Abstract Orogenic gold systems are open, flow-controlled thermodynamic systems and generally occur in mid- to upper crustal environments where there is strong coupling between fluid flow and dilatant plastic deformation. This paper considers the principles involved in such coupling, with an emphasis on the elastic and plastic volume changes and their influence on the fluid, mechanical and thermodynamic pressures. Some misconceptions regarding the magnitudes of these three distinctly different pressures and their influences on fluid flow and chemical equilibrium are addressed, with examples at both the tens of metres scale and the crustal scale. We show that the mean stress is less than twice the lithostatic stress for Mohr–Coulomb materials with cohesion and the thermodynamic pressure only has meaning under isentropic conditions and hence is less than many previously published estimates based on high mean stresses. At the crustal scale, we also include the role of critical behaviour in influencing the geometry and magnitudes of fluid pressure gradients and fluid flow velocities in open, flow-controlled systems.

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 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: 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.

Abstract The 50 km (31 mi) long Hat Creek fault, located along the western margin of the Modoc Plateau in northern California, is a geometrically complex segmented normal fault that offsets Pleistocene lavas by at least 570 m (1870 ft) of cumulative throw. Three subparallel, ∼NNW-trending sets of scarps (Rim, Intermediate, and Recent) reflect a progressive westward migration of surface rupture locations that offset progressively younger Pleistocene volcanic deposits during a ∼1 Myr fault history. The 50 km (31 mi) long Rim scarp comprises predominantly right-stepping segments with a maximum throw of ∼370 m (1214 ft) in ∼925 ka lavas. The 17.5 km (10.9 mi) long Intermediate scarp occurs 0.4 to 3.5 km (0.2–2.2 mi) west of the Rim, comprising left-stepping segments with a maximum throw of ∼177 m (581 ft). The 30.5 km (19 mi) long Recent scarp occurs several tens of meters west of the bases of older scarps, and is composed of left-stepping segments with a maximum throw of 56 m (184 ft). The northernmost segment of the Recent scarp offsets 53.5 ± 2 ka basaltic lavas, whereas the remaining segments offset 24 ± 6 ka basalt flows that erupted into Hat Creek Valley, indicating a youthful scarp system. Vertical propagation of the fault through young lavas produced fault-trace monoclines with amplitudes of up to 30 m (98 ft). The monoclines are commonly breached along their upper hinges by a vertical, dilational fault scarp. Shaking associated with repeated earthquakes progressively broke down these monoclines, causing disaggregation or partial to complete collapse. Fracture patterns and fault segment geometries and linkages were used to deduce the kinematic and stress history. The oldest segments of the Rim and Intermediate systems suggest initial NE-SW to ENE-WSW extension. Later Rim, Intermediate, and Recent segments responded to E-W extension, consistent with the previously documented stress state of the Cascades backarc. Complexity in Intermediate and Recent fault segments near a small shield volcano (Cinder Butte) suggests spatial variability in the stress field caused by a currently dormant magmatic system. Evidence for recent dextral-oblique kinematics along the Recent scarp, implying a slightly WNW-ESE extension, may reflect the transfer of dextral shear into the system from the Walker Lane Belt in western Nevada. Our interpretations require ∼45° of clockwise rotation of the horizontal principal stresses in the vicinity of the Hat Creek fault over the past ∼1 Myr, implying that significant complexity can develop in segmented normal fault systems over relatively short periods of geologic time.