利用噪聲成像解析馬達加斯區域的速度構造

Abstract

馬達加斯加島位處東非外海的西印度洋上。其地質架構複雜，主體可劃分為東西兩側，東側為由岡瓦納古陸所分離的陸塊，約占據全島面積2/3，西側則為自侏儸紀起所堆積的摩倫達瓦盆地(Morondava basin) 及馬哈贊加盆地 ( Majunga basin)，馬島與鄰近海域區域則為白堊紀時馬島脫離於東非、印度所增生的海洋地殼。藉由馬島近年來漸趨完善的測站網，地震學家開始對該區的速度構造有了初步的探究。 本研究透過周遭噪訊層析成像進行研究，範圍概括2012-2013年馬島與鄰近海域，分別為陸上MACOMO，SELASOMA，GEOSCOPE三個測站網，與西印度洋海底地震儀測站網RUM-RHUM。並藉由測站所記錄的連續噪訊量測出測站對之間5-35秒雷利波的相速度，接著以多重尺度小波逆推此區域的地殼三維剪力波速 (Vs) 及其方位非均向性。 反演結果顯示，Vs於淺部地殼( < 10km )具有明顯的側向差異，低速區分佈在馬島西側，速度介於2.5-3.0 km/s，並集中在西南方的摩倫達瓦盆地及西北方的馬哈贊加盆地，高速區則分佈在島嶼中央以東，由泛非造山運動所縫合的克拉通所構成，速度介於3.2-3.6 km/s；馬島區域的地殼非均向性可則分成海陸兩種型態 (1) 馬島陸地的非均向性主要受控於造山時期的擠壓方向且快軸方向平行於島上岩層葉理方向，其反映構造相關的橫向均向性; (2) 馬島周圍海洋地殼的非均向性快軸方向約略平行馬島分離時期的洋殼擴張方向，可能是洋殼生成早期所記錄古老流場導致礦物的排列方向。

We apply ambient noise tomography to Madagascar. The Madagascar Island can be generally divided into two major geological units. The eastern two-thirds of the island is composed of metamorphic basement separated from the east Gondwana, and the western part of the island is mostly covered by sedimentary basins, Morondava basin and Majunga basin. Little was known about the subsurface crustal structure of Madagascar until the recent deployment of seismic networks in and around this area. In this study, we constrain the crustal seismic structure of Madagascar by using the dispersion data of Rayleigh wave extracted from ambient seismic noise. We derive the noise cross-correlation functions (CCF) from the vertical continuous data (2012-2013) recorded by 3 land seismic networks and one OBS network. To ensure the data quality, only the CCFs with inter-station distance less than 700 km are considered for the dispersion measurement. Using the resulting broad-band (5-35 seconds) dispersion data, we implement a wavelet-based multi-scale inversion technique to build 3-D Vs and Vs azimuthal anisotropy model of the crust of Madagascar. In the resulting isotropic Vs model, the lower velocity zones are mainly distributed in the shallow depth ( < 10km ) of Morondava basin and Majunga basin; on the other hand, the metamorphic basement in east Madagascar is mostly covered by higher velocity. The resulting Vs anisotropy model shows that the shallow crust of Madagascar is dominated by structure-related anisotropy, with the fast polarization direction generally parallel to the foliation direction of the metamorphic basement formed during the Pan-African orogeny.