Arakan Yoma Research Articles

Geology and tectonics of Arakan Yoma - a reappraisal

The tectono-stratigraphic evolution of the Arakan Yoma Orogen wi th the Naga and th e Chin Hill s, positi oned between th e Sunda arc and th e Himalayas, is vital in th e framework of convergence of the Indi an plate with Asia. The Orogen is flanked to th e west by the Assam-Burma Cretaceous-Tertiary turbidite-molas e basin and furth er west by the Meghalaya (S hillong) Plateau; to th e east by the Mesozoic-Cenozo ic Cenu·at Burman molasse basin and th e Shan-Tenasserim block east of th e Shan Boundary Fault. From the above tectono-stratigraphic sett in g it is surmised that the present ire of aga Hills - Ar­ akan Yoma- Chin Hills has been marked by a meridjonally trending spit of l<md projecting south ward fro m th e Angara land during Palaeozoic time forming Archipelagos of i land made of basement rocks, th e western s lope of whi ch fe ll away into th e Assam-Bunna trough having oceanic floor in front of th e subductin g Indian Plate. On its ea tern side, ed iments were deposited over a submerged peneplained(?) Pre-Cambrian metamorphic ba ement volcan ic line representing supra Benioff sei mk zone volcanic front. Continuous diastrophism, along a N-S arc uate axis, of the basinal sedim ents on either s ide of the Arakan Yoma and grad ual upheaval of th e same s in ce Eocene indicate th at tJ1i s belt has been under constant and uneven E- W compressive stress du e to eastward subducti on of th e In dia Plate. The peak of deformation and upheaval had been at the close of Oli gocene shapin g th e mega fab ri c of the Arakan Yoma te ctogene. The Bouguer gravity anomaly contou rs run parallel to the axis of the sed imentary basin and th e fold belts. Max im um negative value contours running adjacent to the flysch trough and th e ophiolite belt, 0-anomally contour (po. itive by contrast) along the vo lcani c lin e and the eastward dip of th e seismic zone also uggest th at th e lndian plate has been subductin g eastward be low th e Shan­ Tenasserim bl ock since Eocene.

South Asian Regionalism: Problems and Prospects

The Setting The seven states in South Asia-Pakistan, India, Nepal, Bhutan, Bangladesh, Sri Lanka, and the Maldives-comprise a region inhabited by almost a billion people. The base of mountain wall that extends north from the Kirthar Range of Baluchistan to the Khyber Pass and eastward along the foothills of the Himalayas until it turns sharply south at the Arakan Yoma, defines the northwestern, northern, and eastern boundaries of South Asia. The Bay of Bengal in the east, the main body of the Indian Ocean in the south, and the Arabian Sea in the west complete the demarcation of the region known as South Asia. Afghanistan, though not a formal member of South Asia,' occupies a significant place in any analysis of South Asian regionalism, especially after the Soviet invasion in December 1979.

Earthquake mechanisms and tectonic features of Northern Burma

A seismicity map of northeast India and Burma for the period 1900–1970 is given. A zone of intermediate-depth foci dipping to the east of Arakan Yoma is found to be present underneath the Burmese plains. The seismic zone appears to be of V shape. Six new focal-mechanism solutions of earthquakes in Burma have been determined. These mechanisms, in conjunction with earlier studies of Fitch (1970), Rastogi et al. (1973) and Tandon and Srivastava (1975), indicate that normal as well as thrust faulting takes place in Burma. The compressive stress axis is found to be more nearly vertical than horizontal. The results are discussed in the light of plate-tectonic theory as applied to the Indo-Burma regions.

Seismicity, Earthquake Mechanisms and Tectonics of Burma, 20 N-28 N

Summary Hypocentre data file compiled by the Environmental Research Laboratories (ERL) of the US Geological Survey was used to prepare seismicity maps of the region for the periods, January 1900 to June 1973, and January 1963 to June 1973. Most of the earthquakes occur in the Arakan Yoma range and the Burmese central belt. In the northern part of the region, north of 24N, the foci tend to occur in two zones converging beneath the Chindwin river. One zone dips in the direction S 44 E at an angle of about 29 and the other zone dips in the direction N 50 W at an angle of about 51. South of 24 N, earthquakes occur in a slab dipping due east at an angle of about 47. Focal mechanism solutions for eight earthquakes occurring between January 1964 to December 1971 have been determined by using P-wave polarity and S-wave polarization data. South of 24 N, one normal and two thrust fault plane solutions indicate underthrusting of the Indian plate towards east along the Burmese arc. An exception is the earthquake of 1969 October 17. It has a well-determined thrust fault plane solution and is caused by north-south compression. Further north, due to the complexity of tectonics caused by the syntaxial bend of the Himalayan and Burmese mountain ranges, the axes of maximum compression for two events are parallel rather than perpendicular to the local structural trend. Two shallow focus events occurring near the south-eastern flank of the syntaxis have strike slip mechanism solution.

Some palaeontological data on the Triassic and Cretaceous of the western part of Burma (Arakan Islands, Arakan Yoma, western outcrops of Central Basin)

Some palaeontological data on the Triassic and Cretaceous of the western part of Burma (Arakan Islands, Arakan Yoma, western outcrops of Central Basin)

Quantitative seismicity maps of India

AbstractUsing the Kaila and Narain (1971) method, three quantitative seismicity maps have been prepared for the Indian subcontinent which are compared with regional tectonics. These are the A-value map, the b-value map and the return-period map for earthquakes with magnitude 6 and above where A and b are the constants in the cumulative regression curve represented by log N = A - bM. The A-value seismicity map shows that India can be divided into two broad seismic zones, the northern seismically highly active zone and the southern moderately active zone. In the northern active zone, a number of seismic highs have been delineated such as the Pamir high, the northwest-southeast trending Srinagar-Almora high, the Shillong massif high, the Arakan Yoma high and the West Pakistan highs. These seismic highs are consistent with the Himalayan tectonic trends. Contrary to this, two seismic highs fall in the Tibet plateau region which align transversely to the main Himalayan trend. In the southern moderately active zone, two seismic highs are clearly discernible, the east and the west coast high, the latter being seismically more active than the former. The least active zone encompasses the Vindhyan syncline and the areas of Delhi and Aravalli folding. Between this zone and the east coast high lies another moderately active zone which encloses the Godavari graben, western part of the Mahanadi graben and the Chattisgarh depression. The b-value seismicity map also demarcates the same active zones as are brought out on the A-value map. The return-period map of India for earthquakes with magnitude 6 and above shows a minimum return period of 100 years in the Pamirs, about 130 years in the various seismic highs in the northern active zone, 180 years on the west coast high, 200 years on the east coast high and about 230 years in the least active Vindhyan-Aravalli zone and the Hyderabad-Kurnool area. These quantitative seismicity maps are also compared with the seismic zoning map of Indian Standards Institution and seismicity maps of India prepared by other workers.

Turbidities in the Upper Bhuban Member of the Sitakund anticline, Chittagong, Southestern Bangladesh

The Upper Bhuban Member in the Sitakund anticline, Chittagong, Southeastern Bangladesh is constituted by the light gray, light yellow and greenish gray sandstone and siltstone with dark gray, bluish black and black finely laminated silty shale to shale. The lithofacies of massive sandstone(Sm), turbidite sandstone (ST), flat bedded sandstone-siltstone (Sh), ripple cross laminated sandstone-siltstone (Sr), laminated sandstone-siltstone (Sl), lenticular laminated sandstone-siltstone-shale (Sll), wavy laminated silty shale to shale (Fw), laminated shale (Fl) with sub-facies black shale (Flbk) and blue shale (Flb) and mudstone (Fm) have been delineated in this member. Based on genetic aspects the facies are grouped into (1) turbidity generated - and (2) deep marine basin plain facies association. The medium to fine grained facies Sm and ST with or without Sh, Sr, Sl Sll Fw and Fm facies constitutes the turbidity generated facie association. The deep marine basin plain facies association is characterized by the monotonous hemi-pelagic bluish black shale (subfacies Flb) and pelagic black shale (subfacies Flbk) with rare to scare silty stringers of facies Sh, Sr, Sl, Sll and Fw. The facies Sm is turbidity channel in the inner part of medial fan and the complete ST sequence indicates channelized forms, while incomplete ones have been identified as channel or interchannel deposits in medial to distal fan. The facies Flb and Flbk denote deep marine environments above and below the carbonate compensation depth, where the lithofacies facies Sh, Sr, Sl, Sll and Fw were deposited as distal turbidites. The facies sequence of medial to distal fan and deep marine basin plain were repeated and randomly juxtaposed one upon other due to instability of basin probably by faulting along Dauki and adjoining area accompanied by sea level change. The paleoflow pattern indicates that the dominant source of detrius was the Shillong plateau along with minor contribution from the Himalayas and Arakan Yoma Folded Belt. Keywords: Turbidites; Upper Bhuban Member; Sitakund anticline; SE Bangladesh. DOI: 10.3329/jles.v2i2.7491 J. Life Earth Sci., Vol. 2(2) 1-16, 2007

The Geology of British Oilfields 3. The Oilfields of Burma

The oilfields of Burma lie almost entirely within the elongated Tertiary basin which runs approximately north-south and is bordered on the west by the Arakan Yoma and on the east by the ancient plateau of the Shan Hills. Most of the oil has been obtained from fields lying on a single line of folding and in a belt only 50 miles long between latitudes 20° 20′ and 21° 10′, the remainder being found in small fields situated at various places between latitudes 19° 25′ and 23° 45′.

Notes on the Geology of Burma.

I. Historical . It is the object of this paper to present the results of geological expeditions in the Irawadi valley carried out by my uncle Mr. W. H. Dalton, F.G.S., and myself in the season of 1904–1905, and by myself alone in 1905–1906, and to correlate these observations with those made by previous writers, thus summarizing our present knowledge of the geology of Burma generally and of the Tertiary System of that country in particular. Dr. Fritz Nœtling in 1895 reviewed the work done on the geology of Burma before that date, pointing out the remarkably-accurate hypothetical arrangement of the rocks of Burma made by Buckland in 1828, and the absence of subdivision of the Tertiary System previous to W. Theobald's paper on Lower Burma in 1873. In this summary the ‘Axial Group’ of Theobald is referred to as certainly not of Triassic age—a statement controverted by later examination of the fossils of the Arakan Yoma and other parts of Lower Burma—although found true in the sense that the group is a complex one (as we now know), and neither Triassic nor Cretaceous alone, but assignable to both of these. While Dr. Nœtling's paper thus reviews the greater number of the works published on the geology of Burma, and in particular those relating to the Tertiary of the better-known regions of the Irawadi basin, there are certain other papers to which it is necessary to refer here, as they deal with the outlying districts. Of these the earliest is that

Art. XVI.—The Arakanese Dialect of the Burman Language

It is well known that the people of Arakan are an offshoot of the Burman race, the accepted account being that they first crossed the range of mountains called the Arakan Yoma about b.c. 825 under a Prince Kanruga-gyi. It seems probable that the small portion of the country then inhabited was settled by a few of the advance-guard of the Chin-Lushai or IsTaga tribesmen, with perhaps some colonies of Indians on the sea-coast. These were expelled or absorbed; and the Arakanese kingdom, having its centre in the flat open plains of the Akyab district, gradually extended south as far as the Mawyon-gyaw Hills, in the Sandoway district, and north to Chittagong (a.d. 1450). It was finally crushed by an invasion of Burmans from the east of the Yoma in 1784. The people of Arakan have, however, preserved their peculiar dialect, and in certain customs they differ slightly from the Burmese, against whom in some of the purely Arakanese parts is still cherished a deep hatred, born from the cruel manner in which they were handled at the Burmese conquest. It must be admitted, indeed, that in their intertribal wars the peoples of the Tibeto-Burman race have endeavoured to enforce in the strictest possible manner the modern doctrine of the “survival of the fittest.” Owing to the steady immigration into Arakan of natives of India, principally from the Province of Bengal, which has been going on for centuries, the physical type of the people has been sensibly altered from the pure Mongoloid cast of their first progenitors.

Art. XXI.—Southern Chin Vocabulary (Minbu District)

The accompanying words and phrases of Southern Chin, as spoken at the foot of the Arakan Yoma Mountains in the Minbu district, were taken down a few years since by Major B. A. N. Parrott, I.S.C., who later on presented to me the book in which they were, written, along with others on Oriental subjects. They are interesting as representing the most Northern dialect of this language, which reaches its most Southern point in the Sandoway district. (I pass by the dialect spoken in Bassein and the South of Henzada as being much corrupted by the extended intercourse which has there taken place between the Chins and the Burmans.) In publishing now this Vocabulary of Minbu Chin it has seemed advisable to compare it word by word with that of the Sandoway district, not merely in order to show more clearly the dialectic variations which exist, but also as an assistance towards the elucidation of the still obscure philological laws which obtain in this family of languages. In a speech which has suffered so extraordinarily from phonetic corruption and decay as this latter, all evidence which will in any way tend to join together the scattered links of verbal stems and endings must be considered as scientifically useful.