Mae Ping Fault Research Articles

U–Pb dating for zircons from granitic rocks in southwestern Cambodia

U–Pb dating was conducted for zircons from a total of 14 samples from 13 granite bodies in southwestern Cambodia using LA-ICP-MS. The granitic rock samples were collected from southwestern Cambodia, southwest of the Mae Ping Fault extending from northwest Cambodia via Tonle Sap Lake to southern Vietnam. The studied rock bodies belong to the ilmenite-series, except for three granitic rock bodies. They were identified as I-or A-type. The analysis yielded three distinct age ranges: 295–309, 191–232, and 75–98 Ma. The 295–309 Ma ages are associated with the Paleo-Tethys Sea subduction beneath the Indochina Block. The ages of 191–232 Ma may correspond to the amalgamation period of the Sibumasu and Indochina Blocks during the Indosinian Orogeny. Granitic rocks with ages of 75–98 Ma occur near the southeastern Cambodia-southern Vietnam border. Formation of these granitic rocks was associated with the Paleo-Pacific Ocean Plate (the Izanagi Plate) subduction beneath the Indochina Block. The region in which these granitic rocks occur is part of the Dalat–Kratie Zone.

Geochemical and radiogenic isotopic signatures of granitic rocks in Chanthaburi and Chachoengsao provinces, southeastern Thailand: Implications for origin and evolution

The Chanthaburi, Pliew, Klathing, Khao Cha Mao, and Khao Hin Son granitic bodies in Chanthaburi and Chachoengsao provinces in southeastern Thailand, which are located on the southwestern side of the Mae Ping Fault and eastern side of the Klaeng Fault, were investigated. In this study, magnetic susceptibility measurements, whole-rock chemical composition and Nd-Sr isotope analyses, and zircon U-Pb dating were conducted on these granitic bodies. The surveyed granitic rocks are classified as I- to A-type granites, are of the ilmenite series, and show clearly negative Eu anomalies, which suggest they formed under reducing conditions. Nd-Sr isotope ratios indicate continental crust material involvement in the formation of these granite bodies. The magnetic and geochemical signatures are similar to those of granite bodies in southwestern Cambodia. The study area is thus considered an extensional area of southwestern Cambodia, corresponding to the Sukhothai Zone (the Chanthaburi-Kampong Chhnang Zone). Zircon U-Pb dating yields ages of 208–214 Ma (the Late Triassic) for granite bodies except for the Khao Cha Mao granitic body, which dates to 55 Ma. The former age corresponds to the collision time of the Sibumasu and Indochina terranes, and the latter age is likely related to the collision time of the Indian and Eurasian continents.

Zircon Hf‐isotope constraints on the formation of metallic mineral deposits in Thailand

AbstractThe effects of the Sibumasu–Indochina Terranes collision created several kinds of mineral deposits in Thailand, which include porphyry–skarn copper–gold, epithermal gold and antimony, orogenic gold–antimony–tungsten and tin‐tungsten mineralization among others. The deposits show a distinct spatial zonal distribution and occur in specific tectonic terranes. Combining regional geological data and ore deposit distribution data with Hf‐isotopic data of zircons in igneous rocks can be used to investigate the relationship between crustal construction processes and metallogeny. In this study, we investigated the Sukhothai Fold Belt, which is composed of quartz monzodiorite, granodiorite, syenogranite, and monzogranite of I‐ and S‐type affinities. All granitoids were analyzed for zircon U–Pb geochronology and Lu–Hf isotopic analysis. The granitoids of the Sukhothai Fold Belt yielded U–Pb zircon ages ranging from ~243 to 202 Ma, which mark the timing of subduction to the syn‐collisional stage between the Sibumasu–Indochina terranes at ~243–237 Ma and the timing of post‐collision between the Sibumasu–Indochina terranes during 230–202 Ma. In addition, an age of ~43 Ma in the south of the Sukhothai Fold Belt may indicate intrusion during the sinistral movement of the Klaeng and Mae Ping fault zones resulted from the Indian–Eurasian plate collision. The Doi Tung quartz monzodiorite provided an age of ~350 Ma as a timing of formation of the Sukhothai Fold Belt. The negative and positive initial εHf values (−8.0 to +9.2) with two‐stage depleted mantle model ages (TDMC of 2.2–0.6 Ga) of zircons from the Sukhothai Fold Belt granitoids indicate that the sources of their magma derived from partial melting of old continental crust and young oceanic crust, which probably mixed with a mantle‐derived magma. A zircon Hf‐isotope compilation including the data obtained in this study and previously reported values was used to prepare a map that allows a comparison between magmatic source and mineral deposit distribution in Thailand. The spatial distribution of Hf isotopic data reveals a distinct zonation, with initial εHf values decreasing from the east to the west, that is, from the western margin of the Indochina Terrane or the Loei Fold Belt to the Sukhothai Fold Belt, the Inthanon Zone and the Sibumasu Terrane. The magmatic source for the granitoids in the Loei Fold Belt is dominated by mantle‐derived components, as shown by positive average initial εHf values (+1.0 to +12.7), and contributed to porphyry‐related skarn copper–gold and iron and epithermal gold mineralization. In contrast, magmas in the Sibumasu Terrane and the Inthanon Zone originated from melting of old crustal materials, as indicated by mostly negative average initial εHf values (−15.1 to +0.8), and are responsible for S‐type granite‐related tin‐tungsten mineralization. The average initial εHf values (−5.0 to +11.0) from the intrusions in the Sukhothai Fold Belt suggest mixed sources, including evolved and juvenile magmatic materials, which generated the orogenic gold deposits and other vein‐type antimony, tungsten, fluorite, and base metal deposits. These results imply a close spatial correlation between the source of magmatism in each tectonic terrane of Thailand and different metal ore deposits. These isotope maps can be used as a powerful tool in the exploration for various commodities at the regional scale.

Origin of synchronous extension and inversion in a rift basin: The Phitsanulok Basin, central Thailand

Abstract The Phitsanulok Basin is a Cenozoic rift basin in central Thailand. The basin opened in the Oligocene to Early Miocene with the development of N-S and NNW-SSE trending normal faults. In the Middle Miocene the northern part of the basin continued to experience extension, while the southern part was affected by compressive inversion. Extension resumed in the Late Miocene to Pliocene, followed by post-rift subsidence in the Quaternary. The Middle Miocene anomalous deformation is attributed to activities of the NE-SW Uttaradit Fault and NW-SE Mae Ping Fault, respectively at the northern and southern basin margins. Both faults had limited activities during the Oligocene to Early Miocene rifting, where E-W extension was probably accompanied by vertical maximum principal stress. In the Middle Miocene, the maximum principal stress rotated to a N-S direction and produced sinistral motion on the Uttaradit Fault and dextral motion on the Mae Ping Fault. The strike-slip motions contributed to a counter-clockwise rotation of the continental block west of the Phitsanulok Basin, creating the synchronous extension and contraction. The kinematic model introduces the possibility of local inversion in a rift basin being originated from short-term switch between the maximum and intermediate principal stress axes and reactivation of long-lived strike-slip faults, as opposed to major change in regional stress related to plate movements. The model implies that the segment of the Mae Ping Fault south of the Phitsanulok Basin was inactive during the Oligocene to Early Miocene and was reactivated as a dextral fault in the Middle Miocene.

Cenozoic tectonic evolution of southeastern Thailand derived from low-temperature thermochronology

Low-temperature thermochronological techniques, specifically apatite (U–Th)/He and apatite fission-track dating, were used to reconstruct the thermal history of southeastern Thailand. This area is intersected by vast and complex fault networks related to the Cenozoic Mae Ping and Three Pagodas Faults. These were identified from satellite imagery and confirmed by field observations. New apatite fission-track and apatite (U–Th)/He data were collected from crystalline basement blocks within these fault networks. Ages obtained range from 48 to 24 Ma, with most of the samples clustering between 36 and 24 Ma. Thermal history modelling indicates late Eocene–Oligocene exhumation of the exposed granitic and metamorphic basement rocks in southeastern Thailand. Exhumation was regional and was contemporaneous with sinistral fault activity during the late Eocene–early Oligocene along the Mae Ping Fault and Three Pagodas Fault. Moreover, this exhumation occurred coevally with a synrift phase of intracontinental offshore rift basin and half-graben basin development in the eastern Gulf of Thailand. The phase of exhumation ended in the early Miocene, as a result of the changing plate-tectonic forces along the complex plate boundaries of Sundaland. Supplementary material: AFT data, radial plots and the detailed procedure of thermal history modelling are available at https://doi.org/10.6084/m9.figshare.c.4633064

Petrogenesis and tectonic setting of Late Paleozoic to Late Mesozoic igneous rocks in Cambodia

Emplacement of igneous rocks from the Carboniferous to the Middle Jurassic of Cambodia was controlled by the Indosinian Orogeny, which was related to the sequence of suturing and collision between the continental fragments from Gondwana. To constrain magma genesis, tectonic setting, and relationship with igneous rocks in neighboring countries, we determined the magnetic susceptibility, chemical compositions, Rb-Sr isochron ages and Sr-Nd isotope ratios for igneous rock samples from Cambodia. Igneous rocks in Cambodia can be divided by the inferred Mae Ping fault into magnetite series in the NE region and ilmenite series in the SW region. The igneous rocks in the NE region showed I-type or A-type and metaluminous signatures, and strong involvement of mantle source materials. They are subdivided into older and younger adakitic rocks and non-adakitic rocks. The igneous rocks in the SW region showed non-adakitic and slightly metaluminous to peraluminous signatures. These have significant involvement of source materials with relatively uniform Nd isotope ratios but variable Sr isotope ratios, which may have originated from the lower to middle continental crust. The Late Triassic to Middle Jurassic igneous rocks were formed by the subduction of the Paleo-Tethys Ocean crust associated with the collision of the Sibumasu Terrane with the Indochina Terrane (the Indosinian Orogeny II). The younger granitic rocks showed A-type signature and are regarded as post-orogenic, within the plate or rifting zone that intruded in association with the subduction of the Paleo-Pacific Ocean crust beneath Indochina during the Cretaceous to the Paleogene.

Tectonic evolution and regional setting of the Cuu Long Basin, Vietnam

The Cuu Long Basin is situated off the southeastern coast of Vietnam. This study utilizes all available seismic data in the Cuu Long Basin, Vinh Chau Basin and coastal Mekong Delta to constrain the tectonic history of the area. The linkage between the Mae Ping Fault Zone and deformation in the Cuu Long Basin is also examined in detail. Two major deformational events are observed: a transtensional rifting event from the mid Eocene to mid-Early Oligocene (40–31 Ma) followed by transpression from mid-Early Oligocene to mid-Late Oligocene (31–25 Ma). The data strongly suggest that all the very straight major tributaries of the Mekong River Delta follow northwest trending faults that link into to the southwestern end of the Cuu Long Basin fault system and that the basin opened between where the Mekong Delta faults enter and a point of zero extension at the NE termination of the basin. The Mekong Delta Fault Zone exhibited left-lateral displacement during extension in the Cuu Long Basin and right-lateral displacement during younger transpression. The orientation, age and sense of displacement of the Mekong Delta Fault Zone are consistent with slip on the Mae Ping Fault Zone in Thailand and Cambodia, and hence are considered to be part of the same regional fault system. Based on a Chainat Duplex analogue and other data, the Mekong Delta and Mae Ping fault zones appear to be linked by a NNW-striking step-over zone, referred to here as the Mekong-Ton Le Sap step-over zone. This zone follows the eastern boundary of the Paleocene to Early Eocene Kampot Fold Belt. Based on structural and stratigraphic similarities, it appears the adjacent Vinh Chau Basin shares a common tectonic history with the Cuu Long Basin, but the linkage to the Mekong Delta Fault Zone is less clear.

Permian–Triassic back-arc basin development in response to Paleo-Tethys subduction, Sa Kaeo–Chanthaburi area in Southeastern Thailand

The Sa Kaeo Back-arc Basin in Southeastern Thailand developed behind the Sukhothai Arc along the western margin of the Indochina Block during the Permian to Triassic in response to the subduction of the Paleo-Tethys oceanic plate. To clarify its tectonic evolution of spreading, basin-infilling, and closure, we have investigated the provenance of its clastic sediments, the U-Pb ages of detrital zircons, the geochemistry of its basaltic rocks, and the structures in mélange. The rocks in the Sa Kaeo Back-arc Basin include two contrasting lithological units: the Thung Kabin Mélange, composed mainly of basaltic rocks, chert, limestone, and sandstone, along with mélange; and the Pong Nam Ron Formation, a thick succession of clastic sediments. Our observations suggest that the clastic sediments of the Pong Nam Ron Formation were deposited widely over the oceanic rocks (basaltic rocks, chert, and limestone) of the Thung Kabin Mélange. Geochemical analyses indicate that sandstones in both units (the Thung Kabin Mélange and the Pong Nam Ron Formation) are characterized by a provenance of mostly basaltic rocks. The U-Pb ages of detrital zircons from both units present similar age distributions: the youngest single-grain zircon (YSG) is early Late Triassic in age, the youngest cluster of zircons (YC1σ) is latest Permian to earliest Triassic in age, and there is a range of other Paleozoic and Proterozoic zircons. Based on these geochemical analyses and the U-Pb dating, the sandstones of both units had the same provenance. In addition, the basaltic rocks of the Thung Kabin Mélange are originated from back-arc basin basalts. These new observations have allowed us to reconstruct the tectonic evolution of the basin. In the Early Permian, spreading took place in the basin accompanied by basaltic magmatism. Subsequently, corresponding to an inactive period of the Sukhothai Arc into the Middle Triassic, the Sa Kaeo Back-arc Basin subsided, and a deep-sea environment prevailed with the formation of Permian cherts, Permian limestones, and Middle Triassic cherts. During the Triassic, the basin was filled by clastic sediments and eventually closed. The clastic sediments were sourced directly from the back-arc basin basalts, as well as from the felsic volcanic rocks of the active Sukhothai Arc (which yield Permian to Triassic zircons), and from the continental materials of the Indochina Block (Paleozoic and Proterozoic zircons). After the closure of the Sa Kaeo Back-arc Basin, the discussed area was affected by the termination fault-fan related to the Mae Ping Fault Zone.

Chapter 21 The Kyaukkyan Fault, Myanmar

The Kyaukkyan Fault is an active dextral strike-slip structure that passes 510 km north–south across the western Shan Plateau (e.g. Chhibber 1934; Le Dain et al. 1984; Wang et al. 2014) (Fig. 21.1a). It lies broadly parallel to and about 100–150 km to the east of the central Sagaing Fault. Unlike the rather discrete Sagaing Fault, the Kyaukkyan Fault is characterized by a broad array of splaying segments and basins, dominated by the Inle Lake releasing bend and associated extensional fault systems (Fig. 21.1b). In the north the fault terminates within the complex intersection between the sinistral Kyaukme and Momeik faults, the largely inactive Shan Scarp Fault Zone and the Sagaing Fault. In the south the fault curves to the SW and links with the Mae Ping Fault in Thailand, which itself terminates as it passes east into Cambodia and offshore into the Gulf of Thailand (e.g. Lacassin et al. 1997; Morley 2004; Morley et al. 2011). Several other faults of the western Shan Plateau, such as the Nampun and Taungoo faults, also converge with the western Mae Ping Fault in the region of Papun, indicating that the Mae Ping Fault dissipates or transfers much of the dextral strain of the western Shan Plateau. Fig. 21.1. Kyaukkyan Fault location and overview. ( a ) Tectonic setting of the Kyaukkyan Fault, modified after Morley et al. (2011), Soe Thura Tun & Maung Thein (2012), Wang et al. (2014), Morley & Alvey (2015) and Soe Thura Tun & Watkinson (2017). ( b ) Main faults making up the Kyaukkyan Fault. Solid line: confident interpretation; dashed line: uncertain interpretation; dotted line: other related faults. Earthquake locations from the NEIC and IRIS catalogues 1972–2010. Map location shown in (a). Although it has been devoid of large seismic events for over …

Chapter 16 Karen–Tenasserim Unit

In Bender's comprehensive account of the geology of Myanmar (Bender 1983), he introduced the term Karen–Tenasserim Unit (KTU) for a fault-bounded block extending from the central area southwards to the southern tip of Myanmar. He recognized its stratigraphic distinctiveness from regions to the east (including the Shan Plateau), interpreting the sedimentary rocks of the unit as essentially deep-water deposits, whereas those east of the unit (i.e. his East Kachin–Shan Unit) he saw essentially as shelf deposits. In Thailand, Ridd (2009) identified the Phuket Terrane as a stratigraphically distinct fault-bounded tectono-stratigraphic block that was later traced northwards into Myanmar (Fig. 16.1), adopting the name Phuket-Slate Belt Terrane for the combined entity (Ridd & Watkinson 2013). It is clear that the Phuket-Slate Belt Terrane is the same as Bender's Karen–Tenasserim Unit (with some differences, involving the bounding faults); in recognition of Bender's (1983) priority his term is adopted here. Fig. 16.1. The Karen–Tenasserim Unit of central Myanmar, showing its relations with the equivalent tectono-stratigraphic block in Thailand, the Phuket Terrane. Ridd (2016) has proposed that the boundary of the Phuket Terrane in southern Thailand is not the Khlong Marui Fault as previously thought (Ridd 2009), but is a line further east through Surat Thani. Bender (1983) observed that the unit is bounded on its east by the Panlaung Fault as far south as latitude c. 19° 45′ N, by splays of the Mae Ping Fault Belt beyond that into Thailand, and on its west by what he called the Shan Boundary Fault, but which is now known widely as the Sagaing Fault. Later work (Ridd 2009; Ridd & Watkinson 2013) showed the eastern bounding fault on the Thai border to be the Three Pagodas Fault, and not the Mae Ping Fault Belt. Early accounts of the geology of this belt of country, west …

Chapter 12 Tectonic and metamorphic evolution of the Mogok Metamorphic and Jade Mines belts and ophiolitic terranes of Burma (Myanmar)

The Mogok Metamorphic Belt is a north–south-aligned belt of mainly high-grade metamorphic rocks and granites that extends the length of Burma (Myanmar) along the western margin of the Sibumasu block from the Andaman Sea to the East Himalayan (Namche Barwa) Syntaxis (Figs 12.1 & 12.2). The Mogok metamorphic rocks host some of the world's finest rubies and sapphires, the former mainly from high-grade phlogopite + corundum marbles and the latter from a heterogeneous suite of syenite and charnockites that intrude the metamorphic rocks (Chhibber 1934 a , b ; Iyer 1953; Searle & Haq 1964; Bender 1983; Mitchell et al. 2007; Searle et al. 2007; Themelis 2008). Gem spinels are also found in forsterite-bearing marbles. Gemstones have been mined in the Mogok region since at least the mid-1800s (O'Connor 1888; Gubelin 1965 a , b ; Keller 1982, 1983; Kane & Kammerling 1992). Together with the adjacent Mergui Slate Belt these rocks lie to the west of the Shan Scarp and the Paung Laung-Mawchi Zone, interpreted by Mitchell et al. (2012) as a possible suture zone. We refer to this entire zone of metamorphic rocks, plus the Slate Belt and pre-collisional granites east of the Sagaing Fault and west of the Paung Laung-Mawchi Zone, as the Mogok–Mandalay–Mergui (MMM) Belt; the metamorphic rocks are referred to solely as the Mogok Metamorphic Belt (MMB). Fig. 12.1. Geological terrane map of the Eastern Himalaya, SE Tibet, Burma (Myanmar), Yunnan and Thailand. Lines of cross-sections in Figures 12.2 and 12.5 are shown. NLB, North Lhasa Block; SLB, South Lhasa Block; ITS, Indus-Tsangpo Suture Zone; GHS, Greater Himalayan sequence; LHS, Lesser Himalayan sequence; SH, Shillong Plateau; IBR, Indo-Burman Ranges; CB, Central (Chindwin) Basin; SFZ, Sagaing Fault Zone; TPFZ, Three Pagodas Fault Zone; MPFZ, Mae Ping Fault Zone; ST, Shan-Thai …

Did Oligocene crustal thickening precede basin development in northern Thailand? A geochronological reassessment of Doi Inthanon and Doi Suthep

The Doi Inthanon and Doi Suthep metamorphic core complexes in northern Thailand are comprised of amphibolite-grade migmatitic gneisses mantled by lower-grade mylonites and metasedimentary sequences, thought to represent Cordilleran-style core complexes exhumed through the mobilization of a low-angle detachment fault. Previous studies have interpreted two metamorphic events (Late Triassic and Late Cretaceous), followed by ductile extension between the late Eocene and late Oligocene, a model which infers movement on the detachment at ca. 40Ma, and which culminates in a rapid unroofing of the complexes in the early Miocene. The Chiang Mai Basin, the largest such Cenozoic Basin in the region, lies immediately to the east. Its development is related to the extension observed at Doi Inthanon and Doi Suthep, however it is not definitively dated, and models for its development have difficulty reconciling Miocene cooling ages with Eocene detachment movement. Here we present new in-situ LA-ICP-MS and SIMS U–Pb age data of zircon and monazite grains from gneiss and leucogranite samples taken from Doi Inthanon and Doi Suthep. Our new zircon data exhibit an older age range of 221–210Ma, with younger ages of ca. 72Ma, and 32–26Ma. Our monazite data imply an older age cluster at 83–67Ma, and a younger age cluster of 34–24Ma. While our data support the view of Indosinian basement being reworked in the Cretaceous, they also indicate a late Eocene–Oligocene tectonothermal event, resulting in prograde metamorphism and anatexis. We suggest that this later event is related to localized transpressional thickening associated with sinistral movement on the Mae Ping Fault, coupled with thickening at the restraining bend of the Mae Yuan Fault to the immediate west of Doi Inthanon. Further, this upper Oligocene age limit from our zircon and monazite data would imply a younger Miocene constraint on movement of the detachment, which, when combined with the previously recorded Miocene cooling ages, has implications for a model for the onset of extension and subsequent development of the Chiang Mai Basin in the early mid-Miocene.

Chapter 2 Regional tectonic setting of Myanmar's petroleum basins

The geology of the petroleum-bearing parts of Myanmar (essentially the western part of the country) is dominated by the oblique collision between the fixed Sunda Plate in the east (itself a mosaic of Gondwana-derived blocks, and since the Mesozoic a part of the much larger Asian or Eurasian Plate; Fig. 2.1) and the north-moving India Plate (e.g. Curray et al. 1979; Mitchell 1993; Curray 2005; Socquet et al. 2006; Steckler et al. 2008; Hall 2012; Rangin et al. 2013). Fig. 2.1. Present-day tectonic terranes which make up Myanmar. The Sunda Plate comprises a number of blocks which had amalgamated by the end of the Triassic, subsequently distorted by strike-slip faults and rotations. The present-day Burma Platelet acts as a buffer between the northwards ongoing movement of the India Plate and the relatively static Sunda Plate, and is responding in part by dextral movement on the north–south Sagaing Fault, shown by arrows. The magmatic arc (shown in red) is the Cretacous–Recent Popa-Taungthanlon arc; other older magmatic arcs are omitted. KF: Kabaw Fault; PF: Panlaung Fault; SF: Sagaing Fault; MPF: Mae Ping Fault; TPF: Three Pagodas Fault; KMF: Khlong Marui Fault. Before considering the origin of the component parts of Myanmar it is necessary to identify the main structural units that now comprise the western part of the country. Figure 2.1 shows the importance of the Sagaing Fault and its possible offshore continuation in separating the Burma Platelet from the Sunda Plate to its east. Here we follow the terminology of Rangin et al. (2013) in choosing to call this the Burma Platelet, although earlier it was called the Burma Plate by Curray et al. (1979), the Indo-Burma-Andaman Block by Acharyya (1998) and the West Burma Plate by others (e.g. Hall 2012; Metcalfe 2013). The …

Deformation styles of the Uthai Thani-Nakhon Sawan ridge within the Chainat duplex, Thailand

Uthai Thani-Nakhon Sawan structural ridge is the 35 km mountain length lying in N-S direction within the Chainat duplex at the Central Thailand. The ridge, which is clearly seen from satellite image in the flat areas of the Central Plain, is composed of Triassic mylonitic granite and Silurian-Devonian metamorphosed sedimentary rocks. Satellite image interpretation, geological mapping, field data and microstructure analysis have been integrated for the understanding of structural style of Uthai Thani-Nakhon Sawan ridge. The rocks within the ridge had been deformed under sinistral ductile shear which was shown by foliations, stretching lineations, S-C fabrics, sigmoid clasts, mineral fish and S-folded both in outcrops and under microscope. The major N-S, minor NNW-SSE striking ridges and foliation planes indicate the paleostress of σ 1 in the NW-SE direction. The major sinistral motion of this region probably ended at ∼30 Ma. All structural elements and deformation style of Uthai Thani-Nakhon Sawan ridge relate to the Chainat duplex geometry and Mae Ping fault zone activity during Tertiary.

Timing of metamorphism of the Lansang gneiss and implications for left-lateral motion along the Mae Ping (Wang Chao) strike-slip fault, Thailand

The Mae Ping fault (MPF), western Thailand, exhibits dominantly left-lateral strike-slip motion and stretches for >600km, reportedly branching off the right-lateral Sagaing fault in Myanmar and extending southeast towards Cambodia. Previous studies have suggested that the fault assisted the large-scale extrusion of Sundaland that occurred during the Late Eocene–Early Oligocene, with a geological offset of ∼120–150km estimated from displaced high-grade gneisses and granites of the Chiang Mai–Lincang belt. Exposures of high-grade orthogneiss in the Lansang National Park, part of this belt, locally contain strong mylonitic textures and are bounded by strike-slip ductile shear zones and brittle faults. Geochronological analysis of monazite from a sample of sheared biotite-K-feldspar orthogneiss suggests two episodes of crystallization, with core regions documenting Th–Pb ages between c. 123 and c. 114Ma and rim regions documenting a significantly younger age range between c. 45–37Ma. These data are interpreted to represent possible magmatic protolith emplacement for the Lansang orthogneiss during the Early Cretaceous, with a later episode of metamorphism occurring during the Eocene. Textural relationships provided by in situ analysis suggest that ductile shearing along the MPF occurred during the latter stages of, or after, this metamorphic event. In addition, monazite analyzed from an undeformed garnet-two-mica granite dyke intruding metamorphic units at Bhumipol Lake outside of the Mae Ping shear zone produced a Th–Pb age of 66.2±1.6Ma. This age is interpreted to date the timing of dyke emplacement, implying that the MPF cuts through earlier formed magmatic and high-grade metamorphic rocks. These new data, when combined with regional mapping and earlier geochronological work, show that neither metamorphism, nor regional cooling, was directly related to strike-slip motion.

Development of the Khao Khwang Fold and Thrust Belt: Implications for the geodynamic setting of Thailand and Cambodia during the Indosinian Orogeny

The Indosinian Orogeny in Thailand is often viewed as having developed between strongly linear terranes, which today trend approximately N–S. The terranes were subsequently disrupted by later tectonics, particularly NW–SE trending Cenozoic strike-slip faults. The ENE–WSW to NE–SW striking thrusts and folds in the Khao Khwang Platform area of the Saraburi Group on the SW margin of the Indochina Terrane are not easily explained in the context of this traditional view. Reversal of the clockwise rotation shown to have affected the block north of the Mae Ping Fault zone only enhances the E–W orientation of structures in the fold and thrust belt, and moves the belt further east towards Cambodia. One solution for the trend that fits better with regional understanding from hydrocarbon exploration of the Khorat Plateau is that the Indochina Terrane was actually a series of continental blocks, separated by Permian rifting. During the Early Triassic the early stages of collision (South China-Cathaysian Terrane collision with Vietnam Indochina) resulted in the amalgamation of disparate blocks that now form the Indochina Terrane by closure along the rifts. At the same time or following on from the collision there was closure of the back-arc area between Indochina and the Sukhothai zone. The rift basins, were thrusted and inverted during the early stages of the Indosinian orogeny, and only underwent minor reactivated when later Sibumasu collided with Sukhothai Zone-Indochina Terrane margin during the Late Triassic. The scenario described above requires the presence of a (minor) E–W trending suture in NW Cambodia. Evidence for this suture is suggested by the presence of Permo-Triassic calc-alkaline volcanism.

The Khao Yai Fault on the southern margin of the Khorat Plateau, and the pattern of faulting in Southeast Thailand

A study of Google Earth images has revealed a hitherto-unrecorded gently curved lineament within the southern marginal zone of the Khorat Plateau in eastern Thailand. The lineament, confirmed by digital elevation model (DEM) images, is at least 130 km long and coincides with a dip reversal of the Mesozoic Khorat Group. It is interpreted here as a fault, named the Khao Yai Fault, and it has characteristics which make it unusual within the Khorat Plateau. The fault forms the northern boundary of a belt of several ENE–WSW trending fault splays which are thought to link with the Mae Ping Fault further south; this is interpreted as a left-stepping, sinistral strike-slip duplex about 50 km wide and 150 km long. Apatite fission track data indicate that exhumation began during the earliest Palaeogene. The Khao Yai Fault is considered in its regional context which includes the Cardamomes Mountains of Cambodia, the offshore Phuquoc-Kampot Basin, and the Khao Thalai Red-beds outlier of the Khorat Group in Southeast Thailand. The latter is interpreted as a down-faulted sliver of the Khorat Group in the Tha Mai Fault belt which is thought, in turn, to be a splay of Thailand's other major regional fault, the Three Pagodas Fault. Carboniferous, Permian and Triassic shallow-marine rocks with unusual faunas occur in a limited NNW–SSE trending zone to the west and NNW of the Tha Mai Fault and it is suggested that wrench movement on the fault played a part in the emplacement of these rocks.

Palaeocene–early Eocene inversion of the Phuquoc–Kampot Som Basin: SE Asian deformation associated with the suturing of Luconia

Abstract: The little explored Cambodian and Vietnamese Phuquoc–Kampot Som Basin is a Late Jurassic to Early Cretaceous foreland basin developed in response to the build-up of a palaeo-Pacific magmatic arc. A combination of seismic data, well data and outcrop geology complemented by fission track and U/Pb analysis is used to unravel the basin history. This reveals a hitherto unknown earliest Palaeogene basin inversion associated with the Luconian suturing to SE Asia and the shutdown of palaeo-Pacific subduction underneath SE Asia. The Phuquoc–Kampot Som Basin and the Khorat Basin in Thailand constitute the erosional remnants of a larger basin that covered large parts of SE Asia in Late Mesozoic time, and subsequently became segregated during earliest Palaeogene inversion and erosion. Inversion was focused along the several hundred kilometres long Kampot and Khmer–Chanthaburi fold belts that confine the Phuquoc–Kampot Som Basin and merge with the Mae Ping and the Three Pagodas fault zones. These connections, together with local NW–SE-trending sinistral transpressional faults offshore, indicate a link between initial SE Asian left-lateral strike-slip faulting and the Luconian suturing. The separation between the once unbroken Khmer–Chanthaburi Fold Belt and the Phetchabun Fold Belt in Thailand suggests a 50–100 km Cenozoic left-lateral offset across the Mae Ping Fault Zone.

The kinematic history of the Khlong Marui and Ranong Faults, southern Thailand

The Khlong Marui Fault (KMF) and Ranong Fault (RF) are major NNE-trending strike-slip faults which dissect peninsular Thailand. They have been assumed to be conjugate to the NW-trending Three Pagodas Fault (TPF) and Mae Ping Fault (MPF) in Northern Thailand, which experienced a diachronous reversal in shear sense during India–Eurasia collision. It follows that the KMF and RF are expected to show the opposite shear sense and a slip sense reversal at a similar time to the TPF and MPF. New field data from the KMF and RF reveal two phases of ductile dextral shear separated by Campanian magmatism. Paleocene to Eocene post-kinematic granites date the end of this phase, while a brittle sinistral phase deforms the granites, and has exhumed the ductile fault rocks. The timing of these movements precludes formation of the faults in response to Himalayan extrusion tectonics. Instead, they formed near the southern margin of a Late Cretaceous–Paleocene orogen, and may have been influenced by variations in the rate of subduction ahead of India and Australia. North-south compression prior to reactivation of the subduction zone around southern Sundaland in the Eocene caused widespread deformation in the over-riding plate, including sinistral transpression on the KMF and RF.

Evolution of deformation styles at a major restraining bend, constraints from cooling histories, Mae Ping fault zone, western Thailand

Abstract The c. 500-km-long Mae Ping fault zone trends NW–SE across Thailand into eastern Myanmar and has probably undergone in excess of 150 km sinistral motion during the Cenozoic. A large, c. 150-km-long, restraining bend in this fault zone lies on the western margin of the Chainat duplex. The duplex is a low-lying region dominated by north–south-trending ridges of Mesozoic and Palaeozoic sedimentary, metamorphic and igneous rocks, flanked by flat, post-rift basins of Pliocene–Recent age to the north and south. A review of published cooling-age data, plus new apatite and zircon fission-track results indicates that significant changes in patterns of exhumation occurred along the fault zone with time. Oldest uplift and erosion (Eocene) occurred in the Umphang Gneiss region, west of an inferred thrust-dominated restraining-bend setting. From 36 Ma to 30 Ma, exhumation was strongest north of the duplex, along the NW–SE-trending segment of the fault zone at the (northern) exiting bend of the Chainat duplex. This region of the fault zone is characterized by a mid-crustal level shear zone 5–6 km wide (Lan Sang Gneisses), that passes to the NW into an apparent strike-slip duplex geometry. The deformation is interpreted to have occurred during passage around the northern restraining bend, which resulted in vertical thickening, uplift, erosion and extensional collapse of the northern side of the shear zone. This concentration of deformation at the bends at the ends of the restraining bend is thought to be a characteristic of strike-slip-dominated restraining bends. Following Late Oligocene–Early Miocene extension, there is apatite fission-track evidence for 22–18 Ma exhumation in the Chainat duplex, that coincides with a phase of inversion in the Phitsanulok Basin to the north. The Miocene–Recent history of the Chainat duplex is one of minor sinistral and dextral displacements, related to a rapidly evolving stress field, influenced by the numerous tectonic reorganizations that affected SE Asia during that time.