LITHOS-CAPP-2 (LITHOspheric Structure of Caledonian, Archaean and Proterozoic Provinces)
Final Report Abstract
First, the project aimed to operate a seismic network in Sweden and Finland as part of the international ScanArray consortium. This technical objective was fulfilled as planned. We will now review the progress with regard to the objectives stated in the proposal: Differences between Svecofennian and Archean provinces at mantle depths; Transition between Caledonian collision zone and the Sevecofennian provinces to the east? Generally, the different cratons and Svecofennian domains have distinct lithospheric properties (thickness and shear velocity). Here, the Archaen lithosphere is generally 150-180 km thick. The thickest lithosphere with 250 km is found below the Paleoproterozoic Keitele microcontinent in south-central Finland. The long tectonic history of the Proterozoic and Archeaen domains is also evidenced by the complex fabric of the lithosphere inferred from shear waves splitting measurements. A first order observation, which shows up in both the surface and body wave tomography models in spite of many differences in detail, is a pronounced gradient in the LAB thickness coincident with the Caledonian front, sharp below the southern and northern Scandes, more gradual below the central Scandes. This is somewhat surprising, as the Caledonides are thought to have overthrust the Svecofennian units such that there is no strong lithological contrast in the lower crust below the central and northern Scandes. Support of the high topography of the northern Scandes. The surface and body wave tomography models confirmed earlier studies that the southern Scandes are supported in the mantle by a very thin lithosphere and a hot asthenosphere, possibly initiated or sustained by a long distance outflow from the Iceland plume. A similar mechanism is also at play in the northern Scandes, albeit the lithosphere is somewhat thicker and the temperature not quite as high as in the south. The sharp lithospheric edges uncovered along the Caledonian front east of the northern and southern Scandes are expected to set up small-scale edge-driven convection cells, which would help to sustain these contrasts in the longer term. A detailed quantification of the contributions of the various mechanisms requires a follow-on study incorporating gravity and elastic thickness effects. Correlation between surface geology and anisotropy. The variability of shear wave splitting measurements, which are partially correlated with geological terranes, demonstrates that the lithosphere contributes the bulk of the observed splitting and the asthenospheric signal due to mantle flow is weak below most of Scandinavia. There is no simple correlation with the surface deformation pattern, though, because clear evidence for dipping anisotropy and multi-layer anisotropy was recovered, which demonstrates that even the Archaean and Proterozoic parts of Scandianvia are not simply uniform blocks, but have accreted gradually by stacking ancient subducting plates. In both PhD projects, unexpected discoveries caused a deviation from the original work plan. For the KIT student, this was the observation that the splitting measurements for SKS and SKKS phases can deviate significantly for some backazimuths, which is inconsistent with a localised origin of the anisotropy, the original target in the context of the proposal objectives. Further investigation of this observation led to a discovery of seismic anisotropy patterns in the lower mantle, resulting in a publication in Geology. For the GFZ student, the unexpected discovery was the observation of a 360° periodicity in phase velocity measurements for selected sub-arrays, which is incompatible with standard assumption on surface wave propagation. We could demonstrate that this pattern can arise from strong lateral heterogeneities in the upper mantle. This effect necessitated adaption of the procedures for the determination of the phase velocity maps from surface waves. While important results were obtained and published in both cases, these deviations took time, such that some of the original work plan pertaining to joint inversion of body and surface waves, and receiver functions and surfaces waves could not be carried out for lack of time. Continued work on this topic would still be a worthwhile extension of the work, however, particularly in order to resolve some apparent differences between the different approaches. The determination of azimuthal anisotropy from surface waves was attempted, but it was not possible to obtain robust results, probably related to the short scale variations of seismic anisotropy which are also in evidence in the SK(K)S results.
Publications
- (2020): Another look at the treatment of data uncertainty in Markov chain Monte Carlo inversion and other probabilistic methods. - Geophysical Journal International, 222, 1, 388-405
Tilmann, F., Sadeghisorkhani, H., Mauerberger, A.
(See online at https://doi.org/10.1093/gji/ggaa168) - 2017. Broadband Recordings for LITHOS-CAPP: LITHOspheric Structure of Caledonian, Archaean and Proterozoic Provinces, Sep. 2014 - Oct. 2016, Sweden and Finland. Scientific Technical Report STR-Data 17/02, GIPP Experiment- and Data Archive; Potsdam
Grund, M., Mauerberger, A., Ritter, J.R.R. and Tilmann, F.
(See online at https://doi.org/10.2312/GFZ.b103-17029) - 2017. StackSplit – a plugin for multi-event shear wave splitting analyses in SplitLab. Computers & Geosciences 105, 43-50
Grund, M.
(See online at https://doi.org/10.1016/j.cageo.2017.04.015) - Dataset: 2012. ScanArray Core (1G 2012-2017). The ScanArray consortium. Other/Seismic Network
Thybo, H., Balling, N., Maupin, V., Ritter, J., Tilmann, F.
(See online at https://doi.org/10.14470/6T569239) - 2019. Exploring geodynamics at different depths with shear wave splitting
Grund, G.
- 2019. Widespread seismic anisotropy in Earth’s lowermost mantle beneath the Atlantic and Siberia. Geology, 47 (2), 123–126
Grund, M.; Ritter, J.R.R.
(See online at https://doi.org/10.1130/G45514.1) - 2020. Shear-wave splitting beneath Fennoscandia - Evidence for dipping structures and laterally varying multi-layer anisotropy. Geophysical journal international, 223 (3), 1525–1547
Grund, M.; Ritter, J.R.R.
(See online at https://doi.org/10.1093/gji/ggaa388) - (2021), ScanArray—A Broadband Seismological Experiment in the Baltic Shield, Seismological Research Letters
Thybo, H., Bulut, N., Grund, M., Mauerberger A., Makushkina, A., Artemieva, I. M., Balling, N., Gudmundsson, O., Maupin V., Ottemøller, L., Ritter. J., Tilmann, F.
(See online at https://doi.org/10.1785/0220210015) - (2021): Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia. - Geophysical Journal International, 224, 3, 1684-1704
Mauerberger, A., Maupin, V., Gudmundsson, Ó., Tilmann, F.
(See online at https://doi.org/10.1093/gji/ggaa553)
