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Throughout her career, Professor Sharon Mosher has been a pioneer in the structural analysis of polydeformed rocks and regions. Her work on the evolution of superposed rock fabrics in complexly deformed areas, for example, has greatly improved our ability to determine how faults, shear zones, and orogens evolve over time. Traditionally, sequences of foliations, mineral lineations, folds, and other structural elements have been interpreted in terms of discrete, multiphase deformation events. However, alternative interpretations where structural sequences result from a single, progressive event also are common, especially where changes in stress fields or flow parameters result in non-steady deformation. Here, in honor of Professor Mosher, we present examples of three different types of structural sequences that formed in large seismogenic faults and shear zones in SW New Zealand and southern California. These examples illustrate the different ways in which multiple generations and styles of rock fabrics develop and become preserved in zones of localized deformation. The first example is from a large fault zone located inboard of the Puysegur subduction zone in Fiordland, New Zealand. This zone displays several generations of superposed fabrics that record a history of repeated reactivations over a few tens of millions of years. A second set of examples, from both Fiordland and southern California, illustrates how non-steady deformation can result in parallel ductile and brittle fabrics, including veins of pseudotachylyte, that formed during a single, progressive shearing event. The third example, also from Fiordland, shows how parallel rock fabrics in a large, lower crustal shear zone formed diachronously across a large region as the inboard and outboard belts of the Mesozoic Median batholith converged. Each of these examples displays different structural relationships among rock fabrics in the field. To decipher their histories, we combined structural data with 40Ar/39Ar and U-Pb (zircon, titanite) geochronology. The examples illustrate the utility of combining field observations with both direct and indirect isotopic 

Many large fault zones record multiple reactivations that can be difficult to resolve and interpret in the field. Here, we use examples from Vermont and New Zealand to illustrate how structural data combined with 40Ar/39Ar geochronology can be used to reconstruct fault reactivation histories and interpret their possible origins. In SW New Zealand, the Spey-Mica Burn fault zone parallels a transpressive boundary between the Pacific and Australian plates. Integrated structural and 40Ar/39Ar data obtained from pseudotachylyte, mylonite, and other fault rocks allow us to distinguish successive phases of faulting (i.e., reactivations) from cases where different styles of brittle and ductile deformation occurred simultaneously (or nearly so) in the fault zone. Apparent age spectra from multiple minerals show age gradients that reveal four reactivations spanning ~20 Ma. The style and timing of these events correlate well to times of increased convergence rate and collisions between oceanic ridge segments and a nearby trench. Fault zones in NW Vermont also record different styles of reactivation. The Hinesburg Thrust (HT), which juxtaposes Late Proterozoic-Early Cambrian rift clastic rocks against Ordovician carbonate rocks of the Champlain Valley belt, includes a ~30 m thick zone of mylonite that is cut by a cataclastic fault and deformed by folds. 40Ar/39Ar data suggest the mylonite formed during the Ordovician Taconic orogeny and later was folded into a series of domes and basins during the Late Silurian-Devonian Acadian orogeny. Farther west, the Champlain thrust fault (CT) juxtaposes Cambrian dolostones against Ordovician calcareous shales. Superposed faults within the foot wall of the CT show a progressive change in movement direction from W-directed thrusting, to NW-directed thrusting, to N-S slip, and NE-SW slip. These changing slip directions appear to reflect wholly Taconic motion along a north-dipping lateral ramp between Burlington and Shelburne where the CT cuts up section to the south. Acadian reactivation of the CT appears restricted to late folding similar to the HT. These examples highlight the utility of combining structural data with 40Ar/39Ar geochronology to unravel slip histories in continental fault zones and to distinguish among the different styles and origins of fault reactivation. 

Abstract The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ∼0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity. 

The lower crustal domain of the Ivrea‐Verbano Zone (NW Italy) hosts five ~300‐m‐wide pipe‐like ultramafic intrusions that are metasomatized and exhibit Ni‐Cu‐PGE sulphide mineralization. To better constrain the role of metasomatism in the ore genesis, we studied the best‐preserved pipe at Valmaggia which was emplaced 249 Myrs ago. Phlogopite⁴⁰Ar/³⁹Ar analyses show that the pipe was infiltrated by metasomatic fluids derived from the subcontinental lithospheric mantle (SCLM) in two pulses at ~208 Ma and ~189 Ma which introduced sulphides into the pipe. Consequently, the pipe repeatedly acted as a preferred path for mass transfer from the SCLM into the lower crust over >60 Myrs (i.e., emplacement to second metasomatic pulse). Uplifted block margins, such as the Ivrea‐Verbano Zone, are potentially important exploration targets for magmatic sulphides. We argue that exploration strategies should focus on structures such as pipes that can focus metasomatic agents during ascent through the lithosphere.