Crustal reflectivity near the Archaean Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area

Transcription

1 Geophys. J. Int. (2002) 150, Crustal reflectivity near the Archaean Proterozoic boundary in northern Sweden and implications for the tectonic evolution of the area C. Juhlin, 1 S.-A. Elming, 2 C. Mellqvist, 3 B. Öhlander, 2 P. Weihed 2 and A. Wikström 4 1 Uppsala University, Villavägen 16, SE Uppsala, Sweden. cj@geofys.uu.se 2 Luleå University of Technology, SE Luleå, Sweden 3 SGAB Analytica, Box 511, SE Täby, Sweden 4 Geological Survey of Sweden, Box 670, SE Uppsala, Sweden Accepted 2002 February 5. Received 2002 January 10; in original form 2000 November 20 SUMMARY Sm Nd isotope ratios of Ga granitoids delineate the Archaean Proterozoic boundary in northern Sweden, an important feature in the Fennoscandian Shield. The boundary strikes approximately WNW ESE and is defined as a c. 20 km wide zone with juvenile Palaeoproterozoic rocks to the SSW and Archaean and Proterozoic rocks, derived to a large extent from Archaean sources, to the NNE. It therefore constitutes the strongly reworked margin of the old Archaean craton. Extrapolation of the boundary offshore into the Bothnian Bay and correlation with the marine reflection seismic BABEL Lines 2 and 3/4 indicates that the boundary dips to the south-southwest, consistent with interpretation of the Sm Nd data. In order to tie the BABEL results with onshore surface geology and obtain detailed images of the uppermost crust a short (30 km of subsurface coverage) pilot profile was acquired in the Luleå area of northern Sweden during August The profile consisted of a high-resolution shallow component (1 kg shots) and a lower-resolution deep component (12 kg shots). Both components image most of the reflective crust, with the deep component providing a better image below 10 s. Comparison of signal penetration curves with data acquired over the Trans-Scandinavian Igneous Belt (a large batholith) indicate the transparent nature of the crust there to be caused by geological factors, not acquisition parameters. Lower crustal reflectivity patterns on the Luleå test profile are similar to those observed on the BABEL lines, suggesting the same lower crust onshore as offshore. Interpreted Archaean reflective upper crust in the NE extends below more transparent Proterozoic crust in the SW. This transparent crust contains a number of high-amplitude reflectors that may represent shear zones and/or mafic rock within granite intrusions. A marked boundary in the magnetic field in the SW has been interpreted as being the result of a gently west-dipping contact zone between meta-sediments and felsic volcanic rocks, however, the seismic data indicate a near-vertical structure in this area. By correlating the onshore and offshore seismic data we have better defined the location of the Archaean Proterozoic boundary on the BABEL profiles. Our new interpretation of the crustal structure along the northern part of the BABEL Line 2 shows a more bi-vergent geometry than previous interpretations. Comparison of the re-interpreted crustal structure in northern Sweden with that found in the Middle Urals shows several similarities, in particular the accretion of a series of arcs to a stable craton. Based on this similarity and geological data, we deduce that a continental arc accreted to the southwestern margin of the Archaean craton at c Ga. Shortly thereafter, the Skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi-vergent crustal structure observed today. Key words: Fennoscandian Shield, lower crust, migration, Moho, seismic reflection profile, tectonic evolution. 180 C 2002 RAS

2 Crustal reflectivity in northern Sweden INTRODUCTION Current models of the crustal structure of the northern part of the Fennoscandian Shield are based primarily on geological mapping (e.g. Gaál & Gorbatschev 1987; Lundqvist et al. 1996; Nordkalott Project 1989), Sm Nd isotope studies (e.g. Mellqvist et al. 1999) and the BABEL seismic surveys in the Bothnian Bay (e.g. BABEL Working Group 1993). Combined with observations of present-day plate tectonic processes, these data sets indicate a tectonic evolution consisting of rifting in the Archaean, subduction of its passive margin and later thrusting of volcanic arc rocks on to the Archaean craton (e.g. Nironen 1997). Strong evidence for Archaean rocks underlying Proterozoic rocks of northern Fennoscandia is found in the onshore Sm Nd isotope studies of granitoids and metavolcanics, and the offshore BABEL seismic images. However, it is difficult to correlate the two data sets in detail since the BABEL seismic project was geared to exploring the deep crust, while the mainly Sm Nddefined Archaean Proterozoic boundary is located on the surface. In addition, the onshore Sm Nd data are offset by at least 70 km from the offshore BABEL surveys, raising questions concerning lateral correlation (Fig. 1). Published interpretations of the BABEL surveys in the Bothnian Bay (BABEL Working Group 1990; Öhlander et al. 1993; Lindsey & Snyder 1994; Gohl & Pedersen 1995) generally indicate reflective Archaean crust overlain by transparent crust. This transparent crust has been interpreted as reworked Archaean rocks (Fig. 2) by Öhlander et al. (1993). More reflective volcanic arc rocks of the presumed Skellefte and Arvidsjaur districts are found further south in the interpretations. Also common to these interpretations is the presence of only southwest-dipping structures between the Skellefte arc and the Archaean Proterozoic boundary. In order to better relate geological mapping and isotope studies in northern Sweden to the BABEL seismic images a c. 30 km long onshore reflection seismic test profile was acquired in 1999 August over the Sm Nd-defined Archaean Proterozoic boundary in the Luleå area (Fig. 1). A major objective of the Luleå profile was to investigate whether the seismic images acquired on land resembled those from the BABEL data. If so, then previous correlation of onshore and offshore data could be better justified. Imaging of the reflective lower crust was another important component of the project since many of the deep seismic profiles acquired on land in Sweden show poor lower crustal reflectivity (e.g. Dahl-Jensen et al. 1991; Juhojuntti & Juhlin 1998; Juhojuntti et al. 2001). If the well-defined lower crustal reflectivity below the Bothnian Bay was also observed on the test profile then the lack of lower crustal reflectivity elsewhere could possibly be attributed to geological factors and not necessarily to acquisition methods. Finally, the seismic test profile would also cross a major break in the magnetic anomaly pattern that may represent a Proterozoic southwest-dipping thrust zone (Lilljequist 1980; Öhlander et al. 1993). In order to image both Figure 1. Large-scale map of area showing location of the relevant seismic profiles. That portion of BABEL Line 2 presented in this paper is shown as a thicker black line and referred to as BABEL 2n. The extension of BABEL Line 2 to the southwest is shown as a thin grey line. The approximate location of the Archaean Proterozoic boundary (APB) based on Sm Nd studies is located just north of the Luleå Jokkmokk Zone (LJZ) (Mellqvist et al. 1999) and its interpreted offshore geometry based on the seismic data in this paper is shown as a dashed grey line. The inset shows the location of the map area.

3 182 C. Juhlin et al. Figure 2. Interpretation of BABEL Line 3/4 seismic data in the Bothnian Bay based on Öhlander et al. (1993) and BABEL Working Group (1993). The distance scale refers to the distance along the profile as shown in Fig. 1. Course changes are marked with squares. The approximate along-strike locations of the seismic data presented in this paper are also shown. upper crustal and lower crustal features the seismic test profile consisted of a small-charge higher-resolution shallow component and a larger-charge lower-resolution deep component. In this paper we present results from the seismic test profile and compare them with the northern part of the offshore image of BABEL Line 2 (this northern part is referred to as BABEL Line 2n in the remainder of this paper and is marked in Fig. 1). Detailed images of the near surface are obtained along the high-resolution portions of the profile by applying pre-stack migration and first-break tomography. These images are merged with the conventionally processed deep component section. Reprocessed seismic data from a short (10 km) test profile (Elming & Thunhed 1991), acquired about 100 km SW of the present study area over the southern margin of the Skellefte ore district, are also presented and compared with the Luleå profile. Based on the present results we present an alternative interpretation of the crustal structure along BABEL Line 2n. We then compare the Proterozoic arc continent collision in northern Sweden and Finland with the more modern arc continent collision in the Urals. Finally, we present a possible model for the tectonic evolution of northern Sweden. 2 GEOLOGICAL SETTING 2.1 Regional geology The bedrock of northern Sweden east of the Caledonides is dominated by Palaeoproterozoic rocks. Exposed Archaean rocks occur in relatively small areas consisting primarily of Ga orthogneisses of granodioritic to tonalitic composition (Skiöld 1979; Witschard 1984; Öhlander et al. 1987; Lundqvist et al. 1996). The palaeoboundary of the Archaean continent is delineated by Sm Nd isotope analyses of Proterozoic granitoids (Fig. 1, Öhlander et al. 1993; Mellqvist et al. 1999). North of the Archaean palaeoboundary, narrow supracrustal belts dominated by mafic metavolcanic rocks, metaquartzites and carbonates were formed in association with intracratonic rifting between 2.3 and 2.1 Ga (Skiöld 1987; Huhma et al. 1990; Öhlander et al. 1992). At the final stage of this rifting process MORB-type basalts were emplaced. At present it is not known how far the spreading process went and whether the deep water ocean floor developed before the rift basins were closed again during the Svecokarelian orogeny. At about 2.0 Ga, a rifted continental margin at the southwestern boundary of the Archaean craton formed and turbidites were deposited (Gaál & Gorbatschev 1987, and references therein). Spreading shifted to convergence when subduction initiated beneath the Archaean continent, and juvenile rocks were formed with the peak of igneous activity between 1.89 and 1.87 Ga (Skiöld 1987; Vaasjoki & Sakko 1988; Skiöld et al. 1993). The Skellefte district (Fig. 1) with its very massive sulphide (VMS) ores is probably a remnant of a juvenile volcanic arc (Rickard & Zweifel 1975; Weihed et al. 1992; Billström & Weihed 1996). South of the Skellefte district, in the so-called Bothnian Basin, the supracrustal rocks are dominated by marine metasediments and mafic metavolcanic rocks. Simultaneously with the formation of large amounts of juvenile calc-alkaline intrusive and volcanic rocks southwest of the Archaean palaeoboundary, magmatism with a large component of Archaean source material was widespread within the craton (Huhma 1986; Öhlander et al. 1993; Mellqvist et al. 1999). Later, at c. 1.8 Ga, large amounts of potassic granitoids (Lina type) were again formed in northern Sweden, but coeval volcanic rocks have been found only at one locality. Although the Archaean rocks within the Luleå area (Fig. 3a) are strongly deformed and metamorphosed, evidence for Archaean deformation structures have only been found locally (Wikström & Söderman 2000a). The main deformation in the Luleå area is c Ga in age. In the Skellefte district and towards the south the main deformation and metamorphism is interpreted to be considerably younger at c Ga (Lundqvist et al. 1998; Weihed et al. 1992). Late ductile reverse shear zones with dominantly NNE trends have been dated at c Ga (Weihed et al., unpublished), indicating that these were active more or less at the same time as the intrusion of the major TIB (Trans-Scandinavian Igneous Belt) Revsund granitoid batholiths at c Ga. The N S distribution of these A-I-type granitoids, together with the occurrence of middle crust S-type granites mainly to the east of the A-I-type TIB Revsund granitoids, indicate that the general convergence had shifted from NE SW to E W from c. 1.9 to 1.8 Ga. The 1.80 Ga magmatism, thus, overprints early tectonic features related to Svecokarelian NE directed accretionary processes.

4 Crustal reflectivity in northern Sweden 183 Figure 3. (a) General geological map of the profile area with profile location. Black dots and line refer to the actual profile with station numbers. Light dots and line refer to the processed CDP line and CDP numbers. The trend line of the Sm Nd-defined Archaean Proterozoic boundary (APB) is marked by the dark heavy line. SVF indicates a possible near-vertical fault interpreted from the seismic data. (b) Total field magnetic map over the same area with the CDP line (magnetic data published with permission from the Geological Survey of Sweden).

5 184 C. Juhlin et al. 2.2 Local geology The seismic profile was shot between the river valleys Alterälven and Luleälven with the eastern end located c. 30 km west of Luleå (Fig. 3a). From the western end of the profile to station 1130 (CDP 2020) the profile crosses metasediments (low-amphibolite facies) belonging to the Bothnian supergroup. These cover large areas of central Sweden and are commonly dominated by more or less migmatized greywackes, which were deposited during an extended time period between >1.95 and 1.87 Ga (Lundqvist et al. 1998). In the present study area they are characterized by a low magnetic signature on the aeromagnetic map (Fig. 3b). Further east (east of station 1130, CDP 2020) a fragment-rich complex, mainly Ga pyroclastic volcanic rocks, ranging in composition between rhyolite and andesite with subordinate amounts of more mafic rocks is present (Pålmark unit, Fig. 3a). This complex is highly magnetic and forms a characteristic pattern in the aeromagnetic map (Fig. 3b). The contact with the Bothnian supergroup metasediments is not visible in the field along the profile, but macroscopic structures in outcrop in both metasediments and volcanic rocks in the vicinity are steeply west-dipping. Calculations based on magnetic anomaly profiles indicate a shallower west-dipping contact (Johan Söderman, SGU, personal communication) suggesting the sediments to have been thrusted upon the east-lying volcanic complex (Lilljequist 1980; Öhlander et al. 1993). One of the goals of the present investigation was to test this idea. The volcanic complex is intruded by two major plutonic suites. An older, intensively magma-mingled and mixed, deformed and migmatized calc-alkaline suite is found in dome-shaped structures, where kinematic indicators suggest extensional, core-complex-type features (Wikström & Söderman 2000b). Dating in the central part of such a structure (Riskälen dome, Fig. 3a) gave an age of 1867 ± 19 Ma (Mellqvist, 1999). A younger (c. 1.8 Ga) granite syenite gabbro anorthosite suite is localized within the volcanic complex. In the study area it forms two major massifs, the Vitberget massif in the west (station , CDP ) and the Ale massif (Öhlander & Schöberg 1991) in the east. Both massifs show normal zonation, i.e. more mafic parts in the marginal areas. The Ale massif is located in a structural position near outcrops of Archaean rocks and is possibly a so-called stitching pluton (Wikström & Söderman 2000b), which has intruded along a terrane boundary. The eastern part of the profile was shot over the Bälingsberget unit (stations 1 280, CDP 1 530), which is composed of subvolcanic to volcanic rocks. They are rich in fragments, containing both Archaean and Proterozoic granodiorite. Parts of this unit have previously been classified as conglomerates, but were later reinterpreted as subvolcanic, magmatic breccias (Wikström et al. 1996a). Scattered areas of exposed Archaean bedrock in the area (Fig. 3a) follow the Sm Nd trend line that lies close to the eastern part of the profile. This trend line also approximately coincides with the border between the c Ga magmas in the north containing a large component of Archaean material and similar, more juvenile magmas of the same age in the south (Mellqvist et al. 1999). As shown schematically in Fig. 3(a), relatively large areas of Archaean crust are present in the Vallen Alhamn Mannön area in the southeast and in the Boden area in the northwest. The contacts between the Archaean rocks and surrounding rocks are either tectonic or bounded by intrusions of the 1.88 Ga plutonic rocks. Published ages for the Archaean rocks found so far in the area are Ga (Lundqvist et al. 1996, 2000; Wikström et al. 1996b). Table 1. Acquisition parameters. Parameter Shallow component Deep component Source Dynamite No of channels 200 Station spacing 25 m Nominal shot spacing 100 m 1000 m Nominal fold Nominal spread End on/shoot through Geophones Bunch of six 10 Hz Geophone spacing <1 m Sample rate 2 ms Charge size 1 kg 12 kg Filters 8 Hz lowcut Recording system Sercel 348 Date recorded 1999 August 3 DATA ACQUISITION The objectives of the seismic survey, namely imaging of both upper and lower crustal structure, placed certain constraints on the acquisition parameters. Ideally, a 40 km long survey with 10 kg shots every 100 m would have allowed these goals to have been achieved. However, the limited budget available required a less ambitious programme. In addition, there are only a limited number of roads crossing the interpreted Archaean Proterozoic boundary and we wanted to avoid areas with larger amounts of the younger 1.8 Ga granitic intrusions. These constraints led to splitting of the profile into two shorter shallow component sections and an overlapping longer deep component section (Table 1). The easternmost shallow component section starts at the southern bank of the Luleälven river (Fig. 3a) and runs west-southwest for c. 7.5 km, crossing the Archaean Proterozoic boundary as interpreted from Sm Nd data. Southwest of station 304 (CDP 540) to station 1012 (CDP 1830) only large shots were fired at c. 1 km intervals resulting in low-fold data and poor images of the uppermost kilometres. From station 1012 (CDP 1830) to station 1234 (CDP 2240) (Fig. 3) lies the c. 5.5 km long second shallow component section of the profile, crossing the marked break in the magnetic anomaly pattern at station 1130 (CDP 2020). Southwest of station 1234 (CDP 2240), four additional shot points were located to extend the subsurface coverage in this direction. Owing to the overlapping nature of the deep and shallow components, large charges were substituted for small charges about every kilometre along the shallow component sections. 4 DATA PROCESSING The shallow and deep component data sets were treated as a single data set and standard processing steps (Table 2) were applied to it, resulting in a 30 km long stacked seismic section of the crust (Fig. 4). From this stacked section an automatic line drawing was generated based on the coherence of the reflections (Fig. 4). These picked reflections were then depth migrated (Fig. 4) using a velocity function based on BABEL Line 2n (Gohl & Pedersen 1995). Owing to the closer shot spacing along the shallow component sections on the northeast and southwest portions of the profile shallower (0 2s) features are imaged there (CDP in the NE and CDP in the SW in Fig. 4). The contribution from the shallow and deep components to the stack (processed up to step 13 in Table 2) can be seen in Fig. 5. As expected, a larger-charge size gives a better deep image, but the higher fold of the shallow component data gives a better image at times less than about 10 s.

6 Crustal reflectivity in northern Sweden 185 Table 2. Processing parameters. Number Step 1 Read SEG2 data 2 Trace edits (automatic and manual) 3 Correction for geometrical divergence 4 Elevation statics: datum-100 m 5 Air blast attenuation 6 Spectral whitening Hz at 6 s Hz at 2 s Hz at 3 s Hz at 2 s 7 Bandpass filter at s at s at 2 3s at 2.5 4s at 3.6 6s at 5 8s at 7 20 s 8 Refraction and residual static corrections 9 NMO correction 10 Trace equalization s s s 1 3s s 6 8s 9 20 s 11 Trim statics: max shift 2 ms 12 CMP stacking 13 Trace equalization 14 F-X deconvolution 15 Display Shot gathers in the southwest shallow component data show reflections arriving about ms after the first arrivals, suggesting the presence of shallow subhorizontal reflectors on this portion of the profile (Fig. 6). These reflectors are not imaged using the standard processing steps in Table 2. Therefore, pre-stack migration was applied to these shot gathers after velocity filtering of the first arrivals. Stacking of these migrated shot gathers reveals the presence of two gently southwest-dipping reflectors that appear to project to the surface at about CDP 1800 and CDP 1680 (Fig. 7a). In addition to the above wavefield processing, first-break picks were used for tomographic imaging of the velocity field in the uppermost 500 m along the profile (Fig. 7b). In 1987 October a short seismic reflection test profile was shot near the southern boundary of the Skellefte district (Elming & Thunhed 1991) and is referred to as the Norsjö profile (Fig. 1). Nine of the ten original field tapes were still readable and these data have been reprocessed using a similar processing sequence as that shown in Table 2. A stacked seismic section of the data is shown in Fig. 8. The even shorter length of the Norsjö profile compared with the depths investigated implies that most reflections migrate off the section. Therefore, the migrated line drawing has been extended past the limits of the profile to indicate the spatial location of the deeper events. It was extended 7.5 km to the south and 15 km to the north since the south dipping reflections are steeper and migrate further off the section. Note that the migrated section is highly biased and that there may be several structures present at depth that are not imaged owing to the recording geometry. In order to compare results, a line drawing migration of BABEL Line 2n is displayed on the same scale as the migrated section from the present study (Fig. 4) in Fig. 9. Although the BABEL Line 2n data were acquired with a receiver spacing of 25 m (BABEL Working Group 1993), the data from adjoining receivers were merged prior to processing resulting in an effective receiver spacing of 50 m and a CDP spacing of 25 m. Therefore, the stacked section from which the line drawing of BABEL Line 2n was generated had a trace spacing that is half as dense as that of the onshore stacked sections from the present study and the Norsjö profile. 5 RESULTS 5.1 Near-surface features The pre-stack migrated section (Fig. 7a) shows a layered structure down to c. 500 m between CDP 2060 and CDP 2060 lies about 2 km northeast of the southwestern limit (CDP 2220) of the high-resolution component. Northeast-dipping reflectors appear to bound this layered sequence on both sides. The layering correlates well with the surface exposure of the volcanic rocks in the area. A similar layering is also present east of CDP 600 (Fig. 7a), but its depth and lateral extent is not as well defined as the western layered section. Volcanic rocks are also present in the geological map east of CDP 600 (Fig. 3a). The layered nature of the migrated seismic section in these areas suggests that the total thickness of these volcanic units is limited to less than 500 m in the west and 1 km in the east. Low-velocity zones in the upper m are found associated with the seismic layering (Fig. 7b), but there is not a one-to-one correspondence in their lateral extent. Note that the delays associated with the first arrivals where the reflections intersect the surface (Fig. 6) imply that these reflections originate from low-velocity zones. 5.2 Upper crust After migration, the upper crust (0 20 km) appears more reflective in the northeast than in the southwest (Fig. 4) except for the two pronounced reflectors present below CDP (A and B1 in Fig. 4) and the less pronounced one below CDP 1600 (B2 in Fig. 4) in the upper 5 km. The more reflective crust in the northeast can be roughly separated from the more transparent crust in the southwest by a c. 45 southwest-dipping boundary (APB in Fig. 4). This boundary projects to the surface close to the Archaean Proterozoic boundary as defined from the Sm Nd studies and geological mapping (Fig. 3a), suggesting that the more reflective part of the seismic section corresponds to Archaean and Proterozoic crust containing a large proportion of Archaean rocks, and the more transparent juvenile Proterozoic crust. This interpretation is based on 30 km of profile where any steeply dipping reflectors below c. 10 km will not be imaged. Reflections from out-of-the-plane of the profile will also complicate the image. However, given the available data and observations from the offshore BABEL lines, we believe the above interpretation of the geometry of the Archaean Proterozoic boundary to be the most reasonable one that can be made at present. The marked break in the magnetic anomaly pattern between the volcanic rocks northeast of CDP 2000 (station 1130) and the metasediments to the southwest suggests a different seismic response may be expected from the two rock units. Several features in the seismic section observed northeast of CDP 2060 appear to terminate rather abruptly nearly vertically below this point. These features include (i) the layered reflectivity in Fig. 7(a), (ii) the northeastdipping reflector B1 in Fig. 4 and (iii) the strong reflector or zone of reflectivity marked A in Fig. 4. Termination of these reflectors

7 186 C. Juhlin et al. Figure 4. Panel 1 shows the stacked seismic section processed to step 15 in Table 2, panel 2 shows an automatic line drawing of the stacked seismic section in panel 1, and panel 3 shows the line drawing in panel 2 migrated. The suggested Archaean Proterozoic boundary (APB) is marked as are reflectors A, B1 and B2 that are discussed in the text. Squares near the top of the sections mark where the CDP line changes direction.

8 Crustal reflectivity in northern Sweden 187 Figure 5. Stacked seismic sections down to 20 s processed to step 13 in Table 2. Panel 1 shows the stack with all shots included in the processing, panel 2 the stack with only the large shots included and panel 3 the stack with only the small shots included. The location of the profile is shown in Figs 1 and 3.

9 188 C. Juhlin et al. Figure 6. Shot sections showing near-surface upper crustal reflections before (plots on the right) and after (plots on the left) velocity filtering.

10 Crustal reflectivity in northern Sweden 189 Figure 7. (a) Pre-stack migration of the upper 1 s TWT merged with the upper 3 s of the post-stack migrated data from Fig. 4. (b) Velocity field from first-break tomography.

11 190 C. Juhlin et al. Figure 8. Panel 1 shows the Norsjö stacked seismic section processed with similar parameters to that of the Luleå data, panel 2 shows an automatic line drawing of the stacked seismic section in panel 1 and panel 3 shows the line drawing in panel 2 migrated. The time sections are 7.5 km wide. The migrated section has been extended 7.5 km south of the profile and 15 km north of the profile to better indicate the spatial positions of deep reflections. Location of the profile is shown in Fig. 1. close to or directly below CDP 2060 suggests that the metasediment/volcanic boundary is near-vertical and that it extends down to at least 6 km. Note that the actual acquisition line and the CDP line cross the boundary at different locations, implying that the geology along the CDP line does not correspond exactly to what has been imaged. 5.3 Lower crust Numerous high-amplitude reflections or zones of reflections are present below 5s(Fig.4),manyhaving a concave downward appearance indicating they are diffractions. From c. 12 s down to c. 14 s subhorizontal reflections are present across the entire section (Fig. 4). These reflections are weaker than the higher-amplitude reflections/diffractions above, but have less of a diffractive nature. Below 14 s the seismic section shows some weak east-dipping reflections (15 16 s at CDP in Fig. 4), but is otherwise transparent. We interpret the base of the subhorizontal reflectivity at c. 14 s to correspond to the Moho in accordance with combined reflection and refraction seismic studies in Bothnian Bay (BABEL Working Group 1993; Gohl & Pedersen 1995) and elsewhere. Using the refraction velocities from Gohl & Pedersen (1995) to migrate the

12 Crustal reflectivity in northern Sweden 191 Figure 9. Migrated line drawing of the northern part of BABEL Line 2n with the Luleå and Norsjö profiles plotted at their equivalent estimated along-strike positions. The locations of the profiles are shown in Fig. 1.

13 192 C. Juhlin et al. reflection seismic data from the present study then this termination in reflectivity corresponds to a depth of c. 47 km for the depth to the Moho in this area (Fig. 4). This is approximately the same depth as observed on BABEL Line 2n (Gohl & Pedersen 1995), but c. 5 7 km deeper than indicated on published contoured Moho maps (Riahi et al. 1997; Korja et al. 1993). 6 DISCUSSION 6.1 Crustal structure Based on the near-surface pre-stack migrated image (Fig. 7a) the seismic layering in the volcanic rocks east and west of the gneiss dome (Riskälen dome, Fig. 3a) appear to dip away from the dome, suggesting normal faults at its boundaries. Structural evidence in outcrop data on the periphery of the dome also indicate the presence of normal faults, which suggests that the dome may have been emplaced in an extensional regime some time between 1.87 and 1.8 Ga and subsequently transported to the surface along the Archaean Proterozoic boundary (APB in Fig. 4). The lack of clear southwestdipping reflections along the APB is then somewhat puzzling, but these may have been erased by later intrusion of the 1.8 Ga granites. Reflector A (Fig. 4) may be related to the suturing and had a larger lateral extent prior to the possible extension and intrusions of the 1.8 Ga granites. Note that along the URSEIS profile in the southern Urals that the arc continent suture is not very reflective (Ayarza et al. 2000). Further east, along BABEL Line 2n, there are southwest-dipping reflections that project to the surface somewhat northeast of SP 2500 (Fig. 9), the approximate location of the Archaean Proterozoic boundary as projected on to that line. These reflectors may represent the arc continent suture. The near-vertical nature of the metasediment volcanic boundary at CDP 2060 (Fig. 7a) suggested by the seismic data and outcrops in the area, can be explained by the presence of a subvertical fault separating the igneous rocks to the east from the metasediments to the west (Fig. 3a). Subvertical layering, as observed in the field south of the profile, would explain the absence of any reflected signal from the metasediments. It is difficult to define in which tectonic regime this fault developed, but the juxtaposition of subvertical layered metasediments along a subvertical fault suggests a strong strikeslip component. 6.2 Signal penetration Previously acquired deep seismic onshore profiles in Sweden generally show only weak reflectivity in the lower crust (Dahl-Jensen et al. 1991; Juhojuntti & Juhlin 1998; Juhojuntti et al. 2001). The lack of strong reflectivity has been attributed to the fact that the profiles were mainly acquired over the Trans-Scandinavian Igneous Belt, a large batholith, that has undergone only minor deformation since emplacement (Hurich 1996; Juhojuntti et al. 2001). Signal penetration studies on TIB rocks in the Siljan Ring area in central Sweden indicated that 5 kg charge sizes were possibly insufficient to allow propagation of energy to Moho depths and back, whereas 10 kg charges were sufficient (Juhojuntti & Juhlin 1998). Reflection seismic data acquired over TIB rocks in southern Sweden using 1 kg charges gave consistent signal penetration to only 3 s (Juhlin et al. 2000). These results suggest that the deep component data in the present study should provide reliable images of the lower crust, but the shallow component data may not. However, comparison of the deep component data and shallow component stacks show that they are similar and that reflections from the lower crust are observed on the shallow component data as well (Fig. 5). Amplitude decay curves for the shallow and deep component data have been estimated on traces within the offset range m. The envelope was taken for each trace and the amplitude normalized in the time window s. Diversity stacking with a 25 per cent rejection factor was then applied to all traces for each component to generate average amplitude decay curves (Fig. 10). Inspection of the Figure 10. Amplitude decay curves of raw data from the deep (thick black line) and shallow (thick grey line) component data. Average background noise level from the shallow component data is shown as a horizontal very thin black line. Shown also are the amplitude decay curves from studies in southwestern Sweden for charges of kg (Juhlin et al. 2000) and charges of kg (Dahl-Jensen et al. 1991). The kg curve (thin black line) was amplitude normalized prior to the first arrival and shifted 8 db for the peak to coincide with the small-charge Luleå data. The kg curve (thin grey line) was amplitude normalized in the same s time window as the Luleå data. Note the difference at s compared with the Luleå data.

14 Crustal reflectivity in northern Sweden 193 amplitude decay curves for the shallow and deep component data sets shows that there is still signal at 14 s in the shallow component data, although it is very weak (Fig. 10), and there is a clear drop in signal level at 14 s on the deep component data. The higher fold of the shallow component data results in a coherent image upon stacking (Fig. 5). Comparison of the Luleå shallow component data with kg charges from southwestern Sweden (Juhlin et al. 2000) shows that a greater dynamic range is present in the Luleå data (Fig. 10). This indicates that environmental noise may have affected the small-charge data from southwestern Sweden. Comparison of the deep component Luleå data with kg charges in southwestern Sweden (Dahl-Jensen et al. 1991) shows the amplitude curves to be quite similar, except for the increased values at s in the Luleå data that correspond to lower crustal reflectivity. This is a strong indication that the weak or absent lower crustal reflectivity in the TIB rocks is a result of geological factors (when the charge size is over 10 kg) as suggested by other authors (Hurich 1996; Juhojuntti et al. 2001). 6.3 Comparison with the Norsjö profile Steeply dipping reflections are primarily observed in the upper 10 s in the Norsjö profile (Fig. 8). From s clear subhorizontal reflectivity is present, but it becomes more diffuse below 12 s and gradually terminates at about 13 s. This termination probably corresponds to the base of the crust and is shallower than that observed on the Luleå profile (and also BABEL Line 2n at the latitude of Luleå), but in accordance with crustal thinning towards the south based on refraction data (Gohl & Pedersen 1995). Another difference between the two profiles is the absence of strong subhorizontal reflectivity in the upper and middle crust on the Norsjö profile compared with the Luleå profile. This difference may be partly explained by the fact that the southernmost 5 km of the Norsjö profile was acquired over Revsund granite, suggesting that some of the steeply dipping reflections are from fracture zones within the granite. However, other steeply dipping reflections project up into phyllites lying north of the Revsund granite. Even though much of the subhorizontal reflectivity in the upper and middle crust along the Luleå profile has a diffractive component, this reflectivity, nonetheless, indicates that strong impedance contrasts are present at depth, assuming that the diffractions originate from within the plane of the profile. These strong impedance contrasts in the upper crust appear to be absent in the Norsjö area, indicating a different crustal structure there. 6.4 Comparison with BABEL Line 2n and crustal architecture Determining how to extrapolate surface geology on to BABEL Line 2n is an important objective of the Luleå test profile. Based on the diffractive nature of the Archaean crust as interpreted earlier, the presence of more transparent crust southwest of these diffractors, and the similar thickness of the crust below them, we suggest that the Luleå test profile images similar crust to that from c. SP on BABEL Line 2n (Fig. 1). The Archaean Proterozoic boundary would then be located at the surface close to SP 2500 on BABEL Line 2n and at about 250 km on BABEL Line 3/4 (Fig. 1). This correlation implies that the Archaean Proterozoic boundary either changes orientation offshore or is offset by the Baltic Bothnian Megashear (Fig. 1). Both Nironen (1997) and Berthelsen & Marker (1986) interpret initial dextral movement along the Baltic Bothnian Megashear followed by sinistral movements. Berthelsen & Marker (1986) interpret a total sinistral offset along the zone, suggesting that the Archaean Proterozoic boundary is offset, rather than changing direction. Given that our correlation of the Archaean Proterozoic boundary is correct, the transparent crust, previously interpreted as reworked Archaean rocks (Fig. 2), is more likely to be Proterozoic crust. Seismic images from the more recent, but possibly similar, Uralide arc continent collision (Steer et al. 1998; Juhlin et al. 1998) show island arc rocks there to be generally seismically transparent compared with the East European Craton on to which they were thrusted. Seismic images over the active Banda Arc also show that arc to be seismically transparent (Snyder et al. 1996b). Southwest of this less reflective section on BABEL Line 2n a more reflective middle crust exists between SP 3200 and This more reflective crust appears to be bounded below by a set of 30 NEdipping reflectors (top of the oceanic crust in Fig. 11). Below this set of dipping reflectors the crust is fairly transparent northeast of c. SP 3800, whereas to the southwest of SP 3800 almost the entire crust is highly reflective. This reflective crust southwest of SP 3800, which has an inverted V-shape (Skellefte arc in Fig. 11) may represent the Skellefte island arc. The high reflectivity of the Skellefte island arc contrasts strongly with the more transparent nature of the Figure 11. Interpretation of the present crustal structure along BABEL Line 2n.

15 194 C. Juhlin et al. Palaeozoic Tagil volcanic arc in the Urals (Juhlin et al. 1998) and the modern Banda arc (Snyder et al. 1996b). The more transparent lower crust attached to the northeastern part of the Skellefte arc may be oceanic crust (oceanic crust in Fig. 11) that was not subducted when the Arvidsjaur-Kiruna and Skellefte arcs collided with the Archaean craton. Further to the southwest are the Svecofennian forearc rocks and the relict subduction zone interpreted on the BABEL data (BABEL Working Group 1993; Öhlander et al. 1993). There are two main differences in our interpretation of the crustal architecture in northern Sweden based on the BABEL data from the previous ones of Lindsey & Snyder (1994); Gohl & Pedersen (1995) and Öhlander et al. (1993). First, previous interpretations show only southwest-dipping structures in the Phanerozoic crust in northern Sweden north of the Skellefte arc. We interpret a northeast-dipping boundary separating the Arvidsjaur-Kiruna terrane from possible oceanic crust below (Fig. 11). It is only southwest of the Skellefte boundary that northeast-dipping structures have been shown in previous interpretations. Secondly, the previous interpretations show the Archaean crust continuing below the Proterozoic crust towards the southwest and constituting the base of the crust. We suggest that remnant oceanic crust may exist below the Arvidsjaur- Kiruna terrane and the Archaean crust, forming the base of the crust and resulting in somewhat thicker crust to the northeast (northeast of SP 2500 in Fig. 11). The absence of structure in the upper 10 km of BABEL Line 2n (Fig. 9) is, in all likelihood, a result of acquisition and processing factors, not geological factors. Both the present study and the Norsjö profile show considerable structure in the upper 10 km. Reprocessing of deep seismic marine data acquired along BABEL Line 1 further south resulted in improved images of the uppermost 10 km (e.g. White 1996). However, on BABEL Line 1 the target reflectors were subhorizontal and of high amplitude. Reprocessing of the BABEL Line 2n data may also give a better image even if the target reflectors are more steeply dipping and weaker. 6.5 Comparison with the Middle Urals A possible, more modern, analogy to the Early Proterozoic events in northern Sweden is the Uralian orogeny where island-arc terranes were accreted to the eastern Baltica margin in the Late Palaeozoic (Hamilton 1970; Ivanov et al. 1975; Zonenshain et al. 1990; Sengör et al. 1993). Based on reflection seismic data and geological mapping in the Middle Urals, the following tectonic evolution has been suggested for this part of the orogen (Friberg 2000). Rifting of the Baltica continent began in the Early Ordovician and by the Late Silurian an east-dipping subduction zone east of the spreading centre was active, resulting in the development of two island-arc terranes. The eastern margin of Baltica was now a passive margin. In the preferred evolutionary model, a third island-arc terrane developed in the Early Carboniferous east of the other two arcs as the result of a west-dipping subduction zone even further east. At this time granites intruded in the two westernmost arcs as this composite terrane was sutured to Baltica. Some time in the Permian, the Siberian craton collided with Baltica and the adjoining arc terranes, resulting in uplift. Subsequent to the collision, or possibly contemporaneously with it, N S strike-slip faults were active in the hinterland, controlled mainly by the terrane boundaries (Ayarza et al. 2000). Mesozoic extension resulted in stretching of the crust below the present West Siberian Basin. The extension erased much of the structure in the lowermost crust, but below the Baltica-arc suture remnants of a crustal root remain (Juhlin et al. 1996; Morozova et al. 1999). Northern Sweden shows several similar features to the Middle Urals, but also some important differences. In the Middle Urals, the arc continent suture has a clear seismic signature with well-defined reflectors showing the boundary dipping c. 45 to the east (Juhlin et al. 1998). In northern Sweden, we have defined the boundary as the transition from interpreted reflective Archaean crust to more transparent Proterozoic crust (Fig. 4). Intrusion of 1.8 Ga granites along the suture may explain the lack of reflectivity parallel to the interpreted boundary on the Luleå profile. On the offshore BABEL Line 2n there are some SW-dipping reflectors in the upper crust projecting up to SP 2500, close to where the interpreted Archaean Proterozoic boundary is located (Figs 9 and 11). Furthermore, in the Middle Urals, the arc which was sutured to Baltica is an island arc (Ivanov et al. 1975), whereas the Arvidsjaur-Kiruna terrane is probably a continental arc that was possibly originally rifted from the Archaean craton. The island arc/craton boundary may have a clearer seismic signature than the continental arc/craton boundary. Finally, there is a root zone below the suture in the Middle Urals (Juhlin et al. 1996). A clear root zone is not present below the boundary in northern Sweden, but the crust appears to thicken from c. 45 km in interpreted Proterozoic rock to nearly 50 km in interpreted Archaean rock (Fig. 11). The Archaean crust of northern Sweden may represent an analogous geological setting to the eastern margin of Baltica in the Uralide orogen. 6.6 Tectonic evolution Based on the present-day crustal architecture in Fig. 11 as interpreted in this paper and on comparisons with more recent arc continent collisions the following tectonic evolution for northern Sweden may be proposed (Fig. 12): rifting of the Archaean continent began at c Ga. During the rifting phase, mafic magmas may have intruded into the craton, and cooled at mid-crustal levels. These bodies now produce the diffractions observed in the Archaean rock northeast of the interpreted suture. It is unclear how wide the ocean developed after rifting, but at some point around c Ga, the age of the Pyhäsalmi arc in Finland, southwestward subduction began below the rifted continent resulting in the development of the Arvidsjaur-Kiruna continental arc. Although originally Archaean, the large amounts of Proterozoic magma that invaded the Archaean crust gave it a Proterozoic signature. Simultaneous with the development of the Arvidsjaur-Kiruna arc, the Skellefte island arc developed owing to northeast-, or possibly southwest-directed subduction further to the southwest. At c Ga the Arvidsjaur- Kiruna arc docked with the Archaean craton, but the Skellefte arc was still active. Shortly thereafter, another arc or microcontinent collided with the Skellefte and Arvidsjaur-Kiruna arcs resulting in the thrusting of the Arvidsjaur-Kiruna arc over on to the Archaean craton. It is this collision that produced the relict subduction zone interpreted on the BABEL data (BABEL Working Group 1990). The final major tectonic event in the area was the intrusion of 1.8 Ga TIB Revsund granitoids. A more north south-striking subduction zone further to the west may have provided the heat necessary for the generation of these granitic melts. Alternative scenarios exist to the tectonic evolution outlined above. Snyder et al. (1996a) suggested a subduction polarity reversal at about 1.9 Ga to explain their interpreted reflector geometry on BABEL Line 3/4. The location of this reversal is not given explicitly, but could have occurred between the Arvidsjaur-Kiruna continental arc and the Skellefte arc. At present, there are no constraints on the tectonic evolution of this boundary zone. However, at about 1.87 Ga northeast-directed subduction must have been ongoing

16 Crustal reflectivity in northern Sweden 195 Figure 12. Interpretation of the Proterozoic tectonic evolution of northern Sweden.

17 196 C. Juhlin et al. below the Skellefte arc. Thus, both our suggested tectonic evolution and that of Snyder et al. (1996a) imply southwest-directed subduction below the Arvidsjaur-Kiruna continental arc followed by northeast-directed subduction below the Skellefte arc. 7 CONCLUSIONS The Luleå test profile shows that good-quality seismic images may be obtained of the entire crust using a combination of shallow and deep component acquisition parameters. Even the shallow component shots (1 kg) penetrated to Moho depths. Comparison of signal penetration curves from the present area with those from data acquired over the Trans-Scandinavian Igneous Belt in southwestern Sweden show the lack of lower crustal reflectivity there to be a result of geological factors, not acquisition parameters. The Sm Nd-defined Archaean Proterozoic boundary does not have a corresponding seismic signature, but the seismic image suggests that the Archaean rocks outcropping to the northeast are more reflective than the Proterozoic crust. This implies that the Archaean Proterozoic boundary dips at approximately 45 to the southwest below the profile and that the Proterozoic crust was thrust on to the Archaean crust. Later intrusions of granites may have erased the seismic signature of the suture in the Luleå area. The marked change in the magnetic anomaly pattern in the southwest between volcanic rocks and Bothnian Basin metasediments correlates with a vertical break in the seismic image and indicates the presence of a subvertical fault extending down to at least 6 km. Using the velocities based on wideangle data from BABEL Line 2n and the downward termination of reflectivity as the base of the lower crust, the Moho is estimated to be at c. 47 km below the Luleå profile. Correlation of the present data set with that from BABEL Line 2n shows that many of the features observed in the middle and lower crust are of a similar nature. However, there is much more information from the upper crust in the seismic image from the Luleå profile than from BABEL Line 2n. Differences in acquisition and processing of high-resolution land versus low-resolution marine seismic data are, in all likelihood, the reason for this, not geological factors. In spite of the poor upper crustal image on BABEL Line 2n, it is possible to better locate this line relative to the onshore surface geology using the middle and lower crustal image from the Luleå profile as a guide. Based on the northern part of the marine BABEL Line 2n data, the onshore Luleå and Norsjö profiles and a comparison with the Middle Urals, a modified interpretation of the crustal structure and tectonic evolution of northern Sweden is presented. After rifting of the Archaean craton at about 2.1 Ga a southward-directed subduction zone developed south of the present-day Archaean craton. This subduction resulted in the intrusion of large amounts of Proterozoic rocks into the conjugate rifted margin (Arvidsjaur-Kiruna continental arc) at 1.89 Ga, simultaneous to the development of the Skellefte island arc further south. At present, the polarity of this subduction is not known. At about 1.87 Ga, the northern continental arc, now consisting of mainly Proterozoic rocks, was thrust on to the Archaean craton and the Skellefte island arc was underthrust by the Arvidsjaur-Kiruna continental arc. Attached oceanic crust that was not subducted and is characterized in the seismic data by its low reflectivity in the lower crust below the present-day Arvidsjaur-Kiruna continental arc. ACKNOWLEDGMENTS Field work and processing costs for this study were funded by the Geological Survey of Sweden (SGU) under the auspices of a SGU external Research and Development grant (PROJECT NUMBER /98) and the Swedish Natural Sciences Research Council (NFR). C. Juhlin is also partly funded by the Swedish Natural Sciences Research Council (NFR). H. Palm is thanked for his coordination of the field work. We also thank M. Friberg and two anonymous reviewers for constructive comments on this study. REFERENCES Ayarza, P., Brown, D., Alvarez-Marrón, J. & Juhlin, C., Contrasting tectonic history of the arc continent suture in the Southern and Middle Urals: implications for the evolution of the orogen, J. geol. Soc. London, 157, BABEL Working Group, Evidence for early Proterozoic plate tectonics from seismic reflection profiles in the Baltic Shield, Nature, 348, BABEL Working Group, Integrated seismic studies of the Baltic Shield using data in the Gulf of Bothnia region, Geophys. J. 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