Collisional tectonics of the Baltic Shield in the northern Gulf of Bothnia from seismic data of the BABEL project

Summary Sm–Nd isotope ratios of 1.9–1.8 Ga granitoids delineate the Archaean–Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW–ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south‐southwest, consistent with interpretation of the Sm–Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August 1999. The profile consisted of a high‐resolution shallow component (1 kg shots) and a lower‐resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans‐Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high‐amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west‐dipping contact zone between meta‐sediments and felsic volcanic rocks, however, the seismic data indicate a near‐vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean–Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi‐vergent geometry than previous interpretations. Comparison of the re‐interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c. 1.87 Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi‐vergent crustal structure observed today.

Section: Resultssupporting

confidence: 83%

“…Published interpretations of the BABEL surveys in the Bothnian Bay (BABEL Working Group 1990; Öhlander et al 1993; Lindsey & Snyder 1994; Gohl & Pedersen 1995) generally indicate reflective Archaean crust overlain by transparent crust. This transparent crust has been interpreted as reworked Archaean rocks (Fig.…”

Section: Introductionmentioning

confidence: 99%

Crustal reflectivity near the Archaean-Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area

Juhlin¹,

Elming

Mellqvist³

et al. 2002

“…Lund 1979Lund ,1987Lund ,1990Grad & Luosto 1987;Guggisberg et al 1991;Komminaho & Yliniemi 1992;Riahi & Lund 1994;Gohl & Pedersen 1995). The mean crustal velocity reported by the aforementioned authors for different seismic profiles varies between 6.5 and 6.6 km s-', indicating a rather homogeneous mean velocity in the area.…”

Section: Discussionmentioning

confidence: 76%

Three-dimensional image of the Moho undulations beneath the Gulf of Bothnia using wide-angle seismic data

Riahi

Lund

Pedersen

1997

Self Cite

S U M M A R YThe BABEL marine seismic experiment has been carried out to investigate the lithospheric structure and antecedent tectonic signatures of the Baltic Shield, including the Archaean-Proterozoic collisional structure in the northern part of the Gulf of Bothnia.Lithospheric seismic-reflection streamer data and simultaneously recorded wideangle reflection and refraction data collected in the Gulf of Bothnia as part of the BABEL project have been used for 3-D modelling. The distribution of land stations around the Gulf provides a good 3-D ray coverage of the P,P reflection data recorded at the eight stations in the area and allows an estimation of strikes and dips of the Moho boundary in the area. The traveltimes of reflected phases are calculated using a method that utilizes the finite-difference solution of the eikonal equation. The Moho wide-angle-reflection (PMP) traveltimes are modelled using an inversion method. A 2-D model from the Gulf of Bothnia extended into the third dimension is used as an initial model. During the inversion the velocity is kept constant and only the Moho boundary is allowed to vary. To estimate the strike of the Moho boundary and the stability of the inversion, two initial models with different strikes are examined.The results indicate that the Moho depth in the Gulf of Bothnia undulates and has a maximum depth of 55 km in the south, rising to 42 km in the north. The Moho depth variations seem to be step-like. This change in the Moho depth coincides with the location of the presumed fossil subduction zone in the area. The crustal-thickness variations seem to be well approximated by a nearly 2-D structure striking parallel to a postulated subduction zone immediately to the south of the Skellefte area. The presence of the step at the crust/mantle boundary can be interpreted as a result of a plate-collision event at about 2 Ga.

“…A similar Palaeoproterozoic example is found in the northern Gulf of Bothnia where Archaean crust from the Baltic Shield underlies the Proterozoic northern Svecofennian Province. Deep seismic reflection and refraction data have shown the major features of a collision similar to those observed in SNoRCLE Line 11: a subducted slab, crustal reflections parallel to the slab, a metasedimentary basin, and mafic and ultramafic cumulates between the subducted slab and the overlying Moho (BABEL Working Group 1990; Gohl & Pedersen 1995).…”

Section: Discussionmentioning

confidence: 93%

Lithospheric structure beneath the Archaean Slave Province and Proterozoic Wopmay orogen, northwestern Canada, from a lithoprobe refraction/wide-angle reflection survey

Fernández‐Viejo¹,

Clowes²

2003

SUMMARY The Slave geological province, a relatively small craton in NW Canada that includes the oldest rocks in the world, is one of five cratons that form the Archaean continental core of North America. The Palaeoproterozoic Wopmay orogen, lying between the craton and the Phanerozoic northern Canadian Cordillera, represents crustal growth through addition of a series of north‐south oriented magmatic arcs and accreted terranes. As part of Lithoprobe's multidisciplinary studies, Line 11 of the SNoRE 97 seismic refraction/wide‐angle reflection survey investigated the structure of the lithosphere in this region. Interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling. Results show that crustal thickness at 32–35 km remains relatively constant from the Archaean regions in the east to Proterozoic domains in the west, contrary to average values of crustal thicknesses from global compilations that suggest thicker Proterozoic crust. At the westernmost end of the line, where crustal extension and basin formation are inferred, the crust thins to 30 km. Upper crustal velocities to about 12 km depth lie in the range of 5.8–6.2 km s−1. The middle crust of thickness 8–10 km has velocities between 6.2 and 6.6 km s−1. Average velocities for the lower crust, differentiated as a 10 km thick layer, range between 6.6 and 6.9 km s−1 with a higher velocity region (7.1 km s−1) in the transition zone between the Fort Simpson magmatic arc and Hottah accreted terrane in the Wopmay orogen, where a Palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data. In the same area, anomalously low upper‐mantle velocities (7.5 km s−1) have been found, indicating that the effects of collision, subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure, may have been retained for 1800 Ma. Data from most record sections with offset distances greater than 250 km show continuous wide‐angle reflections from upper‐mantle depths of about 75 km and almost 100 km. Although poorly constrained, velocities in this depth range vary from 8.4 to 8.8 km s−1. Interpretation of these phases and mantle reflections from equivalent depths in coincident, near‐vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the Palaeoproterozoic orogen.