Magnetic fabric stratigraphy of a thick rhyolite lava flow

Abstract

In order to illustrate vertical and horizontal variations in the anisotropy of rock fabrics, particularly in highly viscous silicic lavas, we used anisotropy of magnetic susceptibility (AMS) measurements to investigate a thick rhyolitic lava flow with clearly marked flow structures. We describe the relationships between the geometry of the flow structures and the evolution of magnetic fabrics. Material was taken from three drill cores, located at different distances from the eruptive source. The dips of the AMS foliation planes agree with structural measurements taken throughout the section and with increasing distance from the source. This evolution corresponds to the transition flow structure geometries from stubby to lenticular and further to tabular. All of the flow structure geometries present oblate AMS fabrics, which is a characteristic deformation feature of rhyolitic lava flows.