Crustal motion and block behaviour in SE-Asia
from GPS measurements
Gero Michel*, Christoph Reigber*, Matthias Becker†, Herman Seeger†, Wim
Simons‡, Boudewijn Ambrosius‡, Christophe Vigny§, Nicolas ChamotRooke§, X. Le Pichon§, Peter Morgan||, Saskia Matheussen||
*
GeoForschungsZentrum Potsdam, Germany
†Bundesamt für Kartographie und Geodäsie, Frankfurt am Main, Germany
‡Delft Institute for Earth-Oriented Space Research, Delft, Netherlands
§École Normale Supérieure, Paris, France
||University of Canberra, Canberra, Australia
Current deformation of SE-Asia has been discussed controversially. The
eastern margin of Eurasia, from North China to Indonesia, has been
successively considered as rigidly attached to Eurasia1, continuously
deforming2, or formed by a mosaic of blocks deforming mainly at their
boundaries3-6. Modern space geodetic tools can resolve these controversies
by quantifying crustal deformation. Here we show results acquired using
GPS data taken over a large part of SE-Asia, that indicate that Sundaland,
i.e. Indochina along with the western and central part of Indonesia,
constitutes a stable tectonic block moving approximately east with respect
to Eurasia at a velocity of 12±3 mm yr1. With respect to India and
Australia this block moves due south. Significant motion has not been
detected along the northern boundary to S. China i.e. along the Red River
Fault, whereas nearly 50 mm yr-1 of right lateral motion has to be
accommodated between India and Sundaland in the Andaman-Myanmar
region.
Cenozoic deformation in SE-Asia has been related to the ongoing
convergence of the Indian, Australian, Pacific and Philippine Sea plates towards
Eurasia (Fig. 1). Recent GPS results indicate that North and South China are
decoupled from Eurasia7,8 and move eastward with a velocity of 5 to 11 mm yr–
1
. Further south, previous geodetic results over Sundaland (i.e. Indochina, the
Sunda Shelf, Borneo, Sumatra, and Java) suffered from ill-defined global
reference system9, network restricted to a small region10,11, or concentrated on
seismically active boundaries10,12.
Repeated measurements were made of a GPS network extending over an
area of 4000 by 4000 km (Fig. 1), covering Brunei, Indonesia, Malaysia, the
Philippines, Singapore, Thailand and Vietnam. This geodynamics of S and
SE-Asia (GEODYSSEA) network was measured in 1994, 1996, and 1998. Data
were analysed independently by 5 different groups and combined to reach the
final solution mapped into ITRF9713 (see methods section).
Strain and rotation rates obtained by Delauney triangulation (Fig. 2a-b)
show a remarkably simple pattern: high straining regions are restricted to the
Philippine Mobile Belt to the east and the Sumatra shearing zone to the west,
both areas having opposite sense of rotation. The central part of the network
covering Sundaland shows no deformation and no rotation, and accordingly
displays a coherent velocity field. We chose a subset of 10 stations defined by
small baseline variations (<4 mm yr-1) and small interstation strains (<2x10-8
yr-1) to define a “rigid” Sundaland platelet. Motion of these sites can be
modelled to within 3 mm yr-1 with a single Euler vector (56.0°S, 77.3°E,
- 0.339°Myr-1 in ITRF97) significantly distinct from the NNR-NUVEL1A
frame Eurasian vector14. Both vectors were combined to derive the
Sundaland/Eurasia vector (63.6°S, 107.4°E, - 0.112°Myr-1).
To further explore the accuracy of the obtained Sundaland/Eurasia pole, we
also computed Euler vectors for each individual solution obtained by the four
analysis groups (Fig. 3). Clearly the longitudinal position of the pole is well
constrained whereas the latitudinal position is not. These independent solutions
place constraints on the accuracy of our combined solution and suggest that
although the average east component of Sundaland motion with respect to
Eurasia is distinct at 10 to 13 mm yr-1 (Fig. 3), a small and systematic variation
of this component with latitude and an associated small north component shift
cannot be ruled out. This implies two interpretations: Sundaland platelet escapes
due east with respect to Eurasia at an average rate of 12±3 mm yr1, or
Sundaland rotates clockwise with respect to Eurasia at a velocity increasing
from 10 mm yr-1 in the south to 15 mm yr-1 in the north. Differences between
the extreme solutions represent the uncertainty of our solution in terms of GPS
measurements reduction and reference frame realisation in this area of the
world, where ionospheric noise is high and fiducial stations are scarce.
Moreover part of these discrepancies originate from transient effects, not yet
understood, indicated by the time series in the station velocities at BALI and
BUTU. Velocities of these stations are lower (1994-1996) and higher
(1996-1998) than predicted by the other 8 stations of the Sundaland block.
This new geodetic solution demonstrates that a large piece of the eastern
Eurasian margin behaves to first order as a single block, as suggested previously
by the results of the two first rounds of GEODYSSEA measurements 9,15,16. To
the north, relationship of this Sunda block with the adjacent and previously
defined South China block is still to be established, but in any case differential
motion is small and at the detection limit of our data. Predicted rates on their
assumed boundary, the Red River Fault, are at the 0 to 5 mm yr-1 level. To the
south, the Sundaland block is bounded by the Great Sumatran Fault and the
Sunda Arc. Relative motion between Sundaland and Australia is approximately
north suggesting similar eastward component of motion for both plates along
this boundary. The apparent difference between the NUVEL1A convergence
direction and earthquake slip directions discussed in the literature17 thus appears
to have been resolved. Convergence rates between Australia and Sundaland
reach values between 66 and 72 mm yr-1. Relative velocities of sites on the
fore-arc sliver on Sumatra with respect to Sundaland suggest strike-slip motion
of between 20 and 30 mm yr-1 along the Great Sumatran Fault. Along the
Australian and/or Indian plate interface with Sundaland, intraplate shortening
and/or high rates of elastic strain accumulation at Sumatra is in contrast to
localised deformation on Java and Bali where sites appear to be significantly
less affected by elastic loading and follow the motion of “stable” Sundaland.
Along the Banda Arc, collision occurs between the Australian continental
lithosphere and a complex blocks assemblage in the area of the triple junction
between the Pacific/Philippine Sea plate, the Australian plate and Sundaland.
Here deformation is transferred from the Timor Trench to the north where it is
concentrated on the back-arc thrust system12 as well as influencing motion of
sites within the Banda Sea and Sulawesi area. In this region, a large portion of
the deformation is accommodated by small-scale block rotations at rates of up
to 4° Myr-1. To the east, convergence between the Philippine Sea plate and
Sundaland partitions within the complex Philippine Mobile Belt where
transcurrent motion as well as highly distributed deformation and block
behaviour18 have been observed. To the west, motion of India with respect to
Sundaland is roughly north at a rate of 45 to 52 mm yr–1. Relative motion is
accommodated by opening of the Andaman Sea, strike slip motion on the
Sagaing fault system, oblique shortening along the Andaman and Burma
subduction arc, and rotation about the eastern syntaxis of India.
During the Tertiary19 the Sundaland region was divided into a small
number of distinct provinces each with distinct boundaries and motion. The
apparent change from this regime to one of relative homogeneity that
accommodates the eastward component of the motion of the Indian and
Australian plates is remarkable. One plausible explanation is the general change
in the boundary conditions of E and SE Asia with the onset of subduction and
subsequent roll-back along the Philippine trench in the late Miocene19. The
alternative is that Sundaland eastern motion relates to eastward motion of India
appearing about 8.5 to 10 Ma20, when the Indian equatorial zone of deformation
was initiated and India plate was detached from Australia plate. In any case, this
new evidence for a rather homogeneous motion of E and SE-Asia definitely
rejects any model that includes either intense deformation within the blocks or
large transcurrent displacements between them.
Methods
Data analysis
GPS observations were conducted simultaneously at 42 stations and included data from the
Australian Survey and Land Information Group (AUSLIG) and the International GPS Service for
Geodynamics (IGS). Five analysis groups (BKG, DEOS, ENS, GFZ, and UC) computed daily
station coordinate solutions for the individual campaign data sets. Each group used the latest
version of one of four GPS software packages (i.e. BERNESE v4.0 21, EPOS v3.022, GAMIT
v9.923, and GIPSY-OASIS II v2.524) and applied their favoured analysis strategy9,25. Processing
included the following: fiducial-free network solutions, ionosphere-free linear combinations of
GPS data, tropospheric modelling, and IGS antenna phase centre tables. Consequently this
solution differs from earlier solutions9 by virtue of the additional epoch and the use of additional
global IGS stations. Additionally precise GPS satellite orbits/clocks and corresponding earth
rotation parameters from the IGS, GFZ, the Jet Propulsion Laboratory (JPL), and UC were
applied. For the 1994 and 1996 campaigns these data were newly computed using the ITRF97
reference frame. The daily solutions of each group were combined into multi-day averaged
solutions with the 3D-motion software26. These were then combined into averaged coordinate
solutions for the individual campaigns that had formal errors about half that reported earlier 9.
Accuracy
The daily coordinate repeatabilities of the GEODYSSEA stations for each campaign range
between 2.5 and 6.5 mm for the horizontal components and between 6 and 15 mm in height. The
internal consistency of all contributing multi-day averaged group solutions is 2, 4, and 7 mm in
the north, east and up components. A precise estimate of the horizontal station velocities was
obtained by mapping each campaign solution into the ITRF97 13 frame using 12 global IGS sites,
with long and stable time series that operated during each campaign. This was done using linear
models for all stations not affected by nearby large earthquakes (i.e. Biak and Tomini in
Indonesia). The 1-sigma quality of these models was 2 mm yr-1 in north-south and 3 mm yr-1 in
the east-west direction.
Transient effects
Abundant large earthquakes suggest that transient seismic deformation processes may have
significantly influenced some of the observed site motions15,18. Seismotectonic methods, together
with dislocation modelling were therefore applied to estimate the influence of coseismic
deformation release and interseismic elastic loading on the site motions27. Results indicate that
apart from the eastern perimeter of the study area, where the transition zone between the
adjoining plates and Sundaland extends for up to 1000 km, and the fore-arc sliver at Sumatra, the
sites appear to be almost unaffected by seismic elastic loading and release phenomena.
Acknowledgements
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Figure legends
Figure 1 SE Asian GPS velocity vectors in the Eurasian reference frame. Grey
arrows: permanent stations from IGS and AUSLIG, white and red (stable
Sundaland) arrows: GEODYSSEA. Inset map: triple junction densification area
(vectors relative to Sundaland). Error ellipses are 99% confidence level. White
circles: seismicity from the CMT catalog (1977 to present). Black lines show
main SE Asian faults.
Figure 2 Strain and rotation rates in Delauney triangles within the
GEODYSSEA network. a, Strain rates, horizontal principle axis of the
deformation tensor (red for compression and blue for extension). b, Rotation
rates (red wedges for clockwise rotations and blue for anti-clockwise rotations)
with associated uncertainties (grey wedges). Green dots show rigid Sundaland
sites.
Figure 3 Sundaland motion with respect to Eurasia. a, Positions of plate
rotation poles with associated uncertainty ellipses (99% confidence): Eurasia in
NNR-NUVEL-1A (black star), Sundaland in ITRF97 (open stars), and
Sundaland relative to NUVEL-1A Eurasia (open dots). Group solutions GFZ
(orange), ENS (green), DEOS (yellow) and UC (violet), and combined solution
(grey) are shown. Poles are connected by corresponding great circles (dashed
lines). b, Sundaland stations east velocity sorted by latitude. For each of the
individual solutions and the combined solution, we show actual values
(symbols) and trends (solid lines) predicted by the various Sundaland/Eurasia
poles (same color coding as a). c, Sundaland stations north velocity sorted by
latitude.