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X - 10 SIMONS ET AL.: GPS MEASUREMENTS IN S.E. ASIA TO REFINE SUNDALAND MOTION
that any revision of the Sundaland pole needs to be based on both
more AND more accurate velocity estimates on the rigid part of
Sundaland. If in this paper the same selection of 10 sites would be
used as in Michel et al. [2001], which also included GEODYSSEA
sites in Central Java and in Bali, the pole rate would slow down
slightly and its location would shift towards the one of Michel et al.
[2001].
Therefore only with the significantly expanded, more accurate
and well spread velocity field presented in this paper it made scientific
sense to update the Sundaland motion, thereby clearly identifying
and/or removing sites that are in deforming regions.
4.3. Stable Sundaland rigidity and boundaries deformation
Fig. 7 focuses on both the intra- and inter-plate deformation in
South-East Asia. The strain rate tensors have been computed from
the velocity solution presented in this paper, except for TAIW station
whose velocity was taken from ITRF-2000 because there are
only 2 epochs with poor data quality and relatively short time span
included in our data analysis.
4.3.1. Stable Sundaland definition
From the analysis of the strain rates tensors, it is possible to localize
a zone of very low strain rate (red crosses) which constitutes
the rigid core of Sundaland. This stable core of the Sundaland block
can also be defined from the stations with a residual velocity with
respect to Sundaland smaller than 3 mm/yr (red dots), almost all of
which have been used to compute the pole. Hence, there is a very
good agreement between the analysis of the residual velocities and
the one of the strain rate tensors.
The core of the Sundaland block (green area) covers the Indochina
peninsula to the North, the Malaysian peninsula to the West, the
Sunda shelf, the major part of Borneo to the East and extends to the
South almost until the Java island.
Another zone of small strain rates is the stable South China block,
outlined by a pink area.
Around the Sundaland block, inter-plate deformation rates can reach
very high values (Fig. 7, black crosses) due to the motion of Sundaland
block relative to its neighboring plates. It is possible to see a
progressive increase in the deformation rate from the core of Sundaland
(red crosses) to its boundaries (green and blue crosses). These
zones of medium deformation rate surrounding the rigid core seem
to indicate that high-rate inter-plate deformation is not completely
localized at the boundaries but extends a bit inside the Sundaland
block. However, it is not possible to determine whether the origin
of these medium strain rates is due to elastic coupling on a single
fault accommodating the major part of inter-plate motion or to a
distribution of the deformation on several smaller scale faults in
the boundary areas. Discriminating these two deformation modes
needs locally densified GPS measurements in order to constrain the
mechanical models.
4.3.2. Deformation at the Sundaland boundaries
The highest strain rates are located along the eastern border of
the Sundaland block, in the Philippine belt and Sulawesi island
(Fig. 7). In the latest area, the main compressive strain axis rotates
anti-clockwise from NNE-trending at the NE of the island to
NNW-trending North of the island (Minahassa trench) and until
WNW-trending East of the island (Palu left-lateral strike-slip fault).
To the West, deformation seems to extend, at smaller rates, in the
eastern part of Borneo island. To the south of Sulawesi island the
main compressive strain axis trends generally NNW.
To the South, the Sundaland block is bounded by the arcuate Sunda
trench. There, the main compressive strain axis remains normal to
the subduction axis (fig. 7), and hence rotates slightly clockwise
from NNW-trending to the East (South of Sulawesi and Flores islands)
to NE-trending to the West, in the Sumatra area.
To the West, the Sundaland block is bounded by a right-lateral strikeslip
zone extending from Sumatra to Myanmar. The Sumatra shear
zone is affected by deformation compatible with right-lateral strikeslip
on the Great Sumatran Fault. Further to the North, our results
confirm a right-lateral strike-slip on the Sagaing fault in Myanmar
[Vigny et al., 2003]. Deformation of this area is detailed in Socquet
et al. [2003, subm.] in order to understand how the relative motion
between India and Sundaland is accommodated in Myanmar.
The Northern boundary of Sundaland extends from the Eastern Himalayan
Syntaxis until the South China sea, through the Red River
Fault. There, the main compressive strain directions rotates from
NE-trending to the West, to N-trending to the East. Strain tensors
are compatible with right-lateral shear across the Red River Fault,
the boundary between the Sundaland and the South China blocks.
To the SE, this fault ends in the South China Sea Basin, characterized
by very slow present-day strain rate and that may be considered
as a diffuse boundary zone (fig. 7).
4.4. Motion relative to Eurasia and other plates
The motion of Sundaland with respect to Eurasia is the subject
of discussions since the first GPS measurements in S.E. Asia [e.g.
Tregoning et al., 1994; Genrich et al., 1996; Wilson et al., 1998;
Chamot-Rooke et al., 1998; Simons et al., 1999; Michel et al., 2001]).
Figure 8. Velocities in South-East Asia, with respect to Eurasia
of Calais et al. [2003]. Black arrows are for sites belonging to
Sunda, white ones for South China and gray ones for other areas.
the major tectonic structures of the area are also shown as black
lines.