The causes of spatiotemporal variations in erupted fluxes and compositions along a volcanic arc

Abstract

Decades of study on volcanic arcs have provided insight into the overarching processes that control magmatism, and how these processes manifest at individual volcanoes. However, the causes of ubiquitous and dramatic intra-arc variations in volcanic flux and composition remain largely unresolved. Investigating such arc-scale issues requires greater quantitative comparison of geophysical and geochemical data, linked through sets of common intensive variables. To work towards these goals, we use observed lava compositions to estimate the heat budget associated with Quaternary volcanism in the Cascades Arc and compare this to the heat required to produce the observed geophysical properties of the crust. Here we show that along-strike volcanic variability in the Quaternary Cascades Arc is primarily related to variations in the flux of basalt into the crust, rather than variations in their crustal storage history. This approach shows promise for studying other large-scale frontier geologic problems in volcanic arcs.

Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

“…Similar patterns are observed in surface heat flow measurements (Ingebritsen & Mariner 2010), with higher heat flow in the southern Cascades. These correlations are identified and analysed by Till et al (2019), who show that quantitative agreement exists between our 10-60 s phase velocities and average surface heat flow, erupted volcanic volumes, and the petrologically estimated magmatic heat budget along the entire Cascades arc. These correlations are strongest from 20 to 60 s, the period range primarily sensitive to lower crustal and upper-mantle structure.…”

“…This observation suggests that heat input from magma input in the lower crust and upper mantle associated with extrusive volcanism drives the heating throughout the arc crust. The correlation at 40-60 s implies that the variation in temperature and perhaps melt content extends into the mantle wedge supporting vertically connected arc magmatism, with higher extents of heating and/or melt content in the southern Cascades (Till et al 2019). At periods longer than 60 s, where peak sensitivities are at >100 km depth, these data sets are more weakly correlated.…”

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…To quantify the extent to which composition explains the lateral velocity variations of the lower crust, we calculate Vs at a range of temperatures consistent with surface heat flow for Siletz gabbro compositions, following the calculation of Till et al (). Forearc heat flow of 34 ± 4 mW/m 2 (see the supporting information) is extrapolated to midcrustal depths for an assumed thermal conductivity of 2.0 ± 0.5 W/m/K (Pollack et al, ; van Keken et al, ).…”

“…Forearc heat flow of 34 ± 4 mW/m 2 (see the supporting information) is extrapolated to midcrustal depths for an assumed thermal conductivity of 2.0 ± 0.5 W/m/K (Pollack et al, ; van Keken et al, ). Heat production in gabbros should be negligible and the crust appears to be in thermal steady state so a linear geotherm is appropriate (Till et al, ). The resulting temperatures are then used to calculate Vs at depth in the forearc from petrologic models (Figure ).…”

“…The resulting temperatures are then used to calculate Vs at depth in the forearc from petrologic models (Figure 12). Specifically, we estimate modal mineralogy from major element oxide compositions for a suite of 16 samples of the Crescent-Siletz basalt (Phillips et al, 2017;Sisson et al, 2014) using the Perple_X free-energy minimization 10.1029/2019JB017836 Journal of Geophysical Research: Solid Earth algorithm (Connolly, 2005) and parameters described previously (Obrebski et al, 2015;Till et al, 2019). From these compositions we calculate Vs as a function of pressure and temperature following Abers and Hacker (2016); Figure 12a).…”