Orogeny, geosyncline and continental drift

Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

OROGENY, GEOSYNCLINE AND CONTINENTAL DRIFT

F. AHMAD Department of Geology, Aligarh Muslim University, Aligarh (India) (Received October 5, 1966)

SUMMARY Most of the existing theories agree that mountains are formed in geosynclines. But a eugeosyncline is not, contrary to common belief, a long, narrow, constantly deepening, single basin. It is divided into smaller basins and emergent areas, called intra-geosynclines and intra-geanticlines (Beloussov, 1962). What probably happened was that at the end of a long period of “geosynclinisation” the forces responsible for this activity either shifted or simply ceased. The intra-geanticlines were then already upwarping, the intra-geosynclines started upwarping isostatically. Such epeirogenic upwarping is a very slow process. At best it could result in small isolated peaks, and not in mountain ranges. These isolated peaks could not have attained the heights reached by the Himalayan peaks. It is also pointed out that several ranges in the Tertiary mountain belts, e.g., the Himalayas, the Alps, the Rockies, and the Andes, were formed without geosynclines. Conversely, in the Gulf Coast “geosyncline” an immense thickness of undeformed sediments has accumulated and there is no reason to believe that this will give birth to a mountain range. Thus, it appears that extensive and highly folded ranges can come into being without a geosyncline existing in the area, whereas exceedingly thick sedimentary prisms need not lead to mountain formation. The current concepts are, therefore, in need of revision. Continental drift, on the other hand, appears to explain the phenomenon satisfactorily. Thus, the Tertiary mountain ranges may be divided into three broad categories: (1) Those in which two drifting crustal blocks collided and the sediments in between have been crumpled into a mountain chain. (2) Those in which a drifting crustal block has been resisted by the ocean floor. (3) Those in which an orocline has resulted in extensive block faulting and horst type of mountain on the obtuse side of the “fulcrum”. A geosyncline is essential for none of these and it is contended that the Tertiary mountains, by and large, were formed without the existence of a geosyncline, and were, perhaps, a result of continental drift. Therefore, it follows that unless drift had taken place earlier, there need not have been any mountain ranges on the earth. The intense folding and faulting seen in former geosynclinal belts must, then, be the result of oscillatory movements in the small basins within the geosyncline. From Tectonophysics, 5 (3) (1968) 177-189

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their similarity in faults and folds to the conditions in the existing mountain ranges it need not be concluded that mountains were formed only in former geosynclines.

INTRODUCTION

The Hall-Dana theory that mountain ranges are born of strong compression in geosynclines had not, until a few years ago, been seriously challenged. That it still has support is borne out by the statements of Jeffreys (I952), Landes (1952), Lees (1953), Jacobs et al. (1959, p.360) and others. Yet, Gutenberg (1951, pp.192-193), not long ago, stated that the thermal contraction of the earth probably explains mountain building in part, but other processes play at least equally important and probably more important roles. Goodman (1960, p.216) admitted that “the contraction theory is a good makeshift until something better is discovered”. Daly (1951, p.35), accordingly, concluded that “after wrestling with the erogenic problem for two centuries, geologists are still comparing speculations about it. Now holding the field are the classic contraction theory, the deep convection theory and the crust sliding theory” (see also Lees, 1953, p.250). To these theories may be added the “oscillation” theory of Haarmann (1930), recently restated by Beloussov (1962) and the “regurgitation” theory of Carey (1958). Both envisage considerable vertical movements. Admittedly most of these theories have been inspired by the tectonics of present mountain ranges, projected back in imagination through geological history on the basis of uniformitarianism. Yet, common to all is the supposition that erogenic activity has always been confined to geosynclines, (Bucher, 1951, p.514; Daly, 1951, p.32; Vening Meinesz, 1954, p.144). Recentwork on geochronology has, if anything, emphasized that this is true even of the Precambrian. On the other hand, the theory of continental drift has been in existence for over half a century. Recent work on palaeomagnetism strongly indicates that widespread crustal movement has taken place. Such movement of crustal segments could result in compression if two blocks happened to be moving towards each other, crushing between two giant jaws any sediments on the two continental shelves, and, perhaps accidentally, some on the ocean floor in between. This is believed to have been true of the Himalayas and the Alps. Thus continental drift - or crustal sliding, as Daly puts it could also result in the formation of a mountain chain. In such a case, however, a geosyncline is, perhaps, not strictly needed. It is proposed to consider whether or not a geosyncline is necessary for the formation of mountains. THE GEOSYNCLINE

The most active part of a geosyncline is the eugeosyncline, and orogeny is supposed to be confined to this (Kay, 1947). A eugeosyncline may collect upto 35,000-40,000 ft. or more o! sediments. It contains thick greywacke successions and volcanic rocks, both basic and acidic, is characterised by “Steinmann’s Trinity” and shows widespread mineralisation. 178

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Fig. 1. Queensland-part

of the Tasman geosyncline

(after Hill, 1951).

in that basins of sedimentation rising A eugeosyncline is “cannibalistic” as geanticlines, are rapidly eroded and the erosional products almost immediately buried in adjacent, newly-formed basins, with little transport or alteration. Thus are deposited the “poured in” sediments, petrologically known as greywackes. It has not, however, generally been realized that an extensive eugeosyncline is not, at any stage of its long and complicated history, a single basin. Von Bubnoff (1937) had pointed out that “the turbulent, vicissitudinous history of these troughs is still more important than subsidence, for they were not continuously covered by deeper seas, but comprised shallower Tectonophysics, 5 (3) (1968) 177-189

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parts and island arcs which emerged and subsided in a certain, apparently regular, rhythm”. Krumbein et al. (1949) state “geosynclinal belts exhibit variations, both geographic and temporal, irkthe degree of their subsidence curd Comzi4merzt orogeny, g.ir!irig vise to lomE t~*o-oughso~-~~~~~~ll~~t~o~~~, separated hy avcmte erogenic trends”. Beloussov (1962) designates these areas af subsidence and uplift “intra-geosynclines” and “intra-geanticlines”. The former are from several scores to 200 or 300 km wide and several hundred kilometres long. Dorothy Hill (1951) studied part of the Tasman geosyncline in Queensland and demonstrated that such variable conditions Bctually obtained until the geosyncline disappeared in epi-Permian Period. Hill, thus, pointed out that the Bowen and Maryborough Basins (Fig-l) began to downwarp only in the Carboniferous Period, i.e., in the pre-Carboniferous Period these areas were, perhaps, actually being upwarped. Haarmann (1930) and Beloussov (I 962) termed this repeated up-and-down movement “oscillation”, Brock (1959), “the key-board tectonics”. Harpum (1960, p.204) considered that orogeny followed a “cyclic pattern”. Beloussov (1962), moreover, presents some interesting analyses of erogenic activity in the different mountain ranges and points out that concomitant uplift and subsidence has characterised all the more well-known geosynclines. By the time an intra-geanti~line began its cycle of downwarping it was often so eroded that its granite core was well exposed. Incidentally oscillatory movement of these intra-geosyn- and geanticlines results in intensive folding and widespread unconformities, often found in the relict geosynclines. If the downwarping of a geosyncline is caused by convection currents a similar explanation may apply to the downwarping of the intra-geosynclinal basins and the upwarping of the intra-geanticlines. These convection currents could only be of the “tandem” type. It follows therefore that if the intra-geosynclinal basins and the intra-geanticlines change places, the convection cells must also be shifting, Dietz (1961, p.855) envisages frequent shifting of convection currents.

THE TERTIARY

MOUNTAINS

A geosyncline undoubtedly attracts a thick load of sediments and localizes these up-and-down oscillations, but mere downwarping and athick sedimentary load are not sufficient in themselves to start a cycle of orogenie activity. The geosyncline along the northern coast of the Gulf of Mexico offers an example where concomitant folding does not appear to have taken place although an immense thickness of more or less undeformed sediments is known to exist in the basin (Bucher, 1951, p.514). Hess (1951, p.529)points out that “there is no good reason to suppose, however, that such a geosyncline will localize future mountain building”; instead, he states that “the great thickness of sedimentary rocks of Beltian age in the present Rocky Mountains province persisted undeformed for more than one third of abillion years and seems only by chance to have been transected by the Laramide deformation. Strong deformation to the west of it had taken place earlier in the Mesozoic”. Significantly, in a large part of the Himalayan range, except perhaps in parts of Kashmir, characteristic eugeosynclinal sediments are absent.

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Fig.2. Carey’s

(1954) assembly of Gondwanaland.

On the other hand, andesites are not known from areas outside Baluchistan (Taylor and White, 1965). Similarly also, the lava beds known from the main Himalayan range are all of continental type and trap rocks of marine origin are not known to exist (S.C. Chakravarty, personal communication, 1965). These features indicate that a eugeosynclinal basin did not exist in the area, though such conditions might have prevailed locally. This appears to have been true of the East and West Indies as well (Hess, 1951, p.528), also, perhaps, of the Swiss Alps, where the deformation was initiated in the Late Jurassic, although there was no great thickness of pre-Cretaceous sediments in the area. Similarly, geosynclinal conditions existed only in the Wasatch Range of the Rockies, and not over its entire length (Beloussov, 1962, p.730); whereas Schuchert (1923) thought that the Tethys was not a geosyncline “in the American sense” (Glaessner and Teichert, 1949). Dietz (1963, p.315) admits that the term “geosyncline”, as used for the American geosynclines “cannot be directly applied to Tethyan examples (e.g., Alps and Himalayas) where the collision of sialic blocks seems to havedominated the scheme”. Also, Paleozoic structures of central Asia (Kunlun, Altai, Tien-Shan) were welded to the Tethyan folds, without a geosyncline existing in the area (De Terra, 1933, p.868); whereas the western Cordilleras of the Andes consist of an extensive and almost continuous batholith, with block faulted sedimentary and metamorphic rocks frequently exposing a crystalline core. Thrusting is evident only in the eastern part (Hills, 1947, p.47). These examples from the present mountain ranges could, perhaps, be multiplied. It, thus, appears that extensive and highly folded ranges can comeinto being without a geosyncline existing previously. In other words, the belief Tectonophysics, 5 (3)(1968)177189

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that mountains are formed only where a geosyncline has been folded and thrust need not be correct. On the other hand, if “continental drift” is a reality, perhaps, all the mountain ranges to-day might have been due directly or indirectly to “drift”. S.W. Carey emphasised this possibility in a lecture on the orogenir girdle

Fig.3. Plan of ihe Tertiary Mountain Systems (after Holmes, fig.857 and Du Toit, 1937, fig.45). The arrows show the directions dominant pressure. 182

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of Gondwanaland at the Pan-Indian Ocean Science Congress (Bangalore, India, 1951), and its possibility is brought out in his 1954 reconstruction of Gondwanaland (Fig.2), which he later, however, abandoned. Du Toit (1937) following Holmes (1965), envisaged two “girdles” (Fig.3). In Asia the mountain ranges admittedly belong to the Alpine erogenic activity and these continue southward, into Tonga, New Caledonia, New Zealand, perhaps, the Antarctica, the Andes, the West Indies, the Atlas, and finally, back into the Alps (Fig.2). This Gondwanaland-girdling mountain system may be divided into two broad categories: (1) That in which two crustal blocks, supposedly drifted towards one another deforming littoral and possibly bathyal sediments between them. (2) That in which a drifting continent, over-riding an ocean floor, crushed and warped the sediments on its own continental shelf. The outstanding examples of these two types of mountains are the Himalayas and the Andes. Wilson (1963, p-926) adds that “continental crust is not dragged down and two slabs, on meeting, pile up as in the Himalayas and Tibet. This may ultimately produce slippage beneath the continent. There is certainly a difference between mountains formed where two crustal masses join and those where a continent over-rides an ocean floor as in the Andes.” Carey (1958) accordingly suggests that in Tibet the excessively high negative anomalies are caused by the Gondwana block in this area having been over-ridden by the Angaraland block. He calls it “the double thickness zone”. Taylor (1910, p.191) had concluded, even before the Wegener hypothesis was postulated, that apparently “it was the obstructing action of the Indian peninsula which produced the great Himalayan re-entrant. It was the tremendous resistance offered by this fragment of ancient Gondwanaland which held back the advancing folds to the line of the Himalaya.” Where deformation extends within a continental plate, it “takes place over a much wider belt, is characterized by fracture of the crust, and the typical igneous sequence of the island arcs is not present” (Hess, 1951, p.529). This is as true of the Rocky Mountain area as of many of the fold ranges of central Asia, which, as pointed out above, do not seem to have been formed in geosynclines. The central Asian mountain ranges have been divided by Nalivkin (1960, p.90) into the northern, central and soutern zones. Nalivkin points out that in most of these areas tectonic activity was strong in the Upper Cretaceous and Palaeogene, and continued into the Neogene, and the Quaternary. In the northern area the mountains are not folded and the relief is due entirely to block movement, whereas in the central mountain chains block faulting is masked by folding. The rifts are due mainly to elevation, and rarely to the depression of blocks of Palaeozoic and Precambrian strata along their boundary fractures (Nalivkin, 1960). The Altai mountains, too, were uplifted chiefly during the Quaternary as a broad arch, accompanied by considerable normal faulting (Billings, 1960, p.380). The present relief of Asia is, thus, clearly modern, the movements having started in the Pliocene, are continuing at the present time(Barbour, 1929, p.190; Nalivkin, 1960, p.94). These features of the Asian mountain system would indicate a zone of strong tension which, it may be conjectured, was connected with the Punjab orocline of Carey (1958). Similar large-scale tensional areas existed in the Indian Peninsula from the anticlockwise rotation of the crustal block with the Baluchistan area as its fulcrum, resulting in what Carey called the Tectonophysics, 5 (3) (1968) 177-189

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Baluchistan orocline (Ahmad, 1966). Incidentally, this block movement would provide a third type of mountain building activity that could result from continental drift. There may, then, be no hesitation in treating these Tertiary mountain ranges as having been caused by continental drift. Crushing of the sediments between the two giant jaws of drifting crustal blocks resulted in uplifting the Himalayas and the Alps. Resistance offered by the ocean floor also resulted in mountain building, as in the case of the Rockies and the Andes. Blockfaulting, due to tension caused by the Punjab orocline, produced the ranges of central Asia, the compression, in this case being limited to Kashmir on the Indian side of the fulcrum where the foreland has been involved in folding (West, 1937, p.194). Continental drift can, thus, cause mountain building by three different processes, and all the Tertiary mountain ranges of the earth are the result of one or the other of these tectonic activities. Billings (1960, p.387) points out that “a study of the structure of the various continents indicates that large strike-slip faults have not been important in the past. It is possible that strike-slip faults are much more important now in crustal evolution than they were in the past”. Could not these facts be genetically connected?

GENERAL

DISCUSSION

This brief resume of the tectonism of a geosyncline and that exhibited by the Himalayan-Alpine-Laramide systems helps to bring out some interesting features. Of the foremost importance is the fact that a eugeosyncline is invariably divided into a number of small downwarping intra-geosynclinal basins separated from one another by upwarping intra-geanticlines. These intra-geosynclinal basins vary in dimensions, rate of downwarping and time of initiation. The sediments in these basins, consequently, also exhibit different characteristics and sedimentary structures. Also the sediments in the basins overlap on to the intra-geanticlines and there could be no continuity in sediments from one basin to another, such as exists, for instance, in the Himalayas where many horizons can be traced for adistance of over a thousand miles, without any significant break. Immediately before the final “climacteric” most of these intra-geanticlines will have been upwarped for a considerable time and in most of these, granites and gneisses would be exposed. During this climacteric, then, either the entire geosyncline would be folded, thus, further folding theintrageanticlines and the eugeosyncline would be converted into a mountain range or else only the intra-geosynclinal basins would be involved in the folding. The intra-geanticlines would in that case largely escape deformation because their hard granite cores would not be as affected by tectonic activity as their former sedimentary cover would have been. Moreover, the crust of these intra-geanticlinal belts was no longer being downwarped so that heat did not have an opportunity to impart the plasticity essential for intricate folding. It is, thus, likely that these intra-geanticlinal belts are not additionally folded, or at least, are not equally strongly folded. The strongly folded intra-geosynclinal basins will, then, form isolated, pyramidal heights separated by lower ranges of granites or considerably older and largely altered sediments. In the early stages this may result in chains of islands. 184

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Thus, in the Precambrian and Palaeozoic Eras central Asian region accumulated sediments and effusives, totalling 25-30 km. The Mesozoic Era added another 10-12 km. The Kimmerian orogeny followed and compressed these enormous thicknesses of strata into complex, sometimes iSOdid folds, which formed chains of mountainous islands resembling modern Japan. These islands stood above the Lower and Middle Cretaceous and Palaeogene seas. This was the geosynclinal phase; the “drift” phase of mountain building followed, and reached its maximum intensity in the southern mountain chains, where it created the lofty mountains of the present day (Nalivkin, 1960, pp.92-93). On the other hand, even if the intra-geanticlines were as strongly deformed as the intra-geosynclinal basins, mountain ranges may, nevertheless, not have formed. It is unlikely that the granitic parts of these intra-geanticlines could ever rise as high as the intra-geosynclinal basins that carried thousands of metres of sediments. Nowhere in the existing mountain ranges do these conditions, apparently, obtain. Also, it may appear to be mechanically unlikely that comparatively small, isolated, intra-geosynclinal basins could be upwarped to the dizzying heights of Everest and Kanchenjunga for the resulting pyramids would have been subjected to strong erosion, arid could probably never. have attained any great height. As an example, the Tasman geosyncline in eastern Australia ended in epi-Permian times. The “mountains” that emerged as a result of the supposed folding of the Tasman geosyncline proved exceedingly ephemeral and were soon eroded away. Similarly, the final climacteric in the Appalachian geosyncline was in the Permian-Triassic Period, and yet, late in the Triassic Period basins had appeared in the area and fresh-water sedimentation was taking place. The Triassic Period is believed to have lasted for about 30 million-years and in a fraction of this period the mountains that formed in these geosynclines are believed to have risen and to have been completely eroded away. Taking another example, about 3 km of sediments are estimated to have been removed from the Alpine valleys in about 30 million years (Jeffreys, 1952, p.318); and yet the mountains are, perhaps, still rising. The contrast is significant. Stille (1936, p.850) agreed that “The mountains resulting directly from an orogeny may be very ephemeral. This fact is proved by the consideration that in spite of the erogenic uplift of an area the sea may reappear very soon, in the geologic sense, in the area of uplift”. It may also be significant that none of the great mountain peaks of today, Everest, Kanchenjunga, Matterhorn or Mont Blanc, consist of geosynclinal sediments. Apparently no mountains, in the sense of the Himalayas or the Alps of to-day, were born in geosynclines and the geologist has all along been misinterpreting the data. The beds were no doubt, folded, but the folding happened in the process either of “regurgitation” or of “oscillation”. What, probably, would be equally significant is the likely effect on gravity anomalies. If the intra-geanticlinal belts, already eroded deeply and for a long time, get further upwarped, the mantle is likely to be affected and positive anomalies are likely to come into being, (Jeffreys, 1952, p.32’7). Ahmad (1965) offers this explanation for the “Burrard’s hidden ridge” in the Indian subcontinent. These “hidden ranges”, in the cores of the intra-geanticlines, would be adjacent to the intra-geosynclines where strong negative anomalies would exist simultaneously. The result for the eugeosyncline Tectonophysics, 5 (3) (1968) 177-189

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would, then, be an absence of any anomaly or a comparatively small negative anomaly. This is not found to be the case in the Himalayas. Moreover, in the light of present day thinking, it is uncertain whether “diastrophism” in a geosyncline is caused by compression. It, indeed, appears possible that the earth is not shrinking; but is expanding (Egyed, 1956; Carey, 1958,and others). If this is so, then, perhaps, no geosynclines, not even former ones were affected by compression. Folding of the geosynclinal sediments could, then, have taken place under tension, as envisaged by Carey (1958) or Beloussov (1962). Such “regurgitated” sedimentary beds could not attain Himalayan heights. More important, no “nappes” couldform without large-scale gravity sliding; yet Billings (1960) does not think that gravity sliding could have occurred extensively. The Tertiary mountain ranges were, as suggested above, formed as a result of continental drift. Had the crustal blocks not drifted and their drift not met resistance, these ranges would not have formed. Additionally, Wilson (1963, p.928, fig.6), points out “the inactive mountains making junctions of older continental fragments” and includes the Appalachian Range, and its continuation into Europe, as well as the Urals in this category. The present writer interprets this as mountains formed as a result of earlier continental drift. The Aravallis may also belong here. It is, moreover, noteworthy that the sea trenches, like those present to the south of Indonesia, sometimes referred to as “geosynclines” (although they are being supplied with little sediment, while they downwarp), unlike the normal geosynclines that result from tension, are formed by deformation of island arcs and strong compression (Daly, 1951, pp.28-29; Hess, 1951, p.528). The horizontally directed pressure is produced by the drifting of the crustal blocks. It is, however, not known if these geosynclines acquire other characteristic features of a true geosyncline, but it seems desirable to distinguish them from real geosynclines and to refer them in literature as only “sea trenches”. The few areas where geosynclinal sediments occur in the existing mountain ranges may represent such seatrenches, or may be, locally there were some geosynclines around Gondwanaland. It is, accordingly, suggested that the supposed final climacteric of a geosyncline could not result in the formation of a mountain range and the present mountain ranges need not be taken as the prototype of all geosynclinal folded belts of the past. On the other hand, strong folding informer geosynclinal areas need not indicate the former existence of great mountain ranges along these belts. That strongly folded beds indicate the former existence of a mountain range had been concluded from the malogy of strongly folded and overturned beds observed in the Alps and the Himalayas, and that, similar highly folded beds exist in the former geosynclinal areas. Lees (1953, p.223) points out that “The oldest rocks . . . are the deep levels of one time mountain systems worn down in many cases to mature peneplains”. Mountains, however, result from vertical movements that have no genetic connection with earlier periods of folding (Bucher, 1951, p.516; Billings, 1960, p.393; Harpum, 1960, p.201) and Belouseov (1962, p.500) emphasizes To the present author, then, it appears the role of “oscillatory movement”. probable that there is no activity in a geosyncline which could justifiably be described as the final climacteric or mountain-building episode. What probably happened was that at the end of a long period of “geosynclinization” the stress responsible for this activity either shifted or simply ceased. 186

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The intra-geanticlines were then already upwarping, the intra-geosynclines started toupwarp isostatically. Epeirogenic upwarping is tensional, and is a much slower process than strong tectonic activity observed in the mountains of to-day. Intense folding seen in the former geosynclinal areas was, as suggested above, already there, either from regurgitation or by oscillation. It, then, seems to follow that unless continental drift had taken place earlier as well there probably were never any mountain ranges comparable to th’e Himalayas, the Alps, the Rockies, or the Andes. These ranges, and many others, are a direct result of drifting continental blocks being resisted by other crustal segments or by the ocean floors. It, therefore, seems necessary to separate the terms “diastrophism”, and “orogeny”, and not to use them more or less synonymously. CONCLUSION

All Tertiary mountain ranges, although they differ in details of origin, are due to continental drift. Three different dategories are recognizable due to: (1) Crushing of the sediments between two sliding crustal blocks. (2) Drifting of a single land-mass over ocean floor, so that the bedson the continental shelf are folded. (3) Tension and block faulting caused by oroclinal rotation about a fulcrum. Geosynclines are, thus, not a prerequisite for mountains nor is a great accumulation of sediments in a basin alone sufficient to initiate erogenic activity. The present mountain ranges are, therefore, not representative. On the other hand, a geosyncline is normally subdivided into intra-geosynclines and intra-geanticlines. A geosyncline is, therefore, not a long, narrow, feature, simultaneously downwarping over its entire length and breadth. Upheaval of a geosyncline could only result in isolated peaks, separated by granitic areas, and not in mountain ranges. A chain of peaks may, however, emerge but even this would be ephemeral. The intense folding observed in geosynclinal areas accompanies repeated upwarping and downwarping of intra-geosynclines and intra-geantielines, before the cessation of forces responsible for the main geosynclinal activity. Thereafter the intra-geosynclinal parts upwarp isostatically. The intra-geanticlines were then already upwarping, and would continue to upwarp till the isostatic balance was restored. Thus, no climacteric or mountain building episode is necessary to end the geosynclinal activity. The whole belt thereafter becomes a part of the stable block. This applies to the entire geological history of the earth. Continental drift, therefore, resulted inthe Tertiary mountain formation and unless drift had taken place earlier also there might have been no mountain ranges on the earth. ACKNOWLEDGEMENTS,

The author is grateful to Prof. Patrick A. Hill of the Carelton University, Ottawa and his colleagues Prof. F.K. North and Dr. R.W. Yole for going through the manuscript and offering comments. For the same help he is also obliged to his own colleague, Dr. V.K. Srivastava. Tectonophysics,

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