Oceanization and isostasy: A reply

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OCEANIZATION AND ISOSTASY

difficulties

and requires complex additional

formation

of whole oceans. This does not imply that some process of basification

assumptions,

if it is applied to explain the does

not locally occur; nor is the subsidence of ocean areas denied, but many of such subsidence phenomena are indeed related to plate tectonics, such as the formation of marginal seas behind island arcs by secondary sea-floor spreading (Karig, 197 1) or the submergence of guyots when the plates move away from a ridge.

REFERENCES Artyushkov, E.V., 1971. Rheological properties of the crust and upper mantle according to data on isostatic movements. J. Geophys. Res., 76(5): 137661390. Beloussov, V.V., 1967. Some problems concerning the oceanic earth’s crust and upper mantle evolution. Geotectonics, l(1): l-6. Beloussov, V.V., 1970. Against the hypothesis of ocean-floor spreading. Tectonophysics, Y(6): 489511. Jacoby, W.R., 1970. Instability In the upper mantle and global plate movements. J. Geophys. Res., 75(29): 5671-5680. Karig, D.E., 1971. Origin and development of marginal basins in the western Pacific. J. Geophys. Res., 76(11): 2542-2561. Lliboutry, L.A., 197 1. Rheological properties of the asthenosphere from Fennoscandian data. J. Geephys. Res., 76(5): 1433-1446. Morgan, W.J., 1971. Convection plumes in the lower mantle. Nature, 230(5288): 42-43. Orowan, E., 1969. The origin of the oceanic ridges. Sci. Am., 214(5): 103-119. Ranalli, G., 197 1. The expansion-undation hypothesis for geotectonic evolution. Tectonophysics, ll(4): 261-285. Rankama, K. and Sahama, T.G., 1950. Geochemistry. University of Chicago Press, Chicago, IIl., 9 12 pp. Wilson, J.T., in preparation. Mantle diapirs as a principal driving mechanism and a unifying concept In geonomy.

OCEANIZATION AND ISOSTASY: A REPLY

V.V. BELOUSSOV Soviet Geophysical Committee, Academy of Sciences of the LI.S.S.R., Moscow (U.S.S.R.)

(Accepted for publication March 9, 1972)

ON THE SUBJECT OF W.R. JACOBY’S COMMENTS

W.R. Jacoby has primarily in mind my article “Against the hypothesis of ocean-floor spreading” (1970). This was the article where I enumerated the most significant defects of the ocean-floor spreading hypothesis, many of which, unless shown to be wrong, are fatal. It would, therefore, seem that the best way to defend the hypothesis would be to disprove at least some

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V.V.BELOUSSOV

of my objections. 1 am afraid, an attack on my ideas is not the best argument in favour of the “spreading” hypothesis, I might be all wrong, but the “spreading” hypothesis will be no better for it. . . It would have been of much value, for example, if W.R. Jacoby could have dispelled my doubts as regards the reality of the sinking of the oceanic lithosphere into the mantle, as is claimed by the “spreading” hypothesis, in the zones of deep-water troughs. We would then have seen, how much better this supposition is, than my idea about the scattered, regional sinking of the lithosphere into the mantle. Neither in the article indicated above, nor anywhere else have I given a complete and exhaustive analysis of the “basification” processes, suggested by me. I did not do this because, contrary to the partisans of the “spreading” hypothesis, to whom, evidently, everything is quite simple and clear, I see in natural processes so many complexities and so many difficulties in their interpretation, that I cannot consider the problem of deep processes as a clear one at the modern state of our knowledge. I have, therefore, read with much pleasure that W.R. Jacoby allows at least local manifestations of the basification process. I think at the present stage it is quite enough, since it confirms that we can and must contemplate about this process, And actually my task was in showing the direction of this contemplation. It seems that many scientists would now agree with me, that attempts to explain the history of such inland seas as the Mediterranean, Caribbean or the Gulf of Mexico without any notions about the basification of the continental crust have up to now resulted in constructions very much removed from geological realities (see also: Pannekoek, 1969; Maxwell, 1970; Fox et al., 1971; Trtimpy, 197 1). By applying the hypothesis of “plate tectonics” to the Mediterranean Sea A.G. Smith, a follower of the hypothesis, writes: “From his own field experience in Greece it would be easy for the writer to point out contradictions between field observations and inferences made from the model and the reassembly. However, these inferences and speculations are meant to be assessed on the scale of the diagrams - 1: 1O,OOO,OOO or more - rather if one were viewing the area from a spacecraft. On this scale many of the contradictions of field geology are invisible - for example, an area 100 km* is reduced to a millimeter square. This is the scale on which an internally consistent theory of plate tectonics has been set up and verified. It is obviously invalid on a smaller scale.” (Italics are mine, V.B.) To anyone, who considers geology of some value, these words ring as an eloquent obituary to “plate tectonics”. A similar situation is observed in marginal seas, such as, for instance, the Bering, Okhotsk Seas or the Sea of Japan. In these regions we can also hardly do without basification. W.R. Jacoby refers to the work by D.E. Karig (1971). But as a geologist I cannot avoid seeing strange things in this work as well. First of all, Karig should have convinced us that actual geological relations allow for island arcs to be at first attached to the continent. The basic thing, however, is that Karig applied his theory to the Marianas arc, i.e., to the island arc of the second type (Beloussov and Ruditch, 1961). But the island arcs of the second type are just crevasses in the earth’s crust, through which lava flows and

335

OCEANIZATIONANDISOSTASY forms a range of volcanoes. So what is moved away from the continent by the intrusions which inject themselves between the continent and the arc? The crevasse in the earth’s

crust? While the intrusions are injected not through this crevasse, but are carefully distributed between the crevasse and the continent? And how are we to explain the cases of the continuation

of geological structures

of the second-type

arcs onto the continental

crust? The Marianas arc crosses the major island of Japan, thus forming the Fossa Magna structure, while the Aleutian arc is continued on Kamchatka in the form of a chain of basic intrusions. We shall not overcome the difficulties in discussing these problems until the followers of “plate tectonics” stop viewing the earth “from spacecraft”, and come down to the surface, nearer to real geology. I would like to make now, nonetheless, several remarks as regards the mechanism of basification of the continental crust. It is quite evident that the vertical displacements in the tectonosphere, as in any solid body, cannot take place without horizontal displacements. I understand the process of the rising of material from the mantle (from the asthenosphere) in the form of diapirs, towards which there is a flow in the asthenosphere and which, on coming to the surface of the crust, spread over it. But such asthenospheric diapirs, formed at different times on all the territory of basification, are at last closely pressed and horizontal displacements in each case are small. These displacements are much too far from the displacement of “plates” for thousands of kilometers. A good model might be the regions of salt diapirism where salt flows horizontally at depth, intrudes vertically, but causes no significant horizontal displacement of overlying rocks. The sinking of the continental

lithosphere

into the mantle is neither mysterious,

nor

impossible. It is the same process which, in its time, was excellently described by R.A. Daly in his classical book Igneous Rocks and the Depths of the Earth under the term of stoping. As a matter of fact, the average density of the solid continental lithosphere is greater than the density of the fused material of the mantle (the average density of the solid continental crust, together with the solid upper layers of the mantle several tens of kilometers thick, is probably close to 3.0 g/cm3, whereas the melted material of the mantle has a density of about 2.8 g/cm3). The continental models of this process, though incomplete, are the regions of development of plateau-basalts (for instance the Deccan Plateau in India), where the melted basalt outflowed because its density was less than the density of the solid lithosphere and where, after the outflown basalt solidified, the lithosphere partially subsided into the melted seat which remained in the deep. Seismic data in these cases indicate that the thickness of the crust was below normal. This means that the broken off and subsided blocks of the lithosphere, which included the lower crustal layers as well, were replaced by the intruded material of deeper origin. When melting in the mantle, the subsiding lithosphere must emit water. But water is released not only from the crust, but also from the mantle material which flows onto the surface and intrudes into the crust. Calculations show that the modern volume of the hydrosphere needs a release of water from the mantle layer about 400 km thick.

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V.V. BELOUSSOL

How extensive was or is this process? During each separate period of time the melted seat absorbing the lithosphere

can occupy a rather small area. It is a single asthenospheric

diapir. The area is involved in the process gradually, as the result of the appearance one of new diapirs. As it has been already mentioned, be admitted

one by should

for inland and marginal seas. The channel of low seismic velocities observed

below the western part of the Mediterranean an indication

the process of basification

Sea (Berry and Knopoff,

that here the mantle recently absorbed continental

1967) is possibly

lithosphere.

This is in

complete correlation with geological history. It is quite possible, that the process of basification is continued now within the mid-oceanic ridges, where it is manifested by a subcrustal lens of material with intermediate seismic velocities. Such lenses might be interpreted as the mixing zones of the sinking blocks of the lithosphere and the rising melt. Possibly Iceland, with its high percent of acid rocks, is a part of the previously continental crust not yet completely basificated. As to the whole area of the oceans, this problem might still make a different turn, if the second and third layers of the oceanic crust yield remnants of Paleozoic or even more ancient rocks, which I think quite probable. As I have written in the article which is now under criticism, all my meditations are by no means a “theory”, but simply a hypothesis. It only indicates the direction of further researches which I thought to have more geological perspective than the geometrical exercises with the Poles, around which rotate absolutely rigid plates, or with the mysterious movements of the mid-oceanic ridges which, diving under a continent, miraculously start to produce the continental lithosphere instead of the oceanic. I think we are all in a hurry. We even do not know the profile of the second layer of the oceanic crust, but it seems to us that we know all about the oceans and even more than the geologists have found about the continents during the last two hundred years. Elem&tary knowledge of the history of our science advises us of the necessity to reserve for the future different ways until the factual data will make the most likely choice.

REFERENCES Beloussov, V.V., 1970. Against the hypothesis of ocean-floor spreading. Tectonophysics, g(6): 489511. Beloussov, V.V. and R&itch, EM., 1961. Island arcs in the development of the earth’s structure (especially in the region of Japan and the Sea of Okbotsk). J. Geol.,69(6X 3-23. Berry, M.J. and Knopoff, L., 1967. Structure of the Upper Mantle under the Western Mediterranean Basin. J. Geophys. Res., 72(14): 3613-3626. Daly, R.A., 1933. Igneous Rocks and the Depths ofthe Earth. McGraw Hill, New York, N.Y., 598spp. Fox, P.J., Schreiber, E. and Heezen, B.C., 1971. The geology of the Caribbean crust; Tertiary sediments, granitic and basic rocks from the Aves ridge. Tectonophysics, 12(2): 89-109. Karig, D.E., 197 1. Origin and development of marginal basins in the western Pacific. J. Geophys. Res., 76(11): 2542-2561. Maxwell, J.C., 1970. The Mediterranean, ophiolites, and continental drift. In: H. Johnsoa and B. Smith (Editors), The Megatectonics of Gmtinents and Oceans. Rutgers Univ. Press, New Brunswick, N.J., pp.167-193. Pannekoek, A.J., 1969. Uplift and subsidence in and around the western Mediterranean since the Oligocene: a review. Symposium on the problem of oceanization in the western Mediterranean. Verh. K. Ned. Geol. Mfjnbouwkd. Genoot., 26: 53-77. Trtimpy, R., 1971. Stratigraphy in mountain belts. Q. J. Geol. Sot. Land., 126 (19701, pt.3: 193-318.