Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep...

ELSEVIER Tectonophysics 270 (1997) 259-277

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Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep seismic data

T. Abramovitz *, A. Berthelsen, H. Thybo Geological Institute, Universi~ of Copenhagen, Oster Voldgade 10, DK-1350, Copenhagen K, Denmark

Received 22 November 1995; accepted 26 August 1996

Abstract

A hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal-incidence and wide-angle seismic data along the BABEL profile in the Baltic Sea. This Intermediate Terrane is situated between a Northern Terrane of Svecofennian age and a Southwestern Terrane of Gothian age. It is delimited upwards by two low-angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of ~ 300 km at Moho level. The ~ 1.86 Ga suture against the Northern Terrane is imaged by a prominent almost continuous NE-dipping crustal reflection from 3.5 to 14 s twt over 175 km. Where it downlaps on the Moho, sub-Moho velocities change from 8.2 to 7.8 km/s (___ 0.2) over less than 25 kin. A relatively strong, NE-dipping normal-incidence and wide-angle reflection at 19-23 s twt indicates that the suture extends into the upper mantle. The pervasive NE-dipping reflection fabric of the Intermediate Terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events. High Poisson's ratios suggest a mafic composition or high fluid content. The ~ 1.86 Ga collision was probably succeeded by continental break-up and removal of an unknown continent, except for the Intermediate Terrane. Subsequent formation of an east-dipping subduction zone further to the west led to the emplacement of 1.81-1.77-Ga-old granitoids in the southern part of the Transscandinavian Igneous Belt. The ~ 1.65-1.60 Ga suture against the Southwestern Terrane is defined by a semi-continuous band of strong SW-dipping reflections between 3 and 8 s twt over 65 km, which are interpreted as a low-angle thrust zone along which Gothian crust overrode the Intermediate Terrane. The identification of three individual seismic terranes in the southeastern part of the Baltic Shield provides new evidence for Palaeoproterozoic plate tectonic processes.

Keywords: BABEL deep seismic data; sutures; terranes; Proterozoic; Baltic Shield; crustal evolution; seismic velocity structure

1. Introduction

The concept of continental growth by cont inent - continent collision and terrane accretion is supported by an increasing number of geophysical and geological observations. Palaeomagnetic and palaeobiogeo-

* Corresponding author. E-mail: [email protected]

graphic data have proven to be of particular importance in the studies of Phanerozoic terranes. Deep seismic reflection and refraction data are important tools for recognition of individual terranes and their boundaries especially when dealing with Precam- brian shield areas.

The 2 .1-1.75 Ga assemblages of the Svecokare- lian and Svecofennian domains in the northern and

0040-1951/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PH S0040-195 1(96)002 13-2

260 T. Abramol'itz el al. / Tectonophysics 270 (1997) 259-277

north-central parts of the Baltic Shield have been interpreted in terms of both continent-continent collision and terrane accretion (Marker, 1991; Park, 1991). In this study we use deep seismic data to evaluate crustal structure and to distinguish seismic

terranes by their characteristic reflectivity and by lateral changes in their velocity structures.

Recent geological and geophysical studies con- ducted jointly by East- and West-European re- searchers provide a new and more coherent picture of the gross crustal architecture of the East-European Platform and the Baltic Shield (Gorbatschev and Bogdanova, 1993: Korja, 1995). This improved information provides a foundation for new plate tectonic interpretations of the sinuous Palaeoproterozoic crustal belts that extend from the Svecofennian domains of the central and southeastern Baltic Shield across the Baltic Sea into the concealed basement of the East-European Platform (Fig. 1).

Coincident normal-incidence reflection and wide- angle reflection and refraction data from the BABEL (Baltic and Bothnian Echoes from the Lithosphere) project are of particular importance, as they demonstrate evidence for Palaeoproterozoic plate tectonic processes in the Baltic Shield (BABEL Working Group, 1990, 1993a,b; Meissner et al., 1992; Gohl and Pedersen, 1995). Strong NE-dipping reflections are interpreted as ~ 1.86 Ga collisional structures at the arc-type Skellefte Field of Svecofennian age. These reflections extend through the crust and into the upper mantle, and are associated with strong Moho topography in BABEL profiles 2 and 4 in the northern Gulf of Bothnia. Magnetotelluric measure- ments in Sweden have identified a NE-dipping conductivity anomaly to a depth of 30 km in the crust, with a magnitude of 4000 Siemens (Rasmussen et al., 1987). Mapping similar conductivity anomalies, Korja et al. (1993) interpreted that this presumed collision structure extends to the onshore area east of the Gulf of Bothnia (Fig. 1).

These studies demonstrate that deep reflections and conductivity anomalies, which are associated with orogenic structures of Palaeoproterozoic age may survive over very long time spans after the crust has been cratonized, in spite of later reworking during extensional events and magmatic activity. Evidence for the preservation of old deformational structures in shield crust has been documented by the

Fig. 1. Tectonic basement map of the Baltic Shield and adjacent

platform areas (modified f rom Gorbatschev and Bogdanova , 1993).

Rapakivi massifs are omitted. Legend in upper r ight corner;

/ = m a x i m u m SW-extent of Archean rocks in the crust; 2 = crustal

conduct ivi ty anomalies: 3 = belts of > 1.8-Ga-old Palaeoprotero-

zoic rocks; 4 = major orogenic fronts; 5 = Sorgenf re i -Tornqu i s t

Zone: 6 = Transscandinavian Igneous Belt (TIB, ~ 1 .84-1 .65

Ga): 7 = Gothian crust in the B l e k i n g e - B o r n h o l m Block; 8 =

Sveconorwegian orogen; 9 = pla t form cover. Abbreviat ions: SF = south-central Svecofennian orogenic domains; SA = Skellefte

arc ( ~ 1.86 Ga).

GLIMPCE seismic reflection data from Lake Huron, Ontario, where collisional features from the 1.89- 1.83 Ga Penokean orogeny and the 1.3-1.0 Ga Grenvillian orogeny are preserved (Green et al., 1988).

Here, we present an interpretation of coincident reflection and refraction seismic data from the BA-

o

BEL profile between the Aland Islands and Born- holm which were described previously by the BA- BEL Working Group (1993b). The profile extends from the south-central and southern Svecofennian domains to the Gothian crust of the fault-bounded Blekinge-Bornholm Block. The interpretation pays attention to the onshore geology, tectonic evolution and basement trends of the Baltic Shield and the East-European Platform as well as the results from the neighbouring EUGENO-S 4, FENNOLORA and BALTIC SEA seismic refraction profiles (Fig. 2). The seismic interpretation and geological observations provide evidence for two oppositely dipping, low-angle sutures that divide the crust beneath the profile into three seismic terranes: a Northern Ter-

T. Abramovitz et al. / Tectonophysics 270 (1997) 259-277 261

rane, a concealed Intermediate Terrane and a South- western Terrane. The Jotnian t o Vendian sedimentary sequences and the Early Palaeozoic cover rocks are discussed by the BABEL Working Group (1993b) and the Late Cretaceous-Early Tertiary inversion structures of the Sorgenfrei-Tornquist Zone by Thybo et al. (1994).

2. Seismic evidence

2.1. Reflection seismic interpretation

The BABEL normal-incidence seismic reflection data used here were collected along a.614-km-long NE-SW-striking profile between the Aland Islands

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Transscandinavian Igneous Belt; BBB = Blekinge-Bornholm Block; Sm = Sm~land; STZ = Sorgenfrei-Tornquist Zone; C D F = subcrop of the Caledonian Deformation Front; S N F = Sveconorwegian Front; J - V = down-faulted Jotnian-Vendian sediments. Modified after BABEL Working Group (1993b).

262 T. Abramovitz et al. / Tectonophysics 270 (1997) 259-277

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264 71 Abramm it: el al. / Tectonophysics 270 (1997) 259-277

and Bornholm in the Sorgenfrei-Tornquist Zone (Figs. 2 and 3a). Stacked and migrated sections to 25 s twt (two-way traveltime) for the northeastern part and to 18 s twt for the southwestern part of the BABEL profile are available, c.f. linedrawing in Fig. 3a. The stacking and migration velocities were constrained by results of the wide-angle data modelling (BABEL Working Group, 1993b). The full process- ing sequence is described by the BABEL Working Group (1993a).

The three seismic terranes are identified on the basis of differences in characteristic reflectivity. They are separated by oppositely dipping sutures that are observed as gently dipping reflections. At near- surface levels, the Northern Terrane, which includes Svecofennian rocks and the undeformed, magmatic rocks of the Transscandinavian Igneous Belt, extends from km 190 to km 614 along the profile. The Southwestern Terrane comprises Gothian deformed rocks and extends from km 0 to km 190 (Fig. 2). The Intermediate Terrane is only present at depth below the two inferred sutures. We find no indication of abrupt Moho offsets at steeply inclined shear zones or faults along the profile.

2.2. The NE-dipping suture and reflectit'i~' of the Northern Terrane

The Northern Terrane is interpreted to overlie an almost continuous NE-dipping crustal reflection between 3.5 s twt at km 200 (a in Fig. 3a) and 14.4 s twt at km 350 (b in Fig. 3a), from where lower crustal reflectivity appears to terminate to the north. The dipping reflection a-b has an apparent dip of ~ 15 ° to the northeast. Generally, reflections are stronger and more dense below and southwest of this NE-dipping reflection than above and northeast of it. The NE-dipping reflection is locally interrupted between a and b. A prominent NE-dipping reflection between 18.8 and 23.4 s twt has been identified further north in the otherwise transparent upper mantle at distances km 480-510 (c in Fig. 3a, with enlargement in Fig. 3b).

The upper crust of the Northern Terrane is, in general, non-reflective down to an undulating surface at 3-5 s twt. The transparent nature of the upper crust between km 190 and 330 may be related to the occurrence of undeformed magmatic rocks belonging

to the Transscandinavian Igneous Belt, which are exposed onshore Sweden.

At middle and lower crustal levels in the Northern Terrane, the reflectivity is mainly dominated by a patchwork of reflective zones composed of NE-dipping to sub-horizontal reflections d and relatively transparent regions e in Fig. 3a. Increased middle to lower crustal reflectivity is observed beneath the northeastern part of the profile at km 450-614.

The reflection seismic Moho, defined as the termination of crustal reflectivity, shows lateral depth variations with an increase from 13.0-13.2 s twt beneath the northern part of the profile to ~ 15 s twt beneath the central part of the profile west of Got- land at km 465-485. The lower crust is unreflective between km 350 and 450 where the crust/mantle boundary cannot be determined from normal-incidence data alone.

2.3. ReflectiL, it 3, of the Intermediate Terrane and the SW-dipping suture

Below and southwest of the a-b reflection, the Intermediate Terrane is characterised by a highly reflective middle and lower crust. It is composed of closely spaced NE-dipping bands of reflections that are separated by NE-dipping narrow and elongated non-reflective zones ( f in Fig. 3a). To the southwest, this reflective crust is bounded by a band of SW-dipping reflections from 3-8 s twt, which extend over 65 km horizontal distance (g-h in Fig. 3a). We interpret these SW-dipping reflections as the image of a relict Gothian suture between the highly reflective crust of the Intermediate Terrane to the northeast and the transparent Gothian crust to the southwest.

The SW-dipping g-h and NE-dipping a-b reflection bands both project to the surface between km 180 and 190 in the BABEL profile. The Interme- diate Terrane thus has a triangular shape with a total width of about 300 km at Moho level.

The reflection Moho of the Intermediate Terrane changes its character along the profile. In the southwest, it is composed of several NE-dipping en eche- lon offsets at 11-12 s twt (i in Fig. 3a). In the central part it is characterised by a clear and sharp termination of the reflective lower crust, imaged by short sub-horizontal reflections at 11-12.5 s twt. In

T. Abramot, itz et al. / Tectonophysics 270 (1997) 259-277 265

the northeast, the sharp termination of crustal reflectivity deepens slightly from 13.5 to 14.6 s twt, indicating a northeastward deepening of Moho below the Northern Terrane at distances km 250-350.

2.4. Reflectivity o f the southwestern terrane

The upper crust of the Southwestern Terrane is generally transparent above the SW-dipping g - h reflection band. Exceptions are the sedimentary succession in the Hani5 Bay Basin, the inverted Sorgen- frei-Tornquist Zone and apparently listric NE-dipping reflections in the upper to middle crust.

In the Sorgenfrei-Tornquist Zone between km 0 and 40, the reflectivity of the pre-Triassic basement is influenced strongly by Late Palaeozoic normal faults and Late Cretaceous-Early Tertiary reverse faults (cf. Thybo et al., 1994). We consider the NE-dipping mid-crustal reflection ( j in Fig. 3a) as the downward listric continuation of the reverse Romele fault, which terminates in a detachment zone at the top of the reflective lower crust. The Romele fault forms the southwestern border of the inversion zone at the surface. The NE-dipping listric reflections in the upper to middle crust (k in Fig. 3a) between 3.3 and 6 s twt at km 50-125 are considered as Late Palaeozoic normal faults that were not reactivated or affected by the Late Cretaceous-Early Tertiary inversion tectonics.

Below the southern part of the Hanti Bay Basin and the northern half of the Sorgenfrei-Tornquist Zone, the lower crust appears reflective with subhor- izontal reflections between 7 and 10 s twt. The top of the lower crust shows an up to 1.5 s twt of uplift in the inversion zone at km 0-40.

2.5. Seismic velocity structure

The 2-D velocity model along the BABEL profile is based on ray tracing of P-wave traveltime observations from wide-angle reflection and refraction data recorded by nine onshore mobile seismographs (Figs. 2 and 4). The velocity model is 800 km long, extending further to the north than the 614 km long normal-incidence profile as recordings from a land station placed in the Finnish archipelago are included in this study. The distances along the velocity model

and the normal-incidence profile are identical between 0 and 614 km.

Sedimentary cover is absent or thin in the investi- gation area, except for down-faulted Jotnian-Vendian sediments at km 200-300 and km 435-485 (Fig. 4). Shot gathers of the normal-incidence reflection data have been used to constrain velocities of the sedimentary succession. Forward modelling using a ray tracing algorithm (Thybo and Luetgert, 1990) has been employed to match calculated with observed traveltimes. The discrepancy between observed and calculated traveltimes is generally less than + 150 ms, except at short intervals where up to 300 ms difference occurs.

The velocity structure shown in Fig. 4 is typical for a shield-type, three-layered crust with velocities of 6.0-6.2, 6.5-6.6 and 6.9-7.3 k m / s in the upper, middle and lower crust, respectively. Moho depth varies between 35 and 50 km (10.7-15.3 s twt) with the deepest Moho in the central part of the profile between the t~land and Gotland islands. The shal- lowest Moho is observed at both ends of the profile, being most pronounced below the Han6 Bay Basin.

Sub-Moho velocities are in general 8.2 k m / s in the southern part and 7.8 k m / s in the northern part of the profile. The pronounced lateral velocity change from 8.2 to 7.8 k m / s occurs between km 350 and 375. The velocity change is based on the interpretation and modelling of Pn-arrivals at stations 3, 4 and 5 on 01and and PM P critical reflections at stations 1, 2, 3, 4, 5 and 7 (Fig. 2). Tests have shown that the velocity change is confined to a zone less than 25 km wide in which the lower crust and Moho are non-reflective in the normal-incidence reflection section. The NE-dipping reflection b in Fig. 3a downlaps on the refraction seismic Moho in this region.

No marked velocity change is observed across the interpreted NE-dipping crustal suture. Instead, crustal velocity isolines are conformable with the intra- crustal, sub-horizontal first-order velocity discontinuities, indicative of similar petrophysical properties but not necessarily identical petrological compositions of the two crustal terranes. A slight crustal velocity perturbation is observed in the lower crust, where the vertical velocity gradient in the lower crust is higher in the Northern Terrane than in the Intermediate Terrane.

The transition between the Intermediate and the

266 T Abramovitz et al. / Tectonophysics 270 (1997) 259-277

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Fig. 4. The 800-kin-long, 2-D velocity model along the BABEL profile. (a) Traveltime picks from the wide-angle sections. Small black triangles indicate the position and number of the land stations. (b) P-wave velocity model in km/s . (c) Ray coverage of the velocity model. Lines above/below discontinuities refer to reflections/refraction's respectively, black lines: reverse coverage; grey lines: one-way coverage by multiple stations; light grey lines: single station coverage. (d) The interpreted distribution of geological terranes superimposed on the velocity model.


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268 T. Abramotitz et al. / Tectonophysics 270 (1997) 259-277

Southwestern Terranes coincides with a minor lateral change of mid-crustal velocities from 6.3-6.5 k m / s to 6.5-6.6 km/s .

Fig. 5 illustrates the two-way normal-incidence traveltimes to the first-order discontinuities as calculated from the velocity model superimposed on the reflection seismic section. The intra-crustal, first- order velocity discontinuities show good agreement with the normal-incidence data and separate different reflectivity patterns within the upper, middle and lower crust. Moho depths as determined from the wide-angle data coincide with the base of the reflective crust.

2.6. Documentation of the velocit3, model

The two-dimensional velocity model is based on wide-angle refraction and reflection phases from all crustal levels along the profile. Diving waves from the upper crust, the Pg phase, with apparent velocities of 5.9-6.3 k m / s are observed to offsets of 140-150 km as first arrivals depending on the thickness of the upper crust. Other refracted crustal phases are a P~ phase from the middle crust with apparent velocities of 6.4-6.8 kin/s , and a P2 phase from the lowermost crust with apparent velocities of 6.8-7.3 km/s . Diving waves from the upper mantle, the P, phase, with apparent velocities of 7.75-8.30 km/s . are observed as first arrivals at offsets of 155-310 kin. Intra-crustal wide-angle reflections are also observed. Moho reflections, the PM P phase, are clearly observed with critical offsets between 105 and 170 km depending on the crustal thickness. The PMP phase is observed and modelled to offsets of 280 km.

Modelled traveltimes for the record section from land station 1 at Uusikaupunki, north of the profile in the Finnish archipelago, are illustrated in Fig. 6. Differences between the calculated and observed traveltimes for the crustal phases varies between +80 and - 1 3 0 ms and is better than - 1 0 0 ms for the upper mantle P, phase; a negative difference indicates that the calculated traveltime is earlier than the observed. A relatively strong crustal phase with an apparent velocity of 7.0 k m / s is observed at offsets of 260-325 km. The phase is modelled as a refraction from the middle crust, P~ phase, and as a wide-angle reflection from the top of the lower crust,

P2P phase, at large offsets. The Rum phase at offsets of 345-360 km is interpreted as a wide-angle reflection from a NE-dipping mantle velocity discontinuity at 70-80 km depth; for which discrepancies between observed and modelled traveltimes are less than ± 300 ms. The wide-angle reflector coincides with the source of the normal-incidence reflection c in Fig. 3. The apparent dip of this reflector is ~ 7 ° at km 350-450 and increases to ~ 19 ° at km 450-500. The interpreted crustal suture (a -b reflections in Fig. 3a) intersects the Moho in the interval where the lateral velocity change is observed in the uppermost mantle (Fig. 4).

Modelled traveltimes for the record section from land station 3 on the northern part of Oland match the observed traveltimes of the crustal phases better than +_ 150 ms (Fig. 7). Calculated traveltimes of refractions from the top of the 7.8 k m / s upper mantle wedge, which lies above a NE-dipping velocity discontinuity, fits Pn-observations to within + 70 ms near the critical offset X~t = 170 km. At larger offsets of 270-290 km, however, the calculated traveltimes arrive up to 240 ms later than the observed first breaks (Pn2 in Figs. 7 and 8). The slight curva- ture on the observed P, phase indicates a gradual change in the apparent velocity. Calculated traveltimes for down-dip turning waves from the 8.2 k m / s layer show a difference of less than + 130 ms with Pn-observations at offsets of 270-290 km (Pn~ in Figs. 7 and 8). This indicates that the low velocity upper mantle only extends from the Moho to the NE-dipping mantle discontinuity.

3. Tectonic interpretation

3. I. Crustal age of the seismic terranes

Extrapolation of the orogenic trends from nearby onshore Sweden into the BABEL profile, suggests that the bulk of the crust of the Northern Terrane comprises 1.91-1.87 Ga Svecofennian deformed and metamorphosed plutonic and supracrustal rocks and 1.85-1.75 Ga migmatitic to discordant granite bod- ies (Fig. 1 and 2; St~tlhSs, 1984; Gaal and Gor- batschev, 1987: Patchett et al., 1987; Welin, 1992; Gorbatschev and Bogdanova, 1993; Bergman et al.,

7". Abramovitz et al. / Tectonophysics 270 (1997) 259-277 269

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270 72 Abramot'itz et aL / Tectonophysics 270 (1997) 259-277

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T. Abramovitz et al./ Tectonophysics 270 (1997) 259-277 271

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Fig. 8. Magnification of the ray-tracing results for the diving waves from both the top of the 7.8 km/s wedge (Pn2) and from the top of the 8.2 km/s layer below the NE-dipping mantle discontinuity (P,t) at 220-305 km offsets.

1995; Romer and Ohlander, 1995). In the lower crust there may occur underthrust remnants of the magmatic arcs that supplied 2.1-1.91 Ga detrital zircons to the Svecofennian sedimentary succession (Claes- son et al., 1990, 1993).

The upper crust of the southern part (km 190-328) of the Northern Terrane is presumably an offshore extension of 1.81-1.77 Ga undeformed porphyries and granitoids of the Transscandinavian Igneous Belt, which are exposed in nearby southern Sm~land (Fig. 1 and 2; Mansfeld, 1996). The Intermediate Terrane is possibly also of Palaeoproterozoic age (pre-l.9 Ga?), but cannot be further constrained because no exposed equivalents of this terrane are known in onshore Sweden. The folded basement rocks of the Southwestern Terrane are of Gothian age (i.e. 1.76- 1.60 Ga; EUGENO-S Working Group, 1988, p. 310; Johansson and Larsen, 1989). We consider this terrane to be a relict of a formerly much more extensive Gothian orogen (cf. Fig. 4 in Gorbatschev and Bog- danova, 1993). It probably escaped subsequent Sve- conorwegian reworking because of its protected position in the fault-bounded Blekinge-Bornholm Block to the east of the Sveconorwegian front (Fig. 2).

3.2. The NE-dipping suture and the Northern Ter-

rane

The seismic interpretation of a NE-dipping suture is based on the correlation between crustal reflections (a and b in Fig. 3a) and the extrapolation across the Moho to the prominent NE-dipping reflection between 18.8 and 23.4 s twt (c in Fig. 3a). With this extension, the NE-dipping suture is a 300-km- long lithospheric feature. Additionally, a pronounced lateral change in sub-Moho velocity takes place in a narrow zone below the non-reflective Moho where the crustal reflections projects into the mantle. The NE-dipping mantle reflector is further constrained by a wide-angle reflection from 70-80 km depth recorded at large offsets by the northernmost land station in the Finnish archipelago (the Rum phase in Fig. 6).

The proposed NE-dipping suture is associated with a broad, gentle Moho depression between km 250 and 500 (Fig. 4). It is deepest (50 km) where the suture dips into the mantle (km 350-450). As already pointed out by the BABEL Working Group (1993b), there are no indications of any major vertical offsets of the Moho below the BABEL profile. In an integrated P- and S-wave model for the nearby FENNOLORA profile, a similar smooth Moho relief occurs with Moho depths gradually increasing to ~ 47 km between shotpoints B and C in Fig. 2 (Stangl, 1990). Previous P-wave models from this part of the FENNOLORA profile suggested a large and abrupt offset of Moho approximately midway between shotpoints B and C (Clowes et al., 1987; Guggisberg and Berthelsen, 1987; Guggisberg et al., 1991). Based on the new evidence, we question the validity of this model. There is no indication of such a Moho-offset in BABEL wide-angle reflection data, that were recorded in a fan-type geometry by land stations north and south of the postulated Moho offset with reflection points practically coinciding

Fig. 7. Ray-tracing results for P-wave observations from station 3 in the northern part of ()land (i.e. in the central part of the velocity model). Above: the calculated traveltimes are superimposed on the wide-angle section (8 km/s reduction velocity). The distances km 35-300 refers to the recording distance from the station. Below: ray paths through the velocity model. The distances km 300-800 refers to the distances in the northern part of the model.

272 T. Abramol'itz et al. / Tectonophysics 270 (1997) 259-277

with the FENNOLORA profile (BABEL Working Group, 1993b). Furthermore, Henkel et al. (1990) introduced a smooth density crust-mantle interlace in order to model the observed gravity data. In conclusion, the hypothesis of Guggisberg and Bertheisen (1987) that a crust-mantle fault at the

o

so-called Aseda shear zone has displaced the Moho between FENNOLORA shotpoints B and C should be abandoned.

The close association of a broad Moho depression and the deep-reaching suture in the BABEL profile supports a compressional origin of both features. If the NE-dipping reflection were to image a normal- sense detachment, a Moho rise would be expected instead of a Moho depression. Based on geologically inferred trends in the concealed basement under the Baltic Sea (cf. Fig. 2 in Gorbatschev and Bog- danova, 1993), we correlate this Moho depression in the BABEL profile with another Moho depression modelled in the BALTIC SEA profile by Ostrovsky et al. (1994). With the evidence now available from the BABEL and FENNOLORA profiles, we prefer to interpret the Moho depression observed on all three profiles as representing the same NW-SE-striking feature as illustrated by thin dotted lines in Fig. 2. Such a Moho-depression is considered to be the result of crustal thickening from lateral compression and suturing at the southwestern boundary of the Svecofennian orogenic belt. This is in contrast to the interpretation of the Moho-depression as the result of several deep, nearly vertical faults at Moho level (Ostrovsky et al., 1994).

A genetic relationship between the NW-SE-striking Moho-depression and the NE-dipping suture would preclude the possibility that the suture projects to the surface and outcrops in the E-W-striking shear zone that outlines the northern boundary of the Blekinge-Bornholm Block. This shear zone is discussed below in further detail in connection with the Gothian and later evolution.

The upper frontal part of the suture may extend northwestward from km 180-190 in the BABEL profile into southern Sm~land. No records exist for an outcropping suture across the southern part of the Transscandinavian Igneous Belt. Suturing at the southwest boundary of the Svecofennian crustal domains must therefore have taken place before emplacement of the 1.81-1.77 Ga granitoids and por-

phyries of Sm~land and emplacement of the 1.84- 1.65 Ga northern parts of the Transscandinavian Igneous Belt (Mansfeld, 1991). We propose that magmatic rocks of the Transscandinavian Igneous Belt occupy the transparent upper crustal levels at km 190-330. The seismic transparency of the upper crust in the offshore extension of the Transscandina- vian Igneous Belt may be caused by weak internal reflectivity of the granitoids due to very small velocity and density contrasts between rocks of felsic and intermediate composition. As such, the onset of crustal reflectivity at 3-5 s twt may mark the lower limit of granitoids along the BABEL profile.

Suturing was probably preceded by northeastward subduction of oceanic crust. The mantle-derived 1.91-1.869 Ga plutonics and volcanics in the Sve- cofennian realm are most likely associated with this subduction. Suturing is therefore inferred to have occurred around 1.86 Ga (Fig. 10a), although crustal shortening may have persisted until 1.84 Ga. The Svecofennian fold patterns underwent regional-scale rotation due to the development of WNW- to ENE- trending ductile shear zones with associated retrogressive metamorphism between 1.84 and 1.53 Ga (Stephens and Wahlgren, 1993). If of listric geometry, they may be correlated with the patches of NE-dipping crustal reflections observed in the BA- BEL profile (d and e in Fig. 3a).

Collision of the Intermediate Terrane with the Svecofennian orogen at ~ 1.86 Ga and ceasing of shortening ~ 1.84 Ga, leaves time for continental breakup and establishment of a new plate configura- tion with an E-dipping subduction zone off the western border of the proto-shield, and for ensuing emplacement of the 1.81-1.77 Ga magmatic rocks in the Transscandinavian Igneous Belt (Sundblad et al., 1995: Mansfeld, 1996).

An inferred age of 1.86 Ga for the NE-dipping crust and mantle suture is in agreement with the scenario outlined by Romer and Ohlander (1995) for termination of subduction and continent-continent collision at the southwestern boundary of the Sve- cofennian thickening. migmatitic texis in an crust. This subduction

domains with post-collisional crustal They explain the formation of a 1.846 Ga granite in south-central Sweden by ana- already thickened and thermally matured brackets the termination of Svecofennian to an interval between 1.869 Ga and

T. Abramovitz et aL / Tectonophysics 270 (1997) 259-277 273

1.846 Ga. They stress that large-scale crustal thickening requires that a continent, not just a terrane, collided with the evolving Svecofennian orogenic belt. In agreement with this latter view, we propose that the moderately sized Intermediate Terrane is probably just a remnant of a larger continent, of which the greater part was removed during later continental break-up before the emplacement of the Transscandinavian Igneous Belt.

3.3. The concealed Intermediate Terrane

The Intermediate Terrane is today nowhere exposed onshore in the southeastern Baltic Shield. It might subcrop beneath the Baltic Sea to the southeast of the BABEL profile, but its presence has so far only been inferred from its geophysical expression.

The terrane is characterised seismically by a strongly reflective triangular crustal compartment with different styles of reflectivity at middle and lower crustal levels compared to the surrounding terranes. Its reflectivity is mainly dominated by pervasive NE-dipping reflections that encompass elongated and narrow transparent zones. This reflectivity pattern and the similar but more widely spaced NE- dipping refiectivity in the Svecofennian crust may be primarily conditioned by the development of anastomosing shear zones with alternating high- and low- strain domains. The dipping reflections likely repre- sent highly deformed and sheared areas, whereas the transparent zones are the probable expression of undeformed or less deformed areas. The zones of high normal-incidence reflectivity have not been re- solved by the wide-angle reflection data.

The relict anastomosing shear zones may have originated from ductile shearing and plastic deformation of middle and lower crustal rocks in both compressional and extensional regimes during Protero- zoic times. Post-l.84 Ga tectonic events probably reactivated existing shear zones, leading to enhance- ment of reflectivity by retrogressive mineral reactions and/or input of fluids. The fluids may have originated from metamorphic dehydration reactions in the lower crust or upper mantle and may have migrated along old shear planes (e.g. Fyfe and Ker- rich, 1985; Carter et al., 1990 and Pavlenkova, 1992). As such, the shear zones may have been formed by: (a) the collision of the Intermediate Terrane with the

evolving Svecofennian orogen ~ 1.86 Ga; (b) the Gothian collision (1.65-1.6 Ga), or (c) reworking in connection with Jotnian extensional and rotational tectonics ( ~ 1.4-1.2 Ga).

Complementary support for the existence of a concealed terrane comes from several studies of shear wave velocities and Poisson's ratios along the BABEL profile (Barth and Klemperer, 1996), the FENNOLORA profile (Stangl, 1990), and EUGENO-S profile 4 (Thybo et al., 1993). Barth and Klemperer (1996) found a region with increased Poisson's ratios (>0.28) below the Blekinge- Bornholm Block and further northward in the lower crust beneath the Transscandinavian Igneous Belt and the southernmost Svecofennian crustal domain. This area of increased Poisson's ratio coincides with the middle and lower crustal parts of the concealed Intermediate Terrane. Stangl (1990) found a region of similarly elevated Poisson's ratio in the middle and lower crust between shotpoints B and C on the nearby FENNOLORA profile further to the west. An interpretation of the EUGENO-S profile 4 by Thybo et al. (1993) also shows an area of elevated Poisson's ratio in the middle and lower crust as well as strong lower crustal wide-angle reflectivity that extends northwestward from the BABEL profile to below the W-dipping Sveconorwegian front (Fig. 2).

The high Poisson's ratio could either be explained by the prevalence of mafic rocks, such as amphibo- lite, mafic granulite or gabbro (Barth and Klemperer, 1996), or by an increased content of fluids trapped in the Intermediate Terrane beneath the two oppositely dipping sutures. Neither of these alternatives ex- cludes the other, and both concur with the increased reflectivity of the Intermediate Terrane. Results from a preliminary magnetotelluric survey in the Blekinge region and Smfiland show increasing conductivity with depth, but do not indicate a major low-resistive layering within the crust (Rasmussen, 1987).

If enhanced reflectivity and high Poisson's ratio are indicative of the postulated Intermediate Terrane, it has a considerable lateral extent both to the west and northwest at middle and lower crustal levels, far beyond its image in the BABEL profile.

3.4. The Gothian and later evolution

The seismic interpretation of a SW-dipping suture between the concealed Intermediate Terrane and the

274 T Abramot' i t z et al, / Tectonophysics 270 (1997) 2 5 9 - 2 7 7

Southwestern Terrane is based on differences in reflectivity across a band of SW-dipping reflections (g-h in Fig. 3a) and on lateral variations in middle and lower crustal velocities. The vertical velocity gradient in the middle crust is higher in the South- western Terrane than in the Intermediate Terrane, whereas the opposite applies for the lower crust. Compared to the reflective Intermediate Terrane, the overriding Gothian crust of the Southwestern Ter- rane is relatively transparent.

We interpret the low-angle contact between these two terranes as a Gothian collisional suture that was formed 1.65-1.6 Ga. At this suture, Gothian deformed and metamorphosed plutonic and supracrustal rocks were thrust onto the proto-shield, overrode the Intermediate Terrane and were brought into tectonic contact with the undeformed magmatic rocks of the Transscandinavian Igneous Belt.

The apparent dip of the Gothian suture is south- westerly in the BABEL profile, but according to field relations in Blekinge its true dip may be more westerly. The Gothian fold structures in Blekinge are E-vergent and show moderate NNW- to N-plunges, suggestive of an eastward tectonic transport of the Gothian crust during deformation (Larsson, 1954: SGU, 1982, 1986). With a westerly true dip, the Gothian suture can be expected to subcrop with an approximate N-S trend below the Baltic Sea, east of the BABEL profile and Bornholm.

The deformed Gothian rocks in Blekinge are separated from the Sm~land part of the Transscandina- vian Igneous Belt by an E - W trending shear zone, which is locally stitched by a 1.35-1.4 Ga granite and cut by NNE-trending faults that probably belong to the ~ 0.8 Ga V~ittern fault system (Fig. 2). The shear zone is steeply dipping in outcrops but its attitude at depth is unknown. If it continues offshore with the same strike, it would intersect the BABEL normal-incidence reflection profile at about km 198, in the region where the NE- and SW-dipping sutures approach each other.

With the presently available information it cannot be uniquely determined how these two structurally distinct crustal units north and south of the E -W- striking shear zone became juxtaposed. One possibility is that post-Gothian ( ~ 1.6-1.4 Ga) sinistral movements of ~ 100 km along the E-W-striking shear zone displaced the Gothian folded rocks of the

Fig. 9. Alternative tectonic interpretations of the E-W striking shear zone between the Gothian rocks (transparent) of the Blekinge-Bornholm Block and the Sm~land part of the Transs- candinavian Igneous Belt (crosses). (a) ~ 100 km sinistral strike- slip displacement on a steep post-Gothian shear zone ~ 1.6-1,4 Ga: (b) overthrusting of the Gothian terrane, where a vertical ramp (VR) juxtaposes the most easterly advanced Gothian rocks and the Sm~land granitoids.

Blekinge-Bornholm Block eastwards (Fig. 9a). Al- ternatively, the steep E-W-striking shear zone may have acted as a "vertical ramp" that at depth con- nects with the otherwise W-dipping Gothian collision suture as a sole thrust (Fig. 9b). If the strike-slip hypothesis is correct, the deeper hidden part of the shear zone would be expected to affect the Interme- diate Terrane and would cause a structural "cut out" in the BABEL profile. If the vertical ramp hypothesis is correct, the shear zone would join up with the dipping refections from the Gotbian suture at the base of the transparent crust and would not affect the Intermediate Terrane.

The BABEL profile intersects the projections of two dyke swarms of different trend and age below the Han6 Bay Basin and the Sorgenfrei-Tornquist Zone: the N- to NNE-trending, 0.93 Ga Blekinge dykes (Johansson and Johansson, 1990; Solyom et al., 1992) and the NW-trending Carboniferous-Per- mian dykes (Klingspor, 1976; Bergstr6m, 1982). The intrusion of the 150 km wide Blekinge dyke swarm, which crystallized at near-surface conditions (Rodhe, 1988), was probably accompanied by coeval under- plating that may have contributed to the high P-wave velocities (7.1-7.2 k m / s ) of the lower Gothian crust. Although the relative uplift of the reflective lower crust between km 15 and 60 occurs slightly to the northeast of the Late Cretaceous-Early Tertiary inverted parts of the Sorgenfrei-Tornquist Zone (km 0-40), there is a close match between the uplift and the projection of the Carboniferous-Permian dyke swarm (Tbybo et al., 1994).


4. Concluding remarks

Combined studies of normal-incidence and wide- angle seismic data along the BABEL profile in the Baltic Sea provide evidence for a NE-dipping continent-continent collision suture at the southwestern boundary of the Svecofennian orogen (Fig. 10a). This low-angle suture is inferred to have developed ~ 1.86 Ga due to collision between the evolving Svecofennian orogenic belt and an unknown continent arriving from the southwest. Except for the presently concealed Intermediate Terrane, most of this continent was probably removed during continental break-up of the southwestern and western parts of the proto-shield ~ 1.85 Ga. Subsequent eastward subduction caused magmatic emplacement of the N-S trending Transscandinavian Igneous Belt that now obscures any onshore continuation of the 1.86 Ga suture (Fig. 10b). However, the suture may continue to the southeast in the basement under the Baltic Sea.

The high Poisson's ratio and high reflectivity of the relict and concealed Intermediate Terrane suggest that either mafic compositions predominate over felsic and/or there is a high proportion of fluids.

The Intermediate Terrane is overridden by a presumed allochthonous Gothian terrane that is preserved in the fault-bounded Blekinge-Bornholm Block in the southwest part of the BABEL profile (Fig. 10c). The more westerly parts of the Gothian orogen were intensively reworked during the 1.1-0.9 Ga ensialic Sweconorwegian orogeny (Fig. 10d).

This study emphasises that Palaeoproterozoic crustal and lithospheric structures of collisional ori-

Fig. 10. Plate tectonic cartoon illustrating the proposed Protero- zoic evolution of the southeastern Baltic Shield. (a) ~ 1.86 Ga: continent-continent collision between an unknown continent (UC) and the evolving Svecofennian orogenic belt (SF). This stage was succeeded by continental break-up in the western part of the shield. (NS: Northern suture, B-B: BABEL profile). (b) ~ 1.8 Ga: eastward subduction causes the formation of the Transscandi- navian Igneous Belt (TIB) and obliterates the northwestern part of the Northern suture. (c) ~ 1.6 Ga: collision and overthrusting of the Gothian terrane (G). The E-W-striking shear zone is interpreted as in Fig. 9b. (SS: Southwestern suture). (d) ~ 0.9 Ga: the ensialic Sveconorwegian orogeny (SN) reworks the western parts of the Gothian orogenic belt. Relict Gothian structures are preserved in the Blekinge-Bornholm Block.

gin may survive over long time spans. On the basis of high precision age determinations from the Baltic Shield, the Palaeoproterozoic plate tectonic processes seem to have been as complex and rapid as present- day processes. SengiSr et al. (1993) in their study of the Palaeozoic amalgamation of the Altaid tectonic collage rightly stressed that "magmatic fronts defin- ing the oceanward limit of arc magmatism are useful structural markers for unravelling Precambrian orogenic collages".

B

#f+

276 T. Abramm it: et al. / Tectonophysics 270 (1997) 259-277

Acknowledgements

The authors are grateful to J.R. Hopper (Danish Lithosphere Centre), F. Surlyk (University of Copen- hagen) and S. Zhou (University of Copenhagen) for comments on earlier versions of the manuscript, and to A. Green (Ziirich) and an anonymous referee for their constructive reviews. This work received finan- cial support from the Danish National Science Re- search Council.

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Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep...

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