Large-scale erosional response of SE Asia to monsoon evolution reconstructed from sedimentary records of the Song Hong-Yinggehai and Qiongdongnan basins, South China Sea LONG VAN HOANG1*, PETER D. CLIFT1, ANNE M. SCHWAB2, MADS HUUSE1, DUC ANH NGUYEN3 & SUN ZHEN4 1 Department of Geology & Petroleum Geology, School of Physical Sciences, University of Aberdeen, Meston Building, Aberdeen AB24 3UE, UK 2 Marathon International Petroleum (GB) Ltd, Marathon House, Rubislaw Hill, Anderson Drive, Aberdeen AB15 6FZ, UK 3 Vietnam Petroleum Institute, Yen Hoa, Cau Giay, Hanoi, Vietnam 4 South China Sea Institute of Oceanography, Chinese Academy of Sciences, 164 Xingang Road, Guangzhou, 510301 China *Corresponding author (e-mail: l.v.hoang@abdn.ac.uk) Abstract: The Song Hong-Yinggehai (SH-Y) and Qiongdongnan (Qi) basins together form one of the largest Cenozoic sedimentary basins in SE Asia. Here we present new records based on the analysis of seismic data, which we compare to geochemical data derived from cores from Ocean Drilling Program (ODP) Site 1148 in order to derive proxies for continental weathering and thus constrain summer monsoon intensity. The SH-Y Basin started opening during the Late Paleocene–Eocene. Two inversion phases are recognized to have occurred at c. 34 Ma and c. 15 Ma. The Qi Basin developed on the northern, rifted margin of South China Sea, within which a large canyon developed in a NE–SW direction. Geochemical and mineralogical data show that chemical weathering has gradually decreased in SE Asia after c. 25 Ma, whereas physical erosion became stronger, especially after c. 12 Ma. Summer monsoon intensification drove periods of faster erosion after 3–4 Ma and from 10– 15 Ma, although the initial pulse of eroded sediment at 29.5– 21 Ma was probably triggered by tectonic uplift because this precedes monsoon intensification at c. 22 Ma. Clay mineralogy indicates more physical erosion together with high sedimentation rates after c. 12 Ma suggesting a period of strong summer monsoon in the Mid-Miocene. The history and causes for the East Asia Monsoon are controversial topics, although its proposed links with the uplift of the Tibetan Plateau remain a classic example of how the solid Earth may control atmospheric processes and the climatic evolution of the planet (Prell & Kutzbach 1992; Molnar et al. 1993). Certainly the presence of a large modern plateau plays an important role in driving the present intensity of the summer monsoon (Manabe & Terpstra 1974; Prell & Kutzbach 1992). In contrast, the winter monsoon is characterized by a cold and dry climate caused by air circulation in the reverse direction, although this too is linked with the growth of topography. As well as affecting atmospheric circulation patterns, uplift and deformation of the Tibetan Plateau has also intensified chemical weathering and physical erosion of source rocks as a result of changes in rock physical properties and increasing terrain gradient. This in turn has affected the composition of sediments washed to the oceans by the large rivers that drain the eastern flank of the plateau. In this paper we assess the role of the Asian monsoon in controlling continental erosion. Although precipitation has been recognized as an important control on erosion (Reiners et al. 2003; Wobus et al. 2003) its relative role compared to tectonically driven rock uplift is unclear (Burbank et al. 2003). The South China Sea is a good place to examine the competing effects of these processes because the nature of the monsoon has been partly reconstructed from studies at a series of Ocean Drilling Program (ODP) sites on the rifted southern margin of China (Chen et al. 2003; Jia et al. 2003; Wan et al. 2006, 2007; Clift et al. 2008c). The From: Clift, P. D., Tada, R. & Zheng, H. (eds) Monsoon Evolution and Tectonics –Climate Linkage in Asia. Geological Society, London, Special Publications, 342, 219–244. DOI: 10.1144/SP342.13 0305-8719/10/$15.00 # The Geological Society of London 2010. 220 L. V. HOANG ET AL. weathering and erosion records we present here can be readily compared with monsoon intensities to assess possible linkages. Although studies of the modern Red River suggest a dominant role for tectonically driven rock uplift in driving erosion (Clift et al. 2006b) this may not be the case over longer periods of geological time. Location and geological setting Sediments eroded from eastern Tibet have partly been fed into the SH-Y and Qi basins via the Red River (Fig. 1). Although the Red River is still a large river it has been argued that the present drainage reflects major re-organization caused by re-tilting of eastern Asia towards the east during the Cenozoic (Wang 2004). Prior to this tilting the Red River may have formed the dominant drainage in SE Asia, but would have progressively lost drainage area because of headwater capture into adjacent systems (Brookfield 1998; Clark et al. 2004). Alternative models propose a more stable drainage and explain the curious geometries of river in SE Tibet as reflecting deformation, with the rivers acting as passive strain markers (Hallet & Molnar 2001). Mass balancing eroded and deposited volumes of sediments now present in the SH-Y Basin and onshore in the modern Red River drainage indicated that the original catchment area of the Red River must have been much larger than that observed today (Clift et al. 2006a). Furthermore, Nd isotope values of sediments from the Hanoi Trough, Vietnam (Fig. 1) show a rapid change during the Oligocene (Clift et al. 2006a). Clift et al. (2006a) interpreted these changes as a response to large-scale drainage capture away from the former Red River. Hainan island is not considered to have been a major sediment source until it was uplifted during a period of strong magmatism that started c. 2 Ma (Tu et al. 1991). The influence of local sources along the Vietnamese coast on the total sediment influx is relatively poorly known. The SH-Y and Qi basins together form one of the largest sedimentary systems in SE Asia and are mostly filled by sediment delivered by the Red River. Thus, the sedimentary successions preserved within them record the history of erosion onshore. Although tectonic work can constrain the nature Fig. 1. Location of the research area relative to SE Asia. The SH-Y Basin lies along the SE extension of the RRFZ, while the Qi Basin is situated on the rifted margin of South China Sea. The black straight lines show the location and length of the 2D seismic survey lines that were newly released to this study, while the red lines show those from Clift & Sun (2006). Black-dash lines are seismic lines selected for decompaction and sediment budget estimation. The white circles show the locations of the industrial wells used for the age assignment in this study. The black circle marks the location of ODP Site 1148 (Wang et al. 2000). Locations of seismic profile figures are shown. TECTONIC AND MONSOON EVOLUTION IN SE ASIA of deformation in Tibet and palaeoceanographic studies can reconstruct climate, it is the sediment records in the river deltas and fans that allow us to quantify erosion and so test for any links between climate, tectonics and erosion. Understanding how changing monsoon strength affects terrestrial environments is important not just for scientific reasons, but also because almost two-thirds of the global population are influenced by the monsoonal climate (Clift & Plumb 2008). Determining the relative role of solar insolation and atmospheric chemistry compared to tectonic processes in governing monsoon strength is important to predictions of future monsoon variability. The SH-Y and Qi sedimentary basins are situated within the Gulf of Tonkin, in the northwestern South China Sea (Fig. 1). The SH-Y Basin lies along the southeastern extension of the strike-slip Ailao Shan-Red River Fault Zone (RRFZ); the Qi Basin is situated at the southwestern end of the northern, rifted margin of South China Sea. The basins lie in two different tectonic settings. There is general agreement that formation of the basins was linked to opening of the South China Sea and motion on the RRFZ; whether these are all linked remains controversial. Tapponnier et al. (1982) carried out analogue experiments suggesting that penetration of the rigid Indian Plate into a softer Eurasia led to the extrusion of Indochina to the SE along the left-lateral RRFZ and consequently to the opening of the South China Sea. However, others argue that the opening of the South China Sea was triggered by a subduction force to the south where the Dangerous Grounds underthrust the Borneo Trench (Holloway 1982; Hall 1996; Morley 2002; Clift et al. 2008a). The SH-Y is interpreted as a pull-apart basin developed in a NW –SE orientation and controlled by a series of transtensional faults, especially the RRFZ, whose main trace is located on the SW side of the basin and by the ‘No.1 Fault’ to the NE. Modelling has demonstrated that a moderate degree of strike-slip shear caused by the rotation of Indochina relative to mainland China is capable of forming the basin geometry observed by seismic methods (Sun et al. 2003), without the need for motion .1000 km as had been suggested (Briais et al. 1993; Replumaz & Tapponnier 2003). The timing of deformation within the basin was dated by seismic methods to be before 30 Ma and to have ceased by c. 5.5 Ma (Rangin et al. 1995). Subsequently, Harrison et al. (1996), Wang et al. (1998), Leloup et al. (2001) and Gilley et al. (2003) all used radiometric isotope data to constrain the start of motion on the RRFZ to being close to c. 34 –35 Ma, broadly consistent with an acceleration of tectonic subsidence in the basin at that time (Clift & Sun 2006). 221 In contrast, the Qi Basin was formed by rifting of the continental margin followed by seafloor spreading that started c. 30 Ma, as dated by marine magnetic anomalies in the neighbouring oceanic crust dated back to anomaly 11 (c. 31 Ma) (Taylor & Hayes 1980; Lu et al. 1987; Briais et al. 1993; Zhou et al. 2002), although there is a suggestion that seafloor spreading may date back to 37 Ma in the NE parts of the basin (Hsu et al. 2005). In any case the Qi Basin partly overlies both the continental shelf and continental slope (south of Hainan island), where water depth varies from c. 200–1500 m. Monsoon reconstructions We have some knowledge of how the East Asian summer monsoon varies as a result of studies based on sediment records from ODP sites in northern South China Sea (ODP Sites 1146 and 1148; Fig. 1). Zheng et al. (2004) studied the abundance and ratio of planktonic foraminifera, which is a common proxy for reconstructing palaeoclimate change. They proposed that a decrease of the ratio of planktonic foraminifera Globigerinoides sacculifer/G. ruber and increase of Neogloboquadrina at c. 8 Ma at ODP Site 1146 indicates a lowering of the surface temperature and increased productivity, which are interpreted to have been caused by intensified East Asian winter monsoon winds. Upwelling-related radiolarian palaeomonsoon proxies in the southern South China Sea suggest that the east Asian summer monsoon first initiated close to the middle/late Miocene boundary at c. 12– 11 Ma and reached a maximum strength at 8.8 –7.7 Ma (Chen et al. 2003). This suggestion is consistent with work by Wan et al. (2007) who used sediment grain-sizes at ODP Site 1146 to indicate a stronger winter monsoon at c. 8 Ma and both winter and summer monsoon intensification at c. 3 Ma. However, geochemical data derived from ODP Site 1148 show much earlier intense chemical weathering in SE Asia, which may be linked to monsoon enhancement (Li et al. 2003). Continental weathering intensity is largely controlled by moisture and temperature and thus might be expected to be linked to the intensity of the summer monsoon rains. Wei et al. (2006) used a combination of the traditional, major element based ‘Chemical Index of Alteration’ (CIA) (Nesbitt et al. 1980), together with other geochemical proxies such as Ca/Ti, Na/Ti, Al/Ti, Al/Na, Al/K and La/Sm ratios to suggest that the summer East Asian monsoon has affected South China since the Early Miocene. Curiously, this study suggested that summer monsoon rains have gradually decreased while winter monsoon strength has increased since that time. 222 L. V. HOANG ET AL. Most recently a colour spectral-based analysis of clay mineralogy at ODP Site 1148 has shown more complicated and significant mismatches with other data set (Clift et al. 2008c). This record indicates an initial intensification of weathering after 22 Ma, followed by a period of especially strong summer monsoon from 16– 10 Ma. The summer monsoon would appear to weaken into the Pliocene before experiencing moderate intensification since c. 4 Ma. Earlier monsoon reconstructions sometimes show inconsistent results. In some cases this reflects different timescales and resolution of data and/or grain-size distribution of sediments. Other apparent discrepancies may reflect the fact that the different proxies are measuring different things, for example, upwelling, weathering, wind strength, which in turn may track activity of either winter or summer monsoons. The spectral data of Clift et al. (2008c) are derived from clays, whereas CIA is bulk sediment analysis that can also be affected by grain size. Data sources In this paper we present new constraints on the history of sediment flux from the Red River Basin based on interpretation of newly released 2D multichannel seismic profiles from the SH-Y and Qi basins, which allow us to augment earlier seismic stratigraphic studies of the basins (Rangin et al. 1995; Fang et al. 2000; He et al. 2002; Gong & Li 2004; Clift & Sun 2006). In particular, our data adds greatly to our understanding of the southwestern, Vietnamese parts of the SH-Y Basin compared to the previous regional synthesis of Clift & Sun (2006) (Fig. 1), which was focused much more on the eastern half of the basin. These new data allow us to estimate the flux of sediment into the whole basin through time more completely. We have constrained changing weathering regimes in the area using geochemical data from XRF whole corescanning of sediment from Ocean Drilling Program (ODP) Site 1148 in the northern South China Sea (Fig. 1). Unfortunately, no suitable core exists for the Red River offshore, so here we have exploited a core from the neighbouring Pearl River drainage. This has the additional advantage of imaging a river basin that is largely unaffected by Neogene tectonics or drainage reorganization (Clark et al. 2004), so that changes in weathering regime can be readily related to climate and thus to the monsoon. Methodology Seismic data and workflow In this study, we analysed 2D multichannel seismic profiles provided by BP, PetroVietnam and the Chinese National Offshore Oil Company (CNOOC). In total, 48 lines (c. 5500 km) from the SH-Y Basin and 12 lines (c. 750 km) for the Qi Basin (Fig. 1) were used to constrain the sedimentary evolution. These data augment earlier published data, largely from the Chinese sector. There have been several interpretations carried out by oil and gas companies for different parts of the SH-Y Basin. However, geological correlation across the basin has not previously been possible because this basin straddles the international boundary between Vietnam and China. In this study, we used seismic data, which cover the whole area of the SH-Y Basin (Fig. 1) in order to have better geological interpretation and correlation from the SW to the NE side, as well as towards the SE end, where it meets the Qi Basin (Fig. 1). Once navigation and SEG Y data (SEG Y file format is one of several standards developed by the Society of Exploration Geophysicists for storing geophysical data) were loaded onto a workstation running KingdomTM, software seismic sequence boundaries were picked, based on conventional termination types of seismic reflections (e.g. onlap, downlap, erosion) (Vail et al. 1977; Miall 1991). Age constraints derived from biostratigraphy in industrial wells, typically at the sub-epoch level of resolution were assigned to these horizons prior to time– depth conversion. Geochemical analysis Geochemical data have been used as a proxy for constraining the intensity of chemical weathering, which is controlled by a number of processes including temperature (White et al. 1999) and moisture (Gabet et al. 2006). These in turn may be linked to monsoon strength (Derry & France-Lanord 1996b; Filippelli 1997). In this study, we used cores from ODP Site 1148, located on the deep-water slope offshore the Pearl River (Fig. 1) to examine variations in major element chemistry. Cores from this site were analysed at the Research Centre Ocean Margin (RCOM), University of Bremen, Germany by an X-Ray Fluorescence Core Scanner manufactured by AVAATECH. One half of each core was flattened and covered by plastic film before being positioned under the X-ray beam for scanning. The step-size for each measurement was set up at every 7 cm. However, this resolution was changed in accordance with lithological variation and to avoid fractures within the core. XRF scanning can obtain continuous data at much finer scales than is practical for individual sampling methods. These advantages are especially important for relatively long time series and especially for highresolution analyses on critical boundaries/intervals. XRF core scan data show a significantly higher TECTONIC AND MONSOON EVOLUTION IN SE ASIA signal-to-noise ratio and more consistent holeto-hole agreement than standard logs. Use of XRF core scan data as a tool in palaeoceanographic and stratigraphic studies is well defined and widely accepted (Röhl & Abrams 2000; Tjallingii et al. 2007). Use of major element data to constrain terrestrial weathering intensity often involves use of the CIA which is an established weathering proxy (Nesbitt & Young 1982). However, CIA is dependent on having Al, Na, K and Ca concentration data and is susceptible to variation linked to sediment mineralogy and provenance evolution. Unfortunately, the scanner does not provide any Na concentration data. Even if CIA can be determined, sands yield lower CIA values compared to clays in the same drainage system (Clift et al. 2008b). Results Tectonic and seismic stratigraphic evolution Because the SH-Y and Qi basins are situated in different tectonic provinces, they were interpreted independently. By picking horizons along termination surfaces, fourteen sedimentary packages were defined for the SH-Y Basin, and the sedimentary formations of the Qi Basin were sub-divided into nine packages. The age for each boundary surface was dated by nannofossil-based biostratigraphy provided by the operating company from each industrial well and/or by correlating stratigraphy across regional cross-sections. The cross-section shown in Figure 2 shows the general structure of the SH-Y Basin. It shows a classic pull-apart type basin with a relatively symmetrical shape around an axis developed in a NNW–SSE direction (Dooley & McClay 1997). Because of the limited seismic coverage and lack of drilling data from the basin centre, the morphology of the basement as well as the nature of the oldest sedimentary formations in the basin centre has not been well defined. These deepest formations were estimated as being Paleocene –Eocene, unconformably overlying pre-Cenozoic terrigenous, carbonate sedimentary rocks with minor volcanic rocks (Tran et al. 2003; Mai et al. 2005; Clift & Sun 2006). The basin started to subside after c. 50 Ma, presumably related to the regional crustal extension seen in other parts of the South China Sea (Su et al. 1989; Clift & Lin 2001). This phase was followed by rapid subsidence especially after c. 34 Ma when motion on the RRFZ started (Gilley et al. 2003) and the pull-apart basin developed. Active tectonic subsidence continued until the Late Oligocene–Early Miocene. The extensional faults and carbonate platforms of the pre-Cenozoic basement have created a complicated basement 223 morphology and thus a highly laterally variable sediment distribution (Figs 3, 4 & 5). In Figure 2, most of the syn-rift deposits are observed to be in the basin centre, where they are displaced by numerous transtensional faults on both margins. Offsets of the Palaeogene– Lower Miocene formations across these faults suggest that they were reactivated several times. Active rifting was followed by strong thermal subsidence during the Miocene, gradually weakening, but then accelerating again after Miocene (c. 5.5 Ma), at least in the southern SH-Y Basin (Clift & Sun 2006). Low-angle shoreline trajectories observed in the post-Miocene sedimentary packages demonstrate that little accommodation space was created at that time. As a result, most of the sediments eroded from Hainan island during the Pliocene quickly prograded towards the basin centre with very little vertical aggradation (Fig. 6). During its evolutionary history, the SH-Y Basin has experienced at least two inversion phases. The first uplift period is quite localized and is interpreted to have been triggered by the onset of the motion on the RRFZ at c. 34 –35 Ma. In contrast, the later Mid-Miocene event (c. 15.5 Ma) probably correlates with the generation of the Deep Regional Unconformity in the South China Sea (Hazebroek & Tan 1993; Hutchison 1996; Matthews et al. 1997), the end of the motion on the RRFZ, and with the cessation of seafloor spreading. Evidence for uplift and basin inversion is provided by strong deformation and erosion signatures observed on seismic profiles, especially in the northern part of the SH-Y Basin (Figs 3 & 7). The basin inversion involved not only deformation of basin fill, but also thrust faulting with significant vertical offsets (c. 250 ms of two-way travel time; Fig. 7). Compressional stresses operating during inversion may have played an important role in remobilizing the overpressured, fine-grained sediments to form shale/mud diapir-like structures in the centre of the SH-Y Basin (Hao et al. 2002; Figs 2 & 8b). However, some of these structures penetrate the youngest sedimentary formations that postdate Middle Miocene inversion. This relationship suggests that the structures were initially formed by compressional deformation but may subsequently have been enhanced by sedimentary loading during rapid deposition of the overlying Pliocene –Recent sediments (Clift & Sun 2006). Although these structures could conventionally be interpreted as shale diapirs (Hao et al. 2002; Xie et al. 2003; Clift & Sun 2006) their shapes and the structure of surrounding and overlying seismic reflections do not resemble geometries associated with well documented diapirs elsewhere. A possible alternative interpretation in better agreement with seismic stratigraphic relations may 224 L. V. HOANG ET AL. Fig. 2. Seismic and interpreted cross-section of Line GPGT 93-223 through the SH-Y Basin, showing a pull-apart basin structure. Rifting has strongly disrupted the basement and displaced syn-rift formations with different offsets. A possible shale/mud diapir is intruded into younger layers as a result of the sediment remobilisation initially triggered by tectonic inversion. Alternatively this structure could represent a strike-slip fault zone. Line location is shown on Figure 1. TECTONIC AND MONSOON EVOLUTION IN SE ASIA 225 Fig. 3. Seismic and interpreted cross-section of Line GPGT 93–200 running in a north–south direction through the SH-Y Basin and showing a complicated basement morphology and the increased deformation/erosion towards the north. Line location is shown on Figure 1. 226 L. V. HOANG ET AL. Fig. 4. Seismic and interpreted cross-section of Line GPGT 93–204 in the SH-Y Basin. The presence of a carbonate platform reflects more localized sediment distribution during the early stages of basin opening. The progradation configuration observed here suggests sediments spilled over to the SE during the Plio-Pleistocene. Line location is shown on Figure 1. TECTONIC AND MONSOON EVOLUTION IN SE ASIA 227 Fig. 5. Seismic and interpreted cross-section of Line GPGT 93–225 on the SW flank of the SH-Y Basin. The progradation towards the ENE suggests more sediments spilled over from the northern SH-Y Basin were delivered into the Qi Basin after c. 2 Ma. Line location is shown on Figure 1. 228 L. V. HOANG ET AL. Fig. 6. Cross-section of seismic profile C-65-75 through the Qi Basin. Basement is characterized by strong faulting, which formed a succession of graben-horst structures. Thin layers show that less sediment was delivered to this basin before c. 2 Ma, while steep shelf edge trajectory suggests rapid sediment influx after this time. A large-scale canyon, which incised down through older formations shows an imbalance between the sediment supply and slope stability. Line location is shown on Figure 1. TECTONIC AND MONSOON EVOLUTION IN SE ASIA Fig. 7. Cross-section interpreted from seismic profile GPGT 93– 201 in the northern SH-Y Basin. Strong erosion, deformation and thrust faulting are evidence for several basin inversions. Line location is shown on Figure 1. 229 230 L. V. HOANG ET AL. Fig. 8. Several internal seismic reflection architectures observed within the basin: (a) asymmetrical channel migration; (b) shallow faulting developed above a shale diapir/fault zone; (c) complex-oblique sigmoidal reflections in the progradational package; (d) high amplitude/low frequency of the seismic reflections observed at the bottom of the canyon was likely caused by the presence of a carbonate-cemented sandstone. The underlying reflections are pulled up due to the increase in seismic velocity of infill within the canyon; (e) lateral variation in seismic facies; (f) vertical variation in bedded seismic facies. include combined dip- and strike-slip faulting to create triangular zones of poor seismic imaging overlain by only subtly disturbed, sub-horizontal reflections (Figs 2 & 8b). However, the present data density does not allow a confident interpretation of these structures and their origin is not discussed further here. In contrast to the SH-Y Basin, the Qi Basin overlies the rifted margin of southern China and straddles the continent–ocean transition (Hao et al. 1998). Its shape was strongly influenced by the palaeogeomorphology of the rifted continental margin basement. Seismic characteristics and the interpreted cross-section in Figure 6 show the TECTONIC AND MONSOON EVOLUTION IN SE ASIA large-scale internal architecture of the Qi Basin. Accurate age control is hard to achieve across this section because of a lack of drilling in the deep water. The basement of the Qi Basin is strongly disrupted by a series of normal faults to form a classic graben-horst structure (Fig. 6). Rifting appears to have ceased by 21 Ma, a little earlier than in the SH-Y Basin, but a little younger than seen in the main depocentre east of Hainan island, the Pearl River Mouth Basin. Sediment influx into the Qi basin before the Pliocene was limited and likely derived from southern China and Hainan because older sediment from the Red River was accommodated in the SH-Y Basin. The total volume of sediments deposited in the Qi Basin is much smaller compared to those in the SH-Y Basin. After c. 2 Ma, sedimentation in the Qi Basin became notably faster. The increase in sediment supply during this period is indicated by steep shoreline trajectories and progradation patterns observed on seismic profiles (Figs 6 & 8c). The vertical stacking and horizontal progradation patterns observed in the seismic profiles demonstrate that sediment supply was faster than accommodation space creation at that time. Another striking feature observed in this basin is the presence of a large-scale canyon, which developed in a NE –SW direction, and which widens and deepens towards the SE (Fig. 6b). The maximum observed size of the canyon is estimated to reach c. 30 km wide and c. 1 km deep. The canyon formed after c. 2 Ma and incised older sediments dating back to c. 2.6–3.6 Ma. Slope gradients increase locally associated with the canyon. As a result, more coarse-grained sediments filled the head of canyon, as evidenced by faster seismic interval velocities compared to the surrounding area. This is illustrated by strong amplitude, low frequency of seismic reflections at the base of the canyon and by the ‘pull-up’ effect of the underlying reflections (Fig. 8d). The formation mechanism of this canyon is unclear but may result from relative base level fall as the area around Hainan was uplifted causing down-cutting and cannibalization of the slope stratigraphy. Time – depth conversion Because well data are only available down to limited depths, which are much shallower than basement in the basin centre, we used stacking velocities derived from seismic processing in order to make a time-depth conversion and thus estimate the depths of stratigraphic and basement surfaces. Once the major stratigraphic surfaces were picked, the sections were converted from time to depth using stacking velocities derived from seismic data processing. We calculated the interval velocity 231 for each layer by applying Dix’s equation (Dix 1955): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V22  T2  V12  T1 Vi ¼ T2  T1 where: Vi is interval velocity for each layer V1, V2 are stacking velocity values for the upper and lower layer boundaries T1, T2 are two-way travel time values down to the upper and lower layer boundaries. The layer thickness and the bottom depth are computed by following equations: Li ¼ Vi (T2  T1 ) 2 and Di ¼ Di1 þ Li where: L1 is layer thickness D1, Di-1: are the depths of the upper and lower layer surfaces. The depth of each boundary surface was used to construct isopach maps and to estimate the sediment budget using all the sections shown in Figure 1. Figure 9 shows contoured isopach maps generated for major stratigraphic units within the SH-Y and Qi basins. Seismic interpretation shows that the basin centre has a maximum depth of c. 22 km. The variation in morphology and depth of the basin through time demonstrates that the depocentre was relatively stationary until the Middle Miocene, after which it gradually migrated towards the SE, following strong basin inversion in the north. In contrast, the Qi Basin overlies both continental shelf and slope, and its deposition pattern is different from the SH-Y Basin. Because this basin forms part of a rifted passive margin and most of the sediments were delivered from Hainan island, sediments tended to accumulate quickly on the continental shelf and slope, but more slowly in the deep basin floor. As a result, total sediment thicknesses in the northwestern half of the basin (c. 8.5 km) are much higher than those on the opposite flank (c. 4 km) (Figs 6 & 9a). Within the Qi Basin, the depocentre has not just migrated towards the SW where it intersects with the SH-Y Basin, but also to the SE (Fig. 9a). Sediment budget Decompaction of the depth-converted sedimentary layers allows us to restore them to their original volumes by correcting for the hydrostatic load of the overlying layers (Sclater & Christie 1980). This procedure was executed in 2D using the software FlexDecompTM v.1.0 (Kusznir et al. 1995). 232 L. V. HOANG ET AL. Fig. 9. Isopach maps of the SH-Y and Qi Basins generated for different time periods show that the depocentre has migrated through time: (a) Total sedimentary thickness of the basin; (b) Eocene– Oligocene; (c) Early Miocene; (d) Pliocene– Pleistocene. The varying sediment thickness shows that more sediment has been deposited in the northwestern half of the Qi Basin, under the present shelf and slope than the southern half of the basin. TECTONIC AND MONSOON EVOLUTION IN SE ASIA In this study, we selected eight seismic lines from the SH-Y Basin and seven lines from the Qi Basin for backstripping. These lines are distributed across the whole basin, covering wide stretches in order to ensure their representative character (Fig. 1). The input data consist of depth values for each stratigraphic surface, lithology and age constraints. Because no age control was defined for the oldest sedimentary layers, we assumed the basin started opening at c. 50 Ma. Each individual layer was then decompacted in reverse order of deposition by unloading the overlying packages. The total unloaded area of each stratigraphic unit was normalized to the total area of the whole decompacted sections within the basin in order to define a normalized coefficient of erosion for each individual stratum. The true volume of sediment deposited during any given time period across the whole basin was estimated by multiplying the basin volume by the normalization coefficient. Rates of sediment supply were then derived by dividing by the duration of sedimentation. The sedimentation rate was computed not only for both SH-Y and Qi basins respectively, but was also calculated for the combined SH-Y and Qi basin. Details of the sediment budget estimation are presented in Figure 10 and Table 1, where our results can be directly compared with those from the earlier work by Clift & Sun (2006) and Métivier et al. (1999). Temporal evolution in mass flux From c. 50–29.5 Ma, sedimentation rates in the SH-Y Basin were modest, but increased for the period 29.5 –21 Ma (Fig. 10a). Sedimentation rates fell again between 21 and 15.5 Ma before rising to a higher level at 15.5 –10.5 Ma. Maximum values of sediment supply are calculated for the Plio-Pleistocene, following a period of lower sedimentation at 10.5–5.5 Ma. This general pattern is comparable with that estimated by Clift & Sun (2006), although they predicted peak sedimentation in the Middle, not the Early Miocene, as we do here. In contrast, sedimentation rates in the Qi Basin are quite different (Fig. 10b). Average sedimentation rates were generally low, except for a modest increase around 15 Ma. The sediment flux into the Qi Basin spiked rapidly only after c. 3 Ma. In this general form our reconstruction is close to that of Clift & Sun (2006), although we predict much higher peak values in the Pleistocene. Because the SH-Y Basin is much larger than the Qi Basin, the combined sedimentation rate reconstruction is similar to that of the SH-Y Basin (Fig. 10c), although with an accentuated pulse of sediment delivery since 3 Ma. Our reconstruction differs greatly from the predicted gradual rise in rates 233 predicted by Métivier et al. (1999) since the Eocene, but mostly differs from the budget of Clift & Sun (2006) in emphasizing the increase in rates in the Early Miocene, with a reduced flux in the Middle Miocene. Monsoon weathering reconstructions In order to assess if climate is controlling temporal variations in sediment flux to the ocean we require a detailed history of environmental conditions with which to compare our sediment budget. In this study we assume that sedimentation rates are a proxy for erosion rates in the Red River Basin onshore and that continental weathering intensity can be used as a proxy for summer monsoon rains. We use weathering records derived from the Pearl River Mouth Basin, largely from ODP Site 1148, where the sediments are derived from erosion of largely flat-lying southern China (Li et al. 2003). The general lack of tectonism or major drainage capture means that variations in sedimentary composition largely reflect climatically modulated chemical weathering intensity. In this study, we used published weathering data (Wei et al. 2006; Wan et al. 2007; Clift et al. 2008c) together with new chemical proxies calculated from the XRF scanning data to trace different aspects of the clastic flux. Figure 11 shows a variety of mineralogical and geochemical records, which do not show parallel development. This raises a question concerning the reliability of the scanning data, as well as the other records, which are not in agreement. Kido et al. (2006) pointed out that XRF signal intensity is reduced by the presence of water within sediment and that a thin water film between the sediment surface and the covering film may affect the reliability of the output data. The presence of water between the sediment and the film may be important but it would be equally so for all parts of the core and would not account for coherent longterm trends in the data, although it might explain some of the short duration spikes in values, especially as seen in Al. Progressive dewatering in deeper buried sediments could generate a long-term trend, although we do not see anything that might be suggestive of that in our data. The absorption effect of water is in the following order Al . Si . K . Ti. Consequently, decreasing water content downsection should increase the intensity of Al counts relative to Si, K, and Ti. This may cause a decrease in Al/Si ratios up-section. However, core description and carbonate contents (Clift 2006) show us that the clastic content was relatively stable and only increased after 6–7 Ma, synchronous with the change in Al/Si values. In contrast, porosity changes more progressively, and only shows a 234 L. V. HOANG ET AL. Fig. 10. Sediment budget estimate derived from this study (dark grey) and compared to the earlier work of Clift & Sun (2006) shown in cross-hatched patterns and Métivier et al. (1999) shown with horizontal lines for: (a) the SH-Y Basin; (b) the Qi Basin; (c) total combined SH-Y and Qi Basin. trend to higher water contents at depths shallower than c. 90 m, equivalent to ages of c. 1.3 Ma (Shipboard Scientific Party 2000). We conclude that there is no correlation between porosity and Al/Si values. This indicates that clay content rather than porosity is the primary control on Al/Si values. We further compare the geochemical weathering records with clay mineral assemblages at nearby ODP Site 1146 (Wan et al. 2007) because certain clay minerals, such as kaolinite and smectite are formed by chemical weathering processes, whereas others, such as illite and chlorite, are the products Table 1. Sediment budget estimation for the Song Hong-Yinggehai and Qiongdongnam basins Stratum area/Seismic line (km2) 3555 C-58-79 Song Hong– Yinggehai Basin 0.0– 2.0 52.8 110.9 2.0– 5.5 128.8 237.9 5.5– 10.5 36.0 50.6 10.5– 15.5 91.2 87.8 15.5– 21.0 89.0 38.9 21.0– 29.5 168.9 54.0 29.5– 50.0 75.7 157.7 GPGT 93-207 GPGT 93-211 GPGT 93-215 GPGT 93-219 GPGT 93-223 GPGT 93-225 59.7 294.7 72.4 144.5 65.1 398.2 673.3 85.0 309.3 104.4 228.0 180.5 1003.8 1280.9 109.2 274.9 151.4 344.2 232.2 878.3 802.2 124.4 237.8 278.5 337.7 212.0 538.3 518.6 143.2 262.7 166.5 262.4 145.2 483.4 684.7 183.8 168.9 223.2 231.5 83.9 261.8 278.0 Total area (km2) Real volume (km3) Mean sedimentation rate (km3/Ma) Maximum sedimentation rate (km3/Ma) Minimum sedimentation rate (km3/Ma) 868.9 1915.2 1082.9 1727.4 1046.9 3786.8 4471.1 32.1 70.7 40.0 63.8 38.6 139.8 165.0 16.0 20.2 8.0 12.8 7.0 16.4 8.1 19.2 24.2 9.6 15.3 8.4 19.7 9.7 12.8 16.2 6.4 10.2 5.6 13.2 6.4 1079.0 482.8 117.7 186.7 180.7 141.8 347.9 162.3 121.8 919.4 86.5 38.7 9.4 15.0 14.5 11.4 27.9 13.0 9.8 73.7 43.3 24.2 5.0 3.0 4.4 6.7 5.1 3.8 3.2 3.3 51.9 29.0 6.0 3.6 5.3 8.0 6.1 4.6 3.8 3.9 34.6 19.4 4.0 2.4 3.5 5.4 4.1 3.1 2.5 2.6 Stratum area/Seismic line (km2) C-57-79 C-35-69 Qiongdongnan Basin 0.0– 2.0 39.6 2.0– 3.6 53.4 3.6– 5.5 30.5 5.5– 10.5 8.0 10.5– 13.8 41.2 13.8– 15.5 25.2 15.5– 21.0 22.2 21.0– 24.4 26.9 24.4– 27.5 8.0 27.5– 50 76.0 33.1 37.6 21.5 5.7 25.4 13.2 13.1 18.7 7.4 71.2 44039 C-49-79 C-65-79 C-73-79 C-98-79 76.9 43.7 5.4 14.2 9.4 5.9 19.0 17.1 15.6 113.2 159.3 61.8 8.6 22.7 15.0 14.0 45.0 18.5 16.9 122.7 191.2 77.4 12.5 32.9 21.7 15.0 49.8 20.0 18.2 132.1 267.3 106.4 17.3 45.5 30.0 30.5 98.8 25.0 22.8 165.5 311.7 102.5 21.9 57.7 38.0 38.0 100.0 36.1 32.9 238.7 TECTONIC AND MONSOON EVOLUTION IN SE ASIA Stratum age (Ma) 235 236 L. V. HOANG ET AL. Fig. 11. Geochemical data derived from ODP Site 1148 and showing the temporal variations in the intensity of chemical weathering through the Neogene, interpreted as a monsoon proxy in this study: (a) colour data (CRAT) (Clift et al. 2008c); (b) illite/smectite ratios for ODP Site 1146 (Wan et al. 2007); (c) chemical index of alteration (CIA) (Wei et al. 2006); (d) Al/Si ratio; (e) Ti/Ca ratio; (f) sediment budgets estimated for the whole SH-Y and Qi Basins. TECTONIC AND MONSOON EVOLUTION IN SE ASIA of physical weathering (Thiry 2000). As a result ratios such as illite/smectite can be used to indicate relative strengthening of physical v. chemical processes (Fig. 11b). The data from ODP Site 1146 suggest strengthening of physical weathering after 12 Ma and especially after c. 8 Ma. High-resolution scanning-based weather proxies can also be compared with whole rock XRF analyses of Wei et al. (2006), which allow the CIA to be calculated (Fig. 11c). This proxy shows a general trend to decreasing values through time, which Wei et al. (2006) interpreted to reflect decreasing weathering intensities under the influence of a steadily weakening summer monsoon. This simple interpretation is hard to reconcile with some other monsoon proxies, such as those invoking strong monsoon at 8 Ma (Chen et al. 2003; Zheng et al. 2004). However, a steep decrease in CIA at 8 Ma coincides with a rise in illite/ smectite (Wan et al. 2007), indicating a period of important environmental change. In addition, we plot Al/Si as a measure of the relative proportion of clays compared to quartz sand in the sediment (Fig. 11d). Clays are rich in Al and this proxy has the advantage of not being affected by the large amount of biogenic carbonate in the cores. Most of the core at ODP Site 1148 is very fine grained and no sandy or silty intervals were identified (Shipboard Scientific Party 2000), yet the Al/Si proxy shows strong temporal variations, most notably a steady decrease (i.e. less clay) since c. 8 Ma, after a period of high clay content in the Middle and Late Miocene (Fig. 11d). More sandy flux might indicate stronger physical weathering onshore after that time, consistent with the falling CIA values. We are able to gain further insight into the evolving clay mineralogy itself by reference to the CRAT proxy of Clift et al. (2008c). Clay mineralogy has been used in the past to reconstruct the changing intensity of the monsoon both in South Asia (Derry & France-Lanord 1996a) and in the South China Sea (Clift et al. 2002; Wehausen & Brumsack 2002; Trentesaux et al. 2006). CRAT is calculated based on colour spectral data and end member mixing calculations, and is designed to provide a measure of the mineralogical ratio chlorite/ (chlorite þ hematite þ goethite). The method is remote, but has been calibrated using laboratory standards under the assumption that visible range colour spectra are principally controlled by chlorite, hematite, and goethite, and clearly reflect long-term variations in core chemistry and mineralogy that cannot be linked to diagenesis. The alteration minerals hematite and goethite are largely produced by chemical weathering, whereas chlorite is indicative of physical erosion (Chamley 1989). As a result CRAT measures the relative intensities of chemical 237 weathering and indicates a period of strong chemical weathering in southern China during the Middle –Late Miocene, followed by less intense weathering (and presumably a weaker summer monsoon) in the Late Miocene– Pliocene (Fig. 11a). The period of reducing chemical weathering shown by climbing CRAT values after 12 Ma correlates with increased illite/smectite at ODP Site 1146 (Wan et al. 2007) (Fig. 11b) as might be predicted. In contrast, strong chemical weathering during the Middle Miocene is shown by low illite/smectite ratios and low CRAT values. However, the high CRAT values around 16 Ma show no response in clay mineralogy. Unfortunately, the record at ODP Site 1146 does not extend far enough back in time to see whether illite/smectite was higher before 23 Ma, as might be predicted. Comparison of both CRAT values at ODP Site 1148 and illite/smectite ratios at ODP Site 1146 are hard to reconcile in a simple way with detailed CIA values. However, all three proxies suggest generally weaker summer monsoons and less weathering after the Middle Miocene. CIA does not decrease in a uniform fashion, but shows at least two positive excursions centred at c. 16– 17 and 3– 5 Ma, interpreted as periods of stronger chemical weathering and stronger summer monsoon. The period at 3–5 Ma is noteworthy in having anomalously low illite/smectite ratios, consistent with stronger chemical weathering (Fig. 11b), but this is at odds with the low CRAT value. CIA increases after 5 Ma at the same time that CRAT begins to increase, although since 3 Ma CIA has shown modest decrease at a time that CRAT values decreased, and when illite/smectite ratios were very variable. Differences between these proxies are hard to interpret. Some of the issues may relate to the fact that CRAT examines only the clay mineral fraction of the sediment, whereas CIA looks at the bulk sediment. Furthermore, CRAT values are controlled in part by goethite and hematite and do not factor in smectite, which requires higher degrees of chemical weathering to become abundant. Finally, we consider the Ti/Ca ratio, which is a good proxy for evaluating the relative influence of clastic v. carbonate sediment influx (Fig. 11e). From c. 23 to 6 Ma, the Ti/Ca ratio remained mostly at low levels, but with two short periods of higher values observed at c. 15– 17 and 10.5 – 11.5 Ma. What is most striking is the steady increase in Ti/Ca values since 4 Ma, suggestive of a rapidly increasing clastic flux to the drill site. This is consistent with shipboard core description and CaCO3 measurements (Shipboard Scientific Party 2000). This trend parallels reconstructed trends in monsoon-related foraminifera at ODP Site 1146 on the northern margin of South China Sea (Wang 238 L. V. HOANG ET AL. et al. 2003), as well as winter monsoon dust records from the North Pacific (Rea 1994) and from the Chinese Loess Plateau (An et al. 2001). However, enhanced post 4 Ma clastic flux to the ocean is not only a pan-Asian phenomenon that has been linked to intensifying summer monsoon (Métivier et al. 1999; Clift 2006), but is recognized worldwide and has been linked to fast continental erosion under the influence of variable glacial –interglacial cycles (Zhang et al. 2001). Discussion SE Asia is a classic natural laboratory for examining possible interactions between solid Earth tectonics and climatic evolution. Many researchers have used information from sediment records in the South China Sea, as well as from bedrock onshore, in order to quantitatively model this relationship by reconstructing the timing of East Asian monsoon intensification. However, clear linkages between monsoon intensity, topographic growth and its effects on continental erosion have been hard to find because of uncertainties in all three processes. Results from our study suggest that combinations of geochemical and mineralogical data derived from ODP Site 1148, with sediment budgets from the Pearl and Red Rivers show some correlation, especially in the Pleistocene and Early Miocene, suggesting that climate variability is a controlling factor on continental weathering in SE Asia. The role of topographic uplift is harder to constrain because of the uncertainties in Tibetan and SE Asian topographic growth (Clark et al. 2005; Harris 2006; Schoenbohm et al. 2006a) and the possible influences of drainage reorganization in governing the flux of sediment to the Gulf of Tonkin. Reconstructing the stratigraphic evolution of the SH-Y and Qi Basins is important to studies of monsoon-tectonic coupling relationships for several reasons. These two basins together form one of the largest sedimentary masses in SE Asia and record the erosion flux from a major drainage that cuts the flank of the Tibetan Plateau over a long period of time. Uplift of SE Tibet and Yunnan might therefore be expected to have driven faster mass fluxes as the gorges of the upper Red River were cut. Although this is a region of strong summer monsoon rains the heaviest rains are closer to the coast, not in the areas of strongest surface deformation. A stronger monsoon might be expected to cause stronger run-off, higher erosion and faster sedimentation rates. This area contrasts with the erosional links proposed for the frontal ranges of the Himalaya where the steep topographic gradient results in a close coupling of climate, exhumation and structure (Hodges et al. 2004; Thiede et al. 2004; Clift et al. 2008c). In contrast, the topographic gradient of the edge of the Tibetan Plateau is much more gradual along its southeastern flank. Tectonic and stratigraphic evolution Although the nature of the oldest sedimentary formations within the SH-Y and Qi Basins is still uncertain, there is little doubt that deposition had started during the Eocene –Oligocene (Zhong et al. 2004; Clift & Sun 2006). The basin began to subside strongly after c. 34 –35 Ma when motion on the RRFZ first started (Gilley et al. 2003). This motion and its associated transtensional faulting together controlled formation of the SH-Y Basin as a pull-apart. Motion on the RRFZ probably caused the pre-rift formations to be uplifted, deformed and eroded, at least locally (Figs 3 & 7). This effect is best observed in the northern part of the SH-Y basin. Relatively stationary depocentres during the Oligocene–Early Miocene suggest that most of sediments delivered from the Red River were trapped in the northern and central SH-Y Basin, while little sediment was reaching the Qi Basin. Active faulting and basin subsidence continued until c. 21 Ma and was followed by slower thermal subsidence after an inversion event focused in the northern SH-Y Basin, younging to c. 15 Ma in the south. The cessation of seafloor spreading and the end of left-lateral motion on the RRFZ together resulted in an inversion event in the early Middle Miocene. These processes correlate well with the change in the basin from transtensional to transpressional character. As a result the whole basin was strongly inverted, deformed and eroded before 15 Ma. A significant portion of the pre-uplift formations in the northwestern SH-Y Basin was removed following strong uplift and erosion (Figs 3 & 7). For this reason, very little of the sediment delivered by the Red River was deposited in that part of the system at that time. Instead, sediments were bypassed to the centre and southeastern end of the basin where the basin floor was still deep and where accommodation space allowed preservation. However, the Qi Basin did not experience Mid-Miocene inversion, presumably because of its distance from the RRFZ. After inversion, the SH-Y Basin gradually subsided again. Low-angle shelf edge trajectories observed in some places demonstrate that less accommodation space was created after 15 Ma, consistent with reconstructions of basement tectonic subsidence (Clift & Sun 2006). None the less, the integrated basin-wide sedimentary budget shows faster sedimentation at 11–15.5 Ma compared to the Early Miocene. TECTONIC AND MONSOON EVOLUTION IN SE ASIA Influence of Hainan Clinoforms observed in Figures 4 and 5 show that the northwestern SH-Y Basin was nearly filled after c. 5 Ma, so that most of the younger sediments must have overspilled not only to the SE, but partly into the Qi Basin, as they did during the Last Glacial Maximum (c. 20 ka). Sediments derived from the Red River and eroded from Hainan island were deposited together in the northwestern half of the Qi Basin as a large prograding clastic wedge. The high-angle shelf edge trajectory and the direction of progradation observed in southern Hainan (Fig. 6) suggest that significant volumes of sediments eroded from the island were delivered to the Qi Basin. The increase in sediment supply from that area may be a response to the tectonically driven uplift of the Hainan island, linked to magmatism during the Pleistocene (Tu et al. 1991; Flower et al. 1998). In addition, faster erosion in Hainan and within the Red River Basin driven by an intensified summer monsoon may be responsible for some of the increased erosion. Finally we consider that progressive surface uplift in SW China (Yunnan) and northern Vietnam within the headwaters of the Red River during the Pliocene (Schoenbohm et al. 2006a) may have driven faster erosion by causing enhanced incision of the Red River. Chemical weathering and climate change Intensification of the East Asian monsoon may be one of the most important factors in controlling continental weathering processes. On the continents precipitation, temperature and vegetation are the primary controls on both chemical weathering and physical erosion (White & Blum 1995; Edmond & Huh 1997; West et al. 2005). Over geological timescales, a measure of this is preserved in the chemistry and mineralogy of sediments transported by rivers. Water-mobile elements are easily removed from weathering products and whereas more stable elements tend to be relatively enriched. Based on these principles, we now combine information derived from the sedimentary budget and from geochemical analysis to assess how changing monsoon strength may have influenced erosion within the Red River drainage since the Oligocene. Figure 10c shows that sedimentation flux from the Red River Basin initially increased after c. 29 Ma and subsequently fell again after 21 Ma. Unfortunately, there is no weathering record predating 24 Ma. Clay mineralogical records from ODP Site 1148 show monsoon strengthening after c. 22 Ma (Clift et al. 2008c). If this is true then the higher erosion rates at 21–28 Ma largely predate a strong monsoon, while the first period of strong summer monsoon (22– 17 Ma) correlates with a 239 period of reduced sediment flux to the SH-Y Basin (Fig. 10a). The increase in sediment flux into the basin at 21–29 Ma was thus more likely to have been triggered by the onset of topographic uplift and exhumation related to the Red River Shear Zone, which started at c. 34– 35 Ma (Leloup et al. 2001; Gilley et al. 2003). A decrease in sedimentation rate after c. 21 Ma may indicate erosion of the early topography. Minimum CRAT values and low illite/smectite ratios from c. 15 –10 Ma, together with steady CIA values (Fig. 11a, b, c) indicates that chemical weathering was strong in a climate of intensified summer monsoon rain (Wan et al. 2007). This time correlates with a period of moderately increased sedimentation rates (Fig. 11f ). A positive link between erosion rates and monsoon intensity is suggested at that time, not least because the RRFZ became inactive after 15 Ma, so that the increased rates of erosion are the opposite of those predicted if tectonic forces were the dominant erosion control. Decreased CIA values after 8 Ma indicates that chemical weathering weakened after this time. Similarly, rising CRAT values, high illite/smectite and decreased sedimentation rates all point to weaker chemical weathering in a drying monsoon climate after 8 Ma, correlating with a fall in clastic flux. This drop in sedimentation rates during the Late Miocene parallels similar synchronous trends recognized on the Bengal Fan (Burbank et al. 1993). However, unlike that study we suggest a positive correlation between erosion and monsoon strength because 8 Ma now appears to mark a time of summer monsoon weakening not intensification as previously believed (Derry & France-Lanord 1996b). Tibetan gorge incision Although the RRFZ was reactivated after 5 Ma, albeit in a reverse, dextral sense (Schoenbohm et al. 2006b), the degree of active shear in the Red River drainage never regained the rates seen in the Middle –Early Miocene. However, progressive uplift of eastern Tibet, driving gorge incision along the edge of the plateau may have been an influence on sediment flux to the SH-Y and Qi basins. Thermochronological work in Yunnan and Sichuan in SW China indicates accelerated surface uplift there starting around 11 Ma (Clark et al. 2005). However, this was a time of slower sedimentation in the SH-Y and Qi basins, indicating either that the drier monsoon was the greater influence on erosion or that the gorge incision in SW China was not feeding sediment to the Red River. Reconstructions of the rivers around the eastern Himalayan syntaxis suggest that capture of the Yunnan rivers away from the Red River was completed 240 L. V. HOANG ET AL. before that time (Clark et al. 2004; Clift et al. 2006a), so that any sediment pulse would have been diverted into the East China Sea, consistent with our sediment budgets. Surface uplift in northern Vietnam postdates that in Yunnan, being mostly Pliocene in age and reflecting gradual growth of topography to the SE (Schoenbohm et al. 2006a). This phase of tectonism and associated gorge incision correlates well with the pulse of sediment seen in the SH-Y and Qi basins. The Ti/Ca curve from ODP Site 1148 shows an increase at this time despite the fact that the Pearl River Basin is less affected by topographic uplift. Intensification of the summer monsoon since c. 4 Ma in the South China Sea region may be the dominant control on increasing erosion within this time period (Wan et al. 2006), whereas rock uplift now controls the patterns of erosion within the Red River Basin itself (Clift et al. 2006b). Our climate reconstruction is consistent with regional compilations of increased sediment flux in the Middle Miocene (Clift 2006), although our records favour an earlier start to higher sediment flux, during the Early Miocene. The higher rates of sediment flux at 29–21 Ma are unique to the Red River system and support a local tectonic rather than regional climatic trigger. Our apparent initial monsoon intensification after c. 22 Ma is much earlier than the commonly cited 8 Ma monsoon intensification (Kroon et al. 1991; Prell et al. 1992; Zheng et al. 2004), but is consistent with the revised summer monsoon model of Clift et al. (2008c). Our estimate is older than the c. 15 Ma intensification suggested by Wan et al. (2007), but is consistent with palynology and facies information from China (Sun & Wang 2005). Decreasing humidity from c. 8 –4 Ma and especially a rapid drop between 5 and 4 Ma, charted by falling CIA values (Wei et al. 2006), falling kaolinite contents (Wan et al. 2007) and rising CRAT values (Clift et al. 2008c) (Fig. 10) testify to a weakening monsoon. This resulted in less physical erosion in the mountainous Red River basin and reduced chemical weathering in the flatter Pearl River Basin. Chemical weathering is further reduced by falling global temperatures since the Middle Miocene (Zachos et al. 2001). This change is shown by low sedimentation rate (Fig. 11f) and by decrease in the Al/Si ratios (Fig. 11d). The period of c. 3–4 Ma is marked by falling CRAT values, higher illite/smectite ratios and lower CIA, which are not all in accord regarding the long-term change in summer monsoon strength. Summer monsoon strength varies rapidly over millennial timescales at this time and a longer duration pattern is hard to discern (Clift & Plumb 2008). This period is also accompanied by decreases in the Al/Si and CIA ratios whereas the Ti/Ca increased, suggesting enhanced coarser influx, as well as stronger chemical weathering. Rapid increase in the Ti/Ca ratio after c. 4 Ma, especially after c. 2.7 Ma indicates an increase in clastic sediment influx relative to carbonate sediments, which was probably caused by enhanced continental physical weathering driven by the transition between glacial-interglacial climate states (Zhang et al. 2001). Conclusions Tectonically driven surface uplift of eastern Tibet has commonly been linked to enhancement of the East Asia monsoon. Both these processes have the potential to increase the rates of continental erosion, which should be reflected in the volumes and composition of sediments in river deltas. This study shows that a combination of sedimentary budgets derived from regional seismic stratigraphic and geochemical data can be employed to compare weathering regimes and erosion rates over tectonic time periods .20 Ma. A combination of proxies allows us partially to reconstruct the history of monsoon climate change since 24 Ma. Our work confirms that the SH-Y and Qi basins formed after c. 50 Ma and especially subsided rapidly after c. 34 Ma, coincident with the onset of motion on the RRFZ. The SH-Y Basin experienced two inversion phases that occurred at c. 34 and c. 15.5 Ma, while the Qi Basin seemed not to be affected by these events. 34 –17 Ma motion on the RRFZ correlates with a period of faster sedimentation in the SH-Y Basin. This is despite the initial intensification of the monsoon dating from only c. 22 Ma. We conclude that tectonic forces are dominant in controlling erosion at that time. Geochemical data suggest that chemical weathering has generally decreased since c. 25 Ma, while physical erosion became stronger. A shift to more physically eroded chlorite and increasing sedimentation rates after c. 15.5 Ma points to stronger rains and stronger physical erosion between 15.5 and 10 Ma. In this period climate appears to dominate as the primary erosional control, as motion on the RRFZ had ceased. The period from 10 to 4 Ma saw a reduction in chemical weathering and sediment flux, correlating with a time of weakening summer monsoon. However, the transition to glacialinterglacial climates, surface uplift in northern Vietnam and Hainan island and stronger summer monsoons, at least during the interglacial periods, since 4 Ma correlates with a switch back to stronger erosion of the source rocks, especially physical erosion, which in turn raised the clastic influx into the basins. TECTONIC AND MONSOON EVOLUTION IN SE ASIA We thank the Natural Environment Research Council (NERC) in the United Kingdom and the College of Physical Sciences at the University of Aberdeen for funding and support for this project. We particularly thank BP Exploration for release of new seismic data to our project. We thank PetroVietnam, the Chinese National Offshore Oil Company (CNOOC), and Integrated Ocean Drilling Program (IODP) for additional supporting data. Seismic Micro-Technology Inc. provided use of the KingdomTM seismic interpretation software. We also wish to thank David Heslop, Alan Roberts, Nick Kusznir, Röhl Ursula, Vera Lukies, Prof. Mai Thanh Tan and Tran Thi Kieu Hoa for technical advice. References An, Z., Kutzbach, J. E., Prell, W. L. & Porter, S. C. 2001. Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan Plateau since late Miocene times. Nature, 411, 62–66. Briais, A., Patriat, P. & Tapponnier, P. 1993. Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: implications for the Tertiary tectonics of Southeast Asia. Journal of Geophysical Research, 98, 6299– 6328. Brookfield, M. E. 1998. The evolution of the great river systems of southern Asia during the Cenozoic India-Asia collision; rivers draining southwards. Geomorphology, 22, 285–312. Burbank, D. W., Derry, L. A. & France-Lanord, C. 1993. Reduced Himalayan sediment production 8 Myr ago despite an intensified monsoon. Nature, 364, 48– 50. Burbank, D. W., Blythe, A. E. et al. 2003. Decoupling of erosion and precipitation in the Himalayas. Nature, 426, 652– 655. Chamley, H. 1989. Clay Sedimentology. Springer-Verlag, Berlin. Chen, M., Wang, R., Yang, L., Han, J. & Lu, J. 2003. Development of East Asian summer monsoon environments in the late Miocene; radiolarian evidence from Site 1143 of ODP Leg 184. Marine Geology, 201, 169–177. Clark, M. K., Schoenbohm, L. M. et al. 2004. Surface uplift, tectonics, and erosion of eastern Tibet from large-scale drainage patterns. Tectonics, 23, TC1006, doi: 10.1029/2002TC001402. Clark, M. K., House, M. A., Royden, L. H., Whipple, K. X., Burchfiel, B. C., Zhang, X. & Tang, W. 2005. Late Cenozoic uplift of southeastern Tibet. Geology, 33, 525– 528, doi: 10.1130/G21265.1. Clift, P. D. 2006. Controls on the erosion of Cenozoic Asia and the flux of clastic sediment to the ocean. Earth and Planetary Science Letters, 241, 571–580. Clift, P. & Lin, J. 2001. Preferential mantle lithospheric extension under the South China margin. Marine and Petroleum Geology, 18, 929– 945. Clift, P. D. & Plumb, R. A. 2008. The Asian Monsoon: Causes, History and Effects. Cambridge University Press, Cambridge. Clift, P. D. & Sun, Z. 2006. The sedimentary and tectonic evolution of the Yinggehai-Song Hong Basin and the 241 southern Hainan margin, South China Sea; implications for Tibetan uplift and monsoon intensification. Journal of Geophysical Research, 111(B6, 28), doi: 10.1029/2005JB004048. Clift, P., Lee, J. I., Clark, M. K. & Blusztajn, J. 2002. Erosional response of south China to arc rifting and monsoonal strengthening; a record from the South China Sea. Marine Geology, 184, 207– 226. Clift, P. D., Blusztajn, J. & Nguyen, D. A. 2006a. Large-scale drainage capture and surface uplift in eastern Tibet-SW China before 24 Ma inferred from sediments of the Hanoi Basin, Vietnam. Geophysical Research Letters, 33, doi: 10.1029/ 2006GL027772. Clift, P. D., Carter, A., Campbell, I. H., Pringle, M., Hodges, K. V., Lap, N. V. & Allen, C. M. 2006b. Thermochronology of mineral grains in the Song Hong and Mekong Rivers, Vietnam. Geophysics, Geochemistry, Geosystems, 7, doi: 10.1029/2006GC001336. Clift, P., Lee, G. H., Nguyen, A. D., Barckhausen, U., Hoang, V. L. & Sun, Z. 2008a. Seismic evidence for a Dangerous Grounds mini-plate: no extrusion origin for the South China Sea. Tectonics, 27, doi: 10.1029/ 2007TC002216. Clift, P. D., Hoang, V. L., Hinton, R., Ellam, R., Hannigan, R., Tan, M. T. & Nguyen, D. A. 2008b. Evolving East Asian river systems reconstructed by trace element and Pb and Nd isotope variations in modern and ancient Red River-Song Hong sediments. Geochemistry Geophysics Geosystems, 9, doi: 10.1029/2007GC001867. Clift, P. D., Hodges, K., Heslop, D., Hannigan, R., Hoang, L. V. & Calves, G. 2008c. Greater Himalayan exhumation triggered by Early Miocene monsoon intensification. Nature Geoscience, 1, 875 –880, doi: 10.1038/ngeo351. Derry, L. & France-Lanord, C. 1996a. Neogene Himalayan weathering history and river 87Sr/86Sr: impact on the marine Sr record. Earth and Planetary Science Letters, 142, 59– 74. Derry, L. A. & France-Lanord, C. 1996b. Neogene Himalayan weathering history and river 87 Sr/86Sr; impact on the marine Sr record. Earth and Planetary Science Letters, 142, 59–74. Dix, C. H. 1955. Seismic velocities from surface measurements. Geophysics, 20, 68–86. Dooley, T. & McClay, K. 1997. Analog modeling of pull-apart basins. AAPG Bulletin, 81, 1804– 1826. Edmond, J. M. & Huh, Y. 1997. Chemical weathering yields from basement and orogenic terrains in hot and cold climates. In: Ruddiman, W. F. (ed.) Tectonic Climate and Climate Change. Plenum Press, New York, 330– 353. Fang, H., Li, S., Gong, Z. & Yang, J. 2000. Thermal regime, interreservoir compositional heterogeneities, and reservoir-filling history of the Dongfang gas field, Yinggehai Basin, South China Sea; evidence for episodic fluid injections in overpressured basins? AAPG Bulletin, 84, 607 –626. Filippelli, G. M. 1997. Intensification of the Asian monsoon and a chemical weathering event in the late Miocene-early Pliocene: implications for late Neogene climate change. Geology, 25, 27–30. 242 L. V. HOANG ET AL. Flower, M., Tamaki, K. & Hoang, N. 1998. Mantle extrusion: a model for dispersed volcanism and DUPAL-like asthenosphere in East Asia and the western Pacific. In: Flower, M. F. J., Chung, S.-L., Lo, C.-H. & Lee, T. Y. (eds) Mantle Dynamics and Plate Interactions in East Asia. American Geophysical Union, Washington, DC. Geophysical Monograph, 27, 67–88. Gabet, E. J., Edelman, R. & Langner, H. 2006. Hydrological controls on chemical weathering rates at the soil-bedrock interface. Geology, 34, 1065– 1068, doi: 10.1130/G23085A.1. Gilley, L. D., Harrison, T. M., Leloup, P. H., Ryerson, F. J., Lovera, O. M. & Wang, J. H. 2003. Direct dating of left-lateral deformation along the Red River shear zone, China and Vietnam. Journal of Geophysical Research, 108, doi: 10.1029/2001JB001726. Gong, Z. & Li, S. 2004. Dynamic research of oil and gas accumulation in northern marginal basins of South China Sea. Science Press, Beijing. Hall, R. 1996. Reconstructing Cenozoic SE Asia. In: Hall, R. & Blundell, D. J. (eds) Tectonic Evolution of Southeast Asia. Geological Society, London, Special Publications, 106, 203– 224. Hallet, B. & Molnar, P. 2001. Distorted drainage basins as markers of crustal strain east of the Himalaya. Journal of Geophysical Research, 106, 13, 697– 13, 709. Hao, F., Li, S., Sun, Y. & Zhang, Q. 1998. Geology, compositional heterogeneities, and geochemical origin of the Yacheng gas field, Qiongdongnan Basin, South China Sea. AAPG Bulletin, 82, 1372–1384. Hao, F., Li, S., Gong, Z. & Yang, J. 2002. Mechanism of diapirism and episodic fluid injections in the Yinggehai Basin. Science in China Series D: Earth Sciences, 45, 151–159. Harris, N. B. W. 2006. The elevation of the Tibetan Plateau and its impact on the monsoon. Palaeogeography Palaeoclimatology Palaeoecology, 241, 4– 15. Harrison, T. M., Leloup, P. H., Ryerson, F. J., Tapponnier, P., Lacassin, R. & Wenji, C. 1996. Diachronous initiation of transtension along the Ailao Shan-Red River shear zone, Yunnan and Vietnam. In: Yin, A. & Harrison, T. M. (eds) The Tectonic Evolution of Asia. Cambridge University Press, Cambridge, 110– 137. Hazebroek, H. P. & Tan, D. N. K. 1993. Tertiary tectonic evolution of the NW Sabah continental margin. Geological Society of Malaysia Bulletin, 33, 195–210. He, L., Xiong, L. & Wang, J. 2002. Heat flow and thermal modeling of the Yinggehai Basin, South China Sea. Tectonophysics, 351, 245–253. Hodges, K. V., Wobus, C., Ruhl, K., Schildgen, T. & Whipple, K. 2004. Quaternary deformation, river steepening, and heavy precipitation at the front of the Higher Himalayan ranges. Earth and Planetary Science Letters, 220, 379–389. Holloway, N. H. 1982. The stratigraphy and tectonic relationship of Reed Bank, North Palawan and Mindoro to the Asian mainland and its significance in the evolution of the South China Sea. AAPG Bulletin, 66, 1357– 1383. Hsu, S.-K., Yeh, Y. C., Doo, W. B. & Tsai, C. H. 2005. New bathymetry and magnetic lineations identifications in the northernmost South China Sea and their tectonic implications. Marine Geophysical Researches, 25, 29–44, doi: 10.1007/s11001-0050731-7. Hutchison, C. S. 1996. The ‘Rajang acctionary prism’ and ‘Lupar Line’ problem of Borneo. In: Hall, R. & Blundell, D. J. (eds) Tectonic Evolution of Southeast Asia. Geological Society, London, Special Publications, 106, 247– 261. Jia, G., Peng, P., Zhao, Q. & Jian, Z. 2003. Changes in terrestrial ecosystem since 30 Ma in East Asia: stable isotope evidence from black carbon in the South China Sea. Geology, 31, 1093–1096. Kido, Y., Koshikawa, T. & Tada, R. 2006. Rapid and quantitative major element analysis method of wet sediment samples using a XRF microscanner. Marine Geology, 229, 209–225. Kroon, D., Steens, T. & Troelstra, S. R. 1991. Onset of Monsoonal related upwelling in the western Arabian Sea as revealed by planktonic foraminifers. In: Prell, W. & Niitsuma, N. (eds) Proceedings of the Ocean Drilling Program, Scientific Results. College Station, TX: Ocean Drilling Program, 117, 257–263. Kusznir, N. J., Roberts, A. M. & Morley, C. K. 1995. Forward and reverse modelling of rift basin formation. In: Lambiase, J. J. (ed.) Hydrocarbon Habitat in Rift Basins. Geological Society, London, Special Publications, 80, 33–56. Leloup, P. H., Arnaud, N. et al. 2001. New constraints on the structure, thermochronology, and timing of the Ailao Shan-Red River shear zone, SE Asia. Journal of Geophysical Research, 106, 6657–6671. Li, X., Wei, G. et al. 2003. Geochemical and Nd isotopic variations in sediments of the South China Sea; a response to Cenozoic tectonism in SE Asia. Earth and Planetary Science Letters, 211, 207–220. Lu, W., Ke, C., Wu, J., Liu, J. & Lin, C. 1987. Characteristics of magnetic lineations and tectonic evolution of the South China Sea basin. Acta Oceanologica Sinica, 6, 577–588. Mai, T. T., Le, H. A. et al. 2005. Study on structural setting of Pliocene-Quaternary sediments offshore vietnam, Advances in Natural Sciences, Vietnam, 0-28. Manabe, S. & Terpstra, T. B. 1974. The effects of mountains on the general circulation of the atmosphere as identified by numerical experiments. Journal of Atmospheric Science, 31, 3– 42. Matthews, S. J., Fraser, A. J., Lowe, S. & Todd, S. P. F. J. P. 1997. Structure, stratigraphy and petroleum geology of the SE Nam Con Son Basin, offshore Vietnam. In: Fraser, A. J., Matthews, S. J. & Murphy, R. W. (eds) Petroleum Geology of Southeast Asia. Geological Society, London, Special Publications, 126, 89– 106. Métivier, F., Gaudemer, Y., Tapponnier, P. & Klein, M. 1999. Mass accumulation rates in Asia during the Cenozoic. Geophysical Journal International, 137, 280–318. Miall, A. D. 1991. Stratigraphic sequences and their chronostratigraphic correlation. Journal of Sedimentary Petrology, 61, 497– 505. Molnar, P., England, P. & Martinod, J. 1993. Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon. Reviews of Geophysics, 31, 357– 396. TECTONIC AND MONSOON EVOLUTION IN SE ASIA Morley, C. K. 2002. A tectonic model for the Tertiary evolution of strike-slip faults and rift basins in SE Asia. Tectonophysics, 347, 189– 215. Nesbitt, H. W. & Young, G. M. 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature (London), 299, 715–717. Nesbitt, H. W., Markovics, G. & Price, R. C. 1980. Chemical processes affecting alkalis and alkaline earths during continental weathering. Geochimica et Cosmochimica Acta, 44, 1659–1666. Prell, W. L. & Kutzbach, J. E. 1992. Sensitivity of the Indian Monsoon to forcing parameters and implications for its evolution. Nature, 360, 647– 652. Prell, W. L., Murray, D. W., Clemens, S. C. & Anderson, D. M. 1992. Evolution and variability of the Indian Ocean Summer Monsoon: evidence from the western Arabian Sea drilling program. In: Duncan, R. A., Rea, D. K., Kidd, R. B., von Rad, U. & Weissel, J. K. (eds) Synthesis of Results from Scientific Drilling in the Indian Ocean. American Geophysical Union, Washington. Geophysical Monograph, 70, 447– 469. Rangin, C., Klein, M., Roques, D., Le Pichon, X. & Trong, L. V. 1995. The Red River fault system in the Tonkin Gulf, Vietnam. Tectonophysics, 243, 209–222. Rea, D. K. 1994. The paleoclimatic record provided by eolian deposition in the deep sea; the geologic history of wind. Reviews in Geophysics, 32, 159– 195. Reiners, P. W., Ehlers, T. A., Mitchell, S. G. & Montgomery, D. R. 2003. Coupled spatial variations in precipitation and long-term erosion rates across the Washington Cascades. Nature (London), 426, 645–647. Replumaz, A. & Tapponnier, P. 2003. Reconstruction of the deformed collision zone between India and Asia by backward motion of lithospheric blocks. Journal of Geophysical Research, 108, doi: 10.1029/ 2001JB000661. Röhl, U. & Abrams, L. J. 2000. High-resolution, downhole and non-destructive core measurements from Sites 999 and 1001 in the Caribbean Sea: application to the Late Paleocene Thermal Maximum. Proceedings of the Ocean Drilling Program, Scientific Results, 165, 191–203. Schoenbohm, L. M., Burchfiel, B. C. & Chen, L. 2006a. Propagation of surface uplift, lower crustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau. Geology, 34, 813– 816, doi: 10.1130/G22679.1. Schoenbohm, L. M., Burchfiel, B. C., Chen, L. & Yin, J. 2006b. Miocene to present activity along the Red River Fault, China, in the context of continental extrusion, upper-crustal rotation, and lower-crustal flow. Geological Society of America Bulletin, 118, 672–688. Sclater, J. G. & Christie, P. A. F. 1980. Continental stretching: an explanation of the post Mid-Cretaceous subsidence of the central North Sea basin. Journal of Geophysical Research, 85, 3711–3739. SHIPBOARD SCIENTIFIC PARTY 2000. Site 1148. In: Proceedings of the Ocean Drilling Program, Part A: Initial Reports. 243 Su, D., White, N. & McKenzie, D. 1989. Extension and subsidence of the Pearl River mouth basin, northern South China Sea. Basin Research, 2, 205– 222. Sun, X. & Wang, P. 2005. How old is the Asian monsoon system? Palaeobotanical records from China. Palaeogeography, Palaeoclimatology, Palaeoecology, 222, 181– 222. Sun, Z., Zhou, D., Zhong, Z., Zeng, Z. & Wu, S. 2003. Experimental evidence for the dynamics of the formation of the Yinggehai Basin, NW South China Sea. Tectonophysics, 372, 41– 58. Tapponnier, P., Peltzer, G., Le Dain, G. A. Y., Armijo, R. & Cobbold, P. R. 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology, 10, 611–616. Taylor, B. & Hayes, D. E. 1980. The tectonic evolution of the South China Basin. In: Hayes, D. (ed.) The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands. American Geophysical Union, Washington DC, Geophysical Monograph, 23, 89–104. Thiede, R. C., Bookhagen, B., Arrowsmith, J. R., Sobel, E. R. & Strecker, M. R. 2004. Climatic control on rapid exhumation along the Southern Himalayan Front. Earth and Planetary Science Letters, 222, 791–806. Thiry, M. 2000. Palaeoclimatic interpretation of clay minerals in marine deposits; an outlook from the continental origin. Earth-Science Reviews, 49, 201–221. Tjallingii, R., Röhl, U., Kölling, M. & Bickert, T. 2007. Influence of the water content on X-ray fluorescence core scanning measurements in soft marine sediments. Geochemistry Geophysics Geosystems, 8, doi: 10.1029/2006GC001393. Tran, V. T., Tran, D. T., Waltham, T., Le, D. A. & Lai, H. A. 2003. The Ha Long Bay world heritage; outstanding geological values. Journal of Geology, 22, 1 –18. Trentesaux, A., Liu, Z., Colin, C. J. G. et al. 2006. Pleistocene paleoclimatic cyclicity of southern China; clay mineral evidence recorded in the South China Sea (ODP Site 1146). Proceedings of the Ocean Drilling Program, Scientific Results (CD-ROM), 184, 10. Tu, K., Flower, M. F. J., Carlson, R. W., Zhang, M. & Xie, G. 1991. Sr, Nd, and Pb isotopic compositions of Hainan basalts (south China): implications for a subcontinental lithosphere Dupal source. Geology, 19, 567– 569. Vail, P. R., Mitchum, R. M., Todd, R. G., Widmier, J. M., Thompson, S. I., Sangree, J. B., Bubb, J. N., Hatlelid, W. G. et al. 1977. Seismic stratigraphy and global changes of sea-level. In: Payton, C. E. (ed.) Seismic Stratigraphy – Applications to Hydrocarbon Exploration. American Association of Petroleum Geologists, Tulsa, Memoir, 26, 49– 212. Wan, S., Li, A., Clift, P. D. & Jiang, H. 2006. Development of the East Asian summer monsoon; evidence from the sediment record in the South China Sea since 8.5 Ma. Palaeogeography, Palaeoclimatology, Palaeoecology, 241, 139– 159. Wan, S., Li, A., Clift, P. D. & Stuut, J.-B. W. 2007. Development of the East Asian monsoon: mineralogical and sedimentologic records in the northern South 244 L. V. HOANG ET AL. China Sea since 20 Ma. Palaeogeography, Palaeoclimatology, Palaeoecology, 254, 561–582. Wang, P. 2004. Cenozoic deformation and the history of sealand interactions in Asia. In: Clift, P., Wang, P., Kuhnt, W. & Hayes, D. (eds) Continent-Ocean Interactions in the East Asian Marginal Seas. American Geophysical Union, Washington, DC, 149, 1 –22. Wang, P. L., Lo, C. H., Lee, T. Y., Chung, S. L. & Yem, N. T. 1998. Thermochronological evidence for the movement of the Ailao Shan-Red River shear zone: a perspective from Vietnam. Geology, 26, 887 –890. Wang, P., Prell, W. L. et al. 2000. Site 1148. Proceedings of the Ocean Drilling Program, Part A: Initial Reports, 184, 121. Wang, P., Zhao, Q. et al. 2003. Thirty million year deep-sea records in the South China Sea. Chinese Science Bulletin, 48, 2524–2535. Wehausen, R. & Brumsack, H.-J. 2002. Astronomical forcing of the East Asian monsoon mirrored by the composition of Pliocene South China Sea sediments. Earth and Planetary Science Letters, 201, 621–636. Wei, G., Li, X.-H., Liu, Y., Shao, L. & Liang, X. 2006. Geochemical record of chemical weathering and monsoon climate change since the early Miocene in the South China Sea. Paleoceanography, 21, doi: 10.1029/2006PA001300. West, A. J., Galy, A. & Bickle, M. J. 2005. Tectonic and climatic controls on silicate weathering. Earth and Planetary Science Letters, 235, 211–228, doi: 10.1016/j.epsl.2005.03.020. White, A. F. & Blum, A. E. 1995. Effects of climate on chemical weathering in watersheds. Geochimica et Cosmochimica Acta, 59, 1729– 1747. White, A. F., Blum, A. E., Bullen, T. D., Vivit, D. V., Schulz, M. & Fitzpaterich, J. 1999. The effect of temperature on experimental and natural chemical weathering rates of granitoid rocks. Geochimica et Cosmochimica Acta, 63, 3277–3291. Wobus, C. W., Hodges, K. V. & Whipple, K. X. 2003. Has focused denudation sustained active thrusting at the Himalayan topographic front? Geology, 31, 861–864. Xie, X., Li, S., He, H. & Liu, X. 2003. Seismic evidence for fluid migration pathways from an overpressured system in the South China Sea. Geofluids, 3, 245– 253. Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K. 2001. Trends, rythms and abberations in global climate 65 Ma to Present. Science, 292, 686–693. Zhang, P., Molnar, P. & Downs, W. R. 2001. Increased sedimentation rates and grain sizes 2 –4 Myr ago due to the influence of climate change on erosion rates. Nature, 410, 891– 897. Zheng, H., Powell, C. M., Rea, D. K., Wang, J. & Wang, P. 2004. Late Miocene and mid-Pliocene enhancement of the east Asian monsoon as viewed from the land and sea. Global and Planetary Change, 41, 147– 155. Zhong, Z., Wang, L., Xia, B., Dong, W., Sun, Z. & Shi, Y. 2004. The dynamics of Yinggehai basin formation and its tectonic significance. Acta Geologica Sinica, 78, 302– 309. Zhou, D., Chen, H., Wu, S. & Yu, H. 2002. Opening of the South China Sea by dextral splitting of the East Asia continental margin. Acta Geologica Sinica, 76, 180–190.