enriched lavas
found at a distance from the spreading ridge and the MORB type of lava
erupted on the ridge axis. An interaction between the mantle plume of the
intraplate region and the spreading ridge system itself has been suggested by
several authors including Zhang and Tanimoto (1992), Schilling (1991), Hekinian et al. (1999). For example, it was calculated (Ito and Lin 1995) that
excess mantle temperature anomalies of 50–250 °C could influence hotspotridge interaction, up to a maximum distance of 500 km. Thus the Foundation
Volcanic Chain magmatism, that gives rise to seamounts, has interacted with an
ancient (20–23 Ma) Failed Rift Propagator of the Selkirk microplate and the
more recent (\5 Ma) ridge type magmatism to produce the off-axis linear
The Pacific–Antarctic Ridge at 36–42°S
249
Chain hotspot could become a ‘‘normal spreading center’’.
The last area surveyed during Leg 157 of the R.V. SONNE was the southern
OSC region near 41°35
0 S, which shows an overlap basin (depression) of about
2,800 m separating the two ridge segment propagators (Fig. 7.33). In this region,
the eastern and the western ends of the two major spreading ridge segments north
and south of the 41°35
0 S OSC are en echelon and curving towards each other with
small offsets less than 2 km wide. A tall seamount about 2,210 m in height rises
above the seafloor at 2,420 m deep, and it is about 200 m shallower than the
surrounding topography of the ridge propagator’s tips. The seamount is asymmetrical in shape. Since the base on the eastern (2,600 m) flank propagator is
shallower than the one to the west (2,820 m), it is believed that magma was
channeled from the eastern end (tip) of the OSC propagator to construct the
seamount.
Interaction between off-axis hotspot and spreading ridge
magmatism
As mentioned in another chapter (see Chap. 9), upwelling of magmatic flows
could be either transported laterally at the lithosphere-asthenosphere boundary or
come close to the sea floor underneath spreading ridge segments as well between
diverging plates giving rise to hotspot types of activity.
Two main hypotheses are proposed to explain the volcanic activities giving rise
to the off-axis volcanoes in the vicinities and on the axis of the PAR:
1. The first hypothesis implies that two separate mantle upwelling zones occur:
one deep seated mantle plume underneath the lithosphere ([70 km depths)
giving rise to the hotspot volcanoes, and a second shallower upwelling magmatic zone within the lithosphere (\70 km depth) giving rise to spreading ridge
lava. In this case, the mixing between the two types of magma will give a
hybrid composition during subcrustal magma channeling. This could explain
the compositional variation of the lava erupted such as the alkali enriched lavas
found at a distance from the spreading ridge and the MORB type of lava
erupted on the ridge axis. An interaction between the mantle plume of the
intraplate region and the spreading ridge system itself has been suggested by
several authors including Zhang and Tanimoto (1992), Schilling (1991), Hekinian et al. (1999). For example, it was calculated (Ito and Lin 1995) that
excess mantle temperature anomalies of 50–250 °C could influence hotspotridge interaction, up to a maximum distance of 500 km. Thus the Foundation
Volcanic Chain magmatism, that gives rise to seamounts, has interacted with an
ancient (20–23 Ma) Failed Rift Propagator of the Selkirk microplate and the
more recent (\5 Ma) ridge type magmatism to produce the off-axis linear
The Pacific–Antarctic Ridge at 36–42°S
249