Seismic Imaging of the Scandinavian Caledonides to Define ICDP Drilling

Home , Alum Shale Formation, Scandinavian Caledonides

Tectonophysics 554-557 (2012) 30–41

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Tectonophysics

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Seismic imaging of the Scandinavian Caledonides to deﬁne ICDP drilling sites

Peter Hedin ⁎, Christopher Juhlin, David G. Gee

Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden article info abstract

Article history: A 36 kilometer long high resolution 2D seismic reflection profile was acquired in the summer of 2010 to be Received 5 March 2012 used in the planning of the COSC (Collisional Orogeny in the Scandinavian Caledonides) Deep Drilling Project. Received in revised form 25 May 2012 Two fully cored boreholes, each to c. 2.5 km depth, are planned for the Åre-Mörsil area of west-central Accepted 29 May 2012 Sweden in order to increase our understanding of orogenic processes and, in particular, the tectonic evolution Available online 15 June 2012 of the Scandinavian Caledonides. Besides providing important sub-surface structural information in the vicinity of the potential drill sites, the Keywords: fi Scandinavian Caledonides seismic pro le also provides detailed, high resolution images previously not available for the uppermost few Collisional orogeny kilometers in the region. The subsurface is highly reflective and very complex down to at least 9 km depth Reflection seismic (the limit of decoded data) with clear reflections spanning the entire length of the profile. Borehole Correlation with previous regional reflection seismic and magnetotelluric surveys has been achieved by Migration acquisition of a short (7 km) connecting profile. A clearly defined reflection, present in the new profile at fi Continental Scienti c Drilling depths between c. 2.5 km in the east and c. 4.5 km in the west and with an average westwards dip of c. 3.5°, apparently defines the base of the Lower Allochthon. Closer to the Caledonian front, this sole thrust overlies the Cambrian alum shale formation, which rests unconformably on the autochthonous Precambrian crystalline basement. The latter is remarkable for its deep internal reflectivity which is probably related to mafic intrusions in a dominantly granitic host-rock; their deformation may be of both Caledonian and older (e.g. Sveconorwegian) age. The new high resolution seismic data provide the basis for locating the first borehole in the Seve Nappe Complex. They also demonstrate that the second hole, designed to penetrate the Caledonian basement, will have to be located further east than was originally planned. © 2012 Elsevier B.V. All rights reserved.

1. Introduction from the Laurentian continental margin, in the Scandinavian Caledonides, the rock units that were derived from the Baltoscandian The Caledonide orogen, making up the mountainous margins of platform and margin (Lower and Middle Allochthons) now occur the North Atlantic Ocean, is well exposed throughout most of north- overthrust by Iapetus oceanic terranes (Upper Allochthon), including eastern Greenland and western Scandinavia. The orogen is dominated ophiolites and island-arc assemblages and, uppermost, by fragments by thrust tectonics, with long-transported allochthons emplaced of the Laurentian continental margin. westwards onto the Laurentian platform of eastern Greenland and The nappes of the Middle Allochthon in Scandinavia (Fig. 1A), com- Scotland, and eastwards onto the Baltoscandian margin of Baltica prising metasedimentary and older crystalline rocks from the (Gee et al., 2008). Caledonian orogeny started during the Ordovician Baltoscandian margin, are characterized by inverted metamorphism. with closure of a late Vendian–Cambrian ocean (Iapetus), leading to The highest units were metamorphosed under amphibolite, granulite collision between the paleocontinents Laurentia and Baltica in the and eclogite (locally UHP) facies conditions and the underlying units Silurian and continued contractional deformation well into Devonian. experienced only greenschist facies metamorphism. The latter were Whereas in Greenland, the Caledonian thrust-sheets were all derived emplaced over a Lower Allochthon of Baltoscandian platform and foreland basin sedimentary successions that were separated by a basal décollement (sole thrust) from the underlying autochthon of Cambrian Abbreviations: BL, Byxtjärn-Liten profile, the main profile; CCT, Central Caledonian Transect; COSC, Collisional Orogeny in the Scandinavian Caledonides; ICDP, International black (alum) shales, resting unconformably on Precambrian crystalline Continental Scientific Drilling Program; KF, Kallsjön-Fröå profile, the connecting profile; rocks of the Baltic Shield. About three hundred million years of subse- PTt, Pressure–Temperature–time; SDDP, Swedish Deep Drilling Program; SGU, Swedish quent erosion, followed by Cenozoic uplift during the opening of the Geological Survey; SIST, Swept Impact Seismic Technique; VR, Vetenskapsrådet (Swedish North Atlantic Ocean, has exposed the orogen at mid-crustal levels. Research Council); FK, Frequency–wavenumber domain (in FK-filtering); FX, Frequency– Thus, today, a unique setting is preserved for the study of fossilized space domain (in FX Deconvolution). ⁎ Corresponding author. Tel.: +46 18 471 3322; fax: +46 18 501 110. õorogenic processes; features that are located far beneath the reach of E-mail address: [email protected] (P. Hedin). geologists working in active orogenic regions.

A SCANDINAVIAN CALEDONIDES TECTONIC MAP LEGEND Permain - Oslo rift

Devonian - Old Red Sandstones

Laurentian margin (Uppermost Allochthon) Outboard Iapetus-derived Terranes Köli Nappe Complex KIRUNA (Upper Allochthon) Outermost margin of Baltica Seve & related nappes (upper part of Middle Allochthon)

Outer margin of Baltica (Middle & Lower Allochthon) Precambrian of windows (Lower Allochthon) Sedimentary Autochthon cover MÖRSIL Precambrian basement TRONDHEIM ÖSTERSUND ÅRE

SILJAN RING

OSLO STOCKHOLM

100 km

B SKETCH SECTION FROM TRONDHEIM TO ÖSTERSUND W NORWAY SWEDEN E TRONDHEIM ÖSTERSUND

0 0

BALTICA BASEMENT HIGHER ALLOCHTHONS LOWER ALLOCHTHON, PARAUTOCHTHON and AUTOCHTHON Köli Nappes (Laurentian fauna in uppermost nappe) Baltoscandian Sedimentary cover Seve Nappe Complex (high grade) Baltica basement (allochthons & Baltica autochthonous) Särv Nappes (dyke swarms) outer 0100200 Mylonitic granites & psammites margin Horizontal scale in km

Fig. 1. (A) Tectonic map over the Scandinavian Caledonides. (B) Schematic section along the black line in A (vertical exaggeration×5). A, based on Gee et al. (2010).

A wide range of geophysical surveys have been conducted in the related to the thrust emplaced nappes and the Caledonian deformation, Scandinavian Caledonides over the past few decades. In the central and also the deeper crust down to the Moho. Petrophysical (Elming, part of the mountain belt (Dyrelius, 1985; Dyrelius et al., 1980), along 1980), magnetic (Dyrelius, 1980; Elming, 1980), gravimetric (Dyrelius, the Central Caledonian Transect (CCT), a sequence of deep seismic reflec- 1986; Elming, 1988), magnetotelluric (Korja et al., 2008) and deep seis- tion surveys were carried out during the late 1980s and early 1990s mic sounding surveys (Schmidt, 2000) in the same region have provided (Hurich et al., 1989; Juhojuntti et al., 2001; Palm et al., 1991). The CCT, a wealth of data that forms a basis for today's understanding of the spanning the 300 km width of the mountain range from the Caledonian deeper structure and composition of the mountain belt. front in western Jämtland (Sweden) to the Tröndelag coast of western The reflection seismic surveys, referred to above, correlate well with Norway, provided considerable information on the large scale structures the surface geology of the exposed Caledonian thrust-sheets and basal 32 P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41 décollement, but also show some remarkable images of the previously Western central Jämtland (Fig. 2A) comprises the most complete unknown internal structure of the underlying basement. Understand- profile across the Scandinavian Caledonides. From the thrust front ing the character of the latter is of fundamental importance for the near Svenstavik, with the Cambrian black shales, beneath a major de- interpretation of Caledonian orogeny, as is the need to better under- tachment, deposited directly on the underlying autochthonous crys- stand the inverted metamorphism in the Baltoscandian allochthons talline basement, to the Norwegian–Swedish border near Storlien, and the emplacement of the hot, subduction-generated nappes onto this 150 km long transect provides a section from the Baltoscandian the platform. Further investigation of these features can only be platform, through the entire Lower, and Middle Allochthon into the achieved by deep drilling and it was therefore decided to pursue these ophiolites and overlying Iapetus-derived successions of the Upper investigations in the context of the International Continental Scientific Allochthon. The character of the basal décollement is well established Drilling Program (ICDP). in the eastern front of the orogen, dipping 1–2° west beneath the In 2007, the Swedish Deep Drilling Program (SDDP) was initiated Lower Allochthon, thanks to comprehensive drilling programs (Gee to study fundamental problems of the dynamic Earth system, its nat- et al., 1978), which reached up to c. 30 km westwards from the ural history and evolution (Lorenz, 2010). The following year Sweden front. In the far west, beneath Storlien, the décollement is inferred on joined the International Continental Scientific Drilling Program the basis of the regional seismic profiling to be present at a depth of (ICDP). SDDP focuses on key questions that can only be investigated 5–6km(Palm et al., 1991), but the extent to which the underlying base- by drilling. All projects are of global importance and well defined in ment is imbricated and incorporated in the basal thrust sheets is not Scandinavia; they include phenomena such as post-glacial faulting known. (Juhlin et al., 2010; Kukkonen et al., 2010), ore genesis (Weihed, In this central Jämtland profile, the lithologies comprising the differ- 2010), impact processes (Högström et al., 2010) and mountain build- ent allochthons are particularly well developed. The Jämtlandian Nappes ing processes (Gee et al., 2010). The objectives of the last of these of the Lower Allochthon are dominated by Cambrian, Ordovician and (Collisional Orogeny in the Scandinavian Caledonides, COSC) have Silurian sedimentary successions, metamorphosed in sub-greenschist been referred to above. This project is designed not only to further to low greenschist facies. They record the passage westwards from the our understanding of the Caledonides, but also to test different inter- black shale and carbonate-dominated platform into deeper water, with pretations of collisional orogeny, as proposed and developed for some west-derived greywackes (turbidites) prominent in both the Ordovician of today's active orogenic regions, such as the Himalayas (e.g. Law et and Silurian. The absence of a sandy facies beneath the Cambrian black al., 2006) and the western Pacific, Izu–Bonin–Mariana arc system alum shales in the autochthon is in marked contrast to the presence of (Tamura et al., 2010). Vendian to Early Cambrian quartzites and shales in the Jämtlandian The aim of the COSC project is to drill from the high grade nappes with Nappes, increasing in thickness westwards. their inverted metamorphism, down through the underlying lower grade A prominent mylonite zone separates the folded Jämtlandian allochthons and basal décollement deep into the Precambrian basement. Nappes from the overlying Middle Allochthon. The latter usually in- This 5 kilometer thick section will be divided into two parts, the first cludes highly deformed Early to Middle Proterozoic granitic gneisses (COSC 1) is focused on the higher grade, upper nappes of the Middle (e.g. Tännäs Augen Gneiss Nappe, Vemån Nappe) at the base, overlain Allochthon and the second (COSC 2) is focused on the Lower Allocthon by psammitic formations, remarkable for their very high strains and and basement. These two, c. 2.5 km deep core-holes, will be located in planar foliation (e.g. Offerdal Nappe). Thrust over these lower the Åre-Mörsil area of western Jämtland. In addition to their Caledonian tectonic units are dolerite-intruded sandstones of the Särv Nappes, objectives, they will provide unique information about the deep where the internal structures in the sedimentary formations are hydrogeology and hydrochemistry of the mountain belt, the geothermal well preserved thanks to the presence of the intrusions. In places, gradient and potential for geothermal energy, the paleoclimate evolution the volume of the dolerites exceeds that of the sedimentary rocks. since the last glaciation, and the deep biosphere (Lorenz et al., 2011). High strain zones occur within the Särv Nappes and at their base Although a substantial amount of data are available from the Åre- and top, where the rocks are ductilely deformed to parallel-banded Mörsil area, no high resolution seismic data had been acquired in the lower amphibolite facies tectonites. In the upper contact towards immediate vicinity of the potential drill sites. High quality reflection the overlying, higher grade Seve Nappe Complex, thrusting is often seismic images are a prerequisite for a successful drilling project, es- conspicuous, but in some locations the relationships appear to be pecially one such as COSC where there is major emphasis on the transitional. Thus, in northern Jämtland, some of the quartzites and tectonostratigraphy. Therefore, new reflection seismic data were ac- psammitic gneisses with eclogitized dolerites have been considered quired in the summer of 2010 to allow the COSC boreholes to be op- a part of the Särv Nappes by Strömberg et al. (1994). timally located. These data are presented here, along with a new The Seve Nappe Complex is usually divisible into three main units, interpretation of the Caledonian structure in the area. We also pre- a lower part (the Lower Seve Nappes) dominated by quartzites and sent the optimum location for the westernmost borehole, COSC 1, other, often calcsilicate-bearing, psammites with amphibolitized dol- and discuss the potential location of the more easterly borehole, erites and gabbros, a central higher grade (granulite to eclogite facies) COSC 2. part where the metasedimentary rocks are pelitic and partial melting is conspicuous (e.g. in the Åreskutan Nappe), and an overlying, upper amphibolite facies assemblage of garnet mica schists and amphibo- 2. Geology lites. Ultramafic rocks occur in all units of the Seve Nappe Complex as occasional solitary, isolated bodies, or together with meta-gabbros. The Caledonian geology of central Sweden has been presented by The entire tectonostratigraphic assemblage of allochthons is folded Strömberg et al. (1994, 1:200,000 geological map) and described by by major N-trending antiforms and synforms. Thus, from west to east Karis and Strömberg (1998); with minor modification, it provides (Fig 2), the Skardöra Antiform, along the border between Norway and the basis of Fig. 2. The regional context of the Åre-Mörsil area within Sweden, is flanked by the Tännfors Synform, the Mullfjället Antiform, the central Scandes has been presented by Gee et al. (1985) and sum- the Åreskutan Synform, the Olden Antiform and the Offerdal Synform. marized recently in the ICDP context by Gee et al. (2010). Åreskutan All these late major structures apparently originated during the final Mountain was established by A. E. Törnebohm (1888) to be one of stages of Caledonian contractional deformation, and all show evidence the world's best places to study thrust tectonics, with the high of superimposed extension. The earlier reflection seismic profiling grade allochthons emplaced at least 100 km eastwards from along the transect (Juhojuntti et al., 2001; Palm et al., 1991)hasbeen Tröndelag into Jämtland. Transport distances are now recognized to particularly important in this context, providing clear evidence of imbri- be in the order of, at least, four times this amount. cations lifting the Olden Antiform and prominent flat-lying detachments P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41 33

Åre Storlien Järpen Caledonian Front Mörsil CCT (1987-92)

Östersund

Sweden Norway

calcareous phyllite Silurian formations, undifferentiated B phyllites, greywacke Ordovician formations, undifferentiated Kallsjön Upper gabbro, peridotite, serpentinite Allochthon Cambrian Alum shales Köli Nappe

Lower Lower Neoproterozoic sedimentary formations Byxtjärn CCT (1987-92) Allochthon Fröå migmatitic gneiss, pyroxene granulites Baltoscandian basement, undifferentiated Åre

marble, calc-silicate gneiss, calc-phyllite Cambrian Alum shales KF-Profile amphibolites, metadolerites, peridotite, serpentinite BL-Profile Baltoscandian basement, undifferentiated metapsammite, gneisses, mica schicsts, marbles Järpen Autochthon Seve Nappe Complex Seve Liten Undersåker Mörsil Särv Nappe: Neoproterozoic sedimentary formations CCT, 1987-92 Middle Allochthon

Offerdal Nappe: Acquisition Profiles, 2010 Profiles Neoproterozoic sedimentary formations Seismic CDP Lines, 2010

Fig. 2. (A) Regional geology along the Swedish part of the CCT, and (B) local geology in the survey area along the Byxtjärn-Liten (BL) and Kallsjön-Fröå (KF) profiles. Numbers refer to CDP. Based on Strömberg et al. (1994). beneath the Mullfjället Antiform. Of fundamental importance for under- 2.2. The Byxtjärn-Liten (BL) profile standing Caledonian tectonics has been the interpretation of relationships between the frontal décollement in the east and this evidence of The new main reflection seismic profile is located in the eastern limb basement shortening further west. Testing alternative interpretations of the Åre Synform and reaches from the eastern slope of Åreskutan, can only be done by drilling. near Lake Byxtjärn, southwards, via the old copper mine, Fröågruvan, to the village of Undersåker on the main road (E14) between Östersund and Trondheim; from there, it continues to Liten and then follows along 2.1. The Åre-Mörsil area the southern side of this lake to its southeastern end. The quartzites and subordinate calcsilicate-rich psammitic gneisses and marbles, with The structure of the Åre-Mörsil area is dominated by the Åre abundant amphibolitized dolerites and gabbros and some ultramafites, Synform, with the highest allochthon of the Seve Nappe Complex comprising the Lower Seve Nappe, outcropping in the western part of (Fig. 2B, Middle Allochthon) exposed in the hinge-zone as a klippe on the profile, are highly reflective — a characteristic feature that was rec- the top of the Åreskutan Mountain. Within the core of the flanking anti- ognized in previous seismic investigations across the Åre and Tännfors form to the west on Mullfjället, mid-Proterozoic acid volcanic rocks Synforms (Palm et al., 1991). This gently west-dipping unit overlies of the Baltica basement occur beneath a cover of quartzites, black strongly folded and intensely foliated turbidites of the underlying alum shales (Cambrian) and overlying turbidites, characteristic of the Lower Allochthon. Beneath this formation, prominent reflective units Jämtlandian Nappes of the Lower Allochthon. Formations of quartzites are expected to be present that do not crop out along the profile, an ex- and limestones (early Llandovery), a few meters to tens of meters pectation based on the previously acquired seismic data (Juhojuntti et thick, separate Ordovician from Silurian turbidites of the Jämtlandian al., 2001; Palm et al., 1991). foreland basin. Similar Ordovician and Silurian lithologies outcrop in the eastern limb of the Åre Synform, with the youngest units exposed 2.3. The Kallsjön-Fröå (KF) profile in the vicinity of lake Liten, near Järpen and Mörsil. Both to the north and south of Liten, towards the hinge of the Olden (to Oviken) Antiform, In addition to the newly acquired 36 km long main profile Cambrian alum shales and underlying quartzites are exposed beneath (Byxtjärn-Liten) to define the location of the two drill-holes, a supple- the Ordovician turbidite-dominated succession. mentary line (Kallsjön-Fröå) was also acquired to connect the main The Lower units of the Middle Allochthon are poorly represented one (Fig. 2B) with the CCT profile (Juhojuntti et al., 2001; Palm et al., in the Åre-Mörsil area. In the western limb of the Åre Synform, 1991). This, c. 7 kilometer long line from near Fröågruvan to lake lenticular slices of mylonitic felsic igneous rocks and overlying Särv Kallsjön, is oriented N–S, approximately in the strike-direction of the Nappe psammites and meta-dolerites have been reported to separate Seve bedrock. It is of particular importance for correlation of the struc- the Lower Allochthon from the Seve Nappes. In the eastern limb, only tural features in the Lower Allochthon, the underlying décollement the fine grained mylonitic felsites are present separating the Seve and the basement, from the CCT to the Byxtjärn-Liten profile. from underlying phyllitic turbidites. The axis of the Åre Synform has a slight northerly plunge in the area south of Indalsälven; directly 3. Geophysics up-plunge in the hinge of the synform, south of Ottsjön, the Särv Nappes are particularly well preserved over a vast area, reaching In the mid 1970s, the Swedish contribution to the International south into northern Härjedalen. Geodynamics project included several geophysical studies of the 34 P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41

Caledonian allochthons and upper crustal structures in the western 10 km in the vicinity of the second potential site (CDP 1860–2860 in Jämtland area. The first stages consisted of petrophysical sampling, Fig. 2B). Permission to activate the source along a longer section of the as well as magnetometric, gravimetric (Dyrelius, 1980; Elming, first western half of BL (several smaller parts between CDP 850–1580 1980) and refraction seismic surveys (Palm, 1984). The magnetomet- in Fig. 2B) could not be obtained so a reduced number of source points ric survey revealed a vast magnetic anomaly present below this part were activated along these parts. Furthermore, the rough terrain along a of the Caledonides that has been interpreted as due to magnetite few shorter sections, including the first 124 receiver points of BL, did not rich granites of the Transscandinavian Igneous Belt (TIB) with a thick- permit access with the source and these source points had to be ness of at least 10 km (Dyrelius, 1980). A wide negative Bouguer skipped. Therefore, the fold varies quite significantly between different anomaly was also seen to dominate this region and this is also consis- parts of the main profile. See Table 1 for a more complete overview of tent with the interpretation of granitic TIB rocks being present at the acquisition parameters. depth. The TIB rocks may extend as deep as 20 km based on the gravity data, but the anomaly could also be influenced by density varia- 5. Processing of reflection seismic data tions in the lower crust (Pascal et al., 2007). More local gravity anomalies correlate with the major antiforms and synforms, in agree- The focus of the processing has been to retrieve a high resolution ment with observations from petrophysical sampling. Furthermore, section of the uppermost few kilometers of the crust to allow for ac- widely spaced low-level aeromagnetic profiles in the western curate planning of the two drill holes. For this reason, only the first Jämtland area allowed estimation of the depth to the basement in 3 s of data were decoded, corresponding to a depth of about 9 km. the middle of the CCT to about 2 km, as the allochthons (e.g. Seve) Decoding was done based on the Swept Impact Seismic Technique are essentially non-magnetic (Dyrelius, 1980). (SIST) as described by Cosma and Enescu (2001) and Park et al. The deep seismic reflection survey (CCT) referred to earlier was (1996). In general, 300–400 hammers hits were stacked together at acquired in the years between 1987 and 1992 with the aim of map- each source point to generate the seismograms that were used as ping the entire crust down to Moho and to provide a clear image of input into a standard seismic processing package. the near surface crustal structures related to Caledonian deformation A smoothly curved crooked CDP line with a CDP spacing of 10 m (Juhojuntti et al., 2001; Palm et al., 1991). The seismic sections corre- was defined to follow as closely as possible the actual receiver posi- late very well with the tectonic units as mapped from the surface ge- tions along the profiles while at the same time minimizing the loss ology, as well as with the local gravity anomalies which trace the of data points due to the orientation of the CDP bins. The data were antiforms and synforms. Strong reflections project to the surface in then checked manually and bad shots (e.g. due to bad weather condi- the east where 1.2 Ga old dolerite intrusions are known from the sur- tions) and receivers (e.g. due to environmental noise) were eliminat- face geology. As previously mentioned, the TIB extends NNW beneath ed before being passed on in the processing flow. the Caledonian cover and deep drilling into the TIB in the Siljan Ring Processing of the data along BL followed a standard processing se- area further south linked the strong reflectivity seen in the reflection quence for the most part. The application of a time-variant band-pass seismic section there to a set of 1.0 Ga dolerite intrusions (Juhlin, filter, a median velocity filter, front mute and predictive Wiener 1990). The strong reflectivity seen in the western Jämtland area is, deconvolution, as well as compensating for spherical divergence thus, most likely related to a suite of dolerite intrusions, similar to and using both Automatic Gain Control (AGC) and trace balancing, those found in the TIB in the Siljan Ring area (Juhojuntti et al., served to reduce the noise and give clearer reflections. An example 2001), but subsequently subjected to Caledonian, or perhaps of the pre-stack processing is shown in Fig. 3, where shots 397 and Sveconorwegian, deformation. 858 are shown before and after processing. The crust is thought to extend down to depths of about 40–50 km A substantial amount of effort was spent on conducting a thor- (Juhojuntti et al., 2001), but no clearly defined Moho can be seen on ough velocity analysis in relation to both NMO and DMO corrections. the deep seismic CCT profile. The depth to Moho was confirmed by The DMO corrections provided a much better resolution for most of a deep seismic sounding profile acquired in 1995, partly coinciding the section, the only exception being inside the thick reflective pack- with this same traverse, but extending further east (Schmidt, 2000). age located in the uppermost western end of the profile (Fig. 4). DMO This was then followed by a passive seismic study (England and corrections were, nevertheless, applied to the entire dataset of the Ebbing, 2008) that indicated a crustal thickness on the order of main profile. Stacking velocities are generally increasing with depth 45 km below western Jämtland and magnetotelluric measurements (Fig. 5), however, higher velocities are more quickly reached beneath (Korja et al., 2008) that contributed with more constraints on the subsurface structures. Korja et al. (2008) found a resistive upper Table 1 layer overlying a highly conducting layer with a very well defined Acquisition parameters for the reflection seismic experiment. upper boundary. This boundary coincides with strong reflections in Profile BL (main) KF (connecting) the CCT reflection seismic section and is interpreted to consist of alum shales that separate the Caledonian allochthons from the Pre- Spread type Split spread Split spread – – cambrian basement and that outcrop at the Caledonian front. Number of channels 300 360 300 360 Near offset 0 m 0 m Maximum offset 6804 m 6023 m 4. Acquisition of reflection seismic data Geophone spacing 20 m 25 m Geophone type 28 Hz single 28 Hz single The main profile, BL, and the connecting profile, KF (Fig. 2), were Source spacing 20 m (10 m) 25 m Source type VIBSIST VIBSIST both acquired using a mechanical source, consisting of a rock breaking Hit interval for hammer 100–400 ms 100–400 ms hammer mounted on a construction vehicle (see Juhlin et al. (2010) Sweeps per source point 3–43–4 for details). More than 1800 source points were activated along the BL Nominal fold 150–180 150–180 profile and 255 source points along the KF profile. Recording was Recording instrument SERCEL 408 UL SERCEL 408 UL –– done with 300–360 active channels using 28 Hz geophones with the Field low cut Field high cut –– fi spread being rolled-along continuously along BL, while it was xed on Sample rate 1 ms 1 ms KF. Record length 26 s 26 s The nominal shot and receiver spacing for BL was 20 m. However, Profile length ~36 km ~7 km the shot spacing was decreased to 10 m for almost 2 km in the vicinity Source points 1807 255 Data acquired 30/7–13/8, 2010 28/7–29/7, 2010 of the first potential drill site (CDP 320–500 in Fig. 2B) and for about P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41 35

A B

C D

Fig. 3. Example of two shot gathers from the Byxtjärn-Liten profile before and after processing; source point 397 (A and B) is from the vicinity of the first potential drill site, and source point 858 (C and D) is from the eastern part of the profile. Arrows indicate important reflections (see Sections 6 and 7 for details). 1) Base of Seve; 2) top of shallow reflective package; 3 and 8) the Décollement; 4 and 5) internal reflection and base of shallow reflective package; 6 and 7) steep thrust boundary. the highly reflective western unit. Local lateral variations exist in the Processing of KF generally followed the same procedure as the model (e.g. below 1.5 s around CDP 900); the cause is not known and main profile and the resulting stack and migrated section are shown should be investigated further, but these could be due to several in Fig. 6. However, spectral equalization was used instead of Wiener factors, e.g. to less near-offset data in regions where we could not deconvolution and DMO was not applied as the highly reflective activate the source. Post-stack processing with coherency filtering unit, where DMO was unsuccessful on the main profile, spans this en- (FX-Deconvolution), an FK-filter (applied in the frequency–wavenumber tire section. A complete overview of the processing steps for both pro- domain) and a band pass filter were applied (as specified in Table 2)to files is given in Table 2. reduce the noise and artifacts that had arisen in the pre-stack processing and the stacking. Migration was essential as many reflections are gently dipping 6. Results and a post stack time migration scheme using a finite difference based algorithm was used, as it was deemed the best option of 6.1. General remarks on reflections those migration routines tested (others tested were Kirchhoff, Phase-shift and Stolt). Post-stack migration was chosen mainly be- The stacked sections reveal a highly reflective and very complex cause lateral velocity variations due to the generally sub horizontal subsurface, with generally west dipping features that span the entire tectonic units were not considered significant enough to warrant length of the profile. We interpret most of these reflections to be from pre-stack migration, which requires considerably more computation- nearly within-the-plane of the profile since the profile is oriented al time. Finally, for the purpose of geological interpretation, a depth nearly perpendicular to the strike of the orogen. Events from cross- conversion was applied using a velocity function based on the veloc- dipping out-of-the-plane reflectors are, therefore, expected to be ity analysis following DMO corrections, but smoothed to reduce the minor. This interpretation is supported by the work of Juhojuntti et effects of local lateral variations in our velocity model. The processed al. (2001) on the CCT profile who found that cross-dip corrections stacked section and the migrated and depth converted section are did not render any significant improvement to their stacked section. shown in Fig. 4. Furthermore, the near perpendicular connecting profile (KF) shows 36 P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41

Fig. 4. The Byxtjärn-Liten profile: (A) stacked section, (B) migrated and depth converted section. The solid line outlines the surface. mainly sub-horizontal features, supporting the claim that most of the In the next two sections we focus our attention on observations reflections are from nearly within-the-plane of the profile. along the main profile before correlating the new data with the previ- Some reflections that, at first glance, could be interpreted as mul- ously acquired reflection seismic data. tiples were quickly disregarded as such after simple analysis of travel times. Few, if any, artifacts resulting from the processing are seen in 6.2. Western highly reflective unit on the main profile the stacked section. Some parts in the migrated and depth converted section, however, show signs of minor under- and over-migration, In the uppermost section of the westernmost part of BL a highly most likely as a result of the velocity model used in the migration. reflective unit is observed westwards from about CDP 1100 (Fig. 4).

Velocity model

Fig. 5. Model of the best ﬁt stacking velocities for the BL-proﬁle after application of DMO. In general, velocities increase with depth, although higher velocities are reached more quickly with depth in the west. Contour indicates a velocity of 6000 m/s. P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41 37

Table 2 Processing parameters for the reﬂection seismic experiment.

Byxtjärn-Liten (BL) Kallsjön-Fröå (KF)

Decoding of VIBSIST data Decoding of VIBSIST data Trace edits (manual) Trace edits (manual) Refraction and residual static corrections Refraction and residual static corrections Front mute Front mute Airwave filter Airwave filter Spherical divergence compensation Spherical divergence compensation Wiener deconvolution (filter length of 200 ms; gap length of 17 ms) Spectral equalization (window of 20 Hz; BP-filter of 15–30–120–150 Hz) Median velocity filter Median velocity filter Band pass filter (0–1s,25–50–80–120 Hz; 1.25–3s,20–40–80–120 Hz) Band pass filter (0–1s,25–50–80–120 Hz; 1.25–3s,20–40–80–120 Hz) Trace balancing Trace balancing AGC (window of 200 ms) AGC (window of 200 ms) NMO correction NMO correction DMO correction – CMP stacking CMP stacking Coherency filtering (F-X Deconvolution) Coherency filtering (F-X Deconvolution) FK filter (outside 20 Hz: 0 m−1, 50 Hz: ±0.0175 m−1, 85 Hz: ±0.0175 m−1, FK filter (outside 22 Hz: 0 m−1,50Hz:±0.0155m−1,85Hz:±0.0155m−1, 105 Hz: 0 m−1) 97.5 Hz: 0 m−1) Band pass filter (0–1s,25–50–80–120 Hz; 1.25–3s,20–40–80–120 Hz) Band pass filter (0–1s,25–50–80–120 Hz; 1.25–3s,20–40–80–120 Hz) Trace balancing Trace balancing

Finite difference time migration (padded with 200 traces on each side, θmax: 60°) Finite difference time migration (padded with 200 traces on each side, θmax: 55°) Depth conversion (datum level: 750 m.a.s.) Depth conversion (datum level: 750 m.a.s.)

At the western end of the profile it extends to a depth of about 2.5 km. however, most reflections are generally west dipping, as previously This unit appears to absorb a significant amount of the seismic ener- mentioned, and no additional units of high reflectivity throughout are gy, consequently shadowing the crust underneath, and many of the present. One notable reflection, with a westerly dip of c. 3.5°, stretches reflections clearly seen to the east are much fainter below it. East of the entire length of the profile from a depth of 3.75 km below sea this package the signal quality is good and clear reflections are seen level in the west to about 2 km below sea level in the east. A second re- down to 3 s (c. 9 km), the limit of decoded data (Fig. 4). flection, steeply W-dipping at 25°, becomes visible at 6 km below sea level around CDP 1200 and is cutting through many of the sub- 6.3. Central and eastern part of the profile horizontal reflections in the central part of the section before leveling out and merging with the highly reflective region above 1.5 km further To the east of about CDP 1200, the upper 500 m of the crust are al- to the east. The bedrock to the west of this reflection appears to be most transparent (Fig. 4). Between 250 m and 1.5 km below sea level thrust slightly upwards along the reflection. Another notable, steeply in this region, very strong reflectivity with a complex structure is ob- east-dipping, feature is seen at about 3 km depth just to the east of served that is sub-horizontal with a slight eastwards dip, in contrast CDP 1600, where the two reflections just mentioned cross each other, to the section in general. From 1.5 km below sea level and downwards, however, this is most likely an artifact from the migration.

A B

Fig. 6. The Kallsjön-Fröå proﬁle: (A) stacked section, (B) migrated and depth converted section. The solid line outlines the surface. 38 P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41

6.4. Correlation between old and new data 7.1. Middle Allochthon

The KF profile runs entirely over the highly reflective western unit The boundary to the highly reflective western unit reaches the described above. We also attribute the rather weak reflectivity at surface at a point (CDP 1200) that correlates well with the gently- larger depths along this profile (Fig. 6) as due to absorption of energy dipping tectonic contact that separates the overlying high-grade in the upper reflective part. In spite of this, reflections along the main Seve Nappes from the underlying lower grade allochthons (Fig. 7B). profile can be correlated to the earlier CCT profile via the connecting The strong reflectivity in the Seve Nappes is probably primarily due profile (Fig. 7A). Several of the more prominent reflections can be to the presence of banded amphibolites, psammites and gneisses. A traced through all three sections. Note how the correlation and the thin sheet of felsic gneisses occurring at the base of the Seve, may sub-horizontal nature of the reflections along the connecting profile be a minor, lower component of the Middle Allochthon. The base of support our postulate that most of the reflections observed on the the high reflectivity marks the base of the Middle Allochthon. Korja BL profile are from within the plane of the profile. et al. (2008) also note a transition into a more resistive western unit at this boundary, consistent with this composition.

7. Discussion 7.2. Lower Allochthon

The discussion that follows here considers firstly the surface geo- The Lower Allochthon, comprising several thrust sheets and with a logical evidence along, and in the vicinity, of the profiles, relevant to stratigraphy dominated by thick Ordovician and Silurian greywackes their interpretation, and then treats the more speculative alternative (light green and blue in Fig. 8, low reflectivity), underlain by Cambrian hypotheses for the deeper reflections in the basement. black shales and Cambrian–Vendian quartzites, makes up the BL profile

A E N Z CCT (1987-92) BL-Profile

KF-Profile

Seve Nappes (MA) Ordovician turbidites (LA)

Fig. 7. (A) Correlation of reflections between CCT and the Byxtjärn-Liten profile through the connecting Kallsjön-Fröå profile. (B) Correlation of the western part of the Byxtjärn- Liten profile with surface geology. P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41 39

B COSC 1

Silurian Ordovician Cambrian Mid Proterozoic Cambrian cover Seve Nappes (MA) turbidites (LA) turbidites (LA) quartzites/shales (LA) acid volcanites (LA) Proterozoic basement (A)

Fig. 8. (A) The migrated and depth converted section, as in Fig. 5B, for comparison. (B) Our interpretation of the migrated and depth converted Byxtjärn-Liten proﬁle. The green Cambrian cover represents the basal décollement of the orogenic thrust containing the alum shales.

to the east of CDP 1200. Interpretation of the structure of the Lower profile has been interpreted as corresponding to the Caledonian Allochthon in the BL profile, beneath the uppermost transparent units sole-thrust, or the main décollement (Andersson et al., 1985), with of tightly folded Silurian and Ordovician formations, is mainly based a thin layer of alum shales separating the overlying Caledonian on field evidence from the hills about 5 km south of Undersåker (see nappes from the Precambrian basement. Fig. 2B). In this area, beneath the Ordovician greywackes, Cambrian Correlation of reflections between BL and CCT (Fig. 7A) indicates shales and Cambrian–Vendian quartzites outcrop (dotted blue in Fig. 8, that the reflection entering the western edge at c. 4.5 km on BL can high reflectivity), overlying Mesoproterozoic porphyritic rhyolites, simi- likely be attributed to the décollement. It runs east with an average lar to those in the core of the Mullfjället Antiform, to the west of Åre. western dip of c. 3.5° and is cut around CDP 1800 by a more steeply These quartzite–phyllite and acid vulcanite formations are inferred to ascending reflection. The nature of this crosscutting reflection is not comprise the highly reflective and underlying transparent units, respec- known; however, some thrusting has seemingly occurred along its tively, in the BL profile (Fig. 8), and the entire sequence has been inter- margins, indicating a late shortening of the crust. The décollement preted to overlie the basal décollement. then appears to continue east and reach the eastern edge of the profile at a depth of c. 2.5 km (Fig. 8). 7.3. Caledonian sole thrust (décollement) Further seismic profiling to the east and MT surveying along the profile, as well as the proposed drilling will help to better define which re- The regional seismic reflection profiles acquired in the years be- flections in the main profile can be attributed to the décollement, as tween 1987 and 1992 (CCT in Figs. 1 and 2) indicated a boundary well as the nature of other major reflections. that could be traced from the Caledonian front and through the entire length of the profile (Fig. 1B). Reflections from this boundary corre- spond to a depth of about 3 km in the Åre-Mörsil area and then dip 7.4. Internal basement reflections westwards at about 4° to the Swedish–Norwegian border, where they reach a depth of about 6 km (Palm et al., 1991). The composition of the basement in the study area is unknown, as As mentioned earlier, Korja et al. (2008) observed a clear transi- is the origin of the strong reflectivity at depth and the apparent defor- tion from a resistive upper crust to a conductive layer that correlates mation. Based on the lithologies in the Caledonian front autochthon well with this seismic reflection boundary in the coincident MT pro- to the east and the windows (e.g. Mullfjället) to the west, the domi- file. Based on these observations, this seismic reflection in the CCT nating composition is likely to be granitic or acid volcanic. 40 P. Hedin et al. / Tectonophysics 554-557 (2012) 30–41

As mentioned earlier, aeromagnetic data (Dyrelius, 1980) suggest (KF) demonstrates that most of the reflections are from within-the- that the Transscandinavian Igneous Belt (TIB) continues in under the plane of the profile. Thus, the migrated and depth converted section Caledonian cover, indicating that the basement below the basal of the BL profile gives a reliable representation of the sub-surface décollement is mainly composed of Rätan-type granites. Sandstones structure. In the west, the high grade Seve Nappe Complex is repre- are also a possibility (e.g. the Dala Sandstones that strike northwards sented by a highly reflective unit with an eastern boundary dipping beneath the Caledonian front in Härjedalen). However, this is less at about 20° to the west in the upper kilometer. This surface flattens likely, except as a shallow basin beneath westernmost Jämtland out westwards at about 2.0–2.5 km depth. Within this highly reflec- (Palm et al., 1991); further east, it would create a magnetic anomaly tive unit, it is planned to drill the first borehole, COSC 1. East of the different from that observed. Seve Nappe, lower grade metasediments of the Lower Allochthon The strong reflectivity in the basement is likely to be related to are exposed at the surface. The seismic response of these formations mafic intrusions (Palm et al., 1991) as these are seen to project up to- consists of more flat-lying reflections in the uppermost 2 km, but wards the surface closer to the Caledonian front, where dolerites have with an overall slight westerly dip. Definition of the sole thrust also been mapped in the autochthon (Juhojuntti et al., 2001). Dolerite beneath the Lower Allochthon at c. 4.5 km is based on correlation intrusions were also encountered at depth when drilling in the Siljan with the CCT profile and identification of the Caledonian front Ring area. These sills give rise to strong reflections in the seismic data décollement. The uncertainties in this correlation make it impossible from that area (Juhlin, 1990). The supposed dolerites beneath the to locate the second borehole, COSC 2, at the eastern end of the BL Åre-Mörsil area may well be related to the 1.0 Ga system in the Siljan profile, the main objective of the latter being to pass down through Ring impact structure (Fig. 1), which intrudes the TIB and strikes the sole thrust and autochthonous sediments and penetrate deep NNW in underneath the Caledonian cover; however, the somewhat into the basement reflectors. Therefore, a continuation of the seismic older Åsby dolerites (c. 1.2 Ga), outcropping directly to the east of reflection survey to the east is necessary before the COSC 2 borehole the CCT profile (Söderlund et al., 2006) may also be present. can be positioned. Shear zones with mylonites and, perhaps, intercalated sedimentary rocks are another possible explanation for some of the observed re- Acknowledgments flectivity. Some of the discontinuities and apparent deformation may be Caledonian, but those that influence the basement and do not dis- The COSC Project is a part of the Swedish Deep Drilling Program turb the basal Cambrian unconformity and Caledonian décollement (SDDP) and the seismic reflection component of the project was must be older (e.g. Sveconorwegian). Interpretation of the remark- funded by the Swedish Research Council (VR). P. Hedin is also partly able prominent reflectivity in the basement beneath the Cambrian funded by VR. GLOBE Claritas™ under license from the institute of unconformity is of considerable importance for understanding the Geological and Nuclear Sciences Limited, Lower Hutt, New Zealand Caledonian and, perhaps, Sveconorwegian tectonics. It can only be re- was used to process the seismic data. Seismic figures were prepared solved by deep drilling. with GMT from P. Wessel and W. H. F. Smith (Figs. 3, 4, 5, 6, 7B, 8) and OpendTect under academic license from dGB Earth Sciences BV 7.5. Location of the boreholes (Fig. 7A). Hans Palm (HasSeis), Sverker Olsson and Artem Kashubin (previously Uppsala University) were involved in the seismic acquisi- As previously mentioned, two deep drill holes are planned for the tion, as well as Aurore Carrier, Claire Labonne, Marion Nicolé and project, and the first of these (COSC 1) will be located in the western Philippe Solans (master students from the University of Strasbourg, part of the profile around CDP 400 (±100) (Fig. 8). This hole will start France) and Magnus Juhlin. We would also like to thank Niklas in the Lower Seve Nappe and is expected to reach the underlying Juhojuntti (SGU) for providing processed seismic data and coordi- Lower Allochthon. The overlying granulite facies Åreskutan Nappe is nates for the regional CCT profile and Henning Lorenz for providing well exposed on the top of the mountain and its PTt and micro- geological maps. Alireza Malehmir and the rest of the seismic group structural history is being documented in detail, from its initiation at Uppsala University are thanked for valuable discussions and advice in a Baltoscandian margin subduction system to its high temperature throughout this work. Finally we thank the reviewers for their helpful emplacement over the rocks of the Lower Seve Nappe. The amphibo- comments. lite (perhaps partly granulite) facies metamorphism of the Lower Seve Nappe is less well exposed and thought to be inverted. COSC 1 References seeks to document the PTt and structural history of this underlying nappe in detail and compare this evolution with that of the overlying Andersson, A., Dahlman, B., Gee, D.G., Snäll, S., 1985. The Scandinavian Alum Shales. Åreskutan Nappe. 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