Old sutures and young plumes? – New geophysical investigations of the crust and upper mantle in southwestern Scandinavia

Abstract

This dissertation investigates the physical structure of the crust and upper mantle in southwestern Scandinavia using different seismological and non-seismologic geophysical methods:

1. The S-wave velocity structure of the crust and crust-mantle transition (the Moho) is derived from joint inversion of P-wave receiver functions and Rayleigh wave phase velocities below 42 seismic stations in southern Norway and Sweden.

2. The P- and S-wave velocity structure of the upper mantle in southwestern Scandinavia is investigated using multiscale, finite-frequency, seismic tomography.

3. Temperature and temperature-dependent density of the lithospheric mantle below southern Norway is estimated from thermal and isostatic modelling using several geophysical data sets as constraints (topography, Moho depth, geoid undulations).

The thesis also deals with several practical and theoretical aspects of regional finitefrequency tomography. We present a robust data processing routine for the measurement of relative finite-frequency travel-time residuals and analyze important features of relative finite-frequency Fréchet kernels in a typical regional seismic network.

The work on the crustal and uppermost mantle structure shows that the crust-mantle transition is not necessarily a simple discontinuity. We find a lateral boundary between an area with a sharp and relatively shallow Moho discontinuity and an area with a deep and gradual crust-mantle transition. This boundary is thought to be Proterozoic in age and to have survived the Caledonian orogeny.

The most important features of the upper mantle tomographic models are: a shallow (50-200 km) channel-like low-velocity anomaly between southern Norway and Denmark; a deep (150-350 km) plume-like anomaly with a diameter of ~100km below central southern Norway; and a belt of high velocities below Sweden that delineate the proper Fennoscandian Shield. The wiggly boundary between the low-velocity channel and the high velocities of Sweden follows zones of Carboniferous-Permian rifting and magmatism very closely. The low-velocity channel can be explained by a relatively thin and warm lithosphere (~100 km), whereas the high velocities below Sweden indicate both a thick and cold lithosphere, and a depleted composition of the mantle. The deep plume-like anomaly below southern Norway has structural similarities with small-scale upper-mantle plumes below central Europe, and could be caused by a similar upwelling of material with anomalous temperature and/or composition.

We suggest that episodic erosion of an originally thick mantle lithosphere below southern Norway could have weakened the lithospheric plate and triggered uplift of the area in the Cenozoic.