Paleogeographic History of Western Thailand and Adjacent Parts of South - East Asia - A Plate Tectonics Interpretation

Abstract

The term "western Thailand" is taken to include the western mountain ranges, the central plain and the western part of the Khorat Plateau, covering an area which is more than one-third of the country. Regional stratigraphy is assembled, analysed and interpreted in order to reconstruct the paleogeographic history of this part of Thailand, in context with the surrounding regions in Southeast Asia. Late Paleozoic to Mesozoic paleomagnetism, a gravity transect and additional geochemistry analyses have been studied to aid the reconstruction. The study area includes part of the structurally complex belt of Precambrian, Paleozoic and early Mesozoic rocks on the west, previously known as the Burmese-Thai-Malayan Geosyncline and the contrasting gently folded area of young Mesozoic mollasse on the east, with a broad cenozoic central alluvial plain between them. The stratigraphy of three sub-areas, West Thailand, central and lower North Thailand, and upper North Thailand are described in Chapter 2 to Chapter 4, respectively. The stratigraphy in other areas of Thailand is summarised in chapter 5. Igneous rocks and structural features of Thailand are outlined in Chapter 6 and Chapter 7, respectively. The oldest rocks in Thailand are the stratigraphically dated Precambrian gneiss, schist and calcsilicate of amphibolite metamorphic grade, outcropping along the western Belt from Chiangmai to Kanchanaburi and north Prachuap Kiri Khan and in the eastern Gulf Coast of Thailand at Chonburi, and also probable Precambrian gneiss at Khanom in lower peninsular Thailand. The Precambrian rocks are thought to be mostly metamorphosed marine (flysch) sediments of a continental margin, and were deeply buried, uplifted and deeply eroded before deposition of the Cambrian. Cambrian and Ordovician rocks are also restricted to the Western Belt on the Shan-Thai cratonic fragment. Fossiliferous Upper Cambrian cross-bedded sandstone at Tarutao Island (Tarutao Formation) off the west coast of south peninsular Thailand must overlie continental crust, and is considered to have been derived from a westward source which is now separated. Similar sandstones, lacking fossils, but considered to be Cambrian, outcrop on the island Ko Lan off pattaya on the eastern Gulf, and Cambrian sandstones were deposited on a stable shelf that persisted through the Cambrian and Ordovician from west Kanchanaburi at the southern end of the western Mountains, to chiangmai in the north of the ranges. The fossiliferous Ordovician Thung Song Limestone Formation in the south peninsula is correlated with the Ordovician Sri Chang Limestone on the eastern Gulf, and with limestones in the Western Mountains from a little south of Kanchanaburi to Chiangmai. The Cambro-Ordoviclan outcrops are closely associated with the Precambrian in the western Belt, but cover a wider area. The total thickness of Cambro-Ordovician sandstones and limestones reaches 1600 m. Middle Paleozoic rocks are differentiated into several facies belts from west to east. The fossiliferous carbonate shelf facies persists throughout the Paleozoic in lower peninsular Thailand and in west Kanchanaburi at the southern end of the western Mountains. On the mountains to the north of Kanchanaburi (e.g. Mae Sot, Mae Sariang, Mae Hong Son) scattered remnants of metasediments overlying the Ordovician are mapped as "silurian-Devonian". They were originally clastic sediments, probably quartz sandstone and shale, not exceeding 1000 m in thickness. They are in part carbonate shelf facies and graptolitetentaculite facies conformably overlying Ordovician limestone and conformably underlying Upper Paleozoic rocks. East of the Western Mountains in the Sukhothai Fold Belt, the sediments are strongly deformed and three facies belts are recognised: from west to east, 1, back-arc basin facies; 2, volcanic arc facies; 3, fore-arc, chiefly inner trench slops facies. Along the eastern margin of the western Mountains a fold-thrust zone at the boundary of the back-arc area and the continental margin have been observed at Hod, near chiangmai, at west Tak, at northwest Kanchanaburi, at northeast Rayong in the eastern Gulf, and at Yala in south peninsular Thailand. The Silurian-Devonian back-arc facies includes the Lower Devonian graptolitic black shale and chert between Fang and Chiangmai and unfossiliferous thin bedded cherts and occasional limestones outcropping adjacent to the western Mountains in west Lampang, Thoen, west rak and Kanchanaburi. Similar rocks are found in northeast Rayong in the eastern Gulf and in Yala in south peninsular Thailand. The prevalence of chert relative to limestone suggests that the back-arc area was a fairly deep sea, and evidently shan-Thai had little relief and was partly submerged, and was not supplying large volumes of clastic detritus to the back-arc area. The volcanic arc facies is represented by limited outcrops of amphibolite and mica schists of high green-schist grade mapped as the Den Matum and Mae Ko complexes at Tak and Wiang Pa Pao in south Chiangral, respectively. The volcanic arc facies is also largely represented by agglomerates and fine grained tuffs metamorphosed to greenschist facies at south Lampang, east Tak, Kamphaengphet and Nakhon Sawan in central and north Thailand, Singburi-Suphanburi in western Thailand, and probably at south Narathiwat in peninsular Thailand. They form a mainly north-south trending zone in the Sukhothai Fold Belt west and north of the Chao Phraya Plain. Eastwards the arc-trench gap deposits are marbles and bedded cherts (Thung Saliam Group) mainly outcropping at Sukhothai and Nakhon Sawan. The trench facies is probably represented by flysch-like low grade metasediments of the Pha Som Group, west of Mae Nam Nan, i.e. immediately west of the Pha Som Ultramafic Belt. The lateral sequence of facies in the Sukhothai Fold Belt appears to be consistent with the gravity profile measured between Mae Sot and Khon Kaen. The Silurian-Devonian to late Permian sequence in the Loei Fold Belt contains marine sedimentary rocks locally at Loei, of the same age range as those in the Sukhothai Fold Belt, i.e. Devonian or possibly Silurian to Triassic, in Loei to Nam Pat, in northeast Uttaradit. Until the late Permian the sediments of the Loei Fold Belt were accumulating on a passive continental margin, and were a simple clastic wedge formed from the erosion products shed by the Indochina continent to the east, similar to that of the Atlantic coast of North America at the present day. Late Carboniferous to Permian sediments in the Loei Fold Belt are thicker, changing from massive limestone to a more sandy shaly limestone facies in the direction from Loei to Phetchabun (from east to west) up to 1500 m in thickness. Late permian shales contain Cathaysian plant fossils, and local intercalations of thin limestone bands contain Upper Permian fusulinacean fossils. Carboniferous rocks in the Sukhothai Fold Belt consist of varying lithologies from west to east in west and North Thailand. At places marine shelf sedimentation continued in the west and flysch type sediments in the east, but with local unconformities. In the middle part of the fold belt strong unconformities on the Silurian-Devonian are overlain by thick volcanic agglomerate and possibly marine red-beds underlying Lower Permian limestones. Volcanic rocks are relatively rarer than in the Silurian-Devonian and are represented by agglomerate and tuffaceous rocks with minor shallow acid to basic intrusive rocks near Phrae. West of the Western Mountains passive continental margin sediments continued into the Carboniferous and in places into the Permian, but local paralic red-beds occur near Mae Hong Son. Fossil plants as well as marine fossils are found in the late Carboniferous to Lower Permian strata. Permian rocks are dominantly limestones. The limestones are possibly of different ages in the Western Mountains, the Sukhothai Fold Belt and the Loei Fold Belt. The Permian rocks in the Sukhothai Fold Belt contain minor tuffaceous rocks but these do not appear to represent a volcanic arc. The Permian sequences in all areas are mainly Lower to Middle Permian. Early Upper Permian rocks, mainly sha1e, sandstone and thin limestone are found at places in Lampang, in the Sukhothai Fold Belt, and (with the Cathaysian flora) in Loei and Phetchabun, in the Loei Fold Belt. West of the Western Mountains Middle Triassic rocks are found from south Mae Hong son south through Mae sot to Kanchanaburi. In Mae Sot and Kanchanaburi deposition of shelf carbonates continued from Middle Triassic to middle Upper Jurassic. The Triassic rocks in the Sukhothai Fold Belt cover a wide area from Lampang to east of phrae. Between Lampang and Phrae Triassic sandstone, limestone, greywacke and shale overlie Permo-Triassic basaltic andesite and form three subparallel folds within the great Lampang Basin. At Nam pat east of the Pha Som Ophiolite (east of the Sukhothai Fold Belt) mainly Middle Triassic flysch also overlies Permo-Triassic basaltic andesite. East of this Triassic outcrop in the middle of the Loei Fold Belt at Lom Sak, the Lom Sak Formation is probably related to the Nam pat rriassic. The Permo-Triassic boundary is always an unconformity. Non-marine late Triassic rocks occur on the northwest edge of the Khorat Plateau, in the Nakhon Thai synclinorium, and in the Phayao-Nan Basin, and they are much alike at the three places. The sequence begins with late Triassic paralic grey shale, sandstone and limestone which conformably underlie the continental red-beds of the Khorat Group. The Khorat Group in the Phayao-Nan Basin contains formations as young as Middle to early Upper Jurassic, and provides a record of the upward transition from marine Triassic through paralic facies to continental red-beds. Outside the three major red-bed basins the red-beds are thin, extending as small remnants as far as the middle part of the Western Mountains and locally are overthrusted by strata as old as Ordovician. Cenozoic rocks are mainly fresh-water shale and sandstone in fault bounded intermontaine basins in west, central and north Thailand and often contain lignite and oil shale. The rocks are mainly Upper Tertiary but Lower Tertiary fossils are known from some basins. The upper part of the Cenozoic usually consists of thick coarse-grained deposits. Igneous rocks are widespread, including granitoids, Silurian to Lower Permian metavolcanics, Upper permian to Lower Triassic volcanics, Upper Triassic to Lower Jurassic rhyolites and Upper Cenozoic basalts. Basic to ultrabasic rocks, including spilitic pillow lavas, dunite, serpentinite, dunite and peridotite, are considered to mark a geosuture along the Pha Som, Sra Kaeo and Bentong (in the Malay peninsula) ophiolite lines. Major fold structure in Precambrian to Lower Paleozoic, Middle Paleozoic to Triassic, and Jurassic to cretaceous formations are outlined where they can be recognised in the field. One hundred and four oriented paleomagnetic samples from eighty-five sites in diverse areas of Thailand were examined. They represent Upper Paleozoic to early Cretaceous marine sediments and continental red-beds, and also a few Mesozoic volcanics and Ordovician limestones. Mean declination and inclination directions of magnetisation determined in Carboniferous, Permian, Lower to Middle Triasslc, late Triassic to early Jurassic, Jurassic and early cretaceous rocks are (D, I): 22.6, +24.7; 352.8, +34.6; 315.2, -21.5; 30.3, +20.0; 28.7, +45.8; and .14.8, +11.9. Paleomagnetic reversal stratigraphy from composite sections is correlated with a reference reversal stratigraphy compiled, from published data. The reversal stratigraphy at Mae Sot and at Chumphon provide useful evidence of age. Paleomagnetic pole positions from Carboniferous to Lower Cretaceous provide a tentative polar wander path for Thailand. The results are here used suggest that Thailand rotated progressively clockwise, a total of more than 180 degrees, between the Carboniferous and the Cretaceous, with the most rapid rotation occurring during the Triassic. But are by no mean’s conclusive. Reflected light mineralogy of selected paleomagnetic samples shows hematite to be the dominant magnetite mineral in most lithologies, and magnetite to be an important component in a minority of lithologies. A paleogeographic model is presented which represents Western Thailand as part of a continental fragment, Shan-Thai (eastern Burma, western Thailand and northwestern Malay peninsula), which was a separate microcontinent drifting in the Paleotethys during the late Paleozoic. It is suggested that shan-Thai rifted from Gondwana early in the Carboniferous. It may have lain on the northwest side of Australia, and separated from Gondwana at the same time as Turkey and lran, and possibly Afghanistan and Tibet, at the time when Gondwana rotated rapidly clockwise, moving Australia from a low latitude to a high latitude position in the Southern Hemisphere. The as yet ambiguous paleomagnetic data suggest that Shan-Thai moved from a low latitude southern Hemisphere to a low latitude Northern Hemisphere position, while rotating nearly 180 degrees in the horizontal plane, in the time between early Carboniferous and early Triassic. During the Middle Triassic Shan-Thai sutured nearly simultaneously to Indochina and to South China, the continent-continent collision being a part of the Indosinian Orogeny which resulted from multiple collisions during the Triassic over a very wide belt in southern and eastern Asia. During the collision Indochina tended to underthrust Shan-Thai and the thick marine Paleozoic to Triassic marine sequences on the joined continental margins were folded with consistently west-dipping axial planes. After the collision, mountains arose along the suture, particularly along the overthrusting Shan-Thai margin, and at the same time granites were intruded to high levels in the sediments, and extensive rhyolites were extruded on the land surface. Erosion of the mountains produced mollasse deposits (mostly alluvial plain red-beds) that occur on both sides of the suture, but are most fully developed in the Khorat Basin that formed on the underthrusting west side of the Indochina continent. A parallel is suggested between the Upper Triassic to Cretaceous Khorat Basin and the Cenozoic Indogangetic plain. Tabular cross-bedding directions in the red-beds up to Middle Jurassic in age indicate a paleoslope to the northeast, and in the red-beds of upper Jurassic and cretaceous age a paleoslope to the southwest. Cretaceous marine salt deposits (mainly halite, canellite and sylvite) indicate little supply of detrital sediments, hence general low relief, and submergence of much of the Khorat Plateau, though it was far from the open ocean. Fine-grained detrital and organic-rich Lower and Middle Cenozoic fresh-water sediments in central, western and northern Thailand indicate continuing low relief. Late Cenozoic rudaceous deposits reflect the rise of the present mountains probably during Pliocene and early Quaternary. The present mountains are horsts and tilted blocks bounded by normal faults that generally trend north-south. The normal faulting coincided with the extrusion of high alumina alkaline basalts represented by many small fields in eastern and northern Thailand, and in Indochina. The Gulf of Thailand was formed by rifting during the Cenozoic. It is floored by very thick (up to 5 km) mainly fresh-water sediments that include beds at least as old as Oligocene, and probably considerably older. High heat flow in the sediments suggests a basement of young mantle-derived rocks. Seismic prospecting data indicate many north-south trending normal faults that have moved progressively during deposition of the sediments. Rifting was east-west, at right angles to the trend of the normal faults and the northwest trend of the Gulf of Thailand is not related to the rifting trend, but is inherited from a line of weakness along the Mesozoic northwest trending sinistral Three Pagodas Fault Zone. Approximately northeast trending dextral faults in peninsular Thailand lie roughly at 45 degrees to the east-west sense of extension of the Gulf, and accommodate increasing separation towards the south, of peninsular Thailand from Indochina (the eastern Gulf Coast). From paleomagnetic evidence, the Malay Peninsula has rotated anticlockwise away from Indochina with a pole of rotation near or in western Borneo, and its northern end may be overthrusting southern peninsular Thailand. Intracratonic spreading in the Gulf of Thailand and extensional tectonics in western, central and northern Thailand are probably related to the current subduction of the Indian Plate under Southeast Asia along the Java trench and the Andaman-Nicobar Island chain.