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JOURNAL U
UEUFHYSIGAL
KESEAKGH, VUL. 87, NU. B8, I'AUES
MY7-671U, AUGUST 10, 1982
Geological Evidence For The Geographical Pattern of Mantle Return Flow
and the Driving Mechanism of Plate Tectonics
Department of Geology a n d Geopbsics, University of California, Berkeley, California 94720
Tectonic features at the earth's surface can be used to test models for mantle return flow and to
determine the geographic pattern of this flow. A model with shallow return flow and deep continental roots places the strongest constraints o n the geographical pattern of return flow and
predicts recognizable surface manifestations. Because of the progressive shrinkage of the Pacific
(averaging 0.5 km2/yr over the last 180 m y . ) this model predicts upper mantle outflow through the
three gaps in the chain of continents rimming the Pacific (Caribbean, Drake Passage, AustralianAntarctic gap). In this model, upper mantle return flow streams originating at the western Pacific
trenches and at the Java Trench meet south of Australia, filling in behind this rapidly northwardmoving continent and providing an explanation for the negative bathymetric and gravity anomalies
of the 'Australian-Antarctic Discordance'. T h e long-continued tectonic movements toward the east
that characterize the Caribbean and the easternmost Scotia Sea may be produced by viscous coupling to the predicted Pacific outflow through the gaps, and the Caribbean floor slopes in the
predicted direction. If mantle outflow does pass through the gaps in the Pacific perimeter, it must
pass beneath three seismic zones (Central America, Lesser Antilles, Scotia Sea); none of these
seismic zones shows foci below 200 km. Mantle material flowing through the Caribbean and Drake
Passage gaps would supply the Mid-Atlantic Ridge, while the Java Trench supplies the Indian Ocean
ridges, s o that deep-mantle upwellings need not be centered under spreading ridges and therefore
are not required to move laterally to follow ridge migrations. The analysis up to this point suggests
that upper mantle return flow is a response to the motion of the continents. The second part of the
paper suggests a possible driving mechanism for the plate tectonic process which may explain why
the continents move. This hypothetical driving mechanism has four aspects: (1) the lower mantle
convects, (2) this convection drives the continents by drag o n their deep roots, (3) return flow in
the upper mantle provides a volumetric balance for the motion of the continental masses without
passing under them, and (4) oceanic plates are effectively decoupled from the asthenosphere and
are driven largely by slab pull. This mechanism accounts for the opening of the Atlantic, the ability
of spreading ridges to stay o n the midlines of oceans, and the penetration of India into Asia following their collision. Continental motions strongly imply lower mantle upwelling and divergence'
beneath the Atlantic and southeast Indian Oceans, and convergence and subsidence along the
Tethyan belt. This picture disagrees with the concept, derived from hot spot studies, of an undeforming, absolute reference framework at depth, but weaknesses in current hot spot theory would
make a rejection of the present model o n this ground premature. If the present model is generally
correct, a fairly simple pattern of lower mantle convection, with as few as four cells, may explain
much of the tectonic complexity of the earth.
At the present time the geometry of plate movements is largely understood, but the driving mechanism of plate tectonics remains elusive. There has been
much discussion of convection in the mantle, in some
cases involving the entire mantle Kanasewich, 1976;
Cough, 1977; Davies, 19771 and in other cases with the
upper and lower mantle separate [Richter, 1979; Liu,
1979; Chase, 1979bl. In some treatments, mantle convection is dominated by forces generated in or by the
plates themselves, in particular, slab pull, ridge push,
and sliding of the plate away from the elevated ridge
[Forsyth and Uyeda, 19751. None of the suggested
driving mechanisms seems to be capable of explaining
all the observed features of plate kinematics. The
variety of such features is probably too great to be
explained by any single, simple mechanism. The
answer may well lie in a more complex model, combining various aspects of the simpler models. An
Copyright 1982 by the American Geophysical Union.
Paper number 2B0395.
0148-02271 8212B-0395W5.00
example is the flow-roll model of Richter and Parsons
[I9751 and Richter [19781, which has been discussed
by Marshand Marsh il976, 19781 and Watts [19781.
The present paper gives an alternative model for the
driving mechanism, derived by a two-step procedure.
The first step involves a consideration of the geographic pattern of mantle return flow, without regard
to driving mechanism. A model is developed in which
mantle return flow extends no deeper than the midmantle transition zone (420-670 km; Dziewonski et a!.,
119751) and is probably confined to the asthenosphere.
This flow is restricted to suboceanic paths because of
the presence of continental roots. This model explains
a number of tectonic features, including the unusual
bathymetry south of Australia and eastward movements in the Caribbean and easternmost Scotia Seas.
The second step explores the possibility that the plates
which contain continents are driven by coupling
between lower mantle convection cells and continental
roots. Oceanic plates, underlain by the weak asthenosphere, are considered to be driven primarily by slab
pull. This model appears to be capable of explaining
many aspects of plate kinematics.
At this point the rationale behind the present study
6698
ALVAREZ: MANTLE RETURN FLOW AND
should be emphasized. The physical conditions within
the mantle are controversial, and many conceptual
models for the movement pattern in the mantle have
been proposed. It is generally difficult either to prove
or disprove any model on the basis of existing geophysical data. The approach here is to consider one model
which predicts observable geologic effects at the surface of the earth. At least some of the predicted geologic features are found to exist, and in view of this
partial support, the model deserves further examination.
Models for Mantle Return Flow
The movement of plates at the earth's surface
requires a return flow of mantle material, but the pattern of this flow is unknown. Recent debate on flow
in the mantle has concentrated on the depth to which
convective flow extends [Smith, 1977; Elsasser et al.,
1979; Busse, 19811, with opinion polarized between the
whole-mantle convection view and the shallow convection view. Evidence from observational and theoretical geophysics is ambiguous on this question, and no
consensus has been reached.
A second critical question, which has received less
attention, is the depth to the top of the layer in which
mantle return flow takes place [Chapman and Pollack,
19771. If the lithosphere is everywhere about 100 km
thick, as shown in the conventional cross-sectional
diagrams, then mantle return flow will pass beneath
both continents and oceans. However, Jordan [1975a,
d and S@kin and Jordan [I9751 have made a case that
subcontinental mantle may differ from suboceanic
mantle down to depths of at least 400 km. If this is
so, it indicates that in order to maintain the
continent-ocean contrast, the continents must have
deep, permanent roots and that lithosphere in the tectonic sense (Jordan's 'tectosphere') is several hundred
kilometers thick beneath the continents. A similar
conclusion has been reached on the basis of strontium
isotopic ratios [Brooks et al., 19761 and on the basis of
heat flow considerations [Pollack and Chapman, 1977;
Chapman and Pollack, 19771. However, Anderson
[I9791 disagrees, finding no seismological evidence for
continental roots deeper than 150-200 km. The question will not be debated here; the point is that in view
of this unresolved controversy, one can entertain
models either with or without deep continental roots.
The question of the geographic pattern of mantle
return flow was first investigated by Garfinkel [I9751
and subsequently treated by Chase [1976, 1979~1,
Huger and O'Connell [1976, 19791, Harper [19781,
Alvarez [19781, and Parmentier and Oliver 119791.
These subsequent studies support the conclusion of
Garfunkel [I9751 that return flow cannot be accomplished by closed convection cells; mantle material
must flow from shrinking to expanding reservoirs in
response to seafloor spreading, subduction, and lateral
plate motions. Parmentier and Oliver [I9791 evaluated
the effect of differing lithosphere thickness beneath
continents and oceans. Their results are relevant to
PLATE-DRIVING
MECHANISM
the model considered here and will be discussed at
various places below.
To approach the return flow question from a geological viewpoint, one might ask whether mantle return
flow would leave any traces recognizable at the surface.
The question of depth to the top and bottom of the
return flow layer remains unresolved, and one can
consider, therefore, models based on various
configurations of the return flow layer (Figure 1). If
the whole mantle convects and the return currents are
at great depth, surface manifestations would be
unlikely (Figures 1 c and 1 d. If, however, return flow
occurs only in the upper mantle, it could well leave
traces in surface morphology (Figures 1 a and 16). In
this case the depth to the bottom of the lithosphere is
of great importance, for if subcontinental lithosphere is
several hundred kilometers thick; return flow could be
restricted to suboceanic paths (Figure 1 a).
Mass Balance and the Shrinkage of the Pacific
Mantle return flow must transport material from
areas of lithosphere consumption toward areas where
new lithosphere is being generated. Despite rapid
spreading at the East Pacific Rise, net lithosphere consumption is presently concentrated, in a general way,
in the Pacific, while lithosphere production is dominant
in the hemisphere surrounding Africa. One would
therefore expect mantle return flow to proceed from
the Pacific hemisphere to the African hemisphere.
yes
d e e p roots
no
Fig. 1. Four models for mantle return flow, depending o n
whether the return flow is in the upper mantle (top) or the
lower mantle (bottom), and on whether the continents
have deep lithospheric roots (left) or whether the lithosphere is everywhere about 100 km thick (right). Because
of its severe constraints, mantle return flow according to
the model in Figure l a is the most likely to leave traces at
the earth's surface. The model in Figure l a , in which
crossed circles are flow lines passing through the gap, is
investigated in this paper.
A V A K C L : M A N I L C M 1 1 U K N FLUW A N U I - L A I C - U K I V I N U M C L I l A N I a M
bbW
Viewed in another way, the Pacific Ocean has been
contracting for the last 180 m.y. Since the growth of
the Indian Ocean has roughly balanced the disappearance of the Tethys Ocean during this time [Smith and
Briden, 19771, the Pacific has contracted by roughly the
area of the Atlantic. As a result, the subduction zones
surrounding the Pacific have been forced to retreat
toward the center of the Pacific; Elsasser [I9711 termed
this process 'retrograde motion.' This again leads to
the conclusion that net mantle return flow must move
material from the Pacific to the Atlantic.
Measurements on the equal-area paleocontinental
maps of Smith and Briden [I9771 show that since the
beginning of Atlantic seafloor spreading at 180 m.y.
B.P., the area of the world ocean has decreased by the
area of the former Tethys Ocean (5.0 x 1 0 k m ) , and
increased by the area of the present Atlantic (7.5 x
l o 7 km2) and Indian oceans (6.6 x l o 7 km2). The
net gain in area (9.1 x l o 7 km2) has presumably been
balanced by loss of area from an originally larger
Pacific Ocean over the last 180 m.y., at an average rate
of 0.5 km21yr. Garfunkel [I9751 gives the following
rates of plate area change for the last 5-10 m.y., in
km21yr: Pacific, -0.45; Nazca, -0.11; Cocos, -0.08;
Antarctic, +0.50. Roughly one quarter of the Antarctic plate accretion affects the Pacific Ocean, s o these
2
values indicate a current shrinkage rate of 0.52 km l y r
for the Pacific. Using numbers from Minster and Jordan [1978], one finds that the Atlantic Ocean is
presently growing at a rate of about 0.45 km21yr, while
the Indian Ocean is growing at about 0.15 km21yr
(accretion in the Indian Ocean is nearly compensated
by subduction at the Java Trench); these values
require that the Pacific Ocean be shrinking at a rate of
about 0.6 km21yr. Thus, although the shrinkage rate
of the Pacific has varied through time as spreading
rates changed [Larson and Pitman, 1972; Baldwin et a/.,
19741, the current rate is in good agreement with the
long-term average rate.
Gaps in the Pacific Rims
In a model with a uniform 100-km lithosphere,
material forced away from the Pacific Basin could
escape anywhere around the perimeter. In a model
with upper mantle return flow and thick subcontinental
lithosphere (Figure la), the continental roots would
form a barrier to flow away from the Pacific, and the
escaping material would be funneled through the gaps
between the continental masses that nearly encircle the
Pacific (Figure 2). Continental crust is continuous
between Alaska and Siberia and continuous or nearly
so between the Sunda Shelf and Australia. There are
only three gaps in the continental perimeter of the
Pacific Ocean - the Drake Passage between South
America and Antarctica, the southeastern Indian
Ocean between Australia and Antarctica, and the
Caribbean Sea. Central America has a crystalline basement of lower Paleozoic and possibly older rocks as far
south as central Nicaragua; the rest is underlain by a
younger volcanic basement which cannot be considered continental [Dengo, 19691.
Thick subcontinental lithosphere, if it does exist,
Fig. 2. Oblique cylindrical projection tangent to the globe
along a great circle (a-a') a proximating the perimeter of
the Pacific Ocean (pole at 158N, 0 ' ~ ) . Large, open arrows
show the suggested mantle return flow through gaps in the
Pacific perimeter. C. Caribbean; S. Scotia arc-Drake Passage region; A, Australia-Antarctica gap.
certainly does not underlie all continental crust. This
is shown, for example, by the active, shallow-dipping
slabs moving several hundred kilometers eastward
from the trench off South America at depths up to 700
km [ Barazangi and Dorman, 1969; Stauder, 19751 and
by the probable former presence of similar, shallowdipping slabs under the western United States during
much of the Tertiary [Coney and Reynolds, 19771. The
former slabs reconstructed by Coney and Reynolds
119771 on the basis of volcanic episodes apparently
passed at shallow depth beneath Arizona, an area of
Precambrian cratonic crust [Burchfiel and Davis, 1972,
19751. This situation seems to be peculiar to the
young orogenic belts bordering the Pacific; thick lithosphere, if it exists, would underlie the more ancient
crust of cratons, undisturbed by young tectonic and
thermal events [Brooks et at., 19761.
The third model of Parmentier and Oliver 11979, Figure 71 has very thick subcontinental lithosphere, so it
is comparable to the situation considered here, yet it
shows no outward flow through any of the three gaps
in the Pacific rim. In the case of the AustraliaAntarctic gap this is because the Java Trench provides
a major source of return flow material on the west side
of Australia. Flow lines beginning at the Java Trench
and the Western Pacific trenches stream around the
western and eastern sides of Australia, respectively,
converging on the southern side, where this flow pattern produces a sharp negative perturbation of about
25-30 mGal in the calculated gravity field. Weissel and
Hayes [1974], using the satellite-derived gravity field
of Gaposchkin and Lambeck 119711, show a negative
gravity anomaly of 30-35 mGal immediately south of
Australia, in almost exactly the position predicted by
the third model of Parmentier and Oliver [19791.
Weissel and Hayes [1971, 19741 and Hayes 119761 have
made detailed studies of the Australian-Antarctic
Discordance, a low saddle on the southeast Indian
spreading ridge, immediately south of the negative
gravity anomaly just mentioned. The discordance is
characterized by lineated topography oriented northsouth in a band between the ridge and the south Australian margin, indistinct magnetic anomalies, and an
unusual history of asymmetric seafloor spreading. At
a given crustal isochron, the sea floor is systematically
deeper on the north side than on the south side of the
ridge. These features are not easily explained, but the
topographic saddle at the discordance may mark the
convergence of the flow paths around Australia, the
south-to-north depth variation suggests a component
of asthenospheric flow in that direction [ Weissel and
Hayes, 19741, and the band of lineated topography
may mark the path of this flow. Although Parmentier
and Oliver [I9791 did not discuss the region south of
Australia, its peculiar features provide dramatic support for the way their third model treats this region
and for the closely similar model considered in the
present paper. An important point is that although the
ocean south of Australia is the biggest gap in the rim
of the shrinking Pacific, upper mantle outflow will not
occur there because of the presence of another major
source, the Java Trench, outside the gap.
Outflow through the Caribbean and Drake Passage
gaps would be expected, but in the third model of Parmentier and Oliver 11979, Figure 71 this does not occur,
apparently because their boundary conditions provide
other gaps in the Pacific rim. One gap is through the
Bering Straits, Arctic Ocean, and Norwegian Sea; mantle flow through this route feeds the North and Central
Atlantic ridges. A second gap, through the Mediterranean, feeds the Central and South Atlantic ridges,
with some contribution from flow passing south of
Africa. However, continental crust is continuous
across both these paths; if they are closed at depth by
continental roots, upper mantle return flow will be
forced to pass through the Caribbean and Drake Passage gaps. Geological evidence argues that this is the
case.
The Caribbean and Drake Passage Regions
The Caribbean Sea and the Drake Passage are both
characterized by complex tectonic histories which are
not yet completely understood. Continental reconstructions show that both regions have opened up by
extension since the breakup of Pangea [Billiard et at.,
1965; Barker and Griffiths,19771. However, the north
and south boundaries of the two regions were not simply passive trailing margins; there is much evidence for
lithospheric consumption and compressional tectonics
along these margins at various times during their histories [Bell, 1972; Mattson, 1973, 1979; Maresch, 1974;
Ladd, 1976; Mattson and Pessagno, 19791.
However, here we are most concerned with the evidence for eastward motion in both regions. In the
Caribbean, long-continued eastward motion is shown
by the system of right-lateral faults along the northern
edge of South American" and by the left-lateral fault
system extending from Guatemala to Puerto Rico.
The Lesser Antilles subduction zone and the short
spreading ridge segment in the Bartlett Trough indicate
that most of the Caribbean oceanic crust and parts of
the adjacent continent belong to a Caribbean plate
which is presently moving eastward at about 2 cmlyr
with respect to North and South America [Jordan,
1975 c; Minster and Jordan, 19781.
The situation is less clear in the Drake Passage.
Although the South Sandwich subduction zone
geometrically resembles that of the Lesser Antilles,
there is a north-south spreading center only a few hundred kilometers to the west, so that only the very
small Sandwich plate in the easternmost Scotia Sea is
currently moving east with respect to South America
[Barker, 19721. Magnetic lineations in the Drake Passage show southeastward motion of West Antarctica
away from South America [Barker and Burrell, 19771.
Major eastward movements in the past are required by
the close structural and sedimentological ties between
South Georgia Island and southernmost South America [Dalziel et a/., 1975; Winn, 19781.
Since the Atlantic Ocean is not being subducted
under any other part of North or South America, the
lithosphere of the Caribbean and the easternmost
Scotia Sea must be moving eastward relative to the two
ALVAKLL: M A N ILb
Kbl
UKN t L U W ANU rLAlE.-LJKIVINU IVlCLtlANl>M
American plates, and overriding the Atlantic crust. It
is difficult to consider the Caribbean and eastern Scotia
Seas to be typical marginal basins of western Pacific
type, for the latter are apparently the consequence of
subduction of Pacific lithosphere all along the western
margin of the Pacific Ocean, whereas the former are
local perturbations of a passive margin and require a
different explanation. To emphasize this difference,
the opening of the western Pacific marginal basins can
be considered a second-order effect produced by the
downgoing slab [Karig, 1974, Figures 7 A and 7 D l ; it is
more difficult to explain the eastward motion of the
Caribbean and Scotia Seas relative to South America in
this way, since there would be no downgoing slab
without the eastward motion.
The maximum eastward transport in both the Caribbean and Scotia regions has been about 3000 km. In
the southern Caribbean, right-lateral strike slip motion
began in the Eocene or Oligocene in northern Columbia [Alvarez, 19711, and in the middle Eocene in
Venezuela [Bell, 19721. In the northern Caribbean,
left-lateral motion certainly began by middle Eocene
time, although the geological evidence is also compatible with initiation of this motion as early as 85 m.y.
B.P. [Mattson, 19791. Also in the northern Caribbean,
left-lateral motion on the Cayman Trough apparently
began in the Eocene [Per.fit and Heezen, 19781. In
their tectonic reconstruction of the Caribbean, Malfait
and Dinkelman 119721 show eastward motion of the
Caribbean Plate by the beginning of the Oligocene, but
they show northeastward penetration of Pacific lithosphere into the Caribbean region from the Late Cretaceous on. The situation is less clear in the Drake
Passage-Scotia Sea area. South Georgia Island has
probably moved 1500 kilometers to the east [Dalziel et
a/., 19751 by left-lateral strike slip motion along the
northern side of the Scotia Sea, with separation of
South Georgia and Tierra del Fuego occurring between
the early Oligocene and the middle Miocene [ Winn,
19781. The south margin of the Scotia Sea does not
mirror the north margin; recent reconstructions
require left-lateral strike slip motion on both margins
[Barker and Griffiths, 1972; D e Wit, 19771, and seismic
information shows this to be the present pattern as
well [ Forsyth, 19751.
Jordan 11975~1showed that the Caribbean plate is
nearly fixed with respect to the 'absolute' mesosphere
reference frame deduced from hot spot data, and he
suggested that the subduction zones flanking the
Caribbean plate on the east and the west pin it to the
mesosphere. This may be correct, but an alternative
possibility is that the Caribbean plate is disconnected
from the deeper mantle and moves eastward relative to
the Americas at a rate which happens to compensate
roughly for their westward movement relative to the
hot spot framework. Problems with the 'absolute'
reference frame are discussed below in connection
with the hot spot data. The eastward motion in the
Caribbean and easternmost Scotia Seas is just what is
to be expected if the outflow of Pacific mantle material
required by the contraction of the Pacific is concentrated and funneled through the gaps in the continen-
UIU1
tal barriers. The mantle outflow idea was anticipated
by Hamilton [1963, p. 141, who suggested the possibility that the Scotia Arc
..represents disruption and scattering of continental
material, whereby a sort of subcrustal jet stream, moving eastward from the Pacific, fragmented and stretched
out an initially compact land mass, by strike-slip faulting
and tensional rifting. A similar explanation appears
applicable to the Caribbean Sea.
The outflow hypothesis requires that mantle flow
pass at shallow depth beneath three active subduction
zones - those of the Lesser Antilles and Panama in
the Caribbean region and the South Sandwich subduction zone in the Scotia area (Figure 2 ) . If the earthquake foci marking these subduction zones extended
to several hundred kilometers in depth, this would
clearly invalidate the hypothesis for it would be evident that the descending slabs were passing undisturbed through areas where lateral flow should occur.
It is noteworthy, therefore, that seismicity in these
three zones extends no deeper than 200 km, which
places them among the shallowest of the descending
lithosphere slabs [ Barazangi and Dorman, 1969; TombUn, 1975; Bowin, 1976; Forsyth, 1975; Gutenberg and
Richter, 1949; Rothe, 19691. The seismicity information
thus indicates that there is no lithospheric slab curtain
in the way of the inferred mantle return flow.
In contrast to these shallow seismic zones, the zone
dipping north beneath Indonesia reaches depths of
600-650 km from western Java to Timor [Fitch, 1970;
Cardwell and Isacks, 19781. Although the boundary
conditions in the third model of Parmentier and Oliver
[1979, Figure 71 permit mantle flow beneath Southeast
Asia and Indonesia, the Java-Sumatra lithospheric slab
curtain shows that there is no Pacific mantle outflow in
this region.
Rates of upper mantle flow through the gaps in the
Pacific rim would best be obtained by a calculation of
the type presented by Parmentier and Oliver [19791,
using boundary conditions appropriate to the model
discussed here, but a rough estimate will be sufficient
at present. Using values calculated on the basis of the
work by Minster and Jordan [19781, it appears that
mantle supply required by Indian Ocean spreading
(IND-ANT: 0.52 km2/yr; IND-AFR: 0.11 km2lYr) is
met by input at the Java Trench (0.48 km2/yr) augmented by a contribution from the Pacific, passing
around the east side of Australia. Thus, to a first
approximation, Atlantic expansion (0.45 km2/yr)
would be supplied by flow through the Caribbean and
Scotia gaps, each of which is about 6 0 0 km wide. If
return flow takes place everywhere in the same interval, the average outflow rate through the gaps is about
38 cmlyr. In plate tectonic terms this is a high velocity, but it is still not certain that the Caribbean and
Scotia lithospheres (if they are underlain by normal
oceanic asthenosphere) would be carried eastward by
drag resulting from eastward mantle outflow. A more
effective coupling between mantle outflow and the
lithosphere in gaps may result from the presence of
seismic slabs down to 200 km and from the lateral
6702
ALVAREZ: MANTLE RETURN FLOW AND PLATE-DRIVING MECHANISM
contact between the outflowing mantle and the flanking-continental keels along the north and south margins of the gaps. In favor of this view is the observation that the major strike slip faults bounding the
Caribbean Sea (e.g., the Oca and Cuisa faults. [Alvarez,
19711) are located within the continental crust, 100200 km from the ocean rather than at the oceancontinent boundary.
If there is upper mantle flow passing through the
gaps in the Pacific rim, there should be a difference in
bathymetric level between the upstream and downstream ends of the gap, representing the head that
drives the flow. To test whether this difference in
level should be large enough to detect, the problem
can be treated as flow between parallel plates, ignoring
changes in the north-south direction, a simplification
which introduces much less error than that due to
large uncertainties in the parameters of the problem,
and disregarding motion between the upper plate
(lithosphere) and lower plate (base of the return flow
layer), since this velocity is an order of magnitude less
than the return flow velocity. For these conditions the
discharge per unit width is given by
where a is the thickness of the return flow layer, p is
its viscosity, and dP/dx is the pressure gradient. For
a gradient manifested by a slope in regional bathymetry,
Ludwig, 19771 and gravity field [Bowin, 19761 of the
Caribbean before this could be taken as evidence for
the present hypothesis. When considering the ocean
floor outside the Caribbean, thermal effects are important because the Cocos plate is quite young. Thermal
effects have been removed in the calculation of depth
anomalies [Cochran and Talwani, 19771, which show
that the Pacific Ocean immediately west of Central
America is several hundred meters shallower than
expected and that the Atlantic Ocean adjacent to the
Antilles is slightly deeper than expected. These effects
may be unrelated to the outflow mechanism, but they
are correct in sign and of reasonable magnitude and
would seem to merit further study from this viewpoint.
The presence of an active spreading center within the
Scotia Sea and the lack of depth anomaly information
makes it difficult to look for a gradient in lithostatic
head in this area. Return flow outside the constricted
gaps would be much slower and less likely to be
marked by recognizable depth variations.
Supply of Material to the Ridges
Mass balance requires that mantle material expelled
from the shrinking Pacific Ocean be transported to the
expanding Atlantic Ocean. In the present model,
Pacific mantle outflow escapes only through the gaps,
dP-- p g A h
dx
L
where L is the length of the gap, about 3000 km for
both the Caribbean and Scotia regions, and A h is the
elevation difference between the upstream and downstream ends of the gap. Thus,
where 7 is the average outflow velocity, estimated
above as 38 cm/yr.
The value for A h obtained in this way is model
dependent, and Figure 3 shows the effect of choosing
different values for p and a Elevation differences less
than a few hundred meters would not be detectable;
differences greater than 3 or 4 km would scarcely fit in
oceanic depths. Although a wide range of acceptable
models (for example, 5 of the 7 models used by Hager
and O'Connell [19791) yield values for A h that are
impossibly large, there is a set of other acceptable
models, with viscosities of 1019 to a few times 1 0 P
in return flow layers greater than 100 km thick, which
give A h values that are both possible and detectable.
It is therefore interesting to note that the floor of the
oceanic part of the Caribbean does in fact slope down
from west to east, with mean depths of about 3500 m
in the Colombian Basin and 4500 m in the Venezuelan
Basin [Sounders et a/., 1973, Figure 21. This slope is
consistent with the range of calculated values, but
careful consideration would have to be given to the
crustal structure [Ludwig et at., 1975; Houtz and
19
20
log
u
21
22
Fig. 3. Expected bathymetric difference, Ah over the
3000-km length of the Caribbean and Scotia gaps as a function of return flow layer thickness, a (km), and viscosity, u.
(poise), for an average outflow velocity of 38 cmlyr.
Models used in previous calculations are shown by triangles
[Chase, 1979~1,circles [Hager and O'Cotinell, 19791, and
square [ Parmentier and Oliver, 19791.
I
23
ALVAREZ: MANTLE RETURN FLOWAND PLATE-DRIVING
MECHANISM
and it is this outflow that must feed the spreading
Mid-Atlantic Ridge. Depth anomalies in the North
Atlantic Ocean [ Cochran and Talwani, 19771 suggest
that Iceland may mark an additional source of
material, unrelated to the mechanism under discussion
here.
In the present model, mantle return flow above the
midmantle transition zone feeds the Atlantic and
Indian spreading ridges, but this motion does not drive
the plates apart. If the continents bordering the
spreading oceans are moving apart for other reasons,
as discussed below, mantle return flow simply fills in
the zone of separation. Since the area of passive dike
injection at the rift axis is the hottest and therefore the
weakest place, it will continue to be the locus of
separation, spreading will be symmetrical, and the
ridge will remain on the medial line of the growing
ocean [Morgan, 19711. In a few places, other factors
intervene, producing discontinuous ridge jumps or
their continuous equivalent, asymmetric spreading
[Hayes, 19761.
In the present model, the Atlantic ridge system is
fed by material derived from the Pacific, which has
escaped through either the Caribbean or the Scotia
gap. These gaps are positioned in such a way that they
can feed all parts of the Mid-Atlantic Ridge (Figure 2);
in fact, they flank the SAM-AFR segment which
presently accommodates two thirds of the Atlantic
expansion. However, the geographic pattern of this
flow is obscure downstream from the gaps because of
the effective decoupling of the lithosphere from the
deeper levels that apparently occurs everywhere in the
oceanic regions except possibly above the rapidly moving flow within the gaps. It thus seems probable that
once the gap is traversed, the flow lines pass beneath
the Atlantic Ocean lithosphere in a pattern determined
by the volumetric requirements of the spreading ridges
and by the differential pressure between the gaps and
the various ridge segments.
If the present Atlantic ridge is being fed by mantle
return flow through the gaps in the Pacific rim, how
was the Atlantic ridge fed in its early phases, before
these gaps developed? This problem is most noticeable for the Jurassic opening of the Central Atlantic,
before the North or South Atlantic Oceans or Caribbean gap existed. However, at that time the Tethys,
separating Eurasia from the southern Old World continents, formed a wedge-shaped ocean reaching westward to Spain, where the Central Atlantic opening
began [Smith and Briden, 19771. Early opening of the
Central Atlantic required a connection eastward to
Tethys [Dewey et al., 19731. This oceanic pathway
through Tethys would have allowed a mantle return
flow feed to the new ridge in the Central Atlantic.
THE
DRIVING
MECHANISM
The correspondence between the predicted Pacific
mantle outflow and the geological and geophysical
character of the Australian-Antarctic gap, the Caribbean, and the easternmost Scotia areas provides support for the model of shallow return flow with deep
6703
continental roots, but it does not offer an explanation
for why the plates are moving. This is true also of the
recent models of Harper [19781, Chase [1979al, Hager
and O'Connell 119791, and Parmentier and Oliver
119791, which, like the present paper, treat mantle
return flow as a response to movements of the plates.
Shrinkage of the Pacific and the resulting mantle
outflow suggest that motions of the continents may be
the dominant control on return flow in the mantle.
The evidence for deep continental roots derived from
seismology [Jordan, 1975 a, d, from geochemistry
[Brooks et at., 19761, from heat flow results [Chapman
and Pollack, 19771, and from the considerations given
in this paper suggests a possible driving mechanism
that has apparently not previously been proposed. One
can envision a model in which (1) the lower mantle
(below the midmantle transition zone) undergoes convective overturn, (2) this convection drives the continents by viscous coupling to their roots, ( 3 ) return
flow in the upper mantle provides a volumetric compensation for the motion of the continental masses
without passing under them, and (4) oceanic plates,
underlain by weak asthenosphere, are decoupled from
the lower mantle and are driven largely by slab pull
(Figure 4).
1. Radioactive heat sources distributed through the
mantle and the heat output of the core are sufficient to
ensure that the entire mantle convects [ Verhoogen,
19801, although it is not clear whether the upper and
lower mantles convect separately or together. Plate
motions are sufficiently complex to preclude a simple
pattern of whole-mantle convection; they suggest,
rather, a more complicated pattern, perhaps involving
two-tiered convection. Jeanloz and Richter [I9791 have
considered the thermal profile of the earth; in their
model a thermal boundary layer must be present at the
base of the lower mantle. Furthermore, unless the
core temperature is considerably lower than expected,
a second thermal boundary layer would be required.
This could be either near the base of the lower mantle
or at the top of the lower mantle. Although they
could not choose between these two alternatives, the
latter possibility would suggest that the lower and
upper mantle may be dynamically and chemically distinct systems. Richter [I9791 also reached this conclusion on the basis of seismic evidence from the TongaKerm,adec region. As the question of whole-mantle
versus two-tiered convection remains unsolved, it is
justifiable to consider here whether a two-tiered model
can account for known plate motions. In the present
model, continent-bearing plates are driven by a simple
pattern of lower mantle convection cells, but oceanic
plates are not.
2. The model makes use of the possibility that deep
continental roots may exist. It specifies that the continents are forced to move laterally by viscous coupling
between their roots and the convecting lower mantle.
Oceanic lithosphere included in the same plate as a
continent moves together with the continent (for
example, the western North Atlantic moves with the
North American plate) but is not driven directly by the
lower mantle convection. Oceanic plates with no con-
Fig. 4. Equatorial section through the earth, looking north, showing the mantle flow pattern and driving
mechanism considered in this paper. Arrows show movement direction (not rate) relative to the grid of axes of
convergence and divergence at the top of the lower mantle; this grid should deform slowly compared to the other
motions shown. In the mantle above 670 km, the lower arrows show the subasthenosphere upper mantle,
entrained by movement of the uppermost lower mantle; the upper arrows show return flow in the asthenosphere.
Black: continental crust; vertical lines: lithosphere and thick continental tcctosphcre. The links at 'y' indicate that
South America and Africa move with the lower mantle flow. T h e model places stronger constraints o n the lower
mantle convective pattern in the continental hemisphere than in the Pacific hemisphere, where the pattern shown
is extremely speculative.
tinental crust (Pacific, Nazca, Cocos, Philippine) are
underlain by weak asthenosphere and thus have only a
weak viscous coupling to lower mantle movements.
Viscous coupling between lower mantle and
continental roots could occur in at least two ways. If
continental roots extend to 670 km, they may be
anchored directly to the top of the lower mantle convection cells. In a more realistic model, with one
viscosity increase at the base of the asthenosphere and
another at the top of the lower mantle (e.g., models VI
and VII of Hager and O'Connell [1979, Figure 211, it
would be sufficient for the continental roots to extend
below the asthenosphere and be embedded in the
more viscous upper mantle entrained by lower mantle
movement [cf. Chase, 1979a, Figure 1I. This situation
is shown in Figure 4.
ALVAREZ: MANTLE RETURN PLOW AND FLAT€-URIVI
3. If lower mantle convection does move continents
by drag on deep roots, upper mantle return flow would
be a necessity. This flow would be most likely to produce recognizable effects where it squeezes through
the gaps in the Pacific rim, and the expected effects are
exactly what one observes in the tectonics of the
Caribbean, Scotia and Australia-Antarctica regions.
This subject was discussed extensively in the first part
of the paper.
4. The final feature of the model is that oceanic
lithosphere, being underlain by the asthenospheric low
viscosity layer and having no deep roots (except where
it is attached to seismic slabs), is affected neither by
the lower mantle convection nor by the upper mantle
return flow. Because of this decoupling, the oceanic
lithosphere would behave as in the model examined by
Forsyth and Uyeda [1975], in which the driving forces
for plate motion are generated by the plate itself. In
their analysis the dominant force is slab pull due to the
negative buoyancy of the cold, dense, descending slab.
This downward pull is balanced by a viscuous resistance, and the resulting 'terminal velocity' is 6-9
cm/yr, the observed velocity of plates that are connected to downgoing slabs and which have little or no
continental area. As discussed below, ridge push at
the East Pacific Rise may also help drive the oceanic
plates of the Pacific. Slab pull and ridge push are,
however, simply aspects of thermal convection in a
broad view [ Verhoogen, 1980, Ch. 11. Because of the
effective decoupling of the oceanic lithosphere from
the deeper levels, oceanic plate motions do not reflect
lower mantle convection.
Three kinds of geological evidence can be used to
infer the pattern of movements at depth. Tectonic
disruptions as in the Caribbean may indicate shallow
return flow passing through constrictions. In the
present model, the movements of continents reflect
the motion of the top of the lower mantle. Finally,
hot spot tracks should indicate the motions of the levels where their heat sources reside.
Ideally one would determine lower mantle motions
from continental movements and then use hot spot
information to test whether the pattern was valid.
Unfortunately, this is not possible in view of the
uncertainties in understanding hot spot phenomena, as
discussed below.
Lower Mantle Movementfiom Continental Motions
The motions of continents provide a hazy picture of
the pattern of lower mantle convection. Forsyth and
Uyeda 119751 showed that most plates that contain
continents move at average rates of 1-2 cm/yr with
respect to the approximately rigid hot spot framework;
a major exception is the Indian Plate, carrying India
and Australia, with an average 'absolute' velocity of
6.1 cm/yr. Gordon et a / . [I9791 have shown that rms
'absolute' velocities of continents have been at least 5
cm/yr over periods of about 30 m.y. in the past.
MECHANISM
Figure 4 is an equatorial section through the earth,
and Figure 5 is a sketch map showing the pattern of
movement envisioned in this paper. Ascending lower
mantle limbs apparently underlie the Atlantic and
southeast Indian oceans, with a long, transform-type
offset between Africa and Antarctica. The activity of
this limb began in the central segment of the Atlantic
about 180 m.y. B.P. and more recently extended northward (NAM-EUR: 80 m.y. B.P.) and southeastward
(SAM-AFR: 120 m.y. B.P.; AFR-ANT: 120 m.y. B.P.;
ANT-IND: 80 m.y. B.P.; ANT-AUST: 85 or 53 m.y.
B.P.) [Smith and Briden, 1977; Norton and Sclater,
1979; Weissel and Hayes, 1974; Cande et al., 19811.
This behavior may well reflect the gradual growth of
the region in the lower mantle that was organized into
this particular pair of convection cells. This growth
and the variability in continental 'absolute' velocities
indicate time-dependent lower mantle convection,
which is to be expected [ Verhoogen, 1980, Ch. 51.
The east-west Tethyan zone from Spain to India and
beyond, with its long history of convergence, would be
underlain by a descending limb. This provides an
explanation for one of the most uncomfortable contradictions in current plate tectonic theory - the protracted collision between India and Asia. That the two
continents should collide by subduction of the intervening ocean is reasonable; that India should continue
to drive northward into Asia for some 38 m.y. after
the collision [Molnar and Tapponnier, 19751 is not.
Buoyancy considerations predict that shortly after such
a continent-continent collision, a new subduction zone
should form; in this case the logical place would be
along the southwest coast of India, from Karachi to Sri
Lanka, facing the Carlsberg Ridge. This has not
occurred, and of the apparently important driving
mechanisms for plate tectonics considered by Forsyth
and Uyeda [1975], slab pull clearly cannot be forcing
India deep into Asia, and ridge push is generally
thought to be too weak to accomplish such a task [Forsyth and Uyeda, 19751. The problem is resolved, however, if the two continents are being pushed together
by drag due to a pair of converging lower mantle convection cells. Molnar and Tapponnier [I9751 have
made a strong case that the Asian continental crust
north of the Indus Suture has responded to the indenting action of India by deforming along a set of major
transcurrent faults. Most of these faults trend eastward from the suture zone, indicating easiest relief in
the direction of the nearest oceanic region. Both the
continuing collision and the way in which it is being
accommodated by deformation of Asia are well
explained by the present model.
Information From Hot Spots
Hot spots and their trails would appear to offer an
ideal way to test the model developed here, and early
drafts of this paper contained extensive discussions of
hot spot data. Although the depth of hot spot heat
sources is not known, a position below the asthenosphere is reasonable, and if heat sources reside anywhere from the lower half of the upper mantle down
6706
ALVAREZ: MANTLE RETURN FLOW AND PLATE-DRIVINGMECHANISM
Fig. 5. Convective flow pattern at the top of the lower mantle based on the model developed in this paper.
Open trends are axes of upwelling and divergence in the uppermost lower mantle; solid trends are axes of convergence and sinking. The Atlantic-Indian and Tethyan trends are determined by continental motions; the Pacific
trends are very tentatively suggested, on the basis of much weaker evidence, as explained in the text. Arrows
lower mantle relative to the more slowly deforming grid of convershow inferred flow lines of the umermost
..
gence and divergence axes.
to roughly the middle of the lower mantle, their tracks
would provide valuable information on lower mantle
movements (see Galapagos, in Figure 4). The results
of the hot spot tests were mixed but, on the balance,
more unfavorable than favorable to the present model.
The Chagos-Laccadive and Ninetyeast ridges, attributed to the Reunion and Kerguelen hot spots, are
particularly unfavorable for the present model, in
which India rides northward on a lower mantle cell;
hot spots south of India should be fixed with respect to
the continent and ocean of the Indian plate.
In a number of cases the hot spot data were neither
clearly favorable nor clearly unfavorable, but an
attempt to incorporate them in the model led to a
more and more speculative picture, without adequate
support. For example, the Pacific hot spots seem to
show little or no present motion with respect to Africa
and Europe [Minster and Jordan, 1978; Duncan, 19811.
If a lower mantle upwelling and divergence causes the
2 cm/yr separation rate of the Americas from Africa
and Europe, the Pacific hot spots would imply lower
mantle convergence and descent between the Pacific
and the Americas. If Yellowstone is a valid hot spot,
it appears to be approximately fixed with respect to the
major Pacific hot spots, whereas the Galapagos hot
spot may be approximately fixed with respect to South
America (see below). This would require that the
lower mantle convergence zone pass west of the
Galapagos and between Yellowstone and the rest of
North America, perhaps trending along the East Pacific
Rise and passing just east of the North American
Great Basin (Figures 4 and 5). The presence of a surface divergence above a lower mantle convergence is
to be expected in a model with chemically distinct
lower and upper mantles which do not mix, for
subasthenosphere upper mantle will be entrained by
the converging lower mantle and forced to escape
upward and outward. This raises the possibility of a
fundamental difference between spreading ridges of
the Mid-Atlantic and East Pacific type, with the former
being pulled apart above a lower mantle divergence
and the latter pushed apart above a lower mantle convergence. Ridges of the former type would cease to
function if subducted; the latter type would continue
to spread after subduction. Many authors have suggested that subduction of the East Pacific Rise led to
extension in the Basin and Range, and the present
scenario would agree with this idea. One may picture
the lower mantle convergence presently lying under
Colorado, with entrained upper mantle rising, uplifting
the High Plains and Rocky Mountains [Suppe et a/.,
19751, heating and softening the continental roots, and
escaping westward beneath continent already softened
by passage over the lower mantle convergence. This
pattern is shown in Figures 4 and 5 . It should be
emphasized that this concept is even more speculative
than the rest of the paper, but it suggests that attention should be given to the linkage between flow patterns in lower and upper mantle.
As testing of the model on the basis of hot spot
information proceeded, it became evident that the
quality of information on hot spots was highly varied,
ranging from excellent, as in the case of the
Hawaiian-Emperor chain [ Dalrymple et a!., 19771, to
extremely weak in the cases of many short, poorly
known, or dubious volcanic alignments. Eventually,
the conclusion was inescapable that hot spot
phenomena and the pattern of hot spot movements are
not yet well enough understood to be useful as a test
of the present model. In order to justify this disappointing conclusion, the following points are noted as
weaknesses in the present understanding of hot spots:
1 . It is commonly concluded that hot spot heat
sources are embedded in a fixed, undeforming, absolute frame of reference, that is, one with 'motion of an
order of magnitude less than the relative motion
between plate pairs. In most cases it is concluded that
inter-hotspot movement cannot be discerned for the
period 100 m.y. to Present ...' [Duncan, 19811. This
behavior is unlikely in an earth where thermal considerations apparently require convective overturn at
all depths from the top of the inner core to the base of
the lithosphere [ Verhoogen, 19801.
2. It is not known at what depth the heat sources
reside [Anderson, 19811, or even whether they are all
at the same depth - critical questions in testing a
complex model.
3. Reconstructions of plate positions tens of millions
of years ago are sensitive to the unresolved question
of whether Antarctica should be treated as one or
more than one plate [Jurdy, 1978; Dalziel and Elliot,
19821. This must also be considered in investigating
whether the hot spot framework has deformed over
long periods of time.
4 . Uncertainties in relative plate motions [Minster
and Jordan, 19781 produce uncertainties in the calculated motions of hot spots. These uncertainties are
not often reported but may be substantially greater
than the difference between two alternative
hypotheses. For example, when the motion of the
Galapagos hot spot is evaluated relative to various
reference frames, to test whether it is fixed with
respect to South America, as predicted by Figure 4, or
with respect to some 'absolute' frame, the 95%
confidence limit on Galapagos movement includes the
best value for South America as well as two proposed
absolute reference frames - the 'mantle plate' of Hey
et a / . 119771 and 'AMI-2', the model favored by Minster and Jordan [19781. Figure 7 of Hey 119771 shows
the Galapagos hot spot slowly approaching South
America, but in view of the uncertainties, the hot spot
may just as well have remained fixed with respect to
South America. (A full analysis of this question is
available from the author.)
5. The evidence for the existence and interpreted
motion of hot spots varies widely in quality. The evi-
dence for monotonic age increase along the HawaiiEmperor chain is probably the best available. At the
other extreme, the Line Islands, thought to be concentric and coeval with the Emperor chain [Morgan,
19721, are evidently too old to fit this interpretation
[Haggerty et at., 19811. Duncan 11981, Fig. 11 shows a
control point for the Prince Edward hot spot track on
Walters Shoal at 55 m.y. B.P. This is based entirely on
very weak evidence from DSDP site 2-16, which
recovered about 16 m of lower Eocene sediments,
including a 4.5 m interval in which a volcanic ash component was present, and 0.25 m of volcanic breccia
[Simpson, Schlich et a/., 19741. Migration of the
eastern Australian Cenozoic hot spot is based on
diachronous termination of volcanic activity which had
been nearly constant throughout the province for the
previous 35-70 m.y. [ Wellman and McDougall, 1974;
Piker, 19821. No systematic review of the quality of
the evidence supporting the various proposed hot spots
seems to have been undertaken.
6 . The recent hot spot analyses of Morgan [I9821
and Duncan [I9811 begin by specifying the motion of
Africa over the hot spot frame, with the Walvis Ridge,
interpreted as the track of the Tristan hot spot, providing key evidence. Their published rotations can be
used to predict the path of a plume which might be
located beneath Mount Etna, in eastern Sicily - an
excellent hot spot candidate which has apparently not
yet been evaluated as a hot spot volcano. The parameters in either model predict an Etna path starting about
2700-2900 km north of its present position 100-125
m.y. ago and moving generally southward through the
Cretaceous and Tertiary. However, volcanic rocks
known from outcrop and drill records show that basaltic volcanism, much of it of alkaline character, has
occurred repeatedly in southeastern Sicily, within 125
km of Mt. Etna, over the last 220 m.y. - in the Middle and Late Triassic, the Early and Middle Jurassic,
the Late Cretaceous, the Miocene, Pliocene, Pleistocene and Recent [Cristofolini, 1966; Pichler, 1970;
Romano and Villari, 19731. Yet this part of Sicily, the
Hyblean Plateau, has clearly been an extension of
African continental crust during this entire time
[Channell and Horvath, 1976; Channel! et al., 19791.
Thus, there is at least one probable hot spot source
which has long remained roughly fixed with respect to
Africa and which, therefore, contradicts the hypothesis
of a rigid hot spot framework.
7 . As noted above, the Reunion and Kerguelen hot
spots are usually considered to have produced the
Chagos-Laccadive and Ninetyeast ridges, which do not
fit well in the present model. However, there is some
question whether these features are typical hot spot
tracks, since they flank the major transform faults that
bounded the plate that moved rapidly northward with
India during the early Tertiary [Norton and Sclater,
19791.
These problems are raised not as a criticism of existing work on hot spots, or necessarily as objections to
the conclusion that there is an absolute reference
frame marked by the hot spots. One cannot fail to be
impressed by the number of observations that fit
6708
ALVAREZ: MANTLE RETURN.FLOW AND PLATE-DRIVING MECHANISM
together in the synthesis by Morgan [19821, for example. They are raised as an indication of the uncertainties still remaining in hot spot theory. After an intensive attempt to test the present model on the basis of
hot spot information, I can only conclude that rejection of the model on this basis would be premature.
Lower Mantle Cells
Figure 5 shows the approximate .pattern of movement at the top of the lower mantle, inferred from the
foregoing considerations. The divergences in the
Atlantic and south of Australia and the convergence
along the Tethyan belt are strongly implied by continental motions. The East Pacific-Great Basin convergence is very tenuously suggested by tectonic and hot
spot considerations. Finally, in order to close the
lower mantle convection system, an additional divergence may be postulated to lie in the central Pacific.
between the two convergences; by analogy with the
concentration of hot spots along" the Atlantic divergence, the Pacific divergence could perhaps be responsible for the abundant volcanism of the western
Pacific. Thus a fairly simple pattern of lower mantle
convection cells may explain much of the tectonic
complexity of the earth.
Convergent and divergent axes at the top of the
lower mantle seem to die out poleward and to be offset
or terminated in some places. The prominent 19,000km lineament formed by the Eltanin Fracture Zone
and the Africa-Antarctica transform-ridge system
accommodates the offset of the main divergent zone
from the Atlantic to the southeast Indian Ocean, and
may roughly mark the southward termination of the
Pacific convergence and divergence. Terminations and
offsets would occur over broad belts in the ductile
lower mantle but would be accommodated by sharp
breaks in the rigid lithosphere, and because of the
indirect coupling of oceanic lithosphere to lower mantle, breaks in the lithosphere will only roughly mark
lower mantle features. ,
CONCLUSION
In closing, it is worth stressing both the speculative
nature of the model developed here and the rationale
on which it is based. Geophysical modelers are
developing methods for calculating three-dimensional
flow patterns in the mantle [Harper, 1978; Chase,
1979a; Huger and O'Connell. 1979; Parmentier and
Oliver, 19791 but are hampered by the lack of firm
constraints on many of the physical parameters
involved. Tectonic geologists can contribute to this
endeavor, both by specifying geologically reasonable
boundary conditions and by looking for the tectonic
effects predicted either explicitly or implicitly by calculated flow patterns. The model developed here is an
effort in this direction. It will certainly require strong
modification and perhaps complete rejection, but it
seems to account for a rather wide range of geological
and geophysical observations, and it thus deserves to
be subjected to further testing.
Acknowledgments The starting point of this study was a discussion of Asian tectonics with David W. Simpson in Tadjikistan in 1977. Since then many colleagues have contributed to
the development of these ideas, often by pointing out flaws,
and I thank them all for their help: Subir K. Banerjee, Bruce
A. Bolt, Mark Bukowinski, Lung S. Chan, Clement G . Chase,
Gilles M. Corcos, S. Thomas Crough, Garniss Curtis, Ian
W.D. Dalziel, David Epp, W. Gary Ernst, Raymond Jeanloz,
H. Jay Melosh, J. Bernard Minster, Stephen Morris, David
Simpson, and John Verhoogen. I warmly thank three
anonymous reviewers whose detailed comments led to major
improvements.
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