The Transition of Structural Style in the Foothills in Chiayi Area, Southern Taiwan

• Main Authors: Shu-Lin Huang, 黃姝琳
• Other Authors: Jyr-Ching Hu
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/58971430394428822395

碩士 === 國立臺灣大學 === 地質科學研究所 === 97 === The region of study area extends from Tachienshan and Luku faults in the north to Matoushan and Pulashe faults in the south which includes the internal and external western Foothills in Chiayi area. According to the geological map from CPC, the major faults in study area converge together as a tight structure near Chiayi area and change to different structure style in the southern part of study area. It indicates that the existence of transition zone between the northern and southern part of the Chiayi area. Therefore, the first goal of this paper is to understand the linkage in structures in tight structure zone between north and south of Chiayi area. To this purpose, we first reconstruct a balanced cross-section near Chiayi city and investigated the overall structures by using nine geological cross-sections from CPC to reconstruct the 3D fault geometry by GeoSec 3D software beneath Chiayi area. Finally, we use GOCAD software to integrate 3D relocated seismicity and 3D fault model and show the special relationship between the active structures and distribution of the seismicity. Based on the geological profile across the transition zone, the Matoushan fault is out-of-sequence, and the geometry of Chunglun anticline is controlled by the triangle zone, composed of west-thrusting lower wedge and east-thrusting upper wedge, under the anticline. According to the restoration, the Matoushan fault and the triangle zone probably formed later than Chukou fault and Lunghou fault. It implies the Matoushan fault and the triangle zone are more active than Chukou fault and Lunghou fault and become two of active structures in Chiayi area recently. Preliminary estimating the shortening of geological profile, the shortening of Chukou fault and Lunghou fault are about 2.1 km and 30.4 km, respectively. The southward-decrease of shortening on the Chukou fault indicates the deficit in shortening has been transferred to other structures in this area, such as the Lunghou Fault. And the significant total shortening of about 50 km along the geological profile of this study indicates the total shortening in Tachienshan area and Taichung area are larger than those as we knew in the past or the Tatou fault, tear fault, absorbed the gap of shortening between north and south part of Chiayi area. A 3D fault model demonstrates three important geometries of fault linkage beneath Chiayi area. First, The Chiuchiungkeng, Tachienshan and Luku faults merge in a subsurface of about 5-7 km and form an important detachment in the region of Tachienshan area. The Lunhou , Chukou, Matoushan and Pulashe faults merge at the deeper depth of abut 10 Km and form principle detachment in the region of Lunghou fault. Second, geometry of the frontal part of Tachishan-Chukou fault system is controlled by pre-existing normal fault. The fault consists of a steeper western part as inverted normal fault (ramp), and a gentle-dipping eastern part in the form of detachment thrust (flat). Third, The vertical NW trending Tatou fault, high-angle tear fault, cuts off the Luku Fault and turns into the N-S trending Pulashe Fault dominated by low-angle fault to the south. Based on the analysis of seismicity and geological profile, the tight structure between the Chukou, Lunghou and Matoushan faults reflects the resistance of the border between incoming basement high and Tainan basin in the frontal region. As a result, the hianing-wall of Matoushan fault was out-of-sequence and thrust on the haning-wall rocks of Lunghou fault, and the tight structure formed.