John R. Hopper Leibniz Institute for Marine Science, Kiel, Germany Thomas K. Nielsen Maersk Olie og Gas A/S, Copenhagen, Denmark John R. Hopper Leibniz Institute for Marine Science, Kiel, Germany Thomas K. Nielsen Maersk Olie og Gas A/S, Copenhagen, Denmark Volcanic Productivity during Continental Breakup from Numerical Modeling of Mantle Convection: Application to Atlantic Rifted Margins Volcanic Productivity during Continental Breakup from Numerical Modeling of Mantle Convection: Application to Atlantic Rifted Margins

North Atlantic Rifted Margins ~ <5 0! ? ? ? ? From: Hopper et al JGR

Single transient pulse of anomalous volcanism (double steady-state) Decay to background productivity in ~10m.y. Variation in crustal thickness since 47 Ma ± 10% - 15% Single transient pulse of anomalous volcanism (double steady-state) Decay to background productivity in ~10m.y. Variation in crustal thickness since 47 Ma ± 10% - 15% Summary of Key Observations

Fundamental guiding principle Fundamental guiding principle A successful model of breakup volcanism should naturally evolve to steady-state, plate-driven oceanic accretion.

Model Formulation Citcom: Multi-grid Finite Element Code Consider convection as a corner flow response to a constant velocity surface boundary condition (will also consider edge-driven convection) Viscosity varies with temperature, pressure, and degree of dehydration due to melting Buoyancy sources include thermal, compositional, and retained melt Melting is implemented following the method of Scott, Citcom: Multi-grid Finite Element Code Consider convection as a corner flow response to a constant velocity surface boundary condition (will also consider edge-driven convection) Viscosity varies with temperature, pressure, and degree of dehydration due to melting Buoyancy sources include thermal, compositional, and retained melt Melting is implemented following the method of Scott, 1992.

High Viscosity Case Low Viscosity Case

Varying the mantle reference viscosity Increasing Viscosity Increasing Viscosity

Effect of dehydration viscosity increase Various assumptions about viscosity and buoyancy sources Various assumptions about viscosity and buoyancy sources

Add 50 km thick hot layer beneath the continent 100 °C 200 °C 100 °C, dehyd. 200 °C, dehyd.

Edge Convection Edge Convection + Rifting Edge Convection + Rifting

Melt Production for Edge Convection + Rifting Previous slide 1% thermal perturb. low viscosity case intermediate visc.

Conclusions Models that exhibit small scale convection and edge convection with significant excess melt productivity never evolve to steady-state oceanic accretion. A dehydration induced viscosity increase stabilizes the system, but lacks the time dependent behavior needed to allow small-scale convection followed by state-state spreading. Models that exhibit small scale convection and edge convection with significant excess melt productivity never evolve to steady-state oceanic accretion. A dehydration induced viscosity increase stabilizes the system, but lacks the time dependent behavior needed to allow small-scale convection followed by state-state spreading.

Conclusions Volcanic margin formation like off Greenland seems to require an exhaustible reservoir of anomalous material beneath the lithosphere at the time of breakup. Characterizing the nature of this layer (chemical or thermal?) and understanding how it is emplaced beneath the continent require further work. Volcanic margin formation like off Greenland seems to require an exhaustible reservoir of anomalous material beneath the lithosphere at the time of breakup. Characterizing the nature of this layer (chemical or thermal?) and understanding how it is emplaced beneath the continent require further work.

For Future Work: Require better understanding of the thermal structure of continents prone to rupture Require better understanding of melt extraction/retention during early stages of melt production (pre-rupture) Require better understanding of the thermal structure of continents prone to rupture Require better understanding of melt extraction/retention during early stages of melt production (pre-rupture)