1. 0
Accretionary Prism and Forearc
Interpretation of Seismic Profiles
offshore Central Sumatra
Daniel Cox
2. 1
1 ABSTRACT
The orthogonal subduction rate and obliquity of strike slip represents a distinct geological setting in
which to study the accretionary prism and forearc offshore Central Sumatra. The relatively steep (~3.8 )
and narrow (~117 km) prism deviates from typical taper geometry expected for an accretionary
subduction zone with high sedimentation rates. The taper angle is twice (7-8 ) that of Makran and
Barbados yet with correspondingly high pore pressure values (~0.85). An inverse relationship is found
between surface slope and prism width southwards through the survey area. Prism morphology is also
found to be unusual with important examples of landward verging folds as well as predominantly
seaward verging ones, comparable with Cascadia. The anomalous results highlight the importance of
irregular oceanic basement topography as well as varying sediment input and properties in
understanding accretionary mechanics. An understanding of the structure and evolution of the
accretionary prism and forearc zone in this area also has important implications for large, megathrust
earthquakes.
5. 4
4 INTRODUCTION
The accretionary prism and forearc region of the Sunda Arc, offshore Sumatra (see Figure 1) has
often been cited as one of the classic examples of an accretionary margin (e.g. Moore and
Curray, 1980). The Sunda subduction zone has one of the largest accretionary margins
anywhere on the planet (Chauhan et al., 2009) due to the Bengal-Nicobar Fan sediment source,
particularly in the northern part with the lateral extent of the wedge being from 160 to 180 km
(Singh et al., 2008) comparable to those in Java (Schluter et al., 2002), Makran (Fruehn et al.,
1997; Kopp et al., 2000) and Aleutian margin (Clift and Vanucchi, 2004).
Figure 1: The Central Sumatran convergent zone derived from bathymetry data (from General Bathymetric Chart
of the Oceans grid of primarily satellite-derived bathymetry [Sandwell and Smith, 1997]). It highlights basement
topography and seismic profiles in this study. Convergence vectors (white arrows) and subduction velocities (red
arrows) derived from Simons et al. (2007), Delescluse and Chamot-Rooke (2007), Chlieh et al. (2008) and Kamesh
Raju et al. (2004) in McNeill and Henstock (2014).
6. 5
The Sunda subduction zone, stretching 2600 km from the Andaman Islands to West Java, is an
active convergent margin in the northeast Indian Ocean where the Indian-Australian plate
subducts the Eurasian plate (or more specifically Sunda plate). The parameters considered to
affect the structure of the subduction zone include: (a) age of the subducting plate (in this case
varying from 40-100Ma [McNeill and Henstock, 2014]), (b) convergence rate between the two
plates (considered to be relatively low here offshore central Sumatra - 43-54 mm/yr [Karplus et
al., 2014], with oblique convergence along much of its length), (c) the type of overlying plate
(here, the less dense, continental Eurasian plate) and (d) the regional stress state in the arc and
backarc region (before the 2004 great earthquake considered to be uniformly low due to old,
dense oceanic crust subducting slowly with backarc spreading creating forearc islands).
Figure 2: Structure of a subduction zone, from Kopp (2013).
These subduction zone parameters in turn affect the morphology of the accretionary prism (see
Figure 2), which has usually been modelled in terms of critical wedge theory (Dahlen, 1990;
Wang and Hu, 2006) incorporating some of the following: (i) the dip of the subducting plate
(see Figure 3),
7. 6
Figure 3: Chilean versus Mariana type subduction showing that if young/warm oceanic lithosphere is being
subducted, we would expect a relatively shallow dipping subducting plate i.e. buoyant.
(ii) sediment input to system
and trench (closer to the
terrestrial source and we would
expect high sediment input,
often resulting in a large
accretionary prism/wedge due
to sediment input, although
subduction erosion may occur
or underplated over time
beneath the forearc or continue
down with the slab into the
asthenosphere and contribute
to arc chemical signature), (iii)
shallow versus deep trench
(dependent on sediment input
and the flexure of the oceanic
plate), (iv) the area of plate
contact in shallow crust and
hence potential earthquake
magnitude on the plate boundary fault), (v) the likelihood of extension in the backarc, versus
compression behind the arc (usually a function of absolute and relative plate motions and
dynamics of the subducting slab) and (vi) the physical properties of accreted sediments,
including pore pressures using the Coulomb taper theory of Davis et al. (1983) and Saffer and
Bekins (2002), that affect the wedge taper, stress regime and frictional strengths of wedge and
basal mass (Chauhan et al., 2009).
This paper uses a mechanical model for describing how an accretionary wedge develops (see Figure 4).
Hence, the accretionary prism can be viewed as a wedge controlled by friction, the stress field and
8. 7
gravity – based on the Coulomb’s failure criterion (Davis et al., 1983). The base of the wedge is the plate
boundary fault (décollement) - dipping towards the volcanic arc. The top of the wedge is the seafloor.
Large horizontal, compressional forces maintain the wedge, tapering seawards (sometimes imagined as
a bulldozer shoveling soil or snow). The ‘critical taper’ is defined as equilibrium point and maximum
taper angle before collapse – just on the verge of failure – whilst friction at the wedge base maintains
the taper. Pore fluid pressures along the wedge base are internally important as they reduce effective
stress and friction – a weakening effect. Pore fluid pressures may build up if the fluids cannot escape.
Figure 4: The Coulomb Tapered Wedge (Davis et al., 1983), taken from McNeill & Henstock (2014), showing the
accretionary prism geometry used in this study, where: = dip of surface slope (of wedge), = dip of base of
wedge (slab dip) and + = taper angle.
As the Sunda margin is an example of accretionary subduction zone it has been proposed that
the thickness of sediment accreted is large (e.g. Seely et al., 1974; Moore and Curray, 1980)
9. 8
Figure 5: Showing the accretionary wedge and forearc characteristics of the study area
usually associated with continental margins (close to source) and owing to its varying
orthogonal convergence rate, the growth of its accretionary wedge and forearc make it an ideal
location to study accretionary wedge mechanics (see Figure 5). However, the accretionary
prism has probably the greatest structural complexity of an active arc system (Karig et al., 1980)
and much debate surrounds this zone. For a large accretionary prism, plate convergence is
regarded as responsible for deformation of off-scraped oceanic sediments from the down-going
plate and attachment to the over-riding plate referred to as ‘subduction accretion’ (Scholl et al.,
1980), which can occur either as sediment underplating (to the base of the forearc) or ‘frontal
accretion (Platt, 1989) (e.g. Karig, 1974; Seeley et al., 1974). The same tectonic regime has been
attributed as responsible for the deformation of residual continental plate sediments (e.g.
Scholl et al., 1977) as well as erosional processes on the over-riding plate (von Huene and Scholl,
1991), such as large-scale gravity flows or thrusts (e.g. Hamilton et al., 1977). The process of
removal of sediments entirely by sediment subduction with the down-going oceanic lithosphere
underneath the décollement has been forwarded (Shreve and Cloos, 1986; von Huene and
Scholl, 1991). It is thus the décollement or plate boundary that is seen to partition the incoming
10. 9
sediment section and controls the balance between accreted sections versus underthrust or
subducted sections as the prism initiates. It is also the zone of earthquake generation
(seismogenic zone) and is still not fully understood.
The obliquity of convergence at the Sunda margin has been considered to produce at least two
characteristic features in the arc system: (1) strike-slip faulting parallel to the arc to absorb the
parallel component of convergence at the plate boundary (e.g. Fitch, 1972) and (2) folds and
thrusts with trends ranging from oblique to more perpendicular to that of the arc (e.g. Seely,
1977). Thus the particular tectonic regime here has a unique impact on the relative activity and
style of faulting within the prism. In general, predominant fault geometry within accretionary
prisms is accepted as that of landward-dipping thrusts generating seaward vergent folds with
an imbricate fold-thrust belt building seaward (e.g. Seely, 1977). Seaward-dipping thrusts
generating landward vergent folds are less common (see Figure 6); however, data here shows
that this area provides an important exception, the other most well-known example being the
Cascadia margin. The accretionary wedge usually culminates at the forearc high where it comes
against a backstop buttress of continental or sedimentary origin that consists of a material of
higher yield strength than the wedge material trenchwards of it (Chauhan et al., 2009).
Figure 6: From McNeill
and Henstock (2014),
showing the structural
geometry of varying
thrust vergence in the
study area.
Hence, an understanding of the dynamics of accretionary prisms and forearcs is important for
quantifying factors dictating deformation patterns (Chauhan et al., 2009) and the role the
11. 10
forearc plays in seismogenesis. This paper focuses on the central region offshore Sumatra
between the islands of Nias and Siberut where increased distance from the sediment source
has led to increased dominance of exposed oceanic plate basement topography. This has
allowed correlation of seismically imaged backthrusts within the accretionary prism and forearc
system with bathymetric features and published regional and global deformation patterns in
such regimes to establish the underlying relationships.
5 METHOD
This project used multi-channel seismic (MCS) data acquired by the RV SONNE research cruise in 2008
covering ~2800 km of the Sumatran-Andaman subduction zone. The following CDP lines were copied
into a directory on a UNIX system for processing:
SUMD01
SUMD12
SUMD13
SUMD14
SUME01
SUME03
SUME17
SUME18
SUME19
Additionally, high resolution bathymetric data, from General Bathymetric Chart of the Oceans grid of
primarily satellite-derived bathymetry by Sandwell and Smith (1997), was used. Together with the CDP
lines, a bathymetric map was plotted with the GMT software package (Wessel and Smith, 1991). The
seismic lines were copied into ProMAX where SUMD01, SUMD12 and SUMD14 were converted to SEGY
files. A single bandpass filter was applied to the data with minimum phase 1% additive noise factor to
ensure a clean trace display. A top trace mute was applied after filtering to remove the refracted phases.
Automatic gain control operator length was set at 2500 for optimal grey balance on the sections. A
script was then written using the Generic Mapping Tools (GMT) software to create seismic plots before
being printed out on A1 sheets. The seismic plots were then interpreted on paper before being digitised
using the CorelDraw software package. This involved converting the sumd01.ps, sumd12.ps and
12. 11
sumd14.ps files into .PDF files before importing into CorelDraw. Further interpretation was carried out
in CorelDraw.
6 RESULTS
Figure 7: Seisimc line SUMD12
Figure 8: Seismic line SUMD14
Figure 9: Seismic line SUMD01
The toe of the wedge/prism and surface expression of plate boundary has a vertical exaggeration in
Figure 9 of ~1:8 and ~1:5 for Figure 7 and 8. This means the dips and slopes are relatively squashed, for
example, in seismic profiles. The vertical exaggeration is only strictly valid up to the seafloor, since in
reality this would change with depth as velocity increases.
16. 15
Oceanic Plate Trench
The ‘trench’ in all profiles is not really a depression as typified in models (Seely, 1977) and hence not
strictly a trench since it has been infilled with sediments. The top of the sediment layers now match the
level of the seafloor and obscure the trench slope created by flexure of the subducting Australian/Indian
plate. These sediments have been eroded from the prism and transported along the seafloor in axial
channel systems – typical for a margin of high sediment input (originating from the Bengal-Nicobar Fan,
[Curray and Moore, 1974]). The reflectors of these sedimentary sequences are predominantly parallel,
horizontal/sub-horizontal and consistent with undisturbed stratification. The depth of sediment infill at
the deformation front is up to 2.3 seconds (TWT) deep (Figure 7), although a large portion of that is
background sedimentation and sediment interpreted as being transported from the landward trench
slope represents only 0.5 seconds of this thickness. The younger strata emanating from the base of the
landward trench slope can be delineated by their corresponding seaward dip versus the landward dip of
older sediments being progressively down-lapped beneath. This study did not find evidence of any
lenticular bodies or discontinuous reflections associated with buried channels, as found in previous
studies (Mosher et al., 2008).
Deformation front
The deformation front was measured to be 102, 115 and 136 km respectively away from outer forearc
high in Figures 7, 8 and 9.
Plate boundary fault or décollement
The interpreted contact (plate boundary fault) between heavily deformed accreted sediments/forearc
material above and underlying subducted sediment and basement extends between 72 km (Figure 7),
66.25 km (Figure 8), and 79.5 km (Figure 9) beyond the deformation front into the mantle. The
interpreted contact between the two plates or décollement is well-imaged in all three profiles and
occurs at 8.4, 8.9 and 8.8 seconds (Two-Way Travel time [seconds]) respectively at the deformation
front. Moving into the prism from the trench and oceanic plate, there is velocity ‘pull up’ of reflectors,
where water depth decreases and the seismic waves are passing through more sediment than water and
the average velocity increases. Hence, the subducting plate reflector (top of the oceanic basement)
displays a seaward dip in depth section whereas in reality the reflector continues to dip gently landward.
This is evidenced by the respective angles of the dipping slabs ( ): 4 , 3.7 and 4 .
• Accretionary wedge faults
17. 16
All three seismic profiles show evidence of mixed vergence (see Table 1) although the general trend is a
fold-thrust belt with reverse faults younging seaward and dipping landward/eastwards (i.e. folds verging
seaward). The exceptions to the general trend include:
- Landward verging folds within the wedge. The fold-thrust belt of faults feeding into the plate boundary
fault normally get younger seaward but these profiles also unusually display landward-vergent thrust
folds. The internal structure of these folds shows an upward convex formation interpreted as being the
expression of bathymetric anticlines as suggested in previous studies (e.g. Misawa et al., 2014).
- New faults initiating within the prism ‘out of sequence’
Figure 10: Taken from Figure 7 displaying evidence of a low angle thrust fault
- Low angle thrust faults. Seismic profile (Figure 7 and 10) reveals a clear negative polarity reflector
starting ~CDP 1.4*10^4, indicative of a fluid-filled fault and dipping at 8.9 . This suggests it is a seaward-
vergent low angle thrust fault. Such low-angle faults (although tending to steepen towards the seafloor),
mostly dip/verge in the same direction as the décollement, as is the case here. Figure 9 displays a similar
thrust fault at CDP 2.1*10^4 that starts sub-horizontal but becomes much steeper (~27.1 ). Landward of
this flexure lies relatively undeformed strata that has been uplifted by between ~0.6 km (Figure 9) and
18. 17
~1.9 km (Figure 7) and forms part of the outer forearc high. Since this thrust fault tilts parallel to the
underlying oceanic crustal reflector, then deformation at the toe of the slope can be interpreted as
being separate from that of the oceanic crust and it extends landward between 2.8 - 7.5 km under the
toe of the accretionary prism.
- Evidence of strike-slip faults. Figure 5 displays a fault at CDP 2.5*10^4 that is abutted by sediment
layers in the forearc basin and terminates 1.7 km below the seabed against a strong reflector that is
likely to be an old basement horizon due to its shallow angle (10 ) and position away from the
deformation front. It is suggested here that this comprises evidence of strike-slip motion because of the
position in the forearc basin coinciding with the Mentawai Fault and terminates at the strong basement
reflector rather than the much deeper décollement. In addition, relatively undisturbed sediment layers
sit landward of this fault. However, this may not necessary be the case when compared to CDP 0.9-
1.1*10^4 where similarly parallel horizontal sedimentary layers were sandwiched between reverse
faulting/folding. Neither Figure 8 nor 9 show evidence of this strike-slip motion.
Table 1: Fault Spacing (km) and Dip of Ridges
Sumd12 Sumd14 Sumd01
3.75 53°E 3.75 62°E 3.5 60°E
2.5 54°E 9.22 74°E 10.75 68°E
6.88 78°E 4.69 74°E 17.75 77°E
9.69 78°E 3.75 58°E 6 81°W
10.94 66°W 8.28 70°W 38
33.76 3.28 84°E
32.97
Table 1 shows that main fault spacing causing thrust formation of ridges along the seismic profiles
generally increases away from the deformation front, although there is significant variation in each
profile where these ridges form. The band of thrust fault ridges tends to cease ~35km from the
deformation front. The initial thrust fault ridge uniformly occurs between 3.5 and 3.75 km from the
deformation front. These results also display a landward verging fold ~30 km from the deformation front
(33.76, 29.67 and 32 km respectively). The fault dips also steepen in general, eastwards along the
profiles, starting at ~50 and ending at ~80 . The main thrust faults generally appear to terminate
against the plate boundary décollement indicative that most of the sediment is accreted on the prism
toe. However, the faults may step down deeper landward, resulting in sediments being ‘underplated’.
Better resolution is needed for this kind of analysis.
19. 18
Since the fault ridges become less prominent further landward, this suggests most of the strain
distribution and hence deformation occurs in the initial 30-40 km of the accretionary prism. Exceptions
to this observation can be observed in Figure 7 (and 16) between CDP 0.9-1.1*10^4. Here lies a set of
horizontally parallel sedimentary packages relatively undisturbed by the faults on either side of it.
However, the other profiles do not show this structure suggesting it is an exception.
Forearc high region
Figure 7 (CDP 2 - 2.2*10^4) and Figure 9 (CDP 0.8 - 0.9*10^4) reveal structures of different composition
from the surrounding sediments and likely to be of carbonate composition due to the strong reflectors
in the outer forearc high and the calcareous oozes that make up the sediment. Strong, shallow reflectors
image an anticlinal structure with adjoining steep slopes indicating structural imbrication; however,
coherent deep reflectors are not imaged sufficiently well. This is consistent with earlier work (e.g.
Moore and Curray, 1980; Karig et al., 1980; Silver and Reed, 1988) that observed the presence of
backthrusting along segments of the Sunda Arc but its relationship with the backstop buttress is
undefined, particularly at depth. It is interpreted here that the structure was involved in the creation of
the outer forearc high formed from older, different, deformed accreted material. The sediments either
side of these structures are likely to be the oldest sediments due to the onlapping patterns, which
suggest tilting. Faults are recognized here as discontinuities within these sedimentary packages and are
mostly found on the seaward side of the outer forearc high with both seaward and landward dipping
faults observable as noted in previous research (Misawa et al., 2014). They are interpreted as associated
with extensional flexure of sediment sequences from underlying thrusts.
• Forearc basin
102km (Figure 7), 115 km (Figure 8) and 136 km (Figure 9) behind the deformation front a syncline or
depression between the arc and the prism partially or completely filled with sediments is imaged. These
results display an increasing prism width from the north to the south of the survey area. The forearc
basins are located under ~0.5 seconds water depth (i.e. 750 m deep, using 1.5 km/sec for velocity in
seawater). They have developed with relatively undeformed sediment layers ranging from 0.6 (Figure 8),
1.6 (Figure 9) and 1.9 seconds (Figure 7 + 16). This equates to sediment depths from 900 to 2850 m
(using velocity in sediment of 3.5 km/sec). The seaward flank of these basins, particularly in the case of
Figure 7 and 9, has been uplifted as indicated by landward-dipping reflectors.
20. 19
Figure 9 and 7 (and 16) display strong unconformable reflectors at 2.5 seconds and 3.5 seconds (TWT)
respectively behind the outer forearc highs and in the forearc basins (6875m and 9450m deep
respectively, using velocity in sediment of 3.5 km/sec). These unconformable strong positive reflectors
dip at 4.2 and 10 respectively and are interpreted as old basement related to the formation of the
forearc basin due to their proximal setting to the continental slope and seaward dip direction (opposite
to that of the downgoing oceanic plate). The onlapping sediments above these reflectors also show
pronounced angular unconformities. These unconformities in basin strata suggest coincidental uplift
during sedimentation.
Evidence of slumping and mass transport deposits can be seen in Figure 9 (and 11) ~CDP 0.3*10^ and
Figure 8 0.1 - 0.2*10^4. Since these chaotic features are confined to upper layer (first 1750 m, using
velocity in sediment of 3.5 km/sec), they are more likely to be of sedimentary origin rather than tectonic
because of this spatial constraint. However, no distinctive axial channels indicative of turbidite flows
have been observed.
Figure 11
• Subducting ocean crustal basalt
The subducting Australian/Indian plate shows clearly in the seismic reflection data as a strong
reflector. The average dip of the subducting plate (top of the oceanic basement and base of the
wedge taper, ) is 4 , 3.7 and 4 respectively. The sediment basement reflector is non-uniform with
irregularities and relief of >0.5 seconds in some instances. This may be interpreted as related to the
21. 20
offsets due to faulting, several of which reach the décollement. The Moho was not imaged in any of
the profiles.
Wedge morphology: Topographic Slope ( ), Décollement Dip ( ) and Fluid Pressure Ratio ( )
Figure 12: Showing the bathymetric profile across the region at four different latitudes and striking at an oblique
angle to the deformation front rather than perpendicular as do the seismic profiles.
22. 21
A ridge and trough morphology occurs on the trench slopes (see Figures 12 and 13), with the sediments
accumulating in the troughs. Overall dip of the Sumatran prism ( ) at Figures 7, 8 and 9 is 3.8 , 4.1 and
3.5 respectively (taken from the deformation front to just before the outer forearc high – see Figures 2
and 4). The average dip of the subducting plate (top of the oceanic basement and base of the wedge
taper, ) is 3.7 , 4 and 4 respectively.
Figure 13: An enlarged bathymetric profile along four different latitudes that are closest to the seismic profiles.
Figures 12 and 13 show the irregularities of the incoming Indian-Australian plate that is not the typically
smooth topography of a down-going plate. They also illustrate the uniform steepness of the surface
slope across the study area and broad plateau extending behind the outer forearc high. The forearc
basin is not especially prominent and is growth appears to be hindered by the proximity of surrounding
islands (Nias, Batu and Siberut).
The estimates for and suggest high overall pore fluid pressures for the Sumatran prism: (ratio of
pore pressure to vertical lithostatic pressure) values of 0.87, 0.81 and 0.86 respectively are estimated
(see Table 2). This compares against the established value in the Sunda trench of = 0.7.
Table 2: Properties of Prism and Forearc of Sunda Margin
Seismic
Line Prism Width Surface Slope Slab Dip (β) Taper Angle
(km) Dip (α) (α+β)
Sumd12 102 3.8 3.7 7.5
Sumd14 115 4.1 4 8.1
Sumd01 136 3.5 4 7.5
23. 22
Central 125 2.4
Sumatra
Seismic
Line
Pore Fluid
Pressure Trench Sediment
Trench
Sediment Percent of Total Décollement
(λ)
Thickness (TWT
s) Thickness (km)
Sediment
Accreted Length (km)
at Prism Toe
Sumd12 ~0.87 2.4 3.5 ~95 72
Sumd14 ~0.81 1.2 2.1 ~95 66.25
Sumd01 ~0.86 1.6 2.8 ~95 79.5
Central 1.4-1.7 1.6-2.1 90-100
Sumatra
(using velocity
2.3-2.5 km/s)
7 DISCUSSION
Wedge Structure
The overall dip of the Sumatran prism ( ) at Figures 9, 7 and 8 is 3.5 , 3.8 and 4.1 respectively (taken
from the deformation front to just before the outer arc high in order to avoid the bias of an under-filled
forearc basin). The average dip of the subducting plate (top of the oceanic basement and base of the
wedge taper, = ~3.9 ) is comparable with that reported by Kopp and Kukowski (2003) of 3 and Karig
et al. (1980) of 4 beneath the Central Sumatra margin prism toe, which is a smaller dip than that for the
entire prism (5-8 ). The critical wedge taper geometry does not appear to significantly change along-
strike: 7.5 , 8.1 and 7.5 . The proximity of the survey area to both Nias and Siberut islands is likely to
play a significant role in forearc structure since the oceanic basement highs create a barrier to trench
sediment transport as they subduct. This central section offshore Sumatra is more anomalous than
other parts that do not have outer forearc high islands.
Trench Sediment Thickness as a control on prism geometry and accretion rates
24. 23
Recent research (e.g. McNeill and Henstock [2014]) assert that sediment thickness decreases from north
to south in the Sunda trench whilst water depth increases, although this is highly variable as a result of
basement topography offshore Central Sumatra. Results here confirm a mixed trend in the central
region with a suggested decrease to the south (see Table 1). This correlates well with increasing distance
from the Bengal Fan source and the Ninety East Ridge acting as a barrier reducing sediment transport
southward. The sediment thicknesses here (2.1 – 3.5 km) are less than half those from North Sumatra (7
km) and other thickest global trenches (Makran: 7.5 km [Smith et al., 2012], Southern Lesser Antilles: 7
km [Westbrook et al., 1984]).
The proportion of sediment accreted at the prism toe versus accretion by underplating was estimated at
the deformation front based on the deepest extent of prism compressional faulting. Accurate velocity
values for sediment deeper in the accretionary prism are needed to more accurately define the amount
underplated, since it is more likely to occur further away from the deformation front and display lower
velocity values than the surrounding crustal rock. Nevertheless, the majority of sediment appears to be
accreted on the prism toe.
Prism Width and Surface Slope ( )
These results show a general trend of increasing prism width (102 – 136 km) yet maintaining a broadly
constant prism slope ( ) from north to south (3.8 – 4.4 ), thus correlating against the general trend (e.g.
McNeill and Henstock [2014]), where a strong inverse relationship between prism width and surface
slope exists. This may be a result of variable subducting basement features that may enhance
accretionary underplating. Alternatively, it may be due to highly variable sediment thickness sections
owing to oceanic plate topography, climatic change and prism age.
This also disagrees with global relationships observed between prism width and prism surface slope ( )
(e.g. Clift and Vannuchi, 2004) and generally across the whole Sunda margin (McNeill and Henstock,
2014), where prism surface slope angles are also persistently lower (>1 ) than found in this study.
Accretionary subduction zones have prism widths that vary globally from ~40 km to ~350 km (von Huene
and Scholl, 1991). The prism width is proportional to the amount of incoming sediment, so that the
larger the amount of incoming sediment, the broader the prism. This survey area offshore central
Sumatra is unusual in displaying increasing prism width further away from incoming sediment sources
(Bengal Fan). This compares to prism widths in Cascadia and Nankai (of similar input sediment and plate
convergence rates) that are relatively narrow. In the case of Cascadia, the narrower prism widths can be
25. 24
explained by higher fluid pore pressures leading to major slope collapse (Goldfinger et al., 2000) and
other subduction erosion processes (McNeill et al., 2000). Subduction erosion processes appear to be at
a minimum here partly due to basement topographic irregularities and despite high pore pressures. This
investigation also fails to show a strong correlation between overpressure of sediments leading to lower
angle surface slopes and taper angles, in fact, the reverse is true. In comparison to the Makran and
Barbados which are accreting thick sedimentary sequences with correspondingly low taper angles (~3-
4 ) and high pore pressures (~0.97), the central part of the Sunda trench are producing taper angles
twice the size (~7-8 ) and pore pressure ratios that are still relatively high. This may again be linked to
irregular bathymetric topography or as some authors have suggested (Fruehn et al., 1997; Smith et al.,
2012, 2013) the relationship between sediment and accreted thickness and taper angle is not as
straightforward as previously thought. Rather, changing sediment properties may be responsible for
surface slope steepness.
Prism Sediment Properties
Increasing pore pressures (fluids) at the décollement reduce effective stress along the base of wedge
and decrease traction/friction – the décollement is effectively a plane of weakness. This leads to a
wedge that is likely to fail since it can only maintain smaller dip angles ( + ). The basal internal
coefficient of friction ( b ~ 0.85) and coefficient of friction ( ~1.03, gradient of the failure envelope/line)
also control the prism taper angle, where high friction supports steeper dip, low friction can only
support shallow dipping wedge.
In this study, the ratio of topographic slope versus décollement dip for submarine wedges with respect
to values (from Davis et al., 1983) is high (~0.85 average), which is higher than the established value
for the Sunda trench (0.7) and significantly higher than hydrostatic pressure (0.4) (see Figure 15). Pore
fluid pressure (ratio of pore pressure to vertical lithostatic pressure, ) is considered large if ≈ 1, where
pore pressures are high i.e. lots of trapped fluids. If is small (<< 1), pore pressures are low. Hence,
whilst a relatively high taper angle is likely to mean the prism is relatively well-drained and low pore
fluid pressures exist (Saffer and Bekins, 2002), the value found here is much more comparable to an
accretionary prism in a similar setting of relatively high sedimentation rates that lead to bulk
overpressure, such as the Makran ( = 0.98) being fed by the Indus Fan (see Figure 14). Yet a relatively
high taper angle suggests overpressure is not at work in this location. This illustrates the importance of
decreased sediment rates leading to a suspected lower fluid pressure ratio in Central Sumatra. The
26. 25
absence of a prominent, widespread bottom-simulating reflector in all three profiles also suggests a
dewatered setting and limited overpressure.
Figure 14: Examples of global active accretionary submarine wedges with width and values (from Davis et al.,
1983).
27. 26
Figure 15: Theoretical relationship between taper and for global submarine wedges (from Davis et al., 1983).
Thrust Vergence
Prism width, surface slope and thrust vergence are generally agreed to correlate so that wide prism
widths, low surface slopes and small taper angles lead to landward vergence (McNeill et al., 2012). The
Makran and Barbados wedges, however, are major exceptions with the widest, lowest surface slope
prisms and predominantly seaward vergent thrusts (Smith et al., 2012; Westbrook et al., 1984). The
mixed vergence observed in these results does correspond with observations made in other papers
correlating a sediment thickness of ~1-3 km with such structures along and across strike (McNeill and
Henstock, 2014). However, the wide prism widths, high surface slopes and high taper angles here,
together with landward vergence does not correlate with established research. Increased variation in
oceanic basement topography may be a significant factor. Kopp et al. (2008), for example, show that
thin sediment cover means that the basement interacts more directly with deformation within the prism.
Variation of sediment properties throughout the accretionary prism is also likely to play a significant role
in driving structure and morphology. Gutscher et al. (2001) argue that landward vergence is associated
with rapid prism growth and hence broad prism widths since the décollement is closer to the top of the
28. 27
oceanic basement (Mackay et al., 1992), however, the décollement position does not appear to change
in these results (where it can be confidently interpreted) and hence is not necessarily a corollary of
increasing prism width and slope angle.
Accretionary Wedge Faults
The reverse faults formed on the accretionary wedge are consistent with a compressional tectonic
setting. Additionally, the landward dipping low angle thrust fault most prominent on Figure 7 (and 16)
(CDP 1.4*10^4) confirms this (see also Figure 9, CDP 2.1*10^4). This confirms earlier work by Moore and
Curray (1980) of a seaward-dipping monocline as the shallow expression of this fault. It is likely that a
large proportion of sediment entering this subduction zone atop the oceanic crust is scraped off and
accreted underneath this thrust-thickened sequence. There also appears to be a causal relation between
the thrust angle and the amount accreted beneath the thrust fault; since Figure 5 with an angle a third
as steep as Figure 9 has uplifted three times as high.
Average spacing between the major reverse faults here (ones causing ridge structures) has been found
to be more variable than that in previous studies (e.g. Karig et al., 1980) ranging from 2.5 – 17.75 km
(versus 3-5 km), although faults with an easterly dip are confirmed to predominate. The sharp
bathymetric features on the seafloor that are likely to produce these backthrusts rising from the
décollement also suggest that these faults are active and the inner forearc is continuing to evolve.
Moreover, uplifting along this backthrust belt is likely to explain the presence of forearc islands along
the Sumatran margin helping to constrain their tectonic evolution (Chauhan et al., 2009).
Due to the obliquity of convergence along the Sunda Trench between the Indian-Australian plate and
Indonesian part of the Eurasian plate, it is expected that some strike-slip motion will be accommodated
parallel to the arc. The fault at CDP 2.5*10^4 on Figure 7 appears to be one of these strike slip offsets
and may be a continuation of the Mentawai Fault, which can be traced from the Sumatra fault zone to
Nias island. These strike slip faults developed on the landward side of the Sunda fore arc and obliquely
crossing the forearc basin are well-documented in some areas and strongly suggested in others in order
to absorb part of the dextral motion associated with oblique convergence (e.g. Fitch, 1972; Karig et al.,
1980; Sieh and Natawidjaja, 2000). Coinciding with other research, this study confirms that judging
primarily from the relationships at the discontinuity, the strike slip features are not related to the
downgoing plate but rather pieces of continental margin sliding northwards in response so oblique
29. 28
subduction (ibid). However, the lack of evidence in the other seismic lines needs to be resolved, if it is
indeed a strike slip fault and is laterally extensive across the region.
Figure 16: Taken from Figure 7
Subduction Rates, Obliquity and Prism Morphology
Convergence along the Sunda margin is relatively low and typical of accretionary margins (e.g.
Lallemand et al., 1994; Clift and Vannucchi, 2004). The Indian-Australian and Sunda plates converge
along a NNE angle, therefore, movement is oblique from south Sumatra northward and motion
decreases slightly from south to north (Misawa et al., 2014). However, there appears to be no consistent
relation between convergence rate, obliquity and expected prism morphology. Highest orthogonal
subduction rates correspond to a narrow prism (West Java), whilst the steepest, narrowest prism is
found to the far north, where sediment rates and obliquity are both at a maximum (ibid). However, a
decreasing northward and more orthogonal component in subduction rates moving southwards down
30. 29
the margin leads to a general increase in prism width before a sudden drop within the study area here
between Nias and Siberut islands.
The absence of any clear ‘sediment apron’ (Moore and Curray, 1980) on the outer wedge slope, rather
the development of ponded sediments in slope basins, for example, CDP1*10^4 on Figure 7 (and 17),
and numerous angular unconformities in the forearc basins of Figure 7 (and 16) and 5 confirm previous
research (Seely et al., 1974; Moore and Karig, 1976) that sedimentation and deformation here is almost
synchronous. Moreover, it is postulated here that the across-strike changes in sediment properties
particularly around CDP 2.5*10^4 (Figure 7 and 16) and 0.7*10^4 (Figure 9) play a key role in the
evolution of the prism and forearc morphology in this region. The interpreted strike-slip fault in Figure 7
(and 16) at this point marks an important transition and boundary between seaward, accreted material
and landward, older forearc basement material, leading to a clear backstop.
Figure 17: Showing evidence of ponded sediments
31. 30
8 CONCLUSION
Overall, the results presented in this study suggest that the central Sumatran area is not typical of an
average accretionary prism. The accretionary prism morphology is relatively steep and narrow
compared to accepted models. Pore pressure values ( ) are high, yet this is coupled with surface slope
angles that are unexpectedly steep, therefore deviating from typical taper geometry. The observance of
mixed vergence in this area makes it an important exception to expected seaward vergence in standard
accretionary models. It is concluded that these results are due in part to the irregular oceanic basement
topography affecting sediment thickness and properties along strike. Whilst sediment thickness is
somewhat greater in the northern part of the study area than the southern part, the prism width
increases towards the south. The linkage between forearc structure and tectonics is likely to play a key
role in the formation of these structures through the orthogonal subduction rate and obliquity of strike
slip. These results demonstrate that the central Sumatran accretionary prism and forearc are an
anomalous example that is linked to the complex tectonics of the area evidenced by the megathrust
earthquake rupture in 2004.
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