Results from Prior NSF Support: William Menke. Award Number OCE-11965 Amount $304,958; Period 08/15/98-11/30/00, Title Active Seismic Imaging of Axial Volcano, PI's

Period 08/15/98-11/30/00, Title Active Seismic Imaging of Axial Volcano, PI's William Menke & Maya Tolstoy. The region of Axial Volcano, Juan de Fuca Ridge region provides an excellent opportunity to study the interplay between active "hot spot" and "mid-ocean ridge" magmatic systems. Important questions include how the two magma systems are fed; their magma and heat budgets; the degree of interconnectedness (or interaction) between them; their relationship to seismicity and geodetic strains; the role of each in plate-tectonic spreading and and crustal formation; and their effect on the geochemistry (e.g. mixing, fractionation) of erupted basalts. Information on the physical layout of the magma systems is critical to the study of each of these issues. The purpose of this research was to investigate these questions through the tomographic imaging of the region using seismic data from an active seismic airgun-to-obs experiment. The experiment was remarkably successful, both in the sense that voluminous high-quality data were obtained, and in the sense that very clear signals associated with magma were detected in that data. The key elements of the new three-dimensional compressional velocity model of the Axial and Coaxial magma systems are: 1. A Very Large Axial Magma Chamber; A smaller Coaxial Magma Chamber, unconnected with the one at Axial; 3. Several other small low velocity zones are possibly outlier magma chambers from Axial; 4. Strong thickening of the crust beneath Axial volcano. The crust thickens from about 6 km far from Axial to 8 km near Axial to 11 km beneath the summit (West 2001). Publications: Menke et al., Shallow crustal magma chamber beneath the axial high of the Coaxial Segment of Juan de Fuca Ridge at the "Source Site" of the 1993 eruption, Geology 30, 359-362, 2002. West, The deep structure of Axial Volcano, Ph.D. Thesis, Columbia University, 2001. West et al., Magma storage beneath Axial volcano on the Juan de Fuca mid-ocean ridge, Nature 25, 833-837, 2001.