PETROLOGIC AND TECTONIC CONNECTIONS IN THE CASCADE ARC: WHAT WE KNOW AND WHAT WE'D LIKE TO KNOW

The Cascade arc is unique with its slow subduction of a hot plate and a back-arc plume. The arc merges with the Basin Range in southern Oregon and northern California above the spreading plume head. This setting has given rise to diverse post-plume (ca. 16 Ma) magmatism within the arc, and some direct relationships between rifting and volcanism.

Melt production along the arc correlates with the degree of partial mantle melting. It is highest during rifting, declines as rifting wanes, and is lowest on the arc ends. Brittle structures form as rift-related melting declines. Heat from the spreading plume head may have fueled a propagating intra-arc rift in northern Oregon and southern Washington. Decompression-related magmas associated with rifting are hot and dry. Systematic changes in magmatic patterns with rifting appear to be driven by increased degrees of crustal partial melting and/or shallower differentiation.

Stratocone foci are long-lived, on the order of 10 Ma. Foci are blurred but not erased by rifting. Baker, Glacier Peak, Rainier, Hood, and Shasta are dominantly andesite erupted through non-rifted crust and approximate single vents. Rift-related stratocones are smaller (Jefferson, Middle and South Sisters, Mazama, Lassen) and are surrounded by dome fields where the eruptive products range from basalt to rhyolite. These relationships strongly suggest that evolution of magma in all of these cones is dominated by mixing.

Vanadium data from basalts suggest a minimum of 1mm/a extension is required for significant magma penetration into the shallow crust. But even during rifting most basalt is probably underplated. Significant underplating is not a static situation, and cross sections of the intra-arc rift basins are asymmetric with profound west-side uplift and east tilting of rifted blocks.

Some things we would like to know: 1) How do stratocone foci form? 2) How does rifting relate to heat flow and magma penetration of the crust? 3) What leads to increased degree of crustal partial melting during rifting? 4) What are the subduction vs decompression signals in mafic magma? 5) What is the crustal architecture beneath the arc? 6) What is the volatile/strain budget driving eruptability? 7) Why do most magmas acquire crystal cargo?