利用物理模型探討35Ma以來東亞陸塊的運動演化

Kinematics of East Asia Terranes Since 35 Ma: Implied from Analogue Modeling

李政芳(Cheng-Fang Lee)

指導教授 ： 王維豪

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2015年)

摘要

本研究利用三維物理縮尺模型，模擬32 Ma以來包括中國的羌塘-拉薩、松蟠-甘孜、揚子、華北以及南方的桑德蘭陸塊的運動演化。過去的研究顯示印度板塊與歐亞板塊碰撞，可能會使鄰近陸塊產生塊體脫逸作用、重力擴散與層脫作用，以及下部地殼流動作用現象。然而過去的模擬通常並沒有一併考慮前述古老陸塊的真實幾何形貌及陸塊間可能存在弱帶的影響，因此本研究重建35 Ma時期的東亞陸塊並透過三維物理縮尺模型觀察東亞陸塊在印度板塊碰撞後的運動過程。
根據本研究結果顯示，在35 Ma當印度大陸剛撞上桑德蘭陸塊時，東亞各陸塊幾乎以同一角速度向東脫逸，然而在32 Ma左右桑德蘭陸塊會沿著弧形的嘉梨¬-哀牢山-紅河斷層的先存弱帶做順時針旋轉與向東南脫逸，其旋轉軸由桑德蘭陸塊東南方向西北方移動。桑德蘭陸塊的脫逸造成南海在32-20Ma的張裂以及哀牢山因壓移作用而隆起。在此同時，羌塘-拉薩、松蟠-甘孜與揚子陸塊也向東位移，惟運動速率較桑德蘭陸塊小。由於羌塘-拉薩、松蟠-甘孜陸塊的東移，本研究推測在早第三紀時，在今日帕米爾與巴基斯坦地區曾發生地殼拉張作用；另一方面，華北陸塊雖然也緩慢的向東位移，但因速率較松蟠-甘孜陸塊為慢，因此在柴達木盆地東側會出現地殼壓縮以及在鄂爾多斯與華北陸塊之間出現地殼拉張。
造成桑德蘭陸塊的脫逸主要是因為印度大陸與桑德蘭陸塊初始碰撞的位置是發生在桑德蘭的古大陸棚西北走向的地方，因而產生東北方向的正應力，並在西北西走向的哀牢山－紅河斷層（Ailao Shan-Red River, ASRR）上產生左移的分力，當此分力超過ASRR的強度，桑德蘭陸塊就會向東南脫逸。在此同時，由於印度北北東方向的碰撞，造成緬甸西部陸塊沿北北東走向的實皆斷層右移，並拉張安達曼海。
本研究測試均質的歐亞大陸以及含低強度下部地殼的非均質歐亞大陸兩種模型，研究結果顯示兩者的基本趨勢是一致的，但在模擬非均質歐亞大陸碰撞模型時，桑德蘭陸塊在後期會出現向西的旋轉速度。而這兩個模型都觀察印度板塊的向東隱沒時造成板塊回捲並產生海溝的吸力，致使桑德蘭陸塊旋轉或東西向伸張。

關鍵字

物理模型 ； 東亞

並列摘要

3-D analogue modeling was conducted to simulate motions of East Asian terranes, including China Qiangtang-Lhasa, Songpan-Garze , Yangtze, North China and Sunderland, since 32 Ma. Previous studies have shown that terrane extrusion, gravity spreading and slab delamination as well as lower crustal flow may have occurred during the collision between Indian plate and Eurasian plate. However, most of these studies either ignore the possible effect caused by terrane geometry or the presence of the weak zone between two adjacent terranes. In this study, we conducted physical modeling that used the reconstructed East Asia and India plate at 32 Ma as an initial model to simulate the motions of East Asian terranes ever since. Our results showed that the Sunderland would rotate clockwise and extrude southeastward along the Chiali-Ailao Shan-Red River fault when India collided with the Sunderland at 32 Ma. The rotation pole of the Sunderland migrated from southeast to northwest. The escape of the Sunderland resulted in opening of the South China Sea and uplift of Ailao Shan. In the meanwhile, Qiangtang, Lhasa, Songpan-Garze, Yangtze terranes also moved eastward but in a lower velocity relative to the Sunderland. This movement might have caused crustal extension in Pamir and Pakistan during early Tertiary. On the other hand, the eastward extrusion of Songpan-Garze terrain was faster than that of the North China terrane; as a result, crustal compression was found the eastside of the the Qaidam Basin, and crustal extension was observed in the area between Ordos and North China.
The mechanism response to the extrusion of the Sunderland is because India collided with the Sunderland at the paleo-continental shelf striking NW. As a result, a NE normal stress was acting on the WWN striking ASRR and resulted in left-lateral shear. If the shear force exceeded the friction resistance of ASRR, the Sunderland would extrude southeastward. In the meantime, the fault block of western Burma started to move toward north along the NNE trending Sagaing Fault in response to the NNE collision of the India plate. Consequently, the Andaman Sea opened.
Both homogenous terrains and inhomogeneous terrains with weak lower crust have been tested in this study. The results, however, were quite similar, except that the inhomogeneous model exhibit westward rotation velocity on the Sunderland in the late stage of plate collision. In both cases, we found the eastward subduction of the India Plate would induce slab rollback and trench suction. As a result, the Sunderland would either rotate clockwise or extent EW-ward.

並列關鍵字

Analogue Modeling ； East Asia Terranes

參考文獻

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