Abstract A classical Upper Jurassic fault block in the North Sea, the Fulla Structure, has Brent Group sandstones with good reservoir quality and apparently insignificant fault-related reservoir damage. Core data show high-porous sandstones that extend close to the main faults and there is no evidence of catalase, only of soft-sedimentary deformation. Shear bands are relatively thin with high offsets, and have a texture comparable to the wall rock. To investigate the deformation mechanism and products synthetic Brent Group sands are deformed in a triaxial plane strain box with pre-defined effective consolidation in the range of 100–8000 kPa, simulating a burial depth in the range of 10–800 m. This range covers the burial depth at the time of active faulting for most Jurassic traps in the North Sea, including the Fulla Structure. The experiments demonstrate that grain rolling and grain-boundary sliding are the dominant deformation mechanisms at all the simulated burial depths, and this deformation has no impact on the reservoir quality. The experiments concur with observations from the investigated wells and strengthen an interpretation of limited reservoir damage associated with the Late Jurassic fault activity.

Abstract We perform an integrated analysis of magnetic anomalies, multichannel seismic and wide-angle seismic data across an Early Cretaceous continental large igneous province in the northern Barents Sea region. Our data show that the high-frequency and high-amplitude magnetic anomalies in this region are spatially correlated with dykes and sills observed onshore. The dykes are grouped into two conjugate swarms striking oblique to the northern Barents Sea passive margin in the regions of eastern Svalbard and Franz Josef Land, respectively. The multichannel seismic data east of Svalbard and south of Franz Josef Land indicate the presence of sills at different stratigraphic levels. The most abundant population of sills is observed in the Triassic successions of the East Barents Sea Basin. We observe near-vertical seismic column-like anomalies that cut across the entire sedimentary cover. We interpret these structures as magmatic feeder channels or dykes. In addition, the compressional seismic velocity model locally indicates near-vertical, positive finger-shaped velocity anomalies (10–15 km wide) that extend to mid-crustal depths (15–20 km) and possibly deeper. The crustal structure does not include magmatic underplating and shows no regional crustal thinning, suggesting a localized (dyking, channelized flow) rather than a pervasive mode of magma emplacement. We suggest that most of the crustal extension was taken up by brittle–plastic dilatation in shear bands. We interpret the geometry of dykes in the horizontal plane in terms of the palaeo-stress regime using a model of a thick elastoplastic plate containing a circular hole (at the plume location) and subject to combined pure shear and pressure loads. The geometry of dykes in the northern Barents Sea and Arctic Canada can be predicted by the pattern of dilatant plastic shear bands obtained in our numerical experiments assuming boundary conditions consistent with a combination of extension in the Amerasia Basin sub-parallel to the northern Barents Sea margin and a mild compression nearly orthogonal to the margin. The approach has implications for palaeo-stress analysis using the geometry of dyke swarms. Supplementary material: Details on traveltime tomography model: Resolution tests, traveltime information and ray coverage are available at https://doi.org/10.6084/m9.figshare.c.3783542

Abstract: Analyses of normal faults in mechanically layered strata reveal that material properties of rock layers strongly influence fault nucleation points, fault extent (trace length), failure mode (shear v. hybrid), fault geometry (e.g. refraction through mechanical layers), displacement gradient (and potential for fault tip folding), displacement partitioning (e.g. synthetic dip, synthetic faulting, fault core displacement), fault core and damage zone width, and fault zone deformation processes. These detailed investigations are progressively dispelling some common myths about normal faulting held by industry geologists, for example: (i) that faults tend to be linear in dip profile; (ii) that imbricate normal faults initiate due to sliding on low-angle detachments; (iii) that friction causes fault-related folds (so-called normal drag); (iv) that self-similar fault zone widening is a direct function of fault displacement; and (v) that faults are not dilational features and/or important sources of permeability.

Pseudotachylitic breccias are the most prominent impact-induced deformation phenomenon in the Vredefort Dome, the eroded central uplift of the 2.02 Ga, originally 250-km-wide Vredefort impact structure in South Africa. Controversy remains about the origin of these melt breccias, and the most popular hypotheses are genesis by (1) shearing (friction melting), (2) shock compression melting, (3) decompression melting immediately after shock propagation through the target or slightly later during the modification phase of cratering, (4) combinations of these processes, or (5) intrusion of allochthonous impact melt. A resolution to this problem requires detailed multidisciplinary analysis in order to characterize the nature of different occurrences of such breccias with the aim of identifying the melt-forming process. Past work has focused mainly on orientation and geometry of Vredefort pseudotachylitic breccia veins, besides a few whole-rock geochemical investigations of mostly decimeter- to tens of meter-sized occurrences, whereas detailed geometric and micro-chemical analysis has not yet been adequately related to microdeformation studies of such melt breccias. Here, we report the results of detailed microchemical analyses of small- to meso-scale pseudotachylitic breccias in a polished 3 × 1.5 m granite slab from a dimension stone quarry in the western core of the Vredefort Dome, supplemented by data for samples from the Rand Granite Quarry in the northern sector of the core. The veinlets selected for analysis do not provide textural evidence for shearing/faulting. Electron microprobe analysis of pseudotachylitic breccia groundmass and X-ray fluorescence bulk chemical analysis of both pseudotachylitic breccias and their host rocks reveal that pseudotachylitic breccia commonly displays a close chemical relationship to its direct wall rock. If groundmass compositions are corrected for the inherent microclast content, correspondence of breccia groundmass and immediate host rock composition is further enhanced. For small veinlets (<1 mm width), melting appears to have occurred locally, with compositions of melt and immediately adjacent host rock minerals commonly being identical. It is, thus, suggested that larger breccia zones could be sites of pooling of melt generated in places throughout the wider environs of dilational sites. For millimeter-scale veinlets, local melt formation and also a lack of lateral mixing are indicated. In contrast, pseudotachylitic breccia veinlets <1 mm, and quite possibly also some larger veins, could conceivably have formed by shock or decompression melting. As previous shock experimental work has demonstrated, this local melting could have been accomplished with or without a friction component.

A well-defined axis of maximum dilation within the ca. 148 Ma Independence dike swarm is significantly offset across Owens Valley. Dilation by diking within the axis of maximum dilation is greater than 5%, commonly exceeds 10%, and locally ranges over 40%. Elsewhere in the swarm, dilation rarely ranges above 2%. The axis of maximum dilation steps ∼75–130 km rightward across Owens Valley, although the offset is difficult to measure precisely because the dike swarm is diffuse and intersects the valley at a relatively low angle. Comparison with other recently investigated geologic markers favors 65 ± 5 km of dextral offset since 83.5 Ma and perhaps an additional 10–65 km of offset prior to 83.5 Ma. Although Owens Valley is a locus of modern dextral slip, regional relations suggest that most of the 65 km of dextral displacement accumulated in Latest Cretaceous–early Paleogene (Laramide) time, when Cordilleran subduction was strongly right-oblique. Thermobarometric, structural, stratigraphic, and geochronologic evidence from the southern Sierra Nevada have previously been interpreted to reflect south-directed tectonic unroofing of deep-crustal rocks to form a metamorphic core complex during Laramide time. Large-magnitude Laramide right slip across Owens Valley thus may have been transferred southward into extension in the southern Sierra Nevada. Linked systems of late-Laramide to post-Laramide strike-slip faults and metamorphic core complexes have long been recognized in the plutonic-metamorphic core of the northern Cordillera. Recognition of this tectonic style in California suggests that it may have characterized most of the western Cordilleran orogen at this time.

The Wind River Mountain Range is one of the most prominent Laramide uplifts of the Rocky Mountains deformed foreland. Precambrian rocks in the core of the Wind River anticline were deformed inhomogeneously along a three-dimensional network of brittle and brittle-ductile deformation zones, which are present at all scales, allowing shortening of the basement core by an amount equivalent to that of the Phanerozoic cover. Along the northeast flank of the anticline where the cover sequence was thin (< 5 km), the underlying basement was deformed under brittle conditions, forming an anastomosing network of brittle deformation zones. The Torrey Creek zone is one such large brittle zone that is well exposed. It probably grew into a major zone because of its location between two different lithologies. Shearing within the zone occurred by distributed slip on numerous gently-dipping conjugate fractures with unidirectional slip. The zone has a complex deformation history of successive phases of shearing (involving grain-size reduction by stable and unstable fracturing, gouging, and wear) and dilation (involving vein precipitation) that allowed it to grow in thickness with increasing displacement. Both the development of and sliding on a network of such deformation zones require a significant amount of energy, and are important components of the total deformation in the Wind River Mountains.

The central Andes of northern Chile and northwestern Argentina developed in a largely autochthonous, intracontinental setting during Proterozoic and Paleozoic time through a recurrent sequence of extensional and compressional tectonic regimes. The exposed pre-Andean crust consists of tectonically isolated outcrops of: (1) metamorphosed basement, (2) plutonic bodies, and (3) slightly metamorphosed volcanic and sedimentary strata resting upon and intruded by multiple plutonic units. U-Pb and Nd-Sm geochronology indicates the existence of a Precambrian foundation for the central Andes dating from at least middle Proterozoic time. Three episodes of Precambrian/Paleozoic deformation/magmatism are recognized: (1) the Panamerican at the Precambrian/Cambrian transition, (2) the Caledonian at the Ordovician/Silurian boundary, and (3) the Variscan during the Carboniferous. Intrusive units can be grouped into synorogenic (S-type) and anorogenic (A-type) rocks based on structural, petrographic, and geochemical relations. Mafic lavas of middle Proterozoic to lower Carboniferous age likely were generated during episodes of crustal dilatation.