Sibumasu Research Articles

Tectonic blocks and suture zones of eastern Thailand: evidence from enhanced airborne geophysical analysis

<p>Airborne geophysical data were used to analyze the complex structures of eastern Thailand. For visual interpretation, the magnetic data were enhanced by the analytical signal, and we used reduction to the pole (RTP) and vertical derivative (VD) grid methods, while the radiometric data were enhanced by false-colored composites and rectification. The main regional structure of this area trends roughly in northwest-southeast direction, with sinistral faulting movements. These are the result of compression tectonics (sigma<span style="font-size: 8px;">_1</span> in an east-west direction) that generated strike-slip movement during the pre Indian-Asian collision. These faults are cross-cut by the northeast-southwest-running sinistral fault and the northwest-southeast dextral fault, which occurred following the Indian-Asian collision, from the transpession sinistral shear in the northwest-southeast direction. Three distinct geophysical domains are discernible; the Northern, Central and Southern Domains. These three domains correspond very well with the established geotectonic units, as the Northern Domain with the Indochina block, the Central Domain with the Nakhonthai block, the Upper Southern Sub-domain with the Lampang-Chaing Rai block, and the Lower Southern Sub-domain with the Shan Thai block. The Indochina block is a single unit with moderate radiometric intensities and a high magnetic signature. The direction of the east-west lineament pattern is underlain by Mesozoic non-marine sedimentary rock, with mafic igneous bodies beneath this. The Nakhonthai block has a strong magnetic signature and a very weak radiometric intensity, with Late Paleozoic-Early Mesozoic volcanic rock and mélange zones that are largely covered by Cenozoic sediments. The boundaries of this block are the southern extension of the Mae Ping Faults and are oriented in the northwest-southeast direction. The Lampang-Chaing Rai and Shan Thai blocks, with very weak to moderate magnetic signatures and moderate to very strong radiometric intensities are dominated by marine clastic and igneous rocks or a northwest-southeast trending deformation zone of inferred Precambrian complexes, respectively. It is suggested that these tectonic plates collided against one another in a west-east direction.</p>

Early Middle Triassic mafic dikes from the Baoshan subterrane, western Yunnan: implications for the tectonic evolution of the Palaeo-Tethys in Southeast Asia

The time of final closure of the Palaeo-Tethys and the Sibumasu-Indochina collision in Southeast Asia represents a major unresolved geologic problem. Here, we present zircon chronology, whole-rock elemental, Sr–Nd, and zircon Hf isotopic geochemistry for newly discovered mafic dikes from the northern segment of the Sibumasu terrane, to provide constraints on this issue. Zircon U–Pb data indicate that the dikes were emplaced at 240 ± 3 Ma. These are the earliest Mesozoic magmatic rocks reported so far in the Sibumasu terrane, the late Palaeozoic passive margin of the Palaeo-Tethys. They are subalkaline tholeiites, showing geochemical characteristics similar to those of enriched mid-ocean ridge basalts (E-MORBs). They have 87Sr/86Sr(t) ratios of 0.703161–0.703826, ϵNd(t) of +4.8 to +7.5, and zircon ϵHf(t) of +9.2 to +13.3, implying strong mantle depletion. They were derived by partial melting of asthenospheric mantle and underwent subsequent fractional crystallization and lithospheric assimilation. The geologic–petrologic evidence suggests that the mafic dikes were generated in a collisional setting, when suturing of the Baoshan and Simao subterranes (the two subterranes are part of the Sibumasu and Indochina terranes, respectively) occurred. These early Middle Triassic mafic dikes provide an upper limit for Sibumasu–Indochina collision. In conjunction with previous work, we conclude that the final closure of the Palaeo-Tethys and collision of the Sibumasu and Indochina terranes took place during the late Permian to Early Triassic.

Taxonomy and evolution of the Triassic conodont Pseudofurnishius

Pseudofurnishius is a late Anisian (Pelsonian)—early Carnian (Cordevolian) conodont genus of gondolellid stock, characteristic for the Sephardic Province and restricted to the “Southern Tethys” region of the northern margin of Gondwana. Its most commonly found species, Pseudofurnishius murcianus, appears at the base of the Ladinian (Fassanian). The Ladinian material of Spain reveals its ontogeny characterised by initial lateral protrusions from the carina that may develop first mono-platform and later bi-platform units, finally evolving into elaborated multi-denticulated forms. The late Anisian—early Carnian phylogenesis of Pseudofurnishius priscus → P. shagami → P. murcianus is proposed. At the end of the Ladinian, Pseudofurnishius expanded to the entire “Southern Tethys” shelf and into Cimmerian terranes that drifted away from northern Gondwana, now accreted to Eurasia, such as the Sibumasu terrane (Southwest China—Malayan Peninsula).

Early Permian rugose coral Cyathaxonia faunas from the Sibumasu Terrane (Southeast Asia) and the southern Sydney Basin (Southeast Australia): Paleontology and paleobiogeography

The Sibumasu Terrane consists of allochthonous blocks located between Gondwana and Eurasia and had a tectonic evolutionary history characterized by rifting from Gondwana during the Early Permian, northward drifting during the Mid–Late Permian, and final amalgamation with Eurasia during the Triassic. The rugose coral Cyathaxonia faunas, which are typically composed of small, solitary, and non-dissepimented rugose corals, occur in the Lower Permian of the Sibumasu Terrane in Southeast Asia and the southern Sydney Basin of southeastern Australia. These Cyathaxonia faunas suggest cold or cool water shallow marine conditions, which is also supported by the occurrence of glaciomarine dropstones and diamictites in associated sediments and immediately underlying strata. Significant differences in the composition of coral faunas between the Sibumasu Terrane and the Cathaysian continent (comprising tectonic blocks such as South China and Indochina) indicate that the Sibumasu was probably located at the northern margin of Gondwana during the Early Permian, as opposed to Cathaysia then located near the Paleo-equator in the eastern Paleotethys. The Middle Permian coral faunas in the Sibumasu Terrane are dominated by both solitary and compound Waagenophyllidae. The latter is a common element in the Cathaysian continent. But, some distinctive and endemic taxa occurred in the Sibumasu Terrane during this time, suggesting that it was still an independent (isolated) paleobiogeographical entity, located at a certain distance from the Cathaysian continent. A total of eleven small solitary coral species from the Sibumasu Terrane and the southern Sydney Basin are described in detail. Of these five are new (Ufimia khaokaeonoi, Zaphrentites shidianense, Euryphyllum khaokaeoyai, E. kanchanaburi, and ?Claviphyllum baoshanense).

Evidence for middle Triassic to Miocene dual subduction zones beneath the Shan–Thai terrane, western Thailand from magnetotelluric data

Kanchanaburi province, western Thailand, is in the middle of Shan–Thai terrane. Paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the Shan–Thai terrane. During the late Triassic there was a westward subduction in the east where the Lampang-Chiang Rai block subducted beneath the Shan–Thai terrane. In addition, in the early Tertiary, the western Burma terrane subducted underneath the Shan–Thai in the west resulting in an eastward subduction. A pioneer survey of this region was therefore conducted using magnetotelluric (MT) techniques with the aim of finding deep structures associated with these ancient subduction zones. 39 MT stations were deployed to cover most of the province. Resistivity structures are obtained from the 3-D inversion. The near surface resistivity structures correspond well with the surface geology. The mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3% porosity in which the remnant dehydrated fluid was accumulated during the subduction. In addition, the westward and eastward subduction zones generated mafic/ultramafic intrusions which appear as two conductive zones on the east and west, respectively. These two conductors and the conductive lower crust support the hypotheses of the middle Triassic to Miocene dual subduction zones.

Oroclinal origin of the Simao Arc in the Shan-Thai Block inferred from the Cretaceous palaeomagnetic data

SUMMARY An active oroclinal bending is discovered in the Shan-Thai Block to the south of the eastern Himalayan syntaxis. To investigate the evolution of the Simao Arc using palaeomagnetic techniques, the Middle Cretaceous red beds of the Nanxin Formation were sampled at the Zhengwan (22.8°N, 100.9°E) and Dadugang (22.4°N, 101.0°E) localities in the southern Simao Basin. Most of the studied samples revealed the presence of characteristic remnant magnetization with unblocking temperatures around 680 °C. A primary nature for this magnetization is interpreted on the basis of a positive fold and reversal test. Tilt-corrected mean directions calculated for Zhengwan and Dadugang localities are characterized by large easterly deflected declination; D= 51.8°, I= 47.9°, ks= 45.0, α95= 6.9°, N= 11 and D= 64.1°, I= 48.1°, ks= 36.0, α95= 7.3°, N= 12, respectively. Steep inclination values at both these localities with respect to those expected are in the range previously reported from the Shan-Thai Block, confirming their southward displacement by 6.2°± 1.7° as part of the Shan-Thai Block. Combination of the present data (two localities) with those previously reported from Simao Basin (seven localities) reveals a positive palaeomagnetic oroclinal test, indicating that the present-day arc-like geometry of the Simao Basin was formed by oroclinal bending. Comparison with recent GPS and structural data suggest that formation of the Simao curvature started after the early Pliocene (after 4 Ma) and continuing until the present. Origin of the Simao Arc is ascribed to southwestward movement of the crustal material across the Ailao Shan-Red River Fault (around the eastern Himalaya syntaxis), which was formed by westward movement of the decollement with progressive eastward deepening of the Lanping-Simao Basin. Decoupling between the upper and the middle–lower crusts is a requisite condition for the arc formation on the continent.

Nd isotopic composition of Late Devonian seawater in western Thailand: Geotectonic implications for the origin of the Sibumasu terrane

Conodonts collected from sections near the small towns of Thong Pha Phum and Mae Sariang in the westernmost part of Thailand are used to reconstruct the neodymium (Nd) isotopic composition of seawater during the Late Devonian. The study provides the first Devonian seawater signatures recognized within the Australian shelf of northeastern Gondwana and the adjacent Paleotethys Ocean. At Thong Pha Phum site, the seawater was characterized by very low εNd values (from −13.1 to −18.2) and very high Sm/Nd ratios (between 0.36 and 0.66). In contrast, the seawater at the Mae Sariang site was characterized by much more radiogenic signatures (εNd values from −8.7 to −11.1) and uniform, low Sm/Nd ratios (between 0.20 and 0.23). Extremely low εNd values recognized at Thong Pha Phum attest to a passive margin continental setting and a paleogeographic position very close to a continental area where Paleoproterozoic and Neoarchean rocks were eroded. Thus, the isotopic data provide strong evidence that during Late Devonian time the Sibumasu terrane was situated in the proximity to the Archean cratons of Western Australia, presumably adjacent to the Carnarvon intracratonic basin. Moreover, Sibumasu may not have been situated in an outboard position on the shelf, as previously suggested, but could have been directly attached to the Australian continental crust. By contrast, low and uniform Sm/Nd ratios of seawater at Mae Sariang resemble those of the Variscan and the present-day oceanic seawaters. Therefore, a pelagic setting within the Paleotethys Ocean is postulated for the Mae Sariang succession. This conclusion is also constrained by minor temporal changes in εNd values and suggests that the Paleozoic of Mae Sariang is not part of the Sibumasu terrane but belongs to the Inthanon Zone.

Tectonic deformation of the southeastern tip of the Indochina Peninsula during its southward displacement in the Cenozoic time

The terminal collision of India with Asia brought about significant tectonic deformation into the Asian Continent. In order to depict these deformational features in the southeastern margin of the Indochina Peninsula, Late Cretaceous red beds of the Dak Rium Formation were sampled from Da Lat area (11.7°N, 108.3°E), the Kontum massif, Vietnam. Thermal demagnetization procedure revealed the presence of characteristic remanent magnetization (with unblocking temperatures around 680°C) in most samples. Primary nature of this magnetization is ascertained through positive fold test. A tilt-corrected mean direction calculated for the Dak Rium Formation is Dec/Inc=11.4°/35.4° (ks=331.7 α95=1.7°, N=21), with a corresponding paleopole at 76.5° N, 161.2° E (A95=1.7°). Compared with the European APWP, a negative inclination flattening (−15.2°±4.9°) is observed, indicating southward displacement of the Kontum massif by 9.2°±4.9° since Late Cretaceous. Since, comparable amount of southward displacement has been reported from 16 localities of the Indochina and Shan–Thai blocks, it seems a reasonable assumption to predict that the Indochina Peninsula underwent a southward displacement of 5.1°±2.4° (570km±270km) as a composite unit (including the Shan–Thai Block, Indochina Block and Kontum massif). On the contrary, 27.0°±9.8° counter-clockwise rotation of the Kontum massif with respect to the Indochina Block indicate a significant amount of internal deformation along the tip of south moving Indochina Peninsula. This rotation of the Kontum massif is attributed to 32–17Ma left lateral motion along the East Vietnam Boundary Fault, which makes up the 1000km long seaward continuation of the Ailao Shan–Red River shear zone. Tectonic deformation of the Indochina Peninsula was, thus, partly accommodated by local block rotation (e. g., counterclockwise rotation of the Kontum massif) and to some extent by internal tectonic deformation of the Shan–Thai Block.

Biometry and lifestyle of the Ordovician proetide trilobite Cyamella stensioei Owens, 1979

Biometric analyses of the trilobite Cyamella stensioei Owens, 1979, from carbonate mud mounds of the Upper Ordovician Boda Limestone of central Sweden, along with a comparison of closely allied rorringtoniid taxa from outside Baltoscandia, reveal important systematic and ecological information on this proetide taxon. Biometric analysis of the cephala of C. stensioei shows a number of instars, reflecting several ontogenetic stages, and a meraspis stage of this taxon is figured for the first time. C. stensioei is exclusively found in mass accumulations in the former, open cavities of the carbonate mud mound and was therefore most likely a cavity-dweller, possibly adapted to this lifestyle through chemosymbiosis. Some of the cavities are interpreted as former seepage conduits. Comparison of C. stensioei with other rorringtoniid trilobites suggests possible ancestors occurring in South China and Sibumasu terranes, determined earlier as Decoroproetus sp. and Cyamella sp., all differing from the genus Paracyamella, which is reassessed here. In wider terms of ancestry, they originate from Baltica or Laurentia, where the first rorringtoniids are known from the middle Darriwilian.

The Triassic conodonts of the NW Malayan Kodiang Limestone revisited: Taxonomy and paleogeographic significance

The revision of the Middle and early Late Triassic conodonts in the Nogami (1968) collection from the NW Malaya Kodiang Limestone Formation provides clues to the multi-element reconstruction of Triassic Gondolellacea. Septi- to octomembrate multi-elements characterize the families Gondolellidae and Gladigondolellidae nov. fam. The distribution of cratognathodiform and ozarkodiniform P2 elements supports the hypothesis of sexual dimorphism in Gladigondolella rather than that of two genera. Pseudofurnishius murcianus confers a Southern Tethyan low latitude character to the Kodiang Limestone, a part of the Cimmerian string of terranes that in Triassic times formed a diagonal partition between the gradually closing Paleo-Tethys and the accordingly widening Neo-Tethys. Our findings suggest that such a realm stretched from the Southern Alps (Dinarids) in the West to Malaya (Shan Thai Terrane) in the East. Only the Eastern edge collided with Eurasia in the Late Triassic, forming the platform of Sundaland. Consequently, the Paleo-Tethys closure remained limited to SE Asia, while a substantial Paleo-Tethys still existed in Western direction, ending up in the Pindos and Vardar oceans. The Jurassic Neo-Tethys ocean extended south of the consolidated SE Asia block and Cimmerian string of terranes.

Paleo-environments and tectonic setting of the Mesozoic Thung Yai Group in Peninsular Thailand, with a new record of Parvamussium donaiense Mansuy, 1914

The Thung Yai Group extends over a large area of peninsular Thailand, along the eastern margin of the Shan Thai block. Bound by angular unconformities 300 m thick dominantly detritic brackish to non-marine deposits with few intercalated limestone beds between Triassic marine and Tertiary non-marine sediments, represent the Thung Yai Group that comprises four formations: Khlong Min, Lam Thap, Sam Chom, and Phun Phin Formations. In the Ao Luk–Plai Phraya (ALPP) area, the Khlong Min and Lam Thap formations yield marine, brackish-water and non-marine fossil assemblages. These include trace fossils and for the first time in peninsular southern Thailand, the bivalve Parvamussium donaiense Mansuy, 1914. Based on fossil determinations, the Thung Yai Group has a late Early Jurassic to Early Cretaceous age. Our new observations help unravel the tectonic history of Mesozoic Peninsular Thailand. After the complete closure of the Paleotethys in the Late Triassic, renewed inundation, from the late Early Jurassic to the early Middle Jurassic, brought a regime of shallow to open marine and lagoon sedimentation over northwestern, western and southern peninsular Thailand, in the eastern part of Sundaland bordering the Mesotethys to the west.

A brief review of the Middle Palaeozoic vertebrates from Southeast Asia

Here we review the Silurian and Devonian vertebrates from Southeast Asia and their biogeographic affinities. These vertebrate remains are distributed in the Shan-Thai and Indochina terranes as well as the South China block, including the fossil fishes from western Yunnan (a part of the Shan-Thai terrane). The Devonian vertebrates from the Si Ka, Bac Bun and Mia Le formations of northern Vietnam resemble those from the Xitun Formation of Yunnan, South China, and thus belong to the Xitun fauna. The Late Silurian vertebrates from the Dia Giang Formation of My Duc (central Vietnam) are suggestive of the Xiaoxiang fauna of South China. Fossil vertebrate evidence shows the proximity between the South China block and the Indochina terrane in the Middle Palaeozoic and the close relationship between Shan-Thai and east Gondwana in the Middle Devonian.

Ordovician Sea-Level Change and Paleogeography of the Sibumasu Terrane Based on the Conodont Biostratigraphy

Abstract. Conodont biostratigraphy, paleobiogeography, and depositional environments reveal detailed eustatic changes and paleogeography of the Sibumasu terrane during the Ordovician. Based on lithologic data, the Ordovician sequences on the Sibumasu terrane are classified into two types. One was deposited in the Thong Pha Phum-Li basin, which extends from northern to western Thailand, and the other formed in the Tarutao-Langkawi basin ranging from southern Thailand to northern Malaysia. Five regressive events, RE 1 to 5, and a transgressive event TE 1 are recognized in these basins. On the basis of the conodont biostratigraphy, RE 1, 2, 4, and 5 are correlated with the Peltocare Regressive Event, the Ceratopyge Regressive Event, the Basal Whiterockian Lowstand interval, and the global regression caused by the end-Ordovician glaciation, respectively. TE 1 continued through Darriwilian time. The faunal affinity of Ordovician conodonts makes it clear that the Sibumasu terrane was close to both Australia and...

Darriwilian graptolites of the Shihtien Formation (Ordovician) in west Yunnan, China

West Yunnan in southwestern China formed an important part of the Sibumasu Terrane during the Ordovician. The Ordovician rocks of the region are exposed at many localities but have been strongly affected by several phases of tectonic activity, especially by the most recent Himalayan phase that commenced in the Paleogene. For these reasons, together with the difficult access to the localities, the Ordovician graptolites and biostratigraphy of this region are rather poorly known. Herein, we describe a Darriwilian graptolite fauna from the Shihtien Formation at Baoshan and Shidian, western Yunnan. The fauna comprises 11 genera and 15 species, including Didymograptus artus, D. murchisoni, D. spinulosus, Pterograptus sp., Hustedograptus vikarbyensis, H. teretiusculus, Archiclimacograptus angulatus, A. riddellensis, Haddingograptus oliveri, Proclimacograptus angustatus, and the dendroids Dictyonema, Palaeodictyota, Dendrograptus, Callograptus and Ptilograptus, indicating an age of middle to late Darriwilian. Two graptolite biozones are recognized: the Didymograptus artus and Didymograptus murchisoni biozones, in ascending order. The graptolite fauna is substantially similar to those of Baltica and South China.

Tectonic deformation of the Indochina Peninsula recorded in the Mesozoic palaeomagnetic results

SUMMARY In order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic to Early Cretaceous red sandstones were sampled at three localities in the Shan-Thai and Indochina blocks. Stepwise thermal treatment of most samples revealed the presence of characteristic remanent magnetization, which is generally unblocked by 680 °C. This component from Phong Saly (21.6°N, 101.9°E) and Borikhanxay (18.5°N, 103.8°E) localities yield positive fold tests with Late Jurassic–Early Cretaceous directions of Dec/Inc = 28.8°/32.1° (ks= 15.4, α95= 8.8°, N= 22) and Dec/Inc = 42.1°/46.9° (ks= 20.1, α95= 7.9°, N= 18), respectively. Additionally, a syn-folding mid-Cretaceous characteristic magnetization is observed in the samples of Muang Phin locality (16.5°N, 106.1°E), which gave a mean direction of Dec/Inc = 30.8°/39.9°, k= 102.6, α95= 3.0°, N= 23. This reliable Late Jurassic to Mid-Cretaceous palaeomagnetic directions from three different localities are incorporated into a palaeomagnetic database for Shan-Thai and Indochina blocks. Based on these compilations, tectonic deformation of the Shan-Thai and Indochina blocks is summarized as follows: (1) the Shan-Thai and Indochina blocks experienced a clockwise rotation of about 10° as a composite unit in the early stage of India–Asia collision and (2) following this, the Shan-Thai Block underwent an internal tectonic deformation, whereas the Indochina Block behaved as a rigid tectonic unit during the same period. Comparison of our palaeomagnetic results with seismic tomographic images suggests that the strength of continental lithosphere beneath these blocks played an important role in the process of deformation rather than any other tectonic regime. In contrast to the Shan-Thai Block, an existence of continental roots beneath the Indochina Block prevented its internal deformation.

A new Late Permian brachiopod fauna from Tanggula, Qinghai-Tibet Plateau and its palaeogeographical implications

Weihong He, Jianjun Bu, Zhijun Niu & Yang Zhang, June, 2009. A new Late Permian brachiopod fauna from Tanggula, Qinghai-Tibet Plateau and its palaeogeographical implications. Alcheringa 33, 113–132. ISSN 0311-5518. A brachiopod fauna described from the Late Permian of the Gongri and Lizai villages, northwest of the Qoima Co Lake, Tanggula area, southern Qinghai, northwest China, includes ten species in nine genera. This fauna from the (Qiangtang Block) has a strong affinity to coeval faunas of South China, sharing 74% of its species. In addition, the Late Permian Tanggula brachiopod assemblages also demonstrate a clear link with the Middle Permian brachiopod faunas of neighbouring microcontinents including the Indochina block (Malaysia and Cambodia) and the Shan-Thai block (Thailand), as suggested by the presence of Caricula sp. cf. C. salebrosa, Transennatia termierorum and Strophalosiina. This phenomenon indicates that there were relatively narrow seaways between these microcontinents to enable ready interchange of brachiopods during the Permian, and that brachiopods tended to retreat towards the palaeoequatorial region throughout the period.

Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (I): Geochemistry of ophiolites, arc/back-arc assemblages and within-plate igneous rocks

The Paleo-Tethys Orogen in southwest China is an amalgamation of continental terranes (Sibumasu, Simao and Yangtze), which are bounded by ophiolite belts (Ailaoshan, Jinshajiang and Changning–Menglian) that represent former ocean basins. This study concentrates on ophiolites, arc/back-arc assemblages, and within-plate igneous rocks that occur in this complex orogen. Mainly based on geochemical fingerprinting of basalts, the Paleo-Tethys ophiolites are here classified as MORB and SSZ types. The Ailaoshan ophiolite (NMORB-type; ca. 387–374 Ma) is associated with a non-volcanic segment of the rifted western Yangtze margin. The Jinshajiang ophiolite (EMORB-type; 346–341 Ma), which has ophiolite structure but some samples bear crustal contamination signatures, is associated with a volcanic segment of the rifted Yangtze western margin. These MORB-type ophiolites are interpreted to have formed in the continent-ocean transition zones. The Changning–Menglian ophiolite, which marks the main Paleo-Tethys suture, that is, the boundary between the Gondwana-derived Sibumasu terrane and the Yangtze-derived Simao terrane, has traditionally been thought to have a mid-ocean ridge (MOR) origin. New trace element data of basalts, however, indicate that this ophiolite (ca. 270–264 Ma) was formed at a supra-subduction zone (SSZ). Arc and back-arc assemblages are represented by the Gicha (ca. 311–277 Ma) and Banpo (ca. 288–284 Ma) zoned mafic-ultramafic complexes and the Yaxianqiao arc volcanic rocks (ca. 268–264 Ma). Geochemical data suggest that the Gicha complex originated in a back-arc rift. The Banpo complex makes a plutonic arc that contains low-Ti gabbro and tonalite with volcanic arc granite (VAG) affinity. Within-plate igneous rocks are recognized as xenoliths (amphibolites; ca. 443–401 Ma) in the Jinshajinag ophiolite mélange and the Dalongkai small differentiated mafic-ultramafic intrusion (ca. 247–244 Ma) that intruded the Yaxianqiao Arc. The amphibolite xenoliths are similar to low-Ti continental flood basalts (CFB) in composition. Rocks from the small mafic-ultramafic intrusion have superimposed subduction and within-plate geochemical signatures. These ophiolites, arc/back-arc assemblages and within-plate igneous rocks, are summarized as the magmatic records for the Plaeo-Tethys evolution in the context of a plate tectonic cycle.

Late Palaeozoic and Mesozoic tectonic and palaeogeographical evolution of SE Asia

Abstract SE Asia comprises a collage of continental terranes derived directly or indirectly from the India–Australian margin of eastern Gondwana. The Late Palaeozoic and Mesozoic evolution of the region involved the rifting and separation of three elongate continental slivers from eastern Gondwana and the successive opening and closure of three ocean basins, the Palaeo-Tethys, Meso-Tethys and Ceno-Tethys. The Sukhothai Island Arc System, including the Linchang, Sukhothai and Chanthaburi terranes, is identified between the Sibumasu and Indochina–East Malaya terranes in SE Asia and was formed by back-arc spreading in the Permian. The Jinghong, Nan–Uttaradit and Sra Kaeo sutures represent the closed back-arc basin. The Palaeo-Tethys is represented to the west by the Changning–Menglian, Chiang Mai/Inthanon and Bentong–Raub suture zones. The West Sumatra and West Burma blocks rifted and separated from Gondwana, along with Indochina and East Malaya in the Devonian, and together with South China formed a composite terrane ‘Cathaysialand’ in the Permian. They were translated westwards to their positions outboard of the Sibumasu Terrane by strike-slip tectonics in the Late Permian–Early Triassic at the zone of convergence between the Meso-Tethys and Palaeo-Pacific plates. SW Borneo is tentatively identified as possibly being the missing ‘Argoland’ that separated from NW Australia in the Jurassic. Palaeogeographical reconstructions for the Late Palaeozoic and Mesozoic illustrating the tectonic and palaeogeographical evolution of SE Asia are presented.

The Permian seamount stratigraphic sequence in Chiang Mai, North Thailand and its tectogeographic significance

The widespread Permian carbonate strata outcropped in northwestern Thailand are considered as the evidence for the Late Paleozoic shallow Tethys. Our investigation, however, shows that basalt can be discovered usually under the Permian carbonate sequence in Chiang Mai-Fang area, northwestern Thailand. The basalt belongs to subalkalic basalt and potassic trachybasalt. They are characterized by high P and Ti in major elements, by high content, enriching LREE, lacking δEu anomaly in rare earth elements, and by enrichment of the large ion lithophile element (LILE) (K, Rb, Ba) and high field strength element (HFSE) (Nb, Ta, Zr) in trace elements, which can be compared with the characters of the oceanic island basalt in Three Rivers (Jinsha River, Lancang River, and Nujiang River) area, southwestern China. Therefore, the Permian carbonate in the studied area was deposited on a seamount, rather than on a stable carbonate platform. The oceanic basin is correlated to the Late Paleozoic ocean represented by the Changning-Menglian Belt in southwestern China and they are a major basin of the Paleo-Tethyan Archipelagoes Ocean. The result indicates nonexistence of a Shan-Thai Block in the Late Paleozoic.

Tectonic deformation around the eastern Himalayan syntaxis: constraints from the Cretaceous palaeomagnetic data of the Shan-Thai Block

SUMMARY Lower to Middle Cretaceous red sandstones were sampled at four localities in the Lanpin-Simao fold belt of the Shan-Thai Block to describe its regional deformational features. Most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 °C. Primary natures of magnetization are ascertained through positive fold test. A tilt-corrected formation-mean direction for the Jingdong (24.5°N, 100.8°E) locality, which is located at a distance of 25 km from the Ailaoshan–Red River Fault, revealed northerly declination with steep inclination (Dec./Inc. = 8.3°/48.8°, α95= 7.7°, N= 13). However, mean directions obtained from the Zhengyuan (24.0°N, 101.1°E), West Zhengyuan (24.0°N, 101.1°E) and South Mengla (21.4°N, 101.6°E) localities indicate an easterly deflection in declination; such as Dec./Inc. = 61.8°/46.1°, α95= 8.1° (N= 7), Dec./Inc. = 324.2°/−49.4°, α95= 6.4° (N= 4) and Dec./Inc. = 51.2°/46.4°, α95= 5.6° (N= 13), respectively. The palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the Shan-Thai Block. When combined with geological, geochronological and GPS data, the processes of deformation in the Shan-Thai Block is described as follows: Subsequent to its rigid block clockwise rotation of about 20° in the early stage of India–Asia collision, the Shan-Thai Block experienced a coherent but southward displacement along the Red River Fault prior to 32 Ma. This block was then subjected to a north–south compressive stresses during the 32–27 Ma period, which played a key role in shaping the structure of Chongshan-Lancang-Chiang Mai Belt. Following this some local clockwise rotational motion has occurred during the Pliocene-Quaternary time in central part of the Shan-Thai Block as a result of internal block movements along the reactivated network of faults.