Assembly and breakup of Gondwana - seismological signatures in the lithosphere/asthenosphere system of Southern Madagascar (SELASOMA)

威尔逊旋回中超大陆的组装和破坏是了解海底磁线理记录时间以外的板块构造历史的定义范例之一。在这种情况下，出现了一个问题，即断裂的位置和方式在多大程度上受早期旋回中形成的古组构的控制。补充这个问题的问题是，在何种程度上的主要情节叠加或裂谷早期组构，以及这些早期事件如何影响最近的运动学。虽然这些问题已经得到了广泛的地质调查，这些过程的动力学很可能受到强烈的影响，中地壳和下地壳的岩石圈和地幔部分的流变学，最好的特点是地震调查。马达加斯加岛在冈瓦纳大陆的组装和多阶段分裂中占据了关键区域（Stern 1994），它本身是超大陆泛大陆的一部分。马达加斯加由混合大陆材料组成，包括几个不同的构造单元，最古老的岩石是侏罗纪时代（3.2 Ga）。主要地质边界一般遵循南北走向，反映了岛屿的地理方向。其元古宙至早古生代组构的特征是一个跨越数百公里的剪切带网络，其中一些分支与南北走向倾斜，特别是在该岛南部。 最近，在新近纪期间，该岛中部和北方发生了中度火山活动，并且有迹象表明，在过去10 Ma内，整个马达加斯加东部都有隆起（Roberts等人，2012年）。虽然马达加斯加位于非洲板块内部，远离主要板块边界，但其地震活动表明正在发生一些变形。基于大地测量数据和地震矩张量的现代运动学模型确定了一个分散的南北向小边界，该边界将两个微板块分开，似乎穿过马达加斯加（Stamps等人，2008）。由于它在冈瓦纳大陆的组装中起着关键作用，因此在马达加斯加进行了许多地质调查。然而，由于在马达加斯加进行的地震学研究很少，因此对地下结构的了解相对较少。在本提案中，我们的目标是利用我们从2012年5月至2014年5月进行的被动地震实验的数据来确定马达加斯加南部的岩石圈结构。这项实验包括一个大约东西向分布的全岛宽带站（运行两年）和一个区域阵列的短周期站（运行一年）。剖面图跨越所有主要地质区，短周期阵列将用于绘制当地地震活动图，并对岩石圈/软流圈系统的三维结构提供额外控制。

The assembly and destruction of supercontinents in the Wilson cycle is one of the defining paradigms for understanding the plate tectonic history beyond the time recorded by seafloor magnetic lineations. In this context the question arises to what extent the location and mode of breakup is controlled by ancient fabric formed during earlier cycles. Complementary to this issue is the question to what extent major episodes of suturing or rifting overprint earlier fabric, and how these earlier events influence recent kinematics. Whereas these questions have been addressed extensively by geological investigations, the dynamics of these processes are likely to be strongly influenced by the rheology of the mid- and lower crust and the mantle part of the lithosphere, best characterised by seismological investigations. The island of Madagascar occupied a key region in both the assembly and the multi-stage breakup of Gondwanaland (Stern 1994), itself part of the super-continent Pangaea. Madagascar consists of amalgamated continental material comprising several distinct tectonic units, with the oldest rocks being of Archaean age (3.2 Ga). Major geological boundaries generally follow a N-S trend mirroring the geographic orientation of the island. Its Proterozoic to early Paleozoic fabric is characterised by a shear-zone network spanning hundreds of kilometers, with some of the branches running oblique to the N-S trend, in particular in the south of the island. More recently during the Neogene, moderate volcanism has occurred in the Central and Northern part of the island, and there are indications of uplift throughout Eastern Madagascar over the last 10 Ma (Roberts et al. 2012). Although Madagascar is located within the interior of the African plate and far away from major plate boundaries, its seismic activity indicates that some deformation is taking place. Present-day kinematic models based on geodetic data and earthquake moment tensors identify a diffuse N-S-oriented minor boundary separating two microplates, which appears to pass through Madagascar (Stamps et al. 2008). Because of its key role in the assembly of Gondwana, numerous geological investigations have been carried out in Madagascar. However, comparatively little is known about the sub-surface structure as few focused seismological studies have been carried out in Madagascar. In this proposal we aim to characterise the lithospheric structure of southern Madagascar using data from a passive seismic experiment, which we carried out from May 2012 to May 2014. This experiment included an approximately E-W oriented profile of broadband stations across the island (operating for two years), and an areal array of short-period stations (operated for one year). The profile crosses all major geologic provinces and the short-period array will be used to map local seismicity and provide additional control on the 3-dimensional structure of the lithosphere/asthenosphere system.