THE TREND-LINE OF THE HIMALAYA—ITS NORTH-WESTAND SOUTH-EAST LIMITS¹

D. N. WADIA

The trend-lines of mountains—the directions of extension of the prominent mountain chains of the earth's surface—are not accidental features but possess great significance. They throw light on the primordial structural framework of the outer portion of our planet, and are stamped on it as the outcome of the relief sought along certain well-defined lines of weakness of the earth's surface, as the outer, cooler, shell has, by wrinkling and folding, accommodated itself to the diminishing diameter of the hotter interior. Such wrinkles of the crust, the mountain chains of the continents, and the island-arcs of the oceans, geologists tell us, have been marked out on the face of the earth since very early times and have been followed by periodic compression and folding of the crust during successive geological epochs of the earth's history. It has sometimes happened that old fold-lines are abandoned, but there are few cases where a young mountain chain diagonally traverses an older chain.

Paradoxical as it may seem, mountains denote the weaker belts of the earth's crust, belts that have been depressed below the sea for long ages and have received enormous deposits of marine sediments belonging to long cycles of geological ages. It is these overloaded, and consequently weakened, zones which respond most to the lateral and tangential earth-pressures which follow the cessation of the sinking process and become folded and elevated into mountain chains. Hence has arisen the well-known principle of geology that where areas of the earth have sunk the deepest, they also rise highest. These sunken and loaded belts are called geosynclines in geology, and geosynclines have played a large part in the revolutions of the earth's past geography, stamping upon it the broader features of the continents, mountains, and the ocean-basins. The records preserved in its sedimentary piles reveal the history of the life, deposits, and earth- movements of the various periods in different regions of the earth. During the process of compression of these sunken loaded zones into mountains, the geosynclines are narrowed to from a half to a quarter of their original width; the formation of the Himalaya has, for instance, brought a point in Tibet nearer to a point in Bihar by seventy to eighty miles at least.

Perhaps the largest individual geosyncline on the face of the earth is the one represented by the Himalayan system of mountains—a series of ranges 1,500 miles long and from 150 to 250 miles broad. Geological work of the last few decades in these mountains has proved that, in spite of some local differences, there is an essential unity of structure, composition, and stratigraphy from Kashmir to Assam, which proves clearly that this vast tract of northern India was under the waters of a mediterranean sea—known to geologists as the Tethys—continuously from the end of the Carboniferous period of earth-history to the end of the Eocene. Large portions within this tract had then lately emerged from another older submarine period, extending through the whole length of the Palaeozoic era.

The thousands of feet of marine sediments laid down on the bed of this sea, from the Upper Carboniferous to the Eocene, with their characteristic entombed fossils indicative of the successive ages of deposits, were subjected to protracted compression during later Tertiary ages, as in a vice, between the two stable continental blocks of peninsular India to the south and the table-land of Tibet to the north. The uplifting of the Tethys floor resulting from this compression, its exposure to atmospheric agents, and the sculpturing of time, have produced the youngest, largest, and highest chain of mountains in the world, a chain that is probably still growing in altitude. Late discoveries go to show that the Kashmir portion of the Himalaya has been uplifted at least 5,000 feet since the advent of Man on earth.

The trend-lines of the Himalaya form an interesting study of the mechanism of mountain building and of the reactions of the old stable blocks of the earth's crust against the weaker zones of the earth, the geosynclines. For 1,500 miles from Assam to Kashmir this system of ranges follows one persistent, gently curved alinement, first in an E.-W., later in a SE.-NW., direction, and then appears to terminate suddenly at one of the greatest eminences on its central axis, the peak of Nanga Parbat (26,620 feet). Here the Indus has cut an extraordinarily deep trench in the heart of the chain. In the words of Sir Sidney Burrard, 'this abrupt termination of the Himalaya at the Nanga Parbat peak has for years given rise to surprise and incredulity'. Trigonometrical surveys failed to find any line of heights, or any linear arrangement, to continue the chain to the west of the Indus. Geological study of the region, however, has clearly shown that just at this point the strike of the mountains bends sharply on itself and pursues a south, and then a south-west, course through Chilas, the Indus Kohistan, and north Hazara instead of pursuing a north-westerly course through Chitral. All the geological formations that compose the mountains here and the axis of every individual fold into which these have been plicated show this sharp hairpin curve as if it has been bent round the pivotal point of Nanga Parbat. This extraordinary inflexion affects some hundreds of miles depth of the mountains; it affects the whole breadth of the Himalaya from the foothill zone abutting on the north-west Punjab, and all the mountains beyond the Great Himalaya range, to the Pamir plateau, which is its extreme inner limit and which itself shares in the flexure. On the west side of the syntaxis (as this acutely reflexed bundle of mountain-folds is termed) the Himalayan strike swings from the prevalent north-easterly to a south-north direction in north Hazara and continues so to the latitude of Gilgit (36°); then it turns west-east, the Pamir plateau showing a distinct equatorial disposition of its geological formations. To the south and east of this the main tectonic strike, or the 'grain' of the mountains, in Rondu, Astor, and Deosai quickly takes on a NW.-SE. orientation—a direction which gradually changes, owing to the curved alinement of the Himalaya, to the W.-E. grain north of Sikkim and Bhutan.

The Himalayan Geosyncline and the Gondwana Foreland

The cause of this singular twist in the alinement of the NW. Himalaya was ascribed by Suess to its meeting and conflicting with an opposing mountain system, the Hindu Kush, both systems suffering a deflection and undergoing an acute bend at the contact. Geological work during the last twelve years, however, has proved a remarkable similarity and continuity in the structure, rock-components, and stratigraphy of the two flanks of the Himalayan syntaxis; not only do the rock-series of the Kashmir side pass over into Hazara right round the angle without any discordance whatever, but the individual fold-systems and their structural relations with one another also are identical on the two sides of the loop.

A more probable cause for this phenomenon has been suggested lately,¹ viz. that the Himalayan system of earth-waves as they emerged from the waters of the Tethys has been pressed against, and has moulded its shape on, a tongue-like projection of the Indian peninsular shield—one of the most ancient and rigid blocks of the earth's crust, which has been immune from any folding or deformation since the dawn of palaeontological history. On meeting with this obstruction the northerly earth-pressures were resolved into two components, one acting from a north-east, the other from a north-west, direction against the shoulders of this triangular promontory of the Peninsular block.

Thus it has happened that the Hazara mountains are, in their broad geological constitution, a replica of the Kashmir mountains. This axial continuity is traceable farther westwards as far as Camp- bellpur, beyond which point the persistence of the Himalayan axis is not certainly defined. The Himalayan geosyncline may provisionally be regarded as ending here, though the Himalayan system of orography continues much farther westwards beyond the Baluchistan and Persian highlands. The exact western limit of the Himalaya will be decided by field-work on the Afghan and Perso-Baluch frontiers. The trans-Indus inner Himalaya of Chilas, being tribal country, has not yet been studied geologically.

The eastern limit of the Himalaya beyond Assam is as yet a matter of conjecture only. This part of the range is still largely a terra incognita, geologically speaking; nor is it geographically as much explored as the rest of the range. Nevertheless, the data that are available, mainly from the few scattered geological observations and geographical explorations, permit some tentative views on the subject. Geological mapping has been carried out in patches in the Dafla hills, the Sadiya territory, Fort Hertz, and the Hukawng valley, and farther south in the Arakan Yoma. The tectonic strike and the geological disposition in these rather strongly suggest a deep knee-bend of the mountains from an easterly to a southerly trend. An examination of the skeletal geological map of the region also shows that a considerable depth of the mountains is affected and that the looping of the various fold-axes appears to be more acute and pronounced than at the Indus bend. The obstructing block functioning as the pivot of this syn taxis must be the granite massif of the Assam plateau, whose resistance to all folding movements is manifested in the perfect horizontality of every geological formation that rests against its southern*slopes. Oldham and Pilgrim have stated that the Assam hills, with the Rajmahal hills, formed the old watershed of India in pre-Himalayan times, and this large granite core of these hills may have resisted the southward advance of the Himalayan earth-waves against this promontory of the Gondwana foreland. In the Arakan Yoma the geological strike for several hundred miles is meridional, turning to a NE. direction from Kohima to Fort Hertz. Beyond this point there is an abrupt swing to the NW. at first, and then to an ENE.-WSW. direction for a considerable interval, and finally to E.-W. through north Assam and Sikkim.

Here we may cite some collateral evidence tending to the inference that the Himalayan chain bends southwards and does not extend eastward into China. The following is the most pertinent to this question:

1. The Shan plateau of Burma, built of Palaeozoic and Mesozoic rocks, though of insignificant proportions, is comparable with the Tibet plateau forming the hinterland of the Himalaya. Structurally it has the same relation to the country in front of it as the Tibet plateau has to the Himalaya. This was long ago pointed out by La Touche.
2. The Mesozoic flora of Burma is different from the homotaxial Upper Gondwana flora of India and is more allied to the flora of China, a quite distinct palaeo-botanical province from India. This separation of the life-provinces of Gondwanaland and China, lasting from the Upper Carboniferous to the Cretaceous, is a fact well established by Halle and Sahni. This severance must be due to the interposition of the barrier of the Tethys (Himalayan) sea.
3. The Cretaceous system of Burma reveals no connexion, faunistic or stratigraphic, with the contemporaneous deposits of the closely adjacent Assam province (a part of Gondwanaland), but it possesses marked affinity with the Cretaceous of Sind and Baluchistan—a Tethyan province, covered by a south-west arm of the same sea which covered the Himalaya from the Punjab to the Abor country.
4. According to several palaeontologists, the Tibetan zone of Himalayan stratigraphy extends into Sind on the west side and to Burma on the east. This is explained by the deposits being laid down on the bed of a sea permitting an uninterrupted migration and intercourse of faunas from Sind to Burma during a large section of geological time.

On the other hand, the physiographical evidence is not so decisive on the point. The alinement of the rather well-defined meridional ranges east of the Tsangpo (Brahmaputra), alternating with the gorges of the Irrawaddy, Salween, Mekong, and Yangtze is of a conflicting nature, and so competent an observer as Capt. Kingdon Ward believes that the parallel meridional ranges are not true tectonic ranges but are merely transverse ridges dissected out of the broad eastward extension of the Himalaya beyond the Abor country, carrying on it a line of high snow-peaks. This range extends, according to Kingdon Ward, following Kropotkin and Gregory, across the gorges into China and probably to the Pacific coast. Just where the Assam Himalaya meets the meridional Burma ranges (at the bend from an easterly to a southerly trend, according to the other view) is the high peak of Namcha Barwa (25,445 feet) very much in the position occupied by Nanga Parbat on the bend of the Great Himalaya range at the Indus.

The present writer holds the view that the Great Himalaya range from Nanga Parbat on the Indus to Namcha Barwa on the Brahmaputra denotes the Himalayan protaxis-the axis of original uplift of the floor of the Tethys depression. At both its ends it has undergone sharp southward deflexions owing to the termination of the obstruction offered by the north edge of the Gondwana block.

Kingdon Ward adduces evidence from climate and the present distribution of the living fauna and flora which favours the view of an easterly continuation of the chain. The subject, however, must be regarded as still sub judice and awaiting further exploratory and geological work, though such evidence as exists on the geological side strongly suggests a southward bend of the Himalayan axis after traversing the Tsangpo gorge.

Note by Editor

Captain Kingdon Ward's views mentioned by the author of this paper are given in the Geographical Journal, vol. lxxxiv, p. 369. He bases his views primarily on the fact that the four great rivers, the Tsangpo, the Salween, the Mekong, and the Yangtze cut gorges in a region where all the known snow-peaks fall within a belt about 50 miles wide, which runs diagonally from the great bend of the Tsangpo to the great bend of the Yangtze at Likiang. His conclusion is that these four rivers have cut their courses diagonally across the strike of the main range. The other view is that the rivers lie in trenches cut along the strike of the great mountain bend.

The Trend-Line of the Himalaya—Its NW. and SE. Limits 69 There is nothing inconsistent in Kingdon Ward's view that the Great Himalaya extends beyond the Tsangpo gorge and that the great snowy crest-lines trending south-east between the Rong To Chu and the Salween about lat. 29⁰ 30', long. 97⁰ o', which were observed by him, are part of the Great Himalaya, with the syntaxial hypothesis of Wadia. And it would seem that these snowy crests would then play the part in the flexure of the ranges that Nanga Parbat does in the west, that is to say, that the Great Himalayan syntaxial bend is on the divide between the Rong To Chu and the Salween north-east of Rima.

The discrepancy between the two hypotheses results from the belief of Kingdon Ward that the range from this point bears no relation to the river pattern, a belief which those who have studied the evolution of river systems must find hard to accept. It is possible that a single mature river might have cut its trench diagonal to the strike; it is conccivable that two or even three might do so; but that all these great rivers and many of their tributaries should carve so regular a pattern so utterly discordant to the range trends as visualized by Kingdon Ward is extremely difficult to credit.

It does not seem that the mountain Namcha Barwa bears the same relation to the eastern syntaxis that Nanga Parbat docs to the north-western syntaxis, for in the north-west the Indus in Chilas is truly trans-I limalayan, while the Tsangpo gorge east of Namcha Barwa is carved through the range. On the other hand, if Wadia is correct in his hypothesis, the closely packed trans-Himalayan ranges between the Rong To Chu, the Salween, the Mekong, and the Yangtze are strictly comparable to t he packing of the trans-Himalayan ranges of the Hindu Kush and Karakoram between Nanga Parbat and the Pamirs.

A recent paper by Professor Arnold Heim on 'The Glaciation and Soli- fluction of Minya Gongkar', read before the Royal Geographical Society on Monday, the 10th February 1936, seems to me to add evidence in support of the North-Eastern Syntaxial hypothesis (see also in this volume the reviews of two books on Minya Gongkar, pp. 161-3, infra).

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