- 學位論文
哀牢山地體演化與楚雄前陸盆地形成的關係
Tectonic Evolution of Ailao Shan Mountain Belt in Relation to Formation of Chuxiong Foreland Basin, Yunnan, China
摘要
哀牢山位於中國的雲南省西南部，是華南陸塊與中南半島間的一條地質和地形的不連續帶，長達400公里，寬度小於20公里。由哀牢山剪切帶變質岩定年的結果，指出有兩次大規模的構造事件，分別是中生代印支期印支地塊和華南地塊縫合，以及新生代印度板塊與歐亞板塊碰撞，造成中南半島脫逸的喜馬拉雅期。位於哀牢山的東北方，有一中生代晚三疊紀楚雄前陸盆地，現今的哀牢山與楚雄盆地，呈現山盆位置不耦合的地貌，其特殊性值得深入探討，本研究主要目的是探討哀牢山造山帶從中生代至今地體演化的過程，以及伴隨形成的楚雄盆地的關係。 野外地質考察的結果顯示，哀牢山西側石炭紀玄武岩的存在，說明此區域石炭紀為古特提斯洋；而哀牢山構造帶以變質岩為主，可分為東側的高度變質帶，其變質相為綠色片岩相-角閃岩相，是古元古代揚子地台的結晶基底，還包含強烈剪切的糜嶺岩，為漸新世到中新世左移脫逸構造的證據；西部的低度變質帶，其變質相為綠色片岩相，反應中生代揚子地台的大陸斜坡沉積物，而西側出露的蛇綠岩，更是古特提斯洋閉合的證據。 由楚雄盆地的地層組合以及沉積物厚度變化來分析，盆地內的地層相由下至上，分別為被動大陸邊緣、基底不整合面、複理層及磨粒層，可對應至盆地演化的低填充、填充到過填充的沉積階段，而由西向東為楔形的沉積物厚度側向分佈，都顯示出晚三疊紀楚雄盆地為一標準的周邊型前陸盆地。比對其形成時間，其相對應的造山帶應為印支期揚子板塊與印支板塊碰撞形成的哀牢山造山帶。 本文建立出哀牢山造山帶與楚雄前陸盆地的演化關係共可分為四個階段：(Ⅰ)被動大陸邊緣階段、(Ⅱ)早期聚合階段、(Ⅲ)晚期聚合階段、(Ⅳ)新生代走向滑移階段；在石炭紀，楚雄盆地前身處於揚子板塊被動大陸邊緣，周圍是古特提斯洋；三疊紀早期印支運動開始，揚子板塊與印支板塊碰撞，使古特提斯洋的分支閉合，在哀牢山地區形成了碰撞造山帶，隨後在造山帶荷重作用之下，其東側楚雄前陸盆地隨之形成，堆積了深海相複理層。隨著造山帶的推進，靠近穩定大陸的盆地東側也接收到沉積物，並堆積了淺海相及陸相磨粒層，楚雄盆地一直持續接受沉積物至白堊紀的燕山期結束為止，哀牢山則漸漸被侵蝕。到了新生代，受喜馬拉雅造山運動影響，在漸新世到上新世，現今的哀牢山經歷了左移的脫逸構造，和強烈的抬升及剝蝕，因此形成現今山盆位置不耦合的地貌。而楚雄盆地則被抬升成高原並遭受侵蝕及斷層破壞，只有在哀牢山東側山麓，因紅河斷層的活動，形成了小型的拉張盆地，堆積了第四紀的河相礫岩。
並列摘要
Ailao Shan is located in southwest Yunnan Province (China) characterized by a narrow (<20m) and long (>400km) metamorphic belt. It is a geologic and topographic discontinuity that separates the South China block from Indochina block. According to the dating of metamorphic rocks, the formation of Ailao Shan Shear Zone is related two main episodes: (1)Indosinian Orogeny, a mountain-building event between South China plate and Indochina plate in Triassic and (2)the strike-slip extrusion resulting from India-Eurasia collision in Cenozoic. The Chuxiong basin located in the northeast of the Ailao Shan is considered as a Mesozoic peripheral foreland basin and nowadays it is decoupled with Ailao Shan by topographic characteristics. The aim of this study is to propose a hypothesis relating the tectonic evolution of Ailao Shan mountain belt and to the formation of Chuxiong Foreland Basin. Field investigation of Ailao Shan shows that Carboniferous basalts outcropped along the western Ailao Shan, providing the evidence of the branch of Paleo-tethys. The Ailao Shan metamorphic belt can be divided into two parts: (1)high-grade metamorphic belt to the east is composed of strongly foliated and lineated mylonitic gneisses and (2)low-grade metamorphic belt to the west contains ophiolite (ultramafic rocks). Mylonites is regarded as a result from strike-slip extrusion in India-Eurasia collision in late Oligocene to Miocene. The presence of ophiolite represented the branch of Paleo-tethys that had been emplaced on land. The sedimentary facies in Chuxiong Basin is characterized by sequences composed of passive margin, basal unconformity, flysch and molasse in a ascending order which can be correspond to the depositional stage from under-filled stage to over-filled stage, respectively. Geometric shape of the sedimentary fill in a cross-section view reveals wedge-shaped from west to east. These evidences all indicate that Chuxiong Basin is a typical foreland basin. This study proposes four evolutional stages for the formation of the Ailao Shan and Chuxiong Basin:(a)Passive margin stage, the branch of Paleo-tethys separated South China plate from Indochina Plate in Carboniferous. Pre-Chuxiong Basin was located on the passive margin of South China plate. (b)Early convergent stage, the beginning of collision between South China Plate and Indochina Plate built Ancient Ailao Shan Orogeny during early Triassic. Due to the orogenic loading, Chuxiong Foreland Basin was developed on margin of the south China Plate and accompanied by Ailao Shan to the southwest. Rapid subsidence to deep water environment in the early Chuxiong Basin and then flysch facies was deposited in late Triassic. (c)Late convergent stage, the northeastward progradation of Ailao Shan, the sediment fill in foreland basin and deposition of molasse facies from late Triassic to Cretaceous. (d)Strike-slip stage, since India-Eurasia collision started from Cenozoic, Chuxiong Basin has underwent regional uplift. Now it is a plateau with low-relief, not a foreland basin judged by geomorphology. The Ailao Shan has experienced left-lateral displacement and accompanied by strong exhumation by uplifting at least 18km from Oligocene. The movement of Red-River Fault generated pull-apart basins at the eastern foothill of Ailao Shan and Quaternary fluvial sediment was deposited in pull-apart basins.