Currents in the mantle and the geology of continents

61

Tecronophysics, 187 (1991) 61-67 Elsevier Science Publishers B.V., Amsterdam

Syntheses and Models

Currents in the mantle and the geology of continents J. Two Wilson Department of Physics, University of Toronto, Toronto M5S 1A 7, Canada

(Revised version received by publisher July 31,199(l)

ABSTRACT Wilson, J.T., 1991. Currents in the mantle and the geolokj of continents. In: T.W.C. Hilde and R.L. Carlson (Editors), Silver Anniversary of Plate Tectonics. Tectonophysics, 187: 61-67. This paper is the fist of a series which have considered the possible effects of currents in the mantle upon continents. It mentions effects of aging in oceans upon the direction of slope of coastal peneplains. When oceans are young the coasts are cliffs from which peneplains dip downward on the inland side. As the margins separate from the ridge they cool and the slopes reverse. It suggests how Jeffreys’ arguments against convection currents breaking the lithosphere can be avoided and discusses the possibility that upwelling has penetrated beneath the southwestern United States.

Inaction During this century the opinion of a majority of earth scientists has changed from a belief that the earth’s surface features are essentially fixed in place and only subject to relatively small motions, to the view that the crust is subject to great, if slow movements. That is the result of two sets of proposals. The first, which largely stemmed from the geological evidence which A. Wegener (1966) assembled, is that continents move great distances. It took over thirty years before many earth scientists would agree (Van Waterschoot van der Gracht, 1928; Marvin 1974). The second, which was a result of developments in studying paleomagnetism and the sea floor, led to the recognition of a world-encircling system of mid-ocean ridges from which ocean floors had spread (Ewing and Heezen, 1956). Magnetic and paleomagnetic methods enabled the motions to be precisely measured and dated (Cox, 1973). oo40-1951/91/%03.50

6 1991 - Elsevier Science Publishers B.V.

These developments have been widely acclaimed, but three additions are needed to round out the full picture of the earth’s mobility and to complete the current revolution in the earth sciences. A proposal by H.W. Menard (1960) stimulated my interest in the problem discussed here and elsewhere (Wilson, 1988,1990a, 1990b, 1990~). The first addition that is needed is an answer to H. Jeffreys (1952) objection of sixty years standing {Marvin, 1974) that no known forces are adequate to break the earth’s crust and thus allow motion to occur. The second is a clear statement describing the nature and location of the forces, presumably one or two convection systems, which move the surface. It appears that attention to geological evidence can help to resolve this matter. A third is some explanation of the mobility so evident in the surface of continents and so much more complex than that seen on the ocean floors. The object of this paper is to discuss possible answers to these three questions.

62

How can the earth’s crust be fractured?

Most earth scientists are aware of Harold Jeffreys’ opposition to continental drift, but it was not that he believed that drift is inherently impossible, but because he knew of no forces strong enough to implement it. In 1923, at the first of two symposia held to consider A. Wegener’s proposals, Jeffreys agreed to consider G. Darwin’s suggestion that the moon had separated from the Pacific Ocean and 0. Fisher’s suggestion that the remaining crustal fragments had moved. After consideration he held that such a separation was impossible (Marvin, 1974). Since then he has held the view that no know force due to convection rolls (Holmes, 1931) or compression is sufficient to break the crust. Nevertheless evidence for seafloor spreading is now so overwhelming that consideration should be given to finding some alternative way to generate fractures in the crust. To do this let us trace the evidence for the way fractures have been extended to see if that leads to any indications of their origin. A casual look at any map or globe of the world’s topography reveals a relationship between the widths of expanding oceans and the shapes of the profiles of their coastlines. Thus in the wide part of the Atlantic of Jurassic age between the east coast of North America and the west coast of the Sahara, the surface slopes down gently towards the oceans whether the basement is exposed as in the Nova Scotian peneplain (Goldthwait, 1924) or covered by younger coastal deposits as in the Atlantic States. In contrast, in the narrow northern branches of the Atlantic Ocean the coasts of Labrador, Baffin Island and Ellesmere Island, both coasts of Greenland and those of Norway and Scotland are all highest along the coast where uplands mark the highest residual parts of peneplains which slope gently inland. Evidently, when oceans begin to spread they start with raised coasts which subside as the oceans spread and cool, and as sedimentary deposits collect and depress the coasts. The same principle also applies to the Indian Ocean across which the most active ridge extends from southeast to northwest. Where this part of the ocean is widest between Antarctica and

J.T. WILSON

Australia the coasts are low. In India and Madagascar, each of which is closer to the ridge, the coasts of the Western Ghats of India and of the eastern side of Madagascar are raised {Cogley, 1985) as are the coasts of Arabia, Somalia, Ethiopia and Eritrea, which all face narrow seas. The cliffs overlooking the rift valleys of East Africa are well known to be high and the upland surfaces slope down gently on either side away from the rifts. The conclusion to be drawn is that fracturing of the crust begins in uplifted regions. M. Cloos (1939) pointed out that in East Africa the greatest uplifts of the basement amounting to a few kilometres occur in domes some of which later became connected by the rifts. Each dome is also a volcanic centre. These domes lift and break the continental crust, but they do so in tension and shear, not by compression. This avoids Jeffreys’ objection because neither compression nor convection currents of the type which Holmes proposed are involved in the initial breaking of the crust. Once started, rifts can be extended to permit continental drift to begin. See also P. Morgan and B.H. Baker (1983).

Geological evidence convection currents

concerning

the problems

of

To the domes which Cloos described in Germany and East Africa and those which Fisher (1881) had already noted along the mid-Atlantic Ridge, about forty other hot spots have now been added. They are import~t, for their tracks show that they provide a mechanism for breaking the continental crust as well as the oceanic (Keen and Clarke, 1974; Smith, 1982; Morgan, 1983). Those which become connected by rifts also appear to initiate ocean basins. Minster and Jordan (1978) have shown that hotspots form a nearly fixed frame of reference, for they only move very slowly relative to one another. Since isotopic evidence shows that mantle plumes rise from great depths (O’Nions and Oxburgh, 1983), it seems plausible to suppose that they arise from within the deep lower mantle which is more viscid than the upper layers which would serve to explain their very slow rates of

CURRENTS

IN THE

MANTLE

AND

THE

GEOLOGY

OF CONTINENTS

Fig. 1. Diagram of part of the supercontinent of Gondwanaland showing that, if hot spots (black dots) formed, rifts might join them to outline the future continent of Africa.

displacement (Peltier, 1985). Hotspots show where plumes rise. On the other hand the lavas which flow from rifts and ridges have a different isotopic composition suggesting a shallower source. These lavas are more abundant and arise in rolls, not plumes. Their pattern resembles that advocated by Holmes (1931, 1946), Hess (1962) and others (Marvin, 1974), but as Richter and McKenzie (1981) have advocated, they may be confined to the more fluid upper mantle. Although convection cannot generate breaks in the crust, once plumes have fractured it, convection currents may well up along rifts which have joined hot spots. Once they have

Fig. 2. Diagram of the same region at a somewhat later time showing how spreading of rifts joining four hot spots could have opened the Atlantic and Southern Oceans while they remained on the ridge. If the hot spots move at a slower rate than continents and spreading sea floors, then expansion would have forced the mid-Indian Ocean ridge to move off its founding hot spots.

63

formed in that way they can, and indeed are forced in some oceans to leave their generating hot spots and pursue a life of their own (Martin, 1987). (Compare Figs. 1 and 2). Thus geoiogical evidence supports the view that two convection systems coexist. One consists of at least forty plumes which rise like pipes from the deep mantle and move little. Some live for hundreds of millions of years, but a few die out. The returns flow is presumably a general sinking. The second form of convection appears to be rolls which rise under the mid-ocean ridges and their branches and which descend in subduction zones. These rolls may be confined to the more mobile upper mantle about 700 km thick. These also commonly are active for over a hundred million years each. Contacts between spreading ocean ridges and continents In 1956 M. Ewing and B.C. Heezen used the pattern of earthquake loci in the oceans (Gutenberg and Richter, 1954) to link the mid-Atlantic ridge and shallow regions known in other oceans into a world-encircling system. During the next few years others added details. Among these H.W. Menard (1960) described and named the East Pacific Rise. Since at that time he did not believe in continental drift he held that the rise had been uplifted vertically in place across the Pacific Ocean basin, and that the same rise was continuous beneath the southwestern United States and then emerged again in the northeastern Pacific Ocean. He identified the location by the high elevation of the crest which was also marked by high heat flow and the loci of mjor earthquakes. He held that on land the uplift was part of the Lararnide Revolution of 60 m.y. ago. McKenzie and Morgan (1969) and Atwater (1970) used the theory of triple junctions and some of the first magnetic lineations to show that the collision had occurred only 30 m.y. ago so that the East Pacific rise was not Laramide in age. Atwater held that the San Andreas Fault terminated the rise abruptly at the coast. Subsequently different geologists have supported both views. In 1975 T. Suppe et al. pub-

64

Fig.

J.T

3. Sketch

assumed

map

to illustrate

of the western

United

States towards

and had then been offset along tinue as the Gorda suggested

instead

northern That mantle.

how

H.W.

that the East Pacific Rise. continued

Menard

the crust

the Yellowstone

hot spot

Fault

and Juan de Fuca ridges. T. Atwater that the San Andreas

be true in the crust,

Regions

of Neogene

and

to con(1970)

Fault (SAF) linked the

end of the Past Pacific Rise to the Mendocino

may

(1960)

beneath

the Mendocino

WILSON

but need

not

Lararnide

apply

Uplift

Fault. in the are also

shown.

Fig. 5. Map

showing

areas of greater

by degrees

electrical

of intensity

conductivity

turn is interpreted

as an indication

the highest regions,

the Wasatch

Mountains

of shading

in the mantle of greater

mobility.

uplift and the Southern

are the most active, and the area of Neogene

is more active than the area last disturbed (after Gough,

the

which in

by Laramide

Thus Rocky uplift uplift

1986).

lished evidence for a disturbance and uplift lowing the Wasatch Range and its extensions

folto-

wards the Yellowstone hot spot. They have collected evidence that in the whole region south of the Snake River Plain, which is the trace or track of the Yellowstone hot spot, there have been two periods of uplift, one Laramide and one beginning

Fig. 4. Map after J. Suppe et al. (1975) showing shading

by horizontal

two hot spots and their tracks and by diagonal land over 2.25 km in elevation.

shading

only 30 m.y. ago in Neogene time (Figs. 3 and 4). They also held that spreading from the Wasatch uplift has stretched the crust in the Basin and Range Province and compressed and uplifted it under the Colorado Plateau and Southern Rocky Mountains (Fig. 5). In a recent symposium on metamorphism, W.S. Fyfe (1987) concluded the meeting by pointing out that extensive metamorphism on continents is “a great puzzle” which he suggested might be

CURRENTS

IN THE

MANTLE

AND

THE

GEOLOGY

OF CONTINENTS

related to the style or pattern of convection currents below the crust. P.T. Coney and T.A. Harms (1984) also found in metamorphic core complexes the same distribution of ages as had Suppe. The complexes north of the Snake River Plain are about 60 million years old. Most of those to the south are 30 million years or less, but a few are older. To the north the Rocky Mountains in Canada and in the extreme northern United States are of Laramide not Neogene age, and present a very different appearance from the Colorado Rockies, nevertheless Price and Mountjoy (1970) offered the same explanation for their growth as that of Menard and Suppe. They wrote “upwelling and lateral swelling of a hot mobile infrastructure... in the Western Cordillera and equivalent progressive northeasterly growth of the foreland thrust belt”. The different appearance of the surface geology may be due to the great difference in the thickness of the sedimentary sections in the two areas. Other geologists (Farrar and Dixon, 1984; Eaton, 1986) have advanced views of sub-crustal motions related to these concepts. Geophysicists have also produced three quite separate lines of evidence all supporting the view that hot currents are upwelling below the crust in the western United States. Gough (1984, 1986) has shown that areal studies of magnetovariations lead to the conclusion that partial melting occurs in the mantle and is greatest under the Wasatch and Colorado Rockies, intermediate south of Snake River Plain and less beneath the Canadian Rockies (Fig. 6). D.I. Gough and WI. Gough (1987) have also used a variety of ways to measure stress in the crust and conclude that the results indicate an upflow in the mantle under the general vicinity of the Wasatch Range. See also Zoback and Zoback (1980). J.A. Jacobs et al. (1974) also pointed out that a projection of the magnetic lineations studied by Atwater (1970) showed that if the Pacific Rise had continued to move eastwards relative to the North American plate at a constant rate, it would now he in the proposed region of uplift. There is thus much evidence to suggest that Menrad’s view expressed 28 years ago should be

v

Fig. 6. Map showing how spreading from the Wasatch uplift under the westward moving continent could produce crustal stretching in the Basin and Range province and uplift in the Colorado plateau and particularly in the Rocky Mountain provinces.

revived and re-examined. Elsewhere Wilson (1990b) has pointed out that since all oceans contain long-lived spreading ridges and since all oceans close, that overriding of ridges should have been a recurrent event in geological history. The Kula plate and its ridges provide an example in the northwest Pacific region. A single article cannot establish whether this has been true or not, or describe all the consequences, but it can draw attention to the need for such an examination. Many earth scientists believe that the acceptance of continental drift and of plate tectonics constitutes a revolution in the earth sciences, a new program (Mareschal, 1987) or a change of paradigm, but whatever name is used, the present situation is an incomplete statement of the mobility of the earth. A full theory of the earth requires a clear account of the causative mechanism involved and where at present its elements are situated within the earth. These is also a need to relate that mechanism to what happens on the surface of

66

J.T. WILSON

continents as well as on the sea floor. Why have thrusts, folds, metamorphism, uplift and sinking frequently

affected

are continents

fragmented

particularly torn

These

answers

coasts are

Gough

and

and

and

matters

if Menard,

others,

exotic

terranes

transported

major

regardless

Why

from time to time and

why are slivers

off some

coasts?

the surface of continents?

including

which

require

Atwater,

Suppe,

tackled parts of the problem,

who

are correct

B. and Richter,

and related Hess,

H.H.,

HI.

have

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