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by Lev KRASNY, Corresponding Member, USSR Academy of Sciences, department head at the USSR Geological Research Institute,
Academician Mikhail SADOVSKY, Honorary Director of O. Schmidt Earth Physics Institute, USSR Academy of Sciences
The earth's crust was once thought to be firm and unmoving. It is not for nothing that the Latin continentis means "stable, steady, firm".
In the 20th century geology, an essentially descriptive science, has accumulated an enormous body of facts. New methods of study (e.g., geophysical) have emerged. Employing seismic and blast waves, and also thermal, electric, magnetic and gravitational fields, geologists have learned to see deep into the Earth's interior.
The 1960s of this century saw a geologic revolution. Scientists found that the crust of continents and of the ocean floor differs both in thickness and in composition. This gave rise to the theory of plate tectonics of lithospheric plate movement. The lithosphere was assumed to consist of six relatively rigid plates bounded by seismic belts. Most of the plates occupy entire continents and adjacent areas of the ocean floor as far as the mid-oceanic ridges where the lithosphere spreads causing rifts to open and new oceanic crust to form. Thus identified were the North-and South- American plates with adjacent areas of the Atlantic, the African plate with portions of the Indian and Atlantic oceans, and other plates.
Advocates of this theory considered it to be almost infallible and were convinced that it described the Earth's evolution in its totality and accounted for all its fundamental laws. According to the authors of this article, however, certain patterns of occurrence of mineral resources can be explained more convincingly from the standpoint of the geoblock conception.
That continental and oceanic crustal structures are different cannot be denied. Indeed, the continental crust is normally thick (up to 70 km) and is mainly composed of vast fields and chains of granitoids, while the oceanic crust is typically thin and composed of basalt, the planet's once molten material extruded from the mantle. The boundaries between these two types of crust are called transitals. They mainly consist of intermediate rocks, andesite and andesite- basalt. Of course, the plates move. But do they move as described by the theory of plate tectonics? For the fact is that apart from the plates, there are smaller structures occurring everywhere and making the planet's surface look like a mosaic.
Such structures - geoblocks - were first identified in 1967, in the Eastern Soviet Union, where one of the authors has conducted lengthy investigations. That discovery definitely proved the lithosphere to be inhomogeneous. Similar work was done for the Pacific mobile belt and the Pacific ocean floor. But the pattern of geoblock distribution became clear only after the geologic structure of the Earth's entire outer shell was analyzed.
What are geoblocks? Will not this new theory cancel earlier geologic achievements? Does it "work"?
At first glance the answer is simple. Geoblocks are natural geologic formations with clearly defined boundary systems. On the continents and in transitals their area ranges from 1 to 5 mln km 2 , and in the oceans up to 16 mln km 2 . The marginal regions of geoblocks can be studied by conventional geologic and geophysical methods, including various types of mapping (geologic, mapping of anomalous magnetic and gravitational fields and others), and drilling. All boundary systems can be broken down into three groups.
First, geoblocks are divided by linear troughs in the earth's crust and by extended deep fault zones which have turned in the process of the Earth's evolution into overthrust folding systems. Such systems are best displayed at the junctions of ancient platforms and mobile belts in the Canadian Rocky Mountains, the Zagros Mountains in the southwest of the Iranian Plateau, and in the Sette-Daban mountain range in the southeast of Siberia.
Second, geoblocks are bordered by geosynclines - unstable lithospheric zones which often contain rocks produced as a result of submarine volcanic eruptions. Island arcs associated with deepwater trenches can be regarded as forma-
tions closely related to the boundary systems. Among the intracontinental systems we will just mention the Urals, the Damara-Katanga in Southern Africa, the Elbrus in Western Asia and the Nan-Chang-Tsinling in Eastern Asia. Island arcs extend from Alaska (the Aleutian Arc) all the way to New Zealand.
Third, the boundaries between geoblocks are clearly defined by oceanic and continental rift zones, or zones of deep crustal faults, reaching down to the upper mantle. According to the plate tectonics concept, it is here that new crust is generated. The rift zones abound in complex bodies of solidified magma. In some areas they are formed of highly metamorphic rocks. In the oceans, geoblock boundaries are clearly marked by chains of islands and submarine ridges (e.g. the Emperor Seamounts, the Hawaiian Islands).
Many geoblock boundaries are sites of protracted recurrent geologic processes, i.e., crustal strains and compressions, changing thermal currents coming from the earth's interior, and frequent earthquakes. These are zones favorable for the movement of mineral solutions and melts and the appearance of cavities where minerals are trapped and deposited.
Whereas geoblock margins can be contoured more or less easily, their "bottom" layer is as yet a hypothetical notion. Information on the lower boundaries can be obtained by deep seismic sounding. Additional data are also yielded by studies of the magnetic, electric, thermal and gravitational fields.
Some geoblocks are likely to go down as deep as the Mohorovicic Discontinuity - the boundary between the crust and upper mantle, but most of them, end at the asthenosphere, a viscous mantle layer underlying the lithosphere.
The terms geoblock and lithospheric block are close in meaning. The difference between them is that the first emphasizes the unique nature of each of these structures.
Although the geoblocks are grouped together to form large segments of the crust (oceanic, continental and transitional), they are independent structures that shift their positions relative to one another, while remaining essentially intact. This movement is probably caused by individuality of the physical properties of the blocks. The rock strata making them up differ in thickness and composition, hence in density and the magnetic, electric and gravitational fields. And since all things in nature are tending towards equilibrium, the geoblocks move about to attain it.
Metallogeneticists, studying the laws of distribution of metallic ore deposits, should pay special attention to the geoblock structure of the lithosphere. Since the formation of a particular ore depends on the mineral composition of a particular geoblock, metallogenic provinces do not extend beyond geoblock boundaries. This assumption is supported by data on ore distribution on the continents.
In some cases second-order structures referred to as megablocks can be identified within the geoblocks. These can, in turn, be divided into smaller segments making prospecting for minerals easier. The existence of megablocks can be inferred from helium surveys, a method based on recording jets of the inert gas helium rising from fissures in the crust that reach the upper mantle. These fissures serve as boundaries between individual megablocks.
Crustal blocks differ in size and composition. The data available suggested to us the idea that there must exist a general law governing matter distribution on the planet and, especially, in the lithosphere. Judge for yourself.
According to their geologic characteristics, geoblocks are divided into three groups. The first one includes 39 cratonic blocks. These are massive portions of the earth's crust with areas from 1 to 3 mln km 2 . Their ancient rocks (from 3.8 to 1.6 bln years old) consist of highly metamorphic lavas, tuffs and some specific feldspars (anorthosites). The cratonic geoblocks occur in Canada, Africa, Brazil, Hindustan, Sweden, Finland, and Eastern Siberia. Their distinguishing features are complex folded structures, sometimes in the form of basins or domes, and evidence of recurrent geologic activity. They are rich in deposits of iron, gold, nickel, chromium and other metals.
The geoblocks in the second group (15 in all) are smaller (from 0.6 to 2 mln km 2 ) mobile fold structures of variegated composition formed in the Phanerozoic (600 mln years ago and later). They are represented by the Iberian geoblock in West Europe, the Iran-Baluchistan block in South Asia, and the Amur block in East Asia.
The geoblocks in the third group are 19 young platforms with adjacent shelf areas (e.g. the Western Siberian and the Sakharan geoblocks). They are between 0.6 and 2.6 mln km 2 in size. Since they are sheathed in thick sedimentary covers that almost always contain hydrocarbons, they often harbor deposits of oil and gas.
The transitals include 15 geoblocks. Their specific features are especially pronounced in the Asian-Pacific and Australian-Pacific zones where they cover territories from 1.6 to 3.8 mln km 2 . The largest of them, the Fijian and the Philippine geoblocks are 3.8 and 4.8 mln km 2, respectively In these geoblocks important world deposits of copper, nickel, chromium, silver and complex ores are found. They also contain large oil-and- gas fields; some of them are being commercially exploited (in Indonesia), while others are still to be developed.
And lastly, oceanic geoblocks, numbering 38. Their areas vary quite considerably. The North- Eastern (in the Pacific) and West- Indian ocean geoblocks occupy areas of 16 and 10 mm km 2 , respectively, whereas those in the Arctic Ocean, "only" form 2.5 to 3 mln km 2 . It is interesting to note that in other oceans there are also "small-sized" geoblocks. Most of them resemble continental blocks, since, unlike oceanic geoblocks, they are up to 42 km thick. They are called olands. But despite their thickness, they do not contain rocks typical of continental blocks. The "oland-rich" western part of the Pacific superregion may once have belonged to the enormous graniteless rise - the West Pacifida, the Atlantis of the Pacific. Separated by rift zones, the olands move but never dip under the continents. This proves once again that the earth's largest ocean basin has existed since the late Proterozoic.
Continental crust (microcontinents) also occurs in the oceans. For example, late pre-Cambrian granites (600 mln years old) are found in the Seychelles in the Indian Ocean. Other microcontinents are the New Zealand Plateau and the Chatham Islands with rocks containing Mesozoic granites (188 mln years old).
As the reader has probably noticed, geoblocks increase in size in the continent-ocean direction. Evidently, this is due to the fact that the harder continental crust (the granite-metamorphic rock layer) can be more easily crushed in the course of faulting of various kinds than the thinner and more plastic oceanic crust consisting of basalts.
To understand how geoblocks were formed, let us go back to the moment the Earth and other terrestrial planets were formed. The matter of the primitive nebula from which the solar system originated was most likely inhomogeneous both in chemical composition and particle size. The particles condensed into space objects, the bigger of which formed protoplanets while the smaller and lighter ones fell onto their surface, adding to their material and increasing their mass. The violent bombardment by a solid flow of meteorites heated and melted the surface of the evolving Earth. Its matter began to fractionate according to chemical and physical properties. Thus, low-melting metallic compounds separated from the high-melting silicates. Gradually a thin solid crust, a prototype of the lithosphere, was formed. But the meteorites dept breaking the fragile shell. Subsequently an intricate network of faults emerged, marking the boundaries of the primordial geoblocks.
The difference between the density of the cold solid crust and that of the molten mantle resulted in gravitational instability. Some geoblocks rich in metals sank to the base of the mantle. In this way, an iron-enriched layer evolved. As the earth's core is assumed to consist of metals, the metal components of that layer gravitated towards the core. In other words, the layer became dynamically and chemically unstable. And as soon as its components shifted their positions and it became lighter, it began to move upward, to the planet's surface. In this way, 4.6 to 3.9 bln years ago a pattern of lithospheric geoblocks emerged, based on an intricate network of faults and periodic changes in the chemical composition of the crustal materials.
A large number of hypotheses have been advanced to explain the origin, evolution and composition of the Universe.
Astronomers investigating the spatial distribution of stars and the boundaries of areas of space populated by super-giants, have found that large and small stars usually cluster together according to their size and form separate communities. This pattern seems to resemble the mosaics of the earth's crust: small blocks are confined to the continents and big ones to the oceans.
It is not only in space that matter exhibits discreteness. Look at what happens to the debris of a rock shattered by an explosion. The splinters "arrange themselves" around the epicenter in a strict order according to their size irrespective of the strength of the blast. The same pattern is observed when cracks bounding exposed rock are studied: the discrete blocks resulting from an earthquake shock arrange themselves in accordance with their size.
Curiously, in every case the ratio between the linear dimensions of large and small fragments is virtually constant and is within the two-to-five limits. This, it seems, is a general law governing any object of inorganic matter - from the supergigantic cells of the Universe to the microparticles of the earth's crust.
Apart from their fundamental, theoretical significance, these questions are important for practical geology For instance, if the horizontal dimensions of geological formations are confined to the two-to-five ratio, does this ratio remain the same with respect to the vertical dimensions? Where are the "roots" of the geoblocks to be looked for? It is well known that the bigger a structure, the longer it exists. Isotope analyses show that the life-span of the geoblocks ranges between 3-4 bln and "only" 100 min years. It is possible that the said ratio holds good not only for the extent of the geoblocks, but also for their volume and life-span.
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