Brittle reactivation of ductile precursor structures: The role of incomplete structural transposition at a nuclear waste disposal site, Olkiluoto, Finland

Reactivation of discrete deformation zones that are orientated favourably with respect to the stress field is a well-known phenomenon. What is less clear is the role of other structural features and heterogeneities in localizing deformation. In this paper we describe how brittle deformation structures are localized into zones of incomplete structural transposition inherited from earlier ductile deformation phases. In our example, these zones of incomplete structural transposition are characterised by localised high-strain structures of the latest ductile deformation stage, including short limbs of strongly asymmetric folds and anastomosing networks of minor shear fabrics. When such zones are systematically organized, and orientated favourable with respect to the stress field, they can be very efficient in localizing deformation and forming new fault zones. Applied to the site of the planned geological repository of nuclear waste in Olkiluoto, Finland, the recognized structural inheritance provides tools to understand the geometries, networks and kinematics of the brittle fault zones and the related secondary fracturing which together define the rock mechanical and hydrogeological framework for the repository.</p

3D modelling of the dolerite dyke network within the Siilinjärvi phosphate deposit

Non peer reviewe

Semi-automated geological mapping and target generation from geochemical and magnetic data in Halkidiki region, Greece

This study focuses on using multivariate analyses to generate semi-automated geological maps and exploration targets associated with porphyry Au-Cu mineralization within the Kassandra mining district, Greece. We use principal component analysis (PCA) and self-organizing maps (SOM) to reveal variations in geochemical and magnetic signatures within the input datasets. We visualize the results as pseudo-geological maps reflecting the associated geological processes and their end products. In specific, we utilize the potential of these two methods through an integrated interpretation and comparison of the results. We test the validity of the unsupervised PCAand SOM-derived lithological and prospectivity models by comparing them with existing geological observations and interpretations. The results of this investigation show that both PCA and SOM are able to reproduce the key features of existing geological observations within the study area, but more importantly, also provide useful information that can be used to recognize prospective geological units and exploration targets from previously unknown locations.Peer reviewe

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

Semi-automated geological mapping and target generation from geochemical and magnetic data in Halkidiki region, Greece

This study focuses on using multivariate analyses to generate semi-automated geological maps and exploration targets associated with porphyry Au-Cu mineralization within the Kassandra mining district, Greece. We use principal component analysis (PCA) and self-organizing maps (SOM) to reveal variations in geochemical and magnetic signatures within the input datasets. We visualize the results as pseudo-geological maps reflecting the associated geological processes and their end products. In specific, we utilize the potential of these two methods through an integrated interpretation and comparison of the results. We test the validity of the unsupervised PCAand SOM-derived lithological and prospectivity models by comparing them with existing geological observations and interpretations. The results of this investigation show that both PCA and SOM are able to reproduce the key features of existing geological observations within the study area, but more importantly, also provide useful information that can be used to recognize prospective geological units and exploration targets from previously unknown locations

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

Surface-wave tomography for mineral exploration : a successful combination of passive and active data (Siilinjärvi phosphorus mine, Finland)

Surface wave (SW) methods offer promising options for an effective and sustainable development of seismic exploration, but they still remain under-exploited in hard rock sites. We present a successful application of active and passive surface wave tomography for the characterization of the southern continuation of the Siilinjarvi phosphate deposit (Finland). A semi-automatic workflow for the extraction of the path-average dispersion curves (DCs) from ambient seismic noise data is proposed, including identification of time windows with strong coherent SW signal, azimuth analysis and two-station method for DC picking. DCs retrieved from passive data are compared with active SW tomography results recently obtained at the site. Passive data are found to carry information at longer wavelengths, thus extending the investigation depth. Active and passive DCs are consequently inverted together to retrieve a deep pseudo-3D shearwave velocity model for the site, with improved resolution. The southern continuation of the mineralization, its contacts with the host rocks and different sets of cross-cutting diabase dikes are well imaged in the final velocity model. The seismic results are compared with the latest available geological models to both validate the proposed workflow and improve the interpretation of the geometry and extent of the mineralization. Important large-scale geological boundaries and structural discontinuities are recognized from the results, demonstrating the effectiveness and advantages of the methods for mineral exploration perspectives.Peer reviewe

Constraints over the age of magmatism and subsequent deformation for the Neoarchean Kukkola Gneiss Complex, northern Fennoscandia

The Archean crust in northern Fennoscandia preserves a fragmentary geological record, making direct correlation among Archean domains challenging. This study presents two new zircon U-Pb age determinations from the Archean Kukkola Gneiss Complex (KGC) that straddles the border between Finland and Sweden. The results indicate that crystallization of tonalites within the magmatic core of the complex occurred at 2711 +/- 8 Ma, somewhat earlier than previously considered. A new pulse of magmatism occurred at 2675 +/- 10 Ma as demonstrated by hornblende-tonalites cutting the 2.71 Ga rocks. The results further indicate that the first deformation event responsible for development of penetrative foliations occurred after the first magmatic event at 2.71 Ga and prior to the subsequent tectonothermal event at 2.68 Ga. These findings are in concert with the known major periods of magmatism (2.8-2.7 Ga) and deformation (2.7 Ga) within better-known Archean domains in northern Fennoscandia, and hence support their correlation with KGC. Three complementary age determinations on the Haparanda-suite granites and tonalites were conducted: the results indicate crystallization ages of 1.90-1.89 Ga, overlapping with the known age range of the suite and supporting its predominance over the 1.8 Ga Lina suite granites in the Tornio-Haparanda area

Surface-wave tomography for mineral exploration: a successful combination of passive and active data (Siilinjarvi phosphorus mine, Finland)

Surface wave (SW) methods offer promising options for an effective and sustainable development of seismic exploration, but they still remain under-exploited in hard rock sites. We present a successful application of active and passive surface wave tomography for the characterization of the southern continuation of the Siilinjarvi phosphate deposit (Finland). A semi-automatic workflow for the extraction of the path-average dispersion curves (DCs) from ambient seismic noise data is proposed, including identification of time windows with strong coherent SW signal, azimuth analysis and two-station method for DC picking. DCs retrieved from passive data are compared with active SW tomography results recently obtained at the site. Passive data are found to carry information at longer wavelengths, thus extending the investigation depth. Active and passive DCs are consequently inverted together to retrieve a deep pseudo-3D shearwave velocity model for the site, with improved resolution. The southern continuation of the mineralization, its contacts with the host rocks and different sets of cross-cutting diabase dikes are well imaged in the final velocity model. The seismic results are compared with the latest available geological models to both validate the proposed workflow and improve the interpretation of the geometry and extent of the mineralization. Important large-scale geological boundaries and structural discontinuities are recognized from the results, demonstrating the effectiveness and advantages of the methods for mineral exploration perspectives

LITHOSPHERE 2018: TENTH SYMPOSIUM ON STRUCTURE, COMPOSITION AND EVOLUTION OF THE LITHOSPHERE: PROGRAMME AND EXTENDED ABSTRACTS

Shear zones of various ages and orientations are common in Southern Finland. In the study area, E-W and N-S trending shear zones are the dominant structural feature. Mylonitic foliations were identified from the most intensely sheared rocks. Ductile shearing has mainly been of dip-slip type. Structural mapping revealed several larger map-scale folds, which appear to be relatively continuous across the study area from SE to NW. In the central area, folding interfered with the shear zones causing a complex crustal structure such as associated with the Uunimäki mineralization. Aeromagnetic and lithological maps, field observations, stereographic projections and oriented thin sections were used to determine the structural features of the study area.</p