臺灣-琉球間之隱沒-碰撞複和帶 隱沒板塊與弧前地幔之力學性質與結構

Abstract

頂著歐亞板塊的琉球海溝末端，也是菲律賓海板塊斜向隱沒進入歐亞板塊下方的地點，在台灣外海形成了複雜的隱沒—碰撞複和帶，迄今仍未被詳盡解釋的一個區域。我們結合了來自台灣與日本兩個地區的地震觀測網資料，以期闡明琉球隱沒板塊如何在這個隱沒—碰撞地區隱沒與變形。我們使用同時被兩個地震網所記錄到超過五千個以上的地震資料，並以最佳化的區域一維速度模型及二次差分法進行重新定位。外海的地震分布結果顯示，在隱沒板塊伸至40至80公里深處，有存在間隔約15至20公里寬的雙層地震帶。根據震源機制解的結果顯示，雙層地震帶是肇因於斜向聚合所導致的東西向壓縮。 精確定位後的地震分布也第一次呈現在深度50至100公里處，隱沒板塊水平方向褶皺的曲率大到與整個琉球海溝的曲率異號。這個異常的曲率加上震源機制解的結果，顯示隱沒板塊因為頂住歐亞岩石圈而變形，且這個變形很顯然並非完全彈性。我們針對此變形建立了一個模式，推導出一個不穩定的麥斯威爾黏彈性層，夾在黏滯性介質中的結果。這個不穩定性的構造其特徵波長約為250公里，這個結果符合褶曲的隱沒板塊其黏滯性約為周圍地幔物質的100倍大，且沿著海溝方向的表面彈性應變為0.01；或是隱沒板塊其黏滯性約為周圍地幔物質的1000倍，則表面彈性應變可以大到5%。 本研究中建構了琉球最西側隱沒的菲律賓海板塊以及臺灣東北部地底下的弧前地幔的三維地震層析影像。利用超過5600個同時被臺灣與日本地震觀測網所記錄到的地震資料，將地震重新定位並反演VP、VS、和VP/VS的變化。藉由分析資料的變異量降低對模型變異間的取捨，可以幫助我們建立合適嚴謹程度的正則化，以避免對資料過度解釋或解釋不足。正則化參數的選擇是為了確保人為的VP/VS異常可以被壓制住。隱沒板塊的特徵為高VP、高VS以及中等到低的VP/VS。 值得注意的是在緊鄰隱沒板塊表面，深度約30-80公里處的地幔楔則為高VP/VS異常。過去曾被發現存在有高VP/VS通道連結隱沒板塊與島弧火山之間，因而被解釋作熔融的通道，但在本研究中並未發現。 我們將VS和VP/VS轉換為溫度變化以及地幔中的蛇紋岩化。隱沒板塊的溫度比周圍地幔低約200-400oC，與隱沒帶理論模型的估計一致。在50公里深的地幔，其蛇紋岩化的程度達到約15%，或含水量達2%。我們將此蛇紋岩化的峰值，解釋為由於菲律賓海板塊隱沒，其表面之海洋地殼產生玄武岩—榴輝岩變質反應所釋放的水，與地幔反應所導致。由於本區域現有地震網相對於隱沒帶地震的分佈，存在有空間上的限制，因此限制了完整描述大部分地幔楔產生熔融的型態。層析影像反演的解析度測試則提供了對於解釋較佳解析區域的基本指引。

The termination of the Ryukyu trench against Eurasia and the oblique subduction of the Philippine Sea Plate create a subduction-collision complex offshore Taiwan, which has not previously been elucidated in detail. We combine traveltime data from the seismic networks in Taiwan and Japan to better illuminate how the subducting Ryukyu slab deforms in this subduction-collision zone. More than 5000 events recorded by both networks were relocated with the double-difference method using an optimal regional 1-D velocity model. The offshore seismicity indicates that the double seismic zone, with a gap of 15-20 km, exists in the subducting slab in the depth range of 40-80 km. Focal mechanisms suggest that the double seismic zone is caused by east-west compression resulting from oblique convergence. The improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the Ryukyu trench in the depth range 50-100 km. The anomalous curvature, together with the focal mechanisms, suggests that the slab folds against the Eurasian lithosphere and that this deformation cannot be fully elastic. We model this deformation mode as the developing instability of a viscoelastic Maxwell layer embedded in a viscous medium. The characteristic wavelength of the instability, i.e., ~250 km, is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along-strike elastic membrane strain as small as 0.01, or more than 3 orders of magnitude higher if 5% elastic strain is allowed. Three dimensional tomographic images of the subducting Philippine Sea slab and the forearc mantle beneath NE Taiwan and the westernmost Ryukyu were generated in this study. More than 5600 events recorded simultaneously by seismic networks in Taiwan and Japan were relocated for the inversion for variations in VP, VS, and VP/VS. Analysis of the tradeoff between the data variance-reduction and model-variance helps to determine the appropriate strictness for regularization to avoid either over- or under-interpretation of data. The regularization parameters were also chosen to ensure suppression of artificial VP/VS anomalies. The subducting slab is characterized by high VP, high VS, and intermediate to low VP/VS. Notable in the mantle wedge is the high VP/VS anomalies that abut the surface of the subducting slab at depths of 30-80 km. The previously identified positive VP/VS channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study. We convert VS and VP/VS to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200-400oC relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches ~15%, or 2% water content, at 50 km depth in the forearc mantle. We interpret the peak serpentinization as hydrated by the water released from the basalt-eclogite metamorphic reaction in the oceanic crust of the subducting Philippine Sea plate. The spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge. Resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions.