中國雲南省西南部高黎貢與崇山剪切帶構造史之探討

Abstract

高黎貢與崇山剪切帶在中國雲南省的西南邊。本研究計畫分析此兩條剪切帶間的構造事件說明彼此的構造演化關係，加上利用多種礦物(黑雲母、白雲母及角閃石)之40Ar/39Ar定年結果來決定兩條剪切帶的活動時間，以建立高黎貢與崇山剪切帶對應大地構造在空間與時間上的變化。高黎貢與崇山剪切帶的構造事件包括三期，一期塑性事件、兩期脆性事件。第一期(D1)：高黎貢剪切帶沿著鏟形(listric)斷層面進行右移斷層加逆衝斷層的斜滑移剪切；崇山剪切帶沿著傾向向西的斷層面進行左移斷層加逆衝斷層的斜滑移剪切。第二期( D2)：高黎貢剪切帶由塑性變形轉為脆性變形，相對活動方向依舊是右移斷層加逆衝斷層的斜滑移；崇山剪切帶由塑性變形轉為脆性變形，相對活動方向依舊是左移斷層加逆衝斷層的斜滑移。第三期( D3)：高黎貢剪切帶持續脆性變形，相對活動方向轉變為北段右移，中段左移斷層加正斷層的斜滑移，南段正斷層加左移斷層的斜滑移；崇山剪切帶持續脆性變形，相對活動方向轉變為北段正斷層加左移斷層，南段正斷層。在40Ar/39Ar定年方面，將各礦物之年代與其封存溫度做圖，所得之冷卻曲線顯示高黎貢剪切帶的快速冷卻事件在漸新世晚期到中新世早期(27.7~21.0 Ma)略早於崇山剪切帶的冷卻事件(21.7~19.8 Ma)。故可能的構造演化為：在27 Ma，高黎貢剪切帶成為中南半島脫逸的西界；實皆斷層尚未啟動；崇山剪切帶尚未啟動。21 Ma，高黎貢剪切帶因剪切作用逐漸減弱，使得中南半島脫逸的西界移向實皆斷層；崇山剪切帶啟動；實皆斷層啟動。10 Ma，高黎貢剪切帶被擠壓與旋轉到接近目前的位置，且高黎貢剪切帶南段開始由右移逆斷層轉為左移正斷層；崇山剪切帶已經轉為正斷層；實皆斷層則正式成為中南半島脫逸的西界。

The Gaoligong and Chongshan shear zones (GLSZ and CSSZ) are N-S to NE-SW trending with low to high dipping in GLSZ and almost N-S trending with steep dipping in CSSZ in strike-slip shear zones in south-western Yunnan, China. Field data were measured to reconstruct the structural history of the GLGS and CSSZ . The 40Ar/39Ar thermo geochronology techniques were applied to reconstruct the thermo evolution of these shear zones. The structural and thermo history of the GLSZ and CSSZ are described below: The oldest ductile deformation event namely D1: The GLSZ forms a right-lateral with trust component listric mylonitic shear plane following the present trend of N-S and NE-SW. ; Whereas the CSSZ forms a left-lateral transverse fault with trust component during 27~19Ma. After the shear zones were formed, both shear zones transformed from deformation to brittle deformation during D2. Forming these thrust faults possibly during 19~10Ma. These thrust faults later developed into normal faults during D3 possibly from 10Ma to present. Base on the reconstructed cooling path, the timing of rapidly cooling event for the GLSZ(27.7~21.0Ma) is earlier than CSSZ(21.7-19.8Ma). Base on the structural history reconstructed, I propose that GLSZ was the western boundary of Indo-China extrusion during SE extrusion of Indo-China during 27-21Ma. ; The ceasing of GLSZ right-lateral by 21Ma probably indicate the western boundary of Indo-China extrusion migrated to the present boundary-Sagaing fault. Then the CSSZ and Sagaing fault began activate after 21-19Ma.