Geological aspects of Banda Sea ecosystems and how they shape the oceanographical profile

Abstract

The Banda Sea is a collage of young oceanic basins and fragmented Australian continental crust located at the heart of the Australia–SE Asia collision zone where Australian and Asian biogeographic regions converge. The formation of the sea was governed by the southeastward rollback of the Banda Slab since c. 16 Ma, which in its wake opened new oceanic basins and extended and fragmented Australian crust. These Australian crustal fragments are today either stranded within the Banda Sea where they form the prominent submarine 'Banda Ridges', or now reside as thrust-sheets on the NW Australian shelf after being transported all the way to the southern Banda Arc. The deepest part of the Banda Sea, the 7.2 km Weber Deep, was formed by extreme lithospheric extension that occured in the latter stages of Banda Slab rollback. This extension was accommodated by the vast low-angle 'Banda Detachment', which operated above the subducted fringes of the Australian continental margin.

Full text

IOP Conference Series: Earth and Environmental Science
PAPER • OPEN ACCESS
Geological aspects of Banda Sea ecosystems and how they shape the
oceanographical profile
To cite this article: J M Pownall et al 2018 IOP Conf. Ser.: Earth Environ. Sci. 184 012005
View the article online for updates and enhancements.
This content was downloaded from IP address 181.215.8.236 on 21/08/2018 at 02:37

1
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
Geological aspects of Banda Sea ecosystems and how they
shape the oceanographical profile
J M Pownall1, R Hall2, G S Lister1and A Trihatmojo3
1Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia
2SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham
TW20 0EX, United Kingdom
3Fakultas Ilmu dan Teknologi Kebumian, Institut Teknologi Bandung, Jalan Ganesha 10, Bandung 40132,
Indonesia
E-mail: jonathan.pownall@anu.edu.au
Abstract. The Banda Sea is a collage of young oceanic basins and fragmented Australian continental
crust located at the heart of the Australia–SE Asia collision zone where Australian and Asian biogeographic
regions converge. The formation of the sea was governed by the southeastward rollback of the Banda Slab
since c. 16 Ma, which in its wake opened new oceanic basins and extended and fragmented Australian
crust. These Australian crustal fragments are today either stranded within the Banda Sea where they form
the prominent submarine ‘Banda Ridges’, or now reside as thrust-sheets on the NW Australian shelf after
being transported all the way to the southern Banda Arc. The deepest part of the Banda Sea, the 7.2 km
Weber Deep, was formed by extreme lithospheric extension that occured in the latter stages of Banda Slab
rollback. This extension was accommodated by the vast low-angle ‘Banda Detachment’, which operated
above the subducted fringes of the Australian continental margin.
1. Introduction
The Banda Sea is one of several seas located within the Indonesian archipelago between Australia and
mainland SE Asia. Together, these seas comprise the route for the Indonesian ‘Throughflow’ between
the Indian and Pacific oceans – a crucial gateway for ocean circulation and regulation of global climate
[1, 2, 3]. These seas also mark the boundary for important biological separations. Wallace’s line passes
through the Celebes Sea and west of Sulawesi, while Weber’s line and Lydekker’s line both pass through
the Banda Sea its self. These observations can all be explained when considering the region’s changing
tectonics. The formation of narrow seaways, and the juxtaposition of different flora and fauna, are both
side-effects of the collision of Australia with SE Asia and the closure of the Tethys Ocean that once
separated them [4] (Fig. 1).
One hundred and eighty million years ago (Ma), Australia and India both separated from Antarctica
during breakup of the supercontinent Gondwana, later heading north. India collided with central Asia at
around 50 Ma, ahead of the slower continent of Australia that collided with SE Asia at around 23 Ma [5].
Whereas India–Asia collision commenced early enough to have today produced the Himalaya, Australia–
Asia collision is still in its infancy. In the place of 8000-metre peaks and an elevated continental plateau
is a complex array of oceanic basins, continental fragments, volcanic arcs, and carbonate platforms. The
infancy of the Australia–SE Asia collision zone accounts for why the geography and ecosystems of the
Indonesian archipelago and its intervening seaways are so intricate and diverse.

2
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
Figure 1. Map of India, SE Asia, and Australia. The Banda Sea is located within the SE Asia–Australia
collision zone. India and Australia (Gondwanan; red), which both rifted from Antarctica at c. 160 Ma,
collided with Asia (yellow) at c. 50 Ma and c. 23 Ma, respectively. Note the course of the Indonesian
Troughflow (TF) oceanic current connecting the Indian and Pacific oceans, and the positions of Wallace’s
(Wa), Weber’s (We), and Lydekker’s (Ly) faunal boundary lines within the SE Asia–Australian collision
zone. Figure adapted from Hall (2012) [4].
2. How did the Banda Sea form?
The Banda Sea has a complex and varied oceanographic profile due to the composite nature of the crust
it overlies [6]. Young oceanic crust (12.5–3.5 Ma [7, 8]) contains numerous continental fragments such
as those forming the Banda Ridges, and features the very deep oceanic basins of the North Banda Basin
and Weber Deep (Fig. 2). The Banda Sea lies mainly within the curved island chain of the Banda Arc,
from Timor to Tanimbar to Kai to Seram to Buru. An active inner volcanic arc from Damar volcano to
Banda Api [9, 10] forms an eastward extension to the more mature volcanic islands of Flores, Alor, and
Wetar. The 7.2 km deep Weber Deep [11, 12] is the forearc basin between the inner volcanic and outer
non-volcanic arcs.
So, why is the Banda Arc so tightly curved? It’s shape was dicated by the geometry of the underlying
subducted slab, shown by the location of earthquakes produced within it to have assumed a highly

3
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
Figure 2. Present-day configuration of the Banda Sea, showing the location of the Banda Detachment
flooring the Weber Deep [11]. As in Fig. 4, submarine arcs and oceanic plateaus are shown in pale
blue; volcanic island arcs, ophiolites and material accreted along plate margins are shown in green; and
Australian-affinity continental crust is shown in red.
concave spoon- or half-bathtub-shaped geometry [13, 14, 15] (Fig. 3). This slab, an eastward extension to
the Java and Sumatra slabs, was once located adjacent to southern Sulawesi – a few thousand kilometres
further west of its present location (Fig. 4a). As depicted by the tectonic reconstuctions in Figure 4, the
location of the subduction trench has migrated gradually to the southeast, driven by the sinking of the
slab—the Proto-Banda Sea—into the mantle through a process of ‘slab rollback’ [13].
The Proto-Banda Sea, Jurassic in age, once occupied a ‘Banda Embayment’ within the Australian
continental margin not too disimilar in shape to the modern Banda Sea [5, 13]. As this old, cold, and
dense oceanic lithosphere rolled back into the embayment (Fig. 3b,c), extension of the lithosphere behind
the arc drove oceanic spreading, and the formation of new oceanic crust beneath the modern Banda
Sea. The North Banda Basin and South Banda Basin spread open at different times behind this rolling-
back arc, between 12.5–7.15 Ma [7], and 6.5–3.5 Ma [8], respectively. In addition to forming new
oceanic crust, extension behind the rolling-back slab thinned and rifted apart Australian continental crust
that once enclosed the northern extent of the Banda Embayment. This caused continental slivers to be
standed within the Banda Sea in the form of the Banda Ridges (Fig. 2), as discovered by dredging [17].
Continental slivers may also have been transported immediately behind the rolling-back Banda Trench
right up until the point of arc–continent collision between the Banda Arc and the southern Banda margin,
causing Australian-affinity blocks from north of the Banda Sea to have been accreted onto a different part
of the Australian continental margin in the vicinity of Timor and Babar [3, 6].
Extreme lithospheric extension driven by Banda Slab rollback (Fig. 4) also affected islands in the

4
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
Figure 3. Present-day configuration of the Banda Sea, overlying the curved Banda Slab (the Proto-Banda
Sea), modified from Pownall et al. (2014) [16]. Note the horizontal tear in the Banda Slab beneath Buru
and western Seram.
Figure 4. Tectonic reconstruction of eastern Indonesia, depicting formation of the modern Banda Sea
during rollback of the Banda Arc, at (a) 15 Ma, (b) 7 Ma, and (c) 2 Ma (adapted from Hall, 2012 [4]).
Oceanic crust is shown in dark blue (older than 120 Ma) and mid-blue (younger than 120 Ma); submarine
arcs and oceanic plateaus are shown in pale blue; volcanic island arcs, ophiolites and material accreted
along plate margins are shown in green.

5
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
northern Banda Arc. Hot mantle rocks were exhumed to the shallow sub-surface [11, 16, 18, 19, 20],
driving crustal metamorphism of adjacent rocks under ultrahigh-temperature (UHT; >900°C) conditions
[16, 21]. Volcanic rocks comprising the island of Ambon are shown to be derived primarily from the
melting of these stretched continental crustal rocks [20], and so are disctinct from the volcanic products
of the rest of the arc. However, volcanoes from Banda Api to Damar also record elevated continental
input due to the subduction of continental material from the Australian margin [9, 10].
Exhumation of mantle rocks and HT–UHT metamorphic rocks in the northern Banda Arc was
faciliated, in part, by strike slip faults such as the Kawa Fault of Seram (Fig. 2), and the associated
Kobipoto Mountains pop-up structure [18, 21, 12]. The Kawa Fault is a major structure that has
been fundamental in enabling the Banda Arc to roll back eastwards with respect to the northern
Banda Embayment margin. The fault also forms part of a larger structure, the Seram–Kumawa Shear
Zone, which may have developed in the Jurassic when continental blocks were rifted from the Banda
Embayment [12].
3. SE Asia–Australia collision and the Indonesian Throughflow
Collision of SE Asia with Australia at c. 23 Ma [5] closed the deepwater passageway that existed between
the Indian and Pacific oceans [1, 22]. The Indonesian ‘Throughflow’ describes the resulting oceanic
current that follows a winding and anastomosing course through the narrow and shallow Indonesian
seaways from the SW Pacific to the Indian Ocean (net flow rate: 15 Sverdrups [23]). The primary route
follows the Makassar Strait, Flores Sea, South Banda Sea, and Timor Trench (Fig. 1), although weaker
currents also pass through the Lombok and Ombai straits. Throughflow transport is governed seasonally
by El Ni˜
no–Southern Oscillation and over millennial timescales by sealevel flucuations [22, 24]. Over
geological time, the configuration and strength of the Throughflow has undoubtedly been controlled by
the evolving tectonic configuration, with the uplift of mountain belts on, for instance, Sulawesi restricting
the flow. Due to uncertainties of reconstructing exact palaeogeographies, it is not know exactly how the
passageways of the Indonesian Throughflow evolved through time [22]. However, it is thought that the
passageways were especially restricted between 12 and 3 Ma, with the narrowest gaps occuring at around
10 Ma [22].
4. Isostatic controls on the Banda Sea profile
Structures within slabs beneath the Banda Sea (Fig. 5), and the mechanism by which they were
subducted, have been highly influential in controling on the oceanographic profile through their isostatic
response:
•Within the northern limb of the slab beneath Buru, an aseismic zone is interpreted as a tear that
has caused the slab to peel away from its northern margin [13, 14, 15] (Fig. 3), shown also by
tomographic models [13, 15]. Propagation of this horizontal tear may have contributed to the rapid
uplift rates recorded around the northern Banda Arc [25, 26] as the upper plate rebounded.
•The southern limb of the slab features a band of intense seismicity—the Damar Zone [27]—at 100–
200 km depth, which extends westwards from the Aru Trough and terminates sharply just west
of Romang at the plane of intersection with the ‘Gunungapi Lineament’ (Fig. 2) passing from the
Timor Trough to the North Banda Basin via Gunungapi Volcano. We interpret this ridge to be the
surface expression of a major subvertical slab tear that delineates the actively-rupturing slab on its
eastern side, and which potentially accounts for the positive increase in mean topographic elevation
identified by Sandiford (2008) [27] moving west from Romang to Wetar.
•There is a shallow-dipping section of slab extending to beneath the Weber Deep [11], which likely
is the down-flexed and over-thrust outermost Australian continental margin (Fig. 5). Beneath the
Weber Deep is the location of the continental–ocean transition, at which point the slab steepens
abpruptly. As discussed by Pownall et al. (2016) [11], the incredible depth of the 7.2 km
Weber Deep forearc basin floor (Fig. 6) is likely supported by this shallow-dipping slab segment.

6
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
As discussed in the next section, the Weber Deep was created by the development of a low-
angle detachment system during the final stages of extension behind the rolling-back Banda Slab
[5, 11, 12].
5. The Weber Deep
The floor of the Weber Deep (Fig. 6, 7), beneath 7.2 km water depth, is the deepest part of Earth’s oceans
that does not occur in a trench. So why is this forearc basin so deep? It has been suggested that the
Weber Deep formed as a flexural response to a tightening of the Banda arc’s curvature [6], or in response
to the thrusting of the Banda Sea over the surrounding Australian continental margin [28]. Alternatively,
some authors by interpreting the feature as an extensional basin attributed east-west extension either
directly to north-south shortening caused by the northward advance of Australia [29] or to eastward slab
rollback [13]. The extreme depth of the basin has also been explained simply as the result of sinking of
the underlying Banda slab [6, 30] without requiring rollback.
New high-resolution (15 m) bathymetry [11] has revealed in incredible detail intricate features of
the eastern Banda Sea, including the Weber Deep and Aru Trough [11, 12]. Most notably, the entire
Weber Deep forearc features a set of parallel striations or grooves oriented at 120–300 ±10° (Fig. 6),
which have been generated within a single low-angle detachment fault zone, the ‘Banda Detachment’
[11]. The detachment has a listric geometry, curving from a 12° dip adjacent to the eastern rim of the
basin, becoming horizontal then slightly backrotated (by 1°) approaching the volcanic arc (Fig. 5b). The
grooves’ orientation and length demonstrate a southeasterly slip direction of 120–130°, along which the
Figure 5. (A) Arc–continent collision in the Banda Region. Note how the continent–ocean boundary
between the continental margin of the Banda Embayment (orange) and the Jurassic Proto-Banda Sea
(pink) has been over-thrust by the modern Banda Sea (red and blue). Extreme lithospheric extension
along the Banda Detachment has opened the Weber Deep above the Banda Slab. The location of this cut-
away is shown in Fig. 6 by the line X–X′. (B) An enlargement of the Banda Detachment cross-section
(2×vertical exaggeration) with the highly-extended Sula Spur lithosphere shown in red. Figure from
Pownall et al. (2016) [11].

7
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
10 km
10 km
305˚
125˚
310˚
130˚
Figure 6. Bathymetric map of Weber Deep and Aru Trough (eastern Indonesia), showing the location
of Banda detachment and its relationship to Kawa shear zone on Seram, after Pownall et al. (2016)
[11]. The red areas mark approximate exposures of exhumed upper-mantle–lower-crustal (Kobipoto
Complex) rocks. Multibeam data (15 m resolution) courtesy of TGS (www.tgs.com) and GeoData
Ventures (Singapore). The enlargements, bottom–left, show the parallel lineations present on the Banda
Detachment scarp. Note how the grooved fault scarps exposed in the Weber Deep are parallel also to the
Kawa Shear Zone on Seram.
450 km-long detachment must have slipped >120 km during a massive extensional phase in the arc’s
evolution. The Banda Detachment is the largest identified normal fault system exposed anywhere in the
world’s oceans [11].
Ultramafic rocks and high-temperature metamorphic rocks (diatexites, gneisses, high-grade
amphibolites) are exposed all around the northeastern rim of the Weber Deep, from the Wai Leklekan
Mountains of eastern Seram, to the small islands of Kur and Fadol (Fig. 8) in the east of the arc. Offset
along a major normal fault is the only plausible way of explaining how these lower-crustal–upper-mantle
rocks are exposed adjecent to a 7 km-deep basin [11]. Furthermore, the Banda Detachment fault scarp—
dipping into the Weber Deep at a consistent 12°—has been observed on Fadol (Fig. 8) and in eastern
Seram [11]. There is also a connection between the Banda Detachment and the Kawa Fault Zone,
described previously. Both the Kawa Fault and the grooves on the Banda Detachment fault scarps are

8
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
B
a
n
d
a
D
e
t
a
c
h
m
e
n
t
Weber
Deep
pinnacle
reefs?
B
a
n
d
a
D
e
t
a
c
h
m
e
n
t
slump
Figure 7. Perspective view of the Weber Deep (large vertical exaggeration; looking north) showing the
large submarine landslides and submerged pinnacle reefs.
Figure 8. Sailing south to the island of Fadol in the eastern Banda Arc. The Banda Detachment bounds
the gently-dipping western side of the island (right side in the photo). On Fadol, high-temperature
metamorphic rocks and ultramafic rocks are exposed, capped by uplifted carbonate terraces.

9
1234567890 ‘’“”
International Symposium on Banda Sea Ecosystem (ISBSE) 2017 IOP Publishing
IOP Conf. Series: Earth and Environmental Science 184 (2018) 012005 doi :10.1088/1755-1315/184/1/012005
parallel, thereby demonstrating the two structures are in some way coupled; and so likely acted together
to facilitate southeastward slab rollback plus extreme forearc extension.
The oceanographical profile of the Weber Deep has been also influenced by submerged pinacle reef
structures (alternatively mud volcanoes?; Fig. 7), and by large submarine debris flows that blanket much
of the eastern rise (Fig. 6,7). These flows, some continuous over 100 km, demonstrate the mass transport
of unstable material from the shallow shelf into the abyss. They also provide evidence that the Banda
Detachment is either active, or only recently ceased.
From a geohazards perspective, these mass debris flows may pose a greater tsunami risk than
earthquakes produced by the vast Banda Detachment. As the Banda Detachment is now exposed at the
seabed, it can no longer generate earthquakes other than beneath the volcanic arc, which comprises its
hanging wall (Fig. 5b). Nevertheless, frequent low-magnitude (<5) shallow (<60 km) earthquakes
recorded in the eastern Banda Sea suggest that steeper-angle faults beneath the Banda Detachment,
and dominantly strike-slip faults at the Weber Deep’s northern and southern extents, faciliate continued
extension.
References
[1] Meyers G, Bailey R J and Worby A P 1995 Deep-Sea Research Part I-Oceanographic Research Papers 42 1163–1174
[2] Gordon A L and Kamenkovich V M 2010 Dynamics of Atmospheres and Oceans 50 113–114
[3] Hall R, Cottam M A and Wilson M E J 2011 Geological Society, London, Special Publications 355 1–6
[4] Hall R 2012 Tectonophysics 570-571 1–41
[5] Hall R 2011 Geological Society, London, Special Publications 355 75–109
[6] Bowin C, Purdy G M, Johnston C, Shor G, Lawyer L, Hartono H and Jezek P 1980 AAPG Bulletin 64 868–915
[7] Hinschberger F, Malod J A, R´
ehault J P, Dyment J, Honthaas C, Villeneuve M and Burhanuddin S 2000 Comptes Rendus
de l’Acad´
emie des Sciences - Series IIA - Earth and Planetary Science 331 507–514
[8] Hinschberger F, Malod J A, Dyment J, Honthaas C, Rehault J P and Burhanuddin S 2001 Tectonophysics 333 47–59
[9] Vroon P Z, van Bergen M J, White W M and Varekamp J C 1993 Journal of Geophysical Research 98 22349–22366
[10] Nebel O, Vroon P Z, van Westrenen W, Iizuka T and Davies G R 2011 Earth and Planetary Science Letters 303 240–250
[11] Pownall J M, Hall R and Lister G S 2016 Geology 44 947–950
[12] Hall R, Patria A, Adhitama R, Pownall J M and White L T 2017 Proceedings of the Indonesian Petroleum Association 41
IPA17–91–G
[13] Spakman W and Hall R 2010 Nature Geoscience 3562–566
[14] Pownall J M, Hall R and Watkinson I M 2013 Solid Earth 4277–314
[15] Hall R and Spakman W 2015 Tectonophysics 658 14–45
[16] Pownall J M, Hall R, Armstrong R A and Forster M A 2014 Geology 42 279–282
[17] Silver E A, Gill J B, Schwartz D, Prasetyo H and Duncan R A 1985 Geology 13
[18] Pownall J M and Hall R 2014 Proceedings of the Indonesian Petroleum Association 38
[19] Pownall J M, Forster M A, Hall R and Watkinson I M 2017 Gondwana Research 44 35–53
[20] Pownall J M, Hall R and Armstrong R A 2017 Gondwana Research 52
[21] Pownall J M 2015 Journal of Metamorphic Geology 33 909–935
[22] Kuhnt W, Holbourn A, Hall R, Zuvela M and K¨
ase R 2004 Neogene History of the Indonesian Throughflow (Geophysical
Monograph Series vol 149) (Washington, D. C.: American Geophysical Union)
[23] Gordon A L, Sprintall J, Van Aken H M, Susanto D, Wijffels S, Molcard R, Ffield A, Pranowo W and Wirasantosa S 2010
Dynamics of Atmospheres and Oceans 50 115–128
[24] Tillinger D 2011 Geological Society, London, Special Publications 355 267–281
[25] Fortuin A R, de Smet M E M, Sumosusastro P A, Van Marle L J and Troelstra S R 1988 Geologie en Mijnbouw 67 91–105
[26] de Smet M E M, Fortuin A R, Tjokrosapoetro S and Vanhinte J 1989 Netherlands Journal of Sea Research 24 263–275
[27] Sandiford M 2008 Geophysical Journal International 174 659–671
[28] Hamilton W 1979 USGS Professional Paper 1078 345
[29] Charlton T R, Kaye S J, Samodra H and Sardjono 1991 Marine and Petroleum Geology 862–69
[30] McCaffrey R 1988 Journal of Geophysical Research: Solid Earth 93 15163–15182