Geophysical investigations of the Stuoragurra postglacial fault, Finnmark, northern Norway

Journal of Applied Geophysics, 29 ( 1992 ) 9 5 - 1 1 8

95

Elsevier Science Publishers B.V., A m s t e r d a m

Geophysical investigations of the Stuoragurra postglacial fault, Finnmark, northern Norway Odleiv Olesen a, Herbert Henkel b, Ole Bernt Lile c, Eirik Mauring a and Jan Steinar Ronning a aNorges geologiske undersokelse, P.O. Box 3006, N-7002 Trondheim, Norway bSveriges geologiska unders6kning, P.O.Box 670, S-751 28 Uppsala, Sweden cUniversitetet i Trondheim, NTH, N-7034 Trondheim-NTH, Norway ( Received June 18, 1991; accepted after revision April 30, 1992 )

ABSTRACT Olesen, O., Henkel, H., Lile, O.B., Mauring, E. and Ronning, J.S., 1992. Geophysical investigations of the Stuoragurra postglacial fault, Finnmark, northern Norway. J. Appl. Geophys., 29: 95- l 18. Processed images of aeromagnetic, gravimetric and topographical data and geological maps combined with VLF ground measurements have been interpreted in mapping the main fault structures along the Mierujav'ri-Sv~erholt Fault Zone (MSFZ) in Finnmark, northern Norway. The 230 km long MSFZ is situated in the extensive Proterozoic terrain of Finnmark. Proterozoic albite diabases, which cause characteristic magnetic anomalies in the Masi area, have intruded along the MSFZ. A system of duplexes can be delineated along the MSFZ from the geophysical images. These interpretations have been followed up in the till-covered area with electromagnetic measurements and confirm the existence of the faults interpreted from the geophysical images. The postglacial Stuoragurra Fault (SF) lies within the MSFZ. It is a southeasterly dipping reverse fault and can be traced fairly continuously for 80 km in the Masi-Ie~jav'ri area. Detailed geophysical investigations and drilling have been carried out in the FidnajAkka area 10 km to the south of Masi. A ca. 1 m thick layer of fault gouge detected in the drillholes is thought to represent the actual fault surface. Resistivity measurements reveal low-resistivity zones in the hanging-wall block as well as in the foot-wall block of the SF. These low-resistivity zones lie within a several hundred metre wide belt and are interpreted to be due to fracturing of the quartzites along the regional MSFZ. Within the Fidnaj~kka area, however, the resistivity of the hanging-wall block of the SF is typically lower than in the foot-wall, indicating more intense fracturing in the hanging-wall. Vertical electrical soundings show a low-resistivity layer at depth in the eastern hanging-wall block, which corroborates other evidence that the fault dips to the southeast. The refraction seismic data reveal low seismic velocities along the SF which are interpreted to be caused by faulted and fractured bedrock. Detailed topographical data proved very useful for estimating the dip of the fault zone in the upper part of the subsurface. Additional data from ground-penetrating radar measurements could be used to map the thickness of the overburden and, when combined with the digital topography, these measurements could be used to map the topography of the bedrock surface. Highly reflective bedrock on the ground-penetrating radar records is interpreted to be fractured and weathered quartzite and single reflectors can be interpreted as fault zones. Within the survey area, there exist two possibilities for the course of the postglacial reverse fault at depth: a - - t h e fault is listric with a dip of 50 ° in the uppermost 10 m of the subsurface and a dip of 30 ° at a depth between 25 and 40 m, or b--the fault continues at depth from the surface with a dip of 50-60 °. If model a is correct, the SF will be represented by a 1 m thick gouge which occurs in two drillholes and the fault will be parallel with the foliation at depth. It is concluded that the SF is controlled by older faulting, on a local as well as regional scale. The earliest detectable displacement along the MSFZ is of Proterozoic age and the latest movements occurred less than 9000 yr ago. The postglacial faulting occurs most commonly either along the margins of the duplex structures or within them. Northeast of Ie~jav'ri, however, the young faulting occurs within the main fault zone.

Correspondence to: Dr. O. Olesen, Geological Survey o f Norway, P.O.Box 3006, N-7002 T r o n d h e i m , Norway. Fax: 0477 904494.

96

O. OLESEN ET AI.

Introduction The Stuoragurra Fault (SF) is manifested on the surface as a fault scarp (up to 7 m high) and was discovered in 1983 during follow-up work of geophysical interpretations within the Finnmark Programme and has been reported by Olesen (1988) and Muir Wood (1989). Similar fault scarps (Fig. 1, inset-map), interpreted to be post- or late-glacial, occur in adjacent parts of Finland (Kujansuu, 1964), Sweden (Lagerb~ick, 1979), USSR (Tanner, 1930) and in Troms, northern Norway (Gronlie, 1922; Tolgensbakk and Sollid, 1988). When estimating hazard with respect to the disposal of radioactive waste and the construction of large installations such as offshore platforms, petroleum pipelines and hydropower plants, it is essential to know the neotectonics of the actual area. An improved understanding of the neotectonics in Finnmark may also throw light on the development of the passive continental margin in the Norwegian Sea and the Barents Sea. The Finnmarksvidda area is heavily covered with glacial drift. Regional digital aeromagnetic, electromagnetic, gravity and topographical data are therefore a necessary prerequisite to structural analysis. We have used image processing techniques to analyse and compare different types of data. High-frequency (spatial resolution) and shaded-relief images have proved particularly effective forms of enhancing information from regional geophysical data sets (Henkel, 1988; Broome, 1990; Lee et al., 1990 ). A major structural element, the Mierujav'ri-Sv~erholt Fault Zone (MSFZ) (Olesen et al., 1990 ), is here interpreted in more detail. The relationship between the MSFZ and the postglacial SF is outlined. The SF is easily accessible by roads in the Masi area. Detailed geophysical investigations

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Fig. 1. Simplified geological map of Finnmarksvidda, modified from Siedlecka (1985). Interpreted fault zones are added to the map. A tectonic interpretation of the framed area is presented in Fig. 8. Inset map shows Late Quaternary faults (barbs towards the lower block)in northern Fennoscandia (Lagerb/ick, 1979; Olesen, 1988). M S F Z = Mierujav'ri-SvaerholtFault Zone (Olesen et al., 1990), BKFC= Bothnian-KvaenangenFault Complex. have therefore been carried out to map the SF in detail, with special attention paid to the extent of the fault at depth. At an introductory stage, one area located north of Masi (Fig. 1 ) and another to the south, hereafter named the Fidnaj~kka area, were selected for a reconnaissance study with electrical and electromagnetic methods. It is important to note that the

GEOPHYSICAL INVESTIGATIONS OF THE STUORAGURRA FAULT, FINNMARK

geology to the north of Masi is dominated by mafic and intermediate metavolcanic rocks and mica schists, whereas the Fidnajhkka area is dominated by quartzites. EM-anomalies in the area north of Masi were mainly found to reflect sulphides and graphite in the volcanosedimentary rocks whereas in the Fidnaj~kka area the EM-anomalies reflect faults. Further detailed studies, including ground-penetrating radar, electromagnetic and seismic methods, were therefore carried out in the Fidnajgtkka area to map the fault itself. This work has been summarised in a thesis by Olesen ( 1991 ) upon which the present paper is largely based.

Geological setting Regional geology A simplified geological map of Finnmarksvidda (Siedlecka, 1985 ) is shown in Fig. 1. The Archaean to Early Proterozoic basement on Finnmarksvidda consists primarily of supracrustal sequences, generally referred to as greenstone belts, and of gneiss complexes (Siedlecka et al., 1985). The Kautokeino Greenstone Belt (KGB) in the west is separated from the eastern Karasjok Greenstone Belt by the Jer'gul Gneiss Complex which is partly of Archaean age (Olsen and Nilsen, 1985) and forms the basement to the KGB. However, no clear depositional contact has been found between the gneisses and the supracrustals, presumably because the contacts are either fault-bounded or have been obliterated by younger felsic intrusions. The Tanaelv Migmatite Complex and the Levajok Granulite Complex have been thrusted from the east upon the Karasjok Greenstone Belt. The Kautokeino Greenstone Belt (Siedlecka et al., 1985) can be subdivided into a number of formations. The oldest of these are the G~l'denvarri Formation (Solli, 1983) and the correlative Vuomegielas Formation (Siedlecka, 1985 ), which consists of mafic volcanic rocks metamorphosed in amphibolite facies.

97

The Masi Formation lies above the G~l'denvarri Formation with a supposed angular unconformity and consists of quartzitic rocks. The main body of the greenstone belt is occupied by the Cas'kejas, Suoluvuobmi and Lik'ca Formations, each of which consists mainly of basic volcanites (Fig. 1 ). The bulk of the rock succession within the greenstone belts is of Early Proterozoic age. The G~l'denvarri and Vuomegielas Formations may be of Archaean age. The western area of the Kautokeino Greenstone Belt is dominated by NNW-SSE trending faults which have been interpreted by Olesen and Solli (1985). Berthelsen and Marker ( 1986 ) and Henkel ( 1988, 1991 ) include these faults in the regional Baltic-Bothnian Megashear and Bothnian-Seiland Shear Zone, respectively. We think that the name BothnianKv~enangen Fault Complex (BKFC) will be a more appropriate name since the zone can be traced from the Bay of Bothnia to Kv~enangen. The complex is composed of several fault segments which are well defined from both geological and geophysical data (Holmsen et al., 1957; Olesen and Solli, 1985; Berthelsen and Marker, 1986; Henkel, 1988, 1991; Olesen et al., 1990). In the Kautokeino area 3-4 regional NNW-SSE fault zones have been delineated (Olesen et al., 1990 ). Local faulting along these zones has been mapped by Holmsen et al. (1957).

The Mierujav'ri-Svcerholt Fault Zone The 230 km long Mierujav'ri-Sv~erholt Fault Zone (MSFZ) extends from Mierujav'ri 30 km north of Kautokeino in a northeasterly direction through Masi, Iegjav'ri and Lakselv and then subsurface beneath the Caledonian nappes on the Sv~erholt Peninsula. In the Masi-Iegjav'ri area the MSFZ is parallel to the northwestern margin of the Jer'gul Gneiss Complex and is mostly situated within or at the border of the Masi Formation. Based on interpretation of aeromagnetic and gravity data the

98

MSFZ truncates the Proterozoic Levajok Granulite Belt beneath the Caledonian nappes (Olesen et al., 1990). The Archaean-Lower Proterozoic G~l'denvarri Formation and the correlative Vuomegielas Formation (Siedlecka et al., 1985) have been dextrally displaced 20 km along the MSFZ (Olesen et al., 1990). The albite-diabases have a high magnetite content giving a characteristic anomaly pattern on the aeromagnetic map. To the southwest, the MSFZ is truncated by the Proterozoic BKFC. The MSFZ was therefore mainly active before the last major displacement along the westernmost segment of the BKFC.

The Stuoragurra Fault The postglacial Stuoragurra Fault (SF) runs parallel to the northwestern margin of the Jer'gul Gneiss Complex and is situated within the MSFZ. The fault line is shown in Fig. 1. It can be traced for 80 km, from Fidnaj~kka south of Masi in a northeasterly direction to L~evnjagjhkka northeast of Ie~jav'ri. The SF is made up of numerous small faults with up to 10 m of reverse displacement. The SF is situated mainly within quartzites of the Masi Formation. North of Masi, however, the SF crosscuts amphibolites within the Suoluvuobmi Formation and an albite diabase. Magnetic fabric data has demonstrated that the SF parallels the schistosity of the surrounding bedrock (Olesen et al., 1992). Brecciation is observed in all the locations where the bedrock is exposed in the fault escarpment and such brittle deformation structures are also present along the entire length of the MSFZ in the Precambrian on Finnmarksvidda. Within the survey area the quartzite is folded, with fold axes parallel to the MSFZ. Alternating thin and thick limbs of the folds dip 20 ° to the southeast and almost vertically, respectively (C. Talbot, pers. commun., 1988 ). An axial-plane foliation dips at approximately 30 ° to the southeast. This geological information is in-

O. OLESEN ET AI.

cluded in the final interpretation (Fig. 16 ). The SF transects glaciofluvial deposits northeast of Iegjav'ri and an esker northeast of Masi and, as such, at least certain sections of the zone must have been formed after the deglaciation estimated to 9000 yr BP (Olsen, in prep.). Locally, 1 m wide and 1 m deep cracks parallel to the escarpment can be observed in the hanging wall-block of the SF. These are interpreted to be accommodation faults (R. Gabrielsen, pers., commun., 1990) which were formed during reverse faulting. The earliest detectable displacements along the MSFZ are inferred to be Proterozoic (Olesen et al., 1990) and the latest took place less than 9000 yr ago (Olesen, 1988). Precision levelling data has also shown that the foot-wall has subsided relative to the hanging-wall block by 2.3 m m between 1987 and 1991 (Olesen et al., 1992 ). Along the MSFZ to the northeast of Iegjav'ri, a syn-sedimentary m o v e m e n t during the deposition of the Cambrian Dividal Group and a post-/late Caledonian displacement which cuts the Gaissa Thrust have been reported earlier (Townsend et al., 1989). The MSFZ must consequently represent an extremely long-lived fault-zone.

Regional data sets During 1979-85 the Kautokeino-Masi area was flown at a profile spacing of 200-250 m and a flight altitude of 50 m. A total of 29,000 profile-km of low-altitude measurements have been interpolated to a square grid of 100 X 100 m using the m i n i m u m curvature method (Briggs, 1974; Swain, 1976). The final map shown in Fig. 2 was produced using the shaded relief technique (Drury and Walker, 1987; Erdas, 1990) with illumination from the east. The Norwegian Mapping Authority (Statens kartverk) has provided a 100 × 100 m grid of digital topography from the area. This dataset was derived from digitised 1:50,000 scale topographical maps (M711 series), which had an original contour interval of 20 m. The use

G E O P H Y S I C A L I N V E S T I G A T I O N S OF T H E S T U O R A G U R R A FAULT, F1NNMARK

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of digital topography in tectonic studies has been demonstrated by Henkel ( 1988 ). Glacial processes will enhance or obliterate pre-glacial tectonic imprints depending mainly on the direction and intensity of ice flow. The map shown in Fig. 3 has been produced using the shaded relief technique with illumination from the east. A 3-D perspective model (Fig. 4) of this grid was constructed using the ERDAS T M image processing system.

Methods

Image processing The data were analysed with an ERDAS T M image processing system (Erdas, 1990) running on an Olivetti M380 PC with an Intel

80386 processor and an IMAGRAPH T M 1024X 1024 image processing card. The system can hold three raster images, each with a positive 8-bit range of 0-255 and one 4-bit graphic plane for overlays of vector data. The ERDAS T M system allows data to be swapped in and out of the image buffer and allows multiple data-sets to be easily compared, The gridded gravity, aeromagnetic and topographical data-sets were rescaled into the 0255 range. Histogram-equalised colour, highfrequency filtered and shaded-relief images have been used to enhance the information of the regional data-sets. Shaded-relief presentations, which treat the grid as topography illuminated from a particular direction, have the property of enhancing features which do not trend parallel to the direction of illumination. The illusion of illumination (Figs. 2 and 3) is

100

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Fig. 3. Shaded relief image of the digital topography from the Kautokeino Greenstone Belt ("illumination" from the east). An interpretation of the framed area is shown in Fig. 8.

achieved by the application of two convolution filters of the form 1

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to estimate the change in ground height in the X a n d Ydirections. The dot product of the unit normal on the calculated surface and the sun vector gives a value representing the light reflectance. The location of VLF ground profiles and VLF anomalies from the Masi area have been digitised and transferred to the ERDAS T M system. These data are represented as vector data in the image processing system. The structural analysis of the greenstone belt is subsequently performed by digitising the new interpretation elements directly on the video

monitor using a mouse-controlled cursor. The updated interpretations are kept in the graphics plane while swapping the images from the three data sets. Once completed the final tectonic interpretation is plotted.

Structural interpretation The structural interpretation utilising the image processing technique is similar to the method of Henkel (1988). In Henkel's method, fault zones occurring within magnetic rock units are interpreted from: ( 1 ) linear discordances in the anomaly pattern, (2) displacement of reference structures, ( 3 ) linear or slightly curved magnetic gradients, and (4) discordant linear or curved minima (Henkel and Guzm~in, 1977 ). The digital elevation data can be used to corroborate the interpretation

GEOPHYSICAL INVESTIGATIONS OF THE STUORAGURRA FAULT, FINNMARK

Fig. 4. A three-dimensionalperspectivemodelof the Masi area generatedwith the Erdas 3-D module. The vertical scale is exaggeratedby a factor of 2. The view is towards the southwest along the MSFZ. The Big'gevarri Duplex (Fig. 8) can be seen in the middle of the image to the south of Masi. The rectangulargrey-colouredarea at the southern marginof the duplex structurehas been investigated in detail (Fig. 9). of the magnetic data since fault zones commonly coincide with linear depressions in the topography. Ground VLF and magnetic profiles with lengths varying from 400 m to 4 km have been measured on 28 selected locations across fault zones in the Masi area (Olesen, 1991 ). Five of the profiles in the Big'gevarri-Fidnaj~tkka area are displayed in Fig. 5. The purpose of the measurements was to check and correct the interpretations based on the method just outlined. In regions of resistive soil and bedrock the VLF method can be used to detect large water-containing fracture zones in the bedrock (Henkel and Eriksson, 1980). The shape of the VLF-anomalies was compared with standard curves by Kaikkonen (1979) to estimate the dip of the conductors. The magnetic banding within the Kautokeino Greenstone Belt is generally found to represent primary layering of the volcanosedimentary sequences and metadiabase sills which now mostly show up with steep dips. The dip of albite diabases in the Masi area has been modelled from aeromagnetic data along helicopter-profiles using a modified version (Hes-

101

selstr6m, 1987) of the computer programme by Enmark (1981). The measured magnetic properties (Table 1, Fig. 6 ) are applied in the model. The response of constructed bodies of the magnetic albite diabase is computed and they are further modified to make the magnetic response fit the measured field (Fig. 7 ). The basic model in the programme comprises bodies of polygonal cross-section with limited extension in the strike direction. The susceptibilities from in situ measurements of the highly magnetic albite diabases are shown in Table l and Fig. 6A. Measurements of natural remanent magnetisation, NRM, on hand samples are displayed in Fig. 6B. For comparison, measurements from albite diabases in northern Sweden are included in Table 1. The albite diabases are thought to have intruded along the MSFZ and later been deformed during subsequent movements along the fault zone. The final interpretation map is shown in Fig. 8.

Digital topography A detailed topographic map of the Fidnaj~kka area has been constructed by Fjellanger Wideroe using a photogrammetric method, with contours of 2 m at a scale of 1:2000. Isolines have been digitised and this data-set was then interpolated to a square grid of 5 × 5 m using the minimum curvature method. A 3-D perspective model of this grid was constructed using the ERDAS TM image processing system (Fig. 9). A transect or "fish net" plot over a subset of the data-set was also constructed. This 3-D surface model was used as a base for the interpretation of the ground-geophysical data. If the uppermost part of the postglacial fault is assumed to be a planar surface, the intersection line between the fault and the surface can be used to construct the dip of the fault applying the stratum contour method. The fault scarps in the Fidnajhkka area appear to bend as a response to differences in topographic relief.

O. OLESEN ET AI.

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Fig. 5. V L F ground profiles across the MSFZ. The locations of the profiles are shown i n Fig. 8. Arrows indicate the intersections with the SF. Steep negative gradients of the real component show locations of electrical conductors i n t e r p r e t e d as water-containing faults. VLF data along additional 23 profiles are displayed by Olesen ( 1991 ).

Ground-penetrating radar The two systems employed for ground-penetrating radar measurements were a SIR-7 manufactured by Geophysical Survey Systems Inc. (GSSI) and a PulseEKKO IV from Sensors and Software Inc. The central frequency of the antennas of the two systems are 80 and 50 MHz, respectively, with a maximum depth of penetration of approximately 20 m.

The systems are of impulse radar type and provide profiles of subsurface conditions. An electromagnetic pulse is transmitted from the surface, with reflections from the surface and the subsurface interfaces being displayed on a continuous strip-chart recorder, or on an LCDmonitor (PulseEKKO IV). Travel times for the reflected pulses can be converted to depths to various interfaces if the velocity of propagation is known.

GEOPHYSICAL INVESTIGATIONSOF THE STUORAGURRAFAULT,FINNMARK

103

TABLE 1 Statistical data: density, susceptibility and Q-value for albite diabases in the Masi area and in Karesuando-Lannavaara, northern Sweden (Henkel 1980) Parameters

Masi area

Lannavaara and Karesuando (Henkel, 1980) (n=27)

(n=55) mean

s.d.

mean

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2.91

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2.94

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0.07

0.50

0.13

0.80

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0.44

0.29 ( n = 2 9 )

0.46

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NRM declination and inclination

4° 74 °

45 ° 75 °

Units are in SI. The standard deviation (s.d.) of the magnetic properties is expressed in decades. Magnetic properties have logarithmic mean values

The effective propagation velocity, u, of the pulse through the earth is derived from the equation:

v= 2 O / t

( 1)

where D in the present study is the depth to bedrock, determined from drilling, and t is the elapsed time between the transmitted and received pulse from the bedrock surface. The effective relative dielectric constant, er, of the subsurface material is determined from the equation g~=c2/v 2

(2)

where c is the propagation velocity in free space (3 × 108 ms - l ). The ground-penetrating radar (SIR-7) diagram along profile 807 is presented in Fig. 10A. Note that the vertical scale is exaggerated. Five additional profiles were recorded in the Fidnajfikka area (Olesen, 1991 ). The eastern part of profile 807 was recorded using the PulseEKKO IV system and the radar diagrams is presented in Fig. 10B. The reflectors interpreted from the groundpenetrating radar diagram are illustrated in Fig. 10C. The terrain along the profiles has been surveyed and the line drawings of the reflec-

tors are adjusted according to the levelled topography.

Electrical resistivity measurements Electrical resistivity profiling as well as vertical electrical sounding were undertaken to map the low-resistivity zones representing the brecciated and weathered part of the quartzite. The DC-resistivity profiling data in Fig. 11 and induced polarisation (IP) data were acquired with the NGU-produced IP4 instrument using a gradient array. A potential electrode spacing of 10 m and a current electrode spacing of 660 m were used, with the penetration of the DC-resistivity data estimated to be 100-200 m. This method yields the apparent resistivity perpendicular to the fault. The ABEM Terrameter SAS300 with a Schlumberger electrode configuration was used for the vertical electrical sounding. The apparent resistivity measurements were interpreted using the VESABS computer-programme developed by Kihle (1978). The programme assumes that the ground consists of parallel layers of differing thickness and resistivity. The response of the model is calculated and the

104

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In regions of high resistivity, electromagnetic methods can be used to detect water-containing fracture zones in the bedrock. In situ resistivity measurements using the VLFR and Slingram methods provide information about the soil and the bedrock conductivity. Such measurements allow more detailed studies of the location and width of fault zones because they can discriminate between brecciated and normal crystalline rocks with differing resistivity. Henkel (in press) has documented how the resistivity of crystalline bedrock is dependent on fracturing:

/

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Fig. 6. Magnetic properties of albite diabase in the Masi area. A. Susceptibility spectra of in situ measurements. Logarithmic mean values (rn) and standard deviations (s) expressed in decades are given in SI units. B. Plot of natural remanent magnetization ( N R M ) directions on a Wulff net (lower hemisphere). Filled circles show positive inclination. Dashed line displays the cone of 95% confidence (alpha). The plot shows a smeared distribution around the present Earth magnetic field (declination 4 ° and inclination 77 ° ) and it is concluded that this remanence to a great extent is viscous.

model is adjusted interactively until an optimal fit between the theoretical and measured curves is attained. Information from boreholes and ground-penetrating radar can be used to fix the thickness of the uppermost layers.

Degree of fracturing

Resistivity (ohm-m)

Non-fractured bedrock Fractured bedrock Strongly fractured bedrock Fault gouge

25000 2000-6000 600 30

The following EM instruments were used along profile 807 (Fig. 11 ): the VLF EM16R instrument by Geonics TM using the GYD station with a frequency of 19.0 kHz, and the Geonics EM31 Slingram with a frequency of 9.8 kHz and a coil spacing of 3.66 m. The effective depth of investigation of the EM-31 instrument is 6 m. The skin depth of the EM 16R instrument is 160 and 230 m when the resistivity of the bedrock is 2000 and 4000 ohm.m, respectively, and the applied frequency is 20 kHz. Non-isotropic resistivity conditions within the penetration depth result in a shift of the phase angle. When assuming a horizontal two-layer model, nomograms by Geonics TM ( 1972 ) can be used to estimate depth to the lowermost layer and the resistivity of this layer when the

G E O P H Y S I C A L INVESTIGATIONS O F T H E S T U O R A G U R R A FAULT, F I N N M A R K

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Fig. 7. Aeromagnetic modelling of albite diabases along helicopter profile 390 in the Masi area. Location of the profile is shown in Fig. 8. Modelling along seven additional profiles has been presented in Olesen ( 1991 ).

resistivity of the uppermost layer is known. The VLF resistivity measurements were carried out with 20 m station spacing along profile 807.

Seismics The seismic refraction m e t h o d was used to determine depth to the bedrock and to locate the postglacial fault within the bedrock. Refraction seismics also gave information about the occurrence of fractured and weathered bedrock. The geophone spacing was 5 m except for the four geophones adjacent to the shot points where the spacing was 2.5 m. The measurements were performed using an ABEM TRIO 12 channel analogue seismograph.

Results

Regional geophysical interpretation The prominent, NE-SW-trending, characteristic high-magnetic anomalies in the Masi area are due to Proterozoic albite diabases that have intruded into faults within the MSFZ (see Fig. 2 ). Outside this fault zone the albite diabases have intruded regionally as sills along layering and foliation surfaces. Locally, evi-

dence of deformation of these diabases can be observed, for instance along the main road between Masi and Kautokeino ( U T M coord. 602400-7698500) where the diabase has been transformed to amphibolite. From the aeromagnetic and topographical images a complex system of strike-slip duplexes (Woodcock and Fischer, 1986) can be delineated along the MSFZ from Mierujav'ri to Ie~jav'ri (Fig. 8). Local, weak, magnetic anomalies are caused by small amounts of magnetite in quartzite beds within the Masi Formation. The interpretations have been followed up in the till-covered area with VLF ground profiles. The VLF-profiles (Figs. 5 and 8 ) are located across the MSFZ. The asymmetry of the VLF anomalies with a positive, high-amplitude, real component to the west and an accompanying negative, low-amplitude anomaly to the east indicates that the conductors generally dip to the east along the easternmost border of the MSFZ (Kaikkonen, 1979). The difference between the positive and negative real component of the VLF anomalies is coded with symbols according to the legend in Fig. 8. The highest amplitude anomalies, indicated by crosses, are generally associated with the occurrence of graphite schists. In most of these profiles, the SF coincides with electrical conductors which are interpreted to be fracture zones.

106

O. OLESENET AI.

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GEOPHYSICAL INVESTIGATIONS OF THE STUORAGURRA FAULT, FINNMARK

107

preted as a contractional duplex which we propose to name the Big'gevarri Duplex. A threedimensional perspective model (Fig. 4 ) of the digital topography visualises the Big'gevarri Duplex from the northeast.

Detailed geophysical investigations Digital topography

Fig. 9.3-D terrain model from the Fidnaj~tkka area. This view is towards the southwest. The vertical scale is exaggerated by a factor 3. The survey area (Fig. 16) is indicated by the frame.

Magnetic model calculation along the helicopter profile No. 390 is shown in Fig. 7. The dip estimates of the distinct magnetic anomalies are considered to be within _+5 °. The method, however, is not sensitive to the depth extent of thin and steeply dipping structures. The diabases in the models have therefore been truncated at approximately 1 km depth but they may be much deeper. The modelling of the dips indicates that the albite diabase has either intruded into a positive flower structure (Wilcox et al., 1973) or has been involved in the deformation along a similar structure. Figure 8 shows the dip determination of the diabases. The dip of the central albite diabase along the MSFZ varies from 40 ° SE in the Fidnaj~kka area to almost vertical at the Kautokeino River and to 55 ° NW further east of Masi. This "twisting" of the albite diabase can also be observed on a more local scale along individual segments of the albite diabase along the MSFZ. The inconsistent dip of the albite diabases within the MSFZ is indicating a strikeslip c o m p o n e n t of the displacement along the fault (Christie-Blick and Biddle, 1985 ). The wedge-shaped structure bordered by faults along Fidnaj&kka, Big'gejav'ri, Mazej~kka and the Kautokeino River may be inter-

The postglacial fault cuts through a N-S trending ridge within the framed survey area in Fig. 9. Assuming the fault to be a plane, the stratum contour method can be used to calculate the dip of the fault. It was found to be 50 ° in the uppermost 10 m of the ground. This is consistent with the interpretation of the percussion drilling data, indicating that the fault is listric. In its southernmost section, the SF cuts across a slope and a plain in the terrain (Fig. 9). Application of the stratum contour method in this area gives a dip of ca. 35 °. The height difference between the foot of the slope and the plain is 30 m. From the course of the SF in the topography the main sense of movement along the SF must be reverse. The existence of a minor strike-slip component, however, cannot be ruled out. A dextral component of m o v e m e n t could explain the presence a small lake along the SF in the centre of Fig. 9 as a sag pond (Slemmons and Depolo, 1986 ), but the evidence for this is not compelling. We think that this lake is caused mainly by damming by the reverse fault. This depression may, however, also have formed during glacial erosion prior to the formation of the SF.

Ground-penetrating radar The ground-penetrating radar section taken along profile 807 is presented in Figs. 10A and 10B with the interpretation shown in Fig. 10C. Ground-penetrating radar diagrams for an additional five profiles have been displayed and interpreted by Olesen ( 1991 ). In the interpretations, reflectors are adjusted according to the

108

O. O L E S E N ET AI.

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topography, and coordinate 0 on the profile is situated on the crest of the postglacial fault escarpment. On most of the ground-penetrating radar profiles the bedrock surface appears as a distinct reflector as in Fig. 10. Using the depths from drillholes 4 and 5 (Fig. 10C) and the elapsed times from the ground-penetrating ra-

dar diagram (Fig. 10A, B), the propagation velocity in the glaciofluvial sand is estimated to 0.09 and 0.10 m/ns, respectively, using eq. 1 given above. Applying an average velocity of 0.095 m / n s yields a relative dielectric constant of 10. This corresponds to the upper limit of previously published estimates, 5-10, for nat-

GEOPHYSICAL

INVESTIGATIONS

OF THE STUORAGURRA

109

FAULT, FINNMARK

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ural dry sand in Sweden and Denmark (Berg et al., 1983) and therefore was considered reliable enough to reduce the remaining data. Continuous reflectors within the glaciofluvial deposit are thought to represent layering of material with differing grain sizes. Highly reflective areas within the bedrock, indicated with cross-hatching in Fig. 10C, are situated to the west of the escarpment on the profiles. These areas are thought to be highly brecciated and weathered bedrock. This interpretation is also confirmed by two short drillholes in the foot-wall block of the fault (Olesen, 1991 ). The N - S trending ridge in Figs. 9 and 16 has a core of bedrock which is exposed along the postglacial fault. Resistivity measurements When interpreting the electrical profiling curves in Fig. 11, the differing depth of penetration achieved by the various systems employed must be taken into account. The EM 31 maps the resistivity of the overburden since the

depth of resolution is 6 m while the DC and VLF apparent resistivity methods map the resistivity of the overburden and bedrock down to at least 100 m. The apparent resistivity curves along profile 807 in Fig. 11 show that the eastern (hanging-wall) block generally has a lower resistivity than the western one. The DC-resistivity shows 1000-2000 o h m - m in the hanging-wall block compared with 400020,000 o h m - m in the foot-wall block. This indicates a more fractured bedrock in the hanging-wall which is supported by the VLFR measurements. The low phase angles ( < 45 °) to the east of 50 m in Fig. 11 show that the resistivity increases at depth beneath the hangingwall block. Using the nomograms by Geonics ( 1972 ) along section 50-100 m east on profile 807, assuming a resistivity of 1000 o h m - m for the hanging-wall block, indicates a resistivity greater than 3000 o h m - m for the foot-wall block at a depth of approximately 70 m. The high phase angles ( > 45 °) to the west of the coordinate l0 m show that the resistivity

110

within the foot-wall decreases with depth, supporting the interpretation of another fault-zone outcropping at approximately - 1 0 0 m (see below). A decrease of the apparent DC-resistivity can also be observed at coordinate - 100 m.

Vertical electrical sounding (VES) data (Figs. 12 and 13 ) also show a complex fracturing of the bedrock with low-resistivity zones in the hanging-wall block as well as in the footwall block. The VES data (Fig. 13) show the lowest resistivity layer at some depth in the eastern block, consistent with the observation of eastward-dipping faults. The soundings to the west of the postglacial fault indicate lowresistivity layers at depth, interpreted as zones of fractured bedrock in the diagrams in Figs. 13 and 16. These zones can be traced on the topographical map and approach the postglacial fault to the south (Fig. 9). To the south of the small lake in the centre of Fig. 9, the SF steps approximately 30 m to the right to follow the westernmost fault zone. The resistivity measurements consequently indicate that the SF is situated within a more than 200 m wide fault zone which we interpret to be part of the regional MSFZ. We cannot, however, totally rule out the possibility that the low resistivity, locally, is partly caused by the presence of sulphides. Approximately 25 m to the west of the escarpment, close to the southernmost edge of Fig. 16, there is a spring of water causing a 25 m long and 3-5 m wide zone where there is no vegetation other than the flower Viscaria alpina and some moss. This is caused by naturally copper-contaminated groundwater emerging from one of the westernmost interpreted fault zones in Fig. 16. Chemical analyses of three soil samples from this zone show copper contents of 0.36, 0.50 and 1.34%. Induced polarisation (IP) measurements along profile 807 show, however, that the IP effect is 1-2%, which demonstrates that the content of the sulphides along this profile is very low and therefore cannot explain the observed low-resistivity zones.

o. OLESEN ET AI.

Refraction seismics The interpreted velocities of the quartzites within the survey area are 2400-4100 m s (Fig. 14), which are low compared with published data on fresh quartzites: 4500-6200 ms -l (Carmichael, 1989), 4500-5100 ms -l (Atlas Copco ABEM, 1987) and 5800 ms -I (Reich, 1943 ). Intercalations o f ' f r e s h " albite diabase within the quartzite would have an even higher velocity. We interpret the low velocities to be caused by a generally high incidence of fracturing and faulting of the bedrock within the survey area. Similar low-velocity zones have earlier been reported to coincide with postglacial faults in the Lansj~rv area in northern Sweden (Henkel, 1988). The width of the zone in the Fidnaj~kka area is greater than in the Lansj~irv area, indicating a higher degree of fracturing in the vicinity of the SF as compared with the Lansj~irv Fault. The results from the refraction seismic profiles are therefore generally in agreement with those of the other methods--with the exception of one major discrepancy: the intersection by the drillhole of the bedrock surface is 5 m shallower than the depth obtained from the refraction seismics (Fig. 14). From the percussion drilling we know that the groundwater surface is located in the bedrock at a depth of 35 m along the drillhole. Therefore, no watersaturated sediments with increased velocity exist. The reason for the observed phenomenon is thus believed to be a highly shattered and fractured bedrock in the hanging-wall block. A similar discrepancy between refraction seismics and fault scarp excavations in the hanging-wall block has been reported from the Lansj~irv Fault (Lagerb~ick, 1990), and is also associated with decreased velocity in a layer of shattered and fractured bedrock between the overburden and more competent bedrock. Even if this velocity is higher than that of the overburden it may still not be recognised as a distinct event if the layer is too thin. Such a layer is called the "blind-zone" or the "hidden layer".

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The maximum thickness of this layer may be calculated using the equations by Hawkins and Maggs ( 1961 ). When applying the velocities

800 ms -1 and 3800 m s - l (Fig. 1 4 ) for overburden and "fresh" bedrock, respectively, in addition to the known thickness of overburden

GEOPHYSICAL INVESTIGATIONSOF THE STUORAGURRA FAULT, FINNMARK

113

Fig. 15. Polished breccias of quartzite from the fault escarpment near Fidnajhkka (UTM 596700-7687500). Mineral grains are crushed along fractures. The main secondary minerals present are chlorite (sample A) and ferrohydroxides (sample B). The sample location is the outcropping area shown in Fig. 16.

in drillholes Nos. 4 and 5, we can calculate the m a x i m u m thickness of the blind-zone. On the assumption that the velocity of this zone is 1300-1500 m s - l , the m a x i m u m thickness of the blind-zone in the two drillholes is 11 and 8 m, respectively. If the velocity of the blind-zone is greater than 1300-1500 ms -I , the maxim u m thickness must be smaller. In boreholes Nos. 4 and 5 the well-log indicates a thickness of 29 m and 11 m, respectively, of brecciated bedrock in the uppermost part of the hanging-wall block. These estimates are considerably greater than the calculated m a x i m u m thickness of a possible blind-zone, especially for borehole No. 4. The thickness of the brecciated bedrock is, however, based on visual judgements of the drilling speed and this parameter cannot directly be related to the seismic velocity. We have observed from the drill cuttings (fig. 5.15 in Olesen, 1991 ) that the content of fine-grained material is higher in the uppermost part of drillhole 4, indicating a more fractured and possibly weathered rock. We therefore infer that the discrepancy between the depths obtained from the refraction seismics and from the percussion drilling is most likely caused by highly shattered and fractured bed-

rock in the upper part of the hanging-wall block. A generally intense brittle deformation of the hanging-wall block of the SF is also indicated by open accommodation faults which are situated in the hanging wall-block of the fault (fig. 3C in Olesen et al., 1992). Brecciated quartzite (Fig. 15 ) can also be observed in an outcrop along the fault escarpment 100 m to the south of profile 807.

Percussion drilling Four drillholes are located within the survey area. Drillholes Nos. 4 and 5 are situated along profile 807 in the hanging-wall block, 60 m and 30 m to the east of the escarpment, respectively (Figs. 10C, 13, 14, 16). Two shallow drillholes are located in the foot-wall block immediately to the west of the escarpment (Fig. 16). The depths of the four drillholes were 61 m, 31 m, 13.5 m and 7.5 m, respectively, and the bedrock surface was encountered at depths of 12.0 m, 7.3 m, 4.5 m and 5.5 m. Samples of the drill chips from the bedrock are displayed by Olesen (1991). Fault gouge of approximately 1 m thickness at depths of 37.5 m and 25.5 m in drillholes 4 and 5 is interpreted to represent the postglacial fault at depth. This

114 implies a dip of the postglacial fault of ca. 30 ° to the southeast. At shallower depths the fault plane must dip more steeply, perhaps up to 50 ° (Fig. 10B), in order to outcrop in the escarpment.

Discussion The flat-lying, up to 2 km thick (Olesen, 1991 ), Suoluvuobmi Formation to the west of Masi (Fig. 1 ) is interpreted as having been deposited on a platform at the margins of a deep rift basin. This is supported by the increased volume of sediments in this area (Olesen and Solli, 1985 ). The amphibolites occur adjacent to the Bothnian-Kv~enangen Fault Complex. The MSFZ constitutes the border between this flat-lying sequence to the northwest and the more deformed G~l'denvarri Formation and Jer'gul Gneiss Complex to the southeast (Fig. 1 ). Holmsen et al. (1957) and Solli ( 1983 ) reported that the Masi Formation has locally been inverted and thrusted above the Suoluvuobmi Formation to the west. Thrusting of the quartzite consequently developed, according to our interpretation, mainly beneath and along the border of the Suoluvuobmi Formation. In the present model of the MSFZ we suggest that this thrusting and imbrication of the Masi Formation may be the result of dextral strike-slip movements along the MSFZ. In the area of map-sheet 1933 IV Masi (Solli, 1988) foliation and layering steepen towards the Kautokeino river which, from the geophysical data, are interpreted to represent the central part of the MSFZ (Fig. 8). A palm tree structure (Sylvester and Smith, 1976) slightly tilted to the northwest, can be inferred along the MSFZ by combining the geophysical interpretation and the geological observations. This type of structure typically occurs along strikeslip faults (Sylvester, 1988). A link between flower structures in section and duplex structures in plan view is generally seen in seismic profiles and sandbox experiments (Woodcock and Fischer, 1986).

o. OLESENETAI. Two minor, fault-bounded blocks representing the Suoluvuobmi Formation on ]~rvusvarri and H~i'gadanc~kka are situated close to the centre of the MSFZ. These blocks are interpreted to have been emplaced by normal faulting, which may represent extension across the top of the uplifted and laterally spreading blocks (Sylvester, 1988). The S-shaped MSFZ, representing a restraining bend, is consistent with the observed accumulated dextral displacement of 20 km along the fault zone. The wedge-shaped positive Big'gevarri Duplex interpreted from geophysical images may have formed as a consequence of this dextral strikeslip component. Internal thrusting of the Masi Formation within this structure was reported by Holmsen et al. (1957). Duplex structures within regional shear zones have also been interpreted from aeromagnetic data in the Precambrian of northern Sweden (Henkel, 1988 ) and the Canadian Shield (Broome, 1990). Proterozoic albite diabases have intruded the MSFZ in the Masi area. Locally, deformation features can be observed in these diabases. In the Masi area, Olesen ( 1988 ) reported that the SF follows one old fracture zone and then cuts across to follow another. Interpretations of more detailed geophysical images have resulted in the delineation of a system of duplexes along the MSFZ from Mierujav'ri to Skoganvarre. Albite-carbonate alteration occurs along the borders of the Big'gevarri Duplex. Slight tilting of the MSFZ towards the northwest may have been caused by the doming of the Jer'gul Gneiss Complex. Since the deformation increases towards the MSFZ the folding and imbrication in this area may be associated with movement along the MSFZ. The geophysical methods that we applied in the detailed investigations were capable of detecting the differing properties of the hangingwall and foot-wall blocks in the SF. Information on overburden thickness and bedrock topography was also acquired from the measurements. However, none of the tested geophysical

GEOPHYSICAL INVESTIGATIONS OF THE STUORAGURRA FAULT, FINNMARK

l 15

'°'~O'c/LE" 607

7Z ..? " ~

i

LEGEND L~

J 7~ e

~) D~,~

N

Viscaria Alpina field Digital elevation data Drillhole (percussion drilling)

. . . . Crest of hill YJ///~ Outcrop

Glaciofluvial deposit Fine-grained diabase

~--- ~ Quartzite Highly fractured quartzite Interpreted from drillholes, georadar and vertical electrical sounding .... Fault gouge -Foliation Interpreted by Christopher ~- -- Bedding Talbot from geological mapping in the area Bedrock surface Interpreted from georadar, refraction seismic data and vertical electrical sounding

Fig. 16. Interpretation synthesis of model a (listric fault) for the Stuoragurra Fault as a block diagram, looking south. Note that profile 807 in this diagram is a mirror image of the geophysical profiles along profile 807 in Figs. 10, 11, 13 and 14 and that coordinate 85 in the present figure coincides with coordinate 0 in Figs. 10-14.

methods was able to detect the postglacial fault itself. A dip estimate of 50 ° in the upper 10 m of the subsurface can be inferred from the topographical data. Because of the collapse of the scarp and subsequent erosion we consider that the error in this estimate can be about 10 °. F r o m the percussion drilling, a dip of 30 ° can be interpreted from the intersection with a 1 m thick fault gouge at depths of 37 m and 25 m in drillholes 4 and 5 (Figs. 10C and 16). This would indicate that the fault has a listric form. An approximately 1 m thick zone o f similar fault gouge (R. Gabrielsen, pers. commun., 1990) representing a postglacial fault is ex-

posed in a road-cut in Yrkje, southwest Norway (Anundsen and Gabrielsen, 1990). Faulting m a y not be listric, however, if the drilled fault gouge does not really represent the postglacial fault. This would imply that drillholes Nos. 4 and 5 were a b a n d o n e d prematurely. If the fault is steeper than 55 °, it would not have been penetrated by either of the two drillholes. To find out if the SF represents a listric fault or not we r e c o m m e n d core-drilling a 200 m deep hole at an angle of 60 ° from a location 50 m to the southeast of the scarp at profile 807. We have managed to integrate the results from this detailed survey with the interpreta-

116 tion of aeromagnetic data which shows that the SF in the Fidnaj~kka area is situated within a 1.4 km wide complex zone in which there are four 50-100 m thick albite diabases. The dip of these diabases within the present survey area is 40-50 ° to the southeast which is consistent with the dip of the fault zones. The margin of one of these diabases is encountered in drillhole No. 4 (Figs. 10C and 16). About 3 km to the northeast, the SF is situated along the contact of an albite diabase. The SF then gradually turns to a more northerly trend into the Big'gevarri Duplex. The albite diabases continue in the same NNE direction along the southeastern border of this duplex and gradually become steeper. This group of 1815 _+24 Ma old albite diabases (Krill et al., 1985) can be traced almost continuously for 75 km in a northeasterly direction to Ie~jav'ri and has intruded along the regional MSFZ.

Conclusions (1) The Mierujav'ri-Sva~rholt Fault Zone (MSFZ) constitutes the border between the flat-lying Suoluvuobmi Formation to the northwest and the more deformed G~l'denvarri Formation and Jer'gul Gneiss Complex to the southeast. (2) The wedge-shaped positive Big'gevarri Duplex, interpreted from geophysical images, may have formed as a consequence of this dextral strike-slip component. A palm tree structure slightly tilted to the northwest can be inferred along the MSFZ by combining the geophysical interpretation and the geological observations. (3) Based on the information currently available, within the survey area there are two possibilities for the change in the course of the postglacial reverse fault with depth: a - - t h e fault is listric with a dip of ca. 50 ° in the uppermost 10 m of the subsurface and a dip of ca. 30 ° at a depth between 25 and 40 m, or b--the fault continues at depth from the surface with a dip of 50-60 ° . If model a is correct, the

o. OLESENETAI. Stuoragurra Fault (SF) will be represented by a 1 m thick gouge and the fault will be parallel with the foliation at depth. If model b is correct, the drillholes in the hanging-wall block were abandoned prematurely and did not penetrate the fault. When combining the different geophysical results and geological features we find model a to be the more likely solution. (4) The resistivity measurements show complex fracturing of the bedrock with low-resistivity zones in the hanging-wall block as well as in the foot-wall. The SF is thus interpreted to be situated within a several hundred metre wide fault zone. This zone is part of the Big'gevarri Duplex within the regional MSFZ. (5) Generally, within the Fidnaj~kka area, it can be stated that the resistivity of the hanging-wall block of the SF is lower than in the foot-wall block, indicating a more fractured bedrock than in the foot-wall block. (6) The main sense of m o v e m e n t along the SF is reverse. The existence of a minor strikeslip component, however, cannot be ruled out. (7) When interpreting the refraction seismic data, precautions had to be taken since depth tended to be overestimated. This phenomenon is most likely caused by fractured and weathered bedrock occurring in a "blind-zone" with an intermediate velocity to the overlying overburden and the underlying "fresh" bedrock. ( 8 ) Detailed topographical data proved very useful for estimating the dip of the fault zones close to the surface. (9) Measurements of the physical properties, seismic velocity and electrical resistivity along the SF indicate the location of faulted and fractured bedrock but do not display any detailed 3-D image of the complex faulting. (10) Ground-penetrating radar measurements were used to map the thickness of the overburden and, combined with the digital topography, could be used to map the topography of the bedrock surface. Highly reflective bedrock is interpreted as fractured and weath-

GEOPHYSICAL INVESTIGATIONS OF THE STUORAGURRA FAULT, FINNMARK

e r e d q u a r t z i t e a n d single r e f l e c t o r s c a n b e int e r p r e t e d as fault zones.

Acknowledgements This work was partially financed by the R o y a l N o r w e g i a n C o u n c i l for Scientific a n d I n d u s t r i a l R e s e a r c h ( G r a n t 13220 to O d l e i v Olesen), the Geological Survey of Norway, N o r s a l f i d a.s., a n d t h e N o r w e g i a n I n s t i t u t e o f Technology, University of Trondheim. Norsk teknisk byggekontroll a/s, NOTEBY, kindly p r o v i d e d the S I R - 7 g r o u n d - p e n e t r a t i n g r a d a r s y s t e m for this s u r v e y . L a r s O l s e n , D a v i d R o b erts, J a n S v e r r e S a n d s t a d , A r n e Solli, T r o n d Torsvik and Professors Arne Bjorlykke, Roy Gabrielsen and Christopher Talbot have contributed with valuable advice and discussions. T o all t h e s e p e r s o n s a n d i n s t i t u t i o n s we exp r e s s o u r s i n c e r e t h a n k s . W e are also i n d e b t e d to Drs. P e t e r W a l k e r a n d D a v i d R o b e r t s for critically r e a d i n g the m a n u s c r i p t a n d for sugg e s t i o n s t o w a r d s its i m p r o v e m e n t . W e a r e furt h e r grateful to R a n d i B l o m s o y , G u n n a r G r o n l i a n d T o r b j o r n H a u g e n for p r e p a r i n g the figures.

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