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Abstract
Suao-Donao area, northeastern Taiwan, located in the oblique
convergent belt of Philippines Sea Plate and Eurasian Plate, is
an ideal area to study the structural evolution of the accretionary
wedge around the oblique convergent indented point. The
contact between slate formations and the Tananao Schist,
situated at the northern part and southern part of the study area
respectively, is a mylonitized ductile shear zone. All the rock
units in this area experienced several stages of deformations.
There are dynamic contacts between the different compositional
units. The foliations, which dip to the south in the northern part
and to the north in the southern part, display a fan-structure.
The results of domain analysis, stretching lineation, distribution
of the strain ratio of chlorite-muscovite aggregates and
shear-sense indicators suggested a progressive deformation
defined by two discrete stages in the structural evolution of this
area, D1 and D2. The main ductile structures of the D1 are
E-striking, south-dipping foliations, north-vergent folds and
sub-horizontal stretching lineations that are subparallel to the fold
axes. This stage belongs to the subductional stage. The sinistral
oblique convergence of Philippines Sea Plate toward Eurasian
Plate in this area forced the compressive frontal part of the
accretionary wedge to move gradually into the transpressive root
of the accretionary wedge during this stage. Therefore, the
north-thrusting movement of the fold (D1a) gradually transformed
into the sinistral movement with reverse component (D1b). These
transpressive structures exposed obviously at the contact between
the slate formations and the Tananao Schist. The analysis of
chlorite-muscovite aggregates deformation at the contact are
Rxz=7.77-11.36, Ryz=3.63-4.96, k=0.19-0.45, d=1.82-3.38, which
display apparent flattening. The main ductile structures of the D2
are E-striking, SE-vergent backthrust, NE-striking, SE-dipping
ductile sinistral faults, L-S tectonite and semi-brittle normal faults.
This stage belongs to the collisional stage. On one hand, the
oblique collision of Philippine Sea Plate and Eurasian Plate made
the former underlay the accretionary wedge and resulted in the fan
structure and SE-vergent backthrust (D2a). On the other hand, the
progressive southward propagation of collision pushed this area
gradually out toward the outer side of the indented point and
temporarily released compressive strain. As a result, it caused local
ductile sinistral faults and L-S tectonite (D2b). These transtensional
structures can be obviously observed at 107k ductile shear zone,
and the analysis of the chlorite-muscovite aggregates deformation
at the shear zone are Rxy=4.87-8.81, Rxz=3.63-9.43, k=64-154,
d=5.34-7.78, which display apparent constriction. The
backthrusting caused the uplift of the region and changed ductile
deformation into semibrittle deformation. It resulted in the
development of the semibrittle normal faults with uplifting.
Consequently, the ductile structures in Suao-Donao area are the
results of the complete progressive deformation and reflected
the structural evolution of accretionary wedge around the oblique
convergent indented point.
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