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.
Highlights
One of the driving mechanisms of continental evolution is the assembly and dispersal of supercontinents (BRADLEY 2011)
The high upper crustal velocities of the Kittilaand Savukoski groups are in good agreement with those detected along the POLAR profile by JANIK et al (2009), who detected high values of Vp in the uppermost crust
We suggest that the arrival of the plume initiated a continental spreading centre or triple junction (Fig. 13; Western Karelian craton (WKC), Central Karelian craton (CKC) ? Belomorian Province (BP) and CL (=Central Lapland Granitoid complex (CLGC) ? Central Lapland Greenstone Belt (CLGB))) and resulted in large scale mafic underplating of the crust
Summary
One of the driving mechanisms of continental evolution is the assembly and dispersal of supercontinents (BRADLEY 2011). Crust that forms or that is deformed under extension is stabilized due to strain hardening (KUSZNIR and PARK 1987) and it will be hard to break such crust in later tectonic processes This has been well documented in failed rifts/aulacogens (Midcontinent rift; BEHRENDT et al 1988) as well as in thrust belts where rifted margins have been stacked back on the extended continent e.g. Appalachians (COCORP; COOK et al 1981), Alps (TRANSALP; BERGE and VEAL 2005). In Central Lapland, the change from Archean to Paleoproterozoic lithologies is more complex resulting in additional boundaries in NE–SW and N–S directions It is an area where both Archean and Paleoproterozoic terranes either halt or intertwine with each other and where the lithologies are dominated by rift related volcano-sedimentary sequences (DALY et al 2006; KORJA et al 2006; SLABUNOV et al 2006).
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