Cenozoic evolution of the Vietnamese coastal margin

margin, often characterised by more than 10 km of sedimentary infill (Fig. 1). Greater parts of the margin are still in an early explorational state, although significant petroleum production has taken place in all but the southern Song Hong and the Phu Khanh Basins. This has increased the need for a fundamental understanding of the processes behind the formation of the basins, including analyses of potential source rocks. The basins fringing the Indochina Block provide excellent evidence of the geological evolution of the region, and the basin geometries reflect the collision of India and Eurasia and the late Cenozoic uplift of south Indochina (Rangin et al. 1995a; Fyhn et al. in press). In addition, the basins provide evidence of regional Palaeogene rifting and subsequent Late Palaeogene through Early Neogene sea-floor spreading in the South China Sea. Apart from the regional Cenozoic tectonic record, the basins contain a high-resolution climatic record of South-East Asia due to the high depositional rates, changing depositional styles and large hinterland of the basin (Clift et al. 2004).


Background
Since 1995 the Geological Survey of Denmark and Greenland (GEUS) and the Department of Geography and Geology, University of Copenhagen, have operated jointly in Vietnam aiming to improve the local geoscientific capa city. The work is part of the ENRECA project (Enhancement of Research Capacity in Developing Countries), funded by the Danish International Development Agency (DANIDA). This part of the ENRECA project focuses on an assessment of the hydrocarbon potential of the Vietnamese continental margin, and has led to basin evaluations of the Song Hong and the Phu Khanh Basins (Fig. 1), and to a series of both Vietnamese and Danish M.Sc. projects (Nielsen et al. 1999;Nielsen & Abatzis 2004;Andersen et al. 2005;Boldreel et al. 2005;Fyhn et al. in press). The ongoing second phase of the project focuses both on training Vietnamese M.Sc. and Ph.D. students and on evaluating the hydrocarbon potential of the Vietnamese part of the Malay and Khmer Basins, as well as

Cenozoic evolution of the Vietnamese coastal margin
Michael B.W. Fyhn, Lars Henrik Nielsen and Lars Ole Boldreel the Mesozoic strata underneath and shoreward of these basins. Sampling of source rocks and oil seeps and drilling of two 500 m deep, fully cored wells (ENRECA-1 and 2, Fig. 1) as well as acquisition of shallow seismic data have been carried out as part of the basin evaluations (Bojesen-Koefoed et al. 2005;Petersen et al. 2005). Furthermore, a broader analysis of the structure and stratigraphy of the entire Vietnamese margin is being carried out as a separate Ph.D. study funded by the University of Copenhagen.

Tectonic models
The Indochina Block is situated immediately south-east of the eastern Himalayan syntaxis. The Himalayan Orogeny thus had a major impact on the structural evolution of Indochina, leading to major north-west-south-east crustal shortening in the north and to significant lateral movements along shear zones transecting and bordering the Indochina Block ( Fig. 1; Morley 2002). Some of the largest shear zones are the north-west-trending Red River, Mai Ping and Three Pagodas Shear Zones. South-eastward displacement and rotation of Indochina and adjacent areas produced a total left-lateral offset of several hundreds of kilometres along the three shear zones (Hall 2002). Tapponnier et al. (1982) suggested that the South China Sea and its marginal basins formed due to complex pull-apart mechanisms in response to these left-lateral displacements ( Fig. 2A). Alternatively, Taylor & Hayes (1983) suggested that the formation of the South China Sea was a result of a southward subduction of ocean crust beneath Borneo (Fig. 2B). One of the major differences between the two models is that the subduction model predicts right-lateral displacement across a large part of the Vietnamese margin, whereas the pull-apart model is associated with a left-lateral transform along the margin.

The offshore Red River Shear Zone
The most extensive of the Indochinese left-lateral shear zones is the Red River Shear Zone that passes through South China and northern Vietnam into the Song Hong Basin. Seismic studies of the almost 20 km deep Song Hong Basin indicate that the basin formed in response to major Palaeogene leftlateral offset along the seaward continuation of the Red River Shear Zone (Rangin et al. 1995a;Nielsen et al. 1999;Andersen et al. 2005). Recent studies show that the shear zone continues along the Vietnamese coast in the Phu Khanh Basin further south (  forming a major rift structure filled by thick Palaeogene synrift deposits (Fig. 3). Left-lateral transtension ended during latest Oligocene time in the Phu Khanh Basin, but was followed by earliest Miocene structural inversion. This is interpreted to reflect a change from intense left-lateral trans tension to modest right-lateral movements along the seaward extension in the Red River Shear Zone in the basin, corroborated by a study by Rangin et al. (1995b) showing that left-lateral, coast-parallel wrench faults onshore have been inverted by right-lateral movements. The latest Palaeogene termination of left-lateral movement along the offshore part of the Red River Shear Zone in the Phu Khanh Basin does not support Neogene sea-floor spreading in the South China Sea as a result of left-lateral pull-apart. Consequently, Neogene seafloor spreading cannot have been caused by left-lateral pullapart, but was probably forced by subduction of older oceanic crust beneath Borneo. Palaeogene rifting along the Vietnamese margin was, on the other hand, greatly influenced by left-lateral transtension.

The offshore Three Pagodas Shear Zone
Rifting in the Malay and Khmer Basins south-west of Vietnam was originally linked to left-lateral transtension across a seaward extension of the Three Pagodas Shear Zone ( Fig. 1; Tapponnier et al. 1982). Later models suggested right-lateral faulting along the fault zone as the forcing mechanism (Polachan & Sattayarak 1989), or a combination of forces related to the Indochina extrusion and extension caused by subduction roll-back (Morley 2001), or mantle plume emplacement (Ngah et al. 1996).
Seismic structural analysis of the Vietnamese part of the Malay and Khmer Basins indicates that rifting mainly took place during the Palaeogene, and was controlled by a steep, north-north-west-trending, downward steeping master fault, which is flanked by smaller north-west-trending conjugate normal faults (Fig. 4). The master fault offsets the basement with up to more than 2 sec. TWT and transects the entire study region striking towards the point at which the Three Pagodas Shear Zone enters the Gulf of Thailand. The master fault is therefore interpreted as an offshore fault strand of the Three Pagodas Shear Zone. The fault characteristics indicate Palaeogene left-lateral transtension and thus support a close relation between extrusion of Indochina and rifting in the two basins.

Depositional trends
Sea-floor spreading in the South China Sea did not start until the middle Oligocene, and Palaeogene syn-rift sedimentation was therefore dominated by alluvial and lacustrine deposi-tion. In the Song Hong Basin a gradual marine transgression of the margin started after the onset of sea-floor spreading. During initial transgression siliciclastic deposition in estuaries and narrow marine pathways dominated larger parts of the basins, and carbonate growth took place on inundated highs. Open marine conditions prevailed in most basins during Neogene times as sea-floor spreading propagated to its maximum south-western extension. Extensive carbonate growth took place on many intra-and interbasinal highs south of and along the Vietnamese margin up to c. 16°N during the Neogene, favoured by the open marine environment and climatic conditions. In contrast, sediment supply kept pace with subsidence in most parts of the Malay and Song Hong Basins, preventing long-lasting periods of open marine sedimentation.
During Late Neogene time, central and southern Indochina were thermally uplifted, thus significantly increasing the siliciclastic input to the marginal basins. The increased terrigenous sediment supply inhibited widespread carbonate growth off southern and central Vietnam and resulted in the progradation of a distinct shelf slope, which has led to the present outline of the margin.

Source rocks
One of the main risk factors regarding petroleum exploration in the Vietnamese offshore basins is the presence of adequate source rock intervals. Onshore data from the ENRECA-1 core through the Song Ba Trough in central Vietnam show, however, that thick intervals of excellent oil-and gas-prone lacustrine mudstone and humic coals may develop even in small basins characterised by high sediment input. Although the Song Ba Trough is an order of magnitude smaller than the Vietnamese offshore basins, seismic data in the latter show apparent depositional similarities suggesting the presence of similar high-quality source rocks in the offshore basins (Nielsen et al. 2007;Fyhn et al. in press). In addition, seismic facies analysis as well as oil and gas compositions indicate that other source rock types, such as Neogene fluvio-deltaic coals, carbonaceous shales and fore-reef marls are present in some of the basins and thus testify to the great petroleum potential of the Vietnamese margin (Bojesen-Koefoed et al. 2005;Fyhn et al. in press).