Charles Hurich

The Hardangerfjord Shear Zone in SW Norway and the North Sea : A large-scale low-angle shear zone in the Caledonian crust

The Hardangerfjord Shear Zone is a more than 600 km long low-angle extensional structure that affects the South Norway and North Sea Caledonides. The ductile shear zone, which shows total maximum onshore displacement of the order of 10–15 km, is primarily a basement structure with an associated passive, monoclinal fold structure of the overlying Caledonian nappes. Deep seismic data indicate that the shear zone continues down to the lower crust (20–25 km) at a dip of 22–23°, where it appears to flatten and merge with the general lower-crustal deformation fabric. Onshore, the Hardangerfjord Shear Zone consists of a system of hard-linked ductile shear-zone segments. Brittle faults (the Lœrdal–Gjende fault system) occur in the folded Caledonian allochthons in the NE part of the Hardangerfjord Shear Zone, and reappear in the North Sea. These may represent a high-level brittle response to the Devonian development of the Hardangerfjord Shear Zone, but were reactivated during Permo-Triassic and late Jurassic extensional events. A c. 5 km thick package of seismic reflectors along the Hardangerfjord Shear Zone is presumed to represent a mylonite zone, which is too thick to be formed entirely by 10–15 km of Devonian displacement. Hence the Hardangerfjord Shear Zone is likely to be a Proterozoic shear zone, reactivated during Devonian extension.

Crustal stretching in the Scandinavian Caledonides as revealed by deep seismic data

The normalization of overthickened orogenic crust after a continent-continent collision typically involves several different processes, including erosion, plate divergence, gravity-driven collapse of the orogenic wedge, and viscous flow of the lower crust. As a contribution to this discussion, we here utilize reprocessed deep seismic data to image the Scandinavian part of the major Caledonian continent-continent collision zone where we identify a double set of extensional shear zones: one in the upper to middle crust (Hardangerfjord shear zone) and a major oppositely dipping Moho-offsetting shear zone in the lower crust and upper mantle. The latter shear zone may have a true displacement close to 50 km and appears to offset the Moho vertically by ~10 km, extending downward beyond the 50 km depth limit of the seismic data. While the upper structure offsets the basal Caledonian thrust, the Moho shear zone is more difficult to date. However, they are kinematically consistent and located at the transition zone between thick- and thin-skinned post-collisional tectonics, and we suggest that they may represent a crustal “pinch” or zipper-like structure that separated viscously flowing Caledonian lower crust from cooler rigid basement representative for the rest of the Baltic Shield. Later extension created the North Sea rift, but the general rift axis and the associated riftrelated thinning developed in a narrower zone that runs oblique to the Devonian trend, hence the two generations of extension and thinning can be distinguished at the crustal scale.

Seismic image of the basal portion of the Bjerkreim-Sokndal layered intrusion

A high-resolution seismic profile over the Layered Series of the Bjerkreim-Sokndal layered intrusion provides a high-quality seismic image of one of the rock types frequently cited as a source of dense lower crustal reflectivity. The data interpretation is controlled by a detailed comparison with exposed geology directly up-plunge and has implications for reaching a better understanding of deep crustal reflectivity. Reflections are produced by boundaries between major (≥100 m) lithologic units of moderately differing average composition, and small-scale modal layering (centimeters to several meters) contributes in a minor way. The seismic image provides a standard useful for identifying layered mafic intrusions within the upper and middle crust and for understanding their internal structure. The Bjerkreim data also imply that areas of highly reflective lower crust where underplating is believed to have occurred can result from internal lithologic layering resulting from fractionation and repeated influxes of magma to a closed system. The data show that highly developed modal layering can appear seismically transparent due to destructive interference.

Unified geophysical and geological 3D Earth models

Three-dimensional geological Earth models typically comprise wireframe surfaces of connected triangles that represent geological contacts. In contrast, Earth models used by most current 3D geophysical numerical modeling and inversion methods are built on rectilinear meshes. This is because the mathematics for computing data responses are simpler on rectilinear meshes. In such a model, the relevant physical properties are uniform within each brick-like cell but possibly different from one cell to the next, producing a pixellated representation of the Earth. In principle, arbitrary spatial variations can be represented if a sufficiently fine discretization is used. However, no matter how fine the discretization of the rectilinear mesh, such a mesh is always incompatible with geological models comprising wireframe surfaces. Also, because the computational resources required by 3D numerical modeling and inversion methods increase dramatically as the discretization of a model is refined, it is never really pos...

Mega‐scale Moho relief and the structure of the lithosphere on the eastern flank of the Viking Graben, offshore southwestern Norway

The International Lithosphere Project deep reflection seismic survey in the Norwegian sector of the North Sea has been reprocessed, particularly focusing on the deep crust, the reflection Moho, and the upper mantle. The data display shifting reflection patterns of the crust and the upper mantle parallel to the eastern margin of the Viking Graben. In the upper crust, which is mainly seismically transparent by the processing techniques utilized here, large-scale structural features like detachment shear zones and master faults can be identified. Several of the major onshore faults and shear zones match seismic features in the seismic lines. Many of these structures acted as extensional shear zones in the Devonian. The middle crust is of variable reflectivity, whereas the lower crust is generally strongly reflective and is particularly so in the southern domain. The reflection Moho is identified throughout the study area but is of variable character. The presence of a S(E) dipping structure (Hardanger Moho Offset) that displaces the Moho by approximately 10 km, extends deep into the mantle (below the 50 km line depth), is positioned where the shallower Hardangerfjord Shear Zone, which flattens on the level of the middle crust, is situated. The Hardangerfjord Shear Zone/Hardanger Moho Offset-system coincides with change of the crustal thickness (depth to Moho), a change that also coincides with the transition from thin- to thick-skinned Caledonian deformation. Intramantle reflections are common in the study area, some of which are interpreted as shear zones, whereas others most likely represent magmatic intrusions.

The Nature of Crustal Seismic Heterogeneity: A Case Study From the Grenville Province

Over the range of scales for which most seismic reflection data contain information, lithologic variation is one of the major sources of heterogeneity in the crystalline crust. This is particularly true at depths greater than ~10 to 15 km where most of the fractures and microfractures that contribute to upper crustal heterogeneity are closed. The spatial distribution of lithologic heterogeneity is a function of the range of magmatic and tectonic processes that progressively distribute and redistribute the various lithologic components of the crust (the “tectonic roulette” of Fountain and Salisbury, 1981). Although the seismic reflection wavefield responds indirectly to lithologic variation, it is directly responsive to fluctuations of acoustic impedance that are more closely coupled to mineralogy than lithology. Accordingly, pressure and temperature variations that modify mineralogy, but not bulk chemistry, combine with lithologic variation to play both static and dynamic roles in defining the heterogeneity of the Earth’s crust.

Minimum-structure Borehole Gravity Inversion

The borehole gravity technique has been well established in hydrocarbon exploration geophysics since the 1970s. The concept behind borehole gravity is simply to measure the variation in the Earths gravitational field while traveling along a borehole. Densities both close to and far from the borehole can be derived from such measurements. However, the borehole gravity technique has not yet been routinely used for mineral exploration because gravimeters that fit in the narrower diameter holes used in mineral exploration have not existed. Such gravimeters are now being developed. Complementary investigation and development of interpretation procedures for borehole gravity data in a mineral exploration context are required. Here, results are presented of a study inverting synthetic borehole gravity data for three-dimensional, mineral exploration relevant Earth models. The forward-modelling on which the inversion is based is a finite-difference solution of Poissons equation. The inversion is performed using a standard minimum-structure algorithm for multiple scenarios of varying borehole locations, amount of borehole data and varying model parameters. The intention is to demonstrate what we can expect to determine about the density variation around and between boreholes given varying amounts and locations of down-hole and surface data. It is observed that the benefits of borehole gravity data depend on the locations of the boreholes relative to the anomalous mass. Inversions which produce images of complex subsurface density distributions are attainable with the most successful models resulting from combined surface and borehole data. Minimum-structure borehole gravity inversion is shown to be a beneficial interpretation option which can provide accurate information of an anomalys shape with proper depth resolution and density distribution.

Physical Property Analysis, Numerical And Scale Modeling For Planning of Seismic Surveys: Voisey¿s Bay, Labrador

SUMMARY We present the key results from our analysis of the physical properties data-set from Voisey’s Bay, 2D pre-acquisition model-based studies, and initial results from our 3D modeling studies. The goal is to develop more effective and economical 2D and 3D land seismic techniques for mineral exploration in general, and for an intended campaign at Voisey’s Bay in particular.

IMPROVED DESCRIPTION AND MONITORING OF NEAR SURFACE HAZARDOUS INFILTRATE COMPLEXES BY SHEAR WAVES FOR EFFECTIVE CONTAINMENT REPONSE

Abstract Among numerous causes of fluid releases and infiltration in near surface, resurgence in such anthropic activities associated with unconventional resource developments have brought about a resounding concern. Apart from the risk of an immediate chemical hazard, a long term possible recurrent geo-environmental risk since can also be envisaged as for various prevalent stake holders and broader initiatives. Urgency and exactness for spatiotemporal containment and remediation promotes the devising of efficient methods for monitoring near subsurface flow complexes caused by such spills. Swave (Shear waves) spectral imaging results, in relevant context, of a controlled immiscible fluid displacement monitoring experimental study are analysed and inferred. Against the prospective method as well evaluated, Swave diffraction associated spectral peculiarities are examined, importantly, given background medium characteristics definitions invoking fresh insights of microscale significance alongside macroscale potential.