南沖繩海槽上部地殼的速度構造

Abstract

臺灣東部海域之南沖繩海槽是臺灣弧陸碰撞最早發生的區域，由於碰撞中心逐漸往西南轉移，琉球弧溝系統中的弧後擴張盆地向西南延伸，使得此區域現在為碰撞後期的張裂與崩塌，而南沖繩海槽的地殼構造演化以及現在其地殼性質到底是屬於拉張變薄的大陸地殼亦或是弧後張裂新生之海洋地殼是仍待解決之問題。本研究針對南沖繩海槽中收集到的長支距反射震測資料進行進階震測資料處理，目的在於提升震測剖面的影像品質、增加剖面解釋的可信度及獲得地層正確的速度資訊，藉此來了解南沖繩海槽的速度構造與地殼特性。本研究彙整與分析南沖繩海槽之49條反射震測資料，發覺剖面中構造較複雜之地區常有繞射、訊號雜亂或是被複反射所覆蓋的現象，不僅造成真實的地層形貌與構造無法顯示出來，另一方面也大大的提高了解釋的困難度。因此，本研究首先利用一些特殊的資料處理技術來消除複反射的能量，提高震測資料的訊噪比，然後再利用長支距反射震測資料的優勢，針對特定區域進行進階的震測資料疊前深度移位處理及疊代移位速度分析，以獲得地層正確的速度資訊。研究結果顯示，南沖繩海槽的速度構造大約可分為三層，地層速度由淺至深依序為1.5 km/s -2.35 km/s、2.8 km/s-3 km/s及基盤速度4.5 km/s；從沉積物的分布來看，南沖繩海槽中的沉積物較厚，海槽東側之沉積物較薄。以速度約在2.5 km/s ~3 km/s之間的地層來說，東海大陸棚最厚，且向下梯度增加最多，推側此層應為受到固結而壓密的沉積物層。在火成岩體聚積區域，接近海床表面之鬆散沉積物薄層速度為1.5 km/s-1.9 km/s，然後震波速度以垂直向下梯度0.9為單位增加至3 km/s左右，再逐漸增加至5 km/s。在某些層面中觀察到震波速度突然高起又下降之情形，解釋為火成岩體以岩床形式侵入沉積層中所造成的結果。本研究分析的震測剖面資料經過進階資料處理後，除了可讓深部訊號清楚呈現之外，淺部訊號也變得更清晰，不僅可以了解到火成岩體與沉積物的接觸關係，也可以看到速度在不同地層中所表現的差異。本研究所獲得的速度分析結果顯示南沖繩海槽的地殼速度趨勢落於拉張大陸地殼的速度範圍內，因此本研究認為目前南沖繩海槽的地殼性質，應以拉張性的大陸地殼為主。

The Southern Okinawa Trough (SOT) east of Taiwan is the place where initial arc-continent collision occurred, as the collision center moved southwestward and the Ryukyu backarc extension developed toward Taiwan, the present SOT is in post-collision and extension stage. The structural evolution and the nature of the SOT crust are thus important issues. In this study, we use advanced seismic data processing techniques to process the large offset multichannel seismic reflection data for better imaging and understanding the geological processes and velocity structures of the SOT crust. Radon filter and eigenvector filter have been used to eliminate the multiples on the seismic profiles, and pre-stack depth migration and iterative migrate velocity analysis have been performed to produce better seismic images and correct velocity structures. Our results show that the upper crustal velocity structures in the SOT can be roughly divided into three layers. The top layer consists of the most recent sediments that show velocities from 1.5 km/s to 2.35 km/s；the second layer consists of compacted sediments with velocities from 2.8 km/s to 3 km/s；the third layer is basement with high velocities of 4.5 km/s or greater. The most recent sediment layer is thicker in the central part of the SOT than in the eastern SOT. The layer of compacted sediments is thickest in the East China Sea continental shelf. Velocities of the intrusive volcanic zone show a thin sedimentary layer with velocities from 1.5 km/s-1.9 km/s on top. Velocity of the deeper strata increases from 1.9 to 3 km/s with a gradient of 0.9, and then gradually increased to 5 km/s further deep. In certain areas, velocity may increase suddenly then drop down. This is due to igneous rocks intruded into sedimentary strata in forms of sills. After advanced seismic data processing, signals in the deeper strata are much clear, and that help us not only to observe the contact relationships of igneous rocks and sediments, but also to comprehend the velocity difference of the different strata. Comparing the character of the crustal velocity structure of the SOT derived from this study with the oceanic crust velocity (White et al., 1992) and the extensional continental crust velocity (Christensen and Mooney., 1995), it appears that the crustal velocity character in the SOT is similar to the extensional continental crust. Thus, we suggest that the nature of the crust in the SOT is extensional continental crust.