Astenosphere entrainment Astenosphere entrainment at a subduction zone:at a subduction zone:

numerical and laboratory numerical and laboratory experimentsexperiments

J. Hasenclever*, J. Phipps MorganJ. Hasenclever*, J. Phipps Morgan††, M. Hort*, M. Hort*, L. Rüpke, L. Rüpke‡‡

** Institut für Geophysik, Universität Hamburg Institut für Geophysik, Universität Hamburg† † Cornell University, Ithaca, New YorkCornell University, Ithaca, New York‡ ‡ Leibniz-Institut für Meereswissenschaften IFM-Leibniz-Institut für Meereswissenschaften IFM-Geomar, KielGeomar, Kiel

22

Typical sketch of mantle flow at a Typical sketch of mantle flow at a subduction zonesubduction zone

Mantle flows in concert with the oceanic Mantle flows in concert with the oceanic lithospherelithosphere

In the mantle wedge a corner flow occursIn the mantle wedge a corner flow occurs

33

Typical sketch of mantle flow at a Typical sketch of mantle flow at a subduction zonesubduction zone

Mantle flows in concert with the oceanic Mantle flows in concert with the oceanic lithospherelithosphere

In the mantle wedge a corner flow occursIn the mantle wedge a corner flow occurs

? ?

44

OverviewOverview 1. Hints for an asthenosphere and its 1. Hints for an asthenosphere and its

propertiesproperties 2. Governing equations & numerical method2. 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 implications6. Results and implications

55

Hints for a weakHints for a weak(suboceanic) asthenosphere layer(suboceanic) asthenosphere layer

zone of low seismic velocity zone of low seismic velocity [[GutenbergGutenberg, , 1959; 1959; Dziewonski and AndersonDziewonski and Anderson, 1981], 1981]

and high attenuation and high attenuation [[Widmer et al.Widmer et al., 1991], 1991] postglacial rebound in Icelandpostglacial rebound in Iceland [ [Sigmundsson Sigmundsson

and Einarssonand Einarsson, 1992], 1992]

andand distribution of stresses in oceanic plates distribution of stresses in oceanic plates [[Richter and McKenzieRichter and McKenzie, 1978; , 1978; Wiens and Wiens and SteinStein, 1985], 1985] => 10=> 1018 18 – 10– 1019 19 Pa sPa s

enhanced electrical conductivity at ~100-enhanced electrical conductivity at ~100-300 km depth 300 km depth [[Oldenburg, Oldenburg, 19811981; Constable, ; Constable, 1992]1992]

66

Scenario: plume-fed asthenosphere ?Scenario: plume-fed asthenosphere ?

[[Phipps Morgan et al.Phipps Morgan et al., 1995], 1995]

77

Scenario: plume-fed asthenosphere ?Scenario: plume-fed asthenosphere ?

material originates from mantle plumesmaterial originates from mantle plumes

=> ~200°C hotter than “normal” => ~200°C hotter than “normal” mantlemantle

material might be compositionally material might be compositionally buoyant due to prior melting processesbuoyant due to prior melting processes

depletion and temperature lead to depletion and temperature lead to a ~1% reduced density anda ~1% reduced density anda 10 to 1000-fold reduced viscositya 10 to 1000-fold reduced viscosity

88

The 2-D numerical modelThe 2-D numerical model

Constitutive equation for Newtonian fluidsConstitutive equation for Newtonian fluids

0

uy

v

x

u

i

j

j

iij x

u

x

u

0

iij

ij gx

p

x

Force balance equationForce balance equation

Viscous (Stoke‘s) flow model in Boussinesq approx.Viscous (Stoke‘s) flow model in Boussinesq approx.

Incompressibility constraintIncompressibility constraint

99

Newtonian viscosity lawNewtonian viscosity law

02

2

2

2

z

T

x

T

z

Tv

x

Tu

t

T

M

aD TTR

EDT

11exp,

*

Solved by high precision finite-Solved by high precision finite-difference algorithm (MPDATA)difference algorithm (MPDATA)

Heat advection-diffusion equation Heat advection-diffusion equation

Flow field solved by finite-element algorithmFlow field solved by finite-element algorithm

Buoyancy termBuoyancy term

DTTDT Mo 1,

1010

Simulating the “oceanic side“Simulating the “oceanic side“

oceanic sideoceanic side

1111

1212

1313

1414

Simulating the “overriding plate side“Simulating the “overriding plate side“

OverridingOverridingplate sideplate side

1515

asthenosphereasthenosphere : 10: 102020 Pa s Pa s 1 % less dense1 % less densemesospheremesosphere : 10: 102121 Pa s Pa s age of oceanic lithosphere: 160 age of oceanic lithosphere: 160 MaMa

1616

ResultsResults Entrainment rate increases with plate speed Entrainment rate increases with plate speed

and asthenosphere viscosityand asthenosphere viscosity Age of the slab is important: enhanced Age of the slab is important: enhanced

asthenosphere “freezing“ on top of old slabsasthenosphere “freezing“ on top of old slabs Thickness of entrainend layer amounts to 15 Thickness of entrainend layer amounts to 15

to 35 km and 25 to 75 km at the slab‘s to 35 km and 25 to 75 km at the slab‘s bottom and top side, respectivelybottom and top side, respectively

Suboceanic asthenosphere counterflow Suboceanic asthenosphere counterflow Decoupling of plate motion and deeper Decoupling of plate motion and deeper

mantle flowmantle flow Tilt of the asthenosphere mesosphere Tilt of the asthenosphere mesosphere

interfaceinterface Circulation / stagnation in the mantle wedgeCirculation / stagnation in the mantle wedge

1818

Laboratory experimentsLaboratory experiments

Plexiglas reservoir (50 x 30 x 10cm) Plexiglas reservoir (50 x 30 x 10cm) filled with two unequal layers of filled with two unequal layers of Glucose-syrupGlucose-syrup

Additional water reduces viscosity Additional water reduces viscosity and density of the upper layerand density of the upper layer

Highlighted glass beads visualize Highlighted glass beads visualize flow patternsflow patterns

Plate motion induced by a pulled Plate motion induced by a pulled plastic filmplastic film

1919

Setup of lab experimentsSetup of lab experiments Plastic film (red) is pulled along the upper Plastic film (red) is pulled along the upper

and inclined boundaryand inclined boundary Film is driven by DC motor and reduction Film is driven by DC motor and reduction

gear (0,5 up to 5 cm/min)gear (0,5 up to 5 cm/min)

2020

2121

Results of the lab experimentsResults of the lab experiments

Laboratory experiments support the Laboratory experiments support the results of the numerical simulationsresults of the numerical simulations

Asthenosphere entrainment, Asthenosphere entrainment, counterflow, and interface tilt are counterflow, and interface tilt are observedobserved

Numerical simulations of the laboratory Numerical simulations of the laboratory experiments agree with the measured experiments agree with the measured data in the laboratory (code verified)data in the laboratory (code verified)

On the oceanic side no 3D character of On the oceanic side no 3D character of the subduction process is observedthe subduction process is observed

(2D numerical approximation valid)(2D numerical approximation valid)

2222

Summary & implicationsSummary & implications Simplified model: e.g. dewatering, phase Simplified model: e.g. dewatering, phase

transitions, meltingtransitions, melting Low viscous asthenosphere decouples Low viscous asthenosphere decouples

plate motion and deeper mantle flowplate motion and deeper mantle flow Circulation and stagnation in the mantle Circulation and stagnation in the mantle

wedge may have impact on magma wedge may have impact on magma genesisgenesis

Entrained asthenosphere may be Entrained asthenosphere may be important for the evolution of the mantleimportant for the evolution of the mantle

Laboratory experiments are useful tools Laboratory experiments are useful tools to verify numerical resolution and the to verify numerical resolution and the “dimensionality“ of geodynamic “dimensionality“ of geodynamic processesprocesses

2323

Refined mantle flow and Refined mantle flow and entrainment processes at a entrainment processes at a

subduction zone in the presence of subduction zone in the presence of a buoyant, hot, and weak a buoyant, hot, and weak

asthenosphere layerasthenosphere layer