台灣南部嘉義地區麓山帶構造型態轉換之研究

Abstract

本研究區域的範圍北起大尖山、鹿窟斷層一帶，南至馬頭山斷層及布拉社斷層南端，東西向則包含了內麓山帶及前緣區域。地質圖上，此地區之南北構造線於中端有彎曲並且匯集形成一個緊密的構造帶，且斷層線亦有轉換的現象，這說明緊密構造帶成為連接嘉義地區南北構造的一個重要的構造轉換帶，因此，本研究目的是要了解的連結嘉義地區南北之間的構造。首先重建嘉義市附近橫跨內外麓山帶之地質剖面，初步了解地質構造演化；接著，透過GeoSec3D軟體整合九條地質剖面建立三維模型，一窺嘉義地區深部三維斷層模型；最後將使用GOCAD軟體整合三維斷層模型與3D地震重定位資料，完整呈現深部斷層形貌與地震活動在空間中的分布。 由地質剖面重建結果，馬頭山斷層為一個向西逆衝之逆時序斷層，且中崙背斜形貌由底部分別由向東與向逆衝之上下部楔形體所構成的三角變形帶(triangle zone)控制。根據剖面回復過程，逆時序發育的馬頭山斷層與中崙背斜底部的三角變形帶，其形成時間皆晚於觸口斷層及崙後斷層，暗示馬頭山斷層與三角變形帶其活動性可能高於觸口斷層與崙後斷層，成為嘉義地區近期活躍的構造。 初步估算斷層水平縮短量，在本剖面中水平縮短量已減至 2.1 公里的觸口斷層，崙後斷層高達約30.4公里，顯示觸口斷層其消失的逆衝滑移量已逐漸轉至崙後斷層之上。另外，本剖面異常高的總水平縮短量，約50公里，推測原因有二，一為大尖山地區及台中盆地地區斷層水平縮短量應比過去所認知的更來得大；二為獺頭斷層的剪裂特性可能吸收了南北兩區域逆衝滑移量的差異。 此三維斷層模型顯示嘉義地區地下斷層形貌主要有，一為大尖山斷層與九芎坑斷層、鹿窟斷層匯聚於約 5-7 公里深之滑脫面構成為大尖山斷層一帶主要的基底滑移面，而崙後斷層與觸口斷層、馬頭山斷層、布拉社斷層匯聚構造面則成為崙後斷層及馬頭山斷層一帶主要的基底滑移面，約10公里深；二為大尖山─觸口斷層正斷層地下形貌受到早期正斷層影響淺部呈高角度斷層形貌，深部則為低角度滑移逆衝斷層。；三為獺頭斷層淺部呈高角度斷層至深部則以低角度匯入鹿窟斷層的基底滑脫面之中，往南段轉為布拉社斷層，斷層形貌則以低角度斷層為主。 最後，由地震資料與地質剖面的分析，崙後斷層、馬頭山斷層以及獺頭斷層構成的緊密構造帶反映出麓山帶前緣的台南盆地與構造高區之交界阻擋了崙後斷層往西繼續推進，結果使得上盤的馬頭山斷層逆時序發育且逆衝至崙後斷層上盤之岩系，造成構造線緊密分佈。

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.