through preferential pathways. This displaced magma could form seamounts at
some distance from the axis, which can be found on the flanks (\500 km) and/or
away ([2,000 km) from ridge axis in deep-seated basins ([4,000 m deep). Offaxis seamounts could be formed during axial and/or off-axial volcanism or in deepsea basins within the rigid plates during hotspot activities (See Chap. 9). Evidence
of volcanic activity at a distance away from accreting plate boundary regions,
which are not true intraplate areas, but are rather situated on the flank of ridge
segments, are found along the flanks of the EPR. These off-axial structures differ
from volcanic hotspot chains such as the Society, Austral, Pitcairn and the
Hawaiian volcanoes by their morphology, structure and geological settings. They
do not necessarily form lines of volcanoes, but rather they form isolated edifices as
well as short (\100 km in length) small, ridge segments. They also often differ
from intraplate structures by the presence of a crater and/or caldera at their
summit, which are attributed to the collapse of their magmatic reservoirs when
they have been carried away from the ridge axial-magma supply zone.
The distribution of volcanic edifices across the various segments of the East
Pacific Rise is often related to axial discontinuities (Macdonald 1989). They are
either found in the prolongations of small transforms and/or on overlapping
spreading centers (OSC). They are interpreted as being the remains of volcanic
edifices formed at the tips of accreting ridge segments and which are then moved
away during spreading (Macdonald 1989). Isolated volcanic edifices, which are
found at less than 50 km away from the ridge axis, are formed from axial magmatism lasting for several hundreds of thousand years. Many off-axis seamounts
are generated near the ridge axis and have the same age as the crust (Batiza 1981;
Hekinian et al. 1983) on which they are formed. The rate of seamount formation
decreases with the age of the crust, due to the decrease of magmatic production
(Batiza 1989).
Four off-axis volcanic structures discovered near 12°50
0 N and 11°26
0 N were
investigated between 1980 and 1984 during the three cruises known as ‘‘Clipperton’’, ‘‘Geocyatherm’’ and ‘‘Geocyarise’’ with the surface ships N.O. JEAN
CHARCOT, N.O. LE SUROIT and N.O. NADIR (Figs. 7.15, 7.22). Two off-axis
volcanic seamounts are located on the eastern flank at 12°43
0 N (the Southeastern
Smt) and near 12°50
0 N (Northeastern Smt) and the third is near 11°30
0 N on the
western flank (Western Smt) of the EPR axis. Both eastern seamounts are less than
7 km from the axial graben (Figs. 7.15, 7.16). These two volcanic edifices are
structurally similar; they are both relatively shallow in relief (\350 m in height),
have about the same basal diameter (6–7 km in diameter), and both terminate with
peaks composed essentially of hydrothermal deposits. No central craters were
observed and their southern flanks are characterized by the occurrence of volcanic
aprons resembling funnel shaped protuberances extending down the summit for a
distance of about 1 km. This type of apron structure, also observed on the summits
of some Northwest Pacific Basin seamounts, was interpreted as being due to flank
eruptions injected laterally from shallow magmatic chambers (Vogt and Smoot
1984).
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7 Oceanic Spreading Ridges and Sea Floor Creation
some distance from the axis, which can be found on the flanks (\500 km) and/or
away ([2,000 km) from ridge axis in deep-seated basins ([4,000 m deep). Offaxis seamounts could be formed during axial and/or off-axial volcanism or in deepsea basins within the rigid plates during hotspot activities (See Chap. 9). Evidence
of volcanic activity at a distance away from accreting plate boundary regions,
which are not true intraplate areas, but are rather situated on the flank of ridge
segments, are found along the flanks of the EPR. These off-axial structures differ
from volcanic hotspot chains such as the Society, Austral, Pitcairn and the
Hawaiian volcanoes by their morphology, structure and geological settings. They
do not necessarily form lines of volcanoes, but rather they form isolated edifices as
well as short (\100 km in length) small, ridge segments. They also often differ
from intraplate structures by the presence of a crater and/or caldera at their
summit, which are attributed to the collapse of their magmatic reservoirs when
they have been carried away from the ridge axial-magma supply zone.
The distribution of volcanic edifices across the various segments of the East
Pacific Rise is often related to axial discontinuities (Macdonald 1989). They are
either found in the prolongations of small transforms and/or on overlapping
spreading centers (OSC). They are interpreted as being the remains of volcanic
edifices formed at the tips of accreting ridge segments and which are then moved
away during spreading (Macdonald 1989). Isolated volcanic edifices, which are
found at less than 50 km away from the ridge axis, are formed from axial magmatism lasting for several hundreds of thousand years. Many off-axis seamounts
are generated near the ridge axis and have the same age as the crust (Batiza 1981;
Hekinian et al. 1983) on which they are formed. The rate of seamount formation
decreases with the age of the crust, due to the decrease of magmatic production
(Batiza 1989).
Four off-axis volcanic structures discovered near 12°50
0 N and 11°26
0 N were
investigated between 1980 and 1984 during the three cruises known as ‘‘Clipperton’’, ‘‘Geocyatherm’’ and ‘‘Geocyarise’’ with the surface ships N.O. JEAN
CHARCOT, N.O. LE SUROIT and N.O. NADIR (Figs. 7.15, 7.22). Two off-axis
volcanic seamounts are located on the eastern flank at 12°43
0 N (the Southeastern
Smt) and near 12°50
0 N (Northeastern Smt) and the third is near 11°30
0 N on the
western flank (Western Smt) of the EPR axis. Both eastern seamounts are less than
7 km from the axial graben (Figs. 7.15, 7.16). These two volcanic edifices are
structurally similar; they are both relatively shallow in relief (\350 m in height),
have about the same basal diameter (6–7 km in diameter), and both terminate with
peaks composed essentially of hydrothermal deposits. No central craters were
observed and their southern flanks are characterized by the occurrence of volcanic
aprons resembling funnel shaped protuberances extending down the summit for a
distance of about 1 km. This type of apron structure, also observed on the summits
of some Northwest Pacific Basin seamounts, was interpreted as being due to flank
eruptions injected laterally from shallow magmatic chambers (Vogt and Smoot
1984).
224
7 Oceanic Spreading Ridges and Sea Floor Creation