geological evidence for the geographical pattern of mantle

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B8, PAGES 6697-6710, AUGUST 10, 1982

Geological Evidence For The Geographical Pattern of Mantle Return Flow and the Driving Mechanism of Plate Tectonics

WALTER ALVAREZ

Department of Geology and Geophysics, 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 on 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, Australian- Antarctic 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 northward- moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the 'Australian-Antarctic Discordance'. The 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 loci below 200 kin. 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, so that deep-mantle upweilings 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 on 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 on 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 on 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.

INTRODUCTION

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; Gough, 1977; Davies, 1977] and in other cases with the upper and lower mantle separate [Richter, 1979; Liu, 1979; Chase, 1979b]. 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, 1975]. 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, com- bining various aspects of the simpler models. An

Copyright 1982 by the American Geophysical Union.

Paper number 2B0395. 0148-0227/82/2B-0395505.00

example is the flow-roll model of Richter and Parsons [1975] and Richter [1978], which has been discussed by Marsh and Marsh [1976, 1978] and Watts [1978].

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 al., [1975]) 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

6697

6698 ALVAREZ: MANTLE RETURN FLOW AND PLATE-DRIVING MECHANISM

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 examina- tion.

MANTLE RETURN FLOW

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; Elsasset et aL, 1979; Busse, 1981], with opinion polarized between the whole-mantle convection view and the shallow convection view. Evidence from observational and theoreti-

cal 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, 1977]. 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, b] and Sipkin and Jordan [1975] 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., 1976] and on the basis of heat flow considerations [Pollack and Chapman, 1977; Chapman and Pollack, 1977]. However, Anderson [1979] 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 Garfunkel [1975] and subsequently treated by Chase [1976, 1979a], Hager and O'Connell [1976, 1979], Harper [1978], Alvarez [1978], and Parmentier and Oliver [1979]. These subsequent studies support the conclusion of Garfunkel [1975] that return flow cannot be accom- plished 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 [1979] evaluated the effect of differing lithosphere thickness beneath continents and oceans. Their results are relevant to

the model considered here and will be discussed at

various places below. To approach the return flow question from a geologi-

cal 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 1 b). 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 Pacbqc

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.

a

deep roots yes no

Fig. 1. Four models for mantle return flow, depending on 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 1 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.

ALVAREZ: MANTLE RETURN FLOW AND PLATE-DRIVING MECHANISM 6699

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 disappear- ance of the Tethys Ocean during this time [Smith and Briden, 1977], 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; EIsasser [1971] 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 [1977] 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 10 7 km2), and increased by the area of the present Atlantic (7.5 x 107 km 2) and Indian oceans (6.6 x 107 km2). The net gain in area (9.1 x 107 km 2) 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 km2/yr. Garfunkel [1975] gives the following rates of plate area change for the last 5-10 m.y., in km2/yr: Pacific,-0.45; Nazca,-0.11; Cocos,-0.08; Antarctic, +0.50. Roughly one quarter of the Antarc- tic plate accretion affects the Pacific Ocean, so these values indicate a current shrinkage rate of 0.52 km2/yr for the Pacific. Using numbers from Minster and Jor- dan [1978], one finds that the Atlantic Ocean is presently growing at a rate of about 0.45 km2/yr, while the Indian Ocean is growing at about 0.15 km2/yr (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 km2/yr. Thus, although the shrinkage rate of the Pacific has varied through time as spreading rates changed [Larson and Pitman, 1972; Baldwin et al., 1974], the current rate is in good agreement with the long-term average rate.

. G

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, 1969].

Thick subcontinental lithosphere, if it does exist,

Fig. 2. Oblique cylindrical proiection tangent to the globe along a great circle (a-a') approximating the perimeter of the Pacific Ocean (pole at 15øN, 0øE). Large, open arrows show the suggested mantle return flow through gaps in the Pacific perimeter. C, Caribbean; S, Scotia arc-Drake Pas- sage region; A, Australia-Antarctica gap.

6700 ALYAREZ: MANTLE RETURN FLOW AND PLATE-DRIVING MECHANISM

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 Doretan, 1969; Stauder, 1975] and by the probable former presence of similar, shallow- dipping slabs under the western United States during much of the Tertiary [Coney and Reynolds, 1977]. The former slabs reconstructed by Coney and Reynolds [1977] on the basis of volcanic episodes apparently passed at shallow depth beneath Arizona, an area of Precambrian cratonic crust [Burch/iel and Davis, 1972, 1975]. 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 al., 1976].

The third model of Parmentier and Oliver [1979, Fig- ure 7] 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 Australia-

Antarctic 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 regal in the calculated gravity field. Weissel and Hayes [1974], using the satellite-derived gravity field of Gaposchkin and Lambeck [1971], 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 [1979]. Weissel and Hayes [1971, 1974] and Hayes [1976] 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 north- south in a band between the ridge and the south Aus- tralian 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, 1974], and the band of lineated topography may mark the path of this flow. Although Parmentier and Oliver [1979] 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 Par- mentier and Oliver [1979, Figure 7] 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 Mediter- ranean, 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 Pas- sage 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 [Bu!lard et al., 1965; Barker and Gr.iffiths, 1977]. 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, 1979].

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 cm/yr with respect to North and South America [Jordan, 1975c; Minster and Jordan, 1978].

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, 1972]. Magnetic lineations in the Drake Pas- sage show southeastward motion of West Antarctica away from South America [Barker and Burrell, 1977]. Major eastward movements in the past are required by the close structural and sedimentological ties between South Georgia Island and southernmost South Amer- ica [Dalziel et al., 1975; Winn, 1978].

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


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 7A and 7D]; 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 Carib- bean and Scotia regions has I•een about 3000 km. In the southern Caribbean, right-lateral strike slip motion began in the Eocene or Oligocene in northern Colum- bia [,41varez, 1971], and in the middle Eocene in Venezuela [Be!!, 1972]. In the northern Caribbean, left-lateral motion certainly began by middle Eocene time, although the geological evidence is also compati- ble with initiation of this motion as early as 85 m.y. B.P. [Mattson, 1979]. Also in the northern Caribbean, left-lateral motion on the Cayman Trough apparently began in the Eocene [Per. fit and Heezen, 1978]. In their tectonic reconstruction of the Caribbean, Malfait and Dinkelman [1972] 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 Creta- ceous 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!., 1975] 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 [W inn, 1978]. 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 Griffith& 1972; DeWit, 1977], and seismic information shows this to be the present pattern as well [Forsyth, 1975].

Jordan [1975c] showed 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-

tal barriers. The mantle outflow idea was anticipated by Hamilton [1963, p. 14], 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, tYagmented 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; Tomb- !in, 1975; Bowin, 1976; Forsyth, 1975; Gutenberg and Richter, 1949; Rothk, 1969]. 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, 1978]. Although the boundary conditions in the third model of Parmentier and Oliver

[1979, Figure 7] 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 [1979], 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 [1978], it appears that mantle supply required by Indian Ocean spreading (IND-ANT: 0.52 km2/yr; IND-AFR: 0.11 km2/yr) is met by input at the Java Trench (0.48 km2/yr) aug- mented 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 600 km wide. If return flow takes place everywhere in the same interval, the average outflow rate through the gaps is about 38 cm/yr. 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


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 observa- tion that the major strike slip faults bounding the Caribbean Sea (e.g., the Oca and Cuisa faults [A!varez, 1971]) are located within the continental crust, 100- 200 km from the ocean rather than at the ocean-

continent 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

a 3 dP

Q=12tx dx where a is the thickness of the return flow layer, /x is its viscosity, and dP/dx is the pressure gradient. For a gradient manifested by a slope in regional bathymetry,

dP= pgAh 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,

Ah= 12 gLla 2

p ga

where V 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/z 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 [1979]) yield values for A h that are impossibly large, there is a set of other acceptable models, with viscosities of 10 i9 to a few times 102øP 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 [Saunders eta!., 1973, Figure 2]. This slope is consistent with the range of calculated values, but careful consideration would have to be given to the crustal structure [Ludwig eta!., 1975; Houtz and

Ludwig, 1977] and gravity field [Bowin, 1976] 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, 1977], 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,

6OO

5OO

400

300

200

I00

0 18

f 0 1 at 2821

Ah= 5 km

0

L = 3000 km

= 38 cm/yr

o

IE,Vll

19 20 21 22 log P

Fig. 3. Expected bathymetric difference, 3, h, over the 3000-km length of the Caribbean and Scotia gaps as a function of return flow layer thickness, a (km), and viscosity, • (poise), for an average outflow velocity of 38 cm/yr. Models used in previous calculations are shown by triangles [Chase, 1979a], circles [ttager a,d O'Connel( 1979], and square [Parmentier and Oliver, 1979].

23

ALVAREZ' MANTLE RETURN FLOW AND PLATE-DRIVING MECHANISM 6703

and it is this outflow that must feed the spreading Mid-Atlantic Ridge. Depth anomalies in the North Atlantic Ocean [Cochran and Talwani, 1977] 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, 1971]. In a few places, other factors intervene, producing discontinuous ridge jumps or their continuous equivalent, asymmetric spreading [ Hayes, 197 6].

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 notice- able for the Jurassic opening of the Central Atlantic, before the North or South Atlantic Oceans or Carib-

bean gap existed. However, at that time the Tethvs, 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, 1977]. Early opening of the Central Atlantic required a connection eastward to Tethys [Dewey et al., 1973]. 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 Carib- bean, and the easternmost Scotia areas provides support for the model of shallow return flow with deep

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 [1978], Chase [1979a], Hager and O'Connell [1979], and Parrnentier and Oliver [1979], 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, 1975a, b], from geochemistry [Brooks eta!., 1976], from heat flow results [Chapman and Pollack, 1977], 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 com- pensation 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, 1980], 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 [1979] 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 [1979] also reached this conclusion on the basis of seismic evidence from the Tonga- Ker•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-


o

Africa

O/

r'n •o

// /

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 ot' the uppermost lower mantle; the upper arrows show return flow in the asthenosphere. Black: continental crust; vertical lines: lithosphere and thick continental tectosphere. The links at 'y' indicate that South America and Africa move with the lower mantle ['low. The model places stronger constraints on 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 kin, 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 2]), 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 1]. This situation is shown in Figure 4.


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 resis- tance, and the resulting 'terminal velocity' is 6-9 cm/yr, the observed velocity of plates that are con- nected 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. 1]. Because of the effective decoupling of the oceanic lithosphere from the deeper levels, oceanic plate motions do not reflect lower mantle convection.

GLOBAL MOVEMENT PATTERN

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 Movement Jkom Continental Motions

The motions of continents provide a hazy picture of the pattern of lower mantle convection. Forsyth and Uyeda [1975] 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 al. [1979] 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.

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., 1981]. 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. 5].

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 con- tradictions in current plate tectonic theory -- the pro- tracted collision between India and Asia. That the two

continents should collide by subduction of the inter- vening ocean is reasonable; that India should continue to drive northward into Asia for some 38 m.y. after the collision [Molnar and Tapponnier, 1975] 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 [For- syth and Uyeda, 1975]. 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 [1975] have made a strong case that the Asian continental crust north of the Indus Suture has responded to the indent- ing action of India by deforming along a set of major transcurrent faults. Most of these faults trend east-

ward 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


o.

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 show inferred flow lines of the uppermost lower mantle relative to the more slowly deforming grid of convergence 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, attri- buted 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, 1981 ]. 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 eta!., 1975], 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 eta!., 1977], 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 disap- pointing 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, 1981]. 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, 1980].

2. It is not known at what depth the heat sources reside [Anderson, 1981], 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, 1982]. 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, 1978] 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 al. [1977] and 'AM1-2', the model favored by Min- ster and Jordan [1978]. Figure 7 of Hey [1977] 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 Hawaii- Emperor chain is probably the best available. At the other extreme, the Line Islands, thought to be concen- tric and coeval with the Emperor chain [Morgan, 1972], are evidently too old to fit this interpretation [Haggert?/et a!., 1981 ]. Duncan [1981, Fig. 1 ] 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 2a6, 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 al., 1974]. 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; Pilger, 1982]. 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 [1982] and Duncan [1981] 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 basal- tic 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 Mid- dle and Late Triassic, the Early and Middle Jurassic, the Late Cretaceous, the Miocene, Pliocene, Pleisto- cene and Recent [ Cristofolini, 1966; Pichler, 1970; Romano and Villari, 1973]. Yet this part of Sicily, the Hyblean Plateau, has clearly been an extension of African continental crust during this entire time [Charmell and Horvath, 1976; Charmell et al., 1979]. 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 [1982], for example. They are raised as an indication of the uncertainties still remaining in hot spot theory. After an inten- sive 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 respon- sible 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 19,000- km 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; Hager and O'Connell, 1979; Parmentier and Oliver, 1979] 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 Tadjikis- tan 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, lan 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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(Received March 17, 1980: revised March 4, 1982;

accepted March 12, 1982.)

geological evidence for the geographical pattern of mantle

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