花蓮和平溪下游變質花崗岩之脆韌性與脆性構造研究與其地質意義

Abstract

依目前的地體架構而言，台灣島主要由於歐亞板塊與菲律賓海板塊斜向碰撞而造就今日的主體，並使台灣脊樑山脈呈現東北-西南走向，但大南澳變質雜岩的構造骨幹於脊樑山脈北段有從東北向轉折至東西向的現象，且由於隱沒反轉，使得造成台灣北部受到沖繩海槽的弧後擴張作用。和平溪正位於歐亞板塊與菲律賓海板塊的隱沒反轉帶，使和平溪中下游目前處於北部脊樑山脈構造骨幹旋轉的軸心區域，可能同時受到造山擠壓與弧後張裂作用之影響，為一個研究應力轉換的適當區域。為了重建此地區地質構造與應力演化史，除了於花蓮和平溪中下游的變質花崗岩與大理岩地區進行野外調查，也進行600公尺變質花崗岩岩芯描述，針對區域葉理面發育後的脆韌性構造與脆性構造進行研究與分期。詳細而言，野外調查著重於地質構造之量測與截切關係之判定，進而推演應力演化模式。整合野外與岩心資料，進行構造分期與應力反演，進而推演構造應力演化模型。 研究結果顯示本地區構造先後順序為:(1)區域葉理與早期石英礦脈；(2)假玄武玻璃；(3)壓扁構造；(4)急折帶；(5)斷層擦痕；(6)開口導水裂隙與方解石脈。並且在上述構造中藉由古應力分析結果指示，其應力場順序為:(1)北偏東83o擠壓之逆斷層應力場(區域葉理)；(2)北偏東51o拉張之正斷層應力場(假玄武玻璃)；(3)南北向擠壓之走向滑移斷層應力場(急折帶)；(4)南偏西67o擠壓之走向滑移斷層應力場；(5)北偏西84o拉張之正斷層應力場；(6)於開口填充裂隙中之北偏東69o拉張之正斷層應力場。 彙整上述成果，本研究探討各期構造於花蓮和平地區地體構造演化的可能地質意義:區域葉理應力場反映歐亞板塊與菲律賓海板塊斜向碰撞時期的應力場；假玄武玻璃和壓扁構造可能代表同造山時期之陷落構造的證據；急折帶構造可能為沖繩海槽擴張的證據，並認為沖繩海槽擴張為主要的應力來源，但水平擠壓方向為東南-西北向，並無法排除斜向碰撞的可能性；而之後轉變為拉張性環境之正斷層機制。According to tectonic setting, the NE-trending Taiwan island is the consequence of oblique collision between the Eurasian Plate and the Philippine Sea Plate. Because the subduction flip between two plates results in the southward propagation of Okinawa Trough, the northeastern Taiwan is also influenced by the back-arc extension of the Okinawa Trough. In the northeastern Taiwan, the structural grain of the northern Backbone Range changes from northeast to east-west. Downstream area of the Hoping River, Hualien is located at the rotation axis of structural grain and also is located above the transition zone of subduction flip. As a result, the downstream area of the Hoping River is great place to study the stress interactions between mountain building and back-arc extension. To reconstruct regional structures and stress evolution, beside field investigation on the meta-granite and marble, 600m core examination of meta-granite is also carried out and focused on the study of brittle-ductile and ductile structures after the foliation development. In details, field work emphases on measuring structure attitudes and identifying cross-cutting relationships and furthermore placing constraints on stress evolution model. Study of core examination compensates the lack of outcrops The sequence of structure development associated with stress field in the region is identified as: (1) regional foliation and early quartz veins in reverse faulting stress regime with SE-NW compression; (2) pseudotachylyte in normal faulting stress regime; (3) flattened structure; (4) kink in strike-slip faulting stress regime with N-S compression; (5) fault slip in strike-slip faulting stress regime with SE-NW compression;(6) open filling fluid conduits and calcite veins in normal faulting stress regime with NE-SW extension. Synthesizing the structure characteristics associated with stress field, tectonic meanings of each structure in terms of structural evolution in the Hoping region, Hualien is interpreted. The stress field of regional foliation is reflective of oblique compression between Eurasian and the Philippine Sea Plates. Pseudotachylite and flattening structure may represent evidence of syn-tectonic extension. SE-NW compression inferred from kink band may correspond to the back-arc extension of Okinawa Trough but the compression of oblique collision still can not be ruled out. NE-SW extensional environment of normal faulting stress regime is appeared.