Structural and Climatic Effects of Large-Scale Basaltic Magmatism

This thesis concerns the causes and consequences of magma emplacement in the Earth’s lithosphere during the formation of Large Igneous Provinces (LIPs) and continental rifts. Motivated by geological, geophysical, geochemical and paleoclimate data, I formulate geodynamic models to address the following questions: (1) How were the massive volumes of subaerially erupted lava, described in multi-channel seismic data as seaward-dipping reflectors (SDRs), formed and what can SDRs tell us about the rifting processes? (2) What thermal and rheological conditions are required to produce the contrast in topography of the two youngest LIPs: namely that the Columbia Plateau sits ~0.7 km lower than the surrounding region while the Ethiopian Plateau is ~1.5 km higher than its surroundings? (3) Why does significant global warming occur a few hundred-thousand years prior to the main phase of eruptions of the Columbia River Basalts and the Deccan Traps? The major results of my thesis are: (1) The first two-dimensional thermo-mechanical treatment of SDR formation shows how the lithosphere thickness affects the deformation in response to magmatic loads during volcanic margin formation. I provide a quantitative mapping between the shape of SDRs and the strength of the lithosphere and this mapping reveals weak continental margin lithosphere during the initial continental breakup. (2) Cold and strong crust results in slow lower crustal flow and a persistent high plateau like the Ethiopian Plateau. In contrast, a combination of three things can produce a low plateau like the Columbia Plateau. First, hot and weak lower crust flows fast in response to topographic and magmatic loads. Second, a significant fraction of the magma intruded in the crust freezes onto and becomes part of the strong upper crust. Finally, the bulk of the intrusions occur before the main phase of extrusion to explain the geometry of the Columbia River Basalt lava flows. (3) I argue that the major eruptions of continental flood basalts may require densification of the crust by intrusion of larger volumes of magma than are extruded. Simple models show that magma crystallization and release of CO₂ from such intrusions could produce global warming before the main phase of flood basalt eruptions on the observed timescale.