Analogous Evolution of the Burma Orogen and the Scottish Caledonides

Analogous Evolution of the Burma Orogen and the Scottish Caledonides w^^'x^IJiE^w, 1 Department of Geology and Mineralogy, Parks Road, Oxford OX1 3PR, England W. o. MCKbKKOW ABSTRACT sediment and dacite in the Kalaw syncline, the modern orogen to provide a plate tec- and metamorphic rocks in the west. Granite tonic model for the older one. It should be Thick turbidite sequences derived from of Late Cretaceous and early Tertiary age stressed that, whereas the geology of Scot- the Bengal Delta are accumulating on the intrudes the rocks of the Kalaw syncline land is known in considerable detail (Craig, floor of the Bay of Bengal. Eastward sub- and adjacent areas of the Eastern High- 1965), the geology of many areas of Burma duction has resulted in successive under- lands. The Shan Boundary Fault sepa- still remains to be investigated and the thrusting with tectonic emplacement of the rates the metamorphic rocks in the western generalizations contained in this paper are turbidites above the subduction zone to Eastern Highlands from the Eastern Trough seldom based on more than preliminary re- form the Indo-Burman Ranges of western of the Central Lowlands. West of this fault, connaissance surveys. Burma. The ranges are separated from the the Central Lowlands contain up to 17 km The maximum width of the Burma Eastern Highlands of Burma by the Central of Tertiary marine and fluviatile sediment. orogen from the west of the Indo-Burman Lowlands. The Eastern and Western Troughs of the Ranges to the eastern limit of the late In Scotland, the Upper Ordovician and lowlands are separated by the predomi- Mesozoic granitic rocks in the Eastern Silurian turbidite sequences of the Southern nantly andesitic Burman volcanic arc. The Highlands is 350 km. In Scotland, the dis- Uplands are comparable to those of the sedimentary deposits of the Western tance from the south margin of the South- Indo-Burman Ranges; the Midland Valley Trough are thrust westward over the eastern Uplands to the northern limit of the and the Grampian Highlands, respectively, ern side of the Indo-Burman Ranges, which Paleozoic batholiths is about 300 km. Thus, are similar to the Central Lowlands and the consist largely of turbidite sequences of although Burma is larger than Scotland, the Eastern Highlands. Late Cretaceous and early Tertiary age. orogenic belts in the two regions have a The development of the Midland Valley In northwest Scotland (Fig. 2) the area similar width. and the Southern Uplands can be explained west of the Moine Thrust consists of a by analogy with the Burma orogen. Both Cambrian-Ordovician carbonate succes- REGIONAL SETTINGS orogens show a similar history of sedimen- sion lying unconformably on Precambrian tation, igneous activity, and deformation rocks. The Scottish Highlands east and Burma over a period of 100 m.y., but in Scotland, southeast of the Moine Thrust contain Pre- the equivalent events took place about 400 cambrian (Moine) schist that is thrust over The Indo-Burman Ranges, comprising m.y. earlier than in Burma. There is no the Cambrian-Ordovician carbonate rocks. the Arakan Yoma, Chin, and Naga Hills, known equivalent in Burma of the Early Thè Moine Schists extend southeast of the pass northeastward into a belt of Ordovician Grampian orogeny in Scotland. transcurrent Great Glen Fault to the Gram- northwest-trending structures linking them Our comparison suggests that minerali- pian Highlands, where it is overlain by a with the Himalayan Arc (Fig. 3). South- zation similar to that in the Burman vol- series of upper Precambrian to Lower Or- ward the ranges pass into the Andaman and canic arc should be present in andesite of dovician metasedimentary rocks (the Dal- Nicobar Islands and the Mentawai Islands the Sidlaw anticline. Key words: structural radian). In the Scottish Highlands, the southwest of Sumatra. West of the Arakan geology, paleogeography, orogeny, Scot- Moine and Dalradian rocks are intruded by Yoma, turbidites of the Bengal Delta fan land, Burma. batholiths with a wide range of Paleozoic extend for more than 3,000 km southward ages. The Grampian Highlands are sepa- across the floor of the Indian Ocean (Cur- INTRODUCTION rated from the Midland Valley by the High- ray and Moore, 1971). Farther north, the land Boundary Fault across most of Scot- Shillong Plateau and Mikir Hills consist of The three major structural and mor- land. The Midland Valley contains up to 9 Precambrian metamorphic rocks forming phological belts of Burma are closely com- km of Devonian (Old Red Sandstone) part of the Foreland Spur, considered to be parable with three similar belts in Scotland. fluviatile sedimentary rocks with abundant an extension of the Indian Shield. The The Eastern Highlands, the Central Low- andesitic lava flows; more than 2 km of basement rocks of the spur are overlain by lands, and the Indo-Burman Ranges of lava flows are exposed in the Sidlaw anti- Cenozoic platform sediment that is over- Burma correspond, respectively, with the cline. The Southern Uplands (Fig. 2), south thrust in the east by Tertiary sediment of Grampian Highlands, the Midland Valley, of the Southern Upland Fault, show thick the Naga Hills (Evans, 1964). and the Southern Uplands of Scotland; the sequences of Upper Ordovician and The Central Lowlands (Fig. 1) are di- Shan Boundary Fault occupies a position Silurian turbidites resting conformably on vided into the Eastern and Western analogous to that of the Highland Bound- graptolitic shale, chert, and spilite. Troughs by the Burman volcanic arc, which ary Fault. The aims of this paper are to compare the contains predominantly andesitic rocks In Burma (Fig. 1), the Eastern Highlands modern Burma orogen with the Scottish with local basalt and rhyolite, mostly of contain three successions: the Shan Plateau Paleozoic orogen and to interpret the evolu- Miocene to Quaternary age (Chhibber, carbonate rocks in the east, siliceous clastic tion of the two orogens (Tables 1, 2), using 1934a). Southward the volcanic arc con- Geological Society of America Bulletin, v. 86, p. 305-315, 8 figs., March 1975, Doc. no. 50305. 305 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/305/3433448/i0016-7606-86-3-305.pdf by guest on 01 October 2021 306 MITCHELL AND McKERROW tinues through the active volcanoes of Bar- ren and Narcondam Islands (Fig. 3) in the Andaman Sea into the Sunda Mountain Arc of Sumatra. The Western Trough includes at its southern end the Irrawaddy Delta, which is advancing rapidly into the Anda- man Sea. The floor of the Andaman Sea consists of a complex of rift valleys and seamounts in the west and of the Malay Shelf, continental slope, and marine terrace in the east (Rodolfo, 1969). The Eastern Highlands extend southward into Tenasserim (Fig. 1) and eastward into Thailand. The eastern margin of the Burma orogenic belt is poorly defined, with granitic rocks extending as much as 50 km beyond the eastern limit of the metamorphic rocks. Tectonic and magmatic evidence in Burma requires plate convergence to have taken place during Tertiary time, with westward movement of Burma relative to the Indian plate. Earthquakes of inter- mediate depth beneath the Central Low- lands indicate late Cenozoic eastward subduction, but the scarcity of shallow earthquakes shows that this has recently slowed down or stopped (Molnar and others, 1973). The present northward movement of the Indian plate relative to the China plate (Fitch, 1972, Figs. 2, 4) suggests strike-slip displacement. Scotland The Caledonides include areas of Scot- land and Ireland that underwent metamorphism, deformation, and igneous activity during the Ordovician Grampian orogeny and again in the Silurian to Devo- nian Caledonian orogeny. There is now strong evidence from paleomagnetism, faunal provinces, and ophiolite, graptolitic shale, and turbidite distributions that Scot- land and England were separated by an ocean in early Paleozoic time (Dewey, 1969; McKerrow and Ziegler, 1972a, 1972b). This ocean has been called the Proto-Atlantic, but Iapetus is a better name (Harland and Gayer, 1972). The Precam- brian areas of northwest Scotland were part of the Canadian Shield. In the extreme northwest ofScotland, Cambrian and Lower Ordovician carbonate rocks can be com- pared in detail with similar sequences in western Newfoundland and Greenland (Swett and Smit, 1972). Southeast Ireland, Wales, England, and Norway were linked with the Baltic Shield to the east of the Iapetus Ocean. During much of early Paleozoic time, the margins of both shields adjacent to the Iapetus Ocean lay above active subduction zones. In Scotland, evidence of Benioff Burman Volcanic Metamorphic rocks zones dipping northwestward from the Arc Iapetus Ocean is provided by Ordovician, Granite Ultrabasic rock Silurian, and Devonian calc-alkaline rocks -Ssi or ophiolite (Dewey, 1971). Continental collision, clos- ing the Iapetus Ocean, occurred between Figure 1. Major structural units of Burma (including data from Evans, 1964). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/305/3433448/i0016-7606-86-3-305.pdf by guest on 01 October 2021 ANALOGOUS EVOLUTION OF THE BURMA OROGEN AND THE SCOTTISH CALEDONIDES 307 Norway and Greenland before the close of the Silurian Period (McKerrow and Ziegler, 1972a). There is some evidence that the time of collision was progressively later to- ward the southwest, and there is good evidence that it did not occur until Middle De- vonian time in eastern Canada (McKerrow and Ziegler, 1972a). EASTERN HIGHLANDS OF BURMA AND SCOTTISH HIGHLANDS Both regions contain high-grade metamorphic rocks. In Burma these form a narrow belt, adjacent to the Central Low- lands (Fig. 1). The eastern margin of this metamorphic belt is faulted, locally along a high-angle reverse fault, against the upper Mesozoic sedimentary deposits of the Kalaw syncline and locally against the Shan Plateau carbonate succession (Searle and Haq, 1964; Bateson and others, 1972).

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