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Deformation and metamorphism of the Sukhothai fold belt, northern Thailand

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Singharajwarapan, Sampan (1994) Deformation and metamorphism of the Sukhothai fold belt, northern Thailand. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Rock deformation, Metamorphism (Geology)
Copyright Holders: The Author
Copyright Information:

Copyright 1995 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1995. Includes bibliographical references (p. 302-321)

Date Deposited: 03 Feb 2015 03:22
Last Modified: 11 Mar 2016 05:55
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