| 摘要: | 本研究結合台灣東北部固定地震站與宜蘭平原密集地震網所觀測到的近震初達 P 波走時資料逆推台灣東北部的地下三維速度模型。結果相較於前人研究有較高的解析度,使本研究可以詳細檢視一些較為精細的速度異常構造,並探討他們在地體構造上的意涵。首先,本研究介紹並區分了兩種台灣周遭隱沒方向反轉機制的模型,分別是撕裂模型與斷離模型。兩者針對菲律賓海板塊與歐亞板塊在台灣東部的邊界及菲律賓海板塊隱沒至北台灣之下的西緣有不同的預測。以地體動力學的觀點來看,斷離模型與隱沒擠入體模型較相近,因為隱沒至北台灣底下的歐亞板塊斷離所造成的地函空窗,會使隱沒的菲律賓海板塊像是一個擠入體(indenter)向西移入。接著,本研究自逆推所得的菲律賓海板塊高速異常提取板塊邊界與隱沒的菲律賓海板塊西緣,發現兩者與隱沒擠入體模型描述的一致,因此本研究的觀測較支持斷離模型。此外,本研究在地函楔中觀測到一高速異常,可被解釋為先前隱沒到此區域的地殼產生榴輝岩化,或是其上覆板塊的地殼山根榴輝岩化。而在台北盆地與大屯火山群底下觀測到的低速異常與地函楔相連,暗示菲律賓海板塊在北台灣的隱沒對火山活動的影響,一方面可能是透過山脈垮塌引發地函上湧,也可能是透過隱沒板塊脫水效應產生液體。最後,宜蘭平原底下的低速異常向東連結到沖繩海槽,顯示與弧後擴張的關聯性。;First P-wave arrival-time data from local earthquakes recorded by a dense geophone array deployed on the Ilan Plain and by existing permanent stations were combined to invert for high-resolution P-wave velocity structures under northeast Taiwan. With relatively high resolution, we were able to examine the structures in more detail and to investigate their significance and tectonic implications. We introduce two distinct groups of proposals for mechanisms of subduction polarity flipping in Taiwan, referred to as the “tear model” and “breakoff model”. While the predicted boundaries of the Philippine Sea Plate – the junction with the Eurasian Plate in eastern Taiwan and the west edge beneath northern Taiwan – differ between the two models, those of the breakoff model and the related subducting indenter model are geodynamically similar in that the mantle window opened by the detached Eurasian lithosphere beneath northern Taiwan facilitates the westerly movement of the Philippine Sea slab and thus the subducting indenter in eastern Taiwan. The surface junction as extrapolated and the west edge as determined by the imaged high
Vp anomalous Philippine Sea Plate in this study comply with those predicted by the subducting indenter model and thus favor the breakoff model over the tear model. On the other hand, while the observed high Vp anomalous region in the mantle wedge can be explained as eclogitization of previously subducted crust, eclogitization of the overriding continental crustal roots cannot be ruled out. As for the low Vp anomalies, those beneath the Taipei Basin and the Tatun Volcano Group exhibit a pattern potentially connected to the low Vp anomalies in the mantle wedge, suggesting the involvement of the Philippine Sea slab, either by asthenospheric upwelling due to extensional collapse or by fluid migration due to slab dehydration. Those beneath the Ilan Plain exhibit a low Vp pattern surrounded by high Vp anomalies of the metamorphic rocks in the northern Taiwan mountain belt and extend to deeper origins in the eastern offshore region, suggesting a connection with the opening of the Okinawa Trough. |
