Emplacement and 3D geometry of crustal-scale saucer-shaped intrusions in the Fennoscandian Shield

Sebastian Buntin,Sven Åke Larsson,Hans Thybo,Alireza Malehmir,Annakaisa Korja,Andrzej Górszczyk,Michal Malinowski,Hemin Koyi,Karin Högdahl,Christopher Juhlin

doi:10.1038/s41598-019-46837-x

Abstract

Saucer-shaped intrusions of tens of meters to tens of kilometres across have been observed both from surface geological mapping and geophysical observations. However, there is only one location where they have been reported to extend c. 100 km laterally, and emplaced both in a sedimentary basin and the crystalline basement down to 12 km depth. The legacy BABEL offshore seismic data, acquired over the central Fennoscandian Shield in 1989, have been recovered and reprocessed with the main goal of focusing on this series of globally unique crustal-scale saucer-shaped intrusions present onshore and offshore below the Bothnian Sea. The intrusions (c. 1.25 Ga), emplaced in an extensional setting, are observed within both sedimentary rocks (<1.5 Ga) and in the crystalline basement (>1.5 Ga). They have oval shapes with diameters ranging 30–100 km. The reprocessed seismic data provide evidence of up-doming of the lower crust (representing the melt reservoir) below the intrusions that, in turn, are observed at different depths in addition to a steep seismically transparent zone interpreted to be a discordant feeder dyke system. Relative age constraints and correlation with onshore saucer-shaped intrusions of different size suggest that they are internally connected and fed by each other from deeper to shallower levels. We argue for a nested emplacement mechanism and against a controlling role by the overlying sedimentary basin as the saucer-shaped intrusions are emplaced in both the sedimentary rocks as well as in the underlying crystalline basement. The interplay between magma pressure and overburden pressure, as well as the, at the time, ambient stress regime, are responsible for their extensive extent and rather constant thicknesses (c. 100–300 m). Saucer-shaped intrusions may therefore be present elsewhere in the crystalline basement to the same extent as observed in this study some of which are a significant source of raw materials.

Highlights

Almost three decades ago, a research team from Denmark, Finland, Germany, Sweden and the United Kingdom, through the BABEL (Baltic and Bothnian Echoes from the Lithosphere) project, acquired 2,268 km of marine reflection seismic data and wide-angle refraction data in the Baltic and Bothnian Seas (Fig. 1)
The signals from the reprocessed BABEL lines (1, 6, 7, B and C) acquired in the Bothnian Sea penetrate Paleoproterozoic rocks, two major faults and subsidence-controlled depressions filled with Mesoproterozoic and younger sedimentary rocks
By convention the Mesoproterozoic sedimentary rocks in the Fennoscandian Shield are referred to as Jotnian sandstones (Fig. 1b). Preservation of these sedimentary rocks is to a large part spatially related to the 1.67–1.50 Ga rapakivi intrusions that formed in an extensional setting prior to deposition of the sediments[12]

Summary

Introduction

Almost three decades ago (in 1988–1989), a research team from Denmark, Finland, Germany, Sweden and the United Kingdom, through the BABEL (Baltic and Bothnian Echoes from the Lithosphere) project, acquired 2,268 km of marine reflection seismic data and wide-angle refraction data in the Baltic and Bothnian Seas (Fig. 1). Www.nature.com/scientificreports tectonic processes were active already in the Paleoproterozoic[2] Intriguing features such as 100-km scale dolerite intrusions in the upper crust[3] were observed and tentatively interpreted. We present reprocessing results from BABEL lines 1, 6, 7, B and C (Fig. 1), starting from raw shot gathers and processing to depth converted migrated sections. These lines were chosen due to the great interest in obtaining a better understanding of the tectonic evolution of central Fennoscandia. The felsic rocks originate from partial melting in the lower and middle crust, and the generated granitic magmas were emplaced as 5–10 km thick sheet-like bodies in the upper crust[14]

Methods

Results

Conclusion