3 Evolutionary aspects of the Forsmark site
3.1 Bedrock evolution during the Proterozoic and Phanerozoic eons
Figure 3‑1. Map showing the major tectonic units in the northern part of Europe at the current level of erosion (modified after /Koistinen et al. 2001/). The area referred to in section 3.1.1 and used to provide a regional geological perspective for both the Forsmark and Laxemar-Simpevarp areas is outlined by the rectangle. This area is referred to as the geological reference area /Söderbäck (ed)2008/.
Neoproterozoic and Phanerozoic cover sedimentary rocks and intrusions Caledonian orogen
Fennoscandian Shield Sveconorwegian orogen
Post-Svecokarelian igneous and sedimentary rocks Palaeoproterozoic rocks in Blekinge–Bornholm tectonic belt Pre-Svecokarelian rocks and rocks formed along
Svecokarelian active continental margin (Palaeo–
proterozoic). Younger dolerite dykes/sills not shown
Pre-Svecokarelian rocks (Palaeoproterozoic) Archaean continental nucleus
Archaean continental nucleus – reworked
Rocks affected by the
Svecokarelian orogeny Rocks little affected by Svecokarelian reworking
Oslo Helsinki
Stockholm Forsmark
Olkiluoto
Laxemar-Simpevarp
An overview of the effects of these different, far-field tectonic events in the near-field realm
represented in south-eastern Sweden is described in /Söderbäck (ed) 2008/. These effects gave rise to local igneous activity during the Proterozoic, burial and denudation of sedimentary cover rocks during the Proterozoic and Phanerozoic, and predominantly brittle deformation in the bed rock at different times throughout this long time interval. At least two episodes of pre-Quaternary exhumation of the ancient crystalline bedrock can be inferred, one prior to the Cambrian and the other after the Cretaceous, probably during the Neogene. The current ground surface corresponds to the sub-Cambrian unconformity that morphologically is referred to as the sub-sub-Cambrian peneplain.
In conclusion, it appears that two fundamental types of geological process have made a profound impact on the geological evolution of the geological reference area in south-eastern Sweden (Figure 3-3):
• Igneous activity and crustal deformation along an active continental margin at different time intervals mostly during Proterozoic time.
• Loading and unloading cycles in connection with the burial and denudation, respectively, of sedimentary rocks, around and after c. 1.45 Ga.
As the effects of regional tectonic activity mostly waned in south-eastern Sweden and became prominent solely in the far-field realm, the effects of loading and unloading related to the burial and denudation of sedimentary rocks, respectively, increased in significance (Figure 3-3).
Figure 3‑2. Geological time scale based on the compilation used in /Koistinen et al. 2001/. Age is given in million years (Ma). 1 Ga = 1,000 Ma.
Geological time units
MILLION
YEARS EON ERA PERIOD AGE
CENO- ZOIC MESOZOIC
PHANEROZOIC PALAEOZOIC
NEO
MESO
PALAEO
PROTEROZOIC ARCHAEAN
PRECAMBRIAN RIPHEAN
LATE MIDDLE
EARLY VENDIAN PLEISTOCENE / HOLOCENE
IN QUATERNARY PALAEOGENE / NEOGENE
IN TERTIARY CRETACEOUS
JURASSIC
TRIASSIC
PERMIAN CARBONIFEROUS
DEVONIAN SILURIAN ORDOVICIAN
CAMBRIAN 2
100
200
300
400
500 543
1000
1600
2500
3000
3500
4000
1.635 or older
65
144
206 248
290 360
417 443
490 543
650 1000
1400 1600
2500
4000
3.1.2 Bedrock geological evolution in the Forsmark area
The bedrock geological evolution in the Forsmark area has been evaluated with the help of surface and borehole observational data as well as geochronological data /Söderbäck (ed) 2008/.
The geochronological data are summarised in Figure 3-4. In this area, an older suite of plutonic, calc-alkaline intrusive rocks formed between 1.89 and 1.87 Ga, and the metagranite inside the tectonic lens, where the target volume is situated (see chapter 5), is included within this suite.
Amphibolites that intrude the metagranite and a younger suite of calc-alkaline rocks and granites formed between 1.87 Ga and 1.85 Ga. These two suites of intrusive rocks (Figure 3-4) have been included in separate Svecokarelian tectonic cycles at 1.91–1.86 Ga and 1.87–1.82 Ga, respectively, that have been recognised in south-eastern Sweden /Söderbäck (ed) 2008/.
Deformation in the Forsmark area initiated between 1.87 and 1.86 Ga (Figure 3-4) with the develop-ment of a penetrative grain-shape fabric, with planar and linear components, that formed under amphibolite-facies metamorphic conditions and at mid-crustal depths. The development of broad WNW-ESE to NW-SE belts with higher ductile strain that surround tectonic lenses with generally lower ductile strain also occurred around 1.86 Ga. The amphibolites and other intrusive rocks that Figure 3‑3. Active tectonics (red) and oscillatory loading and unloading cycles (blue) during geological time in the geological reference area (modified after /Stephens et al. 2007/). The detailed evolution during the Quaternary period with several glaciations (loading) and deglaciations (unloading) is not shown.
SvK = Svecokarelian orogeny, G = Gothian orogeny, H = Hallandian orogeny, SvN = Sveconorwegian orogeny, C = Caledonian orogeny.
H
From active tectonics to oscillatory loading / unloading cycles
1.87 Ga
TIME
SvK
c.1.45 Ga c. 900 MaSvN
540 Ma and laterC
Activation and reactivation of deformation zones in connection with tectonic activity along active continental margins.
Significant strike-slip component of displacement along deformation zones
Loading and unloading cycles connected with, for example, the evolution of a sedimentary basin or a passive continental margin, glaciation / deglaciation etc.
Fault reactivation, change in aperture along established fractures, development of new sheet joints
G
Re-exhumations Exhumation
(sub-Cambrian unconformity)
Holocene (after 10,000 years ago)
Bulk crustal shortening
Maximum principal stress at present day related to ridge push from the mid-Atlantic ridge
belts and shortening across them, so-called dextral transpressive deformation, has been inferred. This deformation is related to bulk crustal shortening in an approximately northward direction during oblique subduction of oceanic lithosphere. Subduction occurred beneath the ancient continental margin to the north-east (Figure 3-1).
The brittle deformational history at Forsmark, which initiated some time between 1.8 and 1.7 Ga (Figure 3-4), has been evaluated with the help of three lines of approach:
• The use of low-temperature geochronological data that shed light on the exhumation and cooling history (Figure 3-4).
• The relative time relationships between different fracture minerals and the absolute ages of the low-temperature variant of the mineral K-feldspar, referred to as adularia (Figure 3-4).
• A comparison of kinematic data from brittle structures along deformation zones (section 5.2.6) with the tectonic evolution in a regional perspective (section 3.1.1).
Different generations of fracture minerals have been recognised in the Forsmark area. An early period of precipitation of a high-temperature mineral assemblage, which includes epidote, was followed by a period of hydrothermal precipitation of different, lower temperature minerals, including adularia (older generation), hematite, prehnite, laumontite and calcite. The fractures that bear epidote formed prior to 1.1 Ga, i.e. are pre-Sveconorwegian in age. The effects of Sveconorwegian tectonothermal activity for the evolution of fracture mineral assemblages are evident (Figure 3-4). However, the close relationship between the stability field for the laumontite-prehnite mineral assemblage and the closure temperature at c. 200 to 225°C for the 40Ar/39Ar K-feldspar isotope system (Figure 3-4) illustrates the Figure 3‑4. Summary of the geochronological data that constrain the bedrock geological evolution in the Forsmark area from /Söderbäck (ed) 2008/. 40Ar/39Ar biotite data from depth in boreholes are not shown here. As expected, these data are somewhat younger than the equivalent surface data. Only a selection of (U-Th)/He data are shown. The procedure adopted concerning the interpretation of all the (U-Th)/He data, both corrected and uncorrected, is discussed in /Söderbäck (ed) 2008/. Important geological events that have been recognised in the Forsmark area are shown in pale blue rectangles with accompanying text on the figure.
pumpellyite–
prehnite laumontite–
prehnite Rock fragment
in fault breccia (Bh)Older
adularia (Bh) Younger adularia (Bh) Tectonothermal disturbance
during the Sveconorwegian
orogeny at 1.1-0.9 Ga Post–Cambrian loading by sedimentary cover.
Exhumation of crystalline rocks prior to the Quaternary
Effects of Permian extensional tectonics Greenschist facies
Sub–greenschist facies Amphibolite facies
Age (million years)
Temperature (°C)
U-Pb zircon
U-Pb titanite (550-700oC)
40Ar/39Ar amphibole (c.500oC)
40Ar/39Ar muscovite (c.350oC)
40Ar/39Ar biotite (c.300oC)
40Ar/39Ar K-feldspar (c.200-225oC)
(U-Th)/He apatite (c.70oC)
Rock-forming mineral Fracture mineral
Sample at depth from borehole. Other ages come from surface samples Bh
Bh
Bh
2000 1750 1500 1250 1000 750 500 250 0
1.89-1.85 Ga igneous activity
Penetrative ductile deformation, high-strain belts, folding
More discrete ductile deformation along zones
Age interval for changeover to ductile-brittle and brittle deformation
SC/Bh KFM01A (0–500 m) SU/Bh KFM01A (0–500 m)
Selected and corrected (U–Th)/He age SC
Selected and uncorrected (U–Th)/He age SU
sensitivity of this system for resetting during growth of, for example, laumontite. For this reason, it is not clear whether the older generation of adularia formed during or prior to the Sveconorwegian tectonothermal event. The integrated evaluation that makes use of the different lines of approach outlined above suggests that the different sets and sub-sets of deformation zones in the Forsmark area had formed and had already reactivated during Proterozoic time, in connection with the late Svecokarelian, Gothian and Sveconorwegian tectonic events (see also /Stephens et al. 2007/).
On the basis of the (U-Th)/He apatite data from boreholes, some constraints on when different segments of the bedrock at different crustal levels passed through the c. 70°C geotherm have been attained (Figure 3-4). These data indicate that a sedimentary cover was situated on top of the crystalline basement rocks throughout much of the Phanerozoic. At Forsmark, this cover was possibly c. 3 km thick during the Silurian. A generally slow exhumation rate in the order of 3 to 10 m/Ma occurred during the Late Palaeozoic and Mesozoic, and resulted in a reduction in thickness of the cover to c. 2 km by the Early Jurassic /Söderbäck (ed) 2008/. Some evidence for an increase in exhumation rate during the Permian is apparent.
Several lines of evidence indicate faulting after the establishment of the sub-Cambrian unconformity in the Forsmark area. Furthermore, precipitation of younger low-temperature minerals, including sulphides, clay minerals and calcite, occurred during and probably after Palaeozoic time. Both growth of adularia (younger generation) during the Permian (Figure 3-4) and migration of fluids downwards from the sedimentary cover into the crystalline bedrock have been established. Examples of downward fluid migration include the precipitation of oily asphaltite along fractures in the upper part of the bedrock. The asphaltite was derived from overlying Cambrian to Lower Ordovician oil shale that has now been eroded away. Furthermore, fluids that transported glacial sediment migrated downwards and filled new or reactivated fractures during the later part of the Quaternary period.
The downward migration of different types of water during the Quaternary is addressed in more detail in section 3.4.