Deformation and metamorphism of the Sukhothai fold belt, northern Thailand

A structural and metamorphic study across the Sukhothai fold belt in Northern Thailand provides new evidence for testing many of the tectonic models currently proposed to explain the geological evolution of this part of Southeast Asia. A brief study was also made of the structure and metamorphism of the Doi lnthanon metamorphic complex located about 80 km to the west of the fold belt. The oldest rocks recognised in the Sukhothai fold belt, particularly the eastern part, are arc volcanics and a subduction-accretion complex sequence represented by the Fermo-Carboniferous Pha Som Metamorphic Complex. This complex consists of a metasedimentary unit and a serpentinite melange unit (ophiolite association). The metasedimentary unit is characterised by a multiplydeformed, coherent sequence of metagreywackes and minor phyllites. The metagreywackes are interpreted to have been derived from two different sources, i.e. an accretionary complex/continental magmatic arc source and a volcanic arc source. The serpentinite melange unit consists dominantly of blocks of igneous and meta-igneous rocks that originated in an oceanic arc setting together with less abundant backarc and oceanic within-plate counterparts. Greywacke, argillite, chert and limestone blocks are minor components. Four structural phases were recognised in the coherent metasedimentary unit. The earliest recognisable structure is compositional layering (S 1). D2 structures include tight F2 folds, differentiated layering/phyllitic cleavage (S2) and a stretching lineation (L2). The Dl and D2 structures are interpreted to be the results of deformation in a thrust environment during which diffusional mass transfer was the dominant deformation mechanism. The D1-D2 events are consistent with subduction-accretion models. D3 produced open folds with associated crenulations and crenulation cleavage. D4 kinks and angular folds are probably related to thrusting. A single metamorphic episode was recognised in the metasedimentary rocks. The P-T estimates are constrained via thermodynamic calculations and bo and illite crystallinity studies. A newly calibrated phengite-chlorite geothermometer, applied to this suite of rocks, yields temperatures in the range 300-400 °C. The b0 values of the pelitic rocks are indicative of a low to medium pressure facies series whereas petrogenetic grids constrain the pressures to between 3 and 6 kbar. The peak metamorphic condition was attained during D2. Blocks of igneous rocks within the serpentinite melange have been variably metamorphosed to sub-greenschist through to middle amphibolite facies. A Permian sequence (the Ngao or Phrae Group), distributed mainly in the eastern and central part of the fold belt, is characterised by turbidites and limestones probably deposited in a forearc setting. The deposition of the forearc sediments may have persisted into the Triassic, during which pericxi the upper forearc volcaniclastic sediments and ramp carbonates (the Lampang Group) dominated. Petrographic and geochemical data support the previous interpretations that the Triassic turbiditic sandstones were derived from a volcanic arc source. Only one phase of folding with associated thrusting was recognised in the Permian and Triassic sequences. This deformation event resulted in the upright to inclined folds with axial-plane cleavage, and thrusts. The timing of folding and thrusting is constrained by the age of the youngest rock unit affected by this deformation event (i.e. the middle Carnian-lower Norian Wang Chin Formation) and the age of post-kinematic granites (Late Triassic-Early Jurassic). Metamorphic grades of the Permian and Triassic rocks, based on the illite crystallinity values of pelitic rocks, increase from west to east (i.e. from diagenetic-lower anchizone grade to upper anchizone-epizone grade). The Jurassic strata, east of the Sukhothai fold belt, contain post-orogenic continental sediments. Petrographic criteria and observed sedimentary features suggest that the Jurassic sandstones (the Phra Wihan Formation) were derived from a quartzose recycled orogenic source or craton interior and deposited in fluvial plains. Late Triassic-Early Jurassic S-type granites are widespread in northern Thailand and post-date the regional cleavage in the Permian and Triassic sequences. The Doi Inthanon metamorphic complex is a mantled gneiss dome which is characterised by core orthogneisses and mantling paragneisses detached from the Lower Palaeozoic cover rocks by a low-angle fault surface. The gneisses have been intruded by S-type granites of Late Triassic-Early Jurassic and younger ages. Compositional layering and fQlds in the gneisses are widely overprinted by mylonitic fabrics subparallel to the layering. The compositional layering and folds are likely to be the result of a compressional event as suggested by inclined isoclinal folds in similar gneissic rocks further south, e.g. the Bhumipol Dam area. At this stage, it is uncertain whether this compressional event is related to the Late Triassic folding and thrusting event in the Sukhothai fold belt or some other older events. On the other hand, mylonitisation of the gneisses is probably due to CretaceousMiddle Tertiary t~ctonic unroofing and uplift. The peak metamorphism of the gneissic rocks occurred under low P - high T conditions (i.e. around 4 kbar and 700 °C) which inay be related to the extensive Late Triassic-Early Jurassic S-type granitic intrusions. The peak metamorphism was overprinted by a later retrograde event that is contemporaneous with mylonitisation. During Late Carboniferous to Pennian times, the tectonism of northern Thailand was dominated by subduction and arc volcanism. The accretion of trench and ocean basin sediments and igneous rocks occurred during this time interval and eventually culminated in the collision between the Shan-Thai and the Indochina terranes in Late Triassic. The collision phase is best exemplified by folding and thrusting of the Triassic turbiditic sequences followed by extensive intrusions of Late Triassic-Early Jurassic S-type granites and post-orogenic Jurassic-Cretaceous continental redbeds. There has been extensive mid-Tertiary faulting which resulted in the formation of widespread extensional basins in northern Thailand.