The
Geology of Burma
HL Chhibber’s renowned book, The Geology of Burma, includes a discussion on igneous activity concerning tectonics. Given space limitations, I will highlight some key points regarding “Igneous Activity Relation to Tectonics” of Chapter XXVII as follows: -
Geographical
Distribution of Igneous Occurrences.
The igneous
occurrences of Burma occur in groups along wellmarked lines connected with the
orographical features of the country described in Chapter I. The author
believes that, connected with the geotectonics of Burma, there are at least
five main igneous lines, as described in the sequel and depicted on the map
hereunder.
Shan Plateau
and two Igneous Lines: Connected with the warping of the Shan Plateau and its
southern continuation are two igneous lines: an older and a younger. On the
former lie the siliceous tuffs near the Lagwi Pass situated on the northeastern
frontier of Burma, the Bawdwin lavas and tuffs, and perhaps also the silicious
volcanic rocks of the Elphinstone, Maingay Island, etc., in the Mergui
Archipelago. The younger line comprises the volcanoes of Teng Yueh in
south-west Yunnan, the Loi Han Hun volcano in the Northern Shan States, and
some of the volcanoes of the Mergui Archipelago, including the basaltic flows
of Medaw Island. This line passes farther southwards onto the Malay Peninsula.
Edge of Shan
Plateau: It is now well-known that the edge of the Shan Plateau is formed by an
important boundary fault, which constitutes a major structural feature in the
Burmese Malayan Arc of mountains. Along the edge of the Shan Plateau and the
Tenasserim Yoma lie the lavas and dolerites of the Kabwet area, the Mandalay
and districts, and the rhyolites and rhyolite tuffs of the Thaton district,
etc.
Igneous Line
of Central Belt: The igneous occurrences of the central line connected with the
folding of the Pegu Yoma and their continuation into the northern hill ranges
comprise those of the Jade Mines area in the Myitkyina district, the Wuntho
area in the Katha district, the Lower Chindwin, Shinmadaung, Mount Popa and the
Prome and Tharrawaddy districts. The volcanoes of Barren Island and Narcondam
are situated on the southern continuation of this line.
Western
Igneous Line: Connected with the upheaval of the western hill ranges is the
belt of serpentinised peridotites, which runs from Java and Sumatra through the
Andaman and Nicobar Islands in the south to the frontiers of Burma and Assam in
the north. Associated with these intrusions are volcanic lavas and tuffs found
in almost all the centres of igneous activity. These lines have been depicted
on the sketch map.
Important
Orogenic Periods.
Excluding
the solitary instance of igneous activity during the early Palaeozoic period,
the study of the relationships of igneous activity to tectonics may be said to
have commenced almost with the Tertiary or late Mesozoic orogenic history of
the country. In all, there are the following six important orogenic periods in
the geological history of Burma which were accompanied by igneous activity:
1. Early
Palaeozoic. The age of this orogeny cannot be fixed with certainty;
2. Upper
Cretaceous-early Eocene;
3.
Post-Eocene;
4. Pliocene;
5.
Pleistocene;
6. Recent
and sub-Recent.
Submergence
of Crystalline Floor: After the formation of the crystalline floor, which is
well exposed in the Ruby Mines tract of Mogok, the land was submerged, and a
huge thickness of the unfossiliferous strata of the Chaung Magyi was deposited.
Folding of
the Chaung Magyi and Mergui Series: After the deposition of the Chaung Magyi
and Mergui Series, it appears that the country was upheaved and subjected to
intense pressure. The strata are folded into narrow, closely compressed
corrugation and are extensively faulted. The folding of the Mergui Series is
sharper and more intense than that of the Moulmein Series, which is of a
comparatively gentle type. In the case of the Chaung Magyis, however, evidence
is forthcoming that they were thrown into folds and denuded before the
deposition of the Nyaungkangyis (Ordovician). There is probably an important
hiatus between the Mergui and the Moulmein Series in Tenasserim.
Bawdwin
Lavas and Tuffs: Either towards the end of this period of deposition or during
the interval of upheaval and disturbance that followed it, some exhibition of
explosive volcanic energy took place in the neighbourhood of Bawdwin, resulting
in the emission of lavas and tuffs of an acid type.
Silicious
Tuffs on Chiese Frontiers: Farther north on the Chinese frontier of Burma near
the Lagwi Pass, there occurred at about the same time explosive volcanic
eruptions which deposited dark grey, fine-grained, siliceous tuffs, composed of
highly angular quartz fragments surrounded by finer material containing much
chlorite.
Volcanic
Tuffs and Agglomerates of Mergui Archipelago: The southernmost record of
igneous activity along this line is to be observed on some of the islands of
the Mergui Archipelago. This area, not unlike the Archipelago, appears to have
been another gigantic centre of igneous activity in Burma, where records of all
kinds of igneous manifestation-plutonic, hypabyssal and volcanic-are preserved
in many out of a total of about 900 islands. The Mergui Archipelago owes its
origin to earth movements, aided very substantially by igneous activity. A M
Heron believes that the foci of eruption, which have distributed their debris
widely through the rocks of the Mergui Series lie in the Maingay and
Elphinstone Islands. Hence, he and Sethu Rama Rao consider these rocks to be of
the same age as the Mergui Series.
Acid and
Basic Intrusions in the Mergui Series: The Mergui Series was similarly invaded
by acid and basic intrusions, which Sethu Rama Rao explored near Talobusa in
the Mergui district. According to him, the junction is obscure, and it is
difficult to state whether it is conformable or not. The acid bands are
composed of felsites rich in quartz with a little biotite, while the basic
rocks are rich in hornblende, pyrite, and biotite with very little quartz. The
rocks are undulating and gently folded, and the axes of the folds correspond
with the strike of the sedimentary rocks of the district, i.e. 20° W of N to
20° E, of S.
It is
difficult to fix the age of these intrusives. The original minerals have been
much altered by metamorphism, and they have been subjected to severe crustal
movements, including the folding of the Merguis. Their general appearance
resembles that of gneiss. They were either intruded into the Merguis before the
latter were folded or are older than the Merguis.
Igneous
Quiescence.
When igneous
activity around Bawdwin, etc., became extinct, a remarkable period of
quiescence followed. This interval of rest lasted throughout the later part of
the Palaeozoic and Mesozoic eras of the earth’s history. Though the geological
record of the northern Shan States for this period is fairly complete, no trace
of igneous activity has been discovered during these times. Burma proper was,
of course, under the sea during these ages, and no landmass here existed.
During this vest interval of many changes in the distribution of land and sea,
the evolution of plant and animal life took place. However, it appears that the
earth’s movements during this period were not severe enough to open fissures
for igneous energy to manifest itself from beneath. It is believed that
considerable earth movements took place from time to time during the
accumulation of the stratified deposits. Still, they were probably mainly
tangential, resulting in the folding and squeezing together of the strata. It
was not until near the close of the Mesozoic that great dislocations, which
were more likely to accompany manifestations of igneous activity, began to
develop.
Late
Cretaceous-Early Eocene Movements.
As mentioned
above, it was not until late Cretaceous or early Eocene times that the
subterranean fires once more awoke to vigorous action. It was at this time that
the Arakan Yoma, with its continuations to the north and south, were upheaved
from the bottom of the sea in the form of a long, narrow island.
At about the
same time, the eastern hill ranges of Burma, including the Shan Plateau and
Tenasserim Yoma, were finally lifted into a landmass that has persisted ever
since. It was about this period that Gondwanaland finally broke up, and the
Deccan trap lavas welled out through fissures in Western India. In brief, this
must have been a very important orogenic period when vast changes in land and
sea also occurred in other parts of the world.
Serpentine
Belt of Burma.
As an
accompaniment of the late Cretaceous -early Eocene movements, intrusions of
peridotites, picrites, gabbros and diorites took place in the Archipelagoes of
the Andamans and Nicobars, the Henzada, Bassein, Prome, Thayetmyo and Minbu
districts of Burma. Similar intrusions occur farther north in the Manipur and
Naga Hills, in the Jade Mines area, and in the Hukawng Valley. Volcanic
Eruptions in Early Eocene:
Volcanic
eruptions in the form of explosive activity accompanied or immediately followed
the peridotitic intrusions in most of the areas enumerated above.
Emplacement
of Peridotitic Intrusions: When the intrusions of peridotitic magma took place,
movements both epeirogenic and orogenic were occurring on a tremendous scale
all over the then Southern Continent of Gondwanaland. The rocks in which these
intrusions occur are very tightly compressed, overfolded, and faulted and have
broken-backed anticlinals. These intrusions have taken place either along or
near fault lines, marking weak interformational planes or along tension cracks
formed in the same series. The underground extensions of these intrusions
appear to be like batholiths.
Jade Mines
Area: In the Jade Mines area, the outcrop of peridotites and serpentines forms
a well-marked plateau with an average elevation of 2,500 feet above sea level.
It is, therefore, probable that the magma erupted initially through a fissure.
In places, however, the serpentine hills have a conical appearance; in other
cases, they form well-defined ranges.
In the Lawa
tract of the Kamaing subdivision, Myitkyina district, the writer traced a
narrow band of serpentine, which in places is highly brecciated, for several
miles along the junction of the Crystalline Schists and Older Tertiaries. It
marked almost the position of the fault along which the big earthquake of 28
January 1931 and the numerous aftershocks took place. This was confirmed by the
reversal of the direction of the earth’s sound on either side of the fault.
In passing,
a brief mention of the intrusions of jadeite masses into the serpentines of the
Jade Mines area may be made. The writer has found several outcrops of jadeite,
which all follow linear directions. Both Noetling and Bleeck supposed that they
occurred in the form of dykes. But a section exposed in the Kadondwin mine
revealed that the jadeite intrusion into the serpentine may have taken place in
the form of a sill.
Hukawng
Valley: The serpentine deposits of the Hukawng Valley are similar in their mode
of occurrence, being in reality only a northern continuation of those of the
Jade Mines area referred to above.
References:
Chhibber, H L, 1934: The Geology of Burma, Macmillan, and Co, Limited, St
Martin’s Street, London
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