How many Pacific hotspots are fed by deep-mantle plumes?
Valérie Clouard*
Jeune Equipe Terre-Océan, Université de la Polynésie Française, B.P. 6570, Faaa, French Polynesia
Alain Bonneville*
UMR Géosciences Marines, Institut de Physique du Globe, 4 place Jussieu, 75005 Paris, France
ABSTRACT
The Pacific plate is usually considered to host 14 hotspots, but most of this volcanism
does not seem to originate from deep-mantle plumes. To reach this conclusion, we tried
to establish how many of the seamount alignments on the Pacific plate correspond to
classic hotspots, i.e., long-lived hotspots linked to oceanic basaltic plateaus. We retraced
the tracks of the 14 Pacific hotspots by using (1) the absolute stage poles representing the
Pacific plate absolute motion since 145 Ma, (2) an updated compilation of radiometric
ages of seamounts and oceanic plateaus totaling 266 seamounts or islands, (3) the detailed
bathymetry of the Pacific Ocean, and (4) the present locations of the hotspots. This analysis
allowed us to correlate only three hotspots with the beginning of their tracks possibly
corresponding in space and time to an oceanic plateau: Easter to the eastern Mid-Pacific
Mountains, Louisville to the Ontong Java plateau, and, with less confidence, Marquesas
to Hess Rise and Shatsky Ridge. In addition, the Hawaii hotspot has produced long-lived
volcanism. These four are the only classic hotspots on the Pacific plate. However, seven
hotspots present short tracks (,35 m.y.) that cannot be traced to an oceanic plateau and
thus are not related to any deep-mantle phenomena: Foundation, Macdonald, Pitcairn,
Rarotonga, Rurutu, Samoa, and Society. The two northeast Pacific hotspots, Kodiak-Bowie
and Cobb, and the Caroline hotspot are unclassifiable because of close proximity to a
subduction zone where the prior history of volcanism has been lost.
Keywords: Pacific plate, mantle plume, hotspot track, oceanic plateau.
INTRODUCTION
Along with the fracture zones, transform
faults, and swells, voluminous oceanic plateaus and linear volcanic chains are the main
morphological features of interior of the Pacific plate (Figs. 1 and 2). Oceanic plateaus
and oceanic flood basalts are thick layers of
basalt generated near or at oceanic spreading
centers, usually triple junctions, but not due to
normal seafloor spreading (Coffin and Eldholm, 1994). They were formed by extensive
and voluminous volcanism over a short period
of time (Richards et al., 1989). Their chemical
characteristics are those of enriched midoceanic ridge basalt, and their isotopic analysis shows oceanic-island affinities (Mahoney,
1987). Both oceanic plateaus and intraplate
volcanic chains have been related to hotspot
theory (Morgan, 1971; Richards et al., 1989).
Morgan (1971) first thought these flood basalts to be the result of the arrival of a mantleplume head under the lithosphere. Several experimental or numerical models have
developed this idea (e.g., Griffiths and Campbell, 1990). These authors assumed that the
source of the plume is at the thermal, possibly
chemical, boundary layer, the ‘‘D’’ layer just
above the core-mantle boundary. As the
plume rises, it develops a large head by en*E-mail addresses: Clouard—[email protected];
Bonneville—[email protected].
trainment of the surrounding mantle; the head
can expand to more than 1000 km in diameter
after impingement against the lithosphere.
Then, over a short period of time, the plume
head produces extensive volcanism, and the
plume tail subsequently continues to produce
linear volcanic chains (McDougall and Duncan, 1980) for millions of years as the plate
drifts over the fixed plume tail.
On the Pacific plate, between 308S and
508N, 10 oceanic plateaus have been recognized so far, and 14 hotspots are usually identified (Fig. 1). Our goal was to check how
many of these hotspots are linked to old floodbasalt features or long-lived trails and thus are
plume-fed classic hotspots. For this, we have
reconstructed the different hotspot tracks by
using four updated data sources: a detailed
bathymetric map of the Pacific Ocean, the
ages of seamounts and oceanic plateaus, the
known active hotspot locations, and the stage
poles for plate motion in the hotspot reference
frame.
DATA AND METHOD
Linear volcanic chains, parallel to the absolute motion of the plate when they were created, are supposed to be the trace of a hotspot
on the seafloor. To identify such seamount
alignments, we first needed a complete knowledge of the seafloor topography. We used the
compilation of Smith and Sandwell (1994)
that allows detection of any seamount with a
diameter greater than 15 km. Then the strongest constraint to link a seamount to a presumed hotspot is its age. We have built our
seamount age compilation (Clouard and Bonneville, 2000) from numerous papers, and the
corresponding database, including all the references, is available on line at http://
www.upf.pf/geos/data/agesppacific.pdf. Paleomagnetic ages and foraminifera dates have not
been considered. Only radiometric ages were
thus used, and Ar/Ar ages were preferred over
K/Ar ages, when both exist at the same place.
When several ages separated by less than 2
m.y. exist for the same seamount or island, we
assigned the average age to the feature. The
data set as finalized is composed of 266 ages
on seamounts, islands, and plateaus (Fig. 1);
the oldest feature is 138 Ma. Despite this apparently large-sounding number, age data are
actually very sparse over the Pacific plate and
obviously concentrated on a few seamount
alignments. The present locations of the 14
hotspots usually considered on the Pacific
plate are reported in Table 1 with the age of
their most recent volcanic activity. Hawaii,
Society, Macdonald, and Pitcairn are active,
and there are in situ observations of their volcanism. Historic underwater activity has happened in Ta’u, Samoan Islands. All the other
q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400.
Geology; August 2001; v. 29; no. 8; p. 695–698; 2 figures; 1 table.
695
Figure 1. Locations and ages of seamounts and islands on bathymetric map (equal-area
Hammer projection) of Pacific plate (Smith and Sandwell, 1994). Names of 14 hotspots usually considered on Pacific plate are in white. Present hotspot locations are indicated by
white disks; black star inside disk indicates recent observed activity. Black and white numbers are ages (in Ma) of volcanic features considered in this study. Flood-basalt plateau
names are given in black italics in Figure 2.
hotspots—Caroline, Cobb, Kodiak-Bowie,
Easter, Louisville, Foundation, Rarotonga, Rurutu, and Marquesas—have shown activity
within the past million years. We inferred their
zero-age location by classical backtracking
from the last dated volcanic event.
To reconstruct the apparent path followed
by a hotspot on the seafloor, we have moved
the hotspot’s present location back in time by
using the set of stage poles proposed by Wessel and Kroenke (1997). This data set represents the most updated synthesis for the absolute Pacific plate motion since 145 Ma.
However, for the last interval (0–3 Ma), we
have chosen to use the pole previously proposed by Yan and Kroenke (1993).
HOTSPOT TRACKS
With the chosen set of stage poles in the
hotspot frame of reference, the backtracks
were determined over the past 145 m.y. for
the 14 hotspots (Fig. 2). Our aim was to link
hotspots with oceanic plateaus, but hotspots
whose tracks disappear at a subduction zone
696
usually cannot be so linked. This is the case
for Louisville, Emperor, Kodiak-Bowie, Cobb,
and Caroline hotspots. Because of the unusual
geometry of the Tonga Trench, the inferred
extrapolation of the Louisville hotspot track
can be studied before 100 Ma on the Pacific
plate. The track of the Hawaiian hotspot, if it
is assumed to be a continuation of the Emperor Seamount chain, disappears in the Aleutian subduction zone, but because this hotspot
is used as a reference for the determination of
absolute plate motion by virtue of its continuous volcanism since 65 Ma, it was considered as a classic hotspot in the present study.
Thus, only the northeast Pacific hotspots and
Caroline hotspot were not classified. For each
of the 10 remaining hotspots, the track was
compared to the seafloor topography, and the
predicted ages along the track were compared
to the actual radiometric ages of the seamounts. When the shift between predicted and
radiometric ages is less than 5 m.y. for a feature older than 20 Ma, we concluded that the
hotspot generated the feature.
Easter
Easter hotspot is one of the most puzzling
hotspots of the Pacific plate because of its uncertain location. Its name comes from its initial assumed location beneath Easter Island
(Morgan, 1972). Present volcanism appears in
the island’s vicinity on both sides of the East
Pacific Rise, simultaneously and in several locations. Bonatti et al. (1977) first suggested
that rather than numerous hotspots, there was
a mantle hotline, corresponding to mantle convecting rolls. However, O’Connor et al.
(1995), through the use of new Ar/Ar ages,
supported the idea of a hotspot rather than a
hotline on the eastern side of the East Pacific
Rise. To account for the bathymetry from the
Tuamotu Islands to the Easter microplate, we
located the Easter hotspot to the west of the
East Pacific Rise, in agreement with Okal and
Cazenave (1985). The construction of the
track shows that it could have generated the
Tuamotu Islands and part of the Line Islands,
south of the equator. Epp (1984) pointed out
that these chains could be the result of a hotspot interacting with a ridge, west of the East
Pacific Rise. An 8 Ma seamount is a strong
constraint, as well as Cretaceous ages on the
western Tuamotu Islands. The origin of the
track fits with the eastern part of the MidPacific Mountains, which are composed of
guyots overlying a broad plateau; Morgan
(1972) supposed that they were the continuation of the Line-Tuamotu alignment. A connection with the western Easter hotspot leads
to the same conclusion. According to our reconstruction, the Tuamotu Plateau is not the
end of the track. We thus agree with the interpretation of Ito et al. (1995), who suggested
that this plateau is not a typical oceanic
plateau.
Marquesas
The Marquesas hotspot created the Marquesas Islands between 1.3 and 5.8 Ma. A radiometric age of 0.5 Ma was determined for the
south of the archipelago and provides a strong
constraint on the most recent hotspot location.
The predicted trend of the track disagrees with
the observed volcanic alignment direction.
However, if we consider a control by en echelon fractures oblique to the regional trend,
there is no longer any contradiction. However,
the Line Islands between 58 and 158N exhibit
dates that could agree with generation by the
Marquesas hotspot. The link between the
northern end of the Marquesas chain and the
southern end of the Line Islands chain could
therefore correspond to the Marquesas-Line
swell described by Crough and Jarrard (1981).
From north of the Hawaiian chain to north of
Hess Ridge, the track fits with the Liliuokalani
Ridge, a clear but nondated linear chain. We
thus assumed that this hotspot produced the
GEOLOGY, August 2001
Louisville trail could reappear on the other
side of the Indo-Australian plate, if the Louisville hotspot is that old. There is a seamount
chain north of the Tonga Trench, but no ages
are known. At 125 Ma, the track is close to
the southeast edge of the Ontong Java Plateau.
Isotopic evidence suggests a plume-initiation
model to account for the Ontong Java Plateau
formation (Mahoney and Spencer, 1991), and
the duration of its volcanism is supposed to
be 3 m.y., between 124 and 121 Ma (Tarduno
et al., 1991). However, as Mahoney and Spencer (1991) pointed out, both temporal variations in the isotopic compositions of the lavas
and a southward motion of the Louisville hotspot are needed to connect Louisville with the
Ontong Java Plateau. With this restriction, the
Louisville hotspot and the Ontong Java Plateau could be linked, although a connection
with the Eltanin fracture zone cannot be completely put aside.
Figure 2. Hotspot tracks on Pacific plate. Tracks are represented by continuous gray lines
and have been computed for 0–145 Ma. Dashed lines indicate that no corresponding topographic features have been found on seafloor and thus track segment is questionable. Lines
are not drawn before estimated beginning of hotspot life. Changes in gray along track (legend in upper inset) correspond to successive stage poles used for Pacific plate motion.
Symbols represent dated seamounts and islands.
Hess Ridge ca. 100 Ma and the Shatsky Ridge
between 145 and 125 Ma. From the estimation
of its eruptive rate and magnetic anomalies,
Sager and Han (1993) inferred a mantle-plume
origin for Shatsky Ridge. The Marquesas hotspot could have generated these plateaus.
Louisville
The Louisville chain is, with the HawaiianEmperor chain, the volcanic alignment most
often used to determine the Pacific plate absolute-motion stage poles in the hotspot ref-
erence frame from 66 to 12.5 Ma. Consequently, the track exactly fits the hotspot trail
from 12.5 Ma to the Kermadec Trench. However, the exact location of the active hotspot,
if any, is still a matter of debate (Wessel and
Kroenke, 1997; Géli et al., 1998). With the
pole we used for the recent period, the hotspot
is located near a 0.5 Ma seamount, which is
in good agreement with the interpretation of
Géli et al. (1998). The track that was formed
before 66 Ma has disappeared into the Kermadec Trench. If the Pacific plate is rigid, the
TABLE 1. PACIFIC PLATE HOTSPOTS
Name
Recent
activity (Ma)
Long.
(8E)
Caroline
Cobb
Easter
Foundation
Hawaii
Kodiak-Bowie
Louisville
Marquesas
1.2
0.12
8.0
,1
0
0.02
0.5 or 12.5
0.5
164.0
230.0
245.0
245.0
204.7
225.0
221.0
223.0
5.0
46.0
26.5
36.0
18.9
53.0
50.8
11.2
N
S
S
S
N
N
S
S
0
0
0.2
0
0
1.1
219.7
230.7
208.7
190.5
211.5
201.0
29.0
25.3
23.5
14.3
18.2
21.5
S
S
S
S
S
S
Macdonald
Pitcairn
Rurutu
Samoa
Society
Rarotonga
GEOLOGY, August 2001
Lat.
(8)
Beginning of the activity or
associated oceanic plateau if any
Track .30 Ma subducted
Track .30 Ma subducted
East Mid-Pacific Mountains at 140 Ma
30 Ma at least
Track .75 Ma subducted
Track .20 Ma subducted
East Ontong Java Plateau at 125 Ma
Hess Rise at 110 Ma, Shatsky Ridge
at 145 Ma
19 Ma
Maximum 10 Ma
East Mariana basin at 117 Ma?
14 Ma
4.5 Ma
1.1 Ma
Other Hotspots
All other hotspot tracks considered in our
study vanish after a short period of activity.
For the Society hotspot, there is no evidence
of any volcanic activity prior to that of the
Society Islands. Maupiti is the oldest island
(4.3 Ma), and there is no young oceanic plateau in the vicinity. The Macdonald hotspot
can be backtracked to 19 Ma on the western
side of the Cook-Austral alignment. The socalled Rarotonga hotspot seems to have produced only the Rarotonga island, and the Samoa hotspot trail seems to have vanished after
the Samoan Islands were formed, prior to 14
Ma. Turner and Jarrard (1982) postulated that
the Rurutu hotspot agrees with ages and locations of some of the Austral Islands. Its present location has been determined by a 0.2 Ma
age on a seamount south of Rurutu Island
(Bonneville et al., 2000). Its track is clear over
the past 8 m.y. and reaches the East Mariana
basin, which is covered by a 400-m-thick basalt layer. However, because there is just one
dated volcanic feature between 8 and 117 Ma
along the predicted track of the Rurutu hotspot, we considered it as a short-lived hotspot.
Pitcairn hotspot is an active hotspot responsible for the Pitcairn-Gambier-Moruroa alignment, but it is difficult to extrapolate to older
ages. Because paleopositions of Mid-Pacific
Mountain seamounts are close to the Pitcairn
hotspot, they have been previously linked together (Winterer et al., 1993). However, our
reconstruction indicates a gap of more than 90
m.y. We thus considered Pitcairn as a shortlived hotspot. The Foundation hotspot is a
near-ridge volcanic source of unknown location. From Foundation seamounts, dated between 5 and 21 Ma, its track can be followed
north of the Austral Islands along the 30 Ma
Ngatemato chain. The beginning of the track
697
could be fitted with the Magellan Rise, which
is the smallest Pacific plateau, formed ca. 135
Ma (Tamaki and Larson, 1988). However,
more data from between 30 Ma and 135 Ma
are needed to confirm this hypothesis.
All the tracks of these atypical short-lived
hotspots pass through French Polynesia,
which also corresponds to the South Pacific
superswell. Those hotspots induced a complex
overprinting volcanism and numerous trends
of seamount and island chains since 40 Ma.
However, these trends agree with the absolute
motions of the Pacific plate, and these volcanic sources, like plume-fed hotspots, seem
also to be fixed relative to plate motions, at
least for the time scale considered. Except for
the Foundation hotspot, all of them are located
far from the East Pacific Rise, and all characterized by short-lived volcanism.
Another example of short-lived intraplate
volcanism could be found in the northwestern
Pacific, where there are 120–80 Ma seamounts
on older crust. From gravity and bathymetric
data, Winterer et al. (1993) proposed that
these seamounts were formed on an earlier superswell, the Darwin Rise. The paleolocation
of the Darwin Rise fits well with the present
location of the South Pacific superswell, and
northern Pacific seamounts are often thought
to have originated from French Polynesian
hotspots. Our reconstruction shows a minimum 80 m.y. volcanic gap in the hotspot
tracks. Hence we suppose that these seamounts are not related to any present hotspots,
but are merely due to past, short-lived
hotspots.
CONCLUSIONS
If we restrict the term plume-fed hotspots
to long-lived hotspots and those that generated
flood-basalt plateaus, then with the very conservative hypotheses we used, only three or,
with less confidence, four of the traditionally
admitted Pacific hotspots belong to this category. Hawaii presents evidence of long-lived
volcanism. The Easter hotspot could be linked
to the east part of the Mid-Pacific Mountains,
and the Louisville hotspot could be linked to
the Ontong Java Plateau. The Marquesas hotspot is the most questionable, and could have
produced Hess Ridge at 110 Ma and Shatsky
Ridge at 140 Ma. Seven other hotspots simply
do not show evidence of a continuous volcanic trend between basaltic plateaus and the
present hotspot locations.
As pointed out by McNutt et al. (1997) and
Dickinson (1998), it seems reasonable to think
that most of the volcanic alignments belonging to the South Pacific superswell region are
in fact diffuse and secondary volcanism. De-
698
spite their apparent fixedness, it seems unlikely that they were related to any deep-mantle
phenomena. Therefore, deep-mantle plumes
are unnecessary to explain most, if any, volcanic chains on the Pacific plate, as proposed
with other arguments by Anderson (1998) in
a global study.
ACKNOWLEDGMENTS
We sincerely thank Marcia McNutt and Don Anderson for their careful review. This paper benefited
from the constructive remarks of Anne Davaille,
Paul Wessel, Norman Sleep, and Richard Von Herzen. V. Clouard was supported by a grant from the
ZEPOLYF program, funded by the French State and
the French Polynesian government.
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Printed in USA
GEOLOGY, August 2001