GEOPHYSICAL AND GEOCHEMICAL CONSTRAINTS ON MAGMA STORAGE DEPTHS ALONG THE CASCADE ARC: KNOWNS AND UNKNOWNS

The volcanoes of the Cascade arc stretch from the Lassen Volcanic Center in northern California, through Oregon and Washington, to the Garibaldi Volcanic Belt in British Columbia. Along the arc there are significant differences in the nature of arc volcanism – including the distribution and stles of eruptive vents, the volumes and geochemical compositions of erupted magmas, and the structural setting and geophysical properties of the crust and upper mantle. There is also a factor of two variation in the flux of mantle-derived basalt along the arc between the northern and southern portions of the arc. Despite these differences it remains unclear how these variations are reflected in the structure of the transcrustal magmatic systems that underlie each volcano, and the conditions of magma residence and storage.

To investigate this, we have compiled available petrological, geochemical and geophysical constraints on magma storage depths for each major Cascade edifice. This compilation highlights the variable amount of data available for different edifices, with abundant geochemical and geophysical data available for some systems (e.g. the Lassen Volcanic Center and Mount St. Helens), but very limited data available for others including many volcanoes assessed as high threat (e.g., Glacier Peak, The Three Sisters). In addition, our study reveals that there are considerable uncertainties in the pressures calculated using commonly-used mineral-only and mineral-liquid barometers.

The available data suggests magma storage depths are remarkably constant along the arc, with seismic, geodetic and petrological estimates suggesting storage at pressures of 200 - 400 MPa or less. Our compilation is consistent with previous work suggesting widespread shallow magma storage within the upper crust in many arcs. Although we also see no clear offset in magma storage between the northern and southern portion of the arcs and between arc segments that are in extension, transpression, or compression, large uncertainties in many depth estimates limit our ability to make the finer scale evaluations of storage pressures needed to understand the controls on magma storage in detail.