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Geological Survey of Finland, Special Paper 39 Meri-Liisa Airo The final phase of structural analysis is synthesising the interpreted structures and other available geodata information. The resulting geological interpretation may be highly subjective but nevertheless contains a collection of related information. The interpreted structures are part of the tectonic history, and although it seems that they were formed one system after another, they commonly are linked to each other and represent a continuum of tectonic deformation. In most cases the geometry of earlier structures guides the formation of later structures. If the crustal stress is directed uniformly within a whole structural block, all the structures within the block respond to the stress. The kind of new structures produced, depends on the orientation and competence of the earlier structures, and also on thermal and stress regime in which they occur. The short-wavelength magnetic anomaly field in Finland mostly reflects the continuous, curved patterns formed during the latest regional metamorphism (Svecofennian Orogeny 1900–1800 Ma ago), and the dislocating, brittle structures that formed thereafter. Chemical reactions involving magnetic minerals over large areas need regionally uniform metamorphic conditions under elevated temperatures. Figure 11 represents a tectonic interpretation synthesis of an area of size 170 x 240 km 2 in southern Finnish Lapland. Geophysical data of the area is interpreted to reflect continuous tectonic development between two main tectonic stages, beginning with structures formed under a ductile regime, continuing with a ductile-semi-brittle stage and ending with 194 Structural synthesis brittle structures. Thus the present geophysical picture represents a continuous series of structures caused by the progressive change of the regional stress field magnitude and orientation. Structural patterns commonly evolve incremently, and not always as discrete, unrelated events. Along with the transition from ductile to brittle stages, structures evolve from shear zones to fracture zones (bending –> shearing –> rupturing). Applying this series to the behaviour of magnetite, bending and shearing take place in ductile crustal conditions, where magnetite may be formed or destroyed depending on the nature of fluids present. Then the resulting magnetic texture follows the main structural trend. Sudden rupture of crust at levels deep enough for magnetite reactions may affect mineral stability if oxidizing fluids are present and magnetite may be destroyed. This is why many fault and fracture zones lack magnetite and appear as weakly magnetic lineaments. “Fracture zone” here refers to a zone of broken crust, occurring in brittle conditions, and resulting in partial destruction of magnetite or removal of magnetic rock material from the broken ground surface. At this stage, within the already cooled crust, magnetite reactions on a regional scale are not that obvious. However, minor amounts of hematite or goethite with a weak magnetic expression may be produced due to meteoric fluids in the shear or fracture zones (Mertanen et al., in prep.). To locate these brittle zones is important in mineral exploration, since they may act as fluid channels for mineralisation.
a) b) c) Geological Survey of Finland, Special Paper 39 Regional interpretation of aerogeophysical data: extracting compositional and structural features Fig. 11. Tectonic interpretation of southern Finnish Lapland. Size of area is 170 x 240 km 2 . Blue dashed lines: northeasterly long wave-length trend of granitoid anomalies; light blue: east-west shearing at the major block boundary; yellow: late stage fragmentation of crust, red: shearing and fracturing, still later stage. a) The regional gravity data of 5 km spacing (The Finnish Geodetic Institute) in colour, with aeromagnetic shading (upward-continuated 200m). b) Aeromagnetic upward-continuated data, shading from NE. c) Sketch map the main structures and blocks. Pudasjärvi complex 2 parts: Archaean->Proterozoic; Suomujärvi complex (contains Archean windows); Perä-Pohja schist area (Proterozoic); Central Lapland granitoid (3 parts). 195
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In 1972 GTL started a second nation
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Fig. 4. Geomagnetic total field var
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Figures 11a-b. The Finnish anomaly
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To test the method, the details of
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Fig 14a. Percentage of ferrimagneti
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Data versions and corresponding sma
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Aarnisalo, J., Franssila, E., Eeron
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Time dependent reductions for aerom
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Today, we are able to do numerical
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Increasing magnetic permeability ca
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half-space as geological noise. In
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Interpretation of apparent resistiv
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Thin plate model of Hallaperä mine
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We have demonstrated some character
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three most important radioactive is
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Spatial resolution The resolution o
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Released potassium is dissolved in
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Fig. 2. The Central Lapland granite
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The mineral exploration area in the
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