This paper addresses one of the critical questions of scientific inquiry: How do we know when a given data set is representative of the phenomenon being examined? For paleomagnetists, the question is often whether a particular dataset sufficiently averaged paleosecular variation (PSV). To this aim, we updated an existing PSV dataset that now comprises 2441 site mean directions from 94 individual studies (PSV10-24). Minimal filtering for data quality resulted in 1619 sites from 90 publications. Fitting PSV10-24 with three newly defined parameters as well as two existing ones form the basis of a Giant Gaussian Process field model (THG24) consistent with the data. Drawing directions from THG24 yields directional distributions predicted for a given latitude allowing a comparison between empirical distributions and the cumulative distribution function generated by the model. This tests whether the observed data adequately averaged out PSV according to THG24. Sedimentary datasets that may have experienced inclination shallowing can be corrected using an (un)flattening factor that yields directions satisfying THG24 in a newly-defined, four-parameter space. This approach builds on the Elongation-Inclination (E/I) method of Tauxe and Kent (2004), so the approach introduced here is called SVEI. We show examples of the use of SVEI and explain how to use this newly developed python code that is publicly available in the PmagPy GitHub repository.

The early Miocene unconformity (EMU) occurred during a transition in the tectonic evolution of western North America that impacted faunal diversification. The Railroad Canyon section in the northern Rocky Mountains provides a complete geologic record spanning this period. Our new magnetostratigraphic study in combination with published U-Pb ages from intercalated ash places the end of the EMU at ~20.1 Ma with a duration up to 1.5 Myr. The EMU is marked by an abrupt change in rock color, increase in magnetite concentration, and decrease in calcite abundance. The sudden changes in mineralogy reflect a reorganization in sediment source. Curiously, sedimentation rate remains constant at 6.5±0.1 cm/kyr through the EMU, as do climate proxies. Active uplift as well as drainage network reorganization seemingly contributed to EMU formation, possibly related to a change in tectonic regime such as the arrival of the Yellowstone plume and/or the onset of Basin and Range extension.

Micron to sub-micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid-Atlantic Ridge at 11-17N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite and Pericline) with the bulk magnetic anisotropy of the inclusion-host assembly. In unaltered plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions from hydrothermal alteration shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.

We report palaeomagnetic and K-Ar geochronologic results of two volcanic sections from Ethiopia. One section, dated around 29-30 Ma and spanning ~1 km in thickness, is related to the Oligocene Afro-Arabian traps. The second ~700-m-thick section was emplaced during the Miocene in two pulses around 10-11 and 14-15 Ma. We sampled 67 flows (550 cores) of predominantly basaltic rocks at the Oligocene section and 59 rhyolitic to trachybasaltic flows (500 cores) at the Miocene section. The Oligocene section was correlated to subchrons C11r to C11n.1n, with an average emplacement rate of 1m/kyr and instantaneous rates increasing with time from ~0.5 m/kyr near the base to ~1.36 m/kyr towards the top. We combined our results to the available paleomagnetic studies for the Early Oligocene (N = 4; 167 sites), Middle Miocene (N = 2; 125 sites) and Plio-Pleistocene (N = 8; 249 sites) to better understand how geomagnetic secular variation changed through time in the Afro-Arabian region. Recentred directional distributions for all three periods are elongated in the meridian plane (e = 2.7 ± 0.4, 1.8 ± 0.6, 2.3 ± 0.4, respectively), in coherence with field models for a dipole-dominated field. The angular dispersion S of the virtual geomagnetic poles, representative of the vigour of the palaeosecular variation, was higher during the Early Oligocene (S=14.2°|13.2°15.4°) and the Middle Miocene (S=15.0°|13.8°16.5°) than during the Plio-Pleistocene (S=9.7°|9.0°10.5°). As the reversal frequency f during the Early Oligocene is half that for the Plio-Pleistocene, it appears that S and f are uncorrelated in this near-equatorial region.