Crustal properties of the northern Scandinavian mountains and Fennoscandian shield from analysis of teleseismic receiver functions

Abstract

S U M M A R YThe presence of high mountains along passive margins is not unusual, as shown by their presence in several regions (Scandinavia, Greenland, East US, SW Africa, Brazil, West India and SE Australia). However, the origin of this topography is not well understood. The mountain range between the Scandinavian passive margin and the Fennoscandian shield is a good example. A simple Airy isostatic model would predict a compensating root beneath the mountains but existing seismic measurements of variations in crustal thickness do not provide evidence of a root of sufficient size to produce the necessary compensation. In order to better constrain the physical properties of the crust in northern Scandinavia, two broad-band seismic networks were deployed between 2007 and 2009 and between 2013 and 2014. A new map of crustal thickness has been produced from P-receiver function analysis of teleseismic data recorded at 31 seismic stations. The map shows an increase in crustal thickness from the Atlantic coast (38.7 ± 1.8 km) to the Gulf of Bothnia (43.5 ± 2.4 km). This gradient in thickness demonstrates that the Moho topography does not mirror the variation in surface topography in this region. Thus, classical Airy isostatic models cannot explain how the surface topography is supported. New maps showing variation in Poisson's ratio and Moho sharpness together with forward and inverse modelling provide new information about the contrasting properties of the Fennoscandian shield and crust reworked by the Caledonian orogeny. A sharp Moho transition (R > 1) and low value of V s (3.5 ± 0.2 km s −1 ) are observed beneath the orogen. The shield is characterized by a gradual transition across the Moho (R < 1) and V s of 3.8 ± 0.1 km s −1 which is more typical of average continental crust. These observations are explained by a Fennoscandian shield underplated with a thick layer of high velocity, high density material. It is proposed that this layer has been removed or reworked beneath the orogen.

ScanArray—A Broadband Seismological Experiment in the Baltic Shield

“…The presence of a high velocity lower crust in the Swedish part of the profile may serve to maintain the lower topography in this slightly thinner crust. Crustal thickness and structure derived from receiver functions based on the SCANLIPS2 and SCALIPS3D data around the northern dome do not indicate that crustal Airy isostasy is the cause of the very high topography here [Ben Mansour et al, 2018]. However, eastward crustal thickening may also here be accompanied with the presence of a high velocity, high-density lower crust below the Fennoscandian shield, which is probably absent below the Caledonian Scandes, thereby at least partially compensating the topography through Pratt isostasy.…”

The Moho Architecture and Its Role for Isostasy—Insights From the Lofoten‐Vesterålen Rifted Margin, Norway

“…(2018) used Ps/P amplitude ratio as a measure for Moho sharpness, which shows a very similar regional pattern in the overlapping study area: a sharper Moho along the southern coastline, transitioning to a more diffuse Moho in a S‐N oriented band between ∼15° and 16°E, followed by two regions of alternating sharp and diffuse Moho in the interior of Fennoscandia. Both Ben Mansour et al.’s (2018) and our model have a strong coast/Caledonian‐parallel strike of these Moho sharpness domains, with the sharpest Moho occurring beneath the region of highest topography in the Upper Allochthon, which was not captured to same extent by the other models.…”

Seismic model of the Earth's crust and upper mantle beneath the Fennoscandian Shield

“…First and foremost, mention should be made of the Fennolora [4,5] and Quartz [6] profiles. In the Fennoscandian Shield, over 50 deep seismic profiles were obtained in order to study the deep lithospheric structure of this major unit of the East European Platform [7][8][9][10][11][12][13][14][15][16][17]. The Earth's crust of the region was shown to display a heterogeneous 'mosaic' structure, with no persistent seismic boundaries across the entire shield.…”