On Topographic Subsidence at Initiation of Magmatic Provinces

Increasing the density of parts of the lithosphere is an inevitable result of magma conduit formation at the initiation of a magmatic province. Sufficient volumes of magma will create dikes wide enough to propagate upward without freezing, but some magma will travel through small dikes or by percolation, and will freeze into the lithosphere. Freezing magmas in the eclogite stability region necessarily creates a dense lithosphere. This density increase is likely to cause topographic subsidence.

Numerical models of emplacement over a hot mantle upwelling indicate that this density increase creates a broad region of topographic subsidence. The dense material in the lithosphere is unstable and sinks into the mantle in a series of narrow instabilities. This sequence of instabilities causes variable uplift and subsidence at short wavelength that are superimposed upon the broad basin. Thus basin formation may be a natural result of the initiation of a magmatic province. Increasing evidence from magmatic provinces may support this hypothesis. The Siberian flood basalts and the Ontong Java plateau show evidence for topographic subsidence early in the eruptive process, and recent re-interpretation of the early sequences of clastic deposits in the Emeishan flood basalt province as marine mafic volcaniclastic deposits indicates a similar lack of substantial initial uplift.