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Contents lists available at ScienceDirect
Journal of Geodynamics
jou rnal homepage: http://www.elsevier.com/locate/jog
Propagated rifting in the Southwest Sub-basin, South China Sea:
Insights from analogue modelling
∗
Weiwei Ding , Jiabiao Li
The Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China
a r t i c l e i n f o a b s t r a c t
Article history: How the South China Sea rifted has long been a puzzling question that is still debated, particularly with
Received 18 July 2015
reference to the Southwest Sub-basin (SWSB). Analogue modelling remains one of the most useful tools
Received in revised form 5 February 2016
for testing rift models and processes. Here, we present and discuss a series of analogue modelling exper-
Accepted 11 February 2016
iments designed to investigate the rifting process of the SWSB. Convincing geophysical results were
Available online xxx
compiled to provide realistic constraints to test the experimental results and interpretations. A hetero-
geneous lithosphere model with a varied lithospheric structure showed tectono-morphological features
Keywords:
similar to the natural case of the SWSB, indicating that the initial thermal condition and rheological strati-
Analogue modelling
fication of the lithosphere should have a dominant effect on the rifting process of the SWSB. Rigid tectonic
Rigid blocks
Heterogeneous blocks existed in the continental margin, such as the Macclesfield Bank and the Reed Bank, and they
Propagated rifting played important roles in both the shaping of the continent–ocean boundary and the coupling between
Southwest Sub-basin the crust and mantle. The initial thermal condition and rheological stratification of the lithosphere under
the South China Sea controlled the propagated rifting process of the SWSB. Extension was centred on
the deep troughs between the rigid blocks, and the break-up occurred in these areas between them. The
westward rifting propagation is best explained with a heterogeneous lithosphere model characterized
by varied lithospheric structure, and it was responsible for producing the V-shaped configuration of the
SWSB.
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction attribution. Marginal basins in the western Pacific are often viewed
as products of the initial oceanic opening and thus, they should
Seafloor spreading and oceanic lithosphere formation studies provide key information for the comprehensive understanding of
have played key roles in our understanding of plate tectonics since the issues mentioned above (Benes and Scott, 1994; Goodliffe et al.,
the 1960s (Dietz, 1961; Hess, 1962; Vin and Mattews, 1963). Studies 1997; Okino et al., 1998; Taylor et al., 1996, 1999).
have indicated that a propagation feature can result when con- Among these marginal basins, the South China Sea (SCS), situ-
tinental rifting and oceanic spreading occur diachronously along ated at the junction of the Eurasian, Pacific, and Indo-Australian
strike (Courtillot, 1982; Acton et al., 1988; Wilson, 1988; Morgan plates (Fig. 1), is one of the largest marginal seas in the West
and Sandwell, 1994; Taylor et al., 1996; Manighetti et al., 1997; Pacific. The fundamental elements of the Wilson cycle, including
Tebbens et al., 1997; Fumes et al., 1998; Daessle et al., 2000; magma-poor continental rifting, seafloor spreading and the subse-
Huchon et al., 2001; Galindo-Zaldivar et al., 2006). This propagated quent collision with Borneo to the south, and subduction under the
spreading has been documented widely along modern mid-ocean Luzon Arc to the east (Briais et al., 1993; Hayes and Nissen, 2005;
ridges with various spreading rates, e.g., the East Pacific Rise Cullen et al., 2010; Franke et al., 2014) have all been documented
(Korenaga and Hey, 1996), Mid-Indian Ridges (Dyment, 1998), for the SCS. This has provided compelling insights into the initial
Southeast Indian Ridge (Morgan and Parmentier, 1985), and Mid- rifting and seafloor spreading of the basin. Based on the variations
Atlantic Ridge (LaFemina et al., 2005). The along-strike variation of structural features, the SCS basin has been commonly divided
in timing and rate of spreading causes controversy concerning into three sub-basins: the East Sub-basin (ESB), Southwest Sub-
the interpretation of the opening process and its geodynamic basin (SWSB), and Northwest Sub-basin (NWSB) (Fig. 1). Of these
three sub-basins, the SWSB is particularly suited for studying con-
tinental rifting, seafloor spreading, and its cessation, providing a
∗ case with which to understand the opening process and its geo-
Corresponding author at: The Second Institute of Oceanography, State Oceanic
dynamics in the SCS, and the significance of oceanic opening more
Administration, #Baochubei Road 32, Hangzhou 310012, China.
E-mail address: [email protected] (W. Ding). generally.
http://dx.doi.org/10.1016/j.jog.2016.02.004
0264-3707/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.
0/).
Please cite this article in press as: Ding, W., Li, J., Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue
modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Fig. 1. Morphological features of the South China Sea and adjacent regions with the locations of the major sedimentary basins. The oceanic basin is divided into the NW
Sub-basin, E Sub-basin, and SW Sub-basin. Black lines show the locations of example seismic sections. Purple lines are the locations of wide-angle reflection/refraction seismic
lines. Drilling sites drilled during IODP Expedition 349 and ODP 184 are indicated by yellow circles. Blue broken line shows the boundary between the E Sub-basin and the
SW Sub-basin (referred from Franke et al., 2014).
Two rather different tectonic models (i.e., the “scissor” and rigid blocks and lateral variation in brittle crust thickness to the rift-
“propagation” models) have been invoked to explain the rifting ing process, break-up of the crust and formation of oceanic crust,
process of the SCS. The scissor model (e.g., Zhou et al., 1995; Sun we performed a series of analogue experiments. Independent geo-
et al., 2009) suggests that the SCS can be depicted as opening with a physical data were also compiled to provide realistic constraints
scissor action from the east, incorporating three aspects of predic- for the experimental results and interpretations.
tion: (1) a wider basin should be situated in the eastern portion; (2)
clockwise rotation of the southern continental margin should occur 2. Geological setting
with a pole at the southwestern end of the SWSB; and (3) simul-
taneous seafloor spreading should occur with higher spreading The SWSB is a V-shaped triangular basin in the SCS (Fig. 2).
2
rates to the east. In contrast, the propagation model (e.g., Huchon It is 600-km long and has an area of 115,000 km and water
et al., 2001; Li et al., 2012; Savva et al., 2014) has two aspects of depths of between 3000 and 4300 m. The SWSB is bounded by
prediction: (1) spreading should be initiated in the northeastern several tectonic units: the Paracel Islands, Macclesfield Bank and
part of the SCS and propagate progressively to the southwest, and offshore Vietnam continental margin to the northwest, Reed Bank
(2) accompanying this, tectonic activity and related sedimentation and the Dangerous Grounds to the southeast, and ESB to the east
should reflect such diachronous propagation. (Figs. 1 and 2). The SWSB sea floor comprises many NE-trending
To understand the comprehensive scenarios from initial rifting bathymetric highs with a NE–SW-trending fossil spreading cen-
to subsequent spreading better, including the role of pre-existing tre acting as a central rift. This depression distinguishes the SWSB
Please cite this article in press as: Ding, W., Li, J., Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue
modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Fig. 2. Interpreted structures on the seafloor and selected lineaments in continental margins. Linear morpho-structural interpretations in the oceanic basin are based on
bathymetric and reflection seismic data. The structural lineaments and extents of the rigid massifs are shown in the continental margin according to Sun et al. (2009), Cullen
et al. (2010), Franke et al. (2014), Ruan et al. (2012), Ding et al. (2012, 2013), Pichot et al. (2014), Barckhausen et al. (2014), and Savva et al. (2014).
from the ESB, where the fossil spreading ridge is delineated by a propagated southward and westward (Ding et al., 2012; Franke
seamount chain. et al., 2014; Li et al., 2012; Savva et al., 2014). The episodic rift-
ing continued into the Cenozoic in two main episodes: the early
Paleocene to Eocene and the late Eocene to early Miocene (Cullen
2.1. The continental margin
et al., 2010). The former was widespread and is identifiable in
nearly all offshore basins. Conversely, the latter was less exten-
Since the Triassic the proto-South China Sea margin has
sive but it resulted in the subsequent seafloor spreading in the
undergone long-lasting subduction and was in an Andean-type
SCS. The extensional tectonics of the Cenozoic is evidenced by the
convergent setting caused by the subduction of the paleo-Pacific
development of half-grabens with styles ranging from serial, par-
(Pubellier and Morley, 2014; Shi and Li, 2012; Taylor and Hayes,
allel, to en echelon. Some of these extensional faults merge onto
1980, 1983; Zhou et al., 1995). Important uplift and erosion since
a low angle detachment system, which might have developed in
the Cretaceous exhumed many of the plutons whereas most of the
the early stages of rifting as a decollement system close to the
volcanic products have been eroded (Chen et al., 2010). The sub-
continent–ocean transition area (Ding et al., 2013; Pichot et al.,
sequent extension from the Late Cretaceous took place within this
2014). In the Middle Miocene, the Dangerous Grounds collided
waning configuration. Associated with uplifting shoulders, episodic
with Borneo, transferring the area into a compressional setting
rifting and erosion occurred as a result of subduction slab rollback
(Clift et al., 2008; Cullen, 2010; Hutchison, 2010; Hutchison et al.,
(Hall, 2002; Shi and Li, 2012; Zhu et al., 2012), and interpreta-
2000).
tions of the seismic profiles suggest that the deformation likely
Please cite this article in press as: Ding, W., Li, J., Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue
modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Fig. 3. Comparison of the crustal structures along the profiles run across (a) the Macclesfield Bank (Ding et al., 2012), (b) Reed Bank (Ruan et al., 2012), (c) Zhongjian Massif
(Pichot et al., 2014), and (d) Zhenghe Massif (Qiu et al., 2011). See locations in Fig. 1.
Several basement highs are outstanding features on the bathy- that seafloor spreading occurred from 32 to 16 Ma (magnetic
metric map, including the Reed Bank, Macclesfield Bank, Zhongjian anomalies 11–5C) (e.g., Briais et al., 1993; Taylor and Hayes, 1980,
Massif, and Zhenghe Massif (Figs. 1 and 2). These basement highs 1983). A two-stage process of seafloor spreading in the SCS has
characterize the sinuous continent–ocean boundary (COB). Bore- been widely accepted, i.e., N–S spreading in the ESB, followed by
hole data suggest that the Macclesfield Bank and Zhongjian Massif NW–SE spreading in both the ESB and SWSB after a ridge jump
have a Precambrian metamorphic basement (Qiu et al., 2001; Sun at ∼23 Ma (Taylor and Hayes, 1983; Ru and Pigott, 1986; Briais
et al., 2009), and that the Reed Bank and Zhenghe massif have the et al., 1993; Barckhausen and Roeser, 2004; Cullen, 2010; Li and
same basement as the Macclesfield Bank (Ding et al., 2013; Yan and Song, 2012; Barckhausen et al., 2014; Franke et al., 2014). This tim-
Liu, 2004). Inferred from several wide-angle reflection/refraction ing was confirmed by deep tow magnetic data (Li et al., 2014) and
seismic profiles, the crust thicknesses of these basement highs IODP Expedition drilling results (Expedition 349 Scientists, 2014;
reach 13–20 km (Fig. 3), relatively thicker than in the neighbouring Koppers, 2014). This cessation time is coincident with the collisions
region. These basement highs could represent rigid massifs of between Palawan and Borneo and Mindoro-Central Philippines
continental lithosphere that did not deform internally during the (Hutchison et al., 2000; Yumul et al., 2003; Clift et al., 2008; Cullen,
lithospheric rupture, similar to the Flemish Cap and Galicia Bank 2010; Hutchison, 2010), suggesting a causal relationship between
at the Newfoundland/Iberia margin in the Atlantic (Péron-Pinvidic the spreading cessation and collision events.
and Manatschal, 2009). Based on geophysical and Pb isotopic data,
Yan et al. (2014) suggested that all these basement highs/blocks
3. Analogue modelling
were of continental origin and had an affinity to the South China
Block. The basement highs/blocks were separated from each other
3.1. Model design and experiment materials
by the end of the late Mesozoic due to rifting and spreading in the
region (e.g., Taylor and Hayes, 1980; Holloway, 1982; Clift et al.,
Analogue modelling is a powerful tool with which to evaluate
2008; Hall, 2012; Franke et al., 2014).
and simulate the kinematics, mechanics, and dynamic processes of
natural geological processes, and it has long been used to inves-
2.2. The SWSB tigate the mechanics of deformation and related sedimentation
in areas with either extensional or compressional stress fields
The SWSB is outlined roughly by the 3000-m isobath. The bathy- (e.g., Serra and Nelson, 1989; McClay, 1990; Tron and Brun, 1991;
metric map shows several large seamounts in the east and two Storti and McClay, 1995; McClay et al., 2002; Corti et al., 2003a;
remarkable linear basement highs in central parts of the basin Graveleau and Dominguez, 2008; Sun et al., 2009; Strak et al.,
(Figs. 1 and 2). Seismic data define these linear basement struc- 2011; Wang et al., 2013; Warsitzka et al., 2013). Davy and Cobbold
tures as rift shoulders dominated by parallel fault scarps, separated (1991) considered four main layers for small-scale modelling of
by a NE-oriented central rifted graben (Li et al., 2012). Further lithospheric deformation: (a) the upper brittle crust exhibiting
to the southwest, the seafloor becomes flat with several isolated Coulomb behaviour; (b) lower ductile crust exhibiting a power-law
seamounts or linear structures. The seamounts, composed of alkali behaviour; (c) upper lithospheric mantle, which might be brittle
basalts aged 10–3.5 Ma, represent magma activity after seafloor under some circumstances; and (d) lower ductile part of the litho-
spreading (Yan et al., 2014). spheric mantle (Fig. 4A). The strong brittle upper crust and upper
The spreading age of the SCS has been studied comprehen- mantle are intercalated by the ductile lower crust and lower man-
sively based on analyses of magnetic anomalies, which suggest tle (Brun, 1999; Davy and Cobbold, 1991). When stretched and
Please cite this article in press as: Ding, W., Li, J., Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue
modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Fig. 4. Initial strength profiles of normal and thinned lithospheres and rigid massifs in nature (solid and dotted lines) and the experiments (broken line). BC: brittle crust;
DC: ductile crust; BM: brittle mantle; DM: ductile mantle. Redrawn from Brun (1999).
heated by the upwelling asthenosphere, the crust might become Different materials have been tested for modelling the upper-
thinned with a lower brittle/ductile thickness ratio (Fig. 4B). The most brittle mantle. Keep (2003) used dry sand grains to simulate
old stable lithosphere (rigid blocks) was modelled using only two the upper mantle, while Pubellier and Cobbold (1996) and Corti
layers: an upper brittle layer, including the entire crust and brittle et al. (2003a) modelled the upper mantle using silicone with greater
uppermost part of the mantle, and an underlying ductile litho- viscosity and density than that used for the lower crust. Sims et al.
spheric mantle (Fig. 4C) (Brun, 1999; Mart and Dauteuil, 2000). Dry (1999) and Tron and Brun (1991) used a thin metal board for mod-
quartz sand with a coefficient of internal friction of about 0.55 elling the strong upper mantle. Because an extensional force will
(Liu et al., 1992; Storti and McClay, 1995; Lohrmann et al., 2003; be imposed on the material simulating the upper mantle, the use
Eisenstadt and Sims, 2005), behaves similarly to sedimentary rocks of sand grains is not appropriate. We found it hard to break up the
in the upper crust (Lohrmann et al., 2003), making it a preferred more viscous and denser silicone, and the thin metal board was
analogue with which to model the brittle behaviour of the upper too heavy and tended to sink into the honey when loaded with the
crust (Byerlee, 1978; Krantz, 1991; Lohrmann et al., 2003; Schellart, silicone and sands above. Therefore, we followed Sun et al. (2009)
2000). and used a non-deformable light plastic board with a cut on it to
Based on above-mentioned studies we applied scaled mod- simulate the upper mantle.
elling to simulate the opening process of the SWSB, specifically Details of all these experimental materials are shown in Table 1.
to evaluate the role of the presence of rigid blocks and lateral
variations in crustal thickness on rift initiation and develop-
3.2. Experiment scaling and construction
ment. Lateral variation in crustal thickness has been evidenced
by the variation of depositional environments of the Creta-
Analogue model experiments should conform to the principle
ceous sediments, i.e., the proto-South China Sea margin thickened
of similarity, which demands that the analogue model be prop-
towards the west (Pubellier and Morley, 2014; Savva et al.,
erly scaled to the geometry, kinematics, dynamics, and rheology of
2014). Furthermore, before the opening of the SWSB, the ESB
the system to be evaluated (e.g., Brun, 1999; Corti, 2012). Of these,
has experienced intensive stretching and mantle upwelling. Thus
dynamic similarity is the important link between the scaled and
the proximal western continental margin of the ESB consisted
geological models (Ramberg, 1981), which requires that the model
of relatively hotter and thinner lithosphere than further west.
respects the force balance that is present in the natural prototype.
With these geological evidences we firstly performed four two-
In general, the models are built with typical model-to-nature
− −
layer analogue experiments to establish the most appropriate 5 6
length ratios of 10 –10 (Davy and Cobbold, 1991; Brun, 1999;
model for the actual geological setting of the SWSB, and then
Corti et al., 2002, 2003a). In our models, the linear dimensional
−
we tried to apply four-layer analogue experiments based on 6
ratio was 1 × 10 , where 10 km in nature is represented by 1 cm in
the most appropriate two-layer model to reproduce the entire
the laboratory. Models with overall dimensions of 40 × 40 × 10 cm
lithosphere-asthenosphere system and thus to provide more realis-
were built in a transparent sandbox.
tic constraints into the evolution of rifting and continental break-up
In the presence of ductile layers, the Ramberg number Rm (the
in the study area.
ratio of gravitational to viscous forces acting on the ductile layer)
In the laboratory, we modelled brittle layers using dry quartz
(Ramberg, 1981; Weijermars and Schmeling, 1986) was used as a
◦
sand with low cohesion and a frictional angle of about 30 . Duc-
measure of the dynamic similarity between models and the natural
tile layers were made of silicone putty, which with viscosity of prototypes:
4 −3
10 Pa s and density of approximately 1.2 g cm at room tempera-
◦ gh gh2
ture (25 C) exhibits a Newtonian behaviour. Both sand and silicone = =
Rm = , (A1)
are sensitive to body forces due to normal gravity in laboratory- ε˙ ε˙ v
scale models. Rigid blocks, such as the Reed Bank and Macclesfield or
Bank, have old and rigid crust, as mentioned above, but not as 2 2
mgmhm ngnhn
strong as old craton. For this, we used extra viscous silicone with
Rm = = , (A2)
5 mvm nvn
viscosity of 10 Pa s and patched it onto the normal lower crust. For
the asthenosphere, we used honey with density of approximately
where is the gravitational force, is the viscos-
−3 2
1.4 g cm and viscosity of 10 Pa s, which also acts as a Newtonian 4 21
ity, m = 2 × 10 Pa s, n = 2 × 10 Pa s, is the density,
fluid. −3 −3 −2
m = 1300 kg m , n = 2800–2900 kg m , gm = gn = 9.81 m s
Please cite this article in press as: Ding, W., Li, J., Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue
modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Table 1
Characteristics of experimental materials and prototype layers.
−3
Material and prototype layer Density (kg m ) Viscosity (Pa s) Rheological characteristics
Sand Upper Crust 1200 Mohr–Coulomb behaviour 2600–2800
4
Sand + Red Silicone (1:3 in weight) 1300 2 × 10
21
Normal Lower Crust 2800–2900 2 × 10 Power law 2
Honey 1400 10 Newtonian behaviour Asthenosphere 3200–3300 1019
4
Sand + Green Silicone (1:2 in weight) 1400 6 × 10
× 21
Rigid massif 2900–3000 6 10 Power law
is the gravity acceleration, h is the thickness, v is the extension 3.3. Limitations of the models
rate, vn = 40 mm/yr (Li et al., 2014), ε´ = v/h is the strain rate, and
subscripts m and n represent the model and the prototype, respec- Analogue modelling implies a simplification of the complexities
tively. To achieve dynamic similarity between the scaled models of the natural processes. The natural rheology, strength profiles,
and the natural prototype, the Rm number should be consistent or and geometrical boundary conditions are more complex than the
at least similar (Weijermars and Schmeling, 1986; Bonini, 2001; laboratory replications. Furthermore, we were unable to incor-
Bahroudi et al., 2003). porate the temperature-dependent rock rheology and rheological
According to Eq. (A2) and the parameters mentioned above, changes due to temperature variations during deformation, nor
the kinematic scaled parameter (such as the extension rate v) was to account for the cooling of the newly formed sea floor and
determined. Thus, we can deduce the extension rate in our mod- thermal weakening of the continental crust. It is also difficult to
elling from the Eq. (A2): control precisely the strain rate under which the experiments were
performed.
Since the plastic boards are not deformable, they are not suitable
∗(h∗)2
for reproducing and analyzing the complex structures resulting v = v , (A3)
m ∗ n
from coupling/decoupling of deformation between the crust and
the strong lithospheric mantle. They are introduced to better
−6 −17 extend the models and to better visualize thinning and astheno-
where * = 0.46, h* = 10 , * = 10 , then we can calculate
≈ spheric upraising. Thus our “four-layer” model is a simplification
vm 5 mm/h knowing that vn is about 40 mm/yr.
of the real four-layer model indicated by Davy and Cobbold (1991).
We set up four sandbox models for the two-layer analogue
Despite above limitations, the results of analogue modelling
modelling: two homogenous models with a lithosphere of con-
have been proven to realistically reproduce a wide range of
stant thickness or that is progressively thinned towards the east
first-order characteristics of rifting and thus, be able to provide
to investigate the effects of the initial rheological structure on
important indications of the large-scale structural response of the
the deformation style; and two models similar as described above
brittle/ductile lithosphere during extension (e.g., Brun, 2002; Corti
but defined as heterogeneous because of the presence of rigid
et al., 2003a; Corti, 2012).
blocks. With these models we try to examine the impact of lateral
heterogeneities in the rheology on the deformation of the conti-
nental lithosphere and rifting patterns (Fig. 5). The rigid blocks in 3.4. Modelling results
the heterogeneous models simulated the Macclesfield Bank and
Zhongjian Massif in the northern margin, and the Reed Bank and 3.4.1. Two-layer modelling
Zhenghe Massif in the southern margin, respectively. For these Two-layer model-1 had a uniform normal homogenous crust
experiments, we set two layers: a 2-cm layer of quartz sand, and with a sand and silicone thickness ratio of 2:1 (Fig. 5). Following
a 1-cm layer of red silicone, which simulated the brittle upper the sideward movement of the moving board, a series of parallel
crust and ductile lower crust, respectively. A 0.2-cm-thick plastic and straight faults appeared initially in the middle of the sandbox
board was used as moving-board at the bottom. The astheno- and then propagated to both sides. All these faults were distributed
sphere was not included. For normal lithosphere, the thickness symmetrically between the rifting centres and the fault throw
ratio of the brittle to ductile layers was 2:1, while for the thermally decreased from the middle to the sides.
thinned lithosphere, the ratio was set as 1:1. The rigid blocks were Two-layer model-2 was composed of homogenous layers of sand
simulated using 2-cm-high and 4-cm-diameter cylinders of green and silicone lying above the moving board. As seafloor spreading
silicone. started initially in the ESB after the Middle Oligocene, the crust
Two four-layer analogue modelling experiments were set up in the west region next to the ESB should have been thermally
based on the most appropriate two-layer models. Both were het- thinned prior to the initial opening of the SWSB. Thus, to simu-
erogeneous models with four rigid blocks patched in the western late the thermally thinned crust related to the opening of the ESB,
part. The first was set with normal lithosphere in the west and the thickness ratio of the brittle layer over the ductile layer pro-
thinned lithosphere in the east (Fig. 6A); the other was set with gressively reduced towards the east (Fig. 5). With the sideward
lithosphere of constant thickness on both sides for comparison movement, normal faults occurred initially in the east with sym-
(Fig. 6B). The four-layer analogue models are composed of a 2-cm metrical distribution. With further sideward movement, the rifting
layer of quartz sand, a 1-cm layer of red silicone, a 0.2-cm-thick propagated to the west and formed faults with large displacements
plastic board, and a 7-cm-thick layer of honey, which simulated across the entire area. These major faults were generally parallel to
the brittle upper crust, the ductile lower crust, the brittle upper each other in an E–W trend, and the distance between the opposite-
mantle, and the asthenosphere, respectively. As before, the thick- dipping faults in the west was greater than in the east. The fault
ness ratio of the brittle to ductile layers of the normal lithosphere throw was also larger in the west, because the sand was thicker
was 2:1 and that for the thermally thinned lithosphere was 1:1. there.
The rigid blocks were simulated using green silicone cylinders of Two-layer model-3 had a normal heterogeneous crust with two
similar size to the two-layer model (Fig. 6). pairs of rigid blocks patched into the silicone, which was built
Please cite this article in press as: Ding, W., Li, J., Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue
modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Fig. 5. Two-layer analogue modelling experimental constructions and results. Two-layer model-1 is a uniform normal homogenous lithosphere model with a sand to silicone
ratio of 2:1; two-layer model-2 is a homogenous model with a sand to silicone ratio of 2:1 in the western part, to simulate normal crust, and 1 cm: 1 cm in the east to simulate
a thinned crust; two-layer model-3 is a normal heterogeneous model with two pairs of rigid blocks; two-layer model-3 is an heterogeneous lithosphere model with a varied
crustal thickness.
to explore the influence of the rigid blocks on the deformation 3.4.2. Four-layer modelling
style (Fig. 5). When stretched sidewards, rifting occurred initially Four-layer model-1 (heterogeneous lithosphere model with varied
between the rigid blocks, before developing along the edges of lithospheric structure) was composed of four layers of sand, red sili-
the rigid blocks. As the extension continued, small faults joined cone, a plastic board, and honey. The brittle-ductile thickness varied
together and formed two sinuous symmetrical major faults with from 2:1 in the west to 1:1 in the east (Fig. 6A). Two pairs of green
an E–W trend. The distances between the two faults were similar silicone cylinders with higher viscosity were patched into the red
in both the eastern and western parts. silicone layer to simulate the rigid blocks.
Two-layer model-4 had a heterogeneous crust with different When stretched, normal faulting occurred initially in the eastern
thickness on each side. The thickness ratio of the brittle layer part and along the edges of the rigid blocks (Fig. 7A1 and A2). All
to the ductile layer was reduced to 1:1 in the eastern part to these faults dipped to the rift centre and were generally parallel. It
simulate the thinned crust (Fig. 5). When stretched southward, is notable that the faults in the eastern part did not develop along
fractures appeared both in the eastern part and in between the the cutting edges of the moving board, but about 2–3 cm away from
rigid blocks, and they joined together to form two larger faults. The them.
faults developed along the edges of the rigid blocks and converged As the stretching continued, the faults in the east propagated to
towards the centre. The distance between these two sinuous faults the west and joined with those that developed along the edges of
was much wider in the eastern region than in the west, displaying the rigid blocks and formed two major faults (Fig. 7B1 and B2). Both
a V-shaped opening to the east. faults were about 20 cm in length with a maximum throw of 1 cm.
Compared with the tectonic setting of the SWSB, Two-layer En echelon faults, likely caused by the ductile flow of the silicone
model-4 demonstrated excellent agreement with the observed and honey, developed in the lower eastern part, oblique to the rift
◦
geology, after a 45 anticlockwise rotation, with the following char- centre, which resulted in flattened faults close to the rift centre.
acteristics: (1) the two large faults were sinuous and distributed Boundary effects related to the silicone thinning and shortening
symmetrically with the rift axis; (2) the distance between these in along-axis direction may also play an important role in shaping
two faults was much wider in the east than in the west, and exhib- these faults. The orientations of the faults were sinuous and con-
ited a V-shape; and (3) the rifting propagated southwestwards trolled by the edges of the rigid blocks. The rift, which was confined
(Fig. 2). by the two faults, was much wider in the east than in the west.
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Fig. 6. Four-layer analogue modelling experiment construction. Upper: surface view of the four-layer analogue modelling set up and scale. Lower: side views of (A) the
heterogeneous lithosphere model with a varied lithospheric structure; and (B) the heterogeneous lithosphere model with a normal lithospheric structure for comparison.
The rift width grew when the bulk extension reached about 30% honey now, although some places do not show sinking features
of the total amount. Simultaneously, the honey upwelled to the sur- within the overlying white sands.
face at two points between the two pairs of rigid blocks, as well as in Four-layer model-2 (heterogeneous lithosphere model with nor-
the eastern part with the thinned crust (Fig. 7C1 and C2). Except the mal lithospheric structure) was designed to address the fundamental
faults formed along the edges of the blocks in the western part, two problem of the effect of lithospheric structure. This model also com-
normal faults with east-west-orientation developed in the west- prised four layers of sand, silicone, a plastic board, and honey, but
most area. These two normal faults joined with the axis-dipping the thickness ratio of the brittle layer to the ductile layer was 2:1,
normal faults in the east and formed into two major curving faults. and it was the same on both sides of the model.
The rift confined by these two major faults was still wider in the When stretched southwards, rifting started from the edges of
east and it portrayed a horn shape. opposite rigid blocks and the eastern part with 2–3-cm-long faults,
When the bulk extension reached 50% of the total amount, the which were all E–W trending and distributed symmetrically in par-
area of upwelled honey grew. The two normal faults were oriented allel (Fig. 8A1 and A2).
sinuously with maximum curvatures around the rigid blocks, and When the bulk extension reached 20% of the total amount
more sand accumulated in places between adjacent rigid blocks (Fig. 8B1 and B2), the faults joined together and formed two major
(Fig. 7D1 and D2). As the extension was mostly focused on the faults. In the eastern part, the orientations of these two major faults
rift centre, the silicone between the two boards became extremely were consistent with the edge of the spreading board, whereas in
stretched with a stretching factor >5 (original thickness; 1 cm, the west, they were sinuous and controlled by the edges of the rigid
divided by post-deformation thickness; nearly <0.2 cm). More nor- blocks. The rift confined by these two normal faults was wider in
mal faults developed in the west-most part, dipping to the rift axis. the west than in the east and it displayed a different shape to that
Fig. 7E1 and E2 illustrates a conceptual model with 100% bulk revealed in Four-layer model-1.
extension, which has not occurred in the SWSB. The silicone was As the extension continued, the silicone was broken and
totally broken. Most area between two sinuous is occupied by the honey upwelled to the surface at points between the two
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Fig. 8. Surface view of four-layer analogue modelling experiments (A1, B1, C1, D1,
E1) and the interpretations (A2, B2, C2, D2, E2) of the normal heterogeneous litho-
sphere model.
Fig. 7. Surface view of four-layer analogue modelling experiments (A1, B1, C1, D1,
E1) and the interpretations (A2, B2, C2, D2, E2) of the heterogeneous lithosphere
model with a varied lithospheric structure. The amount of extension is shown at the
bottom of each photo. White broken lines show the boundaries of the two plastic pairs of rigid blocks, as well as at the eastern end (Fig. 8C1
boards. Black lines are the normal faults developed during the extension. Green
and C2). These two major faults were slightly sinuous, espe-
circles show the locations of the patched rigid blocks. The symbols are the same in
cially in the area of the rigid blocks. After the break of silicone,
Fig. 8.
the stretching focused generally on the area between the mov-
ing boards. None of the normal faults changed much in terms
of deformation and orientation (Fig. 8D1 and D2). Finally, the
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honey areas grew and merged into a whole region (Fig. 8E1 2014) is also a benchmark of the breakup in the SWSB (BRU-SW)
and E2). (Fig. 10).
Heterogeneous tectono-sedimentary features can also be
observed in the oceanic basin. Geological observations in the
3.4.3. Comparison between the two Four-layer models
oceanic basin of the SWSB suggested that the NE part is dominated
The analogue modelling shows that Four-layer model-1, the het-
by normal oceanic crust formed in a steady state spreading with a
erogeneous lithosphere model with varied lithospheric structure
high magma budget, indicated by the reasonably flat oceanic base-
(thin in the east and normal in the west), might act as an epitome
ment, increasing sedimentary thickness from the middle to both
of the SWSB. This model generated several important features: (1)
sides, and active post-drift magmatic activities (Ding et al., 2015).
extension developed firstly in the eastern part and then propagated
While in the SW part the oceanic basin was still dominated by
west; (2) two large sinuous and symmetrically normal faults were
subcontinental mantle exhumation with well-developed detach-
formed and generally confined by the edges of the rigid blocks; and
ments, tilted fault blocks, rifted basins, and limited magmatism.
(3) honey upwelling to the surface occurred initially in the east-
ern part with the thinned crust and between the rigid blocks, and
then propagated west. The rift basin is wider in the east than the 4.2. Comparison between analogue modelling and nature
west. The shape of this basin with this model under 50% of the total
extension fits the bathymetric map of the SWSB very well, with a Studies of both the continental margin and the oceanic basin
◦
45 anticlockwise rotation (Fig. 7D1 and 1). In contrast, Four-layer of the SWSB indicate a number of differences in the tectono-
model-2, the heterogeneous lithosphere model with lithosphere of sedimentary fabric from the northeast to southwest. (1) Intense
constant thickness, generated features significantly different to the continental rifting occurred in the Reed Bank area, which reduced
natural appearance of the SWSB, e.g., (1) the simultaneous devel- to the southwest. (2) The time of cessation of rifting, which resulted
opment of normal faults and continental breakup along the strike, from the onset of the opening of the SWSB, became younger from
and (2) a wider oceanic basin in the western part than in the east. the Reed Bank area to the west of the Dangerous Grounds. (3) More
magma budget in the NE part of the ceoanic basin. The realistic
tectono-sedimentary features fit the results of Four-layer model-
4. Constraints from geophysical data
1 very well, suggesting that (1) the rifting and seafloor spreading
should have started in the northeastern part and propagated to
Analogue modelling indicated that the rifting was initiated in
the southwest, and (2) the northeastern part experienced longer
the east, and then propagated to the west. The orientation of
seafloor spreading, implied by the typically wider oceanic basin,
the normal faults was dominated by the rigid blocks. Here, we
whereas the relatively short spreading history in the southwestern
compiled tectono-sedimentary features in the SWSB, which were
part formed a narrow oceanic basin with remarkable continental
constrained by geophysical data, to constrain our experimental
rifting features.
results and interpretations.
4.1. Neogene tectono-variation in the SWSB 5. Discussion
Neogene tectonic and sedimentary variation in the continental 5.1. Rifting model in the SWSB
margin have been well studied by previous scientists (Shipboard
Scientific Party, 2000; Yao et al., 2012; Ding et al., 2013; Franke et al., Zhou et al. (1995) proposed a scissors model to illustrate the
2014; Savva et al., 2014). Fig. 9 shows four seismic sections arranged basin development of the SCS, i.e., the SCS may be depicted as
from east to west in the continental margin (see Fig. 1 for locations). opening from the east with a scissor action, whereby the eastern
Several regional unconformities were identified for the age associ- segment was subject to earlier opening and hence, is a wider basin.
ated with the main tectonic events (Figs. 9 and 10). Tg represents Based on this model, Sun et al. (2009) conducted 3D analogue mod-
the base of the rift infill that is close to the Mesozoic–Cenozoic elling experiments to test the breakup pattern. They established
boundary. T7 marks the onset of seafloor spreading in the ESB and models with a fixed northern plate, and a southeastward moving,
represents the breakup unconformity (BRU-E) with an age about clockwise rotating southern plate with a pole set at the south-
32 Ma. T5 is another regional unconformity forming the top of the western end. With this model, different parts of the edges open
latest syn-rift sequence in the Reed Bank area and the Dangerous simultaneously but with different opening rates, where the slower
Grounds. Franke et al. (2014) named it as the end-rifting uncon- rate of opening occurs in places closer to the pole, resulting in a
formity (ERU). The age is diachronous, which will be discussed in V-shaped basin.
detail in the following. T4 reflects the tectonic result of the col- The prediction of the scissors model is however not consis-
lision between the Dangerous Grounds and Borneo, which halted tent with the magnetic interpretations of previous studies (e.g.,
the spreading in the SCS at about 16 Ma (Schlueter et al., 1996), or Briais et al., 1993; Barckhausen et al., 2014; Li et al., 2014). The
the Middle Miocene Unconformity (MMU) described by Hutchison magnetic anomalies suggested by Briais et al. (1993) match well
and Vijayan (2010). T3 is another unconformity in the post-rift with our new compilation of magnetic data, and this model has
sequence that might have been caused by the continuous uplift been proven by the deep-tow magnetic surveys (Li et al., 2014)
of tilting events after the onset of the collision between the Dan- and drilling results from the IODP Expedition 349 (Expedition 349
gerous Grounds and Borneo (Ding et al., 2013); Savva et al. (2014) Scientists, 2014). The magnetic anomaly model by Briais et al.
named it the Late Miocene Unconformity (LMU, 11.5 Ma). (1993) implies that the initial seafloor spreading age migrated from
Detailed geological interpretation on seismic sections from east the northeastern part (magnetic anomaly 6c, 23.5 Ma near the Mac-
to west imply a general pattern that the thickness of the syn-rift unit clesfield Bank) to the southwest (magnetic anomaly 5e, 18.5 Ma
in the continental margin reduces from east to west and the time near the Zhongjian Massif), indicating a southwestward propa-
of cessation of rifting differs, i.e., it gets younger from east to west gation of seafloor spreading. We calculated the spreading rates
(Figs. 9 and 10). T7 is the BRU of the ESB, and during the drifting between the Reed Bank and Macclesfield Bank, and between the
of the ESB, the SWSB continued to experience continental rifting Zhongjian Massif and Zhenghe Massif using this magnetic anomaly
until the ridge jump at about 23.8 Ma. The so-called ERU (Franke model. The calculated results show that the average half-spreading
et al., 2014) or Oligocene/Miocene unconformity (OMU, Savva et al., rates in the former region are between 18.04 and 19.09 mm/yr, and
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Fig. 9. Seismic data examples illustrating the tectono-sedimentary fabrics in the continental margin of the SWSB. The Cenozoic deposit was divided into syn-rift and post-rift
units. Seismic Section 1 lies in the Reed Bank Basin. Strong extensional tectonic activity is indicated with abundant normal faulting, which resulted in the formation of several
half-grabens and domino-style rotated blocks. A regional unconformity (T5) separates the rifting infill from the draped deposits. Seismic Section 2 lies to the east of the
Dangerous Grounds. Two listric normal faults bounded half-grabens to rotated blocks, which were mainly active in the Paleogene and cut deep into the basement. A rotated
horst block forms the topographic high with reefs presumed on top. T5 is a dissynchronous surface and suggested an age of 19–16.5 Ma for this horizon. Seismic Section 3
lies to the west of the Dangerous Grounds. The syn-rift unit confined under unconformity T5 has thickness ranging between 0.2 and 0.4 s TWT. T5 was assigned an age of
15.5 Ma. Seismic Section 4 lies in the Phuh Khanh Basin, showing a typical half-graben bordered by a deep-rooted normal fault. The extension in this area continued after
15.5 Ma (T5) until 11.5 Ma (T3). For the detailed ages of the unconformities and corresponding tectonic events, see Fig. 10.
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modelling. J. Geodyn. (2016), http://dx.doi.org/10.1016/j.jog.2016.02.004
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Fig. 10. Tectonic stages and rifting evolution in the continental margin of the SWSB from different resources. PKB: Phuh Khanh Basin; WDG: west Dangerous Grounds;
EDG: east Dangerous Grounds; RB: Reed Bank Basin. BRU-E (32 Ma): breakup unconformity of the ESB; BRU-SW (23.8–16 Ma): breakup unconformity of the SWSB; MMU
(15.5 Ma): middle Miocene unconformity; LMU (10.5 Ma): late Miocene unconformity.
between 18.91 and 19.62 mm/yr in the region further southwest. beneath Borneo (Clift et al., 2008; Cullen, 2010; Hall, 2002), the
These half-spreading rates are generally similar in the different Reed Bank and Dangerous Grounds would have moved southward,
regions in contrast to the scissors models. triggering the opening of the SWSB.
Our analogue modelling results have proven that the propaga- Numerical models have proven that the orientation of the nor-
tion model with much earlier faulting along the northeastern SWSB mal faults could influence the stress field, and changes of the
is more likely. The tectono-sedimentary changes in the oceanic orientation would perturb the surrounding stress field (Homberg
basin also indicate a variation in tectonic activity from typical et al., 1997; Kattenhom et al., 2000; Maerten et al., 2002; Rawnsley,
seafloor spreading in the northeast, to initial seafloor spreading in 1992; Corti et al., 2013; Philippon et al., 2014). Our heterogeneous
the southwest, and the latter is featured with more continental rift- analogue modelling experiments showed that the development
ing characteristics (Huchon et al., 2001; Savva et al., 2014). Both the of faults was influenced by the intact rigid massifs with sinuous
analogue modelling and geological studies support the propagation striking. The stress concentrated in areas where the fault strike
model. was bent, which led to deep troughs developing along its edges,
especially in the places between the opposite-moving rigid massifs.
5.2. Process of propagated rifting and its dynamic mechanism Seismic data has proved that the main detachments are local-
ized along these rigid blocks and seem to be active from Upper
Different lithospheric structures might explain this propagated Cretaceous to Paleogene (Savva et al., 2014). Furthermore, the
rifting in the SWSB. Before the seafloor spreading, the proto- lithospheric structure here was thinned and weaker because of its
South China Sea margins thinned towards the east as suggested proximity to the ESB. If the rift is narrower between the rigid blocks,
by the variation of depositional environments of the Cretaceous then thinning is more efficient. This could explain why the breakup
sediments. At about 33 Ma, seafloor spreading firstly occurred in happened firstly at points between the rigid massifs. The existence
the ESB with a southward migrating spreading ridge (Expedition of the rigid massifs, such as the Macclesfield Bank and Reed Bank
349 Scientists, 2014). The oceanic crust in the eastern part of the is the cause of the lithospheric inhomogeneity. Mantle upwelling
SCS occurred when the southwestern portion was still undergoing and melting, in the form of magma intrusion or dikes, first occurred
stretching (Fig. 11A). With continuous subduction of the proto-SCS between the Macclesfield Bank and Reed Bank, or between the
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Fig. 11. Reconstruction of the SWSB since the Late Oligocene based on a free-air gravity map of the SCS. (A) Late Oligocene. Proto-SCS might have still existed in the south
of the Dangerous Grounds and experienced southeastward subduction. The seafloor spreading started in the ESB and NWSB in the Late Oligocene (Red). The Reed Bank and
Macclesfield Bank were still connected, although a deep rift trough developed between them. (B) Early Miocene. With continued subduction of the proto-SCS, the seafloor
spreading propagated to the SW and the SWSB began opening. Breakup occurred at points in between the rigid massifs and propagated to the SW. (C) Middle Miocene.
Spreading continued in the SWSB and formed a V-shaped oceanic basin. The Dangerous Grounds, together with the Reed Bank moved southwards until colliding with Borneo,
and the spreading of the entire SCS halted in the Middle Miocene (Clift et al., 2008; Cullen et al., 2010; Hutchison, 2010; Hutchison et al., 2000). (D) The Philippine Sea Plate
moved northwestwards and the Luzon Arc finally collided with the Eurasian Margin (Hall, 2002, 2012; Sibuet and Hsu, 2004). The SCS subducted under the Philippine Sea
Plate along the Manila Trench. Broken line shows the continental–oceanic boundary (COB). ZS: Macclesfield Bank; ZJ: Zhongjian Massif; RB: Reed Bank; ZH: Zhenghe Massif.
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Zhongjian Massif and Zhenghe Massif during the Early Miocene SWSB was the result of westward propagation towards the west-
rifting. With continued rifting, initial seafloor spreading occurred ern continental margin. The longer spreading history in the eastern
along the deep troughs between the rigid massifs (Fig. 11B). The part of the SWSB induced a wider oceanic basin, and the V-shaped
cooling and consolidation of the upwelled mantle near the seafloor oceanic SCS basin reflects the preceding propagating rift.
could not be simulated in our experiments. This will limit our
understanding of the initial spreading stage of the SWSB, because Acknowledgments
magmatic processes or thermal variations are believed to dominate
the final stages of breakup to a large extent (White, 1992; Turner
We want to thank Dr. Vincent Strak and the Special Issue Editoral
et al., 1994; Corti et al., 2003b; Kendall et al., 2005; Franke et al.,
Board for their great efforts in hosting this special issue. This paper
2014).
benefits from early discussions concerning the model construc-
Hay et al. (1980) proposed a rift propagation model correspond-
tions and geodynamics of the South China Sea with Dr. Zhen Sun,
ing to strain localization within a stretched continental lithosphere,
Dr. Yunfang Zhang from the South China Sea Institute, CAS, China,
where the process of strain localization is a direct consequence of
Prof. Jean-Clause Sibuet, Dr. Dieter Franke and Dr. Udo Barckhause
the relative motion between two lithospheric plates. Their model
from BGR, Prof. Chunfeng Li and Prof. Xixi Zhao from Tongji Uni-
is based on the concept that the lithospheric plate will rotate
versity, and Manuel Pubellier from ENSP. The paper was improved
with a pole and therefore, the breakup should commence away
following comments from Dr. Vincent Strak and two anonymous
from the pole and propagate towards it (Martin, 1984; Huchon
reviewers. This work was financially supported by the National
et al., 2001). Courtillot (1982) presented a different model in which
Natural Science Foundation of China (No. 41376066), the Scientific
the continental rift is characterized by the presence of “locked
Research Fund of the SIO, SOA (No. JT1202), the Public Science and
zones” distributed unevenly along the rift axis. Spreading occurs
Technology Research Funds Project of Ocean (No. 201205003) and
between the locked zones and with a “punctiform initiation” of
the National Program on Global Change and Air-Sea Interaction,
seafloor spreading (Bonatti, 1985). As indicated by the four-layer
SOA (No. GASI-GEOGE-01).
analogue modelling experiments, the V-shaped oceanic basin and
propagated rifting is achieved because of the different lithospheric
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