透過三維板塊重建技術探討伊比利板塊底脫作用於西地中海構造演化之影響

Abstract

在西地中海區域的地質演化十分複雜，也有許多前人對此區域做過研究。而此篇研究中我們使用與以往不同的技術來討論西地中海區域的地質演化與發展。我們使用地震波p波速度構造畫出已隱沒板塊的幾何形狀與板塊還原技術將其還原至地球表面以指示其尚未隱沒時的幾何形狀、大小，作為重建漸新世-現今西地中海地區的板塊還原的新方法。本研究中使用了幾種不同的速度構造模型來進行板塊的繪製，於西地中海區域我們使用了MITP08(Li et al.,2008 ), Bezada et al.(2013), 等不同的速度構造模形，於伊比利半島則是使用BS2000 (Bijwaard and Spakman 2000), Chevrot et al.(2014)。我們會在GOCAD軟體裡進行繪製並進行板塊的還原，在Gplate軟體中將其與現有的板塊一起進行板塊運動的重建。 我們所繪製的板塊共有五塊，西地中海區域有三塊分別為Betic-Alboran, Algerian, and Calabrian 板塊，它們都在前人研究Spakman and Wortel (2004) 中被認定並命名。但他們並沒有確實的描繪出板塊的形狀與大小並將它們還原至地球表面比對是否合理。本研究中確實的將這三個板塊繪出並還原至地表，我們發現此三板塊的形狀與大小正符合30百萬年前-西地中海區域擴張前的板塊邊界。而在南北向方面在板塊的長度與大小也正符合歐亞大陸與非洲板塊的邊界。受限於速度構造的解析度在某些區域並不特別理想，我們繪製的板塊會有一些重疊的區域但還是在解析度的誤差範圍內。而板塊的空間分布與其傾角方向可以支持西地中海的隱沒的方向東-西向的。在伊比利板塊的下方有兩塊板塊，一塊在伊比利板塊的北邊，於Sibuet et al.(2004) 中被指認。這塊板塊可以幫助我們了解明伊比利板塊於中生代寧靜期時的運動模式。另一個在伊比利半島的中部，我們稱為mid-Iberia 板塊。同樣的這塊板塊也被前人(Sibuet et al., 2004)指認過，他們解釋這塊為已隱沒的新特提斯海板塊。與前人研究不同的是，有許多的證據證明mid-Iberia 板塊較有可能為底脫作用而向下沉的大陸岩石圈。而底脫作用的最東端正好是整個西地中海開始擴張的地點，在本研究中我們提出一個新的解釋認為伊比利板塊的底脫作用與西地中海的擴張有著密不可分的關係。

Seismic tomographic images of subducted lithospheric remnants under the western Mediterranean have provided new constraints for Oligocene to present-day plate reconstructions. In this study, we mapped slabs under the western Mediterranean and Iberia from regional seismic tomography (Chevrot et al., 2014; Bezada et al., 2013) and from MITP08 global tomography (Li et al., 2008). A newly developed method was used to unfold (ie. structurally restore) the mapped slabs to a model spherical Earth surface, minimizing area and shape distortion. Slab constraints were input into plate tectonic reconstructions using Gplates software. Our mapping confirms the existence of western Mediterranean slabs including the Betic-Alboran, Algerian, and Calabrian slabs that were previously identified by Spakman and Wortel (2004). When unfolded these mapped slabs fit together in an Oligocene plate reconstruction, within tomographic resolution limits. Slab stretching was not required. Slabs segmentation supports the existence of a North Balearic transform. Here we emphasize the potential importance for western Mediterranean tectonics of another slab under Iberia that we call the ‘mid-Iberia slab’. This slab was first identified by Sibuet et al. (2004) and interpreted to be a Neotethyan suture. We have mapped this slab in detail from the recent regional tomography (Chevrot et al., 2014). Our mapped slab is sub-vertical and strikes E-W under the southern margins of the Duero and Ebro basins. We newly interpret this slab to be delaminated northern Iberian continental lithosphere. We propose that continental delamination occurred during the Oligocene and produced uplifted Iberian Meseta topography, internally-drained basins, and high mean elevations that still persist today. We show how Oligocene northern Iberian continental delamination could have initiated subduction and rollback of the western Mediterranean.