crustal and upper mantle structure of cratons
TRANSCRIPT
Crustal and upper Crustal and upper mantle structure of mantle structure of cratonscratons
Jan BoonenJan BoonenUtrecht UniversityUtrecht University
Department of Earth SciencesDepartment of Earth Sciences Graduation talk, 2 December, 2005Graduation talk, 2 December, 2005
Seismic structure of cratons 2
IntroductionIntroduction Cratons are defined as old, stable
continents Seismic structure beneath cratons is not
well constrained:● Depth of keel; ~200 km or >300km?● Anisotropy; isotropic, SH > SV or SV > SH?
High resolution shear wave models are needed to interpret geological history.
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DataData 11 cratons are sampled Seismograms must be within 7° of the
greatcircle connecting the event and the two stations
Minimum event magnitude: M = 4.9 Minimum event distance: 1000 km 6359 events are used
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Data: sampled regionsData: sampled regionsBaltic shield
Yangtze craton
Gabon craton
Russian platform
Guyana shield
Kazakhstan
São Francisco craton
Qinlin foldbelt
Seismic structure of cratons 5
Methods: Methods: cross-correlation methodcross-correlation method
Data is filtered using multiple filter technique to isolate fundamental mode surface waves
Phase of surface waves from two receivers are cross-correlated
Phase is ambiguous (nπ) → dispersion curves are selected visually
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Methods: Automated Methods: Automated Multimode InversionMultimode Inversion
Phase velocities are computed from synthetic seismograms
Average phase velocity for two receivers is calculated
Dispersion curves are quality checked with the visual curve selection tool
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Data: AMI curve selectionData: AMI curve selectionKEV - LVZKEV - LVZ
phas
e ve
loci
ty [k
m/s
]
frequency [mHz]
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Measurements: two-station Measurements: two-station and AMI combinedand AMI combined
Proterozoic craton Archean craton
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InversionInversion Joint inversion of Rayleigh and Love
wave phase velocities Uncertainties are computed using the
selected curves Small inversion problem:
● 10-15 parameters for depth ● 1 parameter for Moho depth
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InversionInversion Non-linear Gauss-Newton inversion
algorithm Every parameter can be damped
towards the a priori model For every parameter a priori information
can be specified, e.g. vSV
> vSH
, vSH
> vSV
.
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Similarity between Similarity between cratonscratons
Phase velocities fall in 2 groups:● Archean cratons● Proterozoic cratons
Guyana shield does not fit the Archean group at long periods
Archean cratonsGuyana shieldProterozoic cratons
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Results: Results: Average Archean cratonsAverage Archean cratons
Preferred/best fitting model
4.69 km/s2.8%4.62 km/s4.75 km/s
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Results: Results: Average Archean cratonsAverage Archean cratons
Minimum allowed isotropic velocity
4.61 km/s
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Results: Results: Average Archean cratonsAverage Archean cratons
Maximum allowed isotropic velocity
4.86 km/s
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Results: Results: Average Archean cratonsAverage Archean cratons
Maximum allowed anisotropy
6.8%
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Results: Results: Average Archean cratonsAverage Archean cratons
Minimum allowed anisotropy (isotropic)
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Results: depth of lithosphereResults: depth of lithosphere
A LVZ is required below the Guyana shield
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Results: depth of lithosphereResults: depth of lithosphere
Minimum depth where the data can still be fit is 190 km
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DiscussionDiscussion Our method is able to resolve an
accurate velocity profile down to 200 km In regions with strong lateral
heterogeneity the method cannot be applied
Small inversion problem enables to focus on important questions
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ConclusionsConclusions Structure of cratons can be split in two
groups with similar characteristics:● Archean cratons (4.61 – 4.86 km/s)● Proterozoic cratons (~4.50 km/s)
Archean cratons have a keel down to 190 km depth
Archean cratons have anisotropy range of 0 – 6.8 %, preferred: 2 – 3 %
Seismic structure of cratons 26
ReferencesReferences Meier, T., Dietrich, K., Stöckhert, B. & Harjes, H.P.
(2004). One-dimensional models of shear wave velocity for the eastern Mediterranean obtained from the inversion of Rayleigh wave phase velocities and tectonic implications. Geophys. J. Int.,156, 45–58.
Lebedev, S., Nolet, G., Meier, T. & van der Hilst, R.D. (2005). Automated multimode inversion of surface and S waveforms. Geophys. J. Int., 162, 951–964.