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Stress
Fields and the Distribution of Intraplate Volcanism
in the Pacific Basin |
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Abstract
Reconstruction of the Pacific basin for the Early
Cretaceous through Recent shows that oceanic plateaus
formed along ocean ridge systems in the wake of retreating
ridge triple junctions, and that ocean island volcanism
either followed pre-existing fracture zones/lineaments
or was localised along lines of incipient plate tearing
induced by variations in plate margin geometry. The
distribution of volcanism is more regular than would
be expected from deep mantle plumes, but is consistent
with suggestions made over a quarter century ago of
control by stresses transmitted from the boundaries
of the plates.
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Before the concept of hotspots/plumes
was widely invoked, Jackson & Shaw [1975]
proposed the distribution of intraplate volcanism
in the Pacific basin was controlled by plate interactions
inducing changes in the stress field of the plate
(see also Cracks
& Stress page). At the time such concepts
could not be proven as the bathymetry of the ocean
floor and the tectonic history of the basin margins
were poorly known. With the increasing popularity
of the mantle plume concept, interpretations switched
to depicting a random distribution of volcanism generated
from deep-seated thermal anomalies. However, few examples
of intraplate volcanism in the basin have been shown
to conform to the predictions of the plume model.
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Figure:
Reconstruction of the Pacific basin from Early
Cretaceous through Recent based on the paleomagnetic
frame [from Smith, 2003]. Oceanic plateau volcanism
occurred at triple junctions along ocean ridge
systems. Ocean island volcanism is divided into
those chains (mostly Late Cretaceous to Eocene)
which followed pre-existing fractures, and those
which followed propagating fractures induced by
plate margin geometry. The fracture system was
created by the geometry of the ocean ridge systems
as the Pacific plate grew in size. North-northwest/south-southeast
lineaments include the Kashima-Eltanin and Emperor-Easter
megatrends of Smoot [1999] and are traced back
in the reconstruction to transform faulting on
the Pacific-Izanagi and Pacific-Phoenix ridges.
West-southwest/east-northeast lineaments include
the Mendocino and Murray fracture zones and others
created by transform faulting on the Pacific-Farallon
ridge. Island chains following the fracture systems
show the greatest non-linearity in age progression. |
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Potentially only three examples (Easter-eastern
Mid Pacific Mountains, Louisville-Ontong
Java, Marquesas-Hess/Shatsky)
possess the plateau-island chain sequence expected
from the plume head-tail model [Clouard
& Bonneville, 2001]. Non-linear age
progressions are found in many island chains,
including the Cook-Austral-Marquesas,
Marshall-Gilberts,
and Line Islands.
Even the Hawaiian-Emperor
chain, generally considered the archetypical
example of plume volcanism, does not have the
features expected in the plume model. The volcanism
lacks an associated plateau, eruption rates
have increased rather than decreased over time,
and paleomagnetic evidence [Sager,
2002] indicates any hotspot could only have
been stationary for half its history. Consideration
of other models is therefore long overdue.
Correlations between volcanic output along
the ocean island chains can now be correlated
with basin-wide plate re-organisations such
as at 25 and 5 Ma [Kamp, 1991; Smith,
2003] suggesting the model of Jackson &
Shaw [1975] was essentially correct. Reconstruction
of the evolution of the Pacific basin demonstrates
that oceanic plateaus were generated in zones
of tension in the wake of retreating triple
junctions, and that ocean island chains may
be divided on the basis of propagating- or leaky-fracture
origin [Smith, 2003]. The latter, including
the Louisville,
Marshall-Gilbert,
Line Island,
and Cook-Austral-Marquesas
chains are those characterised by non-linear
age progressions. Such volcanism followed pre-existing
north-northwest/south-southeast trending fracture
zones such as the Kashima-Eltanin
and Emperor-Easter
megatrends [Smoot, 1999]. The fracture
zones form part of a pattern of orthogonally
intersecting lineaments which are suggested
to have been initiated by transform faulting
along ridge systems during the early history
of the Pacific plate.
Volcanism attributed to propagating fractures
includes the Sala
y Gomez, Juan
Fernandez, and Caroline
chains which extrapolate to breaks in nearby
subducting slabs suggesting stressing of the
plate by convergent margin geometry [Favela
& Anderson, 1999; Smith, 2003].
The Emperor chain is unique
in the stress field model in representing volcanism
along a propagating fracture induced at a divergent
margin. The location and orientation of this
chain is attributed to the geometry of the
Kula-Pacific ridge
following plate re-organisations at 82 Ma which
prematurely halted triple-junction volcanism
on Meiji seamount.
Subsequent volcanism along the Hawaiian
chain can be explained by re-orientation of
the stress field to control by convergent margin
geometry following abandonment of the
Pacific-Kula ridge,
and does not require a change in Pacific plate
motion at the time of the Hawaiian-Emperor
bend (43 Ma).
The distribution of Pacific intraplate volcanism
is therefore more regular than would be expected
than in the plume model. It can be explained
as a result of shallow volatile-bearing sources
tapped under developing hotcell conditions.
The change from oceanic plateau to island chain
volcanism reflects changes in the stress field
as the Pacific plate changed from having an
intra-oceanic setting bordered by ocean ridge
systems, to subducting beneath the basin margins
[Smith, 2003].
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Clouard, V., and A. Bonneville,
How many Pacific hotspots are fed by deep-mantle
plumes?, Geology, 29,
695-698, 2001.
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Favela,
J., and D.L. Anderson, Extensional tectonics and
global volcanism, in Problems in Geophysics
for the New Millenium, Editrice Compositori,
edited by E. Boschi, G. Ekstrom, and A. Morelli,
pp. 463-498, Bologna, Italy, 1999.
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Jackson, E.D., and H.R. Shaw,
Stress fields in central portions of the Pacific
plate: delineated in time by linear volcanic chains,
J. geophys. Res., 80,
1861-1874, 1975.
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Kamp, P.J.J., Late Oligocene
Pacific-wide tectonic event, Terra Nova,
3, 65-69, 1991.
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Sager, W.W., Basalt core paleomagnetic
data from Ocean Drilling Program Site 883 on Detroit
Seamount, northern Emperor seamount chain, and implications
for the paleolatitude of the Hawaiian hotspot, Earth
planet. Sci. Lett., 199, 347-358,
2002.
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Smith, A.D., A re-appraisal of
stress field and convective roll models for the
origin and distribution of Cretaceous to Recent
intraplate volcanism in the Pacific basin, Int.
Geol. Rev., 45, 287-302, 2003.
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Smoot, N.C., Orthogonal intersections
of megatrends in the Western Pacific ocean basin:
A case study of the Mid Pacific mountains, Geomorphology,
30, 323-356, 1999.
last updated 7th June, 2004 |
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