利用地震學方法討論花東海盆之岩石圈年齡

Abstract

花東海盆（HB）為菲律賓海板塊中，最靠近台灣，橫寬約200 km的狹長海盆。海床磁力異常條帶顯示HB年齡最老達到40百萬年（Ma），然而HB海床上採集輝長岩樣本進行氬-氬定年所得到年齡卻有約125 Ma。因為不同的證據各有優缺點，所以HB的起源至今仍無定論。本論文在台灣寬頻網中選擇測站，針對幾個東北方的遠震組成線性陣列，並以沿陣列之波形變化來檢驗向北隱沒之菲律賓海板塊之幾何效應，進而規範HB的年齡。本論文以2007年4月29日阿留申群島（Aleutian）之地震為例，量取其振幅與走時殘餘，發現二者在線性陣列上都呈現谷狀分佈，暗示隱沒板塊之“散焦”作用。本論文利用簡單的二維隱沒熱模型得到不同年齡板塊之速度構造，再用二維有限差分法，計算合成波形並和觀測之振幅比對。一系列之模擬顯示，對觀測之谷狀分佈擬合較好者，板塊年齡介於20-50 Ma之間，而老於80Ma之模型完全無法解釋觀測到的谷形幾何特徵。此一研究和過去之雷利波研究之結果一致，皆建議HB的年齡應小於50 Ma，其生成年代為始新世。

The Huatung basin (HB), a 200 km cross sliver of seafloor, constitutes the westernmost frontier of the Philippine Sea Plate (PSP). Prior paleomagnetic analyses determined the age of the HB to be about 40 Ma, being tens of Ma younger than the adjacent west Philippine basin (WPB) across the Gagua ridge. However, an Ar-Ar dating for rocks dredged from the HB seafloor suggested an early-Cretaceous age (~125 Ma), thus reversing the age contrast with the WPB. Since then the controversy has remained. In this study, we selected from broadband seismic networks on Taiwan a rough linear array that points to the events in the Aleutian region, and investigated the amplitude variation of P waves along this array. The waves are defocused and focused traveling along the center and the bottom of the subducting slab, respectively, producing a low and high at the center and the far end of the linear array. We employed a 2D finite-difference waveform modeling and experimented with slab models with simple geometry and various ages. The older and thicker the slab, the more distant the high amplitudes appear. We compare observations with model results and found that the best-fit slab ages fall into 20-50 Ma. This suggests an Eocene age for the HB, in contradiction with the early-Cretaceous age proposed previously