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Abstract. The multiscale analysis of lineament patterns helps define the geometric scaling laws and the relationships between outcrop- and regional-scale structures in a fracture network. Here, we present a novel analytical and statistical workflow to analyze the geometrical and spatial organization properties of the Rolvsnes granodiorite lineament (fracture) network in the crystalline basement of southwestern Norway (Bømlo Island). The network shows a scale-invariant spatial distribution described by a fractal dimension D≈1.51, with lineament lengths distributed following a general scaling power law (exponent α=1.88). However, orientation-dependent analyses show that the identified sets vary their relative abundance and spatial organization and occupancy with scale, defining a hierarchical network. Lineament length, density, and intensity distributions of each set follow power-law scaling laws characterized by their own exponents. Thus, our multiscale, orientation-dependent statistical approach can aid in the identification of the hierarchical structure of the fracture network, quantifying the spatial heterogeneity of lineament sets and their related regional- vs. local-scale relevance. These results, integrated with field petrophysical analyses of fracture lineaments, can effectively improve the detail and accuracy of permeability prediction of heterogeneously fractured media. Our results also show how the geological and geometrical properties of the fracture network and analytical biases affect the results of multiscale analyses and how they must be critically assessed before extrapolating the conclusions to any other similar case study of fractured crystalline basement blocks.

The mid-Norwegian passive margin is a multiphase rifted margin that developed since the Devonian. Its geometry is affected by the long-lived activity of the Møre-Trøndelag fault complex, an ENE-WSW−oriented regional tectonic structure. We propose a time-constrained evolutionary scheme for the brittle history of the mid-Norwegian passive margin. By means of remote-sensing lineament detection, field work, microstructural analysis, paleostress inversion, mineralogical characterization, and K-Ar dating of fault rocks, six tectonic events have been identified: (1) Paleozoic NE-SW compression forming WNW-ESE−striking thrust faults; (2) Paleozoic NW-SE transpression forming conjugate strike-slip faults; (3) Carboniferous protorifting forming NW-SE− and NE-SW−striking faults; (4) Late Triassic−Jurassic (ca. 202 and 177 Ma) E-W extension forming approximately N-S−striking epidote- and quartz-coated normal faults and widespread alteration; (5) renewed rifting in the Early Cretaceous (ca. 122 Ma) with a NW-SE extension direction; and (6) Late Cretaceous extensional pulses (ca. 71, 80, 86, 91 Ma ago) reactivating preexisting faults and crystallizing prehnite and zeolite. Our multidisciplinary and multiscalar study sheds light onto the structural evolution of the mid-Norwegian passive margin and confirms the active role of the Møre-Trøndelag fault complex during the rifting stages. Our 62 new radiometric K-Ar ages define discrete episodes of faulting along the margin. The proposed workflow may assist in the interpretation of the structural framework of the mid-Norwegian passive margin offshore domain and also help to better understand fault patterns of fractured passive margins elsewhere.

Abstract. The multiscale analysis of fracture patterns helps defining the geometric scaling laws and the relationships between outcrop- and regional-scale structures in a fracture network. Here, we present a novel analytical and statistical workflow to analyze the geometrical and spatial organization properties of the Rolvsnes granodiorite lineament (fracture) network in the crystalline basement of southwestern Norway (Bømlo Island). The network shows a scale-invariant spatial distribution described by a fractal dimension D ≈ 1.51, with lineament lengths distributed following a general scaling power-law (exponent   = 1.88). However, orientation-dependent analyses show that the identified sets vary their relative abundance and spatial organization with scale, defining a hierarchical network. Lineament length, density, and intensity distributions of each set follow power-law scaling laws characterized by their own exponents. Thus, our multiscale, orientation-dependent statistical approach can aid in the identification of the hierarchical structure of the fracture network, quantifying the spatial heterogeneity of lineament sets and their related regional- vs. local-scale relevance. These results, integrated with field petrophysical analyses of fracture lineaments, can effectively improve the detail and accuracy of permeability prediction of heterogeneously fractured media. Our results show also how the geological and geometrical properties of the fracture network and analytical biases affect the results of multiscale analyses and how they must be critically assessed before extrapolating the conclusions to any other similar case study of fractured crystalline basement blocks.

<p>Fractured crystalline basement units are attracting increasing attention as potential unconventional reservoirs for natural (oil, heat and water) resources and as potential waste (nuclear, CO<sub>2</sub>) disposal sites. The focus of the current efforts is the characterisation of the structural permeability of fractured crystalline basement units, which is primarily related to the geology, geometry, and spatial characteristics of fracture networks. Fracture network properties may be scale–dependent or independent. Thus, a multi–scale characterisation of fracture networks is usually recommended to quantify the scale–variability of properties and, eventually, the related predictive scaling laws. Fracture lineament maps are schematic representations of fracture distributions obtained from either manual or automated interpretation of 2D digital models of the earth surface at different scales. From the quantitative analysis on fracture lineament maps, we can retrieve invaluable information on the scale–dependence of fracture network properties.</p><p>Here we present the results of the quantification of fracture network and fracture set properties (orientation, length, spacing, spatial organisation) from multi– (outcrop to regional) scale 2D lineament maps of two crystalline basement study areas of Western Norway (Bømlo island and Kråkenes). Lineament maps were obtained from the manual interpretation of orthophotos and 2D digital terrain models retrieved from UAV–drone and LiDAR surveys.</p><p>Analyses aimed at the quantification of: (i) scaling laws for fracture length cumulative distributions, defined through a statistically–robust fitting method (Maximum Likelihood Estimations coupled with Kolmogorov–Smirnov tests); (ii) variability of orientation sets as a function of scale; (iii) spatial organisation of fracture sets among scales; (iv) fractal characteristics of fracture networks (fractal exponent). Results suggest that: (i) a statistical analysis considering variable censoring and truncation effects allows to confidently define the best–fitting scaling laws; (ii) the analysis of orientation variability of fracture sets among different scales may provide important constraints about the geometrical complexity of fracture and fault zones; (iii) the statistical analysis of 2D spacing variability and fracture intensity can be adopted to quantify fracture spatial organisation at different scales.</p><p>A statistically robust analysis of the scaling laws, length distributions, spacing, and spatial organisation of lineaments on 2D maps provides reliable results also where only partial or incomplete dataset/lineament maps are available. Such properties are the fundamental input parameters for conceptual (geologic) and numerical (discrete fracture network, DFN) models of fractured crystalline basement reservoirs. Therefore, a statistically robust analysis of fracture lineament maps may help to improve the accuracy of conceptual and numerical models.</p>

<p>The detailed characterization of internal fault zone architecture and  petrophysical and geomechanical properties of fault rocks is fundamental to understanding the flow and mechanical behaviour of mature fault zones. The Goddo normal fault (Bømlo – Norway) accommodated c. E-W extension related to North Sea Rifting from Permian to Early Cretaceous times [1]. It represents a good example of a mature, iteratively reactivated and thus long-lived (seismogenic?) fault zone, developed in a pervasively fractured granitoid basement at upper crustal conditions in a regional extensional setting.</p><p>Field characterization of the fault zone’s structural facies and analysis of background fracture patterns in the protolith have been integrated with in-situ petrophysical and geomechanical surveys of the recognized fault zone architectural components. In-situ air-permeability and mechanical directional tests (performed with NER TinyPerm III air-minipermeameter and DRC GeoHammer, L-type Schmidt hammer, respectively) have allowed for the quantification of the permeability tensor and mechanical properties (UCS and elastic modulus) within each brittle structural facies. Mechanical properties measured parallel to fault rock fabric of cataclasite- and gouge-bearing structural facies differ by up to one order of magnitude from those measured perpendicularly to it (~10 MPa vs. 100-200 MPa in UCS, respectively). Accordingly, permeability of cataclasite- and gouge-bearing facies is several orders of magnitude larger when measured parallel to fault-rock fabric than that perpendicular to it (10<sup>-0</sup>-10<sup>-1</sup> D vs. 10<sup>-2</sup>-10<sup>-3</sup> D, respectively). Virtual outcrop models (VOMs) of the fault zone were obtained from high-resolution UAV-photogrammetry. Field measurements of fracture orientations were used for calibration of the VOMs to construct a statistically robust fracture dataset. The results of VOMs structural analysis allowed for the quantification of fracture intensity and geometrical characteristics of mesoscopic fracture patterns within the different domains of the fault zone architecture.</p><p>Results from field, VOMs structural analysis, and in-situ petrophysical investigations have been integrated into a realistic 3D fault zone model with the software 3DMove (Petex). This model can be used to investigate the influence of mesoscopic fracture patterns, related to either the fault zone or the background fracturing, on the hydro-mechanical behaviour of a mature fault zone. In addition, the evolution of the hydro-mechanical properties through time can be assessed by integrating the progressive development of brittle structural facies and fracture sets developed during the incremental strain and stress history into the model. This contribution proposes a geologically-constrained method to quantify the geometry of 3D fault zones, as a possible tool for models to be adopted in stress-strain analysis, hydraulic characterization and in the mechanical analysis of fault zones.</p><p>[1] Viola, G., Scheiber, T., Fredin, O., Zwingmann, H., Margreth, A., & Knies, J. (2016). Deconvoluting complex structural histories archived in brittle fault zones. Nature communications, 7, 13448.</p>