This study uses gravimetric data integrated with recent seismic data published on south Madagascar to investigate geometry of crust–mantle interface. The regional tectonic framework of Madagascar is characterised by anastomosing network of up to 15-km-wide, 600-km-long and north-oriented high-strain zones, which originated during Neoproterozoic convergence. The studied Bouguer anomalies obtained from the International Gravimetric Bureau were high-pass filtered to emphasise short-wavelength gravimetric variations (shorter than 200 km). The Pan-African high-strain zones coincide with the positive gravimetric anomalies suggesting a link with deep seated high-density material. Considering the present-day thickness of the crust (35 km) and its seismic velocity record, the gravimetric anomalies can be visualised as narrow vertical tabular bodies located at the base of the Moho. Modelling further confirmed that such narrow vertical bodies could be stable over geologic time scale since these structures are relatively small (10 to 30 km wide). The vertical tabular bodies possibly reflect material transfer such as vertical motion of sub-crustal weak and possibly partially molten mantle along vertical deformation zones. It is proposed that these structures were initiated by folding of weak mantle–crust interface characterised by low-viscosity contrast between weak mantle and stronger granulitized lower crust during bulk pure shear-dominated horizontal shortening. It is proposed that the cuspate-lobate “mullion-type” geometry mimics rheological inversions of mafic and felsic rocks and shape of folds of variable scale observed in southern Madagascar. The formation of such mega-mullion structures is possibly an expression of “crème brulée” rheological model, where the deformation of the lithosphere is governed by stronger granulitic lower crust and weaker partially molten and/or hydrated mantle.

这项研究使用重力数据与马达加斯加南部发表的最新地震数据相结合，研究了地壳-地幔界面的几何形状。马达加斯加的区域构造框架的特征是，在新元古代汇聚期间就形成了长达15公里，600公里长和北向的高应变带的吻合网络。对从国际重力局获得的布格异常进行了高通滤波，以强调短波重力变化（短于200 km）。泛非高应变区与正重力异常相吻合，表明与深层高密度物质有关。考虑到地壳的当前厚度（35 km）及其地震速度记录，重力异常可以显示为位于Moho底部的狭窄垂直板状体。模型进一步证实，由于这些结构相对较小（10至30 km宽），因此这种狭窄的垂直体在地质时间尺度上可能是稳定的。竖直的板状体可能反映出诸如垂直沿地下部变形带的亚壳软弱地幔（可能是部分熔融的地幔）的垂直运动之类的物质转移。提出这些结构是通过弱地幔-地壳界面的折叠而开始的，其特征是在大块纯剪切为主的水平缩短过程中，弱地幔与强颗粒状的下地壳之间的低粘度对比。有人提出，呈尖瓣状的“竖框型”几何形状模仿了在马达加斯加南部观察到的镁铁质和长英质岩石的流变反转和可变尺度褶皱的形状。这种巨型竖框结构的形成可能是“crèmebrulée”流变模型的一种表达，其中岩石圈的变形由较强的颗粒状下地壳和较弱的部分熔融和/或水化地幔控制。