Abstract To gain better insight into the structure and complex deformation pattern of Madagascar island, we derived a 3-D heterogeneous and anisotropic velocity model of its crust and uppermost mantle based on ambient noise analyses. The data set comprises ~1 year of seismic ambient noise recorded by 63 broad-band seismic stations deployed in and around the island. Data from Ocean Bottom Seismometers were treated for tilt and compliance noise before applying the standard noise processing routine followed by cross-correlation, dispersion measurement, Rayleigh-wave phase-velocity tomography in period range 5–40 s, and 3-D shear wave velocity modelling to 70 km depth. At shallow to mid-crustal depths, our results show fast velocities all over the Precambrian shield of the eastern two-thirds of the island in contrast to the adjacent west coast that is delineated by slow speeds that correlate with the sedimentary basins. In the lower crust and uppermost mantle, relatively fast velocities preponderate the western third of the island indicating a thinned crust and uplifted mantle. We found complex spatial and depth variation in patterns of azimuthal anisotropy across the island. The orientation of fast azimuth is relatively similar in the upper and middle crust. Aligning mostly N-S in the south and approximately NE-SW in the northern areas, it is attributed to alignment of cracks and metamorphic layering that pervades the Precambrian basement. The fast polarization axis in many sub-regions in the lower crust differs from that in the upper to middle crust and uppermost mantle. Apart from the dominant NE-SW fast axis observed in the uppermost mantle consistent with mantle flow, a semi-circular pattern of anisotropy coinciding with widespread intense slow velocities is observed beneath the central Antananarivo terrane. A delaminating lower crust that is initiated by an upwelling asthenosphere hypothesized for this region is invoked to explain our observation.

中文翻译：

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来自环境噪声断层扫描的马达加斯加岛的深度相关方位各向异性

摘要 为了更好地了解马达加斯加岛的结构和复杂的变形模式，我们基于环境噪声分析推导出了其地壳和最上地幔的 3D 非均质和各向异性速度模型。该数据集包括部署在该岛及其周围的 63 个宽带地震台站记录的约 1 年地震环境噪声。在应用标准噪声处理程序之前，对来自海底地震仪的数据进行倾斜和顺应性噪声处理，然后进行互相关、色散测量、5-40 秒周期范围内的瑞利波相速度断层扫描和 3-D 横波速度建模到 70 公里深度。在浅至中地壳深度，我们的结果显示，与以与沉积盆地相关的低速划定的相邻西海岸相比，该岛东部三分之二的前寒武纪地盾上的速度很快。在下地壳和最上地幔中，相对较快的速度占该岛西部三分之一的优势，表明地壳变薄和地幔隆起。我们发现整个岛屿的方位各向异性模式存在复杂的空间和深度变化。上地壳和中地壳的快方位角方向比较相似。南部主要呈南北走向，北部地区大致呈东北-西南走向，这归因于裂缝的排列和遍布前寒武纪基底的变质层理。下地壳许多子区域的快极化轴与上地壳和中地壳和最上地幔的快极化轴不同。除了在与地幔流一致的最上层地幔中观察到的主要 NE-SW 快轴外，在塔那那利佛中央地体下方观察到与广泛的强烈慢速一致的半圆形各向异性模式。一个由上升流软流圈引发的分层下地壳被用来解释我们的观察结果。