Dan Dyrelius

Deformation of the Baltic continental crust during Caledonide intracontinental subduction: Views from seismic reflection data

Deep seismic profiling in Norway and Sweden provides documentation of the amount and style of deformation of the Baltic crust in response to Caledonian intracontinental subduction. The seismic data demonstrate that the preserved Caledonide allochthon in central Scandinavia is 15 to 20 km thick, considerably greater than the 2 to 5 km predicted from surface geologic data. Much of the thickness of allochthonous rocks results from imbrication of the Baltic basement within a thin-skinned-style thrust complex. Seismic data across the Skardora antiform, one of the basement windows in the central Caledonides, indicate that the window represents an antiformal stack of thrust sheets involving basement and cover rocks. These data suggest that many of the basement windows in the Scandinavian Caledonides may result from similar thrust culminations and that the basement windows do not expose autochthonous Baltic basement. Documentation of the response of the Baltic crust during intracontinental subduction provides new constraints on reconstructions of the Scandinavian Caledonides and insights into the style of deformation occurring in other collisional orogens.

Deep crustal seismic reflection profiling across two major tectonic zones in southern Sweden

Abstract A seismic reflection profile has been shot in Varmland, southwestern Sweden, across two major tectonic zones, the Protogine Zone and the Mylonite Zone. The crustal bedrock units separated by the tectonic zones are clearly distinguished in the seismic profile by changes in the reflection character in the upper 5–6 km. The Protogine Zone is represented by a 15–20 km wide band of dipping structures. The Mylonite Zone dips too steeply to be seen directly and a lack of clear reflectors at depth makes a continuation down through the crust hypothetical. In the uppermost 5–6 km between the two tectonic zones, stronger and more continuous reflections can be seen, and are interpreted as being connected with “hyperite” intrusions, which have been mapped at the surface. The lower crust shows generally little reflectivity, with only short and weak reflectors. An exception occurs at mid-crustal depths where a number of clearly recognizable bands of reflected energy are suggested to be major shear zones formed during Sveconorwegian-Grenvillian thrusting towards the east. A slight increase in reflectivity in the depth range 42–48 km correlates well with the Moho depth determined from refraction seismic studies. The possible relationships between data acquisition parameters and the observed scarcity and low strength of reflections from the deeper crust have been considered in some detail. It is shown that the use of 28 Hz single geophones instead of 10 Hz geophone strings along a major part of the profile did not reduce the signal-to-noise ratio for deep reflections. However, the varying source characteristics, in combination with the different quality of the recordings, made computation of the residual static corrections difficult, which may have resulted in a partly destructive stacking of seismic traces. In this way, the low reflectivity of the deeper crust in this area is further brought out in the stacked seismic sections.

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

Abstract The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

The CABLES project: Imaging deep crustal structures in the Scandinavian Caledonides

The CABLES Project: Imaging deep crustal structures in the Scandinavian Caledonides (abstract)

COSC Geophysical and Geological Site Investigations

Drilling of the first borehole, about 2.5 km deep, for the continental scientific deep drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides) is scheduled to begin in the summer of 2013. Here we present the project, a 3D interpretation of seismic data, combined with surface geology and potential field data, used for locating the most suitable drill site and planning of the drilling. An evaluation of the seismic interpretations by constrained 3D inverse modeling of potential field data shows a good fit to observed data, further supporting the choice of the drill site.

On “Mapping remanent magnetization using the local phase” (J. B. Thurston, 2001, GEOPHYSICS, 66, 1082–1089)

Thurston (2001) proposes a method and presents synthetic and practical results that we find difficult to understand both theoretically as well as practically.