Abstract Epithermal veins and breccias at the Kainantu gold–copper deposit in Papua New Guinea, host gold mineralization in NW–SE steeply dipping lodes. The lodes are parallel to a pre-mineralization dextral strike-slip shear-zone network, which is itself parallel in places to an early greenschist-facies cleavage in basement schists. The cleavage, shear zone and veins are all cut by dextral strike-slip faults. High Au grades correlate with areas of obliquity between the shear-zone fabrics and the cleavage, and plunge at approximately 40° SE in the plane of the lodes – coincident with minor fold axes related to a crenulation cleavage in the basement rocks. This clear structural history shows that gold mineralization was confined to a particular late structural event, but lode geometry was influenced by all previous structures, as well as being displaced by post-mineralization faulting. The north–south shortening recorded through most of the tectonic history can be related to Tertiary convergence along the major plate boundary located approximately 15 km north of the mine. However, mineralization occurred under a different tectonic regime from the current north–south convergence, when there was a change of tectonics between 9 and 6 Ma, possibly related to delamination.

Abstract The distribution of fault rocks interpreted across a modelled fault in oil or gas reservoirs is most often described by its clay content derived from standard industry algorithms such as shale gouge ratio and clay smear factor. These distributions are below the mapping resolution in seismic data, and the actual processes and mechanisms for the fault-rock development are not well understood. A well-exposed and well-preserved low-throw fault in an old railroad tunnel near Salina, Utah provides the access and scale to interpret fault-rock distributions, measure their clay contents and describe the fault-rock development. A significant number of fault-rock elemental compositions were measured quickly on the outcrop surface using a hand-held X-ray fluorescence (XRF) elemental analyser with a novel surface preparation and analysis strategy. The elemental data were converted to clay contents using a small set of samples where elemental composition was calibrated to X-ray diffraction mineralogy. The mineralogy data provide a basis for evaluating the degree of mixing of protolith beds during fault-rock development in the fault core. The fault core is not randomly mixed fragments derived from the protolith (gouge or breccia), but rather discrete, thin layers parallel to the fault surface, many of which can be traced back to a source sandstone or mudstone bed. The mineralogical composition of some fault-rock layers are unchanged from their protolith source bed, but other layers are mechanical mixtures of several source beds. The shale gouge ratio algorithm under-represents the average measured fault-rock clay content. The clay smear algorithm more accurately describes the clay content distribution, but underestimates the clay content heterogeneity along the smear length. A key uncertainty for predicting fault sealing remains prediction of the lengths and continuity of smears.

Abstract: The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting. Several of the reactivated major Devonian extensional structures can be extrapolated offshore into the rift, where they appear as bands of dipping reflectors. They coincide with large-scale boundaries separating 50–100 km-wide rift domains of internally uniform fault patterns. Major north–south-trending rift faults, such as the Øygarden Fault System, bend or terminate against these boundaries, clearly influenced by their presence during rifting. Hence, the North Sea is one of several examples where pre-rift basement structures oblique to the rift extension direction can significantly influence rift architecture, even if most of the rift faults are newly-formed structures.

Abstract: Field analysis shows that fault cores of brittle, extensional faults at a medium to mature stage of development are commonly dominated by lozenge-shaped horses (fault-core lenses) characterized by a variety of lithologies, including intact, mildly to strongly deformed country rock derived from the footwalls and hanging walls, various types of fault rocks of the protocatalasite and breccia series, breccia, fault gouge and clay smear. The lenses are sometimes stacked to form complex duplexes. These structures are commonly separated by high-strain zones of sheared cataclasite, and/or clay smear/clay gouge. The geometry and distribution of clay gouge in high-strain zones sometimes display evidence of intrusion, indicating high fluid pressure. Although the sizes of the horses vary over several orders of magnitude, they frequently display a length:thickness (a:c) ratio of between 1:4 and 1:15. The high-strain zones of fault rocks commonly constitute unbroken, 3D membranes that are likely to constrain fluid communication both across and along the fault zone. There are significant contrasts in fault core architecture that are probably related to processes associated with contrasting fluid pressure, strain intensity and strain hardening/strain softening. Faults associated with strain softening are characterized by less abundant brittle deformation products and are less likely to be conduits for fluid flow compared to those that are affected by strain hardening.