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Westward advance of the deformation front and evolution of submarine canyons offshore of southwestern Taiwan
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Westward advance of the deformation front and evolution of submarine canyons offshore of southwestern Taiwan ⁎
Wei-Chung Hana,b, Char-Shine Liua, , Wu-Cheng Chic, Liwen Chena,b, Che-Chuan Lina, Song-Chuen Chend a
Institute of Oceanography, National Taiwan University, Taiwan GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany c Institute of Earth Sciences, Academia Sinica, Taiwan d Central Geological Survey, Ministry of Economic Affairs, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Deformation front 3D seismic Offshore SW Taiwan Submarine canyon evolution
This study analyzes both 2D and 3D seismic images around the Palm Ridge area offshore of southwestern Taiwan to understand how the deformation front shifted westward and how tectonic activities interact with submarine canyon paths in the transition area between the active and passive margins. Palm Ridge is a submarine ridge that developed on the passive China continental margin by down-dip erosion of several tributaries of Penghu Canyon; it extends eastward across the deformation front into the submarine Taiwan accretionary wedge. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of the westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area.
1. Introduction As an accretionary wedge grows, new thrust faults develop in front of the wedge, and the deformation front (thrust front) is replaced by newly developed thrusts through time. While the deformation front is advancing, a proto-thrust zone may develop in the footwall of the frontal thrust. A proto-thrust zone is usually a transition between the normal fault and thrust zones and is the location of the next frontal thrust. Proto-thrusts have been observed in many subduction systems over the world, such as Manila (Ku and Hsu, 2009), Nankai (Karig and Lundberg, 1990; Leggett et al., 1985), and Cascadia (Cochrane et al., 1994; Adam et al., 2004). With the proto-thrusts developing, a previously extensional environment turns into a compressional one and thus leads to both structural and sedimentary alterations. Submarine canyons are incised into shelf and slope settings in continental margins and are conduits for transporting orogenic sediments to the deep sea (Nittrouer and Wright, 1994). Studies from various continental margins around the world suggest that canyon ⁎
paths can be greatly affected by tectonic activities in tectonically active zones (Clark and Cartwright, 2011; Mountjoy et al., 2009; TuZino and Noda, 2007; Yu and Hong, 2006), whereas those in passive margins cannot (Harris and Whiteway, 2011). However, few studies discuss the role of tectonics on canyon evolution near the transition between active and passive continental margins. Situated in the incipient arc-continent collision zone, the accretionary wedge offshore SW Taiwan was formed by oblique arc-continent collision between the Luzon arc and the China continent (Huang et al., 1997; Liu et al., 1997), which is different from other accretionary wedges that have been created by subduction processes, such as the ones at the Naikai and Cascadia subduction zones. The migration of submarine canyon paths in the area on land and offshore SW Taiwan has been revealed by several studies (Lee et al., 1995; Fuh et al., 1997, 2003; Yu and Hong, 2006), and some authors suggest that it is a result of the growth of the Taiwan orogenic wedge (Yu and Hong, 2006). Their studies note that the submarine canyon paths offshore SW Taiwan have migrated southwestward through time. Yu and Hong (2006)
Corresponding author. E-mail address: [email protected] (C.-S. Liu).
http://dx.doi.org/10.1016/j.jseaes.2017.07.001 Received 16 October 2016; Received in revised form 30 June 2017; Accepted 1 July 2017 1367-9120/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Han, W.-C., Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.07.001
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Fig. 1. Bathymetric map of the study area showing the locations of Palm Ridge, the deformation front of Lin et al. (2008) (black line with teeth) and submarine canyons (dashed blue lines). The deformation front separates the China passive margin to the west and the active accretionary wedge to the east. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the upper reach of Penghu Canyon across the deformation front offshore SW Taiwan (Fig. 1) for both tectonic and sedimentary interests. Palm Ridge is a topographic high surrounded by tributaries of Penghu Canyon; it covers both the China passive margin and the submarine Taiwan accretionary wedge. After distinguishing and mapping the features based on 2D and 3D seismic images (see Fig. 2 for seismic line distribution), the locations of the regional structural and sedimentary features, such as buried canyon deposits, anticlinal ridges and faults, are compiled, and their characters are described and interpreted. Finally, we propose a geological model to illustrate the change of structural styles observed and the possible canyon evolution in our study
further suggest that the shifting axes of Late Pliocene–Pleistocene canyons from onshore SW Taiwan to the present-day position of Penghu Canyon reflect evolving foreland basins with a longitudinal canyon transport system progressively migrating southwestward. Nevertheless, how the deformation front has been shifted and how the canyon paths have been changed are still not clear. The aim of this study is to understand how the growth of the submarine Taiwan accretionary wedge has changed the structural styles and thus the canyon paths in the transitional zone between the active and passive margins. To better characterize these issues, we have conducted both 2D and 3D seismic surveys to investigate Palm Ridge in
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Fig. 2. Shaded relief map showing the location of Palm Ridge (light yellow contour) and locations of the seismic profiles presented in this study. The black line with teeth shows the location of the deformation front of Lin et al. (2008); the red box indicates the area covered by 3D seismic block. The blue lines and the solid white line indicate the locations of the seismic profiles that are shown in Figs. 4–8 and 10. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
features are normal faults, while east of the deformation front, a series of west-vergent folds and thrusts have been observed (Liu et al., 1997, 2004). The location of the deformation front offshore of SW Taiwan has been studied extensively (Lacombe et al., 1997, 2001; Lee et al., 1995; Lin et al., 2008; Liu et al., 1997, 2004; Yu, 2004). Liu et al. (1997) further note that the structure and location of the deformation front are affected by both the growth of the accretionary wedge and the geometry of the China margin basement. Penghu Canyon is an important conduit for transporting terrestrial and shallow marine sediments to the deep sea. In the upper reach of Penghu Canyon, several tributaries (Yu and Lee, 1993; Yu and Chang, 2002) spread over the slope in a roughly N-S direction and finally connect with the Manila Trench (Fig. 1). A few studies conducted in the middle and lower reaches of Penghu Canyon suggest that it is a tectonically controlled canyon rather than a slope canyon dominated by down-slope processes and that it has migrated westward through time (Lee et al., 1995; Yu and Hong, 2006). How tectonic activities affect the canyon paths in the upper reach, however, is still poorly understood. Detailed structural variations are observed across the deformation front in Palm Ridge. In the eastern part of Palm Ridge lies an anticlinal ridge (frontal anticlinal ridge) that has been uplifted by the frontal thrust of the accretionary wedge. The western part of Palm Ridge covers
Table 1 Acquisition parameters of the seismic survey systems used in this study. Cruise
OR1-681
OR1-958
MGL0905
Source Source volume No. channels Channel interval Acquisition vessel
3 air guns 475 in.3 48 12.5 R/V OR1
2 air guns 475 in.3 84 12.5 R/V OR1
Large (40 guns) air gun array 6000 in.3 468 12.5 R/V Marcus G. Langseth
area. 2. Tectonic and geologic background The area offshore SW Taiwan is situated in the transition zone between the Manila subduction to the south and the Taiwan arc-continent collision to the north, or in the incipient arc-continent collision zone (Huang et al., 1997, 2000). The deformation front which separates the China passive margin from the active Taiwan accretionary wedge is defined as the location of the most frontal contractional structures along the convergent boundary between the Philippine Sea Plate and the Eurasian Plate. West of the deformation front, the main structural
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Fig. 3. Flowchart showing the 2D and 3D seismic processing steps used in this study.
regarding the acquisition parameters of the seismic survey systems used in this study is shown in Table 1. All the seismic data were processed at the Institute of Oceanography, National Taiwan University, using the ProMAX software. Typical seismic processing procedures including trace editing, geometry setup, band-pass filtering, amplitude correction, spiking noise removal, velocity analysis, normal moveout correction, water velocity F-K migration and water bottom muting were applied for 2D seismic data (Fig. 3). In addition, eigenvector filtering (Jiao et al., 1999) was applied for multiple attenuation of the MGL data to better image deep structures. The 3D seismic cube used in this study was built by 78 closely spaced (50-m) 2D seismic lines and covers an area of approximately 67 km2 (3.85 km by 17.5 km). Those 2D seismic data used to build the 3D seismic cube image were collected by OR1 during the survey cruise OR1-958 (Table 1). To create the 3D seismic cube, after completing the typical 2D seismic processing procedures noted above, 3D geometry setup, 3D stacking, and two pass water velocity F-K migration were applied (Fig. 3).
the China passive margin, where normal faults are the predominant structural features. The round or palm shape of this ridge could have been caused by a large submarine landslide (Liu et al., 2004) that was then modified by several tributaries at the head of Penghu Canyon.
3. Data compilation and processing The bathymetric map used in this study is produced from a newly compiled digital elevation model with 100-m grid spacing using all the available shipboard single-beam and multi-beam echo soundings offshore SW Taiwan following the compilation procedures described in Liu et al. (1998). In this study, we use 2D multi-channel seismic (MCS) reflection data to construct the regional geologic framework and to recognize deep structures. In addition, we analyze a 3D seismic cube to provide constraints on detailed spatial variations of structural and sedimentary features. MCS data used in this study were collected from 2003 to 2011 by the R/V Ocean Researcher I (OR1) and during the 2009 MGL0905 cruise of the R/V Marcus G. Langseth (MGL) (Fig. 2). Information
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Fig. 4. Uninterpreted (upper) and interpreted (lower) seismic line MGL0905-01. This profile runs across the deformation front, and three structural domains can be observed: the normal fault zone, the proto-thrust zone and the thrust zone. The normal fault zone is characterized by numerous normal faults; the thrust zone is characterized by a series of thrusts and folds; the proto-thrust zone is a transition between the normal fault zone and the thrust zone that lies between the deformation front and the frontal thrust. See Fig. 2 for profile location.
4. Seismic interpretation
4.1. Structural variation from passive margin to accretionary wedge
To understand how the deformation front evolved in the active tectonic region and how tectonic activities affect the development of submarine canyons and ridges near the frontal part of the accretionary wedge offshore SW Taiwan, we use the “KINGDOM” software of IHS to map the 3D geometry of the faults and key reflection horizons from collected seismic profiles. First, we distinguish the structural and sedimentary features from reflection seismic data in the frontal part of the accretionary wedge offshore SW Taiwan. Second, we map these features in the study area, and locations of the frontal thrust, frontal anticline (the hanging wall folding structure of the frontal thrust), proto-thrust zone, paleo-slope surface and buried canyons are then constructed in digital form.
The deformation front offshore SW Taiwan separates the accretionary wedge to the east and the China passive margin to the west. Three structural domains can be distinguished on the seismic section MGL0905-01 that runs across the China passive margin and the accretionary wedge: the normal fault zone, the proto-thrust zone, and the thrust zone (Fig. 4). The normal fault zone is characterized by numerous normal faults. These normal faults offset the sedimentary strata in both the shelf and slope settings of the China passive margin, and some large normal faults even crop out on the seafloor. The thrust zone is characterized by a series of folds and thrusts that are considered to be associated with the decollement system. Near the frontal part of the accretionary wedge, a proto-thrust zone can be recognized as the transition between the normal fault zone and the thrust zone (Fig. 4).
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Fig. 5. Uninterpreted (left) and interpreted (right) seismic sections running across the deformation front showing that the accretionary wedge has encroached on top of the passive continental slope (see Fig. 2 for profile locations). The proto-thrusts are observed west of the previously mapped deformation front of Lin et al. (2008) (dotted gray line). The dotted black line is the deformation front suggested in this study that follows the location of the westernmost proto-thrusts.
boundary unconformity (Figs. 6–8). U1 is the deeper unit, characterized by weak and discontinuous reflectors and some locally bright reflectors (Figs. 6–8). U2 is the upper unit and is characterized by a series of deformed sediments with alternations of very bright and lower-amplitude reflectors (Figs. 6–8). Onlapping features and thrusting/folding structures in U2 indicate that the U2 sediments are syn-convergence deposits, whereas the preexisting normal faults and unfolded strata in U1 imply that U1 consists of pre-convergence passive margin deposits (Figs. 6–8). Since the contraction process in our study area is a result of plate convergence, we interpret this boundary unconformity between U1 and U2 as the paleoslope surface before the frontal thrust developed. We correlate the preconvergence passive margin sediments (U1) with Pliocene-Quaternary deposits deriving from the Taiwan Orogen, which has been suggested by previous studies (Chiang et al., 2004; Chou, 1999). While the frontal thrust was developing, younger sediments (U2) started being deposited and then were deformed.
The frontal thrust is defined as the most frontal (westernmost in this study) thrust of the accretionary wedge that branches from the decollement. The deformation front offshore of SW Taiwan can be considered as a boundary that separates the convergent regime to the east and the extensional regime to the west (Yu, 2004), and thus, it is the location of the most frontal (westernmost) contractional structures of the accretionary wedge. The proto-thrust zone is the area between the frontal thrust and the deformation front. The frontal thrust in our study area is blind, although it emerges to near surface level. It has uplifted the frontal anticline and deformed sedimentary strata. Fig. 5 shows that a large frontal fold can be observed on profile MCS681-09, as this is the place where a N-S-trending anticlinal ridge encroaches on the passive continental margin (Fig. 2). The proto-thrusts are recognized west of the frontal thrust with reverse offsets, although the offsets are small, in several seismic profiles in our study area (Figs. 4–6). 4.2. Seismic unit characterization and interpretation
4.3. Location of buried canyons
Two seismic units (U1 and U2) are identified in our study area. These two units are separated by an unconformity that is characterized by a package of bright reflectors, and it can be observed throughout the study area as the lowest continuous feature distinguishable in the data set (Figs. 6–8). The frontal thrust is interpreted to slip along this
Buried canyons are often characterized by high-amplitude and parallel reflectors that terminate on either side of U- or V-shaped discontinuities in reflection seismic profiles (Antobreh and Krastel, 2006;
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Fig. 6. Inline profile 24 of the Palm Ridge 3D seismic cube running across the frontal part of the accretionary wedge. (a) Uninterpreted seismic profile; (b) Interpreted seismic profile showing structural features and the major unconformity that separates seismic units U1 and U2; (c) Enlarged view of the black box in (a) presenting the location of the proto-thrusts. See Fig. 2 for profile location.
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Fig. 7. Interpreted inline profile 1 of the Palm Ridge 3D seismic cube running across the frontal part of the accretionary wedge. The proto-thrusts are observed in shallow sediments of the slope. Clearly onlapping features and folded strata in U2 sediments imply that the sediments of U2 are syn-convergence deposits, and the unconformity separating U1 and U2 may be the paleo-slope surface. Pre-existing normal faults observed in U1 sediments suggest that the sediments of U1 are pre-convergence deposits. See Fig. 2 for profile location.
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Fig. 8. Interpreted inline profile 32 of the Palm Ridge 3D seismic cube running across the frontal part of the accretionary wedge. In addition to the similar structural/sedimentary patterns that are shown in Fig. 6, two clear buried canyon surfaces (green dotted line) are recognized. These two paleo-canyons cut through the major unconformity (interpreted paleo-slope surface, shown as black dotted line) between U1 and U2. See Fig. 2 for profile location.
base of buried canyons, and the frontal thrust and frontal anticline that are mapped from the 3D seismic cube. The buried paleo-canyons lie on the paleo-slope surface and deepen southwestward. These buried paleocanyons cut the pre-convergence sediments (U1) and then were buried by syn-convergence sediments (U2) (Figs. 8 and 9), which implies that the paleo-canyons formed before the development of fold and thrust structures and then were gradually filled by sediments during the synconvergence stage.
Deptuck et al., 2007; Gong et al., 2011; Pratson et al., 1994; Zhu et al., 2010). Based on the characteristics of buried canyons noted above, several buried canyons (paleo-canyons) can be recognized in our seismic profile images west of the frontal thrust and anticline structures (Figs. 5 and 8–10). We have traced the base of the buried canyons in detail through a series of 2D seismic profiles and the 3D seismic cube (See Fig. 2 for data distribution). Fig. 9 shows the geometry of the paleo-slope surface (the major unconformity shown in Figs. 6–8), the
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Fig. 9. Perspective view within the 3D seismic block showing the geometry of the main faults and horizons. The shown horizon “frontal anticline” (blue) is the top of the frontal anticline that has been deformed by the frontal thrust. The recognized paleo-canyons cut the paleo-slope surface (the major unconformity that is shown in Figs. 6–8) and trend roughly in NE-SW direction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to better constraints due to more seismic profile data are available for Lin et al. (2008) than for previous studies, another reason for this difference is that the previous studies placed the deformation front at the frontal ramp anticlines or the frontal thrusts. However, the detection of proto-thrusts, which are expressions of contractional strain and are located farther west of the deformation front suggested by Lin et al. (2008), implies that the compressional stress has propagated into that region. Since the deformation front is defined as the location of the most frontal contractional structures along a convergent plate boundary, we thus suggest that the location of the deformation front should be placed farther west to where the westernmost proto-thrusts lie in our study area (Figs. 5 and 11).
4.4. Distribution of structural and sedimentary features With the densely distributed 2D seismic profiles and a 3D seismic volume, we identify several important structural/sedimentary features in our study area. A map that shows the main structural and sedimentary features is constructed (Fig. 11). The features shown in this map include the western boundary of the proto-thrusts (the location of the new deformation front defined by this study), the previously mapped deformation front by Lin et al. (2008), and the paleo-flow directions of the buried canyons.
5. Discussion 5.1. Westward advance of the deformation front
5.2. Canyon evolution in the rifted SCS slope
Lin et al. (2008) has placed the deformation front farther west than those suggested by previous studies (Lacombe et al., 1997, 2001; Lee et al., 1995; Liu et al., 1997, 2004; Yu, 2004), at the lowest and flat west limb of the frontal anticline from seismic reflection profiles. In addition
Based on the 3D seismic cube and densely distributed 2D seismic profiles (Fig. 11), we have mapped the locations of buried canyons, and the connection of the buried canyon thalwegs represents the flow directions of these paleo-canyons (Fig. 11). The heads of the buried
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Fig. 10. Uninterpreted (upper) and interpreted (lower) seismic line MCS958-14 running transverse to the strikes of major structures such as the deformation front and the frontal thrust. The paleo-canyon surfaces can be recognized west of the frontal thrust and frontal anticline structures. See Fig. 2 for profile location.
before tectonic loading and canyon incising. In the first stage, several NE-SW-trending normal faults developed and started down-slipping in the study area, probably due to loading from the Taiwan mountain belt (Suppe, 1984; Byrne and Liu, 2002; Huang et al., 2000). In the second stage, as the normal faults caused down-slip of the passive margin blocks, topographic lows were created along the fault outcrops, guiding the canyon thalwegs to run transverse to the slope dip. Similar cases of normal fault-controlled submarine canyons have also been suggested by several studies (Geist et al., 1988; Shepard, 1981; Thornburg et al., 1990). From stages three to four, as the accretionary wedge was developing, the tectonic setting here was changed from extensional to contractional, the normal faults stopped slipping and the canyons began
canyons start on the slope near the shelf break and extend southwestward along the slope, which is transverse to the slope dip and is very different from the present down-slope canyon path directions of NS or NNW-SSE (Fig. 11). Compared with the present-day canyon directions that approximately follow the slope dip (Fig. 1), these two paleo-canyon paths that are transverse to the slope dip seem very unusual. Thus, in terms of canyon evolution, the effects of fault activities cannot be neglected, in addition to the contribution of down-slope processes. We propose a conceptual 6-stage canyon evolution model to illustrate how the canyons have evolved through time in this convergent plate boundary (Fig. 12). The initial stage shows the original China continental slope
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Fig. 11. Map showing the location of the deformation front (black line with teeth). The gray line with teeth indicates the previously mapped deformation front by Lin et al. (2008). The white dotted arrow indicates the possible thalweg of paleo-canyons mapped from 2D seismic profiles (gray lines) and confirmed in the 3D seismic block image (black box).
infilling. In the present day, we can observe that several proto-thrusts have developed and the canyon courses have changed back to the down-slope direction. In terms of canyon evolution, the southwestward migration of the canyons observed from previous studies (Lee et al., 1995; Fuh et al., 1997, 2003; Yu and Hong, 2006) suggest that tectonics prevail over the down-slope processes in the active Taiwan submarine accretionary wedge. In the China passive margin, the down-slope process should generally be dominant, although some of the canyon thalwegs actually run along troughs that were created by normal fault activities. In the transition from passive to active margin, down-slope processes, probably accompanied by normal fault activities as the key factors that controlled the canyon paths, might be gradually replaced by active tectonic processes while the accretionary wedge advanced westward.
(1) The features of a proto-thrust zone are observed in the transition between the normal fault zone and the thrust zone. This observation confirms the westward migration of the compressional stress field as the oblique Taiwan arc-continent collision propagated southward. (2) The proto-thrust zone is recognized west of the previously proposed deformation front. As these proto-thrusts are the most westward contractional structures that can be recognized in the study area, we suggest moving the location of the deformation front to where the westernmost proto-thrusts lie. (3) Paleo-canyon paths show drastically different directions (transverse to the slope dip) from the present canyon paths (down-slope). Our canyon evolution model suggests that the canyon directions have been changed due to the westward migration of the deformation front.
6. Conclusions
Acknowledgements
In this convergent plate boundary offshore of SW Taiwan, westward migration of the compressional stress field has turned previously extensional field into compressional one and changed submarine canyon paths significantly. This study provides an example to show how tectonic activities can affect submarine canyon paths in a transitional setting between the active and passive margins offshore SW Taiwan and may have implications for other areas with similar geologic settings worldwide. Meanwhile, we have identified several features from seismic images that are related to specific interesting geological processes in such a transitional setting in the upper reach of Penghu Canyon. We summarize our results as follows:
We would like to thank Yun-Shuen Wang, San-Hsiung Chung and the Central Geological Survey, Ministry of Economic Affairs, for the projects that made this study possible. The captains and crew of the OR1, technicians of the OR1 Instrumentation Center, and S.D. Chiou of the Institute of Oceanography, National Taiwan University (IONTU), are thanked for helping to collect the seismic data. We acknowledge the teamwork of the Seismic Exploration Laboratory, IONTU. We also grateful to SMT’s Educational Gifts Program for providing the KINGDOM software. This research was accomplished through grants from the Central Geological Survey, MOEA contracts: 103-522690400003-01 and 104-5226904000-02-01.
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Fig. 12. Conceptual model illustrates the processes that affect the canyon paths in our study area. The initial stage shows the original China continental slope before tectonic loading and canyon incising. In the first stage, NE-SW-trending normal faults developed and started down-slipping in the study area (first stage), and this process guided the paleo-canyons to flow along the topographic low (second stage). From stages three to four, the activity of the extensional faults gradually stopped, and then the canyons were abandoned and filled by incoming sediments. In the present day, as the accretionary wedge encroaches upon the passive continental margin, the paleo-canyons have been buried, and some proto-thrusts are developed.
Canyon in the Qiongdongnan Basin, northwestern South China Sea: architecture, sequence stratigraphy, and depositional processes. Mar. Pet. Geol. 28, 1690–1702. Harris, P.T., Whiteway, T., 2011. Global distribution of large submarine canyons: geomorphic differences between active and passive continental margins. Mar. Geol. 285, 69–86. Huang, C.-Y., Wu, W.-Y., Chang, C.-P., Tsao, S., Yuan, P.B., Lin, C.-W., Xia, K.-Y., 1997. Tectonic evolution of accretionary prism in the arc-continent collision terrane of Taiwan. Tectonophysics 281, 31–51. Huang, C.-Y., Yuan, P.B., Lin, C.-W., Wang, T.K., Chang, C.-P., 2000. Geodynamic processes of Taiwan arc–continent collision and comparison with analogs in Timor, Papua New Guinea, Urals and Corsica. Tectonophysics 325, 1–21. Jiao, J., Negut, D., Link, B., 1999. Multiple attenuation using eigenvalue decomposition. In: SEG Technical Program Expanded Abstracts 1999. Society of Exploration Geophysicists, pp. 1052–1055. Karig, D.E., Lundberg, N., 1990. Deformation bands from the toe of the Nankai accretionary prism. J. Geophys. Res.: Solid Earth 95, 9099–9109. Ku, C.-Y., Hsu, S.-K., 2009. Crustal structure and deformation at the northern Manila Trench between Taiwan and Luzon islands. Tectonophysics 466, 229–240. Lacombe, O., Angelier, J., Chen, H.-W., Deffontaines, B., Chu, H.-T., Rocher, M., 1997. Syndepositional tectonics and extension-compression relationships at the front of the Taiwan collision belt: a case study in the Pleistocene reefal limestones near Kaohsiung, SW Taiwan. Tectonophysics 274, 83–96. Lacombe, O., Mouthereau, F., Angelier, J., Deffontaines, B., 2001. Structural, geodetic and seismological evidence for tectonic escape in SW Taiwan. Tectonophysics 333, 323–345. Lee, T.Y., Hsu, Y.Y., Tang, C.H., 1995. Structural geometry of the deformation front between 22 N and 23 N and migration of the Penghu Canyon, offshore southwestern Taiwan arc-continent collision zone. In: International Conference and 3rd SinoFrench Symposium on Active Collision in Taiwan, pp. 219–227. Leggett, J., Aoki, Y., Toba, T., 1985. Transition from frontal accretion to underplating in a part of the Nankai Trough accretionary complex off Shikoku (SW Japan) and extensional features on the lower trench slope. Mar. Pet. Geol. 2, 131–141. Lin, A.T., Liu, C.-S., Lin, C.-C., Schnurle, P., Chen, G.-Y., Liao, W.-Z., Teng, L.S., Chuang, H.-J., Wu, M.-S., 2008. Tectonic features associated with the overriding of an accretionary wedge on top of a rifted continental margin: an example from Taiwan. Mar. Geol. 255, 186–203.
References Adam, J., Klaeschen, D., Kukowski, N., Flueh, E., 2004. Upward delamination of Cascadia Basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux. Tectonics 23, TC3009. http://dx.doi.org/10.1029/ 2002TC001475. Antobreh, A.A., Krastel, S., 2006. Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: a newly discovered canyon preserved-off a major arid climatic region. Mar. Pet. Geol. 23, 37–59. Byrne, T.B., Liu, C.-S., 2002. Geology and geophysics of an arc-continent collision, Taiwan. Geological Society of America Special Paper, vol. 358. Boulder, Colorado, p. 211. Chiang, C.-S., Yu, H.-S., Chou, Y.-W., 2004. Characteristics of the wedge-top depozone of the southern Taiwan foreland basin system. Basin Res. 16, 65–78. Chou, Y.W., 1999. Tectonic framework, flexural uplift history and structural patterns of flexural extension in Western Taiwan Foreland Basin. Ph.D. thesis National Taiwan University 125 p. (in Chinese). Clark, I.R., Cartwright, J.A., 2011. Key controls on submarine channel development in structurally active settings. Mar. Pet. Geol. 28, 1333–1349. Cochrane, G.R., Moore, J.C., MacKay, M.E., Moore, G.F., 1994. Velocity and inferred porosity model of the Oregon accretionary prism from multichannel seismic reflection data: implications on sediment dewatering and overpressure. J. Geophys. Res.: Solid Earth 99, 7033–7043. Deptuck, M.E., Sylvester, Z., Pirmez, C., O’Byrne, C., 2007. Migration–aggradation history and 3-D seismic geomorphology of submarine channels in the Pleistocene Beninmajor Canyon, western Niger Delta slope. Mar. Pet. Geol. 24, 406–433. Fuh, S.C., Liang, S.C., Wu, M.S., 2003. Spatial and temporal evolution of the PlioPleistocene submarine canyons between Potzu and Tainan, Taiwan. Petrol. Geol. Taiwan 36, 1–18. Fuh, S.C., Liu, C.S., Wu, M.S., 1997. Migration of canyon systems from Pliocene to Pleistocene in area between Hsyning structure and Kaoping Slope and its application for hydrocarbon exploration. Petrol. Geol. Taiwan 31, 43–60. Geist, E.L., Childs, J.R., Scholl, D.W., 1988. The origin of summit basins of the Aleutian Ridge: implications for block rotation of an arc massif. Tectonics 7, 327–341. Gong, C., Wang, Y., Zhu, W., Li, W., Xu, Q., Zhang, J., 2011. The Central Submarine
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arc spreading near Taiwan. Memoir Geol. Soc. China 6, V33. Thornburg, T.M., Kulm, L.D., Hussong, D.M., 1990. Submarine-fan development in the southern Chile Trench: a dynamic interplay of tectonics and sedimentation. Geol. Soc. Am. Bull. 102, 1658–1680. TuZino, T., Noda, A., 2007. Tectonic control over topography and channel sedimentation across the forearc slope of the southern Kurile Trench. Geo-Mar. Lett. 27, 1–11. Yu, H.-S., 2004. Nature and distribution of the deformation front in the Luzon ArcChinese continental margin collision zone at Taiwan. Mar. Geophys. Res. 25, 109–122. Yu, H.-S., Chang, J.-F., 2002. The Penghu submarine canyon off southwestern Taiwan: morphology and origin. Terrest. Atmosph. Ocean. Sci. 13, 547–562. Yu, H.-S., Hong, E., 2006. Shifting submarine canyons and development of a foreland basin in SW Taiwan: controls of foreland sedimentation and longitudinal sediment transport. J. Asian Earth Sci. 27, 922–932. Yu, H.-S., Lee, J.T., 1993. The multi-head Penghu submarine canyon off southwestern Taiwan: morphology and origin. Acta Oceanogr. Taiwanica 30, 10–21. Zhu, M., Graham, S., Pang, X., McHargue, T., 2010. Characteristics of migrating submarine canyons from the middle Miocene to present: implications for paleoceanographic circulation, northern South China Sea. Mar. Pet. Geol. 27, 307–319.
Liu, C.-S., Deffontaines, B., Lu, C.-Y., Lallemand, S., 2004. Deformation patterns of an accretionary wedge in the transition zone from subduction to collision offshore southwestern Taiwan. Mar. Geophys. Res. 25, 123–137. Liu, C.-S., Huang, I.L., Teng, L.S., 1997. Structural features off southwestern Taiwan. Mar. Geol. 137, 305–319. Liu, C.-S., Liu, S.-Y., Lallemand, S.E., Lundberg, N., Reed, D.L., 1998. Digital elevation model offshore Taiwan and its tectonic implications. Terrest., Atmosph. Ocean. Sci. 9, 705–738. Mountjoy, J.J., Barnes, P.M., Pettinga, J.R., 2009. Morphostructure and evolution of submarine canyons across an active margin: cook Strait sector of the Hikurangi Margin, New Zealand. Mar. Geol. 260, 45–68. Nittrouer, C.A., Wright, L.D., 1994. Transport of particles across continental shelves. Rev. Geophys. 32, 85–113. Pratson, L.F., Ryan, W.B., Mountain, G.S., Twichell, D.C., 1994. Submarine canyon initiation by downslope-eroding sediment flows: evidence in late Cenozoic strata on the New Jersey continental slope. Geol. Soc. Am. Bull. 106, 395–412. Shepard, F.P., 1981. Submarine canyons: multiple causes and long-time persistence. AAPG Bull. 65, 1062–1077. Suppe, J., 1984. Kinematics of arc-continent collision, flipping of subduction, and back-
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Westward advance of the deformation front and evolution of submarine canyons offshore of southwestern Taiwan ⁎
Wei-Chung Hana,b, Char-Shine Liua, , Wu-Cheng Chic, Liwen Chena,b, Che-Chuan Lina, Song-Chuen Chend a
Institute of Oceanography, National Taiwan University, Taiwan GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany c Institute of Earth Sciences, Academia Sinica, Taiwan d Central Geological Survey, Ministry of Economic Affairs, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Deformation front 3D seismic Offshore SW Taiwan Submarine canyon evolution
This study analyzes both 2D and 3D seismic images around the Palm Ridge area offshore of southwestern Taiwan to understand how the deformation front shifted westward and how tectonic activities interact with submarine canyon paths in the transition area between the active and passive margins. Palm Ridge is a submarine ridge that developed on the passive China continental margin by down-dip erosion of several tributaries of Penghu Canyon; it extends eastward across the deformation front into the submarine Taiwan accretionary wedge. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of the westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area.
1. Introduction As an accretionary wedge grows, new thrust faults develop in front of the wedge, and the deformation front (thrust front) is replaced by newly developed thrusts through time. While the deformation front is advancing, a proto-thrust zone may develop in the footwall of the frontal thrust. A proto-thrust zone is usually a transition between the normal fault and thrust zones and is the location of the next frontal thrust. Proto-thrusts have been observed in many subduction systems over the world, such as Manila (Ku and Hsu, 2009), Nankai (Karig and Lundberg, 1990; Leggett et al., 1985), and Cascadia (Cochrane et al., 1994; Adam et al., 2004). With the proto-thrusts developing, a previously extensional environment turns into a compressional one and thus leads to both structural and sedimentary alterations. Submarine canyons are incised into shelf and slope settings in continental margins and are conduits for transporting orogenic sediments to the deep sea (Nittrouer and Wright, 1994). Studies from various continental margins around the world suggest that canyon ⁎
paths can be greatly affected by tectonic activities in tectonically active zones (Clark and Cartwright, 2011; Mountjoy et al., 2009; TuZino and Noda, 2007; Yu and Hong, 2006), whereas those in passive margins cannot (Harris and Whiteway, 2011). However, few studies discuss the role of tectonics on canyon evolution near the transition between active and passive continental margins. Situated in the incipient arc-continent collision zone, the accretionary wedge offshore SW Taiwan was formed by oblique arc-continent collision between the Luzon arc and the China continent (Huang et al., 1997; Liu et al., 1997), which is different from other accretionary wedges that have been created by subduction processes, such as the ones at the Naikai and Cascadia subduction zones. The migration of submarine canyon paths in the area on land and offshore SW Taiwan has been revealed by several studies (Lee et al., 1995; Fuh et al., 1997, 2003; Yu and Hong, 2006), and some authors suggest that it is a result of the growth of the Taiwan orogenic wedge (Yu and Hong, 2006). Their studies note that the submarine canyon paths offshore SW Taiwan have migrated southwestward through time. Yu and Hong (2006)
Corresponding author. E-mail address: [email protected] (C.-S. Liu).
http://dx.doi.org/10.1016/j.jseaes.2017.07.001 Received 16 October 2016; Received in revised form 30 June 2017; Accepted 1 July 2017 1367-9120/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Han, W.-C., Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.07.001
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Fig. 1. Bathymetric map of the study area showing the locations of Palm Ridge, the deformation front of Lin et al. (2008) (black line with teeth) and submarine canyons (dashed blue lines). The deformation front separates the China passive margin to the west and the active accretionary wedge to the east. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the upper reach of Penghu Canyon across the deformation front offshore SW Taiwan (Fig. 1) for both tectonic and sedimentary interests. Palm Ridge is a topographic high surrounded by tributaries of Penghu Canyon; it covers both the China passive margin and the submarine Taiwan accretionary wedge. After distinguishing and mapping the features based on 2D and 3D seismic images (see Fig. 2 for seismic line distribution), the locations of the regional structural and sedimentary features, such as buried canyon deposits, anticlinal ridges and faults, are compiled, and their characters are described and interpreted. Finally, we propose a geological model to illustrate the change of structural styles observed and the possible canyon evolution in our study
further suggest that the shifting axes of Late Pliocene–Pleistocene canyons from onshore SW Taiwan to the present-day position of Penghu Canyon reflect evolving foreland basins with a longitudinal canyon transport system progressively migrating southwestward. Nevertheless, how the deformation front has been shifted and how the canyon paths have been changed are still not clear. The aim of this study is to understand how the growth of the submarine Taiwan accretionary wedge has changed the structural styles and thus the canyon paths in the transitional zone between the active and passive margins. To better characterize these issues, we have conducted both 2D and 3D seismic surveys to investigate Palm Ridge in
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Fig. 2. Shaded relief map showing the location of Palm Ridge (light yellow contour) and locations of the seismic profiles presented in this study. The black line with teeth shows the location of the deformation front of Lin et al. (2008); the red box indicates the area covered by 3D seismic block. The blue lines and the solid white line indicate the locations of the seismic profiles that are shown in Figs. 4–8 and 10. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
features are normal faults, while east of the deformation front, a series of west-vergent folds and thrusts have been observed (Liu et al., 1997, 2004). The location of the deformation front offshore of SW Taiwan has been studied extensively (Lacombe et al., 1997, 2001; Lee et al., 1995; Lin et al., 2008; Liu et al., 1997, 2004; Yu, 2004). Liu et al. (1997) further note that the structure and location of the deformation front are affected by both the growth of the accretionary wedge and the geometry of the China margin basement. Penghu Canyon is an important conduit for transporting terrestrial and shallow marine sediments to the deep sea. In the upper reach of Penghu Canyon, several tributaries (Yu and Lee, 1993; Yu and Chang, 2002) spread over the slope in a roughly N-S direction and finally connect with the Manila Trench (Fig. 1). A few studies conducted in the middle and lower reaches of Penghu Canyon suggest that it is a tectonically controlled canyon rather than a slope canyon dominated by down-slope processes and that it has migrated westward through time (Lee et al., 1995; Yu and Hong, 2006). How tectonic activities affect the canyon paths in the upper reach, however, is still poorly understood. Detailed structural variations are observed across the deformation front in Palm Ridge. In the eastern part of Palm Ridge lies an anticlinal ridge (frontal anticlinal ridge) that has been uplifted by the frontal thrust of the accretionary wedge. The western part of Palm Ridge covers
Table 1 Acquisition parameters of the seismic survey systems used in this study. Cruise
OR1-681
OR1-958
MGL0905
Source Source volume No. channels Channel interval Acquisition vessel
3 air guns 475 in.3 48 12.5 R/V OR1
2 air guns 475 in.3 84 12.5 R/V OR1
Large (40 guns) air gun array 6000 in.3 468 12.5 R/V Marcus G. Langseth
area. 2. Tectonic and geologic background The area offshore SW Taiwan is situated in the transition zone between the Manila subduction to the south and the Taiwan arc-continent collision to the north, or in the incipient arc-continent collision zone (Huang et al., 1997, 2000). The deformation front which separates the China passive margin from the active Taiwan accretionary wedge is defined as the location of the most frontal contractional structures along the convergent boundary between the Philippine Sea Plate and the Eurasian Plate. West of the deformation front, the main structural
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Fig. 3. Flowchart showing the 2D and 3D seismic processing steps used in this study.
regarding the acquisition parameters of the seismic survey systems used in this study is shown in Table 1. All the seismic data were processed at the Institute of Oceanography, National Taiwan University, using the ProMAX software. Typical seismic processing procedures including trace editing, geometry setup, band-pass filtering, amplitude correction, spiking noise removal, velocity analysis, normal moveout correction, water velocity F-K migration and water bottom muting were applied for 2D seismic data (Fig. 3). In addition, eigenvector filtering (Jiao et al., 1999) was applied for multiple attenuation of the MGL data to better image deep structures. The 3D seismic cube used in this study was built by 78 closely spaced (50-m) 2D seismic lines and covers an area of approximately 67 km2 (3.85 km by 17.5 km). Those 2D seismic data used to build the 3D seismic cube image were collected by OR1 during the survey cruise OR1-958 (Table 1). To create the 3D seismic cube, after completing the typical 2D seismic processing procedures noted above, 3D geometry setup, 3D stacking, and two pass water velocity F-K migration were applied (Fig. 3).
the China passive margin, where normal faults are the predominant structural features. The round or palm shape of this ridge could have been caused by a large submarine landslide (Liu et al., 2004) that was then modified by several tributaries at the head of Penghu Canyon.
3. Data compilation and processing The bathymetric map used in this study is produced from a newly compiled digital elevation model with 100-m grid spacing using all the available shipboard single-beam and multi-beam echo soundings offshore SW Taiwan following the compilation procedures described in Liu et al. (1998). In this study, we use 2D multi-channel seismic (MCS) reflection data to construct the regional geologic framework and to recognize deep structures. In addition, we analyze a 3D seismic cube to provide constraints on detailed spatial variations of structural and sedimentary features. MCS data used in this study were collected from 2003 to 2011 by the R/V Ocean Researcher I (OR1) and during the 2009 MGL0905 cruise of the R/V Marcus G. Langseth (MGL) (Fig. 2). Information
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Fig. 4. Uninterpreted (upper) and interpreted (lower) seismic line MGL0905-01. This profile runs across the deformation front, and three structural domains can be observed: the normal fault zone, the proto-thrust zone and the thrust zone. The normal fault zone is characterized by numerous normal faults; the thrust zone is characterized by a series of thrusts and folds; the proto-thrust zone is a transition between the normal fault zone and the thrust zone that lies between the deformation front and the frontal thrust. See Fig. 2 for profile location.
4. Seismic interpretation
4.1. Structural variation from passive margin to accretionary wedge
To understand how the deformation front evolved in the active tectonic region and how tectonic activities affect the development of submarine canyons and ridges near the frontal part of the accretionary wedge offshore SW Taiwan, we use the “KINGDOM” software of IHS to map the 3D geometry of the faults and key reflection horizons from collected seismic profiles. First, we distinguish the structural and sedimentary features from reflection seismic data in the frontal part of the accretionary wedge offshore SW Taiwan. Second, we map these features in the study area, and locations of the frontal thrust, frontal anticline (the hanging wall folding structure of the frontal thrust), proto-thrust zone, paleo-slope surface and buried canyons are then constructed in digital form.
The deformation front offshore SW Taiwan separates the accretionary wedge to the east and the China passive margin to the west. Three structural domains can be distinguished on the seismic section MGL0905-01 that runs across the China passive margin and the accretionary wedge: the normal fault zone, the proto-thrust zone, and the thrust zone (Fig. 4). The normal fault zone is characterized by numerous normal faults. These normal faults offset the sedimentary strata in both the shelf and slope settings of the China passive margin, and some large normal faults even crop out on the seafloor. The thrust zone is characterized by a series of folds and thrusts that are considered to be associated with the decollement system. Near the frontal part of the accretionary wedge, a proto-thrust zone can be recognized as the transition between the normal fault zone and the thrust zone (Fig. 4).
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Fig. 5. Uninterpreted (left) and interpreted (right) seismic sections running across the deformation front showing that the accretionary wedge has encroached on top of the passive continental slope (see Fig. 2 for profile locations). The proto-thrusts are observed west of the previously mapped deformation front of Lin et al. (2008) (dotted gray line). The dotted black line is the deformation front suggested in this study that follows the location of the westernmost proto-thrusts.
boundary unconformity (Figs. 6–8). U1 is the deeper unit, characterized by weak and discontinuous reflectors and some locally bright reflectors (Figs. 6–8). U2 is the upper unit and is characterized by a series of deformed sediments with alternations of very bright and lower-amplitude reflectors (Figs. 6–8). Onlapping features and thrusting/folding structures in U2 indicate that the U2 sediments are syn-convergence deposits, whereas the preexisting normal faults and unfolded strata in U1 imply that U1 consists of pre-convergence passive margin deposits (Figs. 6–8). Since the contraction process in our study area is a result of plate convergence, we interpret this boundary unconformity between U1 and U2 as the paleoslope surface before the frontal thrust developed. We correlate the preconvergence passive margin sediments (U1) with Pliocene-Quaternary deposits deriving from the Taiwan Orogen, which has been suggested by previous studies (Chiang et al., 2004; Chou, 1999). While the frontal thrust was developing, younger sediments (U2) started being deposited and then were deformed.
The frontal thrust is defined as the most frontal (westernmost in this study) thrust of the accretionary wedge that branches from the decollement. The deformation front offshore of SW Taiwan can be considered as a boundary that separates the convergent regime to the east and the extensional regime to the west (Yu, 2004), and thus, it is the location of the most frontal (westernmost) contractional structures of the accretionary wedge. The proto-thrust zone is the area between the frontal thrust and the deformation front. The frontal thrust in our study area is blind, although it emerges to near surface level. It has uplifted the frontal anticline and deformed sedimentary strata. Fig. 5 shows that a large frontal fold can be observed on profile MCS681-09, as this is the place where a N-S-trending anticlinal ridge encroaches on the passive continental margin (Fig. 2). The proto-thrusts are recognized west of the frontal thrust with reverse offsets, although the offsets are small, in several seismic profiles in our study area (Figs. 4–6). 4.2. Seismic unit characterization and interpretation
4.3. Location of buried canyons
Two seismic units (U1 and U2) are identified in our study area. These two units are separated by an unconformity that is characterized by a package of bright reflectors, and it can be observed throughout the study area as the lowest continuous feature distinguishable in the data set (Figs. 6–8). The frontal thrust is interpreted to slip along this
Buried canyons are often characterized by high-amplitude and parallel reflectors that terminate on either side of U- or V-shaped discontinuities in reflection seismic profiles (Antobreh and Krastel, 2006;
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Fig. 6. Inline profile 24 of the Palm Ridge 3D seismic cube running across the frontal part of the accretionary wedge. (a) Uninterpreted seismic profile; (b) Interpreted seismic profile showing structural features and the major unconformity that separates seismic units U1 and U2; (c) Enlarged view of the black box in (a) presenting the location of the proto-thrusts. See Fig. 2 for profile location.
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Fig. 7. Interpreted inline profile 1 of the Palm Ridge 3D seismic cube running across the frontal part of the accretionary wedge. The proto-thrusts are observed in shallow sediments of the slope. Clearly onlapping features and folded strata in U2 sediments imply that the sediments of U2 are syn-convergence deposits, and the unconformity separating U1 and U2 may be the paleo-slope surface. Pre-existing normal faults observed in U1 sediments suggest that the sediments of U1 are pre-convergence deposits. See Fig. 2 for profile location.
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Fig. 8. Interpreted inline profile 32 of the Palm Ridge 3D seismic cube running across the frontal part of the accretionary wedge. In addition to the similar structural/sedimentary patterns that are shown in Fig. 6, two clear buried canyon surfaces (green dotted line) are recognized. These two paleo-canyons cut through the major unconformity (interpreted paleo-slope surface, shown as black dotted line) between U1 and U2. See Fig. 2 for profile location.
base of buried canyons, and the frontal thrust and frontal anticline that are mapped from the 3D seismic cube. The buried paleo-canyons lie on the paleo-slope surface and deepen southwestward. These buried paleocanyons cut the pre-convergence sediments (U1) and then were buried by syn-convergence sediments (U2) (Figs. 8 and 9), which implies that the paleo-canyons formed before the development of fold and thrust structures and then were gradually filled by sediments during the synconvergence stage.
Deptuck et al., 2007; Gong et al., 2011; Pratson et al., 1994; Zhu et al., 2010). Based on the characteristics of buried canyons noted above, several buried canyons (paleo-canyons) can be recognized in our seismic profile images west of the frontal thrust and anticline structures (Figs. 5 and 8–10). We have traced the base of the buried canyons in detail through a series of 2D seismic profiles and the 3D seismic cube (See Fig. 2 for data distribution). Fig. 9 shows the geometry of the paleo-slope surface (the major unconformity shown in Figs. 6–8), the
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Fig. 9. Perspective view within the 3D seismic block showing the geometry of the main faults and horizons. The shown horizon “frontal anticline” (blue) is the top of the frontal anticline that has been deformed by the frontal thrust. The recognized paleo-canyons cut the paleo-slope surface (the major unconformity that is shown in Figs. 6–8) and trend roughly in NE-SW direction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to better constraints due to more seismic profile data are available for Lin et al. (2008) than for previous studies, another reason for this difference is that the previous studies placed the deformation front at the frontal ramp anticlines or the frontal thrusts. However, the detection of proto-thrusts, which are expressions of contractional strain and are located farther west of the deformation front suggested by Lin et al. (2008), implies that the compressional stress has propagated into that region. Since the deformation front is defined as the location of the most frontal contractional structures along a convergent plate boundary, we thus suggest that the location of the deformation front should be placed farther west to where the westernmost proto-thrusts lie in our study area (Figs. 5 and 11).
4.4. Distribution of structural and sedimentary features With the densely distributed 2D seismic profiles and a 3D seismic volume, we identify several important structural/sedimentary features in our study area. A map that shows the main structural and sedimentary features is constructed (Fig. 11). The features shown in this map include the western boundary of the proto-thrusts (the location of the new deformation front defined by this study), the previously mapped deformation front by Lin et al. (2008), and the paleo-flow directions of the buried canyons.
5. Discussion 5.1. Westward advance of the deformation front
5.2. Canyon evolution in the rifted SCS slope
Lin et al. (2008) has placed the deformation front farther west than those suggested by previous studies (Lacombe et al., 1997, 2001; Lee et al., 1995; Liu et al., 1997, 2004; Yu, 2004), at the lowest and flat west limb of the frontal anticline from seismic reflection profiles. In addition
Based on the 3D seismic cube and densely distributed 2D seismic profiles (Fig. 11), we have mapped the locations of buried canyons, and the connection of the buried canyon thalwegs represents the flow directions of these paleo-canyons (Fig. 11). The heads of the buried
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Fig. 10. Uninterpreted (upper) and interpreted (lower) seismic line MCS958-14 running transverse to the strikes of major structures such as the deformation front and the frontal thrust. The paleo-canyon surfaces can be recognized west of the frontal thrust and frontal anticline structures. See Fig. 2 for profile location.
before tectonic loading and canyon incising. In the first stage, several NE-SW-trending normal faults developed and started down-slipping in the study area, probably due to loading from the Taiwan mountain belt (Suppe, 1984; Byrne and Liu, 2002; Huang et al., 2000). In the second stage, as the normal faults caused down-slip of the passive margin blocks, topographic lows were created along the fault outcrops, guiding the canyon thalwegs to run transverse to the slope dip. Similar cases of normal fault-controlled submarine canyons have also been suggested by several studies (Geist et al., 1988; Shepard, 1981; Thornburg et al., 1990). From stages three to four, as the accretionary wedge was developing, the tectonic setting here was changed from extensional to contractional, the normal faults stopped slipping and the canyons began
canyons start on the slope near the shelf break and extend southwestward along the slope, which is transverse to the slope dip and is very different from the present down-slope canyon path directions of NS or NNW-SSE (Fig. 11). Compared with the present-day canyon directions that approximately follow the slope dip (Fig. 1), these two paleo-canyon paths that are transverse to the slope dip seem very unusual. Thus, in terms of canyon evolution, the effects of fault activities cannot be neglected, in addition to the contribution of down-slope processes. We propose a conceptual 6-stage canyon evolution model to illustrate how the canyons have evolved through time in this convergent plate boundary (Fig. 12). The initial stage shows the original China continental slope
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Fig. 11. Map showing the location of the deformation front (black line with teeth). The gray line with teeth indicates the previously mapped deformation front by Lin et al. (2008). The white dotted arrow indicates the possible thalweg of paleo-canyons mapped from 2D seismic profiles (gray lines) and confirmed in the 3D seismic block image (black box).
infilling. In the present day, we can observe that several proto-thrusts have developed and the canyon courses have changed back to the down-slope direction. In terms of canyon evolution, the southwestward migration of the canyons observed from previous studies (Lee et al., 1995; Fuh et al., 1997, 2003; Yu and Hong, 2006) suggest that tectonics prevail over the down-slope processes in the active Taiwan submarine accretionary wedge. In the China passive margin, the down-slope process should generally be dominant, although some of the canyon thalwegs actually run along troughs that were created by normal fault activities. In the transition from passive to active margin, down-slope processes, probably accompanied by normal fault activities as the key factors that controlled the canyon paths, might be gradually replaced by active tectonic processes while the accretionary wedge advanced westward.
(1) The features of a proto-thrust zone are observed in the transition between the normal fault zone and the thrust zone. This observation confirms the westward migration of the compressional stress field as the oblique Taiwan arc-continent collision propagated southward. (2) The proto-thrust zone is recognized west of the previously proposed deformation front. As these proto-thrusts are the most westward contractional structures that can be recognized in the study area, we suggest moving the location of the deformation front to where the westernmost proto-thrusts lie. (3) Paleo-canyon paths show drastically different directions (transverse to the slope dip) from the present canyon paths (down-slope). Our canyon evolution model suggests that the canyon directions have been changed due to the westward migration of the deformation front.
6. Conclusions
Acknowledgements
In this convergent plate boundary offshore of SW Taiwan, westward migration of the compressional stress field has turned previously extensional field into compressional one and changed submarine canyon paths significantly. This study provides an example to show how tectonic activities can affect submarine canyon paths in a transitional setting between the active and passive margins offshore SW Taiwan and may have implications for other areas with similar geologic settings worldwide. Meanwhile, we have identified several features from seismic images that are related to specific interesting geological processes in such a transitional setting in the upper reach of Penghu Canyon. We summarize our results as follows:
We would like to thank Yun-Shuen Wang, San-Hsiung Chung and the Central Geological Survey, Ministry of Economic Affairs, for the projects that made this study possible. The captains and crew of the OR1, technicians of the OR1 Instrumentation Center, and S.D. Chiou of the Institute of Oceanography, National Taiwan University (IONTU), are thanked for helping to collect the seismic data. We acknowledge the teamwork of the Seismic Exploration Laboratory, IONTU. We also grateful to SMT’s Educational Gifts Program for providing the KINGDOM software. This research was accomplished through grants from the Central Geological Survey, MOEA contracts: 103-522690400003-01 and 104-5226904000-02-01.
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Fig. 12. Conceptual model illustrates the processes that affect the canyon paths in our study area. The initial stage shows the original China continental slope before tectonic loading and canyon incising. In the first stage, NE-SW-trending normal faults developed and started down-slipping in the study area (first stage), and this process guided the paleo-canyons to flow along the topographic low (second stage). From stages three to four, the activity of the extensional faults gradually stopped, and then the canyons were abandoned and filled by incoming sediments. In the present day, as the accretionary wedge encroaches upon the passive continental margin, the paleo-canyons have been buried, and some proto-thrusts are developed.
Canyon in the Qiongdongnan Basin, northwestern South China Sea: architecture, sequence stratigraphy, and depositional processes. Mar. Pet. Geol. 28, 1690–1702. Harris, P.T., Whiteway, T., 2011. Global distribution of large submarine canyons: geomorphic differences between active and passive continental margins. Mar. Geol. 285, 69–86. Huang, C.-Y., Wu, W.-Y., Chang, C.-P., Tsao, S., Yuan, P.B., Lin, C.-W., Xia, K.-Y., 1997. Tectonic evolution of accretionary prism in the arc-continent collision terrane of Taiwan. Tectonophysics 281, 31–51. Huang, C.-Y., Yuan, P.B., Lin, C.-W., Wang, T.K., Chang, C.-P., 2000. Geodynamic processes of Taiwan arc–continent collision and comparison with analogs in Timor, Papua New Guinea, Urals and Corsica. Tectonophysics 325, 1–21. Jiao, J., Negut, D., Link, B., 1999. Multiple attenuation using eigenvalue decomposition. In: SEG Technical Program Expanded Abstracts 1999. Society of Exploration Geophysicists, pp. 1052–1055. Karig, D.E., Lundberg, N., 1990. Deformation bands from the toe of the Nankai accretionary prism. J. Geophys. Res.: Solid Earth 95, 9099–9109. Ku, C.-Y., Hsu, S.-K., 2009. Crustal structure and deformation at the northern Manila Trench between Taiwan and Luzon islands. Tectonophysics 466, 229–240. Lacombe, O., Angelier, J., Chen, H.-W., Deffontaines, B., Chu, H.-T., Rocher, M., 1997. Syndepositional tectonics and extension-compression relationships at the front of the Taiwan collision belt: a case study in the Pleistocene reefal limestones near Kaohsiung, SW Taiwan. Tectonophysics 274, 83–96. Lacombe, O., Mouthereau, F., Angelier, J., Deffontaines, B., 2001. Structural, geodetic and seismological evidence for tectonic escape in SW Taiwan. Tectonophysics 333, 323–345. Lee, T.Y., Hsu, Y.Y., Tang, C.H., 1995. Structural geometry of the deformation front between 22 N and 23 N and migration of the Penghu Canyon, offshore southwestern Taiwan arc-continent collision zone. In: International Conference and 3rd SinoFrench Symposium on Active Collision in Taiwan, pp. 219–227. Leggett, J., Aoki, Y., Toba, T., 1985. Transition from frontal accretion to underplating in a part of the Nankai Trough accretionary complex off Shikoku (SW Japan) and extensional features on the lower trench slope. Mar. Pet. Geol. 2, 131–141. Lin, A.T., Liu, C.-S., Lin, C.-C., Schnurle, P., Chen, G.-Y., Liao, W.-Z., Teng, L.S., Chuang, H.-J., Wu, M.-S., 2008. Tectonic features associated with the overriding of an accretionary wedge on top of a rifted continental margin: an example from Taiwan. Mar. Geol. 255, 186–203.
References Adam, J., Klaeschen, D., Kukowski, N., Flueh, E., 2004. Upward delamination of Cascadia Basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux. Tectonics 23, TC3009. http://dx.doi.org/10.1029/ 2002TC001475. Antobreh, A.A., Krastel, S., 2006. Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: a newly discovered canyon preserved-off a major arid climatic region. Mar. Pet. Geol. 23, 37–59. Byrne, T.B., Liu, C.-S., 2002. Geology and geophysics of an arc-continent collision, Taiwan. Geological Society of America Special Paper, vol. 358. Boulder, Colorado, p. 211. Chiang, C.-S., Yu, H.-S., Chou, Y.-W., 2004. Characteristics of the wedge-top depozone of the southern Taiwan foreland basin system. Basin Res. 16, 65–78. Chou, Y.W., 1999. Tectonic framework, flexural uplift history and structural patterns of flexural extension in Western Taiwan Foreland Basin. Ph.D. thesis National Taiwan University 125 p. (in Chinese). Clark, I.R., Cartwright, J.A., 2011. Key controls on submarine channel development in structurally active settings. Mar. Pet. Geol. 28, 1333–1349. Cochrane, G.R., Moore, J.C., MacKay, M.E., Moore, G.F., 1994. Velocity and inferred porosity model of the Oregon accretionary prism from multichannel seismic reflection data: implications on sediment dewatering and overpressure. J. Geophys. Res.: Solid Earth 99, 7033–7043. Deptuck, M.E., Sylvester, Z., Pirmez, C., O’Byrne, C., 2007. Migration–aggradation history and 3-D seismic geomorphology of submarine channels in the Pleistocene Beninmajor Canyon, western Niger Delta slope. Mar. Pet. Geol. 24, 406–433. Fuh, S.C., Liang, S.C., Wu, M.S., 2003. Spatial and temporal evolution of the PlioPleistocene submarine canyons between Potzu and Tainan, Taiwan. Petrol. Geol. Taiwan 36, 1–18. Fuh, S.C., Liu, C.S., Wu, M.S., 1997. Migration of canyon systems from Pliocene to Pleistocene in area between Hsyning structure and Kaoping Slope and its application for hydrocarbon exploration. Petrol. Geol. Taiwan 31, 43–60. Geist, E.L., Childs, J.R., Scholl, D.W., 1988. The origin of summit basins of the Aleutian Ridge: implications for block rotation of an arc massif. Tectonics 7, 327–341. Gong, C., Wang, Y., Zhu, W., Li, W., Xu, Q., Zhang, J., 2011. The Central Submarine
13
Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx
W.-C. Han et al.
arc spreading near Taiwan. Memoir Geol. Soc. China 6, V33. Thornburg, T.M., Kulm, L.D., Hussong, D.M., 1990. Submarine-fan development in the southern Chile Trench: a dynamic interplay of tectonics and sedimentation. Geol. Soc. Am. Bull. 102, 1658–1680. TuZino, T., Noda, A., 2007. Tectonic control over topography and channel sedimentation across the forearc slope of the southern Kurile Trench. Geo-Mar. Lett. 27, 1–11. Yu, H.-S., 2004. Nature and distribution of the deformation front in the Luzon ArcChinese continental margin collision zone at Taiwan. Mar. Geophys. Res. 25, 109–122. Yu, H.-S., Chang, J.-F., 2002. The Penghu submarine canyon off southwestern Taiwan: morphology and origin. Terrest. Atmosph. Ocean. Sci. 13, 547–562. Yu, H.-S., Hong, E., 2006. Shifting submarine canyons and development of a foreland basin in SW Taiwan: controls of foreland sedimentation and longitudinal sediment transport. J. Asian Earth Sci. 27, 922–932. Yu, H.-S., Lee, J.T., 1993. The multi-head Penghu submarine canyon off southwestern Taiwan: morphology and origin. Acta Oceanogr. Taiwanica 30, 10–21. Zhu, M., Graham, S., Pang, X., McHargue, T., 2010. Characteristics of migrating submarine canyons from the middle Miocene to present: implications for paleoceanographic circulation, northern South China Sea. Mar. Pet. Geol. 27, 307–319.
Liu, C.-S., Deffontaines, B., Lu, C.-Y., Lallemand, S., 2004. Deformation patterns of an accretionary wedge in the transition zone from subduction to collision offshore southwestern Taiwan. Mar. Geophys. Res. 25, 123–137. Liu, C.-S., Huang, I.L., Teng, L.S., 1997. Structural features off southwestern Taiwan. Mar. Geol. 137, 305–319. Liu, C.-S., Liu, S.-Y., Lallemand, S.E., Lundberg, N., Reed, D.L., 1998. Digital elevation model offshore Taiwan and its tectonic implications. Terrest., Atmosph. Ocean. Sci. 9, 705–738. Mountjoy, J.J., Barnes, P.M., Pettinga, J.R., 2009. Morphostructure and evolution of submarine canyons across an active margin: cook Strait sector of the Hikurangi Margin, New Zealand. Mar. Geol. 260, 45–68. Nittrouer, C.A., Wright, L.D., 1994. Transport of particles across continental shelves. Rev. Geophys. 32, 85–113. Pratson, L.F., Ryan, W.B., Mountain, G.S., Twichell, D.C., 1994. Submarine canyon initiation by downslope-eroding sediment flows: evidence in late Cenozoic strata on the New Jersey continental slope. Geol. Soc. Am. Bull. 106, 395–412. Shepard, F.P., 1981. Submarine canyons: multiple causes and long-time persistence. AAPG Bull. 65, 1062–1077. Suppe, J., 1984. Kinematics of arc-continent collision, flipping of subduction, and back-
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