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.

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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 compared 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, closing the Iapetus Ocean, occurred between Figure 1. Major structural units of Burma (including data from Evans, 1964).

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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 toward 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). In northwest Scotland, the Cambrian and Lower Ordovician carbonate strata (the Durness Limestone) on the autochthon west of the Moine Thrust (Fig. 2) appear to be largely unaffected by the Grampian orogeny. They have a similar structural position, away from the contemporary continental margin, to the carbonates of the Shan Plateau (Tables 1, 2). The Jurassic and Lower Cretaceous succession of the Kalaw syncline consists of turbidites and shallow-marine or deltaic deposits overlain locally by dacite (Garson and others, 1975); the turbidites show evidence of eastward thrusting onto the Mid- dle Jurassic continental red beds of the Figure 2. Major structural units of Scotland. Shan Plateau (Garson and others, 1975). In Scotland, Dalradian sediment makes Dalradian rocks just north of the Highland (Fig. 2) and other northeast-trending trans- up the main mass of the Grampian High- Boundary Fault (Shackleton, 1958; John- current faults of Scotland. lands (Fig. 2). The Lower Dalradian is an stone, 1966). No folds closing toward the The folded rocks in both orogens are upper Precambrian succession of domin- west have yet been recorded to the east of overlain by local nonmarine sedimentary antly shallow-water marine sediments, the Shan Boundary Fault. Much of the deposits (Tables 1, 2). while the Upper Dalradian, which is in part thrusting in the Scottish Highlands is prob- Cambrian and Early Ordovician in age ably related to the Grampian orogeny CENTRAL LOWLANDS OF (Downie and others, 1971), consists mainly (about 490 m.y.), when the Dalradian rocks BURMA AND MIDLAND of continental margin turbidites, with some were folded and metamorphosed (Dewey VALLEY OF SCOTLAND local developments of slate, basic volcanic and Pankhurst, 1970). rocks, and chert. The Dalradian rocks can The Eastern Highlands of Burma and the The Ordovician to Devonian succession be compared in their general setting with Scottish Highlands both contain many in the Midland Valley shows broad the Mesozoic marine beds of the Kalaw batholiths (Tables 1, 2). Within the Kalaw similarities to the Tertiary and Quaternary syncline of eastern Burma (Table 2). syncline and Shan Plateau successions, im- sequence in the Central Lowlands of There are also similar structures in the portant deposits of tin and tungsten are as- Burma. two areas. In the northern parts of the sociated with Upper Cretaceous and lower Rocks comparable to the serpentinite, Grampian Higlands, the Dalradian rocks Tertiary granitic rocks in a belt as much as eclogite, and glaucophane schist (Tables 1, are thrust toward the northwest, and on the 50 km wide; they are also present in the 2) near Girvan (Bloxam and Allen, 1960) in Moine Thrust (Fig. 2) the Moine Schists metamorphic belt along the Shan Boundary the southwestern Midland Valley (Fig. 2) (which underlie the Dalradian) are thrust Fault (Chhibber, 1934b). In the Scottish are represented in the northern Central northwestward over the autochthon of Highlands, granodiorite, tonalite, and gab- Lowlands (Fig. 1) by glaucophane schist Cambrian-Ordovician carbonate rocks bro, ranging in age from Cambrian to Early and albite-jadeite dikes of uncertain age lying unconformably on older Precambrian Devonian, are distributed widely, but there within serpentinite (Soe Win, 1968). rocks. This can be compared with the is no known belt of tin-bearing granitic Thick turbidite and shelf successions of high-angle eastward thrusts east of the rocks. Late Ordovician age near Girvan are suc- Kalaw syncline (Tables 1, 2). One apparent Mylonite zones cutting migmatite in the ceeded by Lower Silurian turbidite and difference between the Dalradian structures metamorphic belt (Bateson and others, shelf deposits exposed in inliers along the and those of eastern Burma is the major 1972) are perhaps comparable to crush southern part of the Midland Valley be- overfolding toward the south seen in the zones associated with the Great Glen Fault tween Girvan and the Pentland Hills (Fig.

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2). These marine beds are followed by cal movements predominated, downthrow- in the Southern Uplands of Scotland, major nonmarine sediment of Late Silurian age in ing away from the highlands (Dey, 1968; faults inclined northward toward the Mid- the southern parts of the Midland Valley Win Swe, 1972; George, 1960). land Valley (Walton, 1965, p. 221) separate and then by widespread fluviatile sedimen- The distribution of major structural units turbidite sequences, some of which have tary rocks (Lower Old Red Sandstone) of suggests that the Burma and Caledonian spilite, chert, and graptolitic shale at their Early Devonian age (Table 1). In Burma, orogens are mirror images; if this is so, the base (Peach and Home, 1899). In the thick Upper Cretaceous to Miocene marine large east-trending faults in the Midland southern parts of the Indo-Burman Ranges, (Table 2) and continental sedimentary Valley have a similar trend to the major the ultrabasic rocks and associated deposits are present in the Western Trough, south-southeast-trending faults in the Cen- radiolarian chert (Ngaw Cin Pau, 1973, and Pliocene-Quaternary fluviatile deposits tral Lowlands (Tables 1, 2). personal commun.) occur mostly on the (Table 2) occur throughout the whole of the east side of the ranges toward the Central Central Lowlands (Aung Khin and Kyaw INDO-BURMAN RANGES Lowlands (Chhibber, 1934a), and in the Win, 1969; Maung Thein, 1973). AND THE SOUTHERN Southern Uplands, the spilite and chert In the Midland Valley, there is some evi- UPLANDS OF SCOTLAND occur similarly in the northern sequences dence for a longitudinal advance of non- nearest the Midland Valley. The detailed marine conditions comparable to the Both belts include thick successions of structures are also similar; the beds in each southward migration of the Irrawaddy deformed turbidites associated with some major tectonic unit are progressively Delta during late Tertiary time (Stamp, chert, shale, and spilites (Tables 1, 2), younger away from the Central Lowlands 1922; Rodolfo, 1969). The change from but in the Indo-Burman Ranges, a deltaic or the Midland Valley. The Ordovician marine to nonmarine conditions occurred sandstone unit is present locally in the east. rocks of the Southern Uplands are confined slightly later (in early Wenlock time) in the In the Indo-Burman Ranges, the Upper to the northern areas; the Upper Cretaceous southwest at Girvan (Fig. 2) compared with Cretaceous and Eocene rocks are cut by rocks of the Indo-Burman Ranges occur on a (late Llandovery time) change in envi- thrusts inclined eastward toward the Cen- the eastern portion of the belt. But in both ronment in the east in the Pentland Hills tral Lowlands (Clegg, 1941; U Than Naing, orogens, the successions between each (Cocks and others, 1971). Further to the 1974, personal commun.); within each tec- major fault usually get younger toward the southwest, marine conditions continued tonic unit, local ophiolite or ultrabasic Central Lowlands or the Midland Valley longer in western Ireland; in Galway (Fig. rocks and chert are overlain by turbidite se- (Toghill, 1970), at least as far as the stratig- 2) the transition to nonmarine environ- quences (Brunnschweiler, 1966). Similarly, raphy is known. ments took place in middle Wenlock time

(Laird and McKerrow, 1970), and in the TABLE 1. CALEDONIAN SEDIMENTARY, IGNEOUS, AND TECTONIC EVENTS IN SCOTLAND extreme southwest of Ireland, marine con- Structural Age Event ditions persisted in Dingle until Ludlow Belt

time (Holland, 1969). Early Devonian

Thick sequences of andesite (Tables 1, 2) Pridoll (440-410) E Uplift and erosion. Local fluviatile sediments within the continental Lower Old Red Sand- Ludlow 30-0 (Lower Old Red Sandstone). Silurian D Rise ot Cockburnland. Deformation of turbidites «enlock and ophiolites in north. stone in the Sidlaw anticline (Armstrong SOUTHERN (450-440) 40-30 D Continued deposition of turbidites in south. and Paterson, 1970) are equivalent to the UPLANDS predominantly andesitic rocks, mostly of Ashgi'll C Deposition of turbidites above graptolitic shale, Caradoc (530-450) Ordovician Llandeilo chert and pillow lava. Pliocene-Quaternary age and interbedded 120-40 Llanvirn - Local andesites in Caradoc below Wrae Limestone. with fluviatile sediments, of the Burman Areni g volcanic arc (Chhibber, 1934a). The thick SOUTH OF SIDLAW SIDLAW NORTH OF SIDLAW fluviatile succession in the Strathmore syn- ANTICLINE ANTICLINE ANTICLINE Post- Early Devonian E East-trending faults. cline north of the Sidlaw anticline (Fig. 2) Early Devon îan E Fluviatile sediments and Andesites Fluviatile (435-410) andesitic volcanics (Lower sediments and occupies a tectonic position comparable Pridoll 25-0 Old Red Sandstone). some andesites. with the thick Miocene-Pliocene succession Ê Fluviatile and other non- Silurian Ludlow (445-435) marine sediments, above local ? andesites. in the Eastern Trough of Burma (Fig. 1). Wenlock 35-25 mariM sediments. MIDLAND (460-445) D Turbidites and shelf L'andovery ? andesites. The Upper Devonian and Carboniferous 50-35 sediments. VALLEY D Turbidites and shelf sediments. ? andesites (inferred (490-460) continental and deltaic successions in Scot- Ashgill Faults down-throwing to from detritus in 80-50 Caradoc S.E. at Girvan. Southern Uplands). land are postorogenic deposits, and hence Ordovician L'andeilo Spilites and deep water (520-490) Spilites and cherts near they have no precise equivalents in the -. anvirn - sediments of Highland 170-80 Girvan. still-active Burma orogen. Teschenite and Areni g Border above Dalradian. (520 +) Pre-Middle Arenig B Glaucophane schists. ? Dalradian turbidites. dolerite of late Carboniferous age in the 110 + Midland Valley also postdated the Caledo- Early Devonian (430-410) E Local non-marine sediments (Basement Group of O.R.S.) nian orogeny, but in Burma comparable 20-0 - Granite emplacement. Lome andesites. Silurian C Granite emplacement. Regional uplift. late Cenozoic magmatism in the Shan scarp Ashgi11 (490-430) Caradoc 80-20 east of Shwebo (Chhibber, 1934a) rook c Granite emplacement. Ihrusting to north-west. Crdovician Llandeilo (510-490) Deformation, metamorphism (Grampian orogeny). place at an earlier stage of the orogeny. 100-80 GRAMPIAN Gabbro emplacement There is little evidence for the age of the HIGHLANDS Arenig B Upper Dalradian turbidites, slates, spilites and cherts. Tremadoc Some basic volcanics. earliest movements along either the Shan Late Cambrian (570-510) Granite emplacement on continental side of Boundary Fault or the Highland Boundary 160-100 Dalradian sediments. Middle Cambrian A Carbonate sediments (Durness Limestone) on Fault. Both were probably initiated as continent to north-west of orogen. Early Cambrian (710-570) A Shelf sediments (Lower Dalradian) either strike-slip or transform faults in the Late Precambrian 300-160 early stages of the orogenies. Later in the Note: Letters indicate equivalent events to those in Burma (Table 2). The time scale is given in millions history of both faults (late Tertiary and De- of years (Lambert, 1971; Lambert, 1973, personal commun.} but is still rather uncertain. To assist comparison vonian time, respectively), high-angle verti- with Burma, the estimated ages are shown (in parentheses) above ages with 410 subtracted.

Graywacke detritus in the Midland Val- tinental collision, and because the Southern In Burma, continental collision has oc- ley inliers indicates that the northern parts Uplands have a similar structure, it may be curred in the Naga Hills in the north, but of the Southern Uplands were uplifted in concluded that the deformation occurred, not in the south, and sediment from the Early and Middle Silurian time (Walton, as in southern Burma, above an active sub- Ganges Delta is transported southward. In 1965; Ziegler, 1970). This landmass, duction zone. the Caledonian orogen, continental colli- known as Cockburnland, separated the In the north of the Indo-Burman Ranges, sion occurred earlier (in Norway) to the sedimentary basin of the Midland Valley the deformation is greater. In the eastern northeast, and the supply of Upper Ordovi- from the Iapetus Ocean (Fig. 4); in a similar Naga Hills (Fig. 7), metamorphic rocks cian and Silurian turbidites was from the manner, the Irrawaddy Valley is separated overlain by molasse faciès are thrust northeast. The south end of the Indo- from the Bay of Bengal by the southern northwestward over Upper Cretaceous and Burman Ranges therefore corresponds to parts of the Indo-Burman Ranges, which Eocene turbidites and ophiolites the southwest end of Cockburnland, and were uplifted in Oligocene time. In both (Brunnschweiler, 1966). In the western the decrease in size of the folded turbidite cases uplift appears to have taken place Naga Hills, Tertiary sediment thrust over a belt southward from the Indo-Burman above an active subduction zone on a plate Pliocene shelf succession form a belt of Ranges corresponds to the southwestward margin. schuppen parallel to the Brahmaputra disappearance of the "Southern Uplands" In both orogens, the turbidite successions (Evans, 1964). No such structures are seen in Ireland. Thus the two orogens can only are older than most of the exposed andesitic in the Southern Uplands. be compared directly if one is considered as volcanic rocks occurring in the adjacent a mirror image of the other. structural units of the Central Lowlands RELATIVE ORIENTATION and Midland Valley, respectively, but in OF THE OROGENS PLATE TECTONICS AND THE Scotland many of the turbidites contain an- BURMA OROGEN desitic detritus. Andesite flows are therefore It has been suggested that the Eastern inferred to have been erupted in the Mid- Highlands of Burma correspond to the Permian to Middle Jurassic carbonate, land Valley (or to the northeast of it) while Scottish Highlands, which lie to the north- shale, and minor evaporitic rocks of the Cockburnland (Fig. 4) was being developed west of the Midland Valley; thus east in Shan Plateau were associated with block during Silurian time (Ziegler and McKer- Burma corresponds to northwest in Scot- faulting near the western continental mar- row, 1975). land. However, the two orogens do not gin of Southeast Asia (Fig. 6A) prior to the The southern part of the Indo-Burman merely show a different orientation: they northward drift of India and closure of the Ranges has not suffered any effects of con- are mirror images of each other (Fig. 5). eastern Tethys Ocean (Mitchell and others, 1975). The emplacement of tightly folded turbi- TABLE 2. SEDIMENTARY, IGNEOUS, AND TECTONIC EVENTS IN BURMA dites above Middle Jurassic red beds in the Structural Age Event Kalaw syncline is tentatively related to the Belt start of subduction beneath the continental ARAKAN AND CHIN HILLS NAGA HILLS margin during Cretaceous time. Resulting E Uplift arid erosion North-west overthrusting Quaternary of flysch over shelf compression and elevation of sedimentary 5-0 sediments, and of metamorphic rocks over turbidites deposits was followed by eastward gravity INDO-BURMAN Pliocene gliding of the turbidite sequences onto the RANGES D Rise of mountain belt Miocene 38-5 Eastward underthrusting of turbidites and ophiolites red beds. Alternatively, the folding and Oligocene D Continued deposition ot turbidites in west thrusting could be related to strike-slip Eocene C Deposition of turbidites above chert and ophiolite movement along the Shan Boundary Fault. Paleocene 95-38 Late Cretaceous Emplacement of the Upper Cretaceous to lower Eocene granite and local dacite of the

WESTERN TROUGH VOLCANIC ARC EASTERN TROUGH Eastern Highlands possibly took place 7-0 above a shallow-dipping Benioff zone be- Quaternary E S.S.E. trending Andesite S.S.E. trending faults porphyry faults neath continental crust (Mitchell, 1973). copper E F1uviatile Pliocene Fluviatile sediments The position of the granitic rocks east of the sediments CENTRAL younger Burman volcanic arc could be ex- Miocene 0 Marine and Marine and fluviatile LOWLANDS 38-7 01igocene fluviatile sediments plained if a lower Eocene shallow-dipping sediments Benioff zone approached the surface Eocene 65-38 D Turbidites (?) slightly to the east of the upper Cenozoic Paleocene subduction zone (Fig. 6C). ? Late Cretaceous 95-65 B Jadeite dikes and glaucophane schist The structure and stratigraphy of the Indo-Burman Ranges resemble broadly Post-Early 50-0 E Local non-marine sediments Eocene those described from the Cenozoic success- Early Eocene C Granite emplacement and dacite volcanism; sion in Barbados, where underthrusting of Paleocene 95-50 tin-tungsten mineralization. Eastward thrusting. Late Cretaceous ocean-floor turbidites from the east has Middle Cretaceous ß Kalaw syncline turbidites and shallow marine sediments been related to subduction along a Benioff EASTERN Early Cretaceous 160-95 Late Jurassic HIGHLANDS zone approaching the surface beneath an Middle Jurassic A Shan Plateau carbonate sediments tarly Jurassic 290-160 isostatic anomaly minimum that coincides 1rlasslc with the elevated belt of turbidites Permian (Westbrook and others, 1973). A similar interpretation is shown for the Cenozoic development of the Indo-Burman Ranges (Fig. Note: Letters indicate equivalent events to those in Scotland (Table 1). 6A, 6B), in which the Upper Cretaceous

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and Eocene turbidite sequences consist of Bengal Delta fan and possibly prefan sedimentary deposits (Curray and Moore, 1971) deposited on oceanic crust to the west. These deposits, together with slices of underlying oceanic crust, were progressively thrust beneath similarly emplaced older rocks above the subduction zone, forming a rising tectonic pile of turbidites with minor ophiolite. The ranges thus resulted largely from underthrusting of sediment and oceanic crust of the subducting plate (Fig. 6C). Vertical movements of crustal blocks to the east of the Indo-Burman Ranges cannot be related directly to subduction processes. Elevation of the Eastern Highlands probably began in Eocene or Late Cretaceous time following the start of subduction and was accompanied by uplift of the metamorphic belt. The Western Trough of the Central Lowlands subsided intermit- tently throughout most of the Tertiary Epoch, but subsidence of the Eastern Trough began only in early Miocene time. This late Cenozoic subsidence could be related to incipient rifting indicated by the alkaline igneous activity east of Shwebo. Analogy between the early Eocene igneous activity east of the Shan Scarp and present-day silicic volcanism and transcurrent faulting in Sumatra suggests that the Shan Boundary Fault was initiated as a dex- tral transcurrent fault in Late Cretaceous or early Tertiary time. Subsequently, vertical displacements with downthrow to the west predominated in middle and late Tertiary time. The volcanoes of the Burman volcanic arc provide evidence of subduction from Miocene to Quaternary time, but only minor volumes of volcanic rocks of late Eocene to Miocene age are known (Chhib- ber, 1934a; Aung Khin and Kyaw Win, Figure 3. Regional setting of Burma (including data from Evans, 1964; Searle and Haq, 1964; 1969). In the Indo-Burman Ranges, both Aung Khin and Kyaw Win, 1969). Solid lines = boundaries of morphological features or faults; early Eocene conglomerate and exotic dashed line = plate boundary; arrows = directions of underthrusting of India plate below China plate ophiolite blocks within the Eocene turbidite (these are mostly oblique to convergence direction of plates). sequences indicate local uplift (Brunnschweiler, 1966). The major rubellite schist (Brunnschweiler, 1966) can Indo-Burman Ranges developed over a emergence at the end of Eocene time in the be interpreted as the overthrust western period of about 40 m.y. No continental col- ranges could have resulted from either tec- margin of the continental China plate. The lision was involved, only subduction of tonic thickening of the turbidite belt during present-day subduction zone, by analogy oceanic material covered by a thick turbi- subduction or isostatic elevation following with that under Barbados, is inferred to lie dite sequence. By contrast, the Naga Hills cessation of subduction. Scarcity of late beneath the isostatic anomaly minima in in the northern part of the Indo-Burman Eocene to early Miocene calc-alkaline rocks Burma described by Evans and Crompton Ranges show the effect of more severe could be explained by nonsubduction, by (1946). thrusting, which may be interpreted as the slow subduction, or perhaps by the tectonic Late Cenozoic eastward-directed thrust- result of collision between the India plate emplacement of ocean-floor rocks in the ing in the Western Trough (Fig. 6C) and and the China plate (Fig. 7). Indo-Burman Ranges, resulting in subduc- Pegu Yomas probably resulted from minor Shelf sediments overlying continental tion of only small volumes of oceanic crust. eastward movement of the ranges toward crust of the Shillong Plateau and Mikir In the southern parts of the Indo-Burman the Central Lowlands during underthrust- Hills (Fig. 1) have underthrust the turbi- Ranges, continental crust of the China plate ing of Bengal Delta fan deposits from the dites of the Naga Hills from the northwest. is probably absent beneath the Arakan west. Similar relations are seen northwest of the Yomas and the Western Trough. In the This interpretation suggests that the Brahmaputra (Fig. 7), where rocks of the eastern Chin Hills (Figs. 1, 6C), an area of structures seen in the southern parts of the Eastern Himalayas are thrust southward

that for the greater part of the time between the Cambrian and Middle Devonian Periods, this area lay above an active subduction zone. South of the Highland Boundary Fault, most of the Midland Valley is covered by Devonian fluviatile deposits. But near Gir- van (Fig. 2), Ordovician shelf and slope sedimentary deposits show rapid lateral changes through a belt of normal faults, downthrowing to the south (Fig. 8A). Con- glomerate and limestone in the north of the Girvan area pass southward into a thick (5 km) sequence of turbidites (Williams, 1962). The clastic rocks were derived partly from the Grampian Highlands and perhaps partly from andesitic volcanoes active in the northern part of the Midland Valley (Kel- ling, 1961, 1962). The Southern Uplands can be interpreted in a similar way to the Indo-Burman Ranges. During Late Ordovician time, turbidites began to accumulate on the ocean floor south of, and possibly within, a trench south of Girvan (Fig. 8A). These turbidites were derived from northeast of the South- ern Uplands area, and by analogy with Burma and the Bengal Delta, we suggest that abyssal fan deposits were derived from an Upper Ordovician and Silurian delta on the edge of the Canadian Shield to the north of Scotland (Fig. 4). Some of the sediment supply to this delta may have been derived from rivers flowing from a mountain range (such as the Himalayas), caused by collision of the Canadian Shield and the Baltic Shield in the far north. Rhyolitic lava flows of early Caradoc age (Eckford and Ritchie, 1931), extending along strike over a 30-km belt in the South- ern Uplands below the Wrae Limestone (Fig. 2), indicate island-arc volcanism on the ocean floor prior to, and perhaps during, the turbidite sedimentation; the rhyo- Figure 4. Paleogeography of Scotland in early Wenlock time. Position of delta is uncertain; it may lite may have acted as an additional source have been fed from north by a large river. for volcanic detritus. This suggests the presence of an underlying north-dipping Benioff over Miocene and Pliocene sedimentary de- those of the Ordovician Grampian orogeny zone active during Late Ordovician time posits. in Scotland. (Fig. 8A). Volcanic rocks have been re- On the autochthon northwest of the ported from only one locality within the PLATE TECTONICS AND THE Moine Thrust, the Cambrian and Lower Eocene flysch in the Indo-Burman Ranges. BRITISH PALEOZOIC OROGENS Ordovician carbonate rocks were deposited Subduction beneath the future Southern on the southeastern margin of the Canadian Uplands resulted in northward movement A plate tectonic interpretation of the Shield. To the southeast of this carbonate of oceanic crust and overlying chert, grap- Burma orogen provides a model for the belt, the thick upper Precambrian and early tolitic shale, and turbidites, together with orogenic events in Scotland from Caradoc Paleozoic (Moine and Dalradian) succes- the local Wrae volcanic rocks. As the to Early Devonian time and gives an indica- sions were laid down in shelf and deep- sedimentary pile approached the subduction of the nature of the continental and water environments near the continental tion zone, successive low-angle north- plate margin in Scotland prior to the margin. These rocks were deformed and dipping thrust planes developed, analogous Caledonian orogeny. In Burma, orogeny metamorphosed after Arenig time (Downie to those exposed in the Indo-Burman did not begin before Late Cretaceous time, and others, 1971), possibly during the Ranges. Continued subduction and under- and although evidence for tectonic events Llandeilo Series (Phillips, 1973). Both be- thrusting created a tectonic pile of prior to the late Eocene Epoch is fragmen- fore and after this Grampian orogeny, these ocean-floor sediment and slices of underly- tary, there is little indication of metamor- Moine and Dalradian rocks were intruded ing oceanic crust scraped from the subduct- phic and tectonic events comparable to by numerous batholiths, which suggests ing plate, with progressively younger turbi-

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England may have been emplaced above this south-dipping Benioff zone. It is most improbable that any of the volcanism in England, Wales, or southeast Ireland was related to the same Benioff zones as those on the other side of the Iapetus Ocean, X iñ which dipped northward beneath Scotland and northwest Ireland. o The Norwegian Caledonides also lay on the opposite side of the Iapetus Ocean from LU m< Scotland, but in contrast to Britain, they X t/1 occupied a position where continental collision produced major deformation. Similar deformation of late Cenozoic age is seen in TURBI DITES TURBIDITES O< the Naga Hills of Burma and northern Assam. In Norway, the major nappes were thrust eastward during Early Devonian time; they are analogous to the northwestward overthrusts in the Naga Hills (Fig. 7), which are related to collision of the Indian and China plates in the Pliocene Epoch. The Norwegian nappes probably resulted from collision after the closure of the Iapetus Figure 5. Schematic paleogeography of (A) Burma in Miocene time, and ¡B) Scotland in Silurian Ocean, when the Baltic Shield was partially time. Orogens are mirror images. subducted under the Greenland part of the dites southward in each tectonic unit. The turbidites metamorphosed at depth or by Canadian Shield (Nicholson, 1971; McKer- Wrae volcanic rocks were emplaced with andesitic and sedimentary rocks of the Lake row and Ziegler, 1972b). the turbidites in one of the oldest of the District island arc that was underthrust be- thrust slices. By early Wenlock time, the neath the Southern Uplands prior to cessa- CONCLUSIONS tectonic pile had risen above sea level, form- tion of subduction (Fig. 8C). ing an additional source of detritus known In the Indo-Burman Ranges, there is no The Caledonian events in Scotland show as Cockburnland (Fig. 8B). late Cenozoic granodiorite analogous to the a close similarity to Cenozoic events in the As Cockburnland was forming during granodiorite of Devonian age in the South- Burma orogen. The two major controls on the Silurian Period, an andesitic volcanic ern Uplands and in County Down (Fig. 2). the evolution of the orogens were (1) active arc may have continued in the northern However, granodiorite in a comparable set- subduction of oceanic crust beneath a con- parts of the Midland Valley. Andesitic de- ting occurs in southwestern Alaska on the tinental margin bounded by a strike-slip tritus forms an important constituent of the Shumagin-Kodiak shelf, where plutons of fault; and (2) deposition of turbidites on the turbidites throughout Llandovery and Wen- early Tertiary age intrude a thick succession oceanic crust of the subducting plate, lock time (Ziegler and McKerrow, 1975). of deformed Cretaceous turbidites locally mostly on the ocean floor, but perhaps ini- On the ocean floor south of the Southern underlain by chert and pillow lava (Stonely, tially in the submarine trench. In Burma, Uplands, turbidite deposition ceased by Late 1967; Moore, 1973). The Alaskan and perhaps also in Scotland, the turbidites Silurian time, but continued subduction is granodiorite plutons are interpreted as were derived from a major delta. indicated by the Lower Devonian calc- magmatic arc rocks emplaced during either The relative ages of comparable events in alkaline andesitic lava flows (Fig. 8C) of the steepening or oceanward migration of the the two orogens (Tables 1, 2) are similar; Sidlaw anticline (Fig. 2) comparable to Benioff zone (Moore, 1973). The southern this suggests that rates of subduction of the those of the Burman volcanic arc (Fig. 6C). Upland plutons could thus be associated in ocean plate were also similar. Both orogens Early Devonian subsidence of the Strath- a similar manner with a north-dipping Be- indicate either continuous or intermittent more syncline (Fig. 2), by analogy-with the nioff zone (Fig. 8C), and they add to the subduction for about 100 m.y. Two cm/yr late Cenozoic subsidence of the Eastern evidence from the Lower Old Red Sand- would be a minimum average rate of sub- Trough in Burma, was possibly related to stone andesite of the Midland Valley that duction; continuous movement would incipient rifting. subduction continued through the Early imply that 2,000 km of ocean crust was The structure of the Southern Uplands is Devonian Period. subducted. comparable with the Chin Hills and Arakan Both orogens have the following history: Yoma in the southern part of the Indo- Other Paleozoic Events in (1) tectonic emplacment of turbidites and Burman Ranges, where no continental col- Britain and Norway oceanic crust above the subduction zone lision has taken place. The analogy suggests forming a mountain belt between the conti- that no significant orogeny occurred in The Lower Ordovician Borrowdale Vol- nent and the ocean floor (the Southern Up- Scotland when subduction ceased during canic Group in the Lake District represents lands and the Indo-Burman Ranges), (2) the Devonian Period and that the folding in island-arc volcanism above a south-dipping deposition of a thick sequence of fluviatile both areas occurred only during subduc- Benioff zone. This Benioff zone probably sedimentary deposits in the Midland Valley tion. continued southward under Wales and and the Central Lowlands between the con- The thickness of the crust beneath the southeast Ireland, where further Ordovician temporary continental highlands and the Southern Uplands and its geophysical prop- and Lower Silurian caic-alkaline rocks were turbidite mountain belt, and (3) emplace- erties (Powell, 1971) can be explained by emplaced (Fitton and Hughes, 1970). ment of plutons in the continental high- the presence either of a thick pile of folded Some of the granitic rocks in northern lands (the Grampian Highlands and the

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B MIDDLE MIOCENE

C PRESENT INDO-BURMAN RANGES CENTRAL LOWLANDS EASTERN HIGHLANDS WESTERN TROUGH BURMAN EASTERN SHAN KALAW SHAN PLATEAU Wti VOLCANIC ARC TROUGH SCARPSSYNCLINE 3? (Clastic) {Carbonate) negative isostatic positive anomaly anomaly minima E a -a Mugearitelugearnes anda ^ sji . ç w Dacite Andesites TescheniteTese s ç ~ jVUirMC

:m> ,,mmmm Km X-l'ltäZ1-?. - . ? Alkaline basalt

r.--.Vl Ophiolitic rock ' Continental crust V¿s^f'y'j- |ÖTÖI Fluviatile and shelf sediments |/_~7~1 Carbonate rocks ffifJPfätf Mainly flysch

25 50 75 100 km i 1 1 i Figure 6. Diagrammatic cross sections showing evolution of the Burma orogen. Pc = late Precambrian; D = Devonian; P = Permian; T = Triassic; J = Jurassic; C = Cretaceous; E = Paleocene and Eocene; Ol = Oligocene; Mi = Miocene; PI = Pliocene; Q = Quaternary; L = Lower; M = Middle; U - Upper; PCu = porphyry copper; SnW = tin and tungsten; NiCr = lateritic nickel and chromite.

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geologic history of the ocean floor is pre- served. Thus the Burma orogen and the Sunda Arc provide better models for inter- preting some ancient orogens and related turbidite deposits than other areas (for ex- km ample, the Andes, Japan, or intra-oceanic island arcs) in which much less of the tectonic history of the oceanic plate is pre- served for examination by geologists. Comparison of other ancient orogens with specific modern arc systems may also indicate the location of mineral deposits Figure 7. Diagrammatic cross section through Naga Hills. Key as in Figure 6. that are related to the tectonic history of the orogens. Our comparison of Scotland with Eastern Highlands) and subsequent erup- to the contrasting crustal thicknesses in Burma suggests, in particular, that portion of andesites between the highlands and each region. phyry copper and gold deposits, analogous the ocean. Most old orogens contain thick se- to the Monywa porphyry and gold The three structural belts in each orogen quences of turbidites, usually underlain by mineralization in the volcanic arc of the consist, respectively, of (1) continental crust chert and ophiolite. When ocean floor Central Lowlands of Burma, should be below the highlands, (2) oceanic or thin without a thick cover of sediment is sub- present in the Lower Old Red Sandstone continental crust below a thick succession ducted, little record of the floor will remain andesite of the Sidlaw anticline in the Mid- of fluviatile sediment and andesite in the near the Earth's surface. But when, as in the land Valley of Scotland. central valley, and (3) a thick tectonic pile case of the Burma-Sunda Arc, the ocean of turbidite sequences in the mountain belt floor is covered with a thick pile of sedi- ACKNOWLEDGMENTS bordering the ocean. The contrast in topog- ment, large parts of the floor are tectoni- raphy between these three regions can be cally emplaced above the subduction zone explained by isostatic readjustment related on the plate boundary, and a record of the We are grateful to Dr. H. G. Reading, Dr. E. B. Sellwood, and Dr. W. Hamilton for criticism of the manuscript. 0 50 100 150 200 Permission to quote memoirs in press was kindly given by Dr. Tin Aye, Director A UPPER CARADOC ' ' ' ' ' General of the Directorate of Geological HBF Survey and Exploration in Rangoon.

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