Espectroscopía de resonancia magnética nuclear

The mainland SE Asia comprises several terranes (or microcontinents) including Indochina, South China, Sibumasu (Siam, Burma, Malaysia and Sumatra, and also known as Shan-Thai) and West Myanmar (West Burma) Terranes, all of which are allochthonous terranes that have been rifted from Gondwana in the Palaeozoic and Mesozoic (e.g., Metcalfe, 1999; Fig. 2.1). Their tectonic evolution is fundamentally explained by four main geological processes involving rifting, drifting, amalgamation and post-amalgamation events (e.g., Metcalfe, 1988). This tectonic framework was initially established based on palaeontological and sedimentological data (e.g., Ridd, 1971; Burrett, 1974) and it has been generally accepted since early 1970s, although there are still many uncertainties regarding the detailed geometry and timing of these events.

2.2.1. Rifting

The Indochina Terrane is generally interpreted to have rifted from the Gondwana in the Early to Middle Palaeozoic, along with the South China Terrane. The Sibumasu Terrane is thought to have rifted in the Middle to Late Palaeozoic. This is supported by palaeomagnetic and palaeo-faunal data (e.g., Sengor and Hsu, 1984; Metcalfe, 1988). However, the timing of the

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Fig. 2.1. Tectonic divisions of mainland SE Asia and the surrounding region, consisting of South China, 

Indochina, Sibumasu and West Myanmar Terranes as differentiated by sutures/faults (Modified after 

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rifting event from Gondwana is still widely debated. Metcalfe (1999) indicated that the rifting of the Indochina and South China Terranes from Gondwana occurred in the Devonian, and that the Sibumasu Terrane rifted in the Carboniferous-Early Permian (Fig. 2.2). A number of other scenarios have also been proposed, especially for the South China and Indochina Terranes, and some studies suggest that their rifting commenced as early as the Ordovician or Silurian (e.g., Hutchison, 1989; Charusiri et al., 2002).

2.2.2. Drifting

The Indochina and South China Terranes separated from the margin of Gondwana by the Late Devonian, before the Sibumasu Terrane, and subsequently drifted northwards, with the opening of the Palaeo-Tethys ocean (Metcalfe, 1999; Fig. 2.2). The drifting of these terranes is believed to have been underway by the Early Carboniferous, as there is no faunal or floral similarity or other connections with Gondwana by the Early Permian (Metcalfe, 1999). In contrast, the Sibumasu Terrane was part of Gondwana until Permian. By Late Permian, it was rifted from Gondwana and became a part of the Cimmerian continent drifting rapidly northwards (Metcalfe, 1999). This temporal evolution of the Sibumasu Terrane is demonstrated by the presence of cool water high latitude Carboniferous-Permian faunas, Lower Permian diamictites, and Upper Permian low latitude platform facies limestone within the Sibumasu Terrane (Hutchison, 1989). During this northwards drifting, the Meso-Tethys ocean was formed between the Cimmerian continent and Gondwana (Metcalfe, 1999).

2.2.3. Amalgamation

The amalgamation of the Indochina, South China and Sibumasu Terranes is considered to have occurred during the Late Palaeozoic to Early Mesozoic. It is believed that the Indochina Terrane initially collided with the South China Terrane and subsequently with the Sibumasu Terrane.

The amalgamation of the Indochina and South China Terranes is known to have commenced along the Song Ma Fault in North Vietnam (e.g., Hutchison, 1989; Metcalfe, 1999) where the occurrence of ophiolitic rocks indicates the presence of a closed ocean basin (e.g., Hutchison, 1989). The timing has been poorly constrained by previous studies with proposed amalgamation varying between the Carboniferous (Metcalfe, 1999) to the Triassic (Sengor and Hsu, 1984; Lepvrier et al., 1997). However, recent geochronological studies on magmatic and metamorphic rocks demonstrated that the timing of the amalgamation is confined to a Permian-Triassic time (270–240 Ma; e.g., Owada et al., 2006; Khin Zaw et al., 2010; Halpin et al., 2010).

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The timing and location during the amalgamation of the Indochina and Sibumasu Terranes has also been widely debated. Various timings for this amalgamation have been suggested, from the Carboniferous-Permian (Hutchison, 1989), Triassic (Sengor et al., 1988; Metcalfe, 1988, 1996), and Triassic to Cretaceous (Aduley-Charles, 1983). The boundary between the Indochina and Sibumasu Terranes is generally thought to occur along the Bentong-Raub suture in Malaysia (e.g., Hutchison, 1989), but its continuation into Thailand has been more difficult to determine. Early researchers placed it along the Nan-Uttaradit/Sa Kaeo zones in central Thailand (Hada et al., 1999; Metcalfe, 1999). One study placed this boundary in westernmost Thailand along the Mae-Yuang (Mae Sariang-Mae Hong Son) Fault (Hisada et

Fig. 2.2. Reconstruction maps of the Gondwana‐derived terranes during Cambrian‐Late Triassic period

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al., 2004). Recent studies have favoured the Chiangmai Fault (similar to the Mae-Yuang Fault) as the main boundary of the two terranes (e.g., Sone and Metcalfe, 2008; Khin Zaw et al., 2010).

2.2.4. Post-amalgamation

During and after the major terrane assembly in the region, a number of important extensional episodes occurred, causing subsidence within the Indochina, South China and Sibumasu Terranes (e.g., Hall, 2002). Post-collisional tin-bearing granites were intruded in the Mesozoic throughout the area, and then most of the area subsided with sedimentation occurring in the mainland SE Asia as intracontinental red-bed basins. During the Cenozoic, part of the Indochina Terrane was extruded southeastwards from the remainder of continental Asia, as a result of the collision of the Indian Craton and Eurasia (Hutchison, 1989). The extruded area is bordered by the dextral Sagaing Fault in Myanmar and sinistral Red River Fault in South China and Vietnam (Hall, 2002).

2.3. GEOLOGICAL SETTING AND ORE DEPOSITS OF

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