Mapping Geomorphic Features

Shaded relief maps and relevant 3-D models show abundant geomorphic features associated with recent movement of the active structures in the study area (Figs. 3 and 8). The features include the terrace surfaces, their related rivers, fault scarps, and drainage channel networks developed in response to faulting and folding. The mapped terraces are unevenly distributed on both sides of the Touchien river and are mostly preserved along its southern bank. Based on restorations of the terrace surfaces, they can be divided into five steps (I, II, III, IV, & V in Fig. 4) instead of ten steps as suggested by previous studies (Shih et al., 1985; Chang et al., 1999). The elevations of individual terraces span a wide range where they have been deformed by active structures. Areas of higher terraces are characterized by closely spaced drainage networks and highly dissected terrace surfaces (Figs. 4 and 5). Tilting and back-tilting of terrace surfaces along the long terrace scarp are both common, especially terraces III and IV.

Besides, the fault scarps cutting the terraces, we also mapped the scarps along the mountain-fronts based on their sinuosity and orientation parallel to the regional structural trend (Fig. 4). The terrace risers developed without tectonic relations were

are clearly related to the major fault systems. The first group is distributed around the surrounding area of the previously reported trace of Hsinchu Fault and characterized by several rows of discontinuous scarps, trending ENE (Figs. 4, 6A and 6B). In the southeastern part of the study area, the second group shows two unique traces along the Hsincheng Fault and its associated back-thrust, separating the terrace II, III, and IV, trending NE (Figs. 4 and 6A to 6E). While the third one is striking NW and characterized by two to three continuous and parallel scarps. Also it bounds the terraces and tilts the outer edges backwards (Figs. 4 and 7a to 7f). It obviously bisects the known fault systems, indicating more than two fault systems interplaying over here. The same as the geomorphic surfaces, they are mainly distributed in the southern bank of the Touchien river.

Major rivers are analyzed to examine the responses of the active structures (Fig.

3). Along the Keya river, meandering channel appears on both of hanging walls of the Hsinchu and Hsincheng Faults, and on the back-limb side of the Chingtsaohu anticline. To the south a small stream, Hsingkung river, the same pattern occurs at the small anticline close to the Hsinchu Fault. Upstream across the hinge line of the Chingtsaohu anticline only shows a change of flow direction. With respect to the Hsincheng Fault, promising feature is that the channels of the Touchien river and its tributaries both show the deflections of flow directions across fault traces including the back-thrust. All these features reflect the studied structures are recently active.

Fault-Related Folds

The Chingtsaohu anticline can be clearly identified by folding of the terrace V surface (Figs. 6A and 8). Its recent movement results the meandering and deflection along the Keya and Hsingkung rivers (Fig. 3). The fold-axis derived by the folded terrace trends ENE, parallel to the trace of the Hsinchu Fault. Folding in younger deposits across terraces IV and III is not recognizable at the resolution of the digital elevation model. Surfaces on these terraces instead dip north and may record the geometry of the plunging tip of the anticline, rather than its fore- and back-limbs.

The Poshan anticline, located in the southeastern part of the study area, can only be roughly recognized because of poorly preserved surfaces (Figs. 6A and 6B).

Except for this, no other gomorphological evidence related to its recent movement is found. In addition, two small anticlines are found obliquely attached on the Hsinchu Fault. They are probably related to the strike-slip component of the Hsinchu Fault.

Fault Systems

Boundaries between geomorphic surfaces are typically defined by abrupt

changes in elevations, where single or multiple scarps are preserved (Figs. 3, 4 & 9).

The Hsinchu Fault system partitions slip across at least two fault-stands in western part of the area we mapped (Fig. 4) and splits into numerous discontinuous, subparallel scarps further east near the Touchien river valley (Figs. 6A to 6C). Its recent movement results in meandering developed along the Keya river (Fig.3). The highest terraces are vertically offset over 90 m while the lowest one is offset only less than 10 m (Figs. 4, 6B, and 9a). Given out observation that the Hsinchu Fault offsets the youngest fluvial deposits along the modern flood plain of the Touchien river suggests this fault has recent slip history of a number of seismic events, and that it should be classified as the active fault (Figs. 8 and 9a). The scarps are abruptly terminated at the southern channel wall of the Touchien river. There is only few unclear evidence exists across the northern bank of the Touchien river for recent displacement of young deposits (Fig. 4). We interpret this to indicate that a wrench fault may be burial in the river valley. Another feature in the west, the hill front, formed by up-throwing the hanging wall of the Hsinchu Fault shows an indentation at the Keya river (Figs. 3, 4, and 8b). This probably implies that the wrench fault has been developed and torn apart the Hsinchu Fault. In addition, two topographic highlands are identified as pressure ridges since they only occurred along the suggested fault trace (PR1 & PR2 in Figs. 4 and 6B).

The Hsincheng Fault is defined by relatively simpler scarp that offsets the terraces from IV to I with decreasing scarp heights from 58 to 5 m. Considering the synchronous terrace, its offset is significantly larger than of the Hsinchu Fault.

Associated channel network of the Touchien river strongly reflects that it is recently active (Fig. 3). Recent surface ruptures are well exposed along the Hsincheng Fault where it offsets terrace I deposits (Fig. 9b). It is worthy to note that the scarps parallel or subparallel to the Hsincheng Fault are clear only for ~4 km along its length, which is relatively short compared to its total length measured from geological map.

Recent movement of this active segment also deflects the modern drainage network, especially the Touchien river system (Fig. 3). The fault scarp is not readily apparent to the south and north. A back-thrust is clearly shown in each profile in the hanging wall of the Hsincheng Fault, which has not been previously recognized (Fig. 6).

This subsidiary fault also affects the stream channel development in the highlands south of the main trace of the Hsincheng Fault (Fig. 3). The topographic highland that defines the NW trending pressure ridge (PR2 on terrace IV, Fig. 6B) is bound by the Hsincheng Fault and the associated back-thrust. This orthogonal structural

The wrench fault system mentioned above is composed of two to three parallel but discontinuous surface traces (Figs. 4 and 7). They are all generated from the Hsinchu Fault in the west and all the way truncating the Hsincheng Fault, then reaching the eastern end of the study area. It is very difficult to distinguish this structure using available seismic profiles since the profiles are deep ones worked out for petroleum exploration. The field investigation also failed to find geologic outcrop showing faulting evidence. Nevertheless, in terms of geomorphic features, pressure ridges, back-tilting terraces and pull-apart depressions (albeit they are thought being initiated by bending moment tensile stress associated with the Hsinchu Fault) presented in this study, such a wrench fault system possibly exists. It is characterized by two to three long scarps trending NNW to SSE, which were thought being terrace risers because of their orientations parallel to modern river (Fig. 4).

5. Discussion

New Wrench Fault System and Stress Field Change

We would like to note again that the scarp distribution, both of the Hsinchu and Hsincheng Fault system, becomes unclear across the Touchien river to its northern bank. In contrast, the Hsincheng Fault shows clear features within only a few kilometers in the southern bank (Fig. 3). This implies the shortening of this segment of the Hsincheng Fault is reinforced by the action of the wrench fault system. Since the Hsinchu and Hsincheng are both old fault systems, the newly found wrench fault system might be recently developed and caused by stress turning. As we know, the Hsinchu area is located in a place where the stress changes its orientation (Fig. 1a).

Previous study also addressed this point based on geometry of the major fault systems (Huang, 1987). The collision history of Taiwan suggests that the mountain belt was initiated in the north and propagated to the south (Suppe, 1984; Teng, 1990).

Westward propagation of extensional structures associated with the Okinawa Trough is now interacting with relatively slowly contracting thrust belt in the northern Taiwan.

The study area is contained in the area of abruptly rotated stress fields, provided by the interaction of the thrust belt and the Okinawa rifting. The complex array of thrust faults and strike-slip faults therefore accommodate slow westward propagation of the thrust belt and dextral shear as wrench faults transfer lessened shortening towards the extensional fault systems to the north. Abrupt terminations of the thrust faults and folds we mapped in addition to NW-trending wrench faults support these assumption. Nevertheless, the geomorphic data presented in this study suggest the faults are all active, indicating the shortening wanes but still presents in the study area.

Faulting B eha vi or

As the previously reported, the Hsinchu Fault is a thrust with dextral strike-slip component (Meng, 1965; Tang, 1968; Chang et al., 1998; Lin et al., 2000). The topographic depressions identified in the hanging wall are the structural grabens commonly created by bending moment tensile stresses and occurred along

thrust-associated anticline (Fig. 4; Philip and Meghraoui, 1983). However, currently they are closely attached on the possible trace of the wrench faults. We therefore suggest that they are initiated by Hsinchu Fault but latterly controlled by newly developed wrench fault system. By the graben orientation the dextral slip of Hsinchu and wrench fault is clear. If we further treat the wrench fault as the

antithetic structures, it follows the conclusion very well. However, except for these large-scale features, we did not find much meso-scale geomorphic features showing significant dextral slip. It is probably that the Hsinchu Fault is still dip-slip

dominant. On the other hand, the small anticlines, A2 and A3 in Figure 4 are also suggested to form under the dextral component of the thrust system.

The Hsincheng Fault has also been previously identified as a pure thrust. The field evidence also supports this (Fig. 9b). However, the geomorphic evidence reveals that it was dextrally moved (Fig. 8). As shown in Figures 3, 4, & 8, there is a series of en echlon short scarps generated from the main fault toward due east. We believe the recent fault-slip contains also dextral component, though it was dominated by pure thrusting in early stage of geological time. The hanging-wall back thrust shows simple scarps in the field, indicating it is dip-slip dominant.

Major Active F aults

The structure framework in our study area is built up by dual thrusts and their hanging wall developed anticline (i.e., fault-bend fold). Some of the structures are active but not entirely based on the active landform presented in this paper. For the Hsinchu Fault, the active segment is only the southwestern half, while the Hsincheng Fault has even shorter active segment, a few kilometer in the southern bank of the Touchien river. It is very difficult to explain that the rather short active segment of the Hsincheng Fault if we consider a major seismogenic active fault. In contrast, the newly found wrench fault system is generated from the northern end of the active segment of the Hsinchu Fault, then continuously showing the geomorphic evidence to the eastern border of the study area. We, hence, prefer to treat this wrench fault and the southern segment of the Hsinchu Fault both are the essential active fault systems.

al., 1994; 1996), also indicating a structure termination along the Touchien river.

Slip Rat e

Since no paleoseismology and terrace age are currently available, it is

impossible to derive the slip rates on the short-term time scales. In particular, all the studied faults have significant strike-slip component, the toughest part to determine.

In this paper, we tentatively calculate the long-term dip-slip rate of the Hsinchu and Hsincheng Faults based on the dips and vertical separation of young deposits offset by them and the age of those deposits. As revealed in the profile of Figure 2b,

accumulated dip-slips on Hsinchu and Hsincheng Fault are 900 m and 1400 m, respectively, read out from the offsets of the boundary between Chinshui shale and its overlying Cholan Formation. However, the acting time is unable to derive because of indiscernible sub-units in the youngest sedimentary unit, Toukoshan Formation, which started in 2 Ma. We therefore tentatively compute a minimum rate by

assumption of that the commencing time is the end of the Cholan Formation, ca. 2 Ma, due to the identical thickness measured across the fault planes prior to that time.

Derived dip-slip rates are 0.45 m and 0.7 m per kyr for Hsinchu and Hsincheng, respectively. Even we double the rates, they are still much smaller than the

Chelungpu Fault, the earthquake fault of 1999 Chichi earthquake (Chen et al., 2001).

Unless the strike-slip component is predominant in this two thrust fault systems, they seems relatively stable. Another evidence generally adopted in Taiwan is the youngest lateritic terrace surface. In study area the youngest lateritic terrace is terrace III, generally with a relative height of 40 to 90 m. This height is much lower than the youngest lateritic surface documented in central Taiwan, also indicating a relative low uplifting rate.

6. Conclusions

1. A wrench fault system is newly identified along the Touchien river by geomorphic features, which is considered as a result of the recent change of the stress field.

2. The wrench fault system and the southern segment of the Hsincheng Fault are the major active faults. The hanging wall anticlines, back-thrust and even the Hsincheng Fault are all subsidiary active structures.

3. Both of the active segments on the Hsinchu and Hsincheng Fault show recently dextral slip. The wrench fault system is also dextral.

4. The slip rates of both Hsinchu and Hsincheng are relatively lower than of the central Taiwan.

Acknowledgement

This study is financially supported by National Science Concil, R.O.C under grant number of NSC 91-2119-M-002-010. We also thank the Central Geological Survey and Central Weather Bureau, R.O.C. for their support of field work and the courtesy of seismicity data.

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