Mechanism of Emplacement of Flood Basalt

点击翻译按钮获取中文摘要

9706396 Coe Flood basalts are the second most significant accumulation of mafic igneous material on the surface of the Earth, and their rate of eruption has long been considered to be three to four orders of magnitude greater than occurs at present-day central basaltic volcanoes. This idea depends on the assumption that a single flow is emplaced without significant interruption. Recently it has been proposed that such flows may have grown endogenously, that is, in a multistage process by which lava is injected into the interior of an already solidified flow and jacks up the top surface. If endogenous growth is a common mechanism of emplacement, it would reduce the enormous eruption rates and associated rapid production or storage of vast quantities of melt that are required for single-stage emplacement of very large individual lava flows characteristic of these provinces. Our pilot project is to test the hypothesis of endogenous growth by determining the magnetic fabric of the Birkett flow of the Columbia River Flood Basalts, for which field observations of interior vesicle layers have already suggested the possibility of multistage emplacement. Magnetic fabric analysis can detect zones where more intense shear occurred during flow: for multistage emplacement, two or more shear boundaries should be present inside the flow. If the vesicular layers coincide with shear boundaries, consistent with the injection of lava after initial solidification, the stage will be set for a larger investigation of the role of endogenous growth in continental flood basalts.