Volume 6 Issue 4 - October 31, 2008
Contemporary deformation of tectonic escape in SW Taiwan from GPS observations, 1995-2005
Kuo-En Ching1, Ruey-Juin Rau1,*, Jian-Cheng Lee2, Jyr-Ching Hu3

1Department of Earth Sciences, College of Sciences, National Cheng Kung University
2Institute of Earth Sciences, Academia Sinica
3Department of Geosciences, National Taiwan University
raurj@mail.ncku.edu.tw

Earth and Planetary Science Letters, Vol. 262, 601-619, 2007.

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The concept of tectonic escape (or tectonic extrusion) around the ends of a mountain belt describes the kinematics of the lateral motions of geological units moving toward a free boundary in response to the collision-induced shortening (Figure 1), such as SE Asia and Eastern Alps. For the tectonic escape in large-scale mountain belts, the directions of surface displacements, the sense of shear-wave splitting at the upper mantle, and the strikes of conjugated faults are approximately parallel to the tectonic escaping direction. Therefore, the following mechanisms have been suggested to result in the deformation field of tectonic escape: (1) the indenter shape and convergence direction of collision between plates; and (2) gravity-driven flow dragging the crust because of the gravitational potential energy gradient. At a smaller scale in the Taiwan mountain belt, the southwestward lateral escape was also interpreted to occur in SW Taiwan because of the interaction between the ongoing WNW collisional shortening across the orogen and the presence of a convex crustal indenter, the Peikang basement high of the continental passive margin (Figure 2). Recent studies further proposed that the lateral escape in SW Taiwan is driven by four quasi-rigid blocks moving toward the SSW or S directions along two NNE-SSW-striking (i.e., the Chishan fault and the frontal thrust) and two N-S-striking (the Chaochou fault and the Kaoping fault) structural discontinuities (Figure 2), based on previous studies including GPS observations, seismological data, and structural analyses. The strikes of these four structural discontinuities are nearly parallel to the direction of tectonic escape in SW Taiwan, such as the structures in the large-scale mountain. However, a N140°E-striking sinistral Fengshan transfer fault zone (FTFZ) seemingly intersects the aforementioned major structural discontinuities in SW Taiwan according to the geomorphic analyses from drainage network anomalies, aerial photographs and satellite images (Figure 2). The presence of the FTFZ is visually thought to resist the southwestward extrusion of geological units in SW Taiwan. Therefore a new dense-spaced GPS network in SW Taiwan provides an excellent opportunity to better characterize the motions of these structural discontinuities and to understand the kinematics of tectonic escape in SW Taiwan.
Figure 1. The sketch map of tectonic escape in SE Asia between the Indo-Australian plate and the Eurasian plate. Large red arrows denote the compression direction between two plates and large blue arrows represent the direction of tectonic escape.

Figure 2. Tectonic escape of SW Taiwan. Large red arrows denote the compression direction between the Eurasian plate and the Philippine Sea plate. Yellow arrows represent the direction of tectonic escape. Large white arrows are GPS velocities. Chaochou F.: the Chaochou fault; Chishan F.: the Chishan fault; CTFZ: the Chishan transfer fault zone; FTFZ: the Fengshan fault; Kaoping F.: the Kaoping fault.

In this study, a dense GPS array of ~5 km station-spacing of 102 campaign-mode stations (Figure 3 and Figure 4) has been deployed in SW Taiwan by the Central Geological Survey (CGS) of Taiwan since 1995. The secular velocities of GPS stations with respect to a passive continental margin station, S01R, at Penghu Island (Figure 3 and Figure 4) were estimated based on fitting the coordinate time series in a time span of 10 years from July 1995 to August 2005 with a linear function using the least-square regression method. The estimated horizontal velocities show from east to west a counterclockwise rotation from 42.0 mm/yr to 13.0 mm/yr along the azimuths from 246° to 265° across SW Taiwan (Figure 3). Farther, SW Taiwan can be divided into two areas along the Chishan fault. For the stations west of the Chishan fault, velocities decrease rapidly westwards from 42.0 mm/yr to 13.0 mm/yr with a slight clockwise rotation pattern. In contrast, for the stations east of the Chishan fault, station velocities are quite consistent of about 51.9 ± 6.6 mm/yr with a clear pattern of counterclockwise rotation. The vertical velocities in SW Taiwan indicate that the subsidence rate of 5 to 20 mm/yr is concentrated on the coastal area north of Kaohsiung city and especially the southernmost area of the Pintung plain (Figure 4). The uplift rate of 10 to 20 mm/yr is distributed in the Western Foothills and the Central Range (Figure 4). Based on the analyses of horizontal velocity gradients, large strain rates are revealed in the area west of the Chishan fault and the region along the FTFZ. The remnant region shows little deformation. The Chaochou fault is acting as high-angle reverse faulting with the shallow part of the fault locked during the interseismic period because the Central Range is moving upward with respect to the Pingtung plain (Figure 4). The Chishan fault is acting as oblique reverse faulting with significant right-lateral strike-slip motion (Figure 3). The FTFZ shows a sinistral movement, and extends from the southern tip of the Hsiaokangshan fault and the Gutingkeng fault southeastward to the shoreline of the Pingtung plain (Figure 3).
Figure 3. The GPS horizontal velocity field in SW Taiwan with respect to the station, S01R, in the passive continental margin of the Penghu Island in the Taiwan strait. Black arrows are illustrated as the horizontal velocities. The 95% confidence error ellipse is shown at the tip of each velocity vector. Triangles are GPS station locations. The color scale reveals the degree of horizontal velocities. Gray lines are positions of active faults.

Figure 4. The GPS vertical velocity field in SW Taiwan with respect to the station, S01R. Triangles are GPS station locations. The color scale reveals the degree of vertical velocities. Gray lines are positions of active faults.

Based upon analyses of detailed GPS observations, we propose a revised tectonic kinematic model for the crustal deformation in SW Taiwan (Figure 5). Interaction between the convex Peikang basement high and westward propagation of the accretionary wedge forces the material in SW Taiwan to escape toward WSW and SW, sub-parallel to the edge of the Peikang basement high. The Chishan fault represents a major boundary fault, separating the SW Taiwan into two structural domains: the internally deforming Western Foothills of right-lateral transpression with a clockwise rotation to the west and the relatively rigid Pingtung plain with a counterclockwise rotation to the east. The counterclockwise rotation of velocity field in SW Taiwan is partly accommodated by the left-lateral faulting along the N140°E-striking FTFZ. Finally, the stress regime of E-W shortening and N-S lengthening in SW Taiwan, induced by plate convergence and the lateral spreading of mountain belt, is demonstrated at different depths of the lithosphere: by southward ductile flow in the upper mantle and by a brittle conjugated-type fault system of the NE-SE-striking Chishan fault and the N140°E-striking FTFZ within the crust. In the tectonic escape process of SW Taiwan, the southward upper mantle flow and the southwestward crustal escape imply that the crust and upper mantle are decoupled.
Figure 5. The tectonic kinematic model for SW Taiwan. (a) The tectonic escape model in the crust. Large red arrows and blue arrows show the E-W shortening and N-S extension, respectively. Red lines reveal the conjugated-type fault system formed by the Chishan fault (CHNF) and the Fengshan fault (FTFZ). The large gray arrow denotes the crustal escaping direction. White thin arrows are GPS velocities. Yellow arrows represent the shear senses of transpression area (yellow area) in the Western Foothills. The dark area reveals the location of quasi-rigid block of the Pingtung plain. Green arrows show the opposing rotation directions of the aforesaid two blocks. (b) The tectonic escape model in the upper mantle. Large red arrows reveal the E-W contraction and green arrows represent the direction of upper mantle flow determined by shear wave splitting, plotted as thick bars. Fast polarization directions are given by a bar azimuth from due north. The delay time is proportional to bar length. Because of the 1.5 second splitting time in SW Taiwan, we suggest an anisotropic layer thickness of ~150 km.
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