Two-step Gravity Inversion Reveals Variable Architecture of African Cratons

The lithospheric build-up of the African continent is still to a large extent unexplored. In this contribution, we present a new Moho depth model to discuss the architecture of the three main African cratonic units, which are: West African Craton, Congo Craton, and Kalahari Craton. Our model is based on a two-step gravity inversion approach that allows variable density contrasts across the Moho depth. In the first step, the density contrasts are varied for all non-cratonic units, in the second step for the three cratons individually. The lateral extension of the tectonic units is defined by a regionalization map, which is calculated from a recent continental seismic tomography model. Our Moho depth is independently constrained by pointwise active seismics and receiver functions. Treating the constraints separately reveals a variable range of density contrasts and different trends in the estimated Moho depth for the three cratons. Some of the estimated density contrasts vary substantially, caused by sparse data coverage of the seismic constraints. With a density contrast of Δ ρ = 200 kg/m 3 the Congo Craton features a cool and undisturbed lithosphere with smooth density contrasts across the Moho. The estimated Moho depth shows a bimodal pattern with average Moho depth of 39–40 km for the Kalahari and Congo Cratons and 33–34 km for the West African Craton. We link our estimated Moho depth with the cratonic extensions, imaged by seismic tomography, and with topographic patterns. The results indicate that cratonic lithosphere is not necessarily accompanied by thick crust. For the West African Craton, the estimated thin crust, i.e. shallow Moho, contrasts to thick lithosphere. This discrepancy remains enigmatic and requires further studies.

非洲大陆岩石圈的形成在很大程度上仍未得到充分探索。在本文中，我们提出了一个新的莫霍面深度模型，以探讨非洲三个主要克拉通单元的结构，它们分别是：西非克拉通、刚果克拉通和卡拉哈里克拉通。我们的模型基于一种两步重力反演方法，该方法允许莫霍面深度上存在可变的密度差异。第一步，对所有非克拉通单元的密度差异进行改变，第二步则对三个克拉通分别进行改变。构造单元的横向延伸由区域划分图确定，该图是根据近期的大陆地震层析成像模型计算得出的。我们的莫霍面深度受到逐点有源地震学和接收函数的独立约束。分别处理这些约束条件揭示了三个克拉通的密度差异变化范围以及估计的莫霍面深度的不同趋势。由于地震约束数据覆盖稀疏，一些估计的密度差异变化很大。在密度差异为Δρ = 200 kg/m³的情况下，刚果克拉通具有凉爽且未受扰动的岩石圈，莫霍面处的密度差异较为平缓。估计的莫霍面深度呈现出一种双峰模式，卡拉哈里克拉通和刚果克拉通的平均莫霍面深度为39 - 40千米，西非克拉通为33 - 34千米。我们将估计的莫霍面深度与地震层析成像所显示的克拉通延伸以及地形模式联系起来。结果表明，克拉通岩石圈不一定伴随着厚地壳。对于西非克拉通，估计的薄地壳（即浅莫霍面）与厚岩石圈形成对比。这种差异仍然是个谜，需要进一步研究。