| | 講 題 | Taiwan’s Plate Tectonic Activities, Mountain Building and Earthquakes | | | 講 者 | 美國．紐約州立大學賓漢頓分校地質科學系及南加州大學地球科學系 教授-吳大銘 | | | 日 期 | 2014/11/19 | 長 度 | 00:42:24 | 人 氣 | 545 次 | | | Taiwan’s Plate Tectonic Activities, Mountain Building and Earthquakes
臺灣板塊構造運動、造山與地震
Plate tectonics framework is usually complex in a collision zone, where the continental lithosphere is involved. For the young Taiwan orogeny, with geologic understanding and large new geodetic and subsurface datasets now available, much can be deciphered. Against the background of the commonly accepted view of Taiwan as a southward propagating, self-similar 2-D orogenic wedge, a fully 3-D current structure has been mapped. Along the whole length of the island the convergence of the Eurasian plate (EUP) the Philippine Sea plate (PSP) takes shape with mainly three different plate configurations. In northern Taiwan the convergence led to simultaneous northward subduction of the PSP under EUP and the collision of the oceanic PSP with continental EUP at increasing depth toward the north; in the south, EUP, in the guise of rifted Eurasian continent of South China Sea, subducts toward the east; in central Taiwan collision of oceanic PSP with continental EUP dominates. When relocated seismicity and focal mechanisms are superposed on subsurface P and Vp/Vs velocity images the configurations and the kinematics of the PSP and EUP collision and subduction become clear. In northern Taiwan the subduction/collision explains well the mountains and their dwindling height (accompanied by crustal thinning) toward the north. In the south, mountains rise (and crust thickens) above the east-dipping EUP subduction zone as the Eurasian continental shelf veers toward the southwest, still divergent and separate from the Luzon Arc – raising the question of what is/are the mechanism(s) underlying the southern Taiwan orogeny. In the central section where the PSP and EUP collide to build the main part of the Central Range and its parallel neighbor, the eastern Coastal Range. The Longitudinal Valley (LV) between these two ranges is the main EUP/PSP boundary. Across the LV the PSP lithosphere butts against the EUP lithosphere in the upper 70 km or so, leading to significant thickening of the crust (Fig. 2) to about 55 km (from a continental shelf crust of about 28 km) on the EUP side and about 35 km (from a PSP crust of less than 10 km) on the Coastal Range/Arc side. A steeply east-dipping fault down to a depth of 35 km and associated with the LV defines the edge of the PSP “indenter”. The Coastal Range rises above the thrust fault. Between the two "roots" there is a high velocity rise in the lower crust and the upper mantle; it channels the upper mantle material between the two plates upward as a result of convergence. In the upper mantle, high velocity anomaly and Benioff seismicity coexist under Taiwan in the north and in the south, associated with the two subduction zones. Going north from the southern zone, toward Central Taiwan, the east-dipping high velocity anomaly of the southern zone persists for another 150 km or so, but it is seismically quiescent. Ample GPS and leveling data show a shortening of about 7 cm/year across the Coastal/Central Range with concomitant rising of about 20 mm/yr near the highest peaks; the changes in the patterns of the observations track the plate motions well. Based on our studies, the key implications regarding the occurrence of earthquake include: 1) Central Range is a seismically relatively quiescent region, where large ductile deformation occurs and significant petrological/mineralogical changes may accompany the crustal thickening under the Central Range, 2) The “detachment” under the Western Foothills, commonly credited as the causative fault of the 1999 Chi-Chi earthquake, is most probably driven directly by the exhumation of the metamorphic core of the Central Range, 3) The EUP lithosphere under western Taiwan has a complex rheological structure, indicated, in part, by the depth distribution of seismicity, and 4) Several major potential sources of damaging earthquakes can be envisaged through the analysis of analogous tectonic regimes elsewhere.
The young and very active orogeny in Taiwan makes “a natural observatory for tectonics”. As in any science, to further our understanding of Taiwan tectonics as well as the associated problem of seismic hazards the acquisition of key observations is critical.
古老的造山帶一般而言其板塊構造非常的複雜，不易從現今的地表構造了解其造山過程。然而，臺灣的造山運動從地質定年結果來看非常年輕，用已有豐富的地質資料與近來的地表形變及各種地球物理探測資訊(地殼及上部地函震波層析速度構造，大地電磁測深)可提供了解造山運動與測試各種已提出的造山模式。從新的觀測資料可以清楚看出，臺灣的造山帶是三維的擠壓變形過程造成。過去常用的由北向南菲律賓海板塊(PSP)依序超覆歐亞板塊(EUP)造山的模型很難解釋整個臺灣的地下構造及山脈。根據實際觀測，從北到南，在順著臺灣山脈走向方向的變化上歐亞板塊與菲律賓海板塊兩板塊間有明顯的交互擠壓與隱沒作用(圖1)，其現象可綜合如下：臺灣北部區域菲律賓海板塊(PSP)向北隱沒至歐亞板塊(EUP)之下但由於PSP持續向西北運動的同時愈向北也和EUP產生愈深的碰撞；在南部則是由南中國海張裂後的歐亞大陸板塊向東隱沒至菲律賓海板塊下；臺灣中部是主要受控於兩板塊間的相互碰撞。從地震分佈、震源機制解與震波層析速度構造可以清楚地看出此兩板塊間的地下構造形貌。再就板塊構造與造山的關係而言，在花蓮以西高聳的中央山脈附近碰撞非常激烈但向北到臺北盆地以南的高山就很快降低──此區域為PSP隱沒到了EUP之下──同時地殼也向北逐漸的減薄。南部的山脈則坐落於向東隱沒的歐亞板塊上；在世界各地有這樣隱沒帶上方擠壓造山的例子尚不多，尤其從地形及構造上來看恆春半島以西的大陸棚坡邊界與其東的呂宋島弧(綠島及蘭嶼)未進入碰撞──但山脈是怎樣造的？雖然已有幾個假說，待有更多觀測求證。臺灣的中部是中央及海岸山脈所在區域，前者源自EUP後者為PSP；兩者之間為臺東縱谷。與北部及南部不同是在中部沒有活動的隱沒帶。一個典型的中臺灣剖面(圖2）可看到EUP地殼加厚至55公里左右(未碰撞前的大陸棚坡地殼厚度約為28公里)，而菲律賓海板塊的地殼也加厚至35公里)未碰撞前的菲律賓海板塊地殼厚度約少於10公里)。在縱谷南半段(池上)有清楚向東傾的逆衝斷層。海岸山脈也沿此斷層向上抬昇造成。在恆春半島南部，向東的隱沒的歐亞板塊可從高震波速度異常與班尼霍夫地震帶看出。然而，向北150公里到臺灣中部高速帶異常雖然存在著，但班尼霍夫帶地震已不復見(即上述無活動隱沒帶)。從全臺灣的全球衛星定位與高程測量資料顯示，中央山脈與海岸山脈間的縮短量為每年7公分而上昇速率最高可達每年2公分；大部份變化量可以充分的以上述板塊結構來解釋。在本研究中主要對於臺灣地震的意涵有：(1) 中央山脈為地震相對安靜區，主要受控於在中央山脈下的岩石流變性質(viscosity)及變質作用(例如：石英相變及榴輝岩化)。(2) 在西部麓山下的斷層(所謂滑脫面)主要的作用力極可能自中央山脈變質過程所產生的崛起作用。(3) 從地震分佈可以看出岩石圈為複雜的流變構造。(4) 從整體的研究地體構造環境可以進一步了解可能的造成災害地震之產生。
臺灣是世界地球科學界公認的“造山實驗室”。如欲更進一步了解臺灣的地體構造與其相關的地震災害，用實際的資料來反覆測試是不可或缺的。 | | | 提 供 | 國立自然科學博物館-「地震防災大三角」國際研討會 | | |
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