Proterozoic extensional tectonics of the central Fennoscandian Shield: Results from the Baltic and Bothnian Echoes from the Lithosphere experiment

Abstract

A three‐dimensional crustal structure of the Bothnian Sea is deduced from the near‐vertical reflection data of the crosscutting lines 1, 6, 7, and C of the BABEL (BAltic and Bothnian Echoes from the Lithosphere) experiment. For this study the common depth point stacked sections have been migrated with a velocity 6.5 km/s and displayed as stacked envelope sections to show their enhanced large‐scale crustal structures. The seismic sections show listric shear zones which dip toward SE and flatten at major detachment zones 35–40 km and 48 km in depth. The detachment zones are associated with changes in P wave velocity from 6.8 km/s to 7.0 km/s and from 7.4 km/s to 8.2 km/s and coincide with the middle and lower crustal boundary and the Moho boundary, respectively. The geometry and crosscutting relationship of the detachment zone are well developed on the E–W directed line 7, from where the Moho detachment zone can be interpreted as to be younger. Different shear zone systems operated in the parallel N–S directed lines 1 and 6, and therefore it is not easy to correlate crustal structures between lines 1 and 6. In the thinned southern part of study area, the lower crust has a highly reflective lamellar structure and a strongly reflective subhorizontal Moho. Increased reflectivity of the lower crust is attributed to stretching and mafic underplating of the crust associated with the Subjotnian diabase dykes and rapakivi magmatism. In the upper crust the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures. The distribution of the bimodal association seems to be controlled by the low‐angle listric shear zones. The spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation. The first stage of deformation, which concentrated in the upper and middle crust, records the collapse of the overthickened Svecofennian crust. The second stage involving the entire crust, may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period. The seismic structures predict major post‐Svecofennian deformation/faulting and tilting in the central Fennoscandian Shield. The extensional tectonics operating in the Fennoscandian Shield in the Proterozoic is comparable to modern extensional tectonics.