Geological characterisation of geophysical lineaments as part of the expanded site descriptive model around the planned repository site for high-level nuclear waste, Forsmark, Sweden

Abstract

The Swedish Nuclear Fuel and Waste Management Co. (SKB) started a project in 2020 near Forsmark,
Sweden with the aim to expand the deformation zone model to encompass the entire catchment area around the
planned repository for spent nuclear fuel. This thesis builds upon a lineament model that was constructed for SKB
which utilizes airborne magnetic data supported by airborne Very Low Frequency (VLF) and elevation data. A
selection of the lineaments was investigated in the field to verify whether or not they correspond to deformation
zones, with a subsequent structural analysis for each confirmed deformation zone. My results show that NW-SE
trending ductile deformation zones are characterized by a high degree of localized strain and are locally brittlely
reactivated. These NW-SE trending deformation zones are found in areas inferred to previously have been affected
by high ductile strain. The ductile deformation zones are interpreted to be splays from a regional-scale deformation
zone which makes up the northern tectonic boundary of the so-called Forsmark tectonic lens and have tectonic
precursors that developed during the 2.0-1.8 Ga Svecokarelian orogeny. Tens to hundreds of meters wide strongly
red-stained brittle deformation zones are found on the island of Gräsö, located outside the higher ductile strain area.
Locally, mylonitic fabrics in the cores of the zones are visible, indicating also here a ductile precursor to brittle
deformation. E-W striking red-stained brittle deformation zones hosting a fracture mineral assemblage of calcite,
adularia and laumontite are extensional in nature and are most likely related to the 1.1-0.9 Ga Sveconorwegian
orogeny. Some lineaments in the northern parts of Gräsö that are oriented E-W and NW-SE are parallel to the
gneissic banding of the host rocks and do probably not represent distinct deformation zones.

Abstract (Swedish)

Svenska Kärnbränslehanteringen (SKB) startade ett projekt under 2020 i närheten av Forsmark i
Sverige med målet att utöka deformationszonsmodellen till att inkludera hela dräneringsområdet runt om det
planerade slutförvaret för använt kärnbränsle. Detta arbete bygger på en lineamentmodell som var skapad för SKB.
Denna modell använder sig av magnetiska data tillsammans med höjddata samt data insamlad med den
flyggeofysiska Väldigt Låg Frekvens (VLF) metoden. Ett urval av lineamenten undersöktes i fält för att se om de
stämmer överens med deformationszoner eller inte, och därefter gjordes en efterföljande strukturanalys för varje
bekräftad deformationszon. Mina resultat visar att de nordväst-sydostgående duktila deformationszonerna
karaktariseras av hög lokaliserad strain (resultat av stress) och är lokalt reaktiverade under spröda förhållanden.
Dessa nordväst-sydostgående deformationszoner finns i områden som tidigare har blivit påverkade av hög duktil
strain. De duktila deformationszonerna tolkas vara ‘splays’ som viker av från en deformationszon av regional
storlek som utgör den norra tektoniska gränsen av den så kallade Forsmark tektoniska linsen och har tektoniska
föregångare som bildades under den Svekokarelska orogenesen (2,0-1,8 miljarder år sedan). På Gräsö finn tiotals
till hundratals meter breda, starkt rödfärgade spröda deformationszoner utanför det högre duktila strainområdet.
Lokalt ses mylonit i de inre delarna av dessa deformationszoner som även här tyder duktil deformation före den
spröda deformationen. Öst-västliga rödfärgade spröda deformationszoner med sprickmineralerna kalcit, adularia
och laumontit är extensionella och förmodligen kopplade till den Svekonorvegiska orogenen (1,1-0,9 miljarder år
sedan). Några av lineamenten i den norra delen av Gräsö som sträcker sig öst-väst och nordväst-sydost är parallella
med den gnejsiga bandningen och representerar förmodligen inte distinkta deformationszoner.

Popular Abstract

There are many issues one has to consider when faced with long-time storage of
radioactive waste underground. For example, how will future earthquakes and possible
leakage of nuclear waste affect the waste storage? And what is the importance of
deformation zones (weak zones in rock) in safety assessments regarding the storage? A
team of geologists has been working for decades at the nuclear power plant in Forsmark,
Sweden to answer such questions.

A critical aspect is to know the nature of major deformation zones within and around the target
area since they directly affect the feasibility and the safety of underground radioactive waste
storage. A deformation zone is in essence a 2-dimensional plane along which the bedrock has
been weakened and deformed. One can assume that when new stress is applied to a rock volume
where pre-existing deformation zones are present, the new deformation will preferentially be
partitioned along these zones, basically following the rule of ‘least resistance’. Depending on the
prevailing pressure and temperature (both mainly a function of depth) during deformation, the
deformation zones may be plastic (ductile) or brittle. In a rock volume at Forsmark dominated by
crystalline rocks, the nuclear waste will be stored within a tectonic lens that until now largely has
escaped deformation. A ‘tectonic lens’ that is bordered by older zones of deformation will thus
represent a suitable volume for storage of nuclear waste. After decades of modelling work and
geophysical investigations, we know a lot about the deformation zones in the rock volume at
Forsmark. However much less is known about the deformation zones outside the main target
area and to what extent these deformation zones can be linked to the existing regional model.

My master-project is integrated in an extensive research project lead by Jesper Petersson (GEOS)
& Peter Hultgren (SKB). During two periods of field work in the area around the planned
underground repository for nuclear waste, I have investigated and characterized different type of
deformation zones outside the main target area. In the area near the deformation zone that
borders the Forsmark tectonic lens to the north strongly banded deformation zones are found,
indicating a high ductile strain partitioning. Locally, these zones have brittlely reactivated cores.
On the island of Gräsö which lies in another tectonic lens north of the Forsmark lens, broad
mostly brittle zones of deformation are found which are responsible for broad topographic lows
that cross-cut the island. A narrower ductile fabric is locally present at their core. Steeply dipping
purely brittle deformation zones which consist of red-stained fracture networks are found on
coastal outcrops on western Gräsö. These are suggested to be formed as a result of a much
younger tectonic event than the beforementioned ductile-brittle deformation zones.

• author: Bakker, Aron
• supervisor: Susanne Grigull; Ulf Söderlund; Jesper Petersson; Peter Hultgren
• organization: Department of Geology
• course: GEOR02 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Forsmark, deformation zones, Svecokarelian, Sveconorwegian, ductile-brittle transition, brittle deformation, mylonite
• publication/series: Dissertations in Geology at Lund University
• report number: 623
• language: English
• additional info: External supervisors: Susanne Grigull, SKB, Jesper Petersson, SKB, Peter Hultgren, SKB
• id: 9067297
• date added to LUP: 2021-10-26 15:21:46
• date last changed: 2021-10-26 15:21:46