EOS 410, Final Report – Dec. 3, 2014
Erica Owen, Landon Mutch
Tectonics of the Java Trench
Abstract
The Java Trench is the location of the subduction zone of the Indo-Australian Plate below the
Sunda Plate and lies just south of Indonesia. The area was formed by the collision of the north
moving Indian and Australian Plates with the Eurasian and Sunda Plates. Stresses and strains due
to the change of motion of the Pacific Plate from NNW to WNW caused transform faults in the
Sunda Plate to become subduction zones. Backarc and forearc basins then formed around the
Java Trench. Current tectonic structures include a backarc basin north of the trench, a forearc
basin along the northern margin of the trench, a forearc high, and the trench itself. Fault plane
solutions and GPS measurements reveal that the Australian Plate is orthogonally subducting
below the Sunda Plate at 67 mm/yr N11oE ± 4o relative to the Java Trench, with a small degree
of left-lateral motion. The eastern part of the trench has been closed due to accretion of the
Banda Arc with the Australian continental shelf. Seismicity in the area has increased in
magnitude during the last century. Measurements of relative plate motions indicate the Sunda
Plate is rotating clockwise and moving eastward, that relative to the Eurasian Plate. The
volcanism in the area is divided into four sections based on magmatic composition. There are
three components contributing to this anomaly, characterized by high or low K content and high
or low 87Sr/86Sr values. There is little paleomagnetic data in the area as much of the old oceanic
crust has been subducted. The remaining magnetic lineations show that the plates east of the Java
Trench have undergone major transformations from their original shapes and positions. These
transformations are attributed to the collision of the Indo-Australian Plate with the Eurasian Plate.
Introduction
The Java Trench is located off the southern shore of the Indonesian island of Java (Figure 1). The
Java Trench joins with the Sumatra Trench to the north-west and together they are often referred
to as the Sunda Trench. The Sunda Trench is probably best known for causing the formation of
the volcano, Krakatoa, and the 2004 earthquake which lead to the devastating tsunami. For the
purposes of this paper we will mainly focus on the tectonics of the section of the Sunda Trench
that lies directly south of Java, and from here on we’ll refer to this section as the Java Trench.
Figure 1.
Regional
boundaries of the
Indian, Philippine,
Sunda, and
Australian Plates
and their motions
relative to the
Eurasian Plate to
the north. The Java
Trench is
highlighted in red.
Modified from E.
Baroux et al. (1998).
We will outline the evolution and modern settings of the regional plate boundaries and the plate
movements, the structure and kinematics of the subduction zone where the Australian Plate
descends beneath the Sunda Plate, the resulting Sunda Volcanic Arc and its geochemistry, as
well as the backarc and forearc basins, and some relevant paleomagnetic data. Our aim is to
review the existing information on the tectonics of the Java Trench in some detail and to provide
an overview of the regional tectonics, seismicity, relative plate motions, volcanism,
geochemistry, and paleomagnetism.
Historical chronology leading to the current tectonic settings of Indonesia
To fully understand the formation of the present day Java Trench, the historical movements of
plates in Southeast Asia must first be looked at. To begin, the major plates surrounding the
Sunda Plate include the Eurasian, Indian, Australian, and Philippine Plates (Figure 1). The
Eurasian Plate is the most static in the area, with the only major motion (in an absolute reference
frame) has been a small clockwise rotation during the past one hundred million years (Daly et al.,
1991). The focus will therefore be placed largely on the movement of the Indo-Australian Plate
relative to the Eurasian Plate. The Indian Plate first underwent northward motion toward Eurasia
at a velocity of approximately 120 mm/yr during Chrons 34-21 (70-50 Ma; Figure 2). The Indian
Plate then collided with the Eurasian Plate around Chron 21 and spent the next nine Chrons (5037 Ma) experiencing deceleration of its velocity. The plate has, from Chron 13 to present,
undergone northward motion towards the Eurasian Plate at a velocity of 63 mm/yr (Whittaker et
al., 2007) with a minor counter-clockwise rotational component (Daly et al., 1991). At 50 Ma the
collision of India and Asia began which caused a variety of stresses and strains within the
Eurasian Plate and along its convergent boundary with the Indian Plate.
Meanwhile, the Australian Plate also experienced relative northward movement towards the
Eurasian Plate. During the mid-Cretaceous (~95 Ma) the Australian Plate broke free of the
Antarctica Plate and moved mostly eastward with 50 Ma marking the change of direction of the
plate as it began moving mostly northward toward Eurasia (Figure 2). This was due to increased
ocean seafloor spreading in the Southern Indian Ocean, as India collided with Eurasia (Daly et
al., 1991). This northward motion is known due to magnetic lineations on the seafloor (Daly et
al., 1991) and will be discussed further in the paleomagnetics section.
Further to the east, during 70-40 Ma, the Pacific Plate moved NNW (relative to the Eurasian
Plate) as is recorded by the Emperor Seamount chain. By ~42 Ma the plate began moving WNW
as is recorded by the Hawaiian Ridge (Daly et al., 1991). This change in direction of the Pacific
Plate may have changed transform faults and fracture zones around Southeast Asia into
subduction zones, with associated areas of extension, and likely played a role in the formation of
local microplates such as the Sunda and Philippine Plates. During the Eocene (56-34 Ma) this
backarc extension lead to the creation of a basin north of the Sunda Trench. Later during the lateEocene and early-Oligocene (38-28 Ma), this small basin turned into a minor oceanic basin and
offshore island arc which is the Sumatran forearc (Daly et al., 1991).
Figure 2.
Subduction of the
Indian Plate
beneath the Sunda
Plate, associated
backarc extensions,
and basin
formation around
Sumatra and Java
at 40 Ma. The inset
shows the historical
timescale of the
north-eastward
advance of the
Indo-Australian
Plate and eventual
collision with the
Eurasian Plate
(Indian positions in
Chrons, Australian
positions in Ma)
after Daly et al.
(1991).
This marginal basin closed in the late-Oligocene (23-28 Ma) due to arc reversal, which marked
the beginning of the activation of the Sunda subduction zone. The Sunda subduction zone was
not active until this period because of a lack of volcanic arcs in Sumatra and Java which formed
in the late-Oligocene (Daly et al., 1991). Around 11 Ma, crustal shortening in the backarc basins
of Sumatra began, causing structural inversion that continued until its peak during the Pliocene
(~5 Ma; Daly et al., 1991). This time during the early-Pliocene also saw the collision of the NW
Indo-Australian passive margin with the Sunda Trench and Banda Forearc as the plate continued
northward toward the Eurasion Plate. Basin formation east of the trench also occurred as
subduction rollback contributed to backarc extension. These stresses and strains around the
Sunda Trench, which lead to its present day configuration, are illustrated in Figure 2.
Figure 3.
Absolute plate motions of the
overriding Sunda Plate and the
subducting Indo-Australian Plate
showing reconstructed locations
of the slab window due to the
subduction of the Wharton Ridge.
Modified from Whittaker et al.
(2007).
From 80 to 60 Ma, uniform upper plate motion along the Sunda-Java margin caused compression
in the area. A spreading zone, known as the Wharton Ridge, was active around 70 Ma and was
subducted with the trench under the overriding plate which created a slab window. This slab
window existed from ~70-43 Ma until it was cut off approximately 1000 km from the trench.
Here it reached the 660 km mantle discontinuity and no longer affected the kinematics of the
trench (Figure 3). The slab window may have affected the stresses on the southeastern segment
of the margin by reducing the compressional stress (Whittaker et al., 2007). Plate motion
reconstructions suggest there was mostly compression at this boundary; however, the slab
window may have allowed extension from 50-35 Ma, which agrees with geological evidence
found in the area (Whittaker et al., 2007). The slab window also contributed to extension along
the backarc basin, which was already present due to the retreating upper Sunda Plate.
Modern structure of the Java Trench
Now that we have a good understanding of the events that formed the Sunda subduction zone,
let’s take a closer look at the present day configuration and tectonic activity of the Java Trench.
Some of the major features of the Java Trench subduction zone include a backarc basin north of
the Sunda Volcanic Arc and a forearc basin to its south, a forearc high just north of the trench,
and the Roo Rise to the south of the trench (Figure 4c). These features can be partially attributed
to the events previously discussed: formation of the volcanic arc as a result of the subduction of
the Australian Plate below the Sunda Plate (Pyle et al., 2001), formation of the basins by regional
extension (Daly et al., 1991; McCaffrey, 1991), and formation of the forearc high by the
accretion of high relief seafloor wedges during subduction (Kopp et al., 2006). The modern
structure can be further explained by studies of more recent events and measurements.
The current subduction of the thicker crustal region of seafloor, known as the Roo Rise which is
likely underlain by lower density crust, has caused the forearc high to be further elevated by
1000m (Shulgin et al., 2011; Kopp et al., 2006) and has caused local shallowing and northward
movement of the Java Trench (Kopp et al., 2006). On the other hand, subduction of the Roo Rise
has very likely contributed to a degree of erosion of the accreted wedge (Kopp et al., 2006). The
addition of the volcanic arc mass has also been partly responsible for the deepening of both
basins, due to isostatic loading. In addition, regional erosion has had the effect of filling in the
basins and trench and decreasing their depths.
Figure 4.
a) Earthquake activity >5.0 from 1950-2013 for the Java Trench subduction zone, colored by
depth, with earthquakes between longitudes 112-117oE highlighted and the surface trace of cross
section A-B. b) Inset of map area showing locations of major volcanoes. c) Topographic profile
of cross section A-B, backarc basin (BB), Sunda Volcanic Arc (V), forearc basin (FB), forearc
high (FH), Java Trench (T), and eastern point of the Roo Rise (RR). d) Profile view of depth of
earthquakes along cross section A-B, with nearest earthquakes highlighted, and approximate
surface of subducting plate. (GeoMappApp)
Figure 4a maps the volcanoes and earthquake epicentres in the area and shows a cross section AB along 115oE, which is roughly perpendicular to the trench. Figure 3d shows a profile view of
the depth of earthquakes. Because the trench is not striking exactly east-west in this area, the
profile data is slightly skewed, so the data most proximal to the cross-section is highlighted to
provide a better estimation of the top of the subducting slab. At 115oE, the Sunda Volcanic Arc
is approximately 120 km above the top of the subducting slab, the forearc basin is 4.5 km below
sea level, the forearc high rises to 2km below the surface, the Java Trench descends to 6.8km, the
Roo Rise is 5 km below the surface (although at some points further west it is between 2-2.5 km
below the surface (Kopp et al., 2006), and the average southern abyssal plain is about 5.7 km
below sea level.
To the east, the Sunda Trench has been closed off by the accretion of the Banda Arc due to the
collision with the Australian continental shelf. Here subduction has all but stopped, as is apparent
by the abrupt termination of the Sundra Trench, the lack of recent volcanic activity of the Banda
Volcanic Arc to the north, and the earthquake activity in the area (Audley-Charles, 2004). To the
west, the Sunda Trench continues north-westward off Sumatra, and the subduction zone changes
from orthogonal subduction to right-lateral, oblique convergence to the north-west along the
Sumatra Trench (Malod et al., 1995).
Regional tectonism, seismicity, and relative plate motions
GPS measurements from 1989-1993 comparing the movement of Christmas Island relative to the
city of Jakarta have shown that the Australia Plate is orthogonally subducting at 67 ± 7 mm/yr
N11oE ± 4o relative to the Java Trench (Tregoning et al., 1994).
Since 1840, records have been kept of earthquakes in the Java region. The recorded earthquakes
from 1840-1921 were generally small and caused minimal damage, although a few generated
local tsunamis and did damage to government and public buildings (Newcomb et al., 1987).
Later into the 20th and 21st centuries there seemed to be an increase in the magnitude of
recorded earthquakes, though this is difficult to quantify because early recorded earthquakes
were not assigned magnitudes on the Richter scale (Newcomb et al., 1987). Some of the largest,
Figure 5. Fault plane solutions for the Java Trench region for all published material prior to
1987, modified from Newcomb et al., (1987).
more-recently recorded earthquakes reach magnitudes of M = 7.2–7.9 and include the 2006
earthquake which was assessed as an Mw = 7.8 and Ms = 7.2 event which killed more than 600
people (Ammon et al., 2006; Newcomb et al., 1987). Fault plane solutions for the Java Trench
confirm that oblique subduction of the Australian Plate below the Sunda Plate is occurring with
small amount of left lateral motion (Newcomb et al., 1987; Figure 5). This also explains why,
unlike the Sumatra region, the Java region does not show evidence of extensive strike-slip
faulting (Newcomb et al., 1987). Relative to the Eurasian Plate, the Sunda Plate is rotating
clockwise and moving eastward (Figure 6) around a pole located near south-central Australia
(Chamot-Rooke et al., 1999). This can be seen in Figure 6 where the easterly motions of the
more northerly stations are larger than those of the more southerly stations. The data presented in
Figure 5 for the motion of the Australian Plate is generally within the range of error of the GPS
data previously discussed from Tregoning et al. (1994).
Figure 6.
Motions of the Sunda and
surrounding plates
relative to the Eurasian
Plate (red arrows). Slip
azimuths are marked with
purple lines with lengths
proportional to the
number of earthquakes
used to find the individual
azimuths. Green arrows
indicate vectors of
subducting plates. Blue
arrows represent the
difference vectors (Sunda
Plate motions minus
subduction vectors)
(Chamot-Rooke et al.,
1999).
Volcanism
The Java Trench is a location of active volcanism and is characterized by four different
geochemical types which we will discuss. Plutonism occurred from ~60-50 Ma around the
Sumatra region. Most of the volcanism along the Sunda Trench during the Palaeogene occurred
around south Sumatra, with lesser volcanism to the north and south of the area. Specifically this
lack of volcanism to the south along the Java Trench could be associated with the presence of a
slab window in the area from 70-43 Ma, which was discussed in the history section. The active
volcanism along Sumatra ceased around 50 Ma, when the slab window was established below
southern Sumatra. Volcanism was present again in this area around 30 Ma, when the slab
window was no longer present. The presence of this slab window also explains the lack of
volcanism on Java before 42 Ma (Whittaker et al., 2007).
The current volcanism in Indonesia can be divided into four parts based on compositional
materials, which range from arctholeiitic to high-potassium, undersaturated volcanism and are
labeled in Figure 7 (Varekamp et al., 1989) and is the widest known compositional span of mafic
magmatism from an active arc setting (Wheller et al., 1987). All volcanoes in the Java region of
Indonesia also contain the typical trace element patterns of high Sr and Pb isotopic signatures.
The sections are known as West Java, Bali, Flores, and Banda (from west to east) and are shown
in Figure 7.
Figure 7. A map showing the segmented volcanic sections of West Java, Bali, Flores, and Banda,
which are 1, 2, 3, and 4, respectively (Varekamp et al., 1989).
The eastern Sunda Arc and the Banda Arc are separated by a segment that has been inactive
since 3-4 Ma known as the Alor-Wetar-Romang segment (Figure 7). This could be due to the
continent-arc collision occurring at this location with the possibility of a subduction polarity flip
in the near future (Varekamp et al., 1989). The four segmented sections are divided based on
their differing compositional make up. The range in compositional materials can be explained by
the mantle source below each respective area. For example, the compositions of leucite bearing
volcanics in Indonesia, which have a high-K calc-alkaline content, are sourced partly from the
subduction of sedimentary material from the subcontinental Australian mantle (Varekamp et al.,
1989). Segment 1 is characterized by the subducted Indian oceanic plate and is bounded to the
east by a band of ophiolites which are evidence of a Cretaceous to early-Tertiary subduction
zone. Segment two is marked by older oceanic crust and a different magnetic anomaly
orientation. To the east of this is Segment 3 where backarc thrusts developed behind the
subduction zone. Segment 3 is also where Indian oceanic crust has been subducted and is some
of the oldest oceanic crust on earth. Segment 4 is characterized by arc-continent collision
(Varekamp et al., 1989). The general trend of volcanic compositions from Segments 1-3 become
higher in potassium from west to east, ending in the leucitite volcanoes in the east. However, in
the most easterly section, Segment 4, the volcanoes range from high to low postassium content
from west to east (Wheller et al., 1987). A common mantle source to all the sections is peridotitic
mantle which contains a low K content and low 87Sr/86Sr value. A second component is crustal
material with a low K content but a high 87Sr/86Sr value which is prominent in the Banda section
(Wheller et al., 1987). This is consistent with the proximity of Segment 4 to the Precambrian
Australian continental crust. A third component that contributes the most to compositional
differences along the trench is one that is high in K-group elements with a low 87Sr/86Sr value
and is derived from the mantle. This component is most strongly exhibited in the Bali section
where it is the least overprinted of the crustal material. Bali volcanism shows that their
composition does not include the presence of recently subducted continental material (Wheller et
al., 1987).
Paleomagnetism
While some paleomagnetic data is readily available around the Sunda Trench region, large
amounts of ocean floor in the region from the Paleogene age (66-23 Ma) have undergone
subduction and are no longer readily available. Therefore, some inferences as to the past
magnetic data have to be made to discern historical tectonic plate movement. Many of the
magnetic isochrons in the microplates within the Sunda Plate are debateable. For example, there
are east-west magnetic lines from Chron 11-5D (32-18 Ma) in the South China Sea, implying
that a north-south spreading system existed there during this time period (Daly et al., 1991).
However, looking southwest of the basin, northeast-southwest magnetic lines are present,
indicating a period of northwest-southeast extension, which contradicts the first assumption that
the spreading was north-south. The conclusion to this problem that has been proposed and most
favoured by the literature (Daly et al., 1991) is that the current east-west magnetic lineation
formed as northeast-southwest lines originally and was then altered due to nearby transform
faults (Daly et al., 1991). Another example can be seen east of the Java Trench in the Celebes
Sea, which exhibits magnetic lineations decreasing from Chron 18 to 20, from south to north.
Just north of the Celebes Sea is the Sulu Sea whose oceanic crust is dated back to the Miocene.
This indicates that the Sulu Sea was more recently formed than the Celebes Sea, which is likely
to be trapped oceanic crust from an earlier age – possibly a former piece of the Indo-Australian
Plate (Daly et al., 1991). From these examples, it can be seen that the plates to the east of the
Java Trench have undergone dramatic transformation from their original positions and shapes. A
large proportion of these deformities can be attributed to the collision between the IndoAustralian and Eurasian Plates (Achache et al., 1983). Apart from magnetic lineations, apparent
pole wander paths can also be used as constraints to determine plate motion in the area; although
the mean apparent pole wander paths over time show little latitudinal movement in Indochina
from the Jurassic to Cretaceous periods (Achache et al., 1983).
Conclusion
From the upper-Cretaceous to the present day, the northward advance of the Indo-Australian
Plates and the westward advance of the Pacific Plate, both relative to a stationary Eurasia Plate,
together have shaped the current tectonic settings of the Indonesian region and the Java Trench.
The modern structure of the Java subduction zone entails the Sunda Volcanic Arc with
associated forearc and backarc basins, a forearc high, the Java Trench, and a subducting ocean
rise called the Roo Rise. GPS measurements, relative plate motions, and fault plane solutions
indicate that the Australia Plate is subducting beneath the Sunda Plate and orthogonal to the Java
Trench at 67 ± 7 mm/yr N11oE ± 4o. Volcanism in the area is divided into four distinct segments,
each characterized by the geochemistry of its magma source. These sources range from high to
low K content and high to low 87Sr/86Sr values. Paleomagnetics of the region are sparse and often
contradictory but can provide some insight into basin formation and local plate movements.
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