Elsevier

Tectonophysics

Volumes 554–557, 25 July 2012, Pages 30-41
Tectonophysics

Seismic imaging of the Scandinavian Caledonides to define ICDP drilling sites

https://doi.org/10.1016/j.tecto.2012.05.026Get rights and content
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open access

Abstract

A 36 kilometer long high resolution 2D seismic reflection profile was acquired in the summer of 2010 to be used in the planning of the COSC (Collisional Orogeny in the Scandinavian Caledonides) Deep Drilling Project. Two fully cored boreholes, each to c. 2.5 km depth, are planned for the Åre-Mörsil area of west-central Sweden in order to increase our understanding of orogenic processes and, in particular, the tectonic evolution of the Scandinavian Caledonides.

Besides providing important sub-surface structural information in the vicinity of the potential drill sites, the seismic profile also provides detailed, high resolution images previously not available for the uppermost few kilometers in the region. The subsurface is highly reflective and very complex down to at least 9 km depth (the limit of decoded data) with clear reflections spanning the entire length of the profile.

Correlation with previous regional reflection seismic and magnetotelluric surveys has been achieved by acquisition of a short (7 km) connecting profile. A clearly defined reflection, present in the new profile at depths between c. 2.5 km in the east and c. 4.5 km in the west and with an average westwards dip of c. 3.5°, apparently defines the base of the Lower Allochthon. Closer to the Caledonian front, this sole thrust overlies the Cambrian alum shale formation, which rests unconformably on the autochthonous Precambrian crystalline basement. The latter is remarkable for its deep internal reflectivity which is probably related to mafic intrusions in a dominantly granitic host-rock; their deformation may be of both Caledonian and older (e.g. Sveconorwegian) age.

The new high resolution seismic data provide the basis for locating the first borehole in the Seve Nappe Complex. They also demonstrate that the second hole, designed to penetrate the Caledonian basement, will have to be located further east than was originally planned.

Highlights

► COSC aims to further our understanding of the Caledonides and collisional orogens. ► A reflection seismic site survey reveals a highly reflective subsurface. ► The Seve nappe is easily distinguishable, correlating well with surface geology. ► The basal décollement is found at 1.5 km in the east and at 4.5 km in the west. ► On the basis of the new data, two locations are proposed for the two core-holes.

Abbreviations

BL
Byxtjärn-Liten profile, the main profile
CCT
Central Caledonian Transect
COSC
Collisional Orogeny in the Scandinavian Caledonides
ICDP
International Continental Scientific Drilling Program
KF
Kallsjön-Fröå profile, the connecting profile
PTt
Pressure–Temperature–time
SDDP
Swedish Deep Drilling Program
SGU
Swedish Geological Survey
SIST
Swept Impact Seismic Technique
VR
Vetenskapsrådet (Swedish Research Council)
FK
Frequency–wavenumber domain (in FK-filtering)
FX
Frequency–space domain (in FX Deconvolution)

Keywords

Scandinavian Caledonides
Collisional orogeny
Reflection seismic
Borehole
Migration
Continental Scientific Drilling

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