“Coherent intrusion complexes” in large basaltic volcanoes — a new structural model

Abstract

Highly concentrated “coherent intrusion complexes” consisting of thousands of small mafic intrusions occur in probably all major basaltic volcanoes and play an important role in volcano development. Magma excursions from the high-level chamber travel laterally along a surface of neutral buoyancy at the margin of a complex and cause the complex to grow. The limited distance, however, that narrow intrusions can propagate before becoming blocked causes complexes to be wedge like. Intrusive-dike complexes underlie rift zones, and asymmetric growth of the dike wedge causes rift zones of shield volcanoes to become non-collinear and may initiate a third rift zone in the obtuse angle. Downbowing of stress trajectories across complexes causes dikes to be non-vertical and results in axial subsidence. Intrusive-sheet complexes form instead of dike complexes in volcanic systems that have a restricted ability to expand laterally and accommodate intrusions by expanding vertically instead. Downbowing of stress trajectories causes sheets to be non-horizontal, and this combined with subsidence increasing toward the thicker part of the sheet wedge produces the inward and inwardly increasing dip that characterizes cone-sheet complexes. This mechanism for cone sheets differs considerably from previously proposed mechanisms. Successive injections of sheets at the top of a sheet complex probably offers the most efficient means of powering a high-intensity geothermal field such as the 5000 MW Grimsvotn system in Iceland. It is inferred that similar mechanisms to those in major basaltic edifices operate in spreading ridges; study of basaltic edifices has the potential to contribute significantly to the understanding of spreading ridges.