Geophys. J. R. astr. Soc. (1982) 71, 333-362
The distribution of intraplate volcanism in the Pacific
Ocean basin: a spectral approach
N. M.
R i b e * D e p a r t m e n t of Geophysical Sciences, The University of Chicago,
Chicago, Illinois 6063 7, USA
A. B . Watts
Lamont-Doherty Geological Observatory and Department of
Geological Sciences of Columbia University, Palisades, New York 10964, USA
Received 1982 January 30; in original form 1981 April 16
Summary. Although the major linear island chains in the Pacific Ocean basin
have been extensively studied, relatively little attention has been given to the
origin of the numerous seamounts and oceanic islands that are concentrated
in the western Pacific. Spectral analysis of shipboard gravity and bathymetry
data provides a means for statistically estimating the distribution of this
volcanism in space and time. Estimates of the power spectrum of the bathymetry FBand the average gravitational admittance z"(cross-spectrum correlation of gravity and bathymetry) were obtained as functions of age of the
oceanic lithosphere by dividing the Pacific plate into eight age regions A-H.
The power spectrum of the bathymetry is indicative of the overall bathymetric roughness (primarily volcanic) in each region. This roughness shows
three gradations: regions A-D (< 80 Myr) have few volcanic features and are
relatively smooth; regions E (80-100Myr) and G (120-140Myr) are of intermediate roughness; and regions F (100-120Myr) and H (> 140Myr), with
many large volcanic features, are very rough. The average gravitational
admittance is indicative of the degree of isostatic compensation of bathymetric features in each region and can be used to estimate the proportionh
of total bathymetric power that was emplaced on relatively old lithosphere
and the proportion that formed on young lithosphere at or near a mid-ocean
ridge crest. The highest proportion of bathymetry that formed on old lithosphere occurs in region H (fI- 0.90) and the lowest proportion occurs in
regions G and E (fI 0.40-0.60). The power spectrum of the bathymetry
and the gravitational admittance allow an estimate of the total bathymetric
power PI= ~ I P accounted
B
for by intraplate volcanism ('intraplate power')
and the power PR = (1 - fI)PBwhich is of on-ridge origin ('ridge power'). At
long wavelengths in regions E-G, the amount of on-ridge volcanism exceeds
the amount of intraplate volcanism (PR > PI), indicating the presence of
several large plateaus and rises which formed at ridge crests. This suggests the
-
*Present address: Lamont-Doherty Geological Observatory, Palisades, New York 10964, USA.
334
N. M. Ribe and A. B. Watts
occurrence of a major outpouring of on-ridge volcanism during the Cretaceous
at or near the Pacific/Farallon accreting plate boundary. At intermediate
wavelengths in regions F and H, the amount of intraplate volcanism greatly
exceeds the amount of on-ridge volcanism (PIB PR), suggesting that the
numerous seamounts in these regions are largely of intraplate origin. The age
of this intraplate volcanism cannot be accurately estimated but available
evidence from DSDP sites and geological samples suggest that it may at least
in part be contemporaneous with the Cretaceous on-ridges event.
Introduction
The Pacific Ocean basin and its margins are characterized by a great diversity of volcanic
features. Most of the active volcanism is associated with the boundaries of the Pacific plate:
magmas upwell passively at the East Pacific Rise to form basaltic oceanic crust and are
generated in subduction zones where they give rise to island-arc volcanism. Many of the
largest volcanic features in the Pacific Ocean, however, are located far from plate boundaries,
indicating that they were formed by magma penetration through the lithosphere. A number
of these features (for example, the Hawaiian-Emperor and the Kodiak-Bowie chains) are
linear chains of oceanic islands and seamounts whose ages increase along the chains.
The two main hypotheses proposed to explain intraplate volcanism (Turcotte & Oxburgh
1978) are the ‘hot-spot’ and propagating fracture hypotheses. The former envisions a hot
spot, fmed deep in the mantle, which supplies magmas to the base of the moving lithospheric
plate. The latter proposes that magmas penetrate the plate through fractures caused by
tensile failure of the lithosphere along pre-existing zones of weakness. These models have
been successfully applied to a number of linear seamount chains in the Pacific such as the
Hawaiian-Emperor, Kodiak-Bowie, Society and Caroline chains (for example, Jarrard &
Clague 1977).
The Pacific Ocean basin is known, however, from geological and geophysical studies (for
example, Hess 1946; Menard 1964; Chase, Menard & Mammerickx 1971; Heezen et al. 1973)
to be characterized by numerous bathymetric features of mainly volcanic origin, many of
which are isolated seamounts rather than linear chains. The greatest concentration of these
features occurs on the oldest portions of the Pacific plate, east of the Bonin and Manana
trenches. Their ages and tectonic settings are generally unknown. The apparent ‘western
intensification’ of this volcanism is probably not due simply to the greater age of the western
part of the plate, since the density of seamounts increases abruptly across the 80Myr
isochron. The origin (age) of these seamounts and their pronounced ‘western intensification’
- in short, the spatio-temporal distribution of Pacific volcanism - is of interest for two
reasons. First, this distribution will indicate changes in the rate of volcanic production
through geological time, and may help to determine whether such changes are correlated
with, for example, seafloor spreading rate and volcanism on the plate margins such as in
western North America. Second, the distribution of seamounts in the Pacific may place constraints on dynamical models of intraplate volcanism. Since volcanism is a surface expression
of processes (melting, magma ascent) taking place in the Earth‘s mantle, a definite pattern in
the surface distribution of volcanoes (such as the ‘western intensification’ noted above) may
indicate important features of such dynamical phenomena as mantle convection.
Determination of the age of volcanism in the Pacific Ocean is complicated by the general
lack of geological data from individual features in the basin. Only a few geological samples
have been obtained from the main seamount provinces (Geisha, Magellan, Mid-Pacific,
Musicians, Line Islands) and only a small number of these have been reliably dated (for
Intraplate volcanism in the Pacific Ocean basin
335
example, Heezen et al. 1973). The existing geological studies suggest that a seamount
province may contain volcanic features of widely different ages, some of which may have
been formed near ridge crests and hence should not be considered ‘intraplate’ despite their
present location. Hence, an important first step in understanding intraplate volcanism is to
distinguish features formed in an intraplate setting from those produced on or near ridge
crests.
Recently, Watts, Bodine & Ribe (1980) outlined a method to estimate the tectonic
setting of seamounts and oceanic islands using shipboard free-air gravity anomaly and
bathymetry data. The emplacement of volcanic loads on the lithosphere causes it to deform
over a broad region. The redistribution of mass associated with loading gives rise to a gravity
anomaly whose form depends primarily on the effective elastic thickness T of the oceanic
lithosphere on which the load was emplaced. Studies in the oceans (Watts et al. 1980) have
shown that T closely follows the depth to the 300-600°C oceanic isotherms based on a
cooling plate model. Loads formed on young lithosphere are associated with relatively small
values of T while loads formed on old lithosphere are associated with high values. Thus since
the measured gravity anomaly over seamounts and oceanic islands depends on T , it is
possible to use the anomaly to estimate their tectonic setting.
Watts et al. (1980) estimated the age of the lithosphere at the time of loading for volcanic
features in the Pacific Ocean by ‘anomaly comparison’. They classified 120 volcanic features
in the Pacific as either on-ridge or intraplate, depending on whether a value of T = 5 or
25 km provided the best visual fit between observed and calculated gravity anomalies. This
approach is useful in that it approximates the ‘ideal‘ result, the determination of the tectonic
setting of each volcanic feature. Its major limitation, however, is its inability to minimize the
effects of noise in the gravity data. Watts et al. (1980) found that even a simple binary classification (as on-ridge or intraplate) could not be made for a large proportion (- 50 per cent)
of the available seamount data, which suggests that these effects may be substantial.
An alternative but complementary approach to the problem is an ‘admittance comparison’
approach (for example, McKenzie & Bowin 1976; McNutt 1979; Detrick &Watts 1979), in
which the gravitational admittance is estimated as a function of age of the lithosphere and
compared with theoretical admittances derived from a mathematical model of plate flexure.
The ensemble averaging used to estimate the admittance reduces the effects of noise substantially and includes all available data (not just the largest features) in the analysis. The admittance comparison approach is limited in that its results do not apply to any given seamount,
but represent an average over many. Hence the results cannot be repartitioned to indicate,
say, the number of volcanoes emplaced on the plate during a given age interval before
present. This limitation is not serious, however. The anomaly comparison approach allows
such repartitioning only for on-ridge features, for which the age of the lithosphere at the
time of loading can be determined within about 5 Myr. For intraplate features, the age of the
lithosphere is less closely constrained as a function of T , and the anomaly comparison
approach provides little or no advantage over the admittance comparison approach.
In this paper we apply the admittance comparison approach to shipboard gravity and
bathymetry data to estimate the distribution of intraplate and on-ridge volcanism as a
function of the age of the Pacific plate. The admittance estimated from the data in each of
several age regions will allow us to estimate the proportion of the bathymetric relief that can
be attributed to intraplate rather than on- or near-ridge volcanism. The total amount of
intraplate and on-ridge volcanism present will then be calculated and compared among age
regions. The purpose of the paper is to better understand the volcanic history and tectonic
evolution of the Pacific Ocean basin, and to determine whether the distribution of intraplate
volcanism there may place constraints on dynamical processes in the mantle.
N. M. Ribe and A . B. Watts
336
Gravitational admittance
The gravitational admittance (McKenzie & Bowin 1976) provides a measure of the degree of
isostatic compensation of loads on the Earth’s surface. The admittance can be understood in
terms of the Fourier transform of measured gravity G and bathymetry B :
G (k) = Z (k)B (k)
where k is the wavevector and Z(k) is the linear transfer function or admittance. Z ( k )
represents the gravity anomaly associated with unit (delta-function) bathymetric load.
The admittance can be estimated from either observed gravity anomaly and bathymetry
data (McKenzie & Bowin 1976) or from theoretical considerations based on different isostatic models. Theoretical expressions for the gravitational admittance are always real, but
admittances estimated from observed data generally have non-zero phase due to the effects
of noise. For the long wavelengths of interest in this study, however, this phase is close to
zero and may be ignored. Henceforth the notation Z will refer to the amplitude of the
admittance. Estimates obtained from observed data, and expressions (‘estimators’) used to
obtain such estimates, will be denoted by a tilde (e.g. z“).
The gravitational admittance for the elastic plate model of isostasy was derived by
McKenzie & Bowin (1976) using a full linear theory, and modified to reflect a more realistic
crustal structure by Watts (1978). The form of the admittance used in this study is
Z(k) = 2nGk‘b
X
-
P W ) exp (- Ik I d ) (1 - [exp (- Ik I t z ) ( P 3
(Pm -P3>I/[(Pm
-
- Pb)
+ exp (-lk
I t)
(2.1)
Pb) + 4 ( P m - ~ w ) r n ’ ~ A B - ’ l J
where G is the gravitational constant; t2 is the thickness of crustal layer 2, and t the mean
crustal thickness; pw, Pb, p 3 and pm are the density of seawater, crustal layer 2, layer 3 and
the upper mantle, respectively;M=6E/[gT(pm -pw)] where E is Young’s modulus and the
plate thickness is T; k‘ = TI k I /2 = T ( k z +
A = [(sinh 2k’)/2k’I2- 1 ; and B =
[(sinh 4k’)/4k’] + 1. A schematic diagram of the system described by (2.1) is shown in
Fig. 1, and values of the model parameters are listed in Table 3. The admittance (2.1) repre-
1
+I
7
t
I
elastic plate
(upper mantle)
-
r’n
It
A=2Tk-
Figure 1. Model system described by the admittance (2.1). A gravity anomaly of wavenumber k
arises from mass anomalies at z = - d (the bathymetry), z = - (d + t ) (the crustal layer 2-layer 3 interface) and z = - (d + t ) (the crust-mantle interface). The mean ocean depth is d ; t and t , are the
thicknesses of the crust and crustal layer 2 ; PW. p b , p 3 and pm are the densities of seawater, crustal layer
2, layer 3 and the upper mantle, respectively, and T is the effective elastic thickness of the lithosphere.
Values of these parameters are listed in Table 3.
Intraplate volcanism in the Pacific Ocean basin
I
500
I
200
I
100
I
50
I
25
337
A km
Figure 2. Gravitational admittance (2.1) for several values of the plate thickness 7‘.The admittance for
uncompensated topography (2.2) corresponds to T-t m. For values of T (5 km < T < 30 km) characteristic
of oceanic lithosphere, significant variation of the admittance as a function of T occurs in the ‘compensated waveband’. The admittance estimates of Fig. 4 fa-h) are typically reliable (coherence > 0.75) in the
‘coherent waveband’; useful information on the compensation mechanism (plate thickness) may be
extracted from admittance estimates in the ‘diagnostic waveband’.
sents the sum of contributions from mass anomalies at depths z = - d (the topography),
z = - (d + t z ) (the layer 2-layer 3 interface) and z = - (d + t ) (the crust-mantle interface).
The admittance (2.1) is shown for several values of the plate thickness T in Fig. 2. The
values T = 5 and 25 km are characteristic effective elastic thicknesses of relatively young and
old lithosphere, respectively (Cochran 1979; Watts 1978). Alternatively, the flexural
strength of the lithosphere may be characterized by the flexural rigidity
D=--
ET3
12(1 - a2)’
where u is Poisson’s ratio; D increases by more than two orders of magnitude from ridge
( T = 5 km) to mid-plate ( T = 25 km) lithosphere. For T > 5 km, topography in the ‘compensated waveband’ Ikl < 0.14 km-’(wavelength h > 45 kin) is partially compensated. Topography with I k I > 0.14 km-’does not significantly deform the lithosphere and lies in the
‘uncompensated waveband’, where the admittance has the form ( T +. -)
Zu(k) = 27rC(pb - Pw) exP (- Ikld).
(2.2)
A plot of In Z u. Ikl> 0.14 km should hence yield a straight line with slope - d and
intercept In [27r G ( P -&,)I,
~
allowing the density Pb of the topography to be estimated and
the slope to be checked against the mean depth calculated directly from the bathymetry
records. Also shown in Fig. 2 is the ‘coherent waveband’ 0.025 km-’< Ik I < 0.25 km-’
(250 km > h > 25 km), where the admittance estimates obtained in this and other studies are
typically of high reliability (coherence > 0.75). Useful information concerning differences in
rigidity or thickness of the plate can be extracted only from admittance estimates in the
‘diagnostic waveband’ 0.025 km-’ < lkl< 0.08 km-’ (250 km > X > 80 km), where 2 is
reliable and (2.1) varies substantially as a function of T .
In this study we consider only the flexural or elastic plate model of isostasy, since this
model has been shown to explain gravity anomalies satisfactorily over a number of different
geological features in the world’s ocean basins (for example, Watts & Cochran 1974; Watts &
Talwani 1974; Watts 1978). Furthermore, in using (2.1) we ignore any deviations of the
Pioneer
Argo
1964
1960-62
us
us
1961-77
1967-69
1967 -7 2
Vema
Eltanin
Oceanographer
LaCoste-Romberg
LaCoste-Romberg
Graf Askania Gss-2
Graf Askania Gss-2
LaCoste-Romberg
Graf Askania Gss-2
Graf Askania Gss-2
LaCoste-Romberg
LaCoste-Romberg
AMG NOS3 and 5
Graf Askania Gss-2
Gravimeter
$ Note gravity data obtained on Robert D. Conrad cruises. Vema and Eltanin cruises are also available from the NGDC.
t Russian built gyrostabilized platform.
National Geophysical and Solar-Terrestrial Data Center, Boulder, Colorado. World Data Center A.
National Oceanic and Atmospheric
Administration
Scripps Institution of Oceanography
1963-77
Robert D. Conrad
Lamont-Doherty Geological Observatory
US
USSR
Institute of Oceanology, Moscow
1970
1971-75
1968-70
1971-73
1970-71
us
Hudson
Kana Keoki
Mahi
Dmitri Mendeleyev
Vityaz
Canada
Bedford Institute of Oceanography
Hawaii Institute of Geophysics
Year
Ship
Country
Institution
Table 1. Summary of gravity data used in the Pacific Ocean basin.
Gimbal Mounted
Gimbal Mounted
Anschiitz (196 3-7 3)
Aeroflex (1974-77)
Alidade (1964-77)
Anshiitz
Gimbal Mounted
Anschiitz
Gyrostabilized
Gimbal Mounted
IPEt 1
OSSAt
Stable platform
NGDC (1973)
NGDC (1973)
NGDC (1976)
Von Arx et al. (1971)
NGDC* (1976)
NGDC (1976)
M.G. Kogan, Institute
of Physics of Earth,
Moscow (private
communication)
See note below$
Reference or source
of data
w
w
2
f
2
.b
+*
&
Q
(D
$
3
%
00
Intraplate volcanism in the Pacific Ocean basin
339
rheology of the lithosphere from the simple elastic model. A non-elastic rheology is required
to explain the large curvatures of the flexed lithosphere near trenches (Turcotte, McAdoo &
Caldwell 1978). For the smaller deformations caused by most seamount loads, however, the
approximation of elastic rheology is valid, and little plastic or viscoelastic relaxation of these
deformations appears to occur over times of 106-107yr (Watts 1978; Bodine, Steckler &
Watt 1981).
Data analysis
We have used all available surface ship gravity anomaly and bathymetry data to estimate the
best-fitting gravitational admittance as a function of the age of the Pacific plate. The
principal sources of the data are summarized in Table 1.
We divided the plate into eight age bands labelled A-H, the first seven of which are
20 Myr wide, the last including seafloor of age greater than 140 Myr. The isochrons defining
these regions are shown in Fig. 3. We chose relatively straight ship track segments (Fig. 3)
without data gaps, with no large bathymetric features on or near the endpoints, and of suffcient length (800- 1500 km) to yield reliable spectral estimates. The number of data profiles
and associated mean ocean depths are summarized in Table 2 for each age region. The total
length of ship track data used exceeds 3 x 105km, so use of the Fast Fourier Transform
Figure 3. Map of the Pacific plate showing distribution of ship track data and isochrons used to defiie the
eight age regions A-H.
13
N. M. Ribe and A . B. Watts
330
Table 2. Summary of ship track data in each age region.
Region
Age
Number of gravity Mean ocean
and bathymetry
depth d (km)
profiles N
Estimated*
depth d"(km)
Estimated*
density
PI, (g cm-?
A
B
C
D
E
F
G
H
0-20
20-40
40-60
60-80
80-100
100-120
120-140
> 140
37
26
48
33
78
42
30
40
3.78
4.6 1
5.89
6.04
6.01
4.93
5.91
4.51
2.38
2.29
2.84
2.15
2.58
2.56
2.43
2.55
3.67
4.16
4.57
4.95
5.10
5.08
5.54
5.39
* Estimated from short-wavelengthadmittance (0.14 km-' < k < 0.4 km-').
(FFT) algorithm is a necessity. After removal of the mean and trend from the data they were
cosine tapered, appended with zeros to a fixed length and linearly interpolated at an interval
of - 3 km. The Fourier transform was obtained using the FFT.
For each region, the admittance was estimated from
z t k )=
!G (k)B*( k ) )
(B(k)B*( k ))
and the sample coherence as
-'(k
= Z k)['
'-(
(B(k)B*(k))
(G(k)G*(k))
where B and G are the Fourier transforms of the gravity and bathymetry data, ( ) denotes
ensemble averaging and * denotes the complex conjugate. The formula (3.1) is the best
estimate (in the generalized least-squares sense) of the admittance, interpreted as the
complex 'slope' of the regression of G on B (Munk & Cartwright 1966). The sample coherence gives the average fractinn of gravity power attributable to the bathymetry via the
estimated linear admittance 2. The sample coherence is positively biased by noise in the
gravity data; we follow Munk & Cartwright (1966) in using
7; =--N y 2 - 1
N-l
(3.3)
where N is the number of data profiles (gravity or bathymetry) in the ensemble, as a
relatively unbiased estimate of the true coherence. The reliability of admittance estimates
decreases with increasing values of the 'noise parameter' (Munk & Cartwright 1966)
which is related to the variance of the admittance estimates. The average power spectrum
PB( k ) of the bathymetry was also computed for each age region, according to
where L is the length (before appending with zeros) of the data record and Lo is an arbitrary
constant length. The quotient L/Lo is a normalizing factor to account for differences in
34 1
Intraplate volcanism in the Pacific Ocean basin
profile length. PB ( k ) provides a measure of the total topographic roughness of the seafloor
as a function of wavenumber.
The estimates of the amplitude of the admittance (3.1), the coherence (3.3), and the
bathymetric power spectrum (3.5) are shown for each age region in Fig. 4. For regions C, D
and F, two estimates were obtained for these quantities: one set (solid curves) from all
available data, the other (dotted curves, which coincide with the solid for region F) by
excluding tracks crossing linear volcanic chains (Tuamotu, Cook-Austral and HawaiianEmperor chains respectively for C, D and F). The second set of estimates allows us to study
separately the large component of Pacific volcanism which consists of isolated seamounts.
Comparison of these two estimates will therefore show to what degree, with regard to both
size and isostatic compensation, the linear chains are typical of the seafloor bathymetry as a
whole.
Admittance
Amplitude
~-~
-~~
L
T
'01
Admittance
Amplitude
~
~
I
I
'01
Coherence
N -
Coherence
0.8
08
06
n-
r- 0.4
A
0.6
0.4
0.2
02
o
h
1
d2
03
k km'
04
-,
----
500100
a
50
20
A
km
15
I
00
7
-
500100
b
01
02
03
k km-'
,
50
20
A
04
15
km
Figure 4. (a-h) Amphtude of the admittance (3.1), corrected coherence (3.3), and average power
spectrum P g of the bathymetry (3.5) for the age regions A-H of Fig. 3. Solid curves represent estimates
obtained from all available data; dotted curves, estimates obtained by excluding tracks crossing linear
island/seamount chains noted in the text. The dotted curve for region F essentially coincides with the one
shown.
N. M. Ribe and A. B. Watts
342
n-
10
10
08
08
06
r- 0 4
---
02
4-0i
00
500100
C
n-
06
,-
04
02
4.0
01
02
k krn-’
50
A,
krn
03
I
1
00
04
20
Coherence
01
02
03
k km-’
_r_r-
15
20
A.
Figure 4
-
04
15
km
continued
Bathymetric power spectra and coherence
The total bathyinetric roughness of the seafloor as a function of age can be estimated by
direct comparison of the power spectra in Fig. 4. The two estimates for region F essentially
coincide indicating that the bathymetry of the Emperor seamount chain is typical of that for
the region overall. In C and D, however, the presence of the island chains dominates the
power spectra. Smooth curves representing the spectra of Fig. 4 are shown in Fig. 5; the
curves for regions C and D correspond to the dotted curves in Fig. 4 (c and d). Bathymetry
with k > 0.1 h-’
( h < 63 km) is essentially uncompensated and of little interest here. Fig. 5
shows that the overall roughness of the Pacific basin generally increases with age. The power
spectrum of the roughest region (H; > 140Myr) is about 10 times that of region A, the
youngest and smoothest overall. Fig. 5 suggests that the roughness of the Pacific plate,
exclusive of the island chains, has three major gradations: regions A-D (< SOMyr) are
relatively smooth, regions E (80-100Myr) and G (120--140Myr) are of intermediate roughness, and regions F (100-120Myr) and H (> 140Myr) are very rough. This relationship is
Intraplate volcanism in the Pacific Ocean basin
"1
,
E
343
Admittance
Amplitude
Admittance
Amplitude
m
-
1.0 -
m-
*
0.80.60.4-
0.240
i
30
"E
1
0 2.0
m
-
1.0
00
01
02
k km'
03
04
00
01
02
k
km-'
03
04
,
1
7,
500100
f
Figure 4
50
A
km
20
15
continued
illustrated in the space domain in Fig. 6, which shows typical bathymetric profiles for three
of the age regions (A, E, H).
Except for a few linear island chains, young seafloor (regions A-D, < 80 Myr) contains
little significant (amplitude > 1 km) bathymetric relief. The bathymetry in these regions
consists largely of a low-level 'background' roughness which was probably frozen into the
plate as it formed. We would not therefore expect the corresponding admittance to
accurately reflect a process of 'loading' of an existing lithosphere, as presupposed by the
elastic plate model of isostasy. For these reasons we shall henceforth not consider regions
A-D in this study.
The coherence 7: of Fig. 4 is a measure of the reliability of the admittance estimates:
7: is the (ensemble average) fraction of the power in the gravity accounted for by the
bathymetry via the admittance estimate 2. Fig. 7 shows the observed bathymetry and
gravity anomaly data from four selected cruises in regions E and H together with a calculated
gravity anomaly profie obtained from the observed bathymetry and the appropriate admittance z" from Fig. 4 (e or h). The observed and calculated gravity anomalies agree well for
N. M.Ribe and A. B. Watts
3 44
"1
Admittance
Amolltude
05
'.O 1
Admittance
I
1
'01
Coherence
08-
08
j.l-
H
06
nxr
,-.0 4
060 4 ~
---------
02
02-
4.01
40
,
I
Bathymetry
Power
Bathymetry
Power
E
m
0.0
01
0.2
k.kni'
0.3
r
_
l
-
500 100 50
9
An
20
00
0.4
;p" loo
15
Figure 4
--
01
02
k km-'
50
A
km
03
20
-
04
15
continued
oldest seafloor (region H) as one would expect from the high coherence of Fig. 4(h), and
somewhat less well for ages 80-100Myr (region E), consistent with the lesser coherence for
that region.
Analysis of the admittance amplitude
z"
Admittance amplitude estimates ( k )such as those of Fig. 4 are usually interpreted by comparing them with a set of theoretical admittances Z ( k ; T ) , to determine the value of the
plate thickness T for which Z ( k ; T ) best matc.hes z"(k). In this study we use a different
approach, suggested by the structure of the admittance estimator (3.1), and interpret z"not
in terms of a single value of T but as a weighed sum of admittances for a range of plate thicknesses. Suppose the admittance z" to be estimated for a region of seafloor of mean age t ,
using a large number N of bathymetry and gravity profiles. These may be assumed one-
Intraplate volcanism in the Pacific Ocean basin
345
3'
3.1
am
mP 2 :
-
2s
. -.
1.5
D.
1
.
02
06
04
08
I
k,km-'
500
IAO
260
75
A , km
Figure 5 . Average power spectra of the bathymetry for the age regions A-H of Fig. 3. The curves shown
are smoothed versions of the power spectra in Fig. 4; those for regions C , D and F were obtained by
excluding ship tracks crossing the linear island/seamount chains noted in the text.
dimensional and noise-free with no loss of generality, so that G, ( k ) = Z ( k ;T,) B, (k),where
Z ( k ; T,) is given by (2.1). We thereby assume that all bathymetry in the nth data record of
the ensemble was emplaced on lithosphere of the same elastic thickness, called T,. This
assumption is made to simplify the mathematics and exhibit clearly the structure of the
admittance estimate 2. It is easily shown that for sufficiently large data ensembles the results
will be the same even if bathymetric features with different associated Tare allowed on the
same data record. Now (3.1) can be written as
where
346
N. M.Ribe and A. B. Watts
Figure 6. Typical bathymetry profiies for three age regions of the Pacific plate, illustrating the general
increase in topographic roughness with age. The mean ocean depth d for each profile is shown.
E
H o u l l ~ nRidge
5221
-4617
-4330
- ,0
Id
300
Km
Figure 7. Comparison of observed and predic'ed gravity anomalies for selected ship tracks over the Pacific
plate. The calculated gravity profiies are obtained from the observed bathymetry and the estimated admittance 2 (k) from Fig. 4, as discussed in the text. For region H, the observed and calculated gravity profiles
agree well, indicating that most of the power in the former is predicted by 2. The agreement is less good
for region E, consistent with the lower coherence of z"for that region.
Intraplate volcanism in the Pacific Ocean basin
In the limit N
+ M,
347
(5.1) becomes
Z ( k ) = jomdT{ ( T ;k ) Z ( k ;T )
(5.2)
where
S(T; k ) d T = lim f, (k).
N+-
Formula (5.2) shows that the estimated admittance 2 is a sum of admittances Z ( k ; T ) for a
range of plate thicknesses T, weighted by the fraction [ ( T ;k)dT of the total bathymetric
power emplaced on a lithosphere of thickness between T and T + dT. The function ( ( T ;k )
provides an estimate, as a function of wavenumber, of the fraction fI (‘fractional intraplate
power’) of the total bathymetric power which is of true intraplate origin. If ‘intraplate
volcanism’ is defined as volcanism emplaced on lithosphere of thickness T > To,
The total amount of volcanism on the seafloor, as measured by the bathymetric power
spectrum FB(k), varies greatly among age regions, as shown in Fig. 4. It is convenient to
define the ‘intraplate power’ P I @ ) as the product of the total bathymetric power and the
fraction of that power which is of intraplate origin:
4 ( k ) = fI W F B ( k )
(5.4a)
F1
where fI is given by (5.3) and FBby Fig. 4. is a measure of the total amount of intraplate
volcanism and may be compared among age regions. Similarly, we define the ‘ridge power’
FR
( k ) = [1 - fI
F B (k)
(5.4b)
as a measure of the total amount of ridge volcanism. Higher values of PI ( P R )corresponds to
greater amounts of intraplate (ridge) volcanism per unit area of seafloor.
Although it is not possible in this study to invert the integral (5.2) for { ( T ; k ) , the
fractional intraplate power fI and the intraplate power PI may be estimated using a simplifying assumption. We assume that all Pacific volcanism was emplaced on lithosphere with
effective elastic thickness of either T = 5 km,typical of relatively young lithosphere on or
near ridge crests (Cochran 1979; McNutt 1979) or T = 25 km, for old lithosphere such as
that on which the Hawaiian-Emperor seamount chain was emplaced (Watts 1978). The
corresponding admittances ZR and Z I hence represent geological ‘end-member’admittances
which span the range of observed plate thicknesses T. We may now write (5.1) as
z ( k ) = f I (k)zI ( k ) -f [1 - fI (k)lZR (k).
(5.5)
The theoretical admittances ZI and ZR are easily calculated (for example, from 2.1).
Rearranging (5.9, we obtain the estimate for fI,
where z“ is the admittance estimated from the observed data, shown in Fig. 4. The intraplate
powerFI and ridge power F R for each age region are then calculated by using (5.6) in (5.4).
The mean ocean depth increases with the age of the lithosphere, as shown in Table 2,
which makes direct comparison of admittance estimates from different age regions difficult.
348
N. M. Ribe and A. B. Watts
It is convenient to recast all admittances (both estimated and theoretical) in terms of the
equivalent isostatic response function (IRF), defined as (Walcott 1976)
(5.7)
Zu = 27rG (pb--pw) exp [- k d ] is the admittance for uncompensated bathymetry, and is
asymptotically the same (fork large) for one- and two-dimensional bathymetry (Ribe 1982).
The IRF ranges from 0 to 1 and is the fractional degree (as a function of wavenumber) of
isostatic compensation of the bathymetry. Comparison of (5.7) with the admittance in (2.1)
suggests the IRF has no dependence on the ocean depth d and only slight dependence on
p b . IRFs may therefore be compared directly between age regions.
To transform the admittances of Fig. 4 into IRFs, we used (5.7) with Pb = 2.6 g ~ r n and
-~
d given by the mean depth calculated from the bathymetry data (Table 2). The density
200
1
00
(bl
02
500
A,km
04
200
k,km-l
1,km
60
loo
05
08
100
I(
60
Figure 8. (a-d) Isostatic response functions (IRFs) for the age regions E-H of Fig. 3. Opencirclesareesti- ~ the true mean depth d in (5.7); triangles are the corresponding
mates obtained by using pb = 2.6 g ~ r n and
values obtained by using .Lib and d estimated from the short-wavelength portion of the admittance, and
are probably less reliable. Length of the bars attached to the solid circles is given by the noise parameter u
defined by (3.4), and indicates the relative reliabilities of the estimates as discussed in the text. The
dashed curves are theoretical IRFs for onedimensional bathymetry, for several values of the plate
thickness T and pb = 2.6 g cm-’.
Intraplate volcanism in the Paciflc Ocean basin
(C)
500
02
00
(d)
200
500
04
ZOO
100
X,km
k.km-1
~ , k m
06
100
349
60
08
I
60
Figure 8 - continued
Pb = 2.6 g cm-3 is an average value for the rocks of oceanic crustal layer 2. Although &, and d
are often estimated from the slope of the short-wavelength admittance (McKenzie & Bowin
1976;Watts 1978), this is not a reliable method in general (Kibe 1982).
The estimated IRFs $ ( k ) for regions E-H are shown in Fig. 8. The open circles were
obtained using the measured depth d and Pb = 2.6 gcmV3as discussed above. The solid
triangles were obtained using estimates of d and Pb obtained from the short-wavelength
admittance (Table 2). They are probably less reliable than the open circles and are shown
for comparison only. The vertical bars attached to the open circles indicate the relative
reliabilities of the various estimates. Their length is equal to the noise parameter u given by
(3.4). The solid curves are theoretical IRFs @ ( k T; ) , corresponding to the admittances of
Fig. 2, for various plate thicknesses T.
Fig. 8 shows that the theoretical IRFs 4 (k ;T ) do not in general provide a good fit to the
estimates 6(k). Typically trends across the theoretical curves, intersecting @ ( k ;T ) of progressively higher T as k decreases. Such behaviour is probably a result of the pronounced
two-dimensionality of Pacific plate bathymetry. Kibe (1982) calculated admittances for
bathymetry (such as seamounts) having a characteristic width 1 perpendicular to the ship
6
350
N. M. Ribe and A. B. Watts
04
02
08
01
10
k,km-’
Figure 9. Isostatic response functions for two-dimensional topography, determined as discussed in the
text. Onedimensional IRFs corresponding to (2.1) for T = 10 and 25 km are shown dashed for comparison. As the effective width I of the topography perpendicular to the ship track decreases, so does the longwavelength portion of the IRF, indicating a decreasing degree of isostatic compensation relative to the
onedimensional case 1 = -. IRFs for firnite 1 intersect one-dimensional curves of progressively higher T as
k decreases, suggesting an apparent plate thickness which is too great.
track. IRFs corresponding to these admittances are shown in Fig. 9. The solid curves all have
associated T = 5 km, and the dashed curves are IRFs for one-dimensional bathymetry,
calculated from (2.1). The two-dimensional IRFs trend across the one-dimensional curves in
a manner similar to Fig. 8, suggesting that the effects of two-dimensionality on the IRF
estimates in Fig. 9 are substantial.
In Fig. 10, the IRF estimates are replotted with theoretical IRFs for two-dimensional
bathymetry. The theoretical curves represent linear combinations of IRFs
and $R for
E
02
00
(0)
500
04
200
k,km-l
A,hm
06
loo
08
10
60
Figure 10. (a-d) Estimated isostatic response functions for regions E-H, superimposed on a set of
theoretical IRFs for two-dimensional bathymetry. The notation for the estimates is that of Fig. 8. The
, given by fI @I (k)+ (1 - fr) @R ( k ) ;@R and @I are isostatic
theoretical curves, for several values of f ~ are
response functions characteristic of ridge-produced and intraplate bathymetry as discussed in the text,
and the parameter f1 is the fraction of the total bathymetric power accounted for by intraplate volcanism.
Intraplate volcanism in the Pacific Ocean basin
00
04
02
k,km-l
06
08
I(
(b)
500
ZOO
X . km
100
60
(C)
500
200
k.km
100
60
10,
,
00
(d)
02
500
04
zoo
k.km-1
X,km
06
100
Figure 10 - continued
08
10
60
351
N. M.Ribe and A. B. Watts
352
intraplate and ridge volcanism, respectively:
= f1 @I ( k ) -I-(1
-fi)@R
(k)
(5.8)
which is equivalent to ( 5 . 5 ) . The details of the determination of the appropriate and GR
are discussed in the Appendix. By comparison of the IRF estimates (open circles in Fig. 10)
with the theoretical curves, the fraction fI of the total bathymetric power which is of
intraplate origin may be read directly. This is plotted for age regions E-H in Fig. 11. Using
f~ from Fig. 11 and the bathymetric power spectra of Fig. 4 we can estimate the total power
of intraplate volcanism (‘intraplate power’&) and ridge volcanism (‘ridge power’ PR)in each
age region, which are shown in Fig. 12.
25
r
E
O
F
m
G
O
H
20
1.5
’1
10
...
.-..
.--.’!
. .
.‘
0 0
0
.p l a :
0.5
&
A
.
01
.. .’ .
..
0 . P
A
0 ’ 2
0
d
A.
A
,:i
03
02
04
k,km-‘
I
I
5w
200
I
150
A , km
Figure 11. Fraction f~of the total bathymetric power accounted for by intraplate volcanism, for the four
age regions E-H of Fig. 3. Values off1 are read from the curves of Fig. 10.
Discussion
The estimates of the fraction of bathymetric power which is of intraplate origin (‘fractional
intraplate power’ fI) and the total amount of intraplate volcanism (‘intraplate power’ PI)
and on-ridge volcanism (‘ridge power’ PR) for the oldest portions (regions E-H, > 80Myr)
of the Pacific plate are summarized in Figs 11 and 12. The highest proportion (average 90
per cent of the total power) of intraplate volcanism is found in region H ( > 140Myr),
followed by region F (1 00-- 120 Myr). Regions E and G have roughly comparablefi ( k )> 0.60.
The low values of fi for E and G for k > 0.035 k m ’most likely are due to the effects of
two-dimensionality of the bathymetry, as discussed in the Appendix, and probably do not
indicate an anomalously high proportion of on-ridge volcanism at these wavenumbers.
Intraplate volcanism in the Pacifc Ocean basin
353
The results of Fig. 11, which refer to unit bathymetric power, show a more striking
pattern when multiplied by the bathymetric power spectra of Fig. 4. The resulting total
bathymetric power PI accounted for by intraplate volcanism (‘intraplate power’) is shown in
Fig. 12(a). The estimates of the intraplate power PIfor each region occupy distinct fields
even though the fractional intraplate power fI is roughly the same for regions E, F and G.
Region H (> 140Myr) has the highest intraplate power, followed by F(100-120Myr),
G(120-140Myr) and E (80-100Myr).
The bathymetric relief accounted for by on-ridge volcanism (‘ridge power’ P R ) is determined similarly as the product of the power spectrum PB (Fig. 4) and the fraction
f~= 1 -fI of that power which is of on-ridge origin. Fig. 12(b) shows the ridge power PR for
age regions E-H. The variation of PR among age regions is more complex than the variation
of the intraplate power PI of Fig. 12(a). For short wavelengths (A < 200 km), all age regions
have roughly the same amount of on-ridge volcanism. At longer wavelengths, however,
regions E (80-100Myr), F (100-120Myr), and G (120-140Myr) have substantially greater
PR than does region H (> 140Myr). The difference between the long- and short-wavelength
portions of the spectrum probably reflects the presence (in regions E, F and G) of such large
features as the Shatsky rise, Hess rise, Manihiki plateau and Ontong-Java plateau, which
2000
-1
0
i
e
0
A E
O F
0
0
1000
1I
J
-
0
00
I
0
.01
0
0
k,km‘
500
I
e e
0
.03
02
I
(a)’
Be
O0
200
0
0
.c
.04
I
150
X,km
Figure 12. (a) Bathymetric power accounted for by intraplate volcanism (‘intraplate power’ P I ) for the
age regions E-H of Fig. 3, given by (5.4a). Larger values of PIcorrespond to greater density of intraplate
seamounts per unit area of seafloor. Arbitrary units (length squared) are the same as in Figs 4 and 6 , but
on a linear scale. (b) Bathymetric power accounted for by on-ridge volcanism (‘ridge power’, 5.4b) for age
regions E-H of Fig. 3. Units are the same as in (a).
3 54
N, M. Ribe and A. B. Watts
15003
A
50L
0
A
'
'Oo0I
'
0
00
00
0
(b)
Figure 12 - continued
probably formed at ridges (Watts et al. 1980) and whose bathymetric spectra consist
primarily of longwavelength components.
These results are generally consistent with those of Watts et al. (1980), who classified
about 120 volcanic features in the Pacific Ocean as either formed in an on-ridge or intraplate
setting. Most of the on-ridge estimates of Watts et al. (1980) occur in regions E-G, consistent with Fig. 12(b), and most of their intraplate estimates lie in region H, consistent with
Fig. 12(a).
It should be re-emphasized here that the fractional intraplate power fI ( k ) is to be interpreted as the 'projection' of the IRF estimates ;P"(k)on to the 'basis vector'
representTable 3. Summary of parameters used in model calculations.
Parameter
Value
pb,density of crustal layer 2
p 3 , density of crustal layer 3
pw, density of seawater
pm, density of upper mantle
d, mean ocean depth
t 2 ,thickness of crustal layer 2
t, mean crustal thickness
T, elastic plate thickness
G,gravitational constant
E, Young's modulus
2.6g~m.~
2.9g~m-~
1.03 g cm3.4 g cm-)
5 km
2km
5 km
5 -25 km
6.67 X10~'0dynecm'g~2
lo'* dyne cm-2
Intraplate volcanism in the Pacific Ocean basin
355
ing a typical intraplate gravitational response. Although we assumed for simplicity that all
Pacific volcanism was emplaced on lithosphere of either 5 or 25 km elastic thickness, some
may actually have been emplaced on lithosphere of intermediate thickness. (Some features
may have associated T > 25 km, but this is of little concern, since the gravitational admittance changes little as a function of T > 25 km.) If such bathymetry is present, our method,
by resolving all bathymetry into components characterized by T = 5 and 25 km, will tend to
overestimate the proportion of on-ridge volcanism and underestimate the proportion of
intraplate volcanism. The results of Figs 11 and 12 should hence be interpreted as lower
bounds of jj and PI.
The results in Figs 1 1 and 12 are also affected to some extent by uncertainties due to
noise (interpolation of the data) and large amplitude and two-dimensionality of the bathymetry. Fork < 0.05 km-I,the effects on the admittance of interpolation and large amplitude
are small and of opposite sign, and hence may be ignored (Ribe 1982). Potentially the most
significant source of error is the two-dimensionality of the bathymetry, the extent of which
cannot be determined from the bathymetry data alone. The results of Figs 1 1 and 12 were
obtained using an estimate of the two-dimensionality obtained from the long-wavelength
admittance, as discussed in the Appendix. The consequence of neglecting two-dimensionality
altogether is illustrated in Fig. 13, which is the same as Fig. 1 1 except that the bathymetry is
assumed to be one-dimensional. The estimates in Fig. 13 are determined by replacing the
theoretical curves of Fig. 8 with one-dimensional equivalents calculated from (5.7) and (2.1).
The large values of fr > 1.O in Fig. 13 probably do not represent anomalously high values of
2 5-
A
€
O F
. G
C H
.
.
20-
.
15‘1
10-
.
05-
I
.=’..
I
1.
! & A
1
02
01
..
03
k,km‘
200
500
05
04
,
150
A.km
Figure 13. Same as Fig. 1 1 but assuming one-dimensional bathymetry. The estimates off1 are obtained by
replacing the theoretical curves in Fig. 10 with onedimensional analogues. Values of fI greater than unity
indicate apparent plate thicknesses T > 25 km. High apparent values of T indicate that two-dimensionality
of the bathymetry is pronounced, as shown in Fig. 9.
356
N. M.Ribe and A . B. Watts
T > 25 km; rather, they suggest that the diminution of the long-wavelength IRF by twodimensionality of t.he bathymetry cannot be ignored (see Fig. 9). If the extent of t h i s
two-dimensionality is estimated by the method of the Appendix, however, errors in the
estimates of Figs 11 and 12 arising from two-dimensionality should not be more than about
10 per cent.
The main results of this study are summarized in Fig. 14. The more heavily shaded
regions have higher intraplate power PI,i.e. a greater density of seamounts which are of
intraplate origin. It should be remembered that the Hawaiian-Emperor chain is not included
in these results, since we wish to focus on the isolated seamounts whose ages are unknown.
Regions E, F and G contain some of the largest bathymetric features in the western
Pacific, many of which (Hess rise, Shatsky rise, Manihiki plateau, Ontong-Java plateau) are
over 500km wide. The bathymetric power spectra of these features will thus consist
primarily of long-wavelength components. Fig. 12(b) shows that regions E-G have relatively
high amounts of long-wavelength bathymetry which is of on-ridge origin. This is consistent
with Watts et al. (1980), who showed that the large rises and plateaus in regions E-G
probably formed near ridges, and thus have ages equal to or slightly less than the age of the
adjacent seafloor. The present study substantiates the conclusion of Watts et al. (1980) that
a major outpouring of on-ridge volcanism occurred on the Pacific plate during the
Cretaceous. Tectonic reconstructions of the Pacific (Larson & Chase 1972) suggest that this
volcanism probably occurred on or near the accreting Pacific/Farallon plate boundary.
160"
140"
180"
160"
/
140'
I
I
-
40"
-20"
- 0"
(24
-
v v
A
High intraplate power
Diedqe rocks
?_J
Low intraplate power
@ D S D P sites
Figure 14. Summary map of the western Pacific showing regions of high and low density of intraplate
seamounts. More darkly shaded regions have higher intraplate power PI,and hence greater density of
intraplate volcanism. The solid triangles show the locations of the dredged rocks in Table 4 and the filled
circles show the locations of DSDP sites in Fig. 15. The simplified bathymetry is based on Chase et ul.
(1971).
In traplate volcanism in the Pacific Ocean basin
357
The occurrence of an on-ridge component of volcanism in regions E, F and G is important
since it documents a major volcanic event whose age can be estimated. Fig. 12(b) shows that
most of this on-ridge volcanism probably comprises large plateaus and rises whose spectra
consist primarily of long wavelengths. Comparison of Figs 12(a) and (b) for regions E--G
shows that the long-wavelength ridge power PR is significantly greater than the intraplate
power PI,which probably reflects the large size of the major on-ridge volcanic features. For
shorter wavelengths (A < 400 km), however, the relation between PR and PIis quite different.
Regions E and G have low PI P,, indicating roughly equal portions of on-ridge and intraplate volcanism at intermediate wavelengths (400 km < h < 1 O O k m ) . Regions F and H
though, have much more intraplate than on-ridge volcanism at these wavelengths (PI > PR).
Bathymetric power in the wavelength range 400 km < h < 100 km is contributed primarily by
large seamounts with characteristic uidths of 20-80 km. This is indicated by the bathymetry data, which show that regions F and H contain many large seamounts up to 4.5 km in
height, while the seamounts in regions E and G are fewer and much smaller in general.
Fig. 12(a,b) thus indicates that most of the seamounts in regions F and H are of intraplate origin, and that their total power (roughly equivalent to volume of seamounts per unit
area) greatly exceeds the power of seamounts produced near ridges. Since ihe age of old
(> 60 hlyr) lithosphere is not closely constrained as a function of its elastic thickness T, the
age of this intraplate component is difficult to estimate. Much of this volcanism could be
similar in age to the Cretaceous event on the Pacific/Farallon plate boundary; or it may be
unrelated to ridge crest volcanism and due to younger volcanic events occurring on the
Pacific plate.
There is geological evidence, however, for the age of the volcanism in the western Pacific,
based on bottom samples and Deep Sea Drilling Project (DSDP) sites. Fig. 15, based on
recent compilations by Schlanger & Premoli-Silva (1983, summarizes the occurrence of
-
This
Study
0
0
c
H
-k27
G
F
E
Basalt f l o w s
and extrusives
Figure 15. Summary of basalt flows and extrusives in DSDP sites in the western Pacific, based on a
compilation by Schlanger & Premoli-Silva (1 982).
358
N. M. RibeandA. B. Watts
Table 4. Summary of rocks dredged from seamounts in region H.
Seamount
Latitude
Longitude
Method
Age
Reference
Isakov
Makarov
2413 Guyot
31.6"
29.5"
27.1"
151.2" E
153.5" E
148.7" E
Fossil
Fossil
K-Ar
> Lower Cret.
> Cen-Tur.
> 86-96 Myr
Lamont
2 1.So
159.6' E
Fossil
> M. Eocene
Heezen et al. (1973)
Heezen et al. (1973)
Ozima, Kaneoka &
Aramaki (1970)
Heezen et al. (1973)
volcanic rocks (basalt flows and extrusives) at select DSDP sites in regions E-H. These sites
were all drilled in relatively deep-water locations. Fig. 15 shows that there is evidence of a
series of major deep-water volcanic events in the Cretaceous between 7 5 - 1 0 0 M y r ~ ~in
region E-G and 100-120 Myr BP in region H. For region F-H, these events are significantly
younger than the underlying seafloor, by 10-65Myr. The events documented in Fig. 15
thus occurred on the flanks of the Pacific/Farallon plate boundary, and may be contemporaneous with the volcanism which produced the concentrations of intraplate seamounts in
regions F and H. Unfortunately, there is little evidence for these intraplate volcanic events
from age dating of bottom samples from individual features in regions F-H. Table 4 shows,
for example, that all age determinations in region H are only minimum estimates.
The spectral results of this study suggest that seamounts and oceanic islands formed in an
intraplate setting are, in general, larger than seamounts (not including plateaus and rises)
formed on or near a ridge crest. A comparison of Figs 4 and 11 shows a strong positive correlation between fr and the bathymetric power spectrum P,: regions with greater bathymetric
roughness PB tend to have a larger proportion fI of intraplate volcanism. The bathymetry
data shows that regions F and H contain many large seamounts (up to 4.5 h high) while
those in regions E and G are all significantly smaller. These results are consistent with
geological considerations which suggest that seamounts should be able to build to greater
heights in an intraplate setting, because of the larger water depths available, than in an on- or
near-ridge setting. Since erosion will tend to reduce the overall height of a seamount that
builds above sea-level, a seamount formed on lithosphere of age t can attain a height of at
most d ( t ) , where d ( t ) is the mean depth of the ocean floor expected from thermal models
for the cooling lithosphere. An additional elevation of up to a few hundred metres could
be added to seamounts which do not build quite up to sea-level and are capped by coral
reefs. If on-ridge volcanism is defined as that emplaced on lithosphere of age T < 5 Myr, the
maximum height of on or near-ridge seamounts would be about 3.3 km (using d ( t ) from
Parsons & Sclater 1977). We found that only a few seamounts in regions E and G exceed this
height, but many in regions F and H rise up to 4-4.5 km above the ocean floor.
In summary, we have presented evidence using gravity and bathymetry data for major
outpourings of on-ridge and intraplate volcanism during the Cretaceous. The on-ridge
volcanism probably occurred at the Pacific/Farallon accreting plate boundary, and appears
to consist mostly of large plateaus and rises over 5 0 0 h across, similar to present day
Iceland. Pacific seafloor of ages 100-120 and > 140Myr contains in addition a large concentration of seamounts which were formed in an intraplate setting, which may be contemporaneous with the Cretaceous on-ridge events. The absence of significant volcanic relief on the
younger (< 8OMyr) portions of the Pacific plate indicates that volcanic production at ridges
has been episodic through geological time. The same cannot be said with confidence of the
intraplate volcanism, since its age is still largely unknown. If, as suggested in this study,
Pacific intraplate volcanism is contemporaneous with the Cretaceous on-ridge events, the
'western intensification' of Pacific volcanism is probably a result of the time-dependence of
volcanic production rather than a manifestation of spatial heterogeneity in the structure of
Intraplate volcanism in the Pacific Ocean basin
359
the lithosphere or asthenosphere. Since most of this volcanism is not in the form of linear
island/seamount chains, it cannot be easily explained by either the hot spot or propagating
fracture hypotheses, and a new model may be required.
Conclusions
We have used spectral techniques of analysing gravity and bathymetry data to estimate the
distribution of intraplate volcanism on the Pacific plate. We obtained estimates of the bathymetric power spectrum and gravitational admittance for each of eight regions A-H of the
Pacific plate defined as follows:
A 0-20,
B 10-40,
C 40-60,
D 60-80,
E 80-100,
F 100-120,
G 120-140,
H >140Myr.
The following conclusions can be drawn from the study:
(1) The average power spectrum PB of the bathymetry is a measure of the total bathymetric (primarily volcanic) roughness of the plate. Exclusive of the major linear island
chains, significant volcanic roughness is confined t o portions of the Pacific plate of age
> 80Myr. The bathymetric roughness of the plate has three major gradations: young sea
floor (regions A-D, < 80Myr) is relatively smooth, regions E (80-100Myr) and G
(120-140Myr) are of intermediate roughness, and regions F and H are very rough.
(2) The gravitational admittance representing a best fit to the data provides an estimate of
the proportion fI of the total bathymetric power accounted for by intraplate (off-ridge)
volcanism. The highest values of fI (> 0.7; average 0.90) are found for region H
(>140Myr), followed by region F (100-120Myr), with regions E (80-100Myr) and G
(120-140 Myr) having lower values of fI = 0.40-0.60. These estimates represent lower
bounds. N o estimates were obtained for regions A-D, since there are essentially no volcanic
features there exclusive of the linear island-seamount chains.
(3) The bathymetric power spectrum and gravitational admittance together allow an
estimate of the total bathymetric power PI = frPBaccounted for by intraplate volcanism,
and the power PR = (1 -fr) P B which is oi ijin or near-ridge origin. PI is highest for region
H ( > 140Myr), followed by F (100-120Myr), with regions G (120-140Myr) and
E (80-100Myr) having significantly lower values. At long wavelengths in regions E-G
(80-140 Myr), the amount of on-ridge volcanism exceeds the amount of intraplate
volcanism (PR >PI).This is probably because these regions contain a number of large
features (Hess rise, Manihiki plateau, Shatsky rise) whose bathymetric spectra consist
primarily of long-wavelength components, and which probably formed at ridges. At intermediate wavelengths, the amount of intraplate volcanism in regions F (100 120Myr) and
H (> 140Myr) greatly exceeds the amount of on-ridge volcanism (PI > PR).Since intermediate wavelengths are contributed primarily by seamounts, this result suggests that most
of the seamounts in regions F and H are of intraplate origin.
(4) There is a strong positive correlation betweenfI and the bathymetric power spectrum
PB and fI, such that regions F and H, with greater bathymetric roughness, tend to have a
higher proportion fr of intraplate volcanism. This pattern may reflect the greater ocean
depth associated with intraplate volcanism, which allows intraplate volcanic features to build
to greater heights.
(5) Regions E-G (80-140Myr), which have large amounts of on- or near-ridge volcanism
at long wavelengths, contain some of the most prominent features ir, the Pacific ocean, such
-
360
N. M. Ribe and A . B. Watts
as Shatsky rise, Line Islands ridge, Mid-Pacific mountains and Manihiki plateau. This
suggests, in general agreement with an earlier study by Watts et d.(1980), that the Pacific
Ocean basin was affected by a major Cretaceous volcanic event on or near the Pacific/
Farallon accreting plate boundary. This result, and the lack of comparable volcanic features
on young lithosphere near the East Pacific Rise, indicates that on-ridge volcanism has been
episodic for at least 160 Myr.
(6) Regions F (100-120Myr) and H (> 140Myr), which have the highest densities of
intraplate seamounts, contain prominent features such as the Magellan and Musicians
Seamounts, Marcus-Wake Guyots, Marshall-Gilbert Islands and Cross-Line Islands. The age of
the intraplate volcanism in these regions, as well as the smaller amounts of volcanism in
regions E and G, is not clear at present. Intraplate volcanism in regions F, G and H may be
related to the major Cretaceous volcanic event on or near the Pacific/Farallon plate
boundary. The main evidence for this is the presence in DSDP sites in regions F-H of a
deep-water volcanic event similar in age to the Pacific/Farallon near-ridge event. Since few
reliably dated samples from volcanic features in these regions are available, however, the
relationship of intraplate volcanism to contemporaneous events on or near the ridge cannot
be affirmed with certainty,
(7) Intraplate volcanism in the Pacific ocean consists mostly of seamount provinces and
isolated seamounts, which are strongly concentrated on the older portions of the Pacific
plate. The origin of this ‘western intensification’ of intraplate volcanism is enigmatic at
present. The volcanism may be related to contemporaneous volcanic events at the Pacific/
Farallon accreting plate boundary. If this is so, the ‘western intensification’ is probably a
result of the time-dependence of volcanic production rather than a manifestation of spatial
heterogeneity in the structure of the lithosphere or asthenosphere. Most of the volcanism in
the Pacific ocean is not in the form of linear island-seamount chains, which suggests that it
cannot be satisfactorily explained in terms of the hot spot or propagating fracture
hypotheses, and that a new model may be required.
Acknowledgments
We wish thank Frank Richter and Barry Parsons for the initial suggestion of a spectral study
of Pacific volcanism. We are grateful to Roger Larson for assistance in constructing the
Pacific isochrons. The manuscript benefited greatly from discussions with John Bodine and
the comments of an anomymous referee. NMR was supported by a Robert R. McCormick
Fellowship at the University of Chicago, Office of Naval Research Contract N00014-80C-0098 and National Science Foundation grants EAR 75-17180 and EAR 79-26482 and
ABW by National Science Foundation grant OCE 79-18917 at Lamont-Doherty Geological
Observatory (contribution no. 3352).
References
Bodine, J. H., Steckler, M. S. & Watts, A . B., 1981. Observations of flexure and the rheology of the
oceanic lithosphere, J. geophys. Rex, 86, 3695-3707.
Chase, T. E., Menard, H. W. & Mammerickx, J., 1971. Topography of the North Pacific, ZMR T e c h Rep.
Ser. TR-1Z
Cochran, J. R., 1979. An analysis of isostasy in the world’s oceans, 2. Mid-ocean ridge crests,J. geophys.
Res., 84, 4713-47.
Detrick, R . S. & Watts, A. B., 1979. An analysis of isostasy in the world’s oceans, 3. Aseismic ridges, J.
geophys. Res., 84,3637-3653.
Heezen, B. C. et al., 1973. Western Pacific guyots, in Init. Rep. Deep Sea drill. Proj., 2, 653-723, US
Government Printing Office, Washington, DC.
In traplate volcanism in the Pacific Ocean basin
36 1
Hess, H. H., 1946. Drowned ancient islands of the Pacific basin,Am. J. Sci., 244, 772-791.
Jarrard, R. D. & Clague, D. A., 1977. Implications of Pacific island and seamount ages for the origin of
volcanic chains, Rev. Geophys. Space Phys., 15,57-76.
Larson, R. L. &Chase, C. G., 1972. Late Mesozoic evolution of the western Pacific ocean, Bull. geol. SOC.
Am., 83,3627-3644.
McKenzie, D. & Bowin, C., 1976. The relationship between gravity and bathymetry in the Atlantic Ocean,
J. geophys. Res., 81, 1902-1915.
McNutt, M., 1979. Compensation of oceanic topography: an application of the response function technique to the Surveyor area, J. geophys. Res., 84, 7589-7598.
Menard, H. W., 1964. Marine Geology of the Pacific, McGraw-Ha, New York, 271 pp.
Minster, J. B., Jordan, T. H., Molnar, P. & Haines, E., 1972. Numerical modelling of instantaneous plate
tectonics, Geophys. J. R . astr. SOC., 36, 541-576.
Mu&, W. H. & Cartwright, D. C., 1966. Tidal spectroscopy and prediction, Phil. Trans. R. SOC.A, 259,
533-581.
Ozima, M., Kaneoka, I. & Aramaki, S., 1970. K-Ar ages of submarine basalts dredged from seamounts in
the western Pacific area and of oceanic crust, Earth planet. Sci. Lett., 8, 237-249.
Parsons, B. & Sclater, J. G., 1977. An analysis of the varialion of ocean floor bathymetry and heat flow
with age, J. geophys. Res., 82, 803-827.
Ribe, N. M., 1982. On the interpretation of frequency response functions for oceanic gravity and bathymetry, Geophys. J. R . astr. SOC.,70, 273-294.
Schlanger, S 0. & Premoli-Silva, I., 1982. Tectonic, volcanic and paleogeographic implications of redeposited reef fauna of late Cretaceous and tertiary age from the Nauru Basin and the Line Islands,
in Init. Rep. Deep Sea drill. Proj., 61, eds Scientific Party, US Government Printing Office,
Washington, DC, in press.
Turcotte, D. L., McAdoo, D. C. & Caldwell, J. G., 1978. An elastic-perfectly plastic analysis of the
bending of the lithosphere at trench, Tectonophys., 47, 193-206.
Turcotte, D. L. & Oxburgh, E. R., 1978. Intraplate volcanism, Phil. Trans. R . SOC.A. 288,561.
Von Am, W. et al., 1971. Hudson 7 0 gravity observations 62.9" S to 57.5"N along 150" W, Woods Hole
Oceanogr. Ins. Rep., 71-68, 3 pp.
Walcott, R. I., 1970. Flexural rigidity, thickness and viscosity of the lithosphere, J. geophys. Res., 75,
3941-3954.
Walcott, R. I., 1976. Lithospheric flexure, analysis of gravity anomalies and the propagation of seamount
chains, International Woollard Symposium, Monogr. Am. geophys. Un., 1 9 , 4 3 1-438.
Watts, A. B., 1978. An analysis of isostasy in the world's oceans: part 1. Hawaiian-Emperor seamount
chain, J. geophys. Res., 83,5939-6004.
Watts, A. B., Bodine, J. H. & Ribe, N. M., 1980. Observations of flexure and the geological evolution of
the Pacific Ocean basin, Nature, 283,532-537.
Watts, A. B. & Cochran, J. R., 1974. Gravity anomalies and flexure of the lithosphere along the Hawaiian-Emperor seamount chain, Geophys. J. R . astr. SOC.,38, 119-141.
Watts, A. B., Cochran, J. R. & Selzer, G., 1975. Gravity anomalies and flexure of the lithosphere: a threedimensional study of the Great Meteor Seamount, northeast Atlantic, J. geophys. Res., 80, 13911398.
Watts, A. B. & Talwani, M., 1974. Gravity anomalies seaward of deep-sea trenches and their tectonic
implications, Geophys. J. R . astr. SOC.,36, 57-90.
Appendix: isostatic response functions for two-dimensional bathymetry
Most seamounts on the Pacific plate are roughly equidimensional so that a bathymetric
feature of characteristic width 1 along a ship track is likely to have nearly the same width
perpendicular to the track. Ribe (1982) calculated the gravitational admittance that would
be measured over a bathymetric feature (such as a seamount) whose cross-section normal to
the ship track is a Gaussian of width 1:
Z(k)
=TI,,In
dk,Z(k, k,) exp (- l Z k j / 1 6 )
362
N.M.Ribe and A . B. Watts
where Z(k, k y ) is given by (2.1). This admittance may be recast in terms of the equivalent
isostatic response function (Fig. 9) by using (5.7). For single seamounts, assumed equidimensional, the appropriate value of Z is that which best matches the observed width of the
feature along track. A large ensemble of data, however, as in this study, will exhibit
numerous seamounts with a wide range of characteristic widths. For a large data ensemble
the effects of two-dimensionality are likely to be more complex than allowed for in the
simple model of Ribe (1982). (For example, a ship track may pass over the flank, rather
than the peak, of some seamounts.) The expression (Al) may still be used, however, if Zis
interpreted as a parameter describing the overall degree to which one-dimensional admittances or IRFs are unsuitable for interpreting estimates obtained from the data. Although it
does not reproduce in detail the complex two-dimensionality of the ship track data, the
model of Ribe (1952) parameterizes the effect of two-dimensionality in a mathematically
simple and useful way.
The theoretical IRFs used to interpret the IRF estimates will be similar to the solid curves
of Fig. 9. The appropriate value of Z is not known a priori, but may be estimated as follows.
For a given 1, we calculate typical ridge and intraplate IRFs @R ( T = 5km) and
$1 ( T =25 km), using (Al) in (5.7), and form a set of linear combinations fr $1 + (1 - f l ) $ ~
for various fI. The appropriate value of I is that for which the trend of these curves best
matches the trend of the estimates $ ( k ) for long wavelengths. That is, we attempt to
minimize the oblique trend of $ ( k ) across the theoretical curves, evident in Fig. 8. The
theoretical curves determined by the above method are shown with the estimates $(k) in
Fig. 10. The corresponding values of I are 225, 200, 225 and 250km for regions E-H,
respectively.
In determining I and reading fI from Fig. 10, we have considered only wavenumbers
k < 0.05 km-'(A > 125 km). Fig. 10 shows that, for regions E and G, $ ( k ) does not approach
zero for k > 0.05 km-'. This is probably because most of the short-wavelength bathymetric
power in regions E and G is accounted for by low-amplitude (< 1 km) features of very small
(< 50km) effective width 1. The corresponding short-wavelength IRFs should hence be
similar to the curves of Fig. 9 with Z = 20 or 5Okm. Most of the long-wavelength
(k< 0.05 h-'bathymetric
)
power in these regions is accounted for by large features,
however, for which the iarger values of Z determined in the preceding paragraph should be
appropriate .