Download On the shallow origin of hotspots and the westward drift of the

Geological Society of America
Special Paper 388
2005
On the shallow origin of hotspots and
the westward drift of the lithosphere
Carlo Doglioni*
Dipartimento di Scienze della Terra, Università la Sapienza, Rome 00185, Italy
David H. Green
Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia
F. Mongelli
Dipartimento di Geologia e Geofisica, Università di Bari, Bari 70125, Italy
ABSTRACT
Intraplate migrating hotspots, which are unrelated to rifts or plate margins in
general, regardless of their origin in the mantle column, indicate relative motion between the lithosphere and the underlying mantle in which the hotspot source is located.
Pacific plate hotspots are sufficiently fixed relative to one another to represent an
independent reference frame to compute plate motions. However, the interpretation
of the middle asthenosphere rather than the deep lower mantle as the source for intraplate Pacific hotspots has several implications. First, decoupling between the lithosphere and subasthenospheric mantle is greater than recorded by hotspot volcanic
tracks (>100 mm/yr) due to undetectable shear in the lower asthenosphere below the
magmatic source. The shallower the source, the larger the décollement.
Second, computation of the westward drift is linked to the Pacific plate and assumes that the deep lower mantle, below the decoupling zone, sources the hotspots
above. The Pacific plate is the fastest plate in the hotspot reference frame and dominates the net rotation of the lithosphere. Therefore, if decoupling with the subasthenospheric mantle is larger, the global westward drift of the lithosphere must be
faster than present estimates, and may possibly vary between 50 and 90 mm/yr. In this
case, all plates, albeit moving at different velocities, move westward relative to the
subasthenospheric mantle.
Finally, faster decoupling can generate more shear heating in the asthenosphere
(even >100 °C). This amount of heating, in an undepleted mantle, could trigger scattered intraplate Pacific volcanism itself if the viscosity of the asthenosphere is locally
higher than normal. The Emperor-Hawaiian bend can be reproduced when bent viscosity anisotropy in the asthenosphere is included. Variations in depth and geometry
in the asthenosphere of these regions of higher viscosity could account for the irregular migration and velocities of surface volcanic tracks. This type of volcanic chain
has different kinematic and magmatic origins from the Atlantic hotspots or wetspots,
*E-mail: [email protected].
Doglioni, C., Green, D., and Mongelli, F., 2005, On the shallow origin of hotspots and the westward drift of the lithosphere, in Foulger, G.R., Natland, J.H.,
Presnall, D.C., and Anderson, D.L., eds., Plates, plumes, and paradigms: Geological Society of America Special Paper 388, p. 735–749, doi: 10.1130/2005.2388(42).
For permission to copy, contact [email protected]. ©2005 Geological Society of America.
735
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C. Doglioni, D.H. Green, and F. Mongelli
which migrate with or close to the oceanic spreading center and are therefore platemargin related.
Keywords: shallow hotspots, shear heating, asthenosphere viscosity, westward drift, reference frame
INTRODUCTION
Because Hawaii is the archetypal hotspot, the depth at which
it is sourced is particularly important, not least because its volcanic track constrains the computation of the absolute plate
motions on Earth and it influences greatly models of mantle
dynamics. If the origin of Pacific intraplate hotspots is in the
asthenosphere, what triggers the extraction of the extra melt?
Why are the hotspots isolated, scattered, and sporadic? Their
tracks suggest relatively stationary sources in the mantle (Wilson, 1963, 1973; Morgan, 1971) beneath an overlying lithosphere
that moves independently. In other words the mantle moves
east-southeast along the Hawaiian track (e.g., Ihinger, 1995).
This motion requires a decoupling between the two layers, and
the zone of decoupling is usually thought to be located in the
asthenosphere; that is, the layer with the lowest viscosity in
the mantle (e.g., Anderson, 1989). This decoupling zone is too often disregarded in geodynamic studies.
The nature and depth of hotspots is a matter of debate
(Anderson, 2000; Courtillot et al., 2003). Hotspots form in most
geodynamic environments—along or close to oceanic ridges
and continental rifts (e.g., Iceland, Afar), along transform faults
(e.g., Réunion), in the hanging wall of subduction zones (e.g.,
Yellowstone), and in intraplate settings (e.g., Hawaii). Therefore,
different hotspots may have different depths of origin (Doglioni,
1993). Geochemical, geophysical, and petrological evidence
is growing for a shallow source for hotspots (Bonatti, 1990; Anderson, 1999; Smith and Lewis, 1999; Foulger, 2002). The depth
of origin of Hawaiian magmatism and similar Pacific intraplate
hotspots is also strongly debated. A few authors (Smith, 1993;
Smith and Lewis, 1999; Anderson, 2000; Janney et al., 2000;
Green et al., 2001), argued against the deep mantle plume paradigm (e.g., Schubert et al., 2001). Some authors also propose
crack propagation in the lithosphere along hotspot tracks as the
reason for Pacific intraplate magmatism (Winterer and Sandwell,
1987; Sandwell et al., 1995; Lynch, 1999). Moreover, some seismic tomography results do not show lowered velocity beneath
the Hawaiian hotspot (Wolfe et al., 2002; Foulger et al., 2003).
Many workers still prefer a deep source at the core-mantle
boundary (CMB) for some hotspots (Zhao, 2001; Courtillot et al.,
2003), although there are now fewer adherents to this proposal
than in the past (Ritsema and Allen, 2003). Heatflow does not
show any significant anomaly on the Hawaiian rise (Shaw and
Jackson, 1973; von Herzen et al., 1989; Stein and Stein, 1992,
1993). Deep mantle plumes would be expected to generate a
long-wavelength regional increase in heatflow values (Anderson,
2000). The largest heat dissipation of the mantle occurs along
mid-ocean ridges, and hotspots represent the smallest component of the Earth’s cooling budget.
Hawaiian lavas are thought to come from ~80–120 km depth
or shallower. Petrological estimates of the temperature anomalies beneath Hawaii compared with ridges vary from zero to
a maximum of ~200 °C (Green et al., 2001; Gudfinnsson and
Presnall, 2002). Plume models require temperature anomalies
of 200–600 °C (e.g., Cordery et al., 1997). A small change in
temperature, volatile content, or fertility of the upper mantle can
lead to a large change in the extent of partial melting and melt
volumes (Yaxley, 2000; Green et al., 2001; Asimow and Langmuir, 2003; Foulger et al., 2003). A number of authors (Jackson
et al., 1972; Shaw and Jackson, 1973; Shaw et al., 1980) noted
a periodicity of the magmatism in the Hawaiian-Emperor chain
and its en echelon distribution of volcanoes, and they emphasized the role of the stress field in controlling magma generation.
Shaw (1973) and Shaw and Jackson (1973) proposed that linear
island chains in the Pacific are related to mechanical rather than
thermal anomalies. Three types and depths of mantle source
were proposed by Courtillot et al. (2003) in an attempt to reconcile the different opinions about the origins of hotspots.
In this paper, we explore the kinematic and magmatic implications and consequences of shallow interpretations of hotspot
sources. We show that the shallower the source of the magmatism, the faster the westnorthwest motion of the Pacific and the
entire Earth’s lithosphere relative to the subasthenospheric
mantle. Moreover, the decoupling between lithosphere and subasthenospheric mantle should produce shear heating in the intervening asthenosphere sufficient to generate locally intraplate
Pacific magmatism (e.g., at Hawaii).
ASTHENOSPHERIC SOURCE OF
PACIFIC HOTSPOTS AND WESTWARD
DRIFT OF THE LITHOSPHERE
The Nuvel 1 (DeMets et al., 1990) and the Space Geodesy
ITRF2000 and NASA database models (Heflin et al., 2003) describe plate motions. They are based on the artificially imposed
assumption of a no-net-rotation (NNR) reference frame: Plates
are imagined to move relative to a fixed Earth center and the
lithosphere is stationary relative to the underlying mantle, or
the sum of its movements is zero. However, we know that the
lithosphere moves relative to the subasthenospheric mantle, as
not only suggested by the Hawaiian and similar tracks, but as
kinematically required by the relative migration of plate margins.
On the shallow origin of hotspots and the westward drift of the lithosphere
The ITRF2000 model (e.g., Heflin et al., 2003) is excellent for
describing relative plate motions and is also considered an absolute reference frame (relative to the Global Positioning System constellation or the Earth’s center of mass). However, when
magmatic sources are included in the kinematic analysis, the
movement of the lithosphere relative to the mantle should then
be taken into account in an “absolute” plate motion analysis, and
the NNR should be abandoned. The absolute reference frame
is quite a controversial issue. Typically, the absolute reference
frame is considered to be that of the hotspots.
When plate motions are measured in the hotspot reference
frame, the lithosphere shows a net westward drift (Bostrom,
1971; O’Connell et al., 1991; Ricard et al., 1991). This westward
drift persists when plate motions are computed relative to Antarctica (Le Pichon, 1968; Knopoff and Leeds, 1972). Usually
the drift is computed using a larger number of hotspots on other
plates, where the net rotation of the lithosphere is 49 mm/yr
(0.44 ± 0.11 degrees/m.y.) about a pole at 56° S, 70° E (Gripp
and Gordon, 2002). Most of the hotspots used, however, are not
fixed; nor do they represent a fixed reference frame, because
they are located on plate margins, such as moving ridges (Galapagos, Easter Island, Iceland, Ascension), transform faults (Réunion), above subduction zones (Yellowstone), or in continental rifts (Afar), all features that move relative to one another and
to the mantle. To know whether the hotspots are fixed relative to
one another (Molnar and Stock, 1987; Steinberger, 2002) is crucial for determining a reliable hotspot reference frame. Norton
(2000) grouped hotspots into three main families that have very
little internal relative motion (Pacific, Indo-Atlantic, and Iceland). He concluded that a global hotspot reference frame is not
possible because Pacific hotspots move relative to Indo-Atlantic
hotspots and to Iceland. However, in his analysis, Pacific hotspots are reasonably fixed relative to one another during the past
80 Ma. Present-day plate motions, based on space geodesy data
(Heflin et al., 2003), confirm that Pacific intraplate hotspots
move as a unit, whereas all other so-called “hotspots” move in
various directions and with different velocities.
The westnorthwest motion of the Pacific plate relative to
the underlying mantle is inferred from the Hawaiian and other
major intraplate hotspot tracks (Marquesas, Society, Pitcairn,
Samoan, MacDonald), which show an average velocity of ~103–
118 mm/yr. They also move along the same trend (290°–300°
westnorthwest) and therefore are the only hotspots that appear
to be fixed relative to one another, representing an apparently
reliable reference frame for absolute plate motions. When the
westward drift is computed only taking those Pacific intraplate
hotspot tracks that are relatively fixed during the past few million years (Gripp and Gordon, 2002), the net rotation of the
lithosphere rises to ~90 mm/yr (M. Cuffaro, 2004, personal
commun.), with a pole of rotation still deviated with respect to
the Earth’s axis of rotation.
Provided that the velocity of the Pacific lithosphere, Vl , toward the eastsoutheast (110°–120°) is slower than that of the
underlying subasthenospheric mantle, Vm (i.e., Vm > Vl ), the rel-
737
ative motion corresponding to the westnorthwest delay of the
lithosphere is Vm – Vl = 103 mm/yr. However, assuming that
shear is distributed throughout the asthenospheric channel, a
location for the Hawaiian melting spot in the asthenosphere
instead of in the upper mesosphere would imply a greater displacement between the lithosphere and mesosphere than is
recorded in hotspot traces (Fig. 1). Therefore, if Hawaiian magmatism is in the upper part of the asthenosphere, there should
be a deeper component of shear, which would increase the total
relative velocity. Greater velocity has two basic consequences:
(1) It increases the global westward drift of the lithosphere,
and (2) it increases shear heating in the asthenosphere. If the
source of Pacific hotspots is in the middle of the asthenosphere,
half of the lithosphere–subasthenospheric mantle relative motion is unrecorded, which means that the total relative displacement would amount to ~200 mm/yr. Using this velocity to
compute the net rotation of the lithosphere toward the west, no
one plate still moves eastward relative to the mantle.
There is evidence that the propagation rate of Pacific “hotspots” or seamount tracks has varied over time and even included jumps back and forth and oblique propagation relative to
absolute plate motion. This evidence casts doubt on both the
notion of absolute plate motions computed in the hotspot reference frame, and the nature of the magmatism itself. Different
models have been proposed on the origin of these magmatic overproductions, such as deep plume, or alternatively, shallow plumes
generated by cracks, or boudins of the lithosphere (Winterer and
Sandwell, 1987; Sandwell et al., 1995; Lynch, 1999; Natland
and Winterer, 2003). Compositional heterogeneity in the mantle
has also been invoked, particularly because no significant positive thermal anomaly relative to normal mid-ocean ridge has
been observed (e.g., Stein and Stein, 1992, 1993). Janney et al.
attribute a velocity for the Pukapuka volcanic ridge (interpreted
as either a hotspot track or a leaky fracture zone) in the eastern
central Pacific of ~200–300 mm/yr (see Fig. 2a in Janney et al.,
2000), with a ridge age of between 5 and 12 Ma. They also inferred a shallow mantle source for Pacific hotspots, based on their
geochemical characteristics.
SHEAR HEATING AS A CAUSE FOR
PACIFIC HOTSPOTS?
Kennedy et al. (2002) have shown how mantle xenoliths
recorded shearing possibly located at the lithosphere-asthenosphere interface. This shearing supports the notion of flow in the
upper mantle and some decoupling at the base of the lithosphere
(Russo and Silver, 1996; Doglioni et al., 1999; Bokelmann and
Silver, 2000). The fastest plate on Earth in the hotspot reference
frame (i.e., the Pacific) is the one affected by the most widespread intraplate magmatism. Is there a relation between these
two observations? In Africa, so-called “hotspots” are located
along rift zones, and therefore they are not simple intraplate or
boundary-free magmatic sources. Because the Pacific appears
to be the fastest-moving plate relative to the mantle (e.g., Gripp
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C. Doglioni, D.H. Green, and F. Mongelli
Figure 1. The Hawaiian volcanic track
indicates that there is decoupling between the magma source and the lithosphere, which is moving relatively toward
westnorthwest. If the source is below the
asthenosphere (i.e., in the subasthenospheric mantle, option 1), the track
records the entire shear between the
lithosphere and mantle. In the case of an
asthenospheric source for the Hawaiian
hotspot (option 2), the volcanic track
does not record the entire shear between
the lithosphere and subasthenospheric
mantle, as part of it operates below the
source (deep missing shear). Moreover,
the larger decoupling implies larger
shear heating, which could be responsible for the numerous scattered intraplate
hotspots.
and Gordon, 2002), in the asthenospheric decoupling zone at its
base there should occur the highest shear or viscous heating.
This heating is a function of the lithostatic load acting on the decoupling layer, the friction and velocity of the decoupling (shear
stress), the viscosity of the medium, the velocity of the decoupling, the thickness and thermal gradient between the top and
bottom of the decoupling zone (i.e., the asthenosphere), thermal
diffusivity, specific heat, and so on.
The ability of frictional heating to cause partial melting,
particularly in subduction zones, remains a matter of controversy (Schubert et al., 2001). Yuen et al. (1978) studied frictional
heating on slip zones using thermally activated creep laws for
the deformation. Because rock becomes weak as the solidus is
approached, these authors concluded that the maximum temperatures associated with frictional heating are always less than those
required for partial melting. Shaw (1969, 1973) suggested shear
heating in the asthenosphere generated by the decoupling between lithosphere and subasthenospheric mantle related to tidal
torque (Shaw et al., 1971) as the cause of Hawaiian magmatism. With a different mechanism, such as a combination of
lithospheric flexure and viscous shear heating at the base of the
lithosphere, Hawkins and Natland (1975) and Natland (1980)
proposed a model of magma generation unrelated to a deep mantle plume for the Samoan linear volcanic chain.
It is noteworthy that the fast Pacific plate overlies asthenosphere with the lowest viscosity (5 × 1017 Pa⋅s; Pollitz et al.,
1998), and possibly the least depleted mantle, which is therefore
the most prone to melt. Because of the melting characteristics
of peridotite with minor carbon + hydrogen (the lherzolite–[C +
H+O] system), the asthenosphere is already partly molten (e.g.,
Schubert et al., 2001) and is at a temperature of ~1430 °C
(e.g., Green and Falloon, 1998; Green et al., 2001). A rise in
temperature of only a few tens of degrees will increase the
degree of melting, which, in a deforming material, will migrate
toward the surface. We postulate that locally, the viscosity of the
asthenosphere can also increase (e.g., 1019 Pa(s) due to refractory geochemical anisotropy), or decrease due to locally higher
water activity. Shear stress could be irregularly distributed in
such inhomogeneous material and consequently, higher shear
heating may develop locally.
Assuming a lithosphere 10,000 km long, 100 km thick, and
with a velocity of 200 mm/yr relative to the subasthenospheric
mantle, can the internal shear or viscous heating generate sufficient excess temperature to produce a large amount of melt in the
low-velocity channel? Watson and McKenzie (1991) estimated
that for Hawaiian magmatism, the maximum mantle adiabat has
a potential mantle temperature of 1830 K, the degree of partial
melting is ~6.9%, and the depth range of melting is 72–127 km
(i.e., in the asthenosphere). Recently, Sisson (2003) suggested
that trace and major element features of Hawaiian tholeiites
point to separation from sources shallow in the garnet lherzolite
stability field, at a pressure of 30 ± ~3 kb, corresponding to depths
On the shallow origin of hotspots and the westward drift of the lithosphere
of ~90–100 km. In the same study, a temperature of 1470–1520 °C
(~1740–1790 K) was suggested for the Hawaiian source.
Subsidence related to cooling of the oceanic lithosphere
does not fit with old oceanic crust (>80–100 Ma) where the
bathymetry flattens (Nagihara et al., 1996). Flattened oceanic
bathymetric profiles have been related to radiogenic heating in
convecting mantle (Jarvis and Peltier, 1980) or asthenospheric
flow (Phipps Morgan and Smith, 1992). The apparent constant
lithosphere thickness at 100–80 Ma has also been attributed to
the form of the peridotite–(C + H+O) solidus when it is influenced by pargasite (amphibole) stability at <2.8–3.0 GPa (Green
and Liebermann, 1976). As an alternative hypothesis, extra heating of the lithosphere could be attributed to the shear at the base
of the plate. The hypothesis of shear heating as a mechanism for
generating more melting is tested in the following section. Areas
of higher viscosity, acting as asperities in the asthenospheric
decoupling zone, should generate local tension in the overlying
lithosphere, resulting in thinning of the lithosphere and upwelling of underlying the asthenosphere.
739
is produced by the shear stress:
du
µu0
τ = µ ——– = ——
= τ0 ,
d(–y)
h
where m is the viscosity, u0 is the velocity of the lithosphere
relative to the mantle, h the thickness of the asthenosphere, and
y is the vertical axis. According to Turcotte and Schubert (2002),
the rate of shear heating per unit volume is:
µu02
——
= constant,
h2
because the shear stress is constant and the velocity profile is
linear.
The temperature distribution in the channel is given by the
integration of:
d 2T
µu02
k——
= – ——,
2
dy
h2
THERMAL MODEL
Asthenospheric shear flow may be represented by simple
Couette flow between plane parallel walls, as shown in Figure 2, assuming a simple linear rheology. The upper wall (y =
0), which is the base of the lithosphere, has a velocity u = u0 and
temperature T = T0. The lower wall (y = h) is the top of the subasthenospheric mantle, which has a velocity u = 0 and temperature T = T1. The velocity profile:
where k is the thermal conductivity, with the boundary conditions
T = T0 at y = 0, and T = T1 at y = h. The solution in dimensionless form is (Turcotte and Schubert, 2002):
T – T0
y
µu02/2k
y 2 µu02 /2k
θ = —–—–
= — 1 + ———–
– —–
———– ,
T1 – T0
h
T1 – T0
h2 T1 – T0
{
} (
(
) ( )
)
or
( )
y
PrE
y 2 PrE
θ = — 1 + —— – —–
—— ,
h
2
h2 2
y
u = u0 1 – — ,
h
where
u02
E = —————
Cp(T1 – T0)
is the Eckert number and Cp is the specific heat at constant pressure, and
µ
Pr = —–
ρα
Figure 2. Relative motion of the lithosphere and the subasthenospheric
mantle is imposed. Asthenospheric shear flow may be represented by
simple Couette flow between plane-parallel walls (Turcotte and Schubert, 2002), and the shear stress is assumed to be equal throughout
the asthenospheric section. The upper wall (y = 0), which is the base
of the lithosphere, has velocity u0 and temperature T0. The lower wall
(y = h) is the top of the subasthenospheric mantle and has velocity u =
0 and and temperature T1.
is the Prandtl number, where ρ is the density and α the thermal
diffusivity.
The maximum excess temperature due to frictional heating,
with respect to the linear temperature profile is (Turcotte and
Schubert, 2002):
PrE
θemax = ——,
8
and occurs at y/h = 0.5.
740
C. Doglioni, D.H. Green, and F. Mongelli
TABLE 1. VISCOSITY VARIATIONS IN
THE ASTHENOSPHERE VERSUS AMOUNT OF
DECOUPLING BETWEEN THE LITHOSPHERE
AND SUBASTHENOSPHERIC MANTLE
Viscosity
µ (Pa⋅ s)
Velocity
u0 (cm⋅ yr −1)
Excess temperature
θemax (K)
4 × 1019
1020
4 × 1019
2 × 1020
1020
10
10
20
10
20
12
30
48
60
120
Note: The resultant excess temperatures given in the last column are
determined by the shear heating.
Assuming k = 4 W/m/K, and T1 – T0 = 300 K (the temperature increase of all of the asthenosphere), the variation in viscosity and velocity can result in different excess temperatures
(θemax), as listed in Table 1.
As can be seen from the table, small local viscosity changes
might significantly increase the temperature excess. If the speed
of the Pacific plate or asthenosphere viscosity were greater,
shear heating could be even higher than 120 K (Fig. 3). A Pacific hotspot could represent a local heat source that forms in the
asthenosphere when a combination of shear stress and locally
higher viscosity occur. Melting of the asthenosphere would locally decrease viscosity, but the residual asthenosphere itself
would acquire a higher viscosity as a result of water being extracted (Braun et al., 2000). A decrease in viscosity would decrease shear heating and diminish melt production. Motion over
a new area of higher viscosity would start a new cycle of stronger
shear heating and melt generation. This cyclic process could be
responsible for the fluctuation or periodicity of magmatic sources
recorded at the surface in the volcanic trail (Shaw et al., 1980).
Lower, largely distributed shear heating at the lithospheremantle interface could also explain the observation that the
oceanic floor does not subside as predicted by cooling of the
lithosphere, but flattens out when the lithosphere is older than
the mid-Cretaceous.
PETROLOGICAL MODEL
In major and minor element concentrations, mid-ocean ridge
basalts (MORBs) are inferred to be derived from a well-mixed
“primitive modern mantle” source, although detailed observations including isotopic signatures show that long-lived heterogeneity is required. Different isotopic end-members have been
proposed. In the Hawaiian volcanic chain, differences both within
and between volcanoes occur, including major element, minor
element, trace element, and isotopic signatures. These differences
have led to inferences of greater heterogeneity in this “hotspot”
source. Particular trace element patterns and ratios and distinctive isotopic ratios have been attributed to primordial mantle,
old oceanic crust, sediment, and arc signatures, implying com-
Figure 3. If the viscosity of the asthenosphere is locally higher than
normal, the shear stress and shear heating are also higher, resulting in
an increase in asthenospheric temperature. Variations in lithosphere
velocity (100 or 200 mm/yr) and local increases in viscosity (4 × 1019
or 1020 Pa⋅s) can cause different excess temperatures, ranging from 12 K
to 120 K. Excess temperatures in the higher range of those estimated
could cause extra melting and possibly uprising intraplate magmatism.
plexity and longevity in the Hawaiian “hotspot” source and the
absence of the homogenizing process that has given rise to the
MORB source. The major element compositions of primitive
magmas, and thus the nature of liquidus phases at various pressures and temperatures, have been interpreted as requiring a more
refractory source and harzburgite residues for Hawaiian magmas
compared to primitive MORB (Green and Falloon, 1998; Green
et al., 2001). These petrological arguments have been linked
to geophysical interpretations of the cause of the broad topographic high (the Hawaiian swell) underlying the Hawaiian
volcanic chain to suggest that the buoyancy anomaly reflects a
compositional rather than a thermal anomaly beneath Hawaii.
The phenomenon of selective shear heating linked to viscosity
variations in the asthenospheric mantle obviously influences
these models.
The melting behavior of lherzolitic mantle (pyrolite) containing a little water and carbon (required for sources of either
MORB or hotspot basalts) is defined by the pargasite-bearing lherzolite solidus to ~3 GPa and then by the garnet-lherzolite-(C +
H+O) solidus at deeper levels. The latter solidus is sensitive
to mantle oxidation state (fO2), which determines the CH4/
H2O ratio of potential subsolidus fluid and thus controls solidus
temperature (Green and Falloon, 1998). The asthenosphere is
inferred to have lower viscosity because of a very small melt
fraction—a region of incipient melting (Green and Liebermann,
1976; Green and Falloon, 1998). In Figure 4, the incipient melt
region is contoured for melt fraction assuming 0.1% H2O (and
C content of ~0.01%, equivalent to 400 ppm CO2), and the
oceanic intraplate geotherm depicted lies between 0.5% and 1%
melt contours. If shear stress and heating is distributed throughout the asthenosphere, a temperature increase of ~120 °C caused
On the shallow origin of hotspots and the westward drift of the lithosphere
741
both melting and strain in the boundary zone, augmenting melting by the shear-heating effect.
KINEMATIC MODEL
Figure 4. Solidus and melting behavior of lherzolite mantle with minor
water and carbon contents (0.1% H2O assumed). Anhydrous (volatilefree) solidus and percentage melting contours (10, 20, and 30%) are
shown and compared with the incipient melting regime between the
anhydrous solidus and the solidus for 0.1% H2O. The incipient melting
regime is contoured (heavy lines, 0.5–20%) for percentage melting.
The oceanic intraplate geotherm enters the incipient melting regime at
~90 km, giving rise to the asthenosphere-lithosphere contrast. The possible perturbation of the geotherm by shear heating in the asthenosphere
is illustrated by the dotted curve. Approximately 6% pargasite is inferred below the solidus, and the dashed lines limit fields of plagioclase
lherzolite, spinel lherzolite, and garnet lherzolite residual mineralogy.
Adapted from Green and Falloon (1998) and updated from Green and
Liebermann (1976).
by shear heating would approximately double the melt fraction
to 1.5% melting. If more localized compositional heterogeneity
includes refractory, residual compositions lacking (H2O + CO2),
then increased solidus temperature may produce higher viscosities, locally increased shear stress, and shear heating in the
refractory volume. Alternatively, the fO2 contrasts inferred at
the interfaces between old subducted lithosphere suspended in
the asthenosphere or in the subasthenospheric upper mantle, and
normal MORB mantle (Green and Falloon, 1998) may focus
The Emperor-Hawaiian bend does not correspond to a
change in plate motion of the Pacific plate because there is no
corresponding bend in transform faults at the same time (Norton, 1995; Richards and Lithgow-Bertelloni, 1996; Raymond
et al., 2000). Scholl (2003) noted that between ca. 76 and 43 Ma,
the linear chain of Emperor seamounts records a southwardmoving hotspot across a generally westnorthwest-moving Pacific
plate. This observation means that the so-called “EmperorHawaiian bend” does not record a ~40° change (northnorthwest
to westnorthwest) motion of the Pacific plate but rather, a great
slowing or cessation of hotspot drift. According to the same
authors, while the Hawaiian chain was formed, the hotspot remained fixed with respect to the Earth’s spin axis. The EmperorHawaiian bend has been redated to ca. 47 Ma (Pilger and
Handschumacher, 1981), and the redating of rocks still supports
the picture that the volcanoes increase in age systematically to
the northwest, away from the active volcanoes in Hawaii (Clague,
2003). Using the revised calibration of magnetic anomalies of
Cande and Kent (1995), Norton (2000) pointed out an unexplained variation in plate motion at 47 Ma when comparing
North and South America relative to the Pacific. Shaw et al.
(1980) showed that the Emperor and Hawaiian volcanic chains
must have the same magmatic origin, despite the bend. In other
words, we can argue that the Emperor hotspot migrated obliquely
to the Pacific absolute plate motion, whereas the Hawaiian
chain is parallel to the Pacific absolute plate motion. How is it
possible to reconcile these data in a realistic kinematic model?
Vertical and lateral movements of the magmatic source in the
décollement zone (i.e., the asthenosphere) could account for
these observations (Fig. 5). Anisotropy in the composition and
viscosity of the asthenosphere, lying both oblique and parallel
to the plate motion, can explain the Emperor and Hawaiian
chains (Fig. 5). For an area of higher asthenospheric viscosity
that is oriented northeast-southwest and generating melt, the
overlying plate moving westnorthwestward will record a volcanic track oriented northwest-southeast, such as the Emperor
chain (Fig. 5). In this way, the faster southward migration of the
Emperor hotspot compared with Hawaiian migration rates can
be explained because the oblique vector in a 10-m.y. incremental interval is longer (faster) than the plate motion vector (top
panel of Fig. 5).
Tarduno et al. (2003) suggested a southward motion of the
Emperor hotspot, based on paleomagnetic and radiometric data.
Paleomagnetic data contribute to reconstructing palaeomagnetic
latitude, but not longitude; therefore, the Emperor chain may
also have had an eastward component, although smaller, as the
Hawaiian track shows. In fact, as far as the westward drift is
concerned, plate motions move with the mantle toward the east
only a little slower. The southward component of Emperor hotspot
Figure 5. Kinematic map view model for the bending of the EmperorHawaiian chains, maintaining stable Pacific plate motion. The bend can
be reproduced by introducing a bent, mirrorlike anisotropy in the décollement zone (i.e., the asthenosphere), where locally higher viscosity generates shear heating and related magmatism. The shaded area is
a part of the Pacific plate progressively moving to the westnorthwest
and overriding the asthenosphere. The asthenosphere is assumed to have
a typical viscosity of 1017 Pa⋅s apart from the gray line, where it rises to
~1020 Pa⋅s. In each panel, the red spot indicates the active magmatism.
In the top panel, while the oblique Emperor seamount chain formed,
the velocity of the southeastward-propagating volcanic ridge is faster
because it is oblique to Pacific plate motion (the longer oblique arrow
in the incremental 10-Ma interval).
On the shallow origin of hotspots and the westward drift of the lithosphere
743
Figure 6. The irregular distribution of an area of higher viscosity (e.g., 1020–21 Pa⋅s) in the Pacific asthenosphere (viscosity, 1017 Pa⋅s), such as
that shown by the curved gray band in the pretransit cartoon, could account for the bend in the Emperor-Hawaiian track, in spite of a stable absolute Pacific plate motion direction. The pretransit geometry mirrors the resulting volcanic chain shown in the posttransit cartoon. X, Y, Z, and
H in the asthenospheric asperity correspond to similarly labeled volcanoes in the surface trail. Red letters show the active magmatism. The gray
volcanic track in the left panel indicates inferred, not yet existing volcanoes.
migration is thus not necessarily related to motion of a deep
lower mantle plume, because the surface kinematics can also
be explained by a heterogeneous asthenosphere, in which a viscosity anomaly lies in the low-velocity layer, oblique to the
absolute motion of the overlying plate (Fig. 6). Our model is
clearly ad hoc in the sense that we infer anisotropy in the asthenosphere from the presently observable volcanic trail. Therefore
the model needs to be tested. We postulate two unconstrained
assumptions: (1) the Emperor chain formed obliquely to the
80- to 47-Ma Pacific absolute plate motion; and (2) there are
similar anomalies in the Pacific asthenosphere that generated
the Louisville, MacDonald, and Emperor-Hawaiian volcanic
tracks. In this interpretation, the number of volcanic tracks
would equal the number of anomalies in which the asthenosphere viscosity was and still is high enough to generate extra
melt in the shear zone.
LITHOSPHERE–SUBASTHENOSPHERIC
MANTLE MOTIONS
Relative plate motions can be estimated with great accuracy
with space geodetic data (e.g., Robbins et al., 1993; Heflin et al.,
2003), refining the earlier Nuvel 1 plate-motion model (DeMets
et al., 1990). The east Pacific Rise (EPR), separating the Pacific
and Nazca plates, opens at a rate of 128 mm/yr just south of the
equator (e.g., Heflin et al., 2003). At the same latitude, the shortening along the Andean subduction zone, where the Nazca plate
subducts beneath South America, has been computed to be
~68 mm/yr. These relative motions, when considered in a reference frame in which the Hawaiian hotspot is fixed and positioned in the subasthenospheric mantle, suggest that the Nazca
plate moves eastward relative to the subasthenospheric mantle
at ~25 mm/yr (Fig. 7, option 1). If we assume that the source of
744
C. Doglioni, D.H. Green, and F. Mongelli
the Pacific intraplate hotspots is, instead, in the middle asthenosphere and that half of the relative motion of the lithosphere–
subasthenospheric mantle is missing in the Hawaiian track
(Fig. 1), the rate of motion could be as high as 200 mm/yr, as
is also suggested by some segments of the Pukapuka volcanic
ridge (Janney et al., 2000). Greater speeds, as the Pukapuka
ridge suggests, implies a source from a deeper section of the
asthenosphere relative to the Hawaiian source (Fig. 1). In this
model, the Nazca plate would instead move west relative to the
mantle at 72 mm/yr (Fig. 7, option 2), and therefore all three
plates would move westward relative to the subasthenospheric
mantle. This scenario is in agreement with shear-wave splitting
observations (Russo and Silver, 1994) and the gentle dip of the
Andean slab, which both suggest relative eastward mantle flow.
Similar eastward mantle flow was proposed for the North American plate (Silver and Holt, 2002). The low dip of the Andean
slab has alternatively been attributed to the age of the subducting lithosphere. However, the age has been shown to be insufficient to explain the asymmetry between westerly directed (steep
and deep) versus easterly directed (low dip and shallow) subduction zones. In fact the geographically related asymmetry
persists even where the same lithosphere (regardless of whether
it is oceanic or continental) subducts on both sides, such as in
the Mediterranean orogens (Doglioni et al., 1999). In western
Central America, the dip of the easterly directed slab is steeper
in spite of the young Cenozoic age of the downgoing oceanic
Cocos lithosphere. However, it is known to be shallow, because
most of the seismicity stops at ~200 km, and its dip can be explained by oblique-lateral subduction and by the continental lithospheric thickness of the upper plate. Another counterexample is
the westerly directed Sandwich subduction zone in the southern
Atlantic, where the Cenozoic-to-zero age oceanic lithosphere
dips very steeply and penetrates deep as in the Barbados or Marianas subduction zones.
Jordan (1974) questioned westward drift of the lithosphere
on the grounds that the viscosity required to allow decoupling
of the lithosphere is too low (1011 Pa⋅s) compared with the known
viscosity of the asthenosphere. However, new mechanical models have subsequently been developed to explain the net rotation
of the lithosphere with even higher viscosity values when tidal
drag is coupled with the opposite torque generated by downwelling of heavier materials in the core and mantle and with
asymmetric mantle convection (Scoppola et al., 2005). Another
relevant point raised by Jordan (1974) is that the westward drift
at that time computed by Minster et al. (1974) of 12 mm/yr
was too low compared to the eastward component of a number
of plates (e.g., Cocos and Nazca moving eastward at rates of
>70 mm/yr). This rate would imply that the spreading rate of
the EPR is faster than the absolute westnorthwest velocity of the
Pacific plate, based on the Hawaiian volcanic track, and the
Nazca plate would move relatively eastward faster than the subasthenospheric mantle (Fig. 7, option 1). However, a shallower
source for the origin of Hawaiian magmatism would increase
the calculated westward motion of the Pacific plate to a velocity
Figure 7. Simple kinematic relation between the Pacific, Nazca, and
South America plates. Relative motion vectors (top) after Heflin et al.
(2003). Option 1 (middle) indicates the absolute motions when referred
to the Hawaiian hotspot moving at ~103 mm/yr (Gripp and Gordon,
2002). Option 2 (bottom) is the case for which the hotspot source is
located in the asthenosphere and relative motion between the Pacific
plate and the subasthenospheric mantle is assumed to be 200 mm/yr
(see Fig. 1). In this last configuration, all three plates move westward
relative to the mantle. EPR—east Pacific Rise; S-America—South
America plate.
faster than the spreading rate of the EPR, with the Nazca plate
also moving westward relative to the subasthenospheric mantle
(Fig. 7, option 2).
The lower the asthenosphere viscosity, the faster plates move
westward; that is, there is low coupling between lithosphere and
subasthenospheric mantle. So why does the Nazca plate move
slower with respect to the Pacific plate if it shifts west and travels
above asthenosphere, which was previously underneath the Pacific, allowing fast motion? In other words, when modeling plate
tectonics in the frame of westward drift of the lithosphere, one
unsolved issue is why the eastern plate moves slower relative
to the western plate, if velocity is controlled by lithosphereasthenosphere coupling, which should continuously vary with
the westward motion of the lithosphere. Braun et al. (2000)
show that water extraction during melting results in higher viscosity in the residual mantle of up to two orders of magnitude.
The mantle, once depleted beneath the ridge, should be cooler,
less dense, and more viscous. Therefore, eastward relative movement of the mantle underneath the ridge could explain the higher
seismic velocities detected by mantle tomography in the Nazca
asthenosphere relative to the Pacific asthenosphere in the 140–
300 km interval by Romanowicz and Gung (2002), as well as
the higher Rayleigh and Love wave phase velocities in the
asthenosphere beneath the eastern side of most of the Atlantic
Ocean compared to its western side (Silveira et al., 1998; Silveira and Stutzmann, 2002).
Our model assumes a fixed subasthenospheric mantle,
whereas the lithosphere is moving relatively west, or along
sinusoidal flow lines based on tectonic features (Doglioni, 1990;
On the shallow origin of hotspots and the westward drift of the lithosphere
Doglioni et al., 1999), and confirmed by present-day space
geodetic data (Heflin et al., 2003). This flow coincides quite well
with the fast direction detected for mantle seismic anisotropy in
the asthenosphere (Montagner, 2002; Gaboret et al., 2003), which
supports the model of relative flow in the decoupling zone
(Doglioni et al., 2003). Because the ridge moves west at the sum
of the speeds of the Pacific and Nazca plates divided by 2, the
underlying asthenospheric melting area (where the mantle is
undergoing depletion) is also moving west (Fig. 8). The viscosity of the asthenosphere below the Nazca plate should then be
higher than beneath the Pacific plate; therefore, the Nazca plate
and the subasthenospheric mantle should be better coupled (i.e.,
the Nazca plate moves west slower; Fig. 8). This model predicts
that new fertile asthenospheric mantle is always available along
the ridge, and deep continuous refeeding of the source for generating new oceanic crust is not needed. Kinematically it may
be compared with a mantle bubble that shifts west along the area
of plate separation.
Mantle tomography of the Pacific (Shapiro and Ritzwoller,
2002) shows higher velocities at ~200–250 km beneath Hawaii
(in the middle asthenosphere), whereas velocities are relatively
lower in shallower layers, supporting the idea of a deep “harder”
asthenosphere at the base of the area from which the magmatism
rises. Analysis of shear-wave splitting in the Pacific region shows
a pattern in the asthenosphere that is parallel to the absolute mo-
745
tion of the Pacific. However, as shown by the Hawaiian chain, this
pattern is disturbed by irregularities, testifying to some anisotropy
in mantle flow (Montagner, 2002). This observation supports the
presence of some local asperities (e.g., an increase in viscosity
where higher seismic velocities are reported), which could trigger
local shear heating in the asthenosphere. The proposed asthenospheric anisotropy in the model presented here may be tested in
the future by more refined seismic tomography studies.
A global sinusoidal eastward motion of the subasthenospheric mantle relative to the lithosphere can also be inferred by
the asymmetry of oceanic ridges that are 100–300 m shallower
on their eastern flanks. An eastward-shifting asthenospheric
mantle below the ridges, depleted and therefore lighter, can account for this asymmetry (Doglioni et al., 2003). The asymmetry of subduction zones cannot be ascribed merely to the age of
the downgoing oceanic lithosphere, as it also persists where
continental lithosphere subducts (Doglioni et al., 1999). Flow in
the asthenospheric mantle could also explain the horizontal
asymmetry of seismic anisotropy detected worldwide by Gung
et al. (2003).
CONCLUSIONS
The preferred location of hotspots in a shearing asthenosphere would seem to discount the existence of hotspot reference
Figure 8. Assuming a fixed mantle, the
Pacific lithosphere moves west faster than
the Nazca plate does, because the underlying asthenosphere is less viscous and
decoupling is more efficient. Due to the
increase in viscosity and decrease in
temperature along the rifting area, which
is also moving westward, the asthenosphere below the eastern plate is more
viscous, causing stronger coupling and a
lower steady-state velocity for the Nazca
plate. This kinematic model implies that
new fertile mantle is continually supplied
to the oceanic ridge.
746
C. Doglioni, D.H. Green, and F. Mongelli
frames that include both intraplate and ridge-centered hotspots.
A hotspot reference frame using only Pacific hotspots seems to
be more reliable due to their relative fixity. However, in every
hotspot reference frame, the Pacific plate is the fastest-moving
plate in the world, and it also contains the most diffuse intraplate
magmatism, regardless of its margins. Fastest velocity implies
largest potential shear heating at its base. Classic, deep lower
mantle sources (Morgan, 1971) for Pacific intraplate hotspots
imply ~103–118 mm/yr shearing between the lithosphere and
subasthenospheric mantle, as indicated by the age progression
of the volcanic tracks. In this view, hotspots are generated either
by fluid jets or anomalously high temperatures in the deep
mantle (e.g., Schubert et al., 2001).
Alternatively, an asthenospheric mantle source for the same
hotspots is supported by petrological, geochemical, and kinematic data. A model in which the source of the magma is the
asthenosphere (e.g., Smith, 1993; Anderson, 2000) allows two
important deductions: (1) Shear heating generated in the
asthenosphere by the motion of the overlying Pacific plate is
large enough to generate the scattered and diffuse intraplate magmatism observed (Figs. 3 and 4), and (2) the velocity recorded
in volcanic chains is a function of the depth to the source of the
magmatism in the asthenosphere (Fig. 1) and its trend relative
to plate motion direction (Figs. 5 and 6). The shear between
the lithosphere and subasthenospheric mantle is expected to be
greater than 103–118 mm/yr, because of the missing shear in the
asthenosphere below the hotspot source. This estimate provides
a global net rotation of the lithosphere toward the west that is
larger than the present estimate of 49 mm/yr (Gripp and Gordon,
2002). In this interpretation, all plates on Earth move westward
relative to the subasthenospheric mantle, although at different
velocities, and with a sinusoidal trend (Doglioni, 1993). Therefore, in contrast with the predictions of the classic hotspot reference frame (Fig. 7, option 1), the subasthenospheric mantle
beneath the Nazca plate would move eastward relative to the
lithosphere (Fig. 7, option 2), thus providing a mechanism for
continually providing new fertile asthenosphere to melt beneath
the westward-migrating EPR (Fig. 8). With these velocities relative to the subasthenospheric mantle, the net rotation of the
entire lithosphere at its “equator” could be faster than 90 mm/yr.
Most of the Pacific asthenosphere has a viscosity around
5 × 1017 Pa⋅s (Pollitz et al., 1998). However, lateral variations
in geochemistry and fluid depletion can result in local increases
in viscosity. Greater shear between the lithosphere and subasthenospheric mantle could generate excess heating exceeding
100 °C if the asthenosphere contains local anomalies in which
the viscosity is higher (4 × 1019 to 1020 Pa⋅s). For the maximum
temperature increases, extra melting can be generated, which
may be responsible for scattered superficial magmatism. An increase of ~100 °C would shift the adiabat curve of uprising mantle to the right (Fig. 4), shifting the asthenospheric mantle to the
field of more extensive melting (Green, 2003). This model resurrects the work of Shaw and Jackson (1973) and can explain
Pacific hotspots often attributed to deep mantle plumes as stressgenerated intraplate melt anomalies in the asthenosphere. The
model also explains the observed periodicity and variability in
magma segregation, because strong shear heating decreases
viscosity and therefore also inhibits melting until a higher viscosity is regained and shear heating recommences. The origin
of such hotspots would therefore be different from those persistently located on oceanic ridges and sourced in the asthenosphere, where more abundant water in the mantle (wetspots)
more strongly affects the extent of melting (Bonatti, 1990;
Asimov and Langmuir, 2003). The main differences between
hotspots and wetspots (e.g., Hawaii versus Ascension) may be
summarized in terms of 3D kinematics and petrology:
1. Hotspots. The plate moves relative to a deeper source and
the volcanic trail is independent of any plate margin. The
age of the trail is much younger than the host lithosphere.
The source of the hotspot is in or below the asthenospheric
décollement. The melting temperature is higher (1400–
1500 °C, possibly caused by shear heating).
2. Wetspots. The wetspot moves with the oceanic ridge, and
has an age close to that of the host oceanic crust (i.e., a
speed close to that of the plate boundary). The source of the
wetspot is above the asthenospheric décollement. The melting temperature and pressure are lower (perhaps <1300 °C,
possibly caused by more water in the mantle).
Because wetspots are sourced above the plate-tectonic décollement and do not move with the mantle but rather migrate
at the same velocity as the oceanic ridge, they exhibit different
surface kinematics. For example northeast-trending, water-rich,
parallel anomalies in the asthenospheric mantle generate surface
volcanic trails with the same northeast-trending orientation but
propagating to the southwest (e.g., Walvis and Cameroon ridges,
Fig. 9). Similar but northwest-trending and southeast-propagating
volcanic trails can be generated in the paired plate to the west
(e.g., Rio Grande Rise). The northnorthwest-trending Emperor
hotspot track can be generated by a northnortheast-trending
asthenospheric anomaly located in the décollement surface, and
the resulting magmatism propagates toward the southsoutheast.
In both these cases, there is an angle between absolute plate
motion and mantle structural anisotropy, but the resulting volcanic tracks trend in the opposite directions (compare Figs. 5
and 9). In the case of the Hawaiian track, the directions of plate
motion and mantle anisotropy seem to coincide.
There exist traces that extend from intraplate to ridgecentered locations and vice versa (e.g., Foundation, NazcaEaster-Tuamotu, Cobb), plus ridge-crossing traces (e.g., New
England–Great Meteor, Réunion, Kerguelen). Is each of those
tracks really linked to the same source (e.g., is Réunion really connected to the Chagos-Laccadive system), or are they two independent magmatic systems? Are some of these tracks related to
transtensional tectonics along transform zones? Could shear heat-
On the shallow origin of hotspots and the westward drift of the lithosphere
Westward-migrating MAR
747
ACKNOWLEDGMENTS
Discussions with D. Anderson, D. Bernoulli, D. Boccaletti,
E. Bonatti, E. Carminati, M. Crespi, M. Cuffaro, G. Foulger,
C. Lewis, D. Presnall, B. Scoppola, D. Weeraratne, and J. Winterer were very fruitful. Comments by F. Innocenti, R. Pilger,
and an anonymous reviewer are very much appreciated. Many
thanks again to Gillian Foulger for editing the text. Research
supported by Italian Space Agency ASI and Cofin 2001.
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