海岸山脈奇美斷層的古應力及褶皺分析

Abstract

奇美斷層位於海岸山脈中段，是唯一斜穿海岸山脈的大斷層。斷層上盤為中新統都鑾山層，下盤為更新統八里灣層，被認定為兼具平移活動的逆斷層。以往相關的構造研究多著眼於海岸山脈大尺度的構造演育，尚無人針對奇美斷層進行古應力分析。為重建奇美斷層帶的活動歷史，本研究利用秀姑巒溪剖面，詳細描述斷層帶中的岩體特性，量測小斷層和褶皺的方位，再依據野外的截切關係進行古應力分析。 奇美斷層帶的岩體隨著與主斷層面遠近而呈現不同的產狀，為詮釋這些構造特徵，本研究利用已報導的破裂密度資料，將斷層破壞帶細分為Domain I至Domain IV等四個構造分區。各分區的構造特徵與古應力型態可歸納為三組：(1) 上盤Domain I及Domain II共寬約65公尺，變形集中於被礦脈填充的平移斷層，與其分支和晚期發育的斷層共同呈現東北–西南向壓縮、西北–東南向伸張的平移斷層機制解。(2) Domain III褶皺發達，砂岩常形成脹縮構造及泥質填充脈等軟質變形構造。褶皺軸面極點與褶皺軸反映南北向的壓縮，方向大致與奇美斷層的走向垂直，且波長與兩翼間夾角數據朝主斷層面遞減，顯示近主斷層面的褶皺受到較強烈的變形，反映奇美斷層的變形。後期截切褶皺的斷層亦展現南北向的壓縮，展現一致的古應力型態。(3) Domain IV為脆性破壞帶，與Domain III以P斷層接觸，全區平移斷層及逆斷層發達，整體反映近西北–東南向壓縮的逆斷層機制解。 本研究結合前人報導，推斷上盤中被礦脈填充的斷層形成於都鑾山層早期的熱事件，可能與奇美斷層的構造活動無直接相關。下盤則未發育礦脈，變形時間晚於上盤。Domain III在岩體尚未完全固結時即形成軟質變形的褶皺，指示早期的變形；相對地，Domain IV皆為脆性變形的斷層，可能記錄岩體固結、被抬升至脆性變形區間後的構造活動。總結而言，奇美斷層的變形集中於下盤，且愈遠離主斷層面，構造形成的時間愈晚，先後記錄南北向以及西北–東南向的壓縮。

The Chimei Fault, which thrusts the Tuluanshan Formation over the Paliwan Formation, is the only major fault cutting across the entire Coastal Range. Though previously regarded as a reverse fault with strike-slip component, paleostress analysis has not yet been focused on the Chimei Fault. In order to reconstruct the structural evolution of the Chimei Fault, this study first describes the structure features of the fault rock as illustrated by the excellent exposures along the Hsiukuluan River, and then analysis of fault-slip data and folds are carried out. Fault rock along the exposure indicates more complicated features toward the Chimei Fault plane. This study thus divides the fault damage zone into four structural domains based on reported fracture density data. In the hanging wall, Domain I and Domain II are dominated by strike-slip faults that filled by mineral veins, showing strike-slip movement with NE-SW compression. In the footwall, Domain III and Domain IV are separated by the P fault, dominated by ductile and brittle structures, respectively. Domain III, indicating N-S compression that determined by fold axial planes and axes, consists of asymmetric folds and active deformation structures deformed under insufficiently consolidated condition. Domain IV is crosscut by brittle faults, showing NW-SE compression. The fault slip data within these domains are divided into three sets. The first set, recorded in the Domain I and Domain II, is deemed to related to thermal events of the Tuluanshan Formation and fracture prior to the formation of the footwall since mineral-vein intrusions are confined to the hanging wall. The second set, shown in the Domain III, is demonstrated by the initial deformation of the footwall before the fault rock becomes completely consolidated, followed by the third set that recorded in the Domian IV until the footwall is uplifted to the shallower brittle regime. The second and the third set illustrate the deformation of the Chimei Fault, constructing a two-stage evolution: (1) N-S compression with reverse component; (2) NW-SE compression with reverse component.