Tectono-Stratigraphic Units of Western Yunnan

Continental Dynamics, v et . I, No.2, 1996. 1nstiMe of Geology. CAGS

Tectono-Stratigraphic Units of Western Yunnan, China and Their Counterparts in Southeast Asia Jin Xiaochi Institute ofGeology, Chinese Academy ofGeological Sciences, Beijing 100037. China Abstract The arta west of the Leneangjtaeg (M ekong) River of Yuauan is composed of three te ctcnc-streugraphte units. From east to west, they are the Cbangning-Menglian belt. the Baosban block and the Tengchong block. Th e Changning-Menglian belt is confined by the Lan cangjian g fault zone on the east, a nd the Kejie-Nandinghe faults on the west. It represents th e seer of the Paleotethys in the western Yunnan. T be Baoshan block is confined by the La nca ngjia ng fault zon e and Keji e-Nandinghe faults on the eas t and the Nujiang faull zone on the west The Tengchong block lies west of the Nuji ang fault zone, its western beueday lies in Burma and it is thought to be roughly the line linking Babmo a nd Myitkyina. Both th e Baoshan and T eogchong blocks art believed to be derived from the margin of Gondwana, evidenced by t he presence of Permo-Carboeiferoes gjacle-martne deposits and overlying post-glacial black mudstones, as well as Gondwana affinity fossil assemblage. Furt her south in Southeast Asia, three teetoeo-serat lgrapblc units with differenc evolution histories can also be recognized. T bey are, from west to east, the Karee-Teaasserhn-tbe western J\l alaysla unit, the Shan unit a nd the eastern Burma-the western Thailand unit, corresponding respectively to southern extensions of the Tengchcng block . Baoshan block and C ha ngning-Menglia n belt The Keren-Tenasserim-tbe western Malaysia unit is divided from the rest of the Sibumasu strip by the Nwalabo fault an d the Moei- Uthai-Tbani fault In South east Asia , thick Permo-Carboniferous clastic sequences with pebbly mudstones and diamietites of gtaclc-ma r me environment occur only in thi s unit The Sha n unit is hounded OD th e west by the extension of tbe Nujia ng fallIt a nd the Nwalabo fault . a nd on the east by the southwester n extension of the K ejie-Nandinghe fault, which is thought to run roughly parallel to the Salween River until meeting the Meel-Uthal-Than! fault. Late Paleozoic to Triassic seq uences of the Sha n unit lire almost identical to the southwestern part of the Baoshan block . In be tb regions Carbonifero us deposits a re missing. The eastern Burma-the west ern Thailand unit is the southern continua tion of the Cha ngning-Menglia n belt, and is the scar of the Pa leotet hys in Southeast Asia. The Che ngnleg- Menglian belt an d th e eastern Burma-the west ern Thailand unit fonn a continuous belt of 60 to more than 120 km width. lying west of th e Lancangjiang-Nan -Utt araditliof:.

Key words: tea ono-stratigraphic unit, western Yunnan, Southeast Asia

Introduction Yunnan Province is located at southwestern margin of China and borders Vietnam. Laos and Burma (Fig. I ). Geologically, it constitutes the southeastern extension of the units of the Qinghai-Tibet Plateau, and connects th em with related structures of Southeast Asia. Thu s Yunnan, especially the western part of it, is one of the key regions for the understanding of the geological developmen t of the eastern Tethys system. but unfortunately it is not very well known to the world at large . This is mainly due to the fact that mo st of the geological literature on Yunnan has been published in Chinese. Furthermore, the rugged

nature of the western Yurman, which makes access very arduous and difficult, has hampered the intensity of geological field investigations. After completion of the 1:200,000 scale mapping of the whole province, which was accomplished in 1985, and subsequent work in recent years, the maj or geological subdivisions of Yunnan have been outlined and defined. This is shown in Figure 1. The emphasis of this paper lies on the units west of the Lancangjiang fault zone, namely, the Changning-Menglian belt, the Baoshan block and the Tengchong block. The Changning-Menglian bel t

Jin X.c. Tectonc-Stratigraphic Units, W. Yunnan & S.E. Asia

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Teng chong Block J

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is bounded on the east by the Lancangjiang fault zone and on the west by the Kejie and Nandinghe faults. Immediately west of the Lancengjiang fault zone there is a zone of Middle Proteroz.oic metamorphics with a width of 30-60 km, which were intruded by a large quantity of the latest Penn ian and Mesozoic granites. The Changn ing-Menglian belt in its strict sense is situated west of this zone. The Baoshan block is confined on the east by the Kejie and Nandinghe faults and on the west by the Nujiang fault zone. The Tengchong block is the area west of the Nuj iang fault zone. Its western limit lies in Bunna and is thought at the moment to go through Bhamo and Myitkyina.

Tengchong block The Tengchong block is bounded on the east by the Nujiang fault zone. The western boundary, however, lies in a least studied area in Burma. On present evidence an accurate position of it is not easy to

SCh'S!S. myloMe a nd m arble >6000 m

Fig.2 Composite stratigraphic column or Tengchong block

determine, but it is thought to be the line linking Bahmo and Myitkyina (Fig.7). The basement of the Tengehong block is composed of Middle Proterozoic metamorphics of the Gaoligongshan Group. Th is suite consists mainly of granulites, gneisses, schists, mylonites and marbles with a thickness of more than 6000m. In the eastern part the Gaoligongshan Group forms the prominent crest of the Gaoligongshan (the Gaoligong Mountains), from whence it continues to the south and southwest, where it is exposed over a large area of this region. From there it continues to the area north of Mandalay in Bunna where it is called the Mogok Series or the the Mogok Group (Bender, 1983). Sedimentary sequences of the Tengchong block are basically restricted to the center of this region, extending roughly north-south (Fig.2). The oldest sedimentary strata known belong to the Lower Devonian, possibly also the Silurian. They comprise two formations: the Shizishan Formation at the base consists of clastic deposits of an alternating terres-

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Continental Dynamics. Vol. I. No.2. 1996.Instituteof Geology, CAGS

The Baoshan block is a cratonic terrane. It has a trial-marine facies; the overlying Guanshan Formanearly complete marine sedimentary record from the tion is composed of mudstone, limestone and dolomite. Middle and Upper Devonian rocks have not Sinian to the Middle Jurassic. This means that debeen found (Bureau of Geology and Mineral Re- posits of every system are represented. Apart from sources of Yunnan, 1990) (this is simplified as Bu- the scattered outcrops of Middle Proterozoic metamorphics along the western and northeastern reau. 1990 in the following paragraphs). Deposits of the Carboniferous System are proba- boundaries of the block. the oldest part of the bly restricted to the Late Carboniferous. They are stratigraphic sequence of this region is formed by represented by the Menghong Group, a clastic se- the Gongyanghe Group, which has a thickness of quence with a thickness of about 2000 m. The lower about 10.000m (Fig.S). The slightly metamorphosed boundary is either faulted or covered by Ccnozoic Gongyanghe Group crops out in the southwestern sediments. The Menghong Group starts with 600- part of this block and is thought to have a Sinian to 700 m of shallow marine deposits. In the southern Middle Cambrian age. It extends southward into the part they comprise rather monotonous siltstones and Shan States of Burma where it is called the Chaung mudstones, and in the northern part fine- to medium- Magyi Group. The Gongyanghe Group is a flyschgrained sandstones. These are followed by a coarse like sequence consisting of siliciclastics and a few unit, about 700 to 900 m thick. dominated by dia- siliceous shales and carbonates. The upper boundary mictites and pebbly mudstones. Based on regional of the Gongyanghe Group is defined by Late Camcorrelation and systematic sedimentological study of brian fossiliferous fine-grained c1astics and carbonsedimentary structures, c1ast morphology and com- ates. They have a flyschoid character and reach a position. grain size distribution, heavy minerals etc., thickness of 2,OOOm. However, these deposits are this unit is interpreted as glacio-marine deposits usually not treated as a part of the Gongyanghe which originated during periods of glacier highstand Group but as a separate stratigraphic unit. and glacier retreat (Jin, 1994b).The upper part of the Menghong Group comprises 300-400 m of dark to Baoshan Block black shales and sihstones with some irregular argillaceous limestone beds. This part, especially the top l OOm. is rich in bryozoans, crinoids, brachiopods and bivalves, which are typical of PCIl1lO'- Carboniferous periglacial marine deposits. Brachiopods indicate a Penno-Carboniferous age. The Menghong Group is overlain by Permian limestones, dolomitic limestones and marbles of the Dadongchang (or Yanzipo)Fonnation . Fusulinids have been found in S n.le. ..11$1""" the basal part, which indicate the latest Carboniferwonl aO/'l$ ous to the earliest Permian age (Bureau, 1990; Jin,1994a. b). The top part of the Pennian is unknown. ln the northern part of the Tengchong block, on the western flank of the Gaoligongshan, there are some dolomitic and argillaceous limestones which have been mapped as Middle Triassic, and very probably these successions also contain Lower Triassic strata. The whole block emerged by the end of the Triassic. Jurassic and Cretaceous sedimentary • ~ca 10000 m rocks are missing. During the Mesozoic the whole region was intruded by large quantities of granites. In the Cenozo ic this region experienced volcanic activities associated with the Himalayan orogeny. »1'I(I~wne

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126

Jin X.C. Tectono-Slratigraphic Units, w. Yunnan &. S.E. Mm

The Ordovician System consists mainly o f [0 5-

limestones of (late) Qixian to Wujiapingian (7) age.

siliferous shallow water siliciclastics and some argillaceous limestones, revealing a relatively stab le environment. The Low er 'Silurian is represen ted dominantly by graptolite-bearing shales. The Middle and Upper Silurian are composed of fossiliferous argil laceous limestones. The Devonian starts with fine silicid astics with some argillaceo us limestone intercalations, contai ning abundant tentaclilids and conodo nts (Fig.4). Fro m the Midd le Devonian 10 the end of the Early Carbonifero us, deposition of various fossilifero us carbonates prevailed. Then the whole block eme rged. Sedimentation was resumed in the Late Carboniferous with the deposition o f the Dingj iazhai Form ation , which lies uncon fonnably on the

Baoshan Block

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Lower Carboniferous or older sequences. The Dingjiazhai Form ation begins with shallow marine mudstones and siltstones, which reach a thickness of 15 - 40 m. They are overlain by diamictites, which are in turn overlain by pebbly mudsto nes. This coarse part usua lly has a th ickness o f 20 - 50 m and is interp reted as depo sits of mostly reworked tills at the glacier front by mean s o f mass flow and slumping in a glacio-marine environm ent. Ice rafted materials are additional contributors to the sediments (Jin. I994b). Larninites with dropstones above the coarse part ,..'ere depo sited in the glacial retreat period as the ice rafting processes provided only the coarse-grained input. The upper part of the Dingjiazha i Formation comp rises black or dar k-co lored mudstones and shales of abo ut 50 m thickness. Bryozoan s, crinoids, brachiopods and bivalves are abundant in this dark part and in the pebbly mudsto nes. The top o f the Dingj iazhai Formation consists o f several meters of bioclast lc limestones of coquina facies, containing abundant bryozo ans , crinoids, brachiopods, bivalves and fusulini ds. The deposition of the Dingjiazhai Formation was tenninated by the eruption of the Woniusi Basalis. The Woni usi Formation has a thickness of 500 - 900 m and consists mainly of basaltic lava flows, tuffs, pyroclastics and a few sedimentary intercalations in the basal and middle parts, which contain the earliest Pennian fusullnids. After the eruption of the Woniusi Basalts, there w'as a short period of exposure in the Baoshan block. Th is is evidenced by a hard ground on the top of the Woni usi Form ation seen at a few o utcrops. Red beds of late Sakmarian to early Arti nskian age with a thickness rang ing from 0 to 70 meters in most areas lie discon form ably on the Won iusi Basalts. The red beds are ov er lai n by ar gill aceo us and dolom itic

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The so uthwestern part o f the Baoshan block emerged dur ing the Carbo nifero us. Permian red beds and carbonates, which lie in other parts o f the block o n the basalts of the Woniusi Formation. rest here uncon form ably on Devonien carbonates or older strata. Further so uth. nearly identical strata are observed in the Shan States of Burma, where the Permo-Triassic "Upper Shan Dolom ite" lies unconfonnably on the Devonian Lower Shan Dolomite or older strata (Bender, 1983) . The Lower and Middle Triassic are composed of dolom itic limesto nes and dolomitcs. There is possibly a hiatus between the Permian and Triassic, but the exte nt o f it is not easy to determine, because both of them are composed of dolomit ic limestones . and so me Scyth ian conodonts have been found recently (Bureau, 1990). In the early Cam ian ande sitic

Co ntinental Dynamic s, Vol. I, No.2, 1996,lnstitUleo f Geo logy, CAGS

volcanics erupted in the eastern and southern parts of this bloc k. Below the volcanics there are up to 8 m hematite and baux ite beds. After that, only some limestones and siliciclastics of restricted extent were deposited . Due to the Indosinian orogeny in the Late Triassic, the Latest Triassic sediments and Early Jurassic sediments are absen t. A Middle Jurassic molasse sequ ence with basic volcanic intercalations is conflned to the western margin of this block. Upper Jurassic and Cretaceous red beds are restrict ed to the southwestern part of the Baoshan block.

Differen ces betw een Tengc hong a nd Baosb a n blocks Both the Tengchong and Baoshan blocks were located at the Tethyan marg in of Go ndwana and within the reach of glaciers at their highstands in the period o f Late Carbo niferous to earliest Pcrmian. This is indicated by the glacio-marine deposits and Gondwana affinity fossils. The following differences. however. may indicate that the two blocks were separate units during the Late Paleozoi c and that the prese nt j uxtaposition was achieved by later tectonic movements. First, the Tengchong block has a basement made up of Middle Proterozoic metamorphics. These metamo rphics are widely distributed and extend southwestwards to the area north of Mandalay, Burma, where they are called the Mogok Series. In the Baoshan block Middle Proterozoic metamorphics have only been found along the northeastern margin and the western margin. Whether these metamorphi cs belong to, or are exposures of, the baseme nt of the Baoshan block remains unclear. The oldest unit known in the Baoshan block is the Go ngyanghe Group . a thick Sinian 10 Middle Cambrian clastic sequence. It is widely exposed in the southwestern part and extends into the Shan States. where it is called the Chaung Magyi Group. Second , the Upper Proterozoic rocks are unknown in the Tengchong block. Early Palcozoic strata are poorly developed. and only a few Early Devo nian (poss ibly some Silurian) deposits are exposed . In the Baoshan block. sedimentary sequences from Sinian 10 Early Carboniferous are much more comp lete and widely distributed. Late Carboniferous deposits of the Tengchong block, the Menghong Group. have a thickness of more than 2000m. On the other hand, the Dingjia-

121

zbai Formation of the Baoshan block has a much smaller thickness of less than 300 m. Th is indicates that at that time, the Baoshan block was more stable and was subsiding rather slowly, whereas the Tengchong block was a rapidly subsiding region. They were formed in different structural environments. The composition of the diamictites of the Dingjiazhai Formation and the Menghong Group is significantly different. Clasts of the diamictite of the Baoshan block are mainly carbonates from the Lower Carboniferous and Devonian. The matrix has also a high content of carbonate. This reveals a source area with abundant carbonate rocks. Clasts of the diamicntes of the Tengchong block comprise quartz, gran. ite, metamorphic rocks, cherts and silicic1astics. The matrix is composed mainly of quartz and mica; feldspars are present as a minor component. The composition of the diam ictites indicates that the source area was a crysta lline terra in. Th ird, the erupt ion of the Woniusi Basalts was an important event in the history of the Baoshan block. After the eruption red beds were deposited on the whole block. In the Tengchong block. however. there is no trace of volcan ic activity at that time and the carbonate formation was not interrupted. Red beds were not developed.

C ha ngning-Me nglian belt The Changni ng-Menglian belt diffe rs from the Tengchong and Baoshan blocks mainly in two ways: I) There are neither Pcrmo-Carbon iferous glaciomarine deposits nor associated Gondwana affinity fossil asse mblages; 2) Late Paleozoic to Triassic lithological assemblages of the Changning-Menglian belt indicate that they were formed in a more "geosync1inal" environment. The belt has been interpreted by many authors (e.g. Zhang et aI., 1985; Liu et al., 1993; Wu et al., I995) as the suture of the

Paleotethys. The oldest strata in this region are metamor phosed Prccambrian flysch-like and volcanic formations, located immediately west of the Lancangjiang fault zone. They are probably of Middle Proterozoie age. Early Paleozoic slightly metamorphosed silicic1astics and carbonates are restricted 10 the southwest of this belt, from which Ordovician fossils have been obtained. The barren sequence below the Ordov ician fossiliferous beds is thought to be of Cambrian age (Bureau, 1990). Silurian rocks have not been found in this region.

"8

Jin

x.c. TcdonO-StratigntphieUnits, W. YUllI'IIIn A S.E. Asia

Rocks of the Devonian System have a large thickness and are thought to rest disconfonnably on the Lower Paleozoic, because no contact with the underl ying

rocks has been observed(Fig.5). Devonian sediments Changning-Menglian Belt

Fig. 5 Composite I.ntignphic column (Dt'vonlanTriusic) oftbe C b. ngninc-Mcngliaa HII

consist of fine graptolite-bearing clastics, siliceous shal es and cherts. Rifl ing and sed imentary env ironment differentiation began in the Late Devon ian and was intensified during the Permo-Carboniferou s periods. Consequently, two major different lithological

near Yiliu of Lancan g County indica te a To urna isian age (Lan Chaohua et al., 1983; Bureau, 1990). In the south of Menglian County, bedded cherts have been found intercalated within the Yiliu Formatio n. Radio larians extracted from these cherts do not contradict the Early Carboniferou s assignment (Liu Benpei et al., 1993; Wu Haoruo et al.• 1995). Liu Benpei et al . (1993) declared that 14 rad iolarian zones have been recognised in the Changning-Menglian belt and claimed that these identify a continuous Devonian to Anisian chert succession. Wheth er the cherts which appear in different parts of this belt, mostly with other siliciclastics, represent a continuous chert succession is still doubtful, but at least it indicates the presence of bedded cherts o f Devonian to An isian age within the Changning-Menglian bell. Possible Perm ian basalts with or without cherts have also been reported in the central part of the Changning-Menglian belt (Zhang Qi et al., 1985; Liu Benpei et al.•1993 ). Ophiolites, typical or nontypical. hitherto described in the Changn ingMenglian belt refer mostly to the occurrence of basa lts and cherts in the central zone. tho ugh there are some basic o r ultrabasic rock bodies in the bell. Early Triass ic sed iments are not widely d istributed. Besides the cherts repo rted by Liu Benpei et al. ( 1993), there are a few siliciclastics. Late Triass ic to Middle Jurass ic sequence is represented by lacustrine and fluviat ile strata, red beds and red lagoo nal evapori tes. The description above shows that the Ten gchong block , the Baoshan block and Changn ing-Mengl ian belt in the western Yunnan are three distinctive teetono-stratigraphic units with differe nt evoluti on histories. All of these three units extend southward across the Chinese border into Burma, Laos and Thailand.

Burnta

successions were developed. One, present in the eastern part and in some places of the western part of the belt, consists of thick. more or less continuous, fine siliciclastics and bedded cherts. The other lithofaci es association is rep resen ted by basa lts with intercalations of bedded cherts overlain by a fossiliferous limes tone succession containing Middle Carboniferous to Late Perm ian fusulini ds and other fossils. It was formed in the central part of the reg ion. The basal tic volcanics are called the Yiliu Formation . Fossil s from the intercalated limestone lenses in the upper part of the vo lcanics in the type local ity

Generally speak ing, Burma can be d ivided into two tectonic domains, the Indo-Bunnese Rang e and the Sino- Burme se Range separated by the northsouth striking Shan boundary fault. The first named domain occ upies the western part of the country. It represents a Late Meso zoic-Terti ary basi n with a rapid ly subsid ing floor. The other domain occu pies the eastern part of the country and comprises consolidated Precambrian to Mesozoic metamorphics, sedi mentary strata and igneous rocks. Like the western Yunnan, the Sino-Bunnese

Continental Dynamics. Vol. I, No.2, 1996.lnstitutc of Geo logy, CAGS

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mainly of Cambrian age. but some younger Paleozoic rocks are also recognized. In regional geological terms this unit corresponds to the KarenTenasserim unit of Bender ( 1983). The Karen-Tenasserim unit which extends down to the western Sumatra is the area where the PermoCarboniferous diamictites, pebbly mudstones and dark-colored fine c1astics occur, which are interpreted as glacio-marine and post-glacial deposits (e.g. Stauffer & Lee, 1986). This three-fold sequence with a thickness of 1600 to more than 2000 meters is called the Lebyin Group and the Mergui Group in central Burma and pen. , insular Burma, the Kaeng K.rachen Group and the Phuket Group in southwestern Thailand and peninsular Thailand, and ......... P,_. Singa Formation in Langkawi Island, Malaysia. These sequences resemble the Menghong Group not only in architecture and thickness. but also in the composition of diamictites and pebbly mudstones " (Stauffcr & Lee, 1986; Stauffer & Mantajit, 1981). Thin sections of samples of the Singa Formation examined by the author show nearly identical features to those of the Menghong Group. The Precambrian basement of this unit has little surface expression in the peninsular part, but is well • exposed in the northern part, from about Mandalay through the Mogok into the Tengchong block in the the western Yunnan. In peninsular Burma and the western Malaysia the sedimentary sequence starts with Late Cambrian clastic sediments. Ordovician and Silurian sediments com• prise shallow water clasties and some limestones. Devonian sediments are relatively widely distributed, and they are mainly shallow marine carbonates and some clasties (Metcalfe, 1988; Stauffer & Lee, 1986). A prominent feature of this unit is the extensive exposure of Carboniferous to Triassic sediments (Bender, 1983). This is another point in common with the Tengchong block. In fact, the Tengchong block consititutes the northern continuation of this unit ( Fig. 7). , -->.-- _. ' In the Shan States the slightly metamorphosed flysch-Iike sediments of the Chaung Magyi Group. about 3000 m in Fig. 6 Tec1ono-slnti2n phic un its of Burm . (fro m & ndtr, 1983) thickness, are the oldest strata exposed.

Range is also composed of several units. Bender (1983) noted the sharp contrast between the Late Paleozoic sequences of the strip of Karen, Tenasserin and the western Malaysia and the Paleozoic sequences of the Shan States. He divided the SinoBurmese Range into the West Kachin unit, the East Kachin-Shan unit and the Karen-Tenasserim unit (Fig.6). The West Kachin unit is a sparsely studied area. It comprises mainly metamorphics and granitic intrusives. Limited clastic sediments are thought to be

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Jin X.C. Tectcno-Strstigraphic Units, W. YUMan & S.E. Asia

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not been observed in this region. The Upper Shan Dolomite, a Permo-Triassic carbo nate sequence, lies unconformably upon the Lower Shan Dolomite or older strata. The Upper Shan Dolomite has a time range from Artinskian to Middle Triassic. It comprises dolo mites and limestones, the upper part norm ally showing a brecciated appearance. The presence of Late Permian and Early Triassic fossils within the Upper Shan Dolomite may indicate continuous deposition across the Permo-Triassic boundary, though in different localities a break may occur (Brunn schwe iler, 1970; Bender, 1983 ). Upper Tr iassic sediments consist mainly of carbonates, but there are also evaporites and c1astic s. In the Early Jurassic, carbonate formation cont inued in some places. Later, sediments of an alternating marine-terrestrial environment became dominant. Contine ntal red beds were formed in the Late Jurassic and Cretaceou s (Bender 1983). These are continuous into the southwestern Baoshan

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Fig.7 Tedono-stratigrapbic units which constit ute western Yuo nan and ana further south

They are believed to have an age of Sinian to Middle Cambrian. The oldest age shown by trilobites from the overlying format ion is Late Cambrian (Bender, 1983). Late Cam brian to Middle Ordoviciao sed iments are represented by mudstones and

In Figure 6 the eastern boundary of the Shan unit is not shown. It is not easy to dete rmine, due to the low research level in this region. Considering the fact that the Baoshan bloc k and the Shan unit are cont iguous and their stratigraphic formations are continuo us and that both parts are typically cratonic, it is possible that the eastern boundary of the Baoshan block extends down to Burma and lies rough ly parallel to the Salween River until it meets the Moe i-Uthai-Thani fault (Fig . 7). This line is roughly the eastern boundary of the craton . This will also be touched on in the following paragraphs.

Northern Thailand

sandstones with volcanic intercalations. In the Late Ordovician, carbonates with subordinate fine c1astics were depo sited. Silurian strata con sist mainly of black grapto lite shales and sandstones (Bender. 1983). In the Devonian, the lower part of the Shan Dolomite was formed, which comprises dolomites, limestones with local biostrome s, argillaceous limestones and subordinate shales. Thereafter, the who le region emerged. Depos its of Carbo niferous age have

Frequently the opinion has been expre ssed that Thailand is essentially composed of two crato nic blocks separated by a suture. These two cratons are the Indochina craton and the Shan-Thai craton, and the suture is the Nen-Uttaradit suture (e.g . Helmoke, 1985, 1986; Metcalfe, 1984, 1988, 1993), A simpler interpretation is that the Nan-Uttaradit suture represents the Paleotethys, the area east of it having been

Continental Dynamics. Vol. I. No.2, 1996. Institute o f Geology, CAGS

a part of Eurasia and the area west of it a part of Gondwana during the Late Paleozoic. This oversimplified model ignores the fact that in northern Thailand Permo-Carboniferous limestones rich in warm Tethyan fusulinids and corals are widely distrib uted to the west of the Nan- Uttaradit suture. If it is therefore assumed that the Sban-Thai craton was an Eurasia affinity block. two other facts are overlooked: first. northern Thailand does not only have Perm o- Carbon iferou s limestones with warm-water fossils of the Tethys, but it also exposes Devonian to Early Triassic radlolari tes, cherts, siliceous shales, and siliciclastic deposits, as well as Carboniferous basa lts; second, the Penno-Carboniferous three-fold clastic sequence which correl ates well with the sequences of the same period in other Gondwana regions does occu r in the Baoshan block, which is continuous with the Shan States as shown in the previous discussion. At the moment this creates a dilemma. The solution of it is provided by the understanding of the geological features of northern Thai land. Jin Xiaochi (1994b) argued against the assignment of northern Thailand to a cratonic block and poin ted out that the eastern boundary of the Shan craton here follows more or less the course of the Salween. Baum et al. ( 1970) noted that the diversification of the depo sitional enviro nment of northern Thai land began in the Devonian, and became more pronounced in the Carboniferous and Permian. On the one hand. sed imentation of siliciclastics and .cherts lasted in varied lateral extent into the Triassic. On the other hand, Carboniferous basic inuu sives and volcanics normall y associated with cherts (often interpreted as ophiolites) were formed over a large area . Northeast of Chiang Mai the volcanic series is overlain by mudstones and carbonates, containing a Namurian B to Westphalian fauna (Baum et al., 1970). The formation of carbonates continued with varied lateral extent into the Triassic. This situation is nearly the same as that observed in the Changn ing-Menglian belt in Yunnan . Sashida et al. ( 1993) have found Early Carboniferous and Early to Middle Pennian radiolarians in the "Fang Chert" exposed between Chiang Mai and Fang along the road linking the two places, which was assigned to a Late Devonian age. This discovery may indicate that a chert environment was sustained at least into Penno-Carboniferous times. Fontaine et al. (1993 ) have found that the widespread Late Visean to Late Pennian limestones containing Tethyan fusulinids occur in the Sop Pong area of

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northwestern Thailand. which they call an "unexpected lithology and fossils". Caridroil (1993) concluded, after examining the radiolarians in the Chiang Dao area, that three stratigraphic sequences can be described: I) an Upper Paleozoic to Triassic radiolarite series; 2) a limestone series mainly dated on the foram inifers and algae Carbon iferousPennian (probably also Triassic); and 3) a detrital series thal is probably the matrix of an olistostrome. Tofke et al. ( 1993) found Triassic syn-orogenic siliciclast ics in the area of Mae Saring. All these observations indicate the complexity of northern Thailand and further confirm its similarity to the C han gni ng-Menglian belt. The lithological assemblages preclude northern Thailand from being simply a part of a cratoni c block during Late Paleozoic-Triassic times. On the co ntrary, the varied facies associations indicate that these sequences originated in a mobi le tectonic zone. Northern Thailand has the same Late PaleozoicTriassic Iithological assemblage and tectonic features as those of the Changning-Menglian belt in the western Yunnan. Geologically, it constitutes the southern extension of the C hangni ng-Menglian belt. S ib u m asu The Sibumasu block as defined by Metcalfe (1984, 1988) is bounded on the west by the Shan Boundary fault. and on the east by the Nan-Uttaradit and the Bentong-Raub line in Thailand and Malaysia and by the Changning -Menglian belt in the western Yunnan (Metcalfe, 1995, 1996). But the connection of the Changning-Menglian belt with the NanUttaratdit line is unconvincing . As shown above, the Sibumasu under this definition is composed of three different units, namely the TengchongTenasse rim-the western Malaysia-the western Sumatra unit. the Baoshan-Shan unit and the southern extension of the Changning-Menglian belt. though the Changning-Menglian belt itself is excluded by definition (Fig. 7). These three units have different evolutionary and tectonic histories., thus they cannot be treated as a single unit. It is therefore suggested that the name "Sibumasu" be used only for the Tengchong-Tenasserim-the western Malaysia-the western Sumatra unit. This will retain the literal mean ing of "Sibumasu (China-Burma-Malaysia-Sumatra)" and avoid the cumbersome listing of individual units. The Baoshan-Shan block and the southern extension ofChangning-Meng lian belt are explicitly excluded.

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Jin X.C. Tectono-Stnltigraphic Units, W. Yunnan '" SE . Al;ia

Conclusions The area west of the Lancangjiang (Mekong) of Yunnan is composed of three tectono-stratigraphic units. They are, from east 10 west. the Changn ingMenglian belt, the Baoshan block and the Tengchong block. Together with thei r counterparts in Southeast Asia.

they are Tengchong-Tenasserim-the western Malaysia-the western Sumatra unit, the Baoshan-Shan unit, and the Changning-Menglian belt and its southern exte nsion . The Sibumasu block as de fined by Metcalfe ( 1984. 1988, 1995, 1996) is a composite of different units. It is here suggested that the name "Sib umasu" be redefined to denote the unit consisting of Tengchong, Tenasserim, the western Malaysia and the western Sumatra. The area of 60 10 120 km width west of the Lancangj iang and the Nan-Uttaradit lines is the Changning-Menglian belt and its southern extension, which was not a part of a craton during the Late Paleozoic and Triass ic, but originated from the closure of the Paleotethys in this region.

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Jin Xiaochi Bachelor of Science. Nanj ing University 1982; Master of Science. Graduate School of Chinese Academy of Geological Sciences 1985; Doctor of Natural Sciences. University of Cologne, Gennan y 1994. He is presently active in the fields of sedimentology, stratigraphy and paleogeography of Yunnan and the Qinghai-Tibet Plateau.