The influence of inherited continental margin structures on the stress
and strain fields of the south-central Taiwan fold-and-thrust belt
Cristina Biete1, Björn Lund2,
Dennis Brown1, Joaquina Alvarez-Marron1, Yih-Min Wu3,4,5,
Hao Kuo-Chen6, Chun-Wei Ho6,7
Fold-and-thrust belts, the frontal-most part of a
Mountain Range, have been studied for decades for its economic and social
interest due to its importance on the exploration of natural resources. It has also
been studied how and which could be the effects of incorporating in the
deformation of the fold-and-thrust belts rocks that have been through a
previous deformation history, however, this field is still under debate. The
active Taiwan fold-and-thrust belt is incorporating in the deformation the
rocks of the Eurasian continental margin (Fig. 1). This margin went through a
previous deformation history that during a rifting period developed extensional
faults with east-northeast orientations that now are at a high angle to the north-south
grain fold-and-thrust belt. Moreover, since Taiwan fold-and thrust belt is
currently active, it provides an un-parallel location to study the effects of
this oblique collision on the current stresses driving the deformation and on
the surface deformation.
 |
| Figure 1: Tectonic setting of the Taiwan orogen. |
In
this post we want to show our research on the contemporaneous stress and strain
fields in south-central Taiwan fold-and-thrust belt and how these may be
influenced by the inherited structure and morphological features from the
Eurasian continental margin, which are at a high angle to the Taiwan
fold-and-thrust belt grain (Fig. 1).
To
estimate the current stress field, we use earthquake focal mechanism, which are
represented as beach balls (stereographic projection of the two possible
rupture planes of the brittle deformation that produce earthquakes (see Fig. 2a
for the graphic explanation of the beach balls)).
 |
Figure 2: a) Schematic illustration of the three
general tectonic regimes and the according orientations of the principle stress
axes (after Anderson, 1951, and Zoback, 1992) (Modified from Barth et al., 2008
of the World Stress Map Project Guidelines). b) Earthquake focal mechanisms in
south-central Taiwan study area.
|
From
earthquake focal mechanisms recorded between 1994 and 2014 (Fig. 2b), we
estimate the principal stress directions (S1, S2 and S3) and the resultant
maximum compressive horizontal stress (SH), what gives a view of
which forces are acting in the crust. This process is done through the
inversion of earthquake focal mechanisms, from which we also obtain which are
the most likely active fault planes orientations. To investigate if there are
any differences in the principal stress directions in depth through the crust,
we divide the focal mechanisms in three depth levels, the upper one to
investigate the sedimentary cover and the fold-and-thrust belt, and two depth
levels within the basement (Fig. 3). The results obtained for the stresses
distribution throughout the crust are compared with the strain in the surface
(Fig. 4).
 |
| Figure 3: Direction of the maximum compressive horizontal stress (SH) for each cluster at their respective depth level and colored depending on fault type. |
To
investigate the strain field in south-central Taiwan, we use data from the
Taiwan GPS network, which is composed by the velocity vectors from the period
2005 through 2009 of each GPS stations in the study area (Fig. 4a). Since the
strain is the derivative of the velocities, we obtain the strain from the
velocity vectors of each station by the grid-nearest neighbor interpolation
method using SSPX software. As a result, we obtain the grids for dilation,
vertical rotation and shear strain rates, as well as the compressive strain
orientations and the most probable shear planes throughout south-central Taiwan.
 |
| Figure 4: a) GPS horizontal velocity vectors. b) Dilatation strain rates. Blue colors representing compression and red extension. c) Summary of the stress results for the basement, the SH is shown with colored arrows, with their most likely fault planes. |
The
comparative between the south-central Taiwan horizontal displacement field
(Fig. 4a), the maximum compressive horizontal stress azimuth (SH) (Fig. 4c) and the
compressive strain orientation (Fig. 4b) show an overall similar pattern, in
the north of the study area they are roughly sub-parallel to the absolute plate
motion vector NW directed, whereas in the south they rotate nearly 45º
counterclockwise. These rotations are produced in the center of the study area,
where the Eurasian continental margin is entering in the deformation of the
Taiwan fold-and-thrust belt (Fig. 4c). Also in this area the results show that
the orientations of the most likely fault planes at depth and the shear planes
in the surface are very similar to those orientations of the Eurasian margin
faults, east-northeast striking, which are at a high angle to those faults and
structure characteristic of the fold-and-thrust belt, with roughly north-south strike.
The east-northeast striking active faults are typically
reactivated as strike-slip faults and located in the basement, whereas newly
formed faults in the fold-and-thrust belt are commonly thrusts or oblique
thrusts. In
the south of the study area, the results show an east-northeast oriented high
shear strain area again with similar orientations to those found in the
structures of the Eurasian continental margin.
Therefore,
we interpret the southward change in the SH
azimuth, in the compressive strain axis azimuth, and in the horizontal
displacement field to be related to the reactivation of east-northeast
striking faults inherited from the Eurasian continental margin.
Author’s
institutions:
1) Institute of Earth
Sciences, Jaume Almera, ICTJA, CSIC, Lluis Sole i Sabaris s/n, 08028 Barcelona,
Spain. cbiete@ictja.csic.es. 2) Department of Earth
Sciences, Uppsala University, Uppsala, Sweden. 3) Department of Geosciences,
National Taiwan University, Taipei 10617, Taiwan. 4) Institute of Earth
Sciences, Academia Sinica, Taipei 11529, Taiwan. 5) NTU Research Centre for
Future Earth, National Taiwan University, Taipei 10617, Taiwan. 6) Department
of Earth Science, National Central University, Zhongli District, Taoyuan City,
Taiwan. 7) Central Weather Bureau, Taipei, Taiwan
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