Geophysical evidence for silicic crustal melt in the continents: where, what kind, and how much?

Abstract:

A fundamental question about restless silicic magma systems is the distribution, composition, and amount of potentially eruptible magma. Geophysical observations provide constraints on these properties, but they suffer from limited resolution and inherent non-uniqueness. To overcome some of the limitations, we can use different, yet complementary, geophysical approaches -- including electromagnetic methods, gravity, geodesy, imaging both seismic S and P- wave velocities, receiver functions, and seismic attenuation -- combined with petrological, laboratory, and geochemical measurements. In this talk, we summarize some the lessons learned from such integrated studies that have recently been conducted or are ongoing in the central Andes, Mt. St. Helens, and a few other transcrustal magmatic systems. A common result is geophysical anomalies extending from the base of the crust to the surface indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. Fluid movements in these different reservoirs can produce a range of ground deformation as measured with geodesy and geomorphology over timespans of years to > 10,000 years – from permanent uplift related to magmatic intrusions to cycles of reversible uplift and subsidence caused by magma mush reorganization. The characteristics of the geophysical anomalies differ somewhat depending on the technique used - reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, temperature, connectivity, and volatile content, and highlighting the need for integrated, multi-disciplinary studies. Taking a global perspective on large silicic systems reveals that several have >10% partial melt over large areas (10s of km2), and there may be localized zones with 50% or more. We discuss reasons why some of these estimates might be too low or too high. While no certain evidence currently exists of large volumes of eruptible magma, we should not be satisfied that no such evidence can be found. To identify these zones with confidence, further work is needed, including: deployments of multi-sensor dense networks of geophysical instruments; improved inversion techniques; and lab experiments to determine the subsurface characteristics that match the geophysical observations.