Serpentine Belt of Burma.
This is a continuation of Episode 47, which HL Chhibber described in 1934 as the serpentine belt of Burma.
Naga Hills: In the Naga Hills, Pascoe mapped a few bands of serpentine, which have an NE to SW trend, coinciding with the country’s general strike. The bands are most probably in the form of sills. Some occur in the Makwari Beds, while others are found along the junction of the Chimi conglomerate (Cretaceous) and the Makwari Beds. At the same time, a few are intrusive into the Disang Shales (Cretaceous), which have been correlated with the Negrais Series of Theobald.
Manipur Hills: In the Manipur Hills, the serpentine forms a long dyke about 32 miles in length, which was intruded along a fault in the Axial Series. R D Oldham has marked another linear outcrop along the Axial-Nummulitic boundary on the map accompanying his paper. The rock occurs in dykes of varying sizes, the main axis of the intrusion forming a band some mile or two in breadth, which runs throughout the whole of the district examined in a general north and south direction. He states that it is further worthy of note that the mode of occurrence of this serpentine is the same as in Pegu; namely, that the serpentine outburst is confined not merely to the east of the main range but to the neighbourhood of the eastern limit of the hill rocks.
Prome and Thayetmyo Districts: Father South, in the Prome and Thayetmyo districts, serpentine masses have erupted along or very close to the Nummulitic-Negrais or Nummulitic-Axial Series boundary, again on the eastern slopes of the Arakan Yoma. Theobald, however, has shown on his map a few outcrops of these intrusions in the Axial Series, which the author thinks is the site of tension cracks in the formation.
Henzada and Bassein Districts: The serpentine intrusions of the Henzada and Bassein districts occur in the eastern foot-hill of the Arakan Yoma, either on the important fault marking the Negrais-Nummulitic junction or very close to it.
Andaman and Nicobar Islands: In the Andaman and Nicobar Islands, these intrusions occur mostly on the eastern side of the islands. In the middle Andamans, a solitary outcrop of serpentine occurs in the central part of the island. The slopes on the eastern side are much steeper than those on the west, and Tipper found that in the case of North Andaman Island, this persisted as a submarine feature. He believed that to the east of the island, there is a fault and that in the neighbourhood of the fault, there are the volcanoes of Barren Island and Narcondam.
Granites of the Eastern Hill Ranges.
The granites of Tenasserim and other districts farther north most probably also belong to the late Cretaceous-early Eocene period of igneous activity. The Malayan and Dutch geologists are decisive that they are of late Cretaceous age. The orogenic history of Burma supports such a conclusion, and these intrusions must have taken place when the Indo-Malayan arc of mountain chains was upheaved on the east. According to Sethu Rama Rao, after the deposition of the Mergui and Moulmein Series, there followed an upheaval by crustal movements, which caused folds (whose axes are parallel to the strike of the sedimentary rocks), and produced lines of weakness through which the granites were intruded in the form of large and small bosses. Two types of granite have been recognised; one is a coarse porphyritic variety devoid of tin, and the other is tin-bearing. The former is the older of the two. The age of this granite has long remained uncertain. L D Stamp brought forward some evidence tending to prove the Mesozoic age of the granite of Tenasserim, a point which the Geological Survey of India has lately confirmed. J Coggin Brown and others now believe that the granites that stretch from Mergui into the hilly eastern edges of the Yamethin and Kyaukse districts are identical in composition, age, and mineral association and are comparable with the granites of British Malaya, Sumatra, Borneo, and the Dutch East Indies.
Post-Eocene Orogenic Period and Igneous Activity.
The third important orogenic period was some time post-Eocene, as seen in the Andaman Islands, where, according to Tipper, the Miocene beds, which rest unconformably on the Eocene rocks, only slightly disturbed so that the main portion of the movement must have ceased before their deposition. Cotter recorded similar testimony in the Pondaung range. In the Maw Valley, near Tilin, there are deposits of gently dipping upper Tertiary gravels (Maw Gravels), a local facies of the Irrawaddian, resting upon the upturned edges of the Lower Eocene with strong unconformity and discordance. They prove that the Pondaung fold is very much older than the Maw Gravels and, therefore, pre-Irrawaddian, and, perhaps, even lower Tertiary. During these movements, the volcanic eruptions of Mount Loimye in the Kamaing subdivision, Myitkyina district, and the Wuntho area in the Katha district perhaps took place.
Mount Loimye: The tectonics of Mount Loimye are simple. The oldest in the history of this volcano is a greenish-black basaltic tuff that extends in a north-south direction for over three miles. It is undoubtedly an aqueous-bedded deposit, appearing in places to be interbedded with the Tertiaries and to have participated in the folding of the area. North of the Loimye kha, gabbro was intruded into the tuff in the form of a composite sill or sheet. There is little doubt that the tuff initially erupted along a fault running in a north-south direction, which subsequently provided an outlet for gabbro and the later rocks of the volcano. The existence of a fault along this line is supported by the presence of salt springs on it, etc.
Wuntho Region: The tectonics of the Wuntho area are far from adequately known. Noetling visited it in 1894, and according to him, the most remarkable phenomenon within the area of the volcanic ash on the western side of the Mainthong hill tract is that of salt springs. It is, therefore, highly probable that these salt springs follow a line of faulting, which seems, in the main, to run parallel to the cerussite vein. Looking at Noetling’s map, it appears that this fault is important, as it can be traced back to about 60 miles. It is, therefore, not unlikely that the igneous rocks of Wuntho have erupted along the fault. The floor through which the igneous rocks have erupted is nowhere exposed in the area so far explored. The idea of the existence of faults in this region is further substantiated by the occurrence of local earthquakes. The writer, while collecting accounts of the shocks felt on the north-eastern frontier of Burma, received several reports of shocks that had originated and were felt locally in the Wuntho subdivision of the Katha district. Some of them were indeed quite severe. It is, therefore, apparent that there are weak tectonic lines in this region that have not yet attained their equilibrium.
Pliocene Contemporaneous Igneous Activity.
There is no doubt that at the close of the Peguan epoch, orogenic movements occurred, which folded the Pegu Series. These movements must have also operated for some time during the deposition of the Irrawaddians. Accompanying these movements were manifestations of igneous activity, proofs of which we have in the Shwebo and other districts of Burma.
Shwebo District: In the southwestern corner of sheet 84 M/3, flows of basic rocks were found by L A N Iyer in the Kyauktan chaung. They are very numerous and vary in thickness from five to twenty feet. They possess low westerly dips. Their presence among the softer sandstones and shales of the Irrawaddian Series often gives rise to waterfalls. They are associated with tuffs and, in one case, at least bear interbedded bands of clay.
Kabwet Area, Shwebo District: Passing eastwards, we find similar evidence in the Kabwet area of the Shwebo district. Here, very decomposed lava, described by Dr Stamp and the author as the “older lava”, occurs interbedded with the Irrawaddians and is also involved in the folding. Where it was traced on the western side of the anticlinal fold, it dips very steeply but is much better displayed on the eastern side, where it forms a long and conspicuous scarp.
Mount Popa Area: In the Myingyan district, these movements continued throughout the Irrawaddian period right to its close, while in the Mount Poa area, we have black basic tuffs and ashes interbedded with almost the highest of the Irrawaddian beds, of which the plateau, on which Mount Popa stands, is built. As was noted by Sir Edwin Pascoe and the author, volcanic activity commenced towards the close of Irrawaddian times, but the later and larger flows of augite- and hornblende-andesites and the clastic material derived therefrom are in all probability post-Irrawaddian.
Pleistocene Orogenic Period and Igneous Manifestations.
The Pleistocene marked the next orogenic period in Burma when the Irrawaddian was folded and faulted. During this diastrophic period, volcanic and other igneous activity manifested itself in several centres in Burma, especially in the Central Belt and the Shan Plateau. It was at this time that some of the volcanoes of the Jade Mines area, the Lower Chindwin Region, Mount Popa, etc., were active. The dolerites of the Pegu Yoma were also most probably intruded at that time. The olivine-dolerites, teschenites, and olivine-basalts (mugearites) of the Mandalay and Shwebo districts were also erupted as a result of these tectonic movements.
Jade Mines Area: The volcanic rocks of the Jade Mines area can be classified as follows:
A. The Mabaw siliceous agglomerate.
B. Andesite and basalt outcrops in the neighbourhood of Namyong (25° 40’, 96° 25’).
The red siliceous agglomerate, which the writer has designated the Mabaw siliceous agglomerate, begins near Sankha village and continues in a south-easterly direction for about three miles as far as the Moschen hka. The width of the outcrop does not exceed half a mile. It appears that the volcano burst through the floor of the serpentine since lapilli of that rock, which are sometimes the size of small blocks about 18 inches long, are included in the breccia. Moreover, the reddish colour of the paste confirms the view that it must have been derived from the soil or subsoil of the serpentine.
There is no doubt that the formation in question is interbedded with the late Tertiaries since boulders of the Tertiary conglomerate are seen in the breccia itself, and a thin band of the Tertiaries is generally seen at the base of the breccia. The eruption probably took place along the fault, marking the junction of the serpentine and the agglomerate. This is confirmed by the occurrence of salt springs on the western boundary of the formation in question.
Several outcrops of andesites and basalt occur in the neighbourhood of Namyong. The volcanoes in each case have burst through the Tertiaries, which consist of soft sandstones with quartz pebbles and fossil wood, leaving little doubt that they cannot be older than the Irrawaddy Series. The Tertiaries have been thrown into anticlinal and synclinal folds, and most probably, the lavas reached the surface along the axes of the former.
The Lower Chindwin Region: The general trend of folding in the Tertiary hills lying to the west of the Chindwin River in the latitude of Monywa is north and south. There are three folding lines in the area.
(a) The line of folding, including the Medin and Myaing inliers of Pegu Series is continued
both to the north and to the south. Southwards, it assumes a trend east of south and includes the inliers of Pagan, Ngashandaung, and Yedwet. Northwards, the trend becomes east of north, and the line passes into Shwebo Hills inlier.
(b) The fold line that passes through Mount Popa is seen to include Shinmadaung and the explosive craters of the Chindwin. When traced still farther north, it passes to the east of the Shwebo hills into the Katha district and possibly includes the volcanics of the Wuntho region (Mingin Range).
(c) The volcanic occurrences in the hill belt to the east of Monywa, which run in a general direction of a few degrees west of north, appear to lie along an additional tectonic line to the east and add to the width of the belt of vulcanicity. (to be continued).
References: Chhibber, HL, 1934: The Geology of Burma, Macmillan, and Co Limited, St Martin’s Street, London.
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