Abstract. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The high spatial resolution of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional isotope dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-millimetre scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and “hit or miss”. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; our examples are drawn from the dating of fracture-filling calcite, but our discussion is relevant to all carbonate applications. We review several limitations to the method, including open-system behaviour, variable initial-lead compositions, and U–daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging and image-based dating that utilises LA-ICP-MS elemental and isotopic map data.

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the onset of fluid-assisted, grain size-sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal-scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal-scale faults containing interconnected networks of phyllosilicate-bearing fault rocks will be characterized by long-term relative weakness and shallow (-5 km)

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Repeated reactivation of structures and reworking of crustal volumes are characteristic, though not ubiquitous, features of continental deformation. Reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long-lived fault zones at different palaeo-depths. Ancient basement fault systems typically comprise heterogeneous, superimposed assemblages of fault rocks formed at different times and depths for which down-temperature thermal histories are most common. Several lithological and environmental factors influence the evolution of fault rock fabrics and rheology, but most fault/shear zone arrays appear to develop as self-organized deformation systems. Once mature, the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault/shear zone network.A case study from the crustal-scale Great Glen Fault Zone (GGFZ), Scotland, reveals a complex evolution of mid- to upper-crustal deformation textures formed adjacent to the frictional-viscous transition. Fluid influx in the mid-crust has led to reaction softening of the rock aggregate as strong pre-existing phases such as feldspar are replaced by fine-grained, strongly aligned aggregates of weak phyllosilicates. In addition, a grainsize-controlled switch to fluid-assisted diffusional creep occurs in the highest strain regions of the fault zone. It is proposed that this led to a shallowing and narrowing of the frictional-viscous transition and to long-term overall weakening of the fault zone relative to the surrounding wall-rocks. Cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime, by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain-scale dilatancy. Equivalent processes are likely to occur along many other long-lived, crustal-scale fault zones.

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Continental breakup between Greenland and North America produced the small oceanic basins of the Labrador Sea and Baffin Bay, which are connected via the Davis Strait, a region mostly comprised of continental crust. This study contributes to the debate regarding the role of pre‐existing structures on rift development in this region using seismic reflection data from the Davis Strait data to produce a series of seismic surfaces, isochrons and a new offshore fault map from which three normal fault sets were identified as (i) NE‐SW, (ii) NNW‐SSE and (iii) NW‐SE. These results were then integrated with plate reconstructions and onshore structural data allowing us to build a two‐stage conceptual model for the offshore fault evolution in which basin formation was primarily controlled by rejuvenation of various types of pre‐existing structures. During the first phase of rifting between at least Chron 27 (ca. 62 Ma; Palaeocene), but potentially earlier, and Chron 24 (ca. 54 Ma; Eocene) faulting was primarily controlled by pre‐existing structures with oblique normal reactivation of both the NE‐SW and NW‐SE structural sets in addition to possible normal reactivation of the NNW‐SSE structural set. In the second rifting stage between Chron 24 (ca. 54 Ma; Eocene) and Chron 13 (ca. 35 Ma; Oligocene), the sinistral Ungava transform fault system developed due to the lateral offset between the Labrador Sea and Baffin Bay. This lateral offset was established in the first rift stage possibly due to the presence of the Nagssugtoqidian and Torngat terranes being less susceptible to rift propagation. Without the influence of pre‐existing structures the manifestation of deformation cannot be easily explained during either of the rifting phases. Although basement control diminished into the post‐rift, the syn‐rift basins from both rift stages continued to influence the location of sedimentation possibly due to differential compaction effects. Variable lithospheric strength through the rifting cycle may provide an explanation for the observed diminishing role of basement structures through time.