Crustal and upper mantle structure of cratons

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.


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


Data: sampled regionsData: sampled regionsBaltic shield

Yangtze craton

Gabon craton

Russian platform

Guyana shield

Kazakhstan

São Francisco craton

Qinlin foldbelt


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


Data: visual curve selectionData: visual curve selection


Data: visual curve selectionData: visual curve selection


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


Data: AMI curve selectionData: AMI curve selectionKEV - LVZKEV - LVZ

phas

e ve

loci

ty [k

m/s

]

frequency [mHz]


Measurements: two-station Measurements: two-station and AMI combinedand AMI combined

Proterozoic craton Archean craton


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


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

.


Results: Archean cratonsResults: Archean cratons


Results: Proterozoic cratonsResults: Proterozoic cratons


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


Results: Results: Average Archean cratonsAverage Archean cratons

Preferred/best fitting model

4.69 km/s2.8%4.62 km/s4.75 km/s



Minimum allowed isotropic velocity

4.61 km/s



Maximum allowed isotropic velocity

4.86 km/s



Maximum allowed anisotropy

6.8%



Minimum allowed anisotropy (isotropic)


Results: depth of lithosphereResults: depth of lithosphere

A LVZ is required below the Guyana shield


Results: depth of lithosphereResults: depth of lithosphere

Minimum depth where the data can still be fit is 190 km


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


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 %


ConclusionsConclusions


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.

Technology

Crustal and upper mantle structure of cratons