從熱定年學看東喜馬拉雅山結外圍主要斷裂帶的活動歷史及其大地構造演示

Abstract

新生代印度與歐亞板塊的聚合使得亞洲大陸產生了顯著的地殼變形，透過測地學對現今地表變形之觀察可知，在東喜馬拉雅山結外圍有一呈現順時針旋轉的地塊運動正在進行，其邊界恰與山結外圍的嘉黎(Jiali)、高黎貢(Gaoligong)、鮮水河(Xianshuihe)、小江(Xiaojiang)及奠邊府(Dien Bien Phu)斷裂帶所處的位置一致。前人提出諸多地體構造演化的模式解釋此一陸陸聚合和亞洲岩圈變形的因果關係，但缺乏這些邊界斷裂帶的構造與定年資料提供對比。本研究於嘉黎、高黎貢及奠邊府斷裂帶的範圍中採集變質岩及相關侵入岩樣本，透過鈾-鉛與氬-氬定年學的方式嘗試建立這些斷裂帶的變形歷史，並探究它們在新生代東亞構造演化的意義及其隱喻。 從分析結果來看，位於順時針旋轉塊體西界的嘉黎與高黎貢剪切帶，其主要剪切帶活動時間記錄分別落在22到11Ma及18到13 Ma之間。對比嘉黎、高黎貢的活動記錄與紅河哀牢山帶和Sagaing斷層之前人研究結果，其中，嘉黎和左移的紅河哀牢山剪切帶與右移的Sagaing斷層在早中新世的剪切活動記錄一致，暗示著當時三者可能共同建構一個左移的邊界斷層使中南半島得以延之向東南脫逸。在紅河哀牢山剪切帶的左移活動趨於靜止之時，高黎貢剪切帶的活動記錄開始活躍(~18 Ma)，嘉黎剪切帶亦可能在此時改變了其剪切方向成右移，讓變形由嘉黎向南與高黎貢剪切帶連接，並自此形成今日所見東喜馬拉雅山結外圍的順時針旋轉地塊的西南邊界斷層。嘉黎剪切帶可能的構造反轉發生的時間恰與藏南東西向拉張產生的正斷層時間相符，同時亦與中南半島河系演化有一致性。地質証據的相關性顯示在陸陸聚合影響下，西藏岩石圈可能因增厚而促發了深部地殼的流動性造成山結外圍的右移剪切活動和藏南拉張環境的出現，此外，於此時印度向北移動的方向略為順時針向東旋轉使斜碰撞的效應更加明顯，可能亦是影響機制之一。 而在北越地區的奠邊府剪切帶雖是今日順時針旋轉塊體的左移東界之一，但在地質圖上卻明顯曾以右移方式錯移了馬河縫合帶(Song Ma suture)。本研究提供了第一批此斷裂帶活動的氬氬定年結果，並發現奠邊府斷裂帶曾於早侏儸紀時就出現活動記錄。糜嶺岩中的雲母礦物的氬氬年代落於198~158 Ma，相當於印支期時華南陸塊與中南半島聚合後，再與Sibumasu、Simao塊體聚合的時間，而陸塊之間斜碰撞的相對運動方向則可能正好提供了奠邊府斷裂帶右移的應力來源。奠邊府斷裂帶得以保存其印支期活動記錄，可見其內部的新生代地殼變形並未重置其內部歷史，其在東亞大地構造演化的意義仍待後續探討。

The Jiali, Gaoligong, and Dien Bien Phu shear zones form the major boundaries for the clockwise rotation zone of crustal materials around the Eastern Himalayan Syntaxis (EHS), regarded as the most significant deformation zones in Southeast Asia. A number of models have been proposed to explain the mechanism of crustal deformation in this region. However, additional geochronological and structural studies from these major shear zones are required to provide more constraints on such models. With the aim of revealing the deformation history of the shear zones and exploring the implications of these findings for the Cenozoic tectonics of Asia, the present study involved a systematic collection and thermochronological analysis of deformed metamorphic and associated plutonic complexes within the Jiali, Gaoligong, and Dien Bien Phu shear zones. Geochronological data show that the main phases of deformation along the Jiali and Gaoligong shear zones occurred in the periods of ~22–11 and ~18–13 Ma, respectively. A comparison of the deformation recorded in the Jiali, Gaoligong, and other major shear zones around the EHS reveals that sinistral shearing along the Jiali shear zone was synchronous with that along the Ailao Shan–Red River shear zone, forming a northern boundary to the southeastward extrusion of the Indochina block during the Early Miocene. From the Middle Miocene (~18 Ma), the Jiali shear zone may change to dextral displacement, becoming linked with the dextral Gaoligong shear zone that developed as a consequence of continued northward indentation of the Indian plate into Asia. Since this time, the Jiali and Gaoligong shear zones have been united, defining the southwestern boundary of the EHS during clockwise rotation of the eastward-extruding Tibetan block, as present-day tectonics in southeastern Tibet. The temporal comparison between plausible dextral shearing records within the Jiali and Gaoligong shear zones, and dates of normal faulting in southern Tibet, suggests that lower crustal flow resulting from thickened Tibetan lithosphere and oblique convergence between India and Eurasia may explain the change in deformation style around the EHS at ~18 Ma. Furthermore, the temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern Himalayan drainage system in the area of the EHS. In NW Vietnam, the NNE–SSW-trending Dien Bien Phu shear zone, which represents part of the eastern boundary for the extruding continental blocks, dextrally offsets the NW–SE-trending Song Ma suture. This study reports the first 40Ar-39Ar geochronological data obtained for metamorphic rocks within the Dien Bien Phu shear zone. 40Ar-39Ar ages of 194 to 212 Ma for mica schists and 158 to 198 Ma for mylonites suggest that the fault was active during the Early Jurassic, making it significantly old and longer-lasting. Temporal correlations indicate that collision between the South China, Indochina, Sibumasu and Simao blocks, representing the late Triassic to early Jurassic Indosinian Orogeny, may have led to the initiation of movement along the shear zone and associated fault systems in the Indochina region. The preservation of a geological record of the Indosinian Orogeny in the Dien Bien Phu shear zone indicates that Cenozoic shearing has not apparently reset the isotopic systems of the metamorphic complex distributed along the fault.