Taiwan mountain belt - 應力場變化對於三維裂隙系統演化的影響：以台灣東北部為例

According to previous works, the crustal thickness decreases from > 33km to <

18km affiliated with the topographic drop from northern Taiwan to southern Ryukyu (Wu, 1978; Lee et al.,1980; Yen et al., 1995). Moreover, the formation of Okinawa Trough was associated with the Pliocene-Quaternary andesites of orogen strata.

Extensional structures resulted from actively numerous submarine volcanoes located in the southwest Okinawa Trough (Lee and Wang, 1988) cut across the compressional structure by the andesites. Therefore, the Taiwan mountain belt involves the kinematic evolution of the different orogenic processes that evidential for the transition of the EP continental to PSP oceanic subduction from south to north of NE Taiwan (Malavieille et al., 2019).

Figure 2.1: 3D model of the tectonic setting of Taiwan. In southwestern Taiwan, the oceanic crust of the South China Sea (2) subducted eastward under the PSP and created LVZ (4) along the transition area China continental margin. In eastern Taiwan, the northwestward movement of the western edge of PSP (3) collided with LVZ and overrode the EP (5) in the south, whereas it subducted beneath EP in the north along the Ryukyu arc (6) of NE Taiwan. (Malaivielle et al., 2019).

12 2.1.2 Kinematics in NE Taiwan

NE Taiwan, where is experienced in different tectonic environments from the collision in the south to extension in the north (Fig 2.2), indicated that the stress regime has evolved from the southern to the northern part (Wu et al., 2008). The collision primarily constructed orogen and explained the existence of the thrust sheet, fold, and other compressional structures. Teng et al. (2000) supposed that the Luzon arc in the early Pliocene overrode the continental margin rapidly during the movement of PSP-EP collision system. As the eastward subduction of the continent margin, the collision the extensional process, which is highly connected with the post-orogenic collapse. The break-down of NE Taiwan orogen is referenced by the reduction of topography in Ilan Plain and the eastern of oceanic crust of southern Okinawa Trough. The fast rifting caused the southward bending of the mountain belt in nearly E-W striking (Hsu et al., 2009). Therefore, numerous extensional and compressional focal mechanisms were recorded and addressed in northeastern Taiwan orogen (Wu et al., 2008; Huang et al., 2012) (Fig 2.2).

The compressional stress regime (thrust and strike-slip faulting) is dominant in most of Taiwan (Yeh et al., 2014) (Fig 2.2). However, local tectonic variation in NE Taiwan induced the different distribution of the stress regime. In particular, Wu et al.

(2008) and Wu et al. (2010) suggested the focal mechanism range from thrust and

strike-slip faulting in the Hualien to normal faulting in the transition area and Ilan Plain.

Especially, Wu et al. (2008) indicated that the trend of the tensile axis near Ilan Plain is close to NW-SE trending, whereas the NW-SE orientation of the compression axis is dominant in the southern part of the Heping area (Fig 2.2). Therefore, it is significant to evaluate the evolution of the stress pattern in NE Taiwan.

Huang et al. (2012) determined regional stress patterns at NE Taiwan from the focal mechanism in NE Taiwan by relocated EQ events with magnitude >4.0 (depth

<30km) (Fig 2.3). Particularly, six different stress groups were identified from 169 focal mechanisms. Within them, three groups are associated with our studies. Firstly, the thrust faulting regime with NW-SE principal stress ( 1) is dominant in the Hualien area (Domain III), which is displayed by the red circle. In the Heping area (Domain II), Huang et al. (2012) found the two different clusters of strike-slip faulting regimes, the first strike-slip faulting in the southern Ilan Plain (Domain I) has 3 orientation consistent with the opening direction of Okinawa Trough, whereas 1 orientation of the second strike-slip faulting regime in the Heping area might have corresponded with arc-continent collision. The light green square stress regime (Fig 2.3) of the Heping area represented the earthquake events with an NNW-SSE orientation of 3 and an ENE-WSW orientation of 1. Meanwhile, in southern Ilan Plain, the light green triangular stress regime (Fig 2.3) has a trans-tension stress pattern.

Figure 2.2: The kinematics of Taiwan island from stress tensor inversion. Surface projection of the 1 and 3 axes (left) and the different principal axes in the equal-area projection of the lower hemisphere (Wu et al., 2008).

Figure 2.3: The evolution of the stress state in NE Taiwan. The result of the focal mechanism analysis from Huang et al. (2012) emphasized the stress state evolution in Northeastern Taiwan. In this figure, the normal-faulting focal mechanism cluster illustrated by the blue dots is dominant in Ilan Plain. High density of reverse-faulting cluster was discovered in Hualien county with the red dots and the strike-slip faulting cluster highlighted by green dots located in the central part of study area.

Geological Background

Taiwan orogen can be classified by six narrow geologic units parallel to the long axis of the island (Fig 2.4A). From west to the east, they comprised of the Coastal Plain (CP), the Western Foothill (WF), the Hsueshan Range (HR), the Central Range (CeR);

the Longitudinal Valley (LV), and the Coastal Range (CoR). Inside, CeR contains the Tananao Schist (TC) (Ho, 1988). The Tananao Schist is the oldest metamorphic basement of Taiwan, experienced several phases of orogenic deformation, magmatism, and metamorphism (Wang, 1966; Ernst et al., 1983; Stanley et al., 1981). Especially, the higher metamorphic grade that occurred in the northeastern part of Tananao schist might be caused by the strong influences of the tectonic evolution of Taiwan.

Furthermore, Tananao Schist can be divided into two metamorphic belts: the western Tailuko belt and the eastern Yuli belt (Ho, 1986). This research addressed the northeastern Tailuko belt (red box, Fig 2.4B).

NE Tailuko belt is mainly composed of Pre-Tertiary metamorphic rock such as pelitic schist, gneiss, migmatite, metamorphosed limestone, greenschist, and amphibolite. The detail will be described below.

Figure 2.4: (A) Geological units of Taiwan (Ho, 1988), (B) The Geological map of NE Taiwan (Geological Central Survey, 1995).

Table 2.1: Lithological units in the study area (Central Geological Survey, 1995).

Stratigraphical division Lithology

C E N OZO IC

Nansuao formation Slate; Metastone and slate in alternation Terrance deposit Gravel; sand and mud

Alluvial deposit Gravel; sand and mud

Suao Formation Slate and argillite with a thin bed of metastone

LA TE M ES OZO IC

Yuantoushan gneiss Gneiss or metamorphosed granodiorites

Metabasite Metabasite

Kangyang gneiss Green-greenish granite and gneiss Dekeli gneiss Green-greenish granite and gneiss Fanpaochienshan gneiss Gneiss

Amphibolite Amphibolite

LA TE P A LEOZ OI C TO M ESOZ OI C

Paijang Schist Greenschist; siliceous schist and marble interlayer

Happen Marble Thick – bedded marble

Fongshushan amphibolite Amphibolite and horblende schist

Tungao Schist Marble; Amphibolite; Graphite schist and quartz-mica schist

Wuta Schist Marble and chlorite schist; Chlorite schist; Quarzt- mica schist

Chiuchiu Marble Thick-bedded marble, dolomite and chlorite schist intercalation

Nanaoling Schist Quarzt schist; paragneiss

Kuyan schist Phyllite; micaceous schist and quartz to mica schist

19 2.2.1 Pre-Tertiary metamorphic rock

The wide-spread lithological units in the basement can basically be classified into several complex series of geological features as listed below:

Kuyam schist belongs to Nanao Peak Schist (124-125.5K) located in the western Central Cross-Island Highway.

Nanaoling Schist incorporated with Tungao Schist became Donao Schist at 115-122.6 K along Central Cross-Island highway. In the Suao-Nanao area, they are lenticular bodies that interbedded with other kinds of metamorphic rocks.

Chiuchiu Marble is accompanied by the Happen marble located in the Suao-Hualien highway, north of the Heping area. They are lenticular metamorphosed limestone, which intercalated in the schistose rock (Ho, 1988). It is characterized by massive and thick-bedded and ranged from fine to coarse-grain.

Paijang schist was featured by lenticular greenschist, siliceous schist, and marble alternation. The greenschist are derived from mafic volcanic flows or pyroclastic rocks.

Especially, the chlorite schist of Wuta schist formation is easily accompanied by the greenschist, and it has fine to medium-grain.

Amphibolite is elongated lenticular bodies that well-foliated rock distributed in the northern Nanao area. Its foliation plane is generally parallel to that of surrounding schist.

Six gneiss bodies were discovered by Yen (1954) contained two large gneiss bodies in the north and others contacted with metamorphosed limestone in the south.

Granitic texture, which explained for the intrusion of the granite mass into the pervasive greenschist facies metamorphism. (Fuh, 1962) is characterized as the orthogneiss.

2.2.2 Tertiary metamorphic rock and sediment deposit

Nansuao and Suao formation located in the north of the study area. The rocks contained black to dark grey argillite, slate, and phyllite, and occasional interbeds with dark gray compact sandstone are characterized by the low metamorphic grade. Gravel, sand, and mud were deposited as alluvial and terrace that were distributed in the stream area.

METHODOLOGY

The methodology of our research was briefly summarized in Fig.3.1. There are three major parts: (1) Using the 5-meter Digital Elevation Model to identify surface fracture planes and calculate their attitudes; (2) Operating the stress inversion program to obtain the stress state at NE Taiwan from focal mechanisms data of 1991-2016; and (3) the correlation of surface fracture pattern with a given stress state can help us creating 3D Fracture Nucleation Map.

Several geological definitions will be clarified in the first part of this chapter.

Figure 3.1: Flowchart of the method.

Geological definitions

3.1.1 Foliation

In general, foliation is a geological term defined for any planar fabric or curve structure (Fossen, 2016). There are two types of the foliation: primary foliation was formed during the formation of magmatic rock or deposition of sedimentary rocks, and secondary foliation is tectonic foliation responded to the deformation process.

Tectonic foliation is the penetratively structural fabric defined by zones of different grain size and flattened object or recrystallized tabular grain with the uniform orientation (Fossen, 2016). Therefore, the planar, parallel, triangular faces observed from the topography of metamorphic terrain is characterized as the existence of specified sheet texture in the 3D environment of Arc Sense. Under the surface processes, the foliated rock may easily split when the force overcomes its cohesion.

3.1.2 Fracture

Fracture in geology is a common term composed of structure features such as faults, joints, veins, dikes, and sills (Schultz and Fossen, 2008; Peacock et al., 2016).

However, in this study, we proposed that the tectonic fracture is composed of only faults and joins, and they can reveal in digital elevation data. The study of Pollard and Aydin (1988) aimed to distinguish the joint and fault by the opening model and shear model.

▪ Extension fracture: where the fracture walls move apart, opening model, and may contain joints or faults.

▪ Shearing-mode fractures: a general term for discontinuity features along the wall rocks move sub-parallel to the plane of the fracture. These include fault and fault zones.

3.1.3 Stress tensor

Stress tensors are a specialized mathematic equation that can be used to define the Earth’s forces. Individual components in a stress tensor are tractions acting perpendicular or parallel to three orthogonal planes. The normal axes to the three orthogonal planes define a Cartesian coordinate system. The stress tensor has nine components. Each of them has an orientation and a magnitude. Three of these components are normal stresses perpendicular to plan, whereas the other six are shear stresses, and their traction is applied along the plane in a particular direction.

Methods

3.2.1 DEM Database

In recent years, spaceborne satellite images, high attitude aircraft images, and their products, such as DEM, are highly efficient dataset able to characterize geologic features on the landform. Furthermore, it also is a reasonable source of information for the detection of crustal deformation patterns caused by faults and fractures (Wladis, 1999; Morelli and Piana, 2006). Because geology fieldwork is a costly, time-consuming, and sometimes dangerous undertaking, thus any beneficial techniques supported tool for effortless and convenient fieldwork should be considered. Therefore, in our research, we

investigated the DEM of 5m raster cell data to analyze the surface fracture pattern along the coastline area of NE Taiwan under the 3D software environment. The surface patterns of the fracture plane can be recognized by geological lineaments. Therefore, we try to mapping lineaments through 2D view of the Red Relief Image Model (RRIM).

Finally, 12536 lineaments were identified and picked through RRIM.

3.2.2 Red Relief Image Model (RRIM)

RRIM is a new conception of the geomorphology analysis that took a new approach of examining surface to classify geological structures. Compared with other topographic visualization methods such as contour map and shaded relief, RRIM eliminates the dependency of incident light direction and shows the 3D image without shadow (Chiba et al., 2008; Chiba and Hasi, 2016). Two landform element layers resulted in the RRIM (Chiba et al., 2008) are the topographic openness map and slope map. We used the Relief Visualization Toolbox (RVT 1.3) to create the openness map and slope map from DEM data. The RRIM was further created in Arc Map by overlap the topographic openness map and slope map. The adjustment of contrast, brightness, and transparency parameter of each layer in Arc Map software is required to clearly display RRIM.

3.2.2.1 Topographic Openness Map

The topographic openness map, including positive and negative openness maps, is an essential component to produces RRIM. Openness expressed the degree of dominance or enclosure of location on an irregular surface. The positive openness map

refers to a calculation of concave-downward zenith angles. Yokoyama et al. (2002) concluded that positive openness represented the topographic ridges. On the others, negative openness referred to an evaluation with concave-upward nadir angles and characterized the valley (Fig 3.2A). Therefore, the concept of openness strongly emphasizes the dominance of concavities and convexities on the landscape. The zenith and nadir angles were computed the subtraction 90^○ to slope angles and calculated in eight different azimuth directions (D= 0; 45; 90; 135; 180; 225; 270; 315). The positive and negative values of the openness map were obtained by averaging angle for each direction. The openness map is designed by the gray tone to enhance its optimal detail and contrast. Positive openness takes a higher value of gray tones and generally encountered an expanse of terrain by one or several elevated relief features such as ridges (Fig 3.2B), while negative openness expressed the degree closure of the lower location as valley, river, or crater by elevated surroundings (Fig 3.2C) (Yokoyama et al., 2002). To successfully display the positive and negative map into ArcMap software, we need to adjust the contrast, brightness, and transparency parameters of each map. And the adjustments are continuously repeated until we observe clearly the topography. In summary, openness parameters highly support to highlight the different terrain features such as ridge, crests, gullies, or valleys (Fig 3.2D).

Figure 3.2: Elements of topographic openness map. (A) Using the nadir point to calculate the negative openness and zenith point to identify the positive value on openness map analysis (modified from Yokoyama et al., 2002). (B) Negative openness map. (C) Positive openness map. (D) Diagram of the concave and convex topography based on the concept of topographic openness map

27 3.2.2.2 Slope Map

The slope map was computed as the ratio of relief to irregular terrain surface and can be roughly identified topographic features of the area. If the openness map displays the gray tone in RRIM, the slope map layer is usually generated by a red color pattern because the red color has the richest tones for human eyes (Chiba et al., 2008). In similar, the contrast, brightness, and transparency parameters of the slope map also need to adjust. The slope map should be overlap with the topographic openness map and repeatedly adjust the parameter until the clearly observe topography (Fig 3.3). The yellow lineaments were mapped along the gullies and primarily indicated for the surface fracture traces in the 2D view of ArcMap software.

Figure 3.3: Red Relief Image Model. Yellow lineaments presented in the RRIM are emphasized the convex topography. Lineaments correspond to the straight or slightly curved edge of convex topography and might be affected by the geological features, such as join, fault or cleavage, etc. (Koike et al., 1998).

29 3.2.3 Lineament Map

Geological lineaments can be associated with the rupture of Earth’s surface from a different origin such as structure, lithology, surface processes, etc. They commonly demonstrate a particular type of fracturing (Hobbs, 1904; Wise et al.,1985; Twidale et al., 2007). The topographic expression of the rupture area with the offset of the surface, differential erosion of juxtaposed units, or erosion of damaged rock could help us to classify the geological structures such as faults, fault zones, or joints, etc. Therefore, mapping lineaments is a necessary step for interpreting and examining the fracture plane.

Typically, lineaments displayed by straight or slightly curved components can be observed and recognized through high-resolution remote sensing data. The identification of lineaments distribution in the DEM is strongly dependent upon the effect of illumination such as azimuth, imagery declination angle, and view direction (Koike et al., 1998). However, RRIM enhances lineament quantity by the independent illumination for any viewing direction (Chiba et al., 2008).

The lineament map of NE Taiwan was generated by hand-picking the lineaments through RRIM. In this study, we recognized and picked lineaments along gullies, straight river banks, or broken ridge. It basically displays in RRIM by yellow dash lines (Fig 3.3). In total, we mapped 12536 lineaments in the coastline area of NE Taiwan.

The fracture planes can be identified based on the distribution of lineaments in the 3D view.

30 3.2.4 Extraction of Fracture Attitude

RRIM overcomes the shortcoming of the illumination problem to identify the lineaments, which might be considered as a trace of the fracture plane. Fracture planes are three-dimensional and are commonly represented in the spatial by attitude (Fig 3.4).

In the field investigation, the attitude of fractures is examined on the surface by using the clinometer. The strike and dip direction of the fracture surface are commonly measured.

In recent years, the development of the reformative resolution of digital elevation data was able to obtain the strike and dip of the sedimentary bedding (Yeh et al., 2014). It follows the same traditional principle method that the dip angle was calculated by measuring the angle between bedding and horizontal plane (Fig 3.5). They defined that the sedimentary bedding planes in the LiDAR are measurement triangles.

Each measurement triangle contains more than three points to compute a regression plane. The strike and dip of the bedding planes are acquired from the attitude of the regression planes (Fig 3.6).

In this study, we tried to use the DEM with 5 meters of resolution to conduct the 3D fracture map. We also assume that fracture planes in the spatial are shaped as the triangle and should be fitted with the regression planes. At least three measurement points defined the triangle, but more than three points are needed to obtain the smooth and correctly triangle. To conduct the 3D fracture map, we floated the lineaments map into the 3D environment of Arc Sense. After that, hand-picking the triangle planes (S), which are defined by the group of green dots (Fig 3.5) in 3D environments. The attitude

of fracture planes was calculated by measuring both the intersection and the inclined angle between a triangle (S) and the horizontal plane (H) (Fig 3.5).

In the TM2 coordinate system, the x and y gave the longitude and latitude of the point, while z is the elevation above a given datum. The regression plane can be represented by z= ax+ by+ c. To derive the strike and dip of the measurement triangle, we take the cross product of the normal measurement vector and the normal horizontal plane vector (Fig 3.6). Calculating the gradient of two-variable function can reveal the dip direction of any point on the surface: 𝑣⃗_𝑑𝑖𝑝 = 𝛻𝑧 = (𝜕𝑧 𝜕𝑥⁄ )𝑖⃗ + (𝜕𝑧 𝜕𝑦⁄ )𝑗⃗ , where 𝑣⃗_𝑑𝑖𝑝 is the fracture plane dip direction and 𝑖⃗, 𝑗⃗⃗⃗ are the unit vector of x, y-direction. The dip angle was calculated by the following equation: 𝑆_𝑑𝑖𝑝 = ‖𝑢⃗⃗_𝑑𝑖𝑝‖ = ‖𝑣⃗_𝑑𝑖𝑝⁄‖𝑣⃗_𝑑𝑖𝑝‖‖. The strike vector can be calculated at any point over the regression surface using the following equation: 𝑣⃗_{𝑠𝑡𝑟𝑖𝑘𝑒} ^{= 𝑛}⃗_𝑑𝑖𝑝 ^{× 𝑛}⃗_{𝑧𝑠𝑢𝑟𝑓𝑎𝑐𝑒}^{, where}^𝑣⃗𝑠𝑡𝑟𝑖𝑘𝑒 is the strike direction,

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