Title Mariana melting and thermal structure James A. Conder Dept. of Earth and Planetary Sciences Washington University James A. Conder Dept. of Earth and Planetary Sciences Washington University

Subduction system melting & thermal components From Zyzzy2 on Wikipedia (aka Bob Stern) backarc arc forearc slab

First order thermal controls Convergence rate moderate/? Backarc spreading slow/cold Slab age old/cold

Geophysical constraints Wiens, Kelley, and Plank, 2006 Mariana mantle is relatively cool

Secondary thermal controls Forearc depth extent 2D vs 3D flow 3D/warm?

Temperature-difference grids for different numerical models Conder, 2005, PEPI Temp-dep viscosity - isoviscous Yield strength - Temp-dep visc

- Seismicity begins west of Big Blue Seamount -Shallow seismicity ends at ~55 km depth Courtesy of Erica Emry Seismicity with depth near Big Blue (18ºN)

serpentinized forearc: receiver functions Tibi and Wiens, submitted

Yield strength + serpentine

SSTs Top 350m above wet basalt solidus Likely extent of crustal dewatering Basalt facies & solidus from Hacker et al., 2003 “…melting of fluid-saturated subducted sediment or basalt in normal, steady-state subduction may be inevitable, rather than impossible.” Kelemen et al., 2003

Kincaid and Griffiths mantle motion Kincaid & Griffiths, 2003 Rollback Slab temperatures Slab temperatures

Melting Decompression melting + rehydration to 120km Wedge melting structure

Melting + rehydration to 200km + rehydration to 120km Wedge melting structure

Crustal addition rate Diamalanta et al., 2002 Tori-Shima (Izu), 2002

Backarc-Arc relationship Pagan vs. Guam

Pagan seismic attenuation structure Pozgay, 2007

Pagan Backarc Spreading movie

Pagan melt structure

Backarc-Arc relationship Pagan vs. Guam

Guam Backarc Spreading movie

Summary Forearc/Slab: Serpentinization of the deep Mariana forearc mantle extends the decoupling zone defining the forearc, potentially leading to warmer slab surface temperatures and possible slab melting. Arc: Melting beneath the Mariana arc may have significant contributions from multiple mechanisms: decompression melting ~20 km 3 /Myr/km; decomp+flux ~45; decomp+flux+slab <~ 50. Backarc: Arc and backarc melting have a history of intermixing with melting loci that become more distinct and less intermingled as spreading matures. Pagan is in the latter stages with distinct melting loci for arc and backarc. Melting loci are less distinct near Guam with more intermixing of melting regimes. Forearc/Slab: Serpentinization of the deep Mariana forearc mantle extends the decoupling zone defining the forearc, potentially leading to warmer slab surface temperatures and possible slab melting. Arc: Melting beneath the Mariana arc may have significant contributions from multiple mechanisms: decompression melting ~20 km 3 /Myr/km; decomp+flux ~45; decomp+flux+slab <~ 50. Backarc: Arc and backarc melting have a history of intermixing with melting loci that become more distinct and less intermingled as spreading matures. Pagan is in the latter stages with distinct melting loci for arc and backarc. Melting loci are less distinct near Guam with more intermixing of melting regimes. Anatahan, N. Mariana, 2003