«September 1992 ^ _ O^ 2, THE PROTOGINE ZONE. GEOLOGY AND MOBILITY DURING THE LAST 1.5 Ga Per-Gunnar Andréasson, Agnes Rodhe September 1992 This ...»
The Protogine Zone.
Geology and mobility during the
last 1.5 Ga
Per-Gunnar Andréasson, Agnes Rodhe
^ _ O^ 2
THE PROTOGINE ZONE.
GEOLOGY AND MOBILITY DURING THE LAST 1.5 Ga
Per-Gunnar Andréasson, Agnes Rodhe
This report concerns a study which was conducted
for SKB. The conclusions and viewpoints presented
in the report are those of the author(s) and do not necessarily coincide with those of the client.
Information on SKB technical reports from 1977-1978-(TR 121), 1979 (TR 79-28), 1980 (TR 80-26), 1981 (TR 81-17), 1982 (TR 82-28), 1983 (TR 83-77), 1984 (TR 85-01) 1985 (TR 85-20), 1986 (TR 86-31), 1987 (TR 87-33) 1988 (TR 88-32), 1989 (TR 89-40), 1990 (TR 90-46) and 1991 (TR 91-64) is available through SKB.
T 1I The Protogine Zone.
Geology and mobility during the last 1.5 Ga Per-Gunnar Andréasson and Agnes Rodhe Lund, September 1992
ui SUMMARY iv
INDEX MAP OF GEOGRAPHICAL NAMES
2 GEOLOGY OF THE PROTOGINE ZONE (PZ)
22 THE PZ AS A GEOLOGICAL UNE OR BOUNDARY22.1 The aligned intrusions 5 222 The aligned hydrotherrnal mineralizations 5 22.3 The metamorphic break 5
2.3 GEOPHYSICAL ANOMALIES 2.3.1 Moho 2.3.2 Bouger anomaly patterns 2.3.3 Aeromagnetic patterns 2.3.4 Palaeomagneiism 2.3.5 Seismkity
2.4 THE ZONE AS A MORPHOTECTON1C FEATURE2.4.1 The aligned palaeo-depressions 11 2.4.2 Ongoing subsidence 15 2.4.3 The break in regional rock block patterns along the PZ IS 2.4.4 Tilted and broken peneplains 15 2.4.5 Features of the PZ landscape 15
3 PZ STRUCTURES IN THE "SCHISTOSITY ZONE" TYPE AREA 17
3.1 TECTONITES OF MODERATE STRAIN 19 3.1.1 Fabric description 19 3.1.2 Geom
SUMMARYThe Protogine Zone (PZ) is tretated here as the western margin of the Southeastern Megablock (SEM). The PZ has a complex structure with a long tectonic history. In addition to being a system of NNW, N-S and NNE trending deformation zones, it is the site of magmatic intrusions, metamorphic breaks, hydrothermal mineralizations and is characterized by geophysical anomalies such as a gravity low and aeromagnetic breaks and lows. The evidence of repeated deformation over a period of more than 1000 Ma is in marked contrast to the history of the adjacent granite region of eastern Småland (i. e. the SEM).
In the type area of the "Schistosity Zone", typical structures include a foliation (the "schistosity**) striking c. N-S and dipping steeply to the west and a conspicuous stretching lineation defined by quartz and feldspar plunging steeply towards WSW. The sense of net regional displacement is inconclusive, both as regard vertical and horizontal movements. The geometry of small-scale faults is not uniform and does not comply with trends and dips of foliations and mylonite zones. Ductile and brittle structures are often intimately associated and ductile structures also pass gradually into brittle ones suggesting continuous deformation during uplift.
Three generations of mafic dyke swarms of remarkably similar, specific composition indicate repeated E-W tension along the PZ at c. 1500,1200 and 900 Ma. Syenitic and granitic rocks intruded along the zone in connection with the 1200 Ma event. The typical PZ foliation as seen in the field and described in Chapter 3 of the report probably formed during uplift of the gneiss complex of SW Sweden following deep crustal (35 km) metamorV" phism of the gneisses c. 915 Ma ago. Deformation continued under more 1 shallow and thus brittle conditions as the Vättern Basin opened from c. 850 Ma. The following deformation was essentially related to uplift and resulted in regional tilting of blocks and local faulting. The PZ was most probably affected and reactivated in connection with the large deformational events during which the Caledonian, Hercynian and Alpine mountain belts formed.
These events strongly affected the southwestern margin of the shield and must have triggered movements along the PZ. Faulting occurred during deglaciation at least locally. Since such evidence requires considerable detailed and systematic mapping in order to be demonstrated, it may be much more frequent than is known so far and may be directly measurable with modern satellite techniques. Historical seismicity and the pattern of the relative uplift suggest that the PZ may be a significant zone of ongoing movement.
Horizontal displacement may be significant even if vertical scarps and throws have not been detected. Future work should inter alia investigate, if east-west running lineaments within the SEM like the Nömmen-Oskarshamn fault zone are connected with the PZ, and if movements along the PZ can reactivate these zones.
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ACKNOWLEDGEMENTSThe Protogine Zons has been a popular subject of discussion at our department during the last two decades, and we acknowledge many stimulating and constructive talks with our colleagues. Discussions with participants on national and international field excursions along the PZ have also increased and refined our knowledge. The systematic study of structures presented in Giapter 3 was carried out mainly by Dr. Joseph Hull. Prof. David G. Gee and Dr. Carl-Henric Wahlgren kindly read an early version of the manuscript and contributed valuable information and critizism. Mrs. Christin Andréasson made the illustrations.
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The background of this report is the increasing evidence that the bedrock of eastern Småland belongs to the most stable in the country and as such is potentially suitable for a repository of Sweden's toxic waste. The relatively stable Southeastern Megablock (SEM) is delimited to the west by the Protogine Zone (PZ), a system of defomation zones for which there is a variety of evidence for movement over the last 1000 Ma.
It is well-known that old zones of weakness in the Earth's crust often are reactivated during younger deformational events. For instance, the ongoing transform faulting in the Red Sea reactivates Pan-African (c. S00 Ma old) faults in the Arabian Shield, as evidenced by microseimic activity along these faults. With regard to the stability of the SEM, the ideal function of the PZ would be that the regional stress is localized to the zone and released within it. Possible faults extending eastwards across the megablock could deviate and transfer movements from the PZ. Therefore, the relations between the PZ and east-west trending fracture zones of southeastern Sweden are important subjects for future investigation.
The PZ is a composite feature and this report provides a review of the various geological and geophysical characteristics of the zone (Chapter 2), followed by a structural study across the zone in its type area (Chapter 3).
The published evidence of mobility along the zone is summarized in Chapter 4. Possible relations between the PZ and other major structures of southern Sweden are discussed in Chapter 5. A comprehensive bibliography of specific subjects is provided as an appendix.
GEOLOGY OF THE PROTOGINE ZONE (PZ)
2.1 DEFINITIONS Geological maps of Sweden (a. o. Magnusson et al. 1958) describe the PZ as an approximately N-S trending zone of strong schistosity and a boundary between two bedrock provinres of different age (Fig. 2-3). To the east is the belt of 1.75-1.5 Ga old Småland-Värmland granitoids and volcanics (SVG) with remnants of Svecofennian ( 1.8 Ga) supracrustal rocks and intrusions. To the west of the zone occurs the gneiss complex of southwest Sweden (SGC) which underwent strong deformation and metamorphism mainly in Sveconorwegian time (1.25 - 0.9 Ga). The N-S trending zone of strong deformation ("Förskiffringszonen") that has also been used as a criterion to define the PZ does not coincide precisely with the lithological boundary. Mapping by the Geological Survey of Sweden to the west of Lake Vättern has demonstrated that granitoids of the SVG can be mm*
Figure 2: Major bedrock provinces traced across the PZ (Gorbatschev 1971; Lundcgårdh 1977; Larsson et al.
1986; Wahlgren and Rönnlund 1988). Andréasson and Rodhe (1990) demonstrated, that the lithological PZ and the tectonic PZ south of Lake Vättern only locally coincide in the field.
In this report, the Protogine Zone is defined as a zone of intense deformation which runs from Scania in the south northwards between Lake Vättern and Lake Vänern and along the upper reaches of the Klarälven River into Norway. The term is used in its widest sense and irrespective of age of deformation. This is done with regard to the task, and because of frequent evidence that old fractures in the crust frequently regenerate.
In the type area south of Lake Vättern, the zone is a 20-30 km wide belt of anastomosing, steep shear zones striking NNW, N-S and NNE. Each shear zone is normally less than 100 m wide; it passes laterally into undeformed granite. Similar descriptions have been given for the zone in Scania (Kornfält in Kornfält et al. 1978; Wikman in Wikman et al. 1983);
west of Lake Vättern (Johansson in Westergård et al. 1926) and east of Lake Vänern (Wahlgren & Stephens 1990); however, in the latter area, zones are less steep. Geological and aeromagnetic patterns indicate that PZ structures extend into Bergslagen at least as far as the Grythytte Field (Fig.
1,3; Wahlgren & Stephens 1990).
Structures identical to those in the type area of the tectonic PZ south of Lake Vättern with regard to intensity and style of deformation and metamorphic grade can be observed along several lineaments further west like the Nissan and Lagan valleys (Fig. 3). This report suggests that these lineaments can be regarded as splays of the main PZ. Moreover, it has been known for a long time that PZ structures occur on the Romele and Linderöd Horsts in Scania, and also along Lake Immeln in eastern Scania.
Thus, as defined in this report, the PZ is a system of deformation zones rather than a single zone.
Both ductile and brittle structures are observed along the discrete shear zones. To the former belong foliations, lineations and their mylonitic equivalents. The most conspicuous lineation is a steeply pitching, strong stretching lineation defined by quartz and feldspar. Brittle structures include breccias, slickensides and slickensidelines. Metre-wide zones of gouz* and clay occur along several topographically marked fault scarps like, for instance, the boundary faults of the Lagan valley (Tofteryd).
While regional scale off-set of rock units is very rare, kinematic indicators abound (Andréasson and Rodhe 1990). In general they indicate dip-slip movements; only very locally have strike-slip movements been proved.
A systematic study of structures within the PZ type area is reported in Chapter 3.
. 'S * * * * * * * * * \ \ \ \ \ •\ \ \ s t********* \ \ \ \ \ »\ \ s
Protogine Zone (PZ) Unmetamorphosed (Visingsö, Almesåkra) or very low grade (Jotnian) Low-pressure, high-temperature (Svecofennian metasedimentary rocks) g|^f High-pressure (Southwest Swedish Granulite Region) C. 9 0 0 Age in Ma of metamorphism (or formation, if unmetamorphosed) Figure 6: Metamorphism of selected units 1000 Ma apparently restricted to the SGC. This has been taken to indicate Sveconorwegian uplift of the western block (a. o. Welin and Blomqvist 1966). Systematic Ar-Ar-dating across the zone is in progress (L-G. Jarl, Riksmuseet, Stockholm). According to Bylund (1981), the PZ may represent an eastern limit of Sveconorwegian regeneration of palaeomagnetism.
The pervasive regional foliations within the PZ ("protoginförskiffringen") are typically defined by mineral assemblages of epidote-amphibolite fades grade. Foliated mafic dykes may cany hornblende and garnet. The mineralized mica schists at Västanå, Hökensås and Hålsjöberg carry complex assemblages indicative of a polyphase low- to high-grade metamorphic evolution which, however, appears to be restricted to the mineralized zones.
2.3 GEOPHYSICAL ANOMALIES Several geophysical features run parallel to, or are bounded by, the lithological PZ. Again, there is no complete coincidence with the tectonic PZ. For instance, the tectonic PZ in Scania overlaps areas with contrasting geophysical characters (Henkel in Wikman et al. 1983).
2.3.1 Moho Moho contour maps show a marked N-S trending ridge to the west of the PZ (Fig. 7a; Thybo et al. 1989). The EUGENO-S refraction seismic profile between Karlskrona and Gothenburg shows two breaks in the Moho beneath Shotpoint 23 (Fig. 7b) which have been interpreted as the roots of Sveconorwegian listric thrusts. The EGT reflexion seismic profile across the PZ north of Lake Vättern (Dahl-Jensen et al. 1991) found shallow reflectors possibly corresponding to the changing dip of PZ foliation east of Lake Vänern (ef. 3.4). The deep reflectors which had been interpreted to represent Sveconorwegian listric thrusts could not be traced to the Earth's surface or connected with the inferred thrust front ("Sveconorwegian Front"; Berthelscn 1988; cf. Fig. 3). The reflections terminate against a blank, vertical wall at the boundary to the Småland Granites. The authors concluded that considerable vertical movements, with westerly downthrow, must have occurred along the wall after the inferred overthrusting.
Bouguer anomaly patterns 2.3.2 These show a gravity low along the PZ (Fig. 8; Wideland 1970) which has been interpreted as an evidence of crustal thickening (Gorbatschev 1980;
Thybo et al. 1989) or the opposite (Mörner 1977). Stephansson (1978) emphasized the importance of the trench and also the break in anomaly patterns along the zone as an expression of "a major, deep-seated structure".
7a: Moho contours of southern Sweden From Thyboctal. (1989).
7b: Jumps in the Moho beneath Shotpoint 23 (location: cf. Fig. 7a).
From EUGENO-S Working Group (1988).