Astenosphere entrainment at a subduction zone: numerical and laboratory experiments J. Hasenclever*, J. Phipps Morgan †, M. Hort*, L. Rüpke ‡ * Institut für Geophysik, Universität Hamburg † Cornell University, Ithaca, New York ‡ Leibniz-Institut für Meereswissenschaften IFM-Geomar, Kiel

2 Typical sketch of mantle flow at a subduction zone Mantle flows in concert with the oceanic lithosphere Mantle flows in concert with the oceanic lithosphere In the mantle wedge a corner flow occurs In the mantle wedge a corner flow occurs

3 Typical sketch of mantle flow at a subduction zone Mantle flows in concert with the oceanic lithosphere Mantle flows in concert with the oceanic lithosphere In the mantle wedge a corner flow occurs In the mantle wedge a corner flow occurs ? ?

4 Overview 1. Hints for an asthenosphere and its properties 1. Hints for an asthenosphere and its properties 2. Governing equations & numerical method 2. Governing equations & numerical method 3. Simulating the “oceanic side“ 3. Simulating the “oceanic side“ 4. Simulating the “overriding plate side“ 4. Simulating the “overriding plate side“ 5. Setup and conduction of the laboratory 5. Setup and conduction of the laboratory experiments experiments 6. Results and implications 6. Results and implications

5 Hints for a weak (suboceanic) asthenosphere layer zone of low seismic velocity [Gutenberg, 1959; Dziewonski and Anderson, 1981] zone of low seismic velocity [Gutenberg, 1959; Dziewonski and Anderson, 1981] and high attenuation [Widmer et al., 1991] postglacial rebound in Iceland [Sigmundsson and Einarsson, 1992] postglacial rebound in Iceland [Sigmundsson and Einarsson, 1992] and distribution of stresses in oceanic plates [Richter and McKenzie, 1978; Wiens and Stein, 1985]=> – Pa s enhanced electrical conductivity at ~ km depth [Oldenburg, 1981; Constable, 1992] enhanced electrical conductivity at ~ km depth [Oldenburg, 1981; Constable, 1992]

6 Scenario: plume-fed asthenosphere ? [Phipps Morgan et al., 1995]

7 Scenario: plume-fed asthenosphere ? material originates from mantle plumes material originates from mantle plumes => ~200°C hotter than “normal” mantle material might be compositionally buoyant due to prior melting processes material might be compositionally buoyant due to prior melting processes depletion and temperature lead to depletion and temperature lead to a ~1% reduced density and a 10 to 1000-fold reduced viscosity

8 The 2-D numerical model Constitutive equation for Newtonian fluids Constitutive equation for Newtonian fluids Force balance equation Force balance equation Viscous (Stoke‘s) flow model in Boussinesq approx. Incompressibility constraint Incompressibility constraint

9 Newtonian viscosity law Newtonian viscosity law Solved by high precision finite-difference algorithm (MPDATA) Solved by high precision finite-difference algorithm (MPDATA) Heat advection-diffusion equation Heat advection-diffusion equation Flow field solved by finite-element algorithm Flow field solved by finite-element algorithm Buoyancy term Buoyancy term

10 Simulating the “oceanic side“ oceanic side

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14 Simulating the “overriding plate side“ Overriding plate side

15 asthenosphere: Pa s1 % less dense mesosphere: Pa s age of oceanic lithosphere: 160 Ma

16 Results Entrainment rate increases with plate speed and asthenosphere viscosity Entrainment rate increases with plate speed and asthenosphere viscosity Age of the slab is important: enhanced asthenosphere “freezing“ on top of old slabs Age of the slab is important: enhanced asthenosphere “freezing“ on top of old slabs Thickness of entrainend layer amounts to 15 to 35 km and 25 to 75 km at the slab‘s bottom and top side, respectively Thickness of entrainend layer amounts to 15 to 35 km and 25 to 75 km at the slab‘s bottom and top side, respectively Suboceanic asthenosphere counterflow Suboceanic asthenosphere counterflow Decoupling of plate motion and deeper mantle flow Decoupling of plate motion and deeper mantle flow Tilt of the asthenosphere mesosphere interface Tilt of the asthenosphere mesosphere interface Circulation / stagnation in the mantle wedge Circulation / stagnation in the mantle wedge

17 Asthenosphere entrainment: Numerical vs. analytical solution

18 Laboratory experiments Plexiglas reservoir (50 x 30 x 10cm) filled with two unequal layers of Glucose-syrup Plexiglas reservoir (50 x 30 x 10cm) filled with two unequal layers of Glucose-syrup Additional water reduces viscosity and density of the upper layer Additional water reduces viscosity and density of the upper layer Highlighted glass beads visualize flow patterns Highlighted glass beads visualize flow patterns Plate motion induced by a pulled plastic film Plate motion induced by a pulled plastic film

19 Setup of lab experiments Plastic film (red) is pulled along the upper and inclined boundary Plastic film (red) is pulled along the upper and inclined boundary Film is driven by DC motor and reduction gear (0,5 up to 5 cm/min) Film is driven by DC motor and reduction gear (0,5 up to 5 cm/min)

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21 Results of the lab experiments Laboratory experiments support the results of the numerical simulations Laboratory experiments support the results of the numerical simulations Asthenosphere entrainment, counterflow, and interface tilt are observed Asthenosphere entrainment, counterflow, and interface tilt are observed Numerical simulations of the laboratory experiments agree with the measured data in the laboratory (code verified) Numerical simulations of the laboratory experiments agree with the measured data in the laboratory (code verified) On the oceanic side no 3D character of the subduction process is observed On the oceanic side no 3D character of the subduction process is observed (2D numerical approximation valid)

22 Summary & implications Simplified model: e.g. dewatering, phase transitions, melting Simplified model: e.g. dewatering, phase transitions, melting Low viscous asthenosphere decouples plate motion and deeper mantle flow Low viscous asthenosphere decouples plate motion and deeper mantle flow Circulation and stagnation in the mantle wedge may have impact on magma genesis Circulation and stagnation in the mantle wedge may have impact on magma genesis Entrained asthenosphere may be important for the evolution of the mantle Entrained asthenosphere may be important for the evolution of the mantle Laboratory experiments are useful tools to verify numerical resolution and the “dimensionality“ of geodynamic processes Laboratory experiments are useful tools to verify numerical resolution and the “dimensionality“ of geodynamic processes

23 Refined mantle flow and entrainment processes at a subduction zone in the presence of a buoyant, hot, and weak asthenosphere layer