bernhard steinberger mantle evolution and dynamic topography of the african plate deutsches...
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Bernhard Steinberger
Mantle evolution and dynamic topography of the African Plate
Deutsches GeoForschungsZentrum, Potsdam and
Physics of Geological Processes, Univ. Osloand
Center for Advanced Studies, Oslo
Understanding the mantle contribution to surface uplift and subsidence over time on a large scale
Motivation
•Dynamic topography influences which regions are below sea level, and at what depth, and therefore where sediments and related natural resources may form•Before attempting to compute uplift and subsidence in the geologic past, we must first understand present-day dynamic topography
Present-day topography
•Dynamic topography influences which regions are below sea level, and at what depth, and therefore where sediments and related natural resources may form•Before attempting to compute uplift and subsidence in the geologic past, we must first understand present-day dynamic topography
Present-day topography + 200 m
•Dynamic topography influences which regions are below sea level, and at what depth, and therefore where sediments and related natural resources may form•Before attempting to compute uplift and subsidence in the geologic past, we must first understand present-day dynamic topography
Present-day topography minus 200 m
Outline
Mantle flow models based on seismic tomographyDynamic topography for present-day – computation and comparision with observationsInferring uplift and subsidence in the past from backward-advection of density anomalies and plate reconstructions
Seismic tomography
S-wave models (here: tx2007 of Simmons, Forte and Grand)
Seismic tomography
S-wave models (here: tx2007 of Simmons, Forte and Grand)
• Conversion factor ~ 0.25 (Steinberger and Calderwood, 2006) – 4 % velocity variation ~~ 1 % density variation Remove lithosphere
Seismic tomography
Converted to density anomalies
•Conversion factor ~ 0.25 (Steinberger and Calderwood, 2006) – 4 % velocity variation ~ 1 % density variation Remove lithosphere
Computation of dynamic topography•radial viscosity structure based on mineral physics and optimizing fit to geoid etc. (Steinberger and Calderwood, 2006)
•Computation of dynamic topography through topography kernels (above: stress-free upper boundary; below: normal-stress-free with zero horizontal motion)
Actual topography What to compare computations to for present-day
Actual topography
MINUSIsostatic topography
What to compare computations to for present-day
Actual topography
MINUSIsostatic topography
Non-isostatic topography
=
What to compare computations to for present-day
Comparision non-isostatic vs. dynamic topographyTX2007 tomographyLithosphere removed (cutoff 0.2%)
Non-isostatic topography What to compare computations to for present-day
Non-isostatic topography
MINUSThermal topography
What to compare computations to for present-day
Non-isostatic topography
residual topography
MINUSThermal topography =
What to compare computations to for present-day
Comparision residual vs. dynamic topographyTX2007 tomographyLithosphere removed (cutoff 0.2%)Sea floor cooling removed
Comparision residual vs. dynamic topographyTX2007 tomographyLithosphere not removedSea floor cooling removed
Correlation globally
Correlation on African plate
Correlation and ratio of dynamic vs. residual topography
Ratio globally
Ratio on African plate
Best fit (in terms of variance reduction)
Correlation globally
Correlation on African plate
Correlation and ratio of dynamic vs. residual topography
Ratio globally
Ratio on African plate
Best fit (in terms of variance reduction)
Further improvements by combination with surface tomography models, or ...
Correlation globally
Correlation on African plate
Correlation and ratio of dynamic vs. residual topography
Ratio globally
Ratio on African plate
Best fit (in terms of variance reduction)
Mixing tomography models – here: Princeton P and S models
PRI-P05 PRI-S05
TOPOS362D1 J362D28-P
4 6
TX2007 S20RTS 9 1
4 6
6 4
SAW24B16 SAW642AN
PRI-S05 PRI-P05Harvard Princeton
Berkeley «smean»
2 8
7 3
East West
6 4
Further improvements possible by using other lithosphere modelsBest results when using lithosphere thicknesses from Rychert et al.(based on seismic observations of Lithosphere-Asthenosphere-Boundary) where data are available ...
Further improvements possible by using other lithosphere modelsBest results when using lithosphere thicknesses from Rychert et al.(based on seismic observations of Lithosphere-Asthenosphere-Boundary) Where data are available -- and the lithosphere model TC1 of Irina Artemieva (based on heat flow) elsewhere
Comparision residual vs. dynamic topographyMIX-A tomographyLithosphere from Rychert et al. (2010) and Artemieva (2006)Sea floor cooling removed
How much of the discrepancy is due to errors in observation-based “residual topography” and how much due to errors in modelled “dynamic topography”?What are the regional differences in this discrepancy?How does the agreement depend on spherical harmonic degree?
Instead of looking at dynamic topography “in isolation” we hope to gain insight through also considering the geoid:
Can we match the “expected” correlation and ratio of geoid and topography?
Model prediction for no-slip surface
Model prediction for free-slip surface
Geoid / uncorrectedtopography
Geoid / residual topography
In degree range 16 to 31→ expect high correlation→ expect geoid-topography ratio around 0.01
residual topography too high above degree 10, too low below degree 6 ?
In degree range 16 to 31→ expect high correlation→ expect geoid-topography ratio around 0.01
Higher correlation indicates better residual topography model
In degree range 16 to 31→ expect high correlation→ expect geoid-topography ratio around 0.01
Ratio about 1.4 % indicates better residual topography model
9 58 87 1.19
45
Joint consideration with geoid indicates that discrepancies are, to a larger degree, caused by inaccuracies of residual topography model (e.g. inappropriate crustal model)
9 58
87 1.19 45
geoid
-topogra
phy r
ati
o
Geoid / residual topography
Model predictions
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
CongoAfar
South Africa
KufraChad
Taoudeni
Afar
CongoSouth Africa
KufraChad
Taoudeni
Conclusions→ Present-day dynamic topography computed from mantle density anomalies inferred from tomography→ Need to “cut out” lithosphere→ Better fit through «mixing» tomography models→ Further improved fit with lithosphere models based on thermal and (where available) seismic data→ Joint consideration of geoid and topography indicates that much of the remaining misfit is due to errors in residual topography. → Past dynamic topography through combining plate reconstructions in absolute reference frame with backward-advected density and flow→ Problem: signal decays back in time→ Possible solution (partially): adjoint methods