A couple of fault zones are particularly critical in the interpretation of Taiwan's structural history. I present studies of two such zones; see the geologic terrane map for fault locations (figure 3).
| Lee et al. (1997) present an excellent analysis of the Lishan Fault. The Lishan Fault separates the Hsuehshan terrane from the Central Ranges and has previously been interpreted as a normal fault that separated sub-basins in the South China Sea or as the suture between Gutiawan (Tananao complex) and rocks deposited in the South China Sea. |
The fault separates Oligocene sandstone and slate of the Hsuehshan terrane on the west from Miocene slate of the Central Ranges on the east. These are all sediments of the South China Sea. Metamorphic grade increases from west to east across the fault except for a small break in this increase directly over the fault. The fault is east-vergent to vertical in some localities (figure 12) and exhibits overprinting of structures of many different orientations on and near the fault. A paleostress inversion was performed on these structures to try and unravel the many different events that created them. This inversion takes structure orientations and cross-cutting relationships as input and creates a best-fit stress history that lumps structures of like orientations and ages in distinct groups. The results of this inversion are shown in figure 13.
Figure 12 (left): Serial cross-sections across the Lishan fault. Ss = Szeleng Sandstone; LF = Lishan Fault; Lu = Lushan Formation. After Lee et al. (1997).
| Early history of the fault shows weak extensional events in many directions (figure 13 a and b). Subsequently, a stronger NW-SE extension affected structures just off of the Lishan fault (figure 13 c). This event was followed by strong compression in the NW-SE direction (figure 13 d and e). A late extension in the NNW-SSE direction (figure 13 f) may be related to syn-compression extension or later back-arc spreading of the Okinawa trough. |
The westward dip of the fault is somewhat incongruent with the Lishan fault's history as principally a reverse fault. However, Lee et al. (1997) interpret this dip as due to the flower-like structure of the Hsuehshan Range (figure 14). Lee's model explores the likelihood that the Lishan fault is actually a west-vergent thrust fault with an early extensional history and subsequent development into a flower-like structure with late extension due to uplift.
Figure 13 (right): Evolution of paleostress regimes across the Lishan fault. Stereoplots are Schmidt's projection, lower hemisphere. After Lee et al. (1997).
|Figure 14: Tectonic evolution of the Lishan fault since the Paleogene. After Lee et al. (1997).|
The other major fault zone and source of much dispute is the Longitudinal Valley which separates the Central Range from the Coastal Range in eastern Taiwan (figure 3). A recent analysis of the Lichi Melange in this valley was completed by Chang et al. (2000).
Previous studies interpreted the Lichi Melange as an olistostrome or as part of a subduction complex on the western side of the volcanic arc remnant from the subduction of the South China Sea. The Lichi Melange contains sediments from both coastal Chinese and Luzon arc provenance. It is quazi-conformable with rocks of the Eastern Central Range (Takangao formation).
|Figure 15: Structural evolution of one section of the Lichi Melange. The initial configuration for the bottom of the forearc basin is considered a simple horizontal plane. Geometrical restoration was carried out based on consideration of length of bedding interfaces. Because of the lack of subsurface data in the area, decollement surfaces could not be located. To account for compaction and water escape in compressed mudstones, and also for non-planar shear deformation, a surface reduction of about 20% is applied between stage 1 and stage 5. After Chang et a. (2000).|
A structural analysis of the melange, though not definitive, shows that structures in the melange have accommodated a minimum of 10 km total shortening (figure 15). Chang et al. (2000) point out that to understand the origin of the melange and the shortening it accommodated, one only needs to look offshore of southern Taiwan at the most active part of the orogenic belt. This region contains a fore-arc basin not previously represented in the interpretation of the stratigraphy of Taiwan. They interpret the Lichi Melange as a remnant of the sediments once contained in a forearc basin between the Coastal Range and the Western Central Ranges. This basin collapsed upon the growth of the Hsuehshan and Central Ranges (interpreted as the accretionary prism) as shown in figure 16.
|Figure 16: Structural evolution of the Lichi Melange and the Longitudinal Valley fault zone. Profiles are based on the structural evolution represented at different latitudes in the Taiwan orogeny (upper right corner). To illustrate the structural evolution better, the topography and geographic distribution of the Lichi Melange in profile C were exaggerated. After Chang et al. (2000).|
The authors admit that ophiolites contained within the melange may be somewhat incongruent with their interpretation, but they argue that the ophiolites could have been back-thrusted from the accretionary wedge (Chang et al., 2000). Alternatively, they could also have been upthrust from the oceanic crust that presumably underlined the basin when it collapsed.
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