Characterisation of brittle structures of bedrock in southern Finland: Unravelling the evolution and properties of fault and fracture systems and their prediction at different scales

The aim of this thesis is to improve the understanding of the brittle tectonic evolution in Fennoscandia, which is known to be complex and contain multiple stages of fault nucleation and reactivation. Building on prior work, this work provides new structural descriptions and temporal constraints of fractures and faults from the Paleoproterozoic bedrock of southwestern Finland. Integration of structural and isotopic data from Olkiluoto, the site of planned nuclear waste disposal, allowed constraining the geometric and temporal relationships of specific sets of brittle faults within the fault network at local scales. However, the Olkiluoto dataset leaves a substantial period of uncertainty in crustal evolution at 1.6-1.3 Ga, and this thesis aims to close this gap through examining novel regional structural datasets from the 1.58 Ga Åland rapakivi batholith for paleostress analysis. Moreover, this work displays the results of multiscale lineaments mapped from southern Finland, providing information on the scalability of brittle structures, localization of regional deformation, and insights into the evolution of the brittle crust under various stress conditions. According to the results of this study, the tectonic development of the brittle crust within southwestern Finland includes at least six major tectonic stages: 1) brittle deformation initiated under NW–SE to NNW–SSE compression at the brittle-ductile transition at around 1.75 Ga. Two successive stages of regional extension followed: 2) N–S extension at around 1.64 Ga during the emplacement of the pre-1.6 Ga rapakivi granites, and 3) E–W to NW–SE extension during the intrusion of the 1.58 Ga Åland rapakivi, diabase dykes, and the onset of the development of a sedimentary basin where the Bothnian Sea is now located. This extensional period was succeeded by strike-slip tectonics including 4) WNW–ESE to NNW–SSE compression between 1.55–1.4 Ga and 5) NE–SW compression at around 1.3–1.2 Ga. 6) NE–SW to ENE–WSW compression during the early phases of the 1.1 Ga Sveconorwegian orogeny resulted in the formation of low-angle faults in Olkiluoto. Any later tectonic events were restricted to reactivations of previously formed faults. The absolute age of the overprinting regional joints remains unknown, but a minimum age of ca. 540 Ma can be inferred from Cambrian sedimentary infillings observed within some of the joints.---- Tämän opinnäytetyön tavoitteena on kehittää ymmärrystä Fennoskandian kilven hauraasta tektonisesta evoluutiosta, jonka tiedetään olevan sekä monimutkainen että sisältävän useita siirrosten synty- ja reaktivoitumisvaiheita. Tämä työ täydentää aiempia julkaisuja Lounais-Suomen kallioperän rakennekehityksestä uusien rako- ja siirrossystemien karakterisoinnin ja ikämääritysten muodossa. Olkiluotoon sijoitettavan ydinjätteen loppusijoituspaikan kallioperän osalta näiden uuden tulosten keskinäisen integroinnin perusteella kyettiin määrittämään sekä hauraiden siirrosten suhteelliset ikäsuhteet että absoluuttiset iät. Olkiluodon aineisto ei kuitenkaan kata ajallisesti maankuoren evoluutiota mesoproterotsooisen kauden alkuosassa, jonka vuoksi ajanjaksoon 1,6-1,3 Ga liittyy merkittäviä epävarmuuksia. Tällä opinnäytetyöllä pyritään edellä mainitusta syystä täyttämään tätä aukkoa tietämyksessä laatimalla uusiin alueellisiin rakenneaineistoihin perustuvia malleja paleojännityskentästä 1,58 Ga Ahvenanmaan rapakivibatoliitista. Lisäksi tässä työssä esitellään Etelä-Suomesta kartoitettujen monimittakaavaisten lineamenttitukintojen tulokset, jotka antavat tietoa hauraiden rakenteiden skaalautumisesta, alueellisten muodonmuutosten lokalisoitumisesta sekä ymmärrystä hauraan kuoren kehityksestä erilaisissa jännitysolosuhteissa. Tämän tutkimuksen tulosten perusteella maankuoren hauraan tektonisen kehityksen vaiheet Lounais-Suomessa ovat seuraavat: 1) hauras muodonmuutos alkoi NW–SE - NNW–SSE puristuksessa noin 1,75 Ga sitten, kun kallioperä oli jäähtynyt ja hauraiden rakenteiden muodostuminen oli mahdollista. Tätä puristusta seurasi kaksi peräkkäistä ekstensiovaihetta: 2) N–S ekstensio vaikutti yli 1,6 Ga ikäisten rapakivigraniittien asettumiseen ja 3) E–W - NW–SE ekstensio 1,58 Ga Ahvenanmaan rapakiven paikoilleen asettumiseen sekä, diabaasijuonten ja sedimenttialtaan kehitykseen. Näitä vaiheita seurasi kulkusiirrostektoniikka, mukaan lukien 4) WNW–ESE - NW–SSE puristus välillä 1,55–1,4 Ga ja 5) NE–SW puristus noin 1,3–1,2 Ga. 6) 1,1 Ga Svekonorjalaisen orogenian alkuvaiheessa NE–SW - ENE–WSW puristus johti kaateiltaan loiva-asentoisten siirrosten muodostumiseen Olkiluodossa. Kaikki tätä myöhäisemmät tektoniset tapahtumat rajoittuivat jo aiemmin muodostuneiden siirrosten uudelleenaktivoitumiseen. Kallioperästä havaittujen tensiorakojen absoluuttinen ikä on tuntematon, mutta niiden vähimmäisiäksi voidaan päätellä n. 540 Ma perustuen niissä esiintyviin kambrikautisiin sedimenttitäytteisiin

A new subsampling methodology to optimize the characterization of two-dimensional bedrock fracture networks

This paper introduces a new subsampling method to determine the empirical relationships between the areal sampling coverage and the topological-geometric parameters resulting from characterization of two-dimensional outcrop fracture networks. We further developed these relationships into correlations between the total sampling coverage and variance of the characterization result, hence providing insight to the objective uncertainties related to fracture network characterization. The analyses were conducted using two new open-source Python packages: fractopo and fractopo-subsampling, designed for fracture network analysis and subsampling, respectively. We conducted the study on the well-exposed crystalline outcrops of Getaberget, Åland Islands, Finland, where a total of 42499 fracture traces were manually digitized from 13 circular target areas. For the purposes of subsampling, we conducted fracture network characterization for randomly located and sized subsample areas, which locate within the larger target areas.Based on our subsampling results we provide recommendations for the preliminary optimization of areal coverage used in outcrop fracture sampling and the use of our subsampling method for assessing the precision related to the areal fracture network characterization in other previously uncharacterized areas. As an example, we recommend using a total sampling area of 8000with 8 circular sampling areas to define the power-law exponents of fracture traces when conducting outcrop fracture network characterization with drone-based methodology done with similar initial sampling setup in comparable geological environments, as this coverage has shown an acceptable level of precision.</p

Precambrian fault reactivation revealed by structural and K-Ar geochronological data from the spent nuclear fuel repository in Olkiluoto, southwestern Finland

Integrated structural/geochmnological studies help unraveling complex brittle deformation histories. We have analysed the structural geological database of brittle faults from the ONKALO (TM) underground facility for spent nuclear fuel in Olkiluoto in southwestern Finland. Based on the structural geological data from eleven representative fault zones, we classify the Olkiluoto brittle structural features into four fault systems, referred to as Fault system I to IV. The classification is based on their structural properties and tectonic history, crosscutting relationships, fault rock mineralogical characterization and 3D modelling. Some constraints on the timing of faulting are provided by K-Ar dates on synkinematic illite from fault gouge samples. Our results show that the bedrock in southwestern Finland experienced numerous brittle deformation phases between ca. 1.75 and 0.9 Ga. N-S strike-slip faults (Fault systems I and II) formed at mid-crustal levels ca. 1.79-1.75 Ga ago in response to NW-SE/NNW-SSE compression soon after the Svecofennian omgeny. Later E-W striking oblique dextral/normal faults (Fault system III) are tentatively associated with the Gothian omgeny 1.6 Ga ago. These three fault systems were reactivated during NE-SW compression ca. 1.3-1.2 Ga ago, coeval with intrusion of a regional swarm of olivine diabase sills. E-W compression at the onset of the Sveconorwegian omgeny ca. 1.1-1.0 Ga ago resulted in the formation of SE dipping low-angle thrust faults (Fault system IV) and the selective reactivation of fault system II and III. Overall E-W extension during the collapse of the Sveconorwegian orogen ca. 0.97-0.87 Ga ago caused the localised reactivation of fault systems III and IV. Our research approach, which is integral to the siting process of repositories for spent nuclear fuel, demonstrates that the basement in southwestern Finland experienced repeated reactivation since the Mesoproterozoic, suggesting that future deformation localization is likely to be also accommodated by reactivation of existing brittle structures rather than formation of new faults

Fault-induced mechanical anisotropy and its effects on fracture patterns in crystalline rocks

We used drone-acquired orthophotographs to map the 2D-networks of fractures transecting the Mesoproterozoic Rapakivi granites in southern Finland. The work aims at understanding i) how discrete faults and the kine-matically linked syn-fault extension fractures originate in crystalline rocks, and ii) how the resulting structural anisotropy controls the patterns of later-formed regional fractures. We recognized incipient faults with associated distinct damage zones (DZ; wall-, tip-, bend- and linking damage) indicating that faulting occurred within mesoscopically isotropic material devoid of  pre-existing fabrics. Moreover, fault-induced extension fractures occur also outside the normal DZs indicating much wider DZs than that predicted by scaling laws. Extension fractures within the wide DZs are the result of linkage between sub-parallel segments of the dominantly sinistral and ~N-S trending (sub-) vertical faults. The resulting syn-fault network of N-S, NW-SE and NE-SW trending fracture sets contributed to the mechanical anisotropy that controlled the patterns of the later-formed regional fractures. Specifically, i) narrow fault reactivation damage zones caused deviations in the patterns of otherwise systematic near-orthogonal regional fractures, and ii)  the  NNW-SSE trending regional fractures are  poorly developed in  areas where either several N-S  faults or  sub-parallel NW-SE syn-fault extension fractures are dominant. The latter applies in particular to a large left step-over zone between sinistral faults, which we infer to provide the first-order control over the development of contrasting fracture domains. The results of this study further indicate that a strike-slip paleostress event affected the crust after ca. 1.65 Ga.</p

Hyperspectral outcrop models for palaeoseismic studies

The traditional study of palaeoseismic trenches, involving logging, stratigraphic and structural interpretation, can be time consuming and affected by biases and inaccuracies. To overcome these limitations, a new workflow is presented that integrates infrared hyperspectral and photogrammetric data to support field-based palaeoseismic observations. As a case study, this method is applied on two palaeoseismic trenches excavated across a post-glacial fault scarp in northern Finnish Lapland. The hyperspectral imagery (HSI) is geometrically and radiometrically corrected, processed using established image processing algorithms and machine learning approaches, and co-registered to a structure-from-motion point cloud. HSI-enhanced virtual outcrop models are a useful complement to palaeoseismic field studies as they not only provide an intuitive visualisation of the outcrop and a versatile data archive, but also enable an unbiased assessment of the mineralogical composition of lithologic units and a semi-automatic delineation of contacts and deformational structures in a 3D virtual environment. Résumé L'étude traditionnelle des tranchées paléosismiques, impliquant l'enregistrement des coupes et l'interprétation stratigraphique et structurelle, peut prendre beaucoup de temps et être entachée de biais et d'inexactitudes. Pour surmonter ces limites, une nouvelle méthodologie est présentée, intégrant des données photogrammétriques et hyperspectrales infrarouges en appui aux observations paléosismiques de terrain. Comme étude de cas, cette méthode est appliquée à deux tranchées paléosismiques creusées à travers un escarpement de faille post-glaciaire dans le nord de la Laponie finlandaise. L'imagerie hyperspectrale (HSI) est corrigée géométriquement et radiométriquement, traitée à l'aide d'algorithmes classiques de traitement d'images et d'apprentissage machine, et recalée sur un nuage de points photogrammétrique. Les modèles virtuels d'affleurements améliorés par HSI constituent un complément utile aux études paléosismiques de terrain, car ils fournissent non seulement une visualisation intuitive de l'affleurement et une archive de données facile d'emploi, mais permettent également une évaluation non biaisée de la composition minéralogique d'unités lithologiques ainsi qu'une délimitation semi-automatique des contacts et des structures de déformation dans un environnement virtuel 3D

Hyperspectral imagery-enhanced virtual outcrop models of two palaeoseismic trenches in northern Finnish Lapland

The traditional study of palaeoseismic trenches involving logging, stratigraphic and structural interpretation can be time-consuming and affected by biases and inaccuracies. To overcome these limitations, we present a new workflow that integrates infrared hyperspectral and photogrammetric data to support field-based palaeoseismic observations. As a case study, this method is applied on two palaeoseismic trenches excavated across a post-glacial fault scarp in northern Finnish Lapland. The hyperspectral imagery (HSI) is geometrically and radiometrically corrected, processed using established image processing algorithms and machine learning approaches, and co-registered to a Structure-from-Motion point cloud. HSI-enhanced virtual outcrop models are a useful complement to palaeoseismic field studies as they not only provide an intuitive visualisation of the outcrop and a versatile data archive, but also enable an unbiased assessment of the mineralogical composition of lithologic units and a semi-automatic delineation of contacts and deformational structures in a 3D virtual environment. Uploaded data: 14 individual 3D point clouds (ascii format) from two palaeoseismic trenches, including two structure-from-motion photogrammetric RGB point clouds and 12 hyperspectral-enhanced point clouds. Data headers contain point coordinates in m (ETRS89/UTM35N), RGB color (0–255), and point normals (only for SfM RGB point clouds) in the following order: X, Y, Z, Red, Green, Blue, Nx, Ny, Nz