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Crustal Architecture of the Inverted Central Lapland Rift Along the HUKKA 2007 Profile |
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| Authors: | Timo Tiira Tomasz Janik Elena Kozlovskaya Marek Grad Annakaisa Korja Kari Komminaho Endre Heged?s Csaba Attila Kovács Hanna Silvennoinen Ewald Br?ckl |
| Institution: | 1. Department of Geosciences and Geography, University of Helsinki, POB 68, 90014, Helsinki, Finland 2. Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, 01-452, Warsaw, Poland 3. Sodankyl? Geophysical Observatory/Oulu Unit, University of Oulu, POB 3000, 90014, Oulu, Finland 4. Institute of Geophysics, Faculty of Physics, University of Warsaw, Pasteura 7, 02-093, Warsaw, Poland 5. E?tv?s Loránd Geophysical Institute of Hungary, 1445, Budapest, Hungary 6. Institute of Geodesy and Geophysics, Vienna University of Technology, Gusshausstrasse 27-29/1282, 1040, Vienna, Austria |
| Abstract: | We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km. |
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