雲南點蒼山變質岩帶應變分析與構造演化史之研究

劉怡岑(Liu yi chen)

指導教授 ： 李通藝 博士 ； 葉孟宛 博士

國立臺灣師範大學/理學院/地球科學系/碩士(2009年)

摘要

新生代以來，印度和歐亞大陸的碰撞作用及其引發的地體構造運動，不但控制了東亞地區的地體架構及新生代構造演化，並造成喜馬拉雅山及西藏高原的抬升與東亞地區和其周邊大量走向滑移斷層的活動；紅河哀牢山剪切帶便為其中一條主要走向滑移斷層。至今東亞地體演化模式仍有許多爭論，因此紅河剪切帶上的塑性變形構造重建，對於還原東亞地體演化史扮演非常重要的角色。本研究選定紅河哀牢山剪切帶之西北段，中國雲南西南方點蒼山地區的塑性剪切變形構造作為研究對象。藉由野外構造、露頭測量及定向標本進行顯微構造分析來重建構造史，利用Rf/φ方法(分析應變橢圓之軸率與位態)、礦物拉張線分析及斷層機制解分析，計算測量顯微薄片的應變橢圓軸率及位態，進而了解此區的應變狀態，配合上氬氬定年之結果來討論點蒼山變質岩帶異時剪動的變形情況。
根據大地構造及野外資料分析結果得知，點蒼山地區之主葉理面的走向大致平行於西北-東南向之紅河剪切帶，且至少經歷三期的塑性變形作用。根據本研究定年結果加上比對前人定年研究的成果，認為第一期塑性變形約為三疊紀印支時期之前或同時發生，為一高角度褶皺作用，其褶皺軸面約為南北走向 ( ~N10°W)，向北傾斜，傾角約為63°；第二期為次水平方向剪切事件，剪切方向為上層往西北，下層往東南，為一偃臥褶皺，形成的葉理大致呈西北-東南走向( ~N19°W)，向北傾角約為20°；第三期可能為紅河哀牢山剪切帶左移期，可能發生在中新世早期，由本研究白雲母礦物氬氬定年得出結果約為21 Ma左右，屬於高角度褶皺，葉理大致呈西北-東南走向( ~N29°W)，向北傾斜，向北傾角約為55°，伴隨東西向的擠壓；第四期為東西向( ~N88°E)之向南傾（43°）的葉理，但只在點蒼山段發現此現象，可能為重力垮塌事件，同時可能為紅河哀牢山剪切帶右移期，約在中新世晚期到上新世早期，由本研究鉀長石礦物氬氬定年得出結果約為發生於4 Ma左右：之後則屬於脆性變形（D5）之右移正斷層。
目前Rf/φ分析方法及礦物拉張線分析結果顯示點蒼山北段與中段應變Z軸傾角近垂直軸，南段應變Z軸傾較為近水平，顯示點蒼山主應力由北中段到南段之轉變為水平應力轉成垂直應力，說明點蒼山中段到南段發生上部地殼與下部地殼偶合（Coupled）轉成不偶合（Decoupled）的現象。

關鍵字

紅河哀牢山剪切帶 ； 點蒼山 ； Rf/φ 分析法

並列摘要

The Cenozoic collision of the Indian and Eurasia plate had controlled the East Asian tectonic movement, and the uplift of the Himalaya Mountains and the Tibet plateau. One of the major shear zones formed due to this plate collision is the Red River shear zone (RRSZ). However, the deformation mechanism is still under heavy debate. This research focus on the Diancang Shan metamorphic belt. Reconstruction of the structural evolution, strain pattern and thermal history of the Diancang Shan Metamorphic Belt via field work, numerous strain analysis with 40Ar/39Ar geochronology was performed in order to gain insite into the structural and strain history of the Diancang Shan metamorphic belt, and their tectonic significance.
Field data analysis revealed the Diancang Shan had experienced 5 deformation events. The first deformation event (D1) forms upright fold with N-S striking (~N10°), steep dipping (63°), axial planes probably during the Triassic Indosinian orogeny. The second deformation event (D2) forms recumbent fold with NW-SE striking (~N19°), shallow to subhorizontal dipping( 20°) axial plane. The third deformation event (D3) forms upright fold with NW-SE striking (~N29°), inclined dipping (55°) axial planes, possibly resulted from the left-lateral strike-slip movement of the RRSZ during the early Miocene (21 ~ ? Ma), As suggeated by the first rapid cooling event according to the decides the dating by this research 40Ar/39Ar dating result . The fourth deformation event (D4) forms upright fold with E-W striking (~N88°), inclined dipping (43°) to ward south axial plane is only observed in the Diancang shan south section, and the strain pattern suggested a right-lateral shear regime; that I correlated the D4 event to the right-lateral shearing of the ASRR. The D4 event possibly formed during 5 Ma as the second cooling event of the 40Ar/39Ar dating result. The uplifting continued under the right-lateral shear regine, which the Diancang Shan metamorphic belt (D5) are mostly normal fault with right-lateral displacement.
Of the Rf/φ, Fry, and center to center strain analysis methods applied in this research. The Rf/φ is the most suitable method or strain analysis for med to high grade metamorphic rocks. As the foliation orientation calculated base on the Rf/φ analysis method better corrects with the actual foliation orientation. The strain pattern calculated
from the N-S and E-W sections suggested that the Diancang Shan recorded a right lateral shear event with thrusting component. further more, the mineral lineation distribute a concentric circle pattern, which suggests the southern portion of Diancang shan metamorphic belt might have decoupled tectonic activity between the upper crust from the lower crust, and the deformation activity is driven by a lower crustal flow.

參考文獻

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