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The current model for the end-Permian terrestrial ecosystem crisis holds that systematic loss exhibited by an abrupt turnover from the Daptocephalus to the Lystrosaurus Assemblage Zone (AZ; Karoo Basin, South Africa) is time equivalent with the marine Permian–Triassic boundary (PTB). The marine event began at 251.941 ± 0.037 Ma, with the PTB placed at 251.902 ± 0.024 Ma (2σ). Radio-isotopic dates over this interval in the Karoo Basin were limited to one high resolution ash-fall deposit in the upper Daptocephalus AZ (253.48 ± 0.15 (2σ) Ma) with no similar age constraints for the overlying biozone. Here, we present the first U-Pb CA-ID-TIMS zircon age (252.24 ± 0.11 (2σ) Ma) from a pristine ash-fall deposit in the Karoo Lystrosaurus AZ. This date confirms that the lower exposures of the Lystrosaurus AZ are of latest Permian age and that the purported turnover in the basin preceded the end-Permian marine event by over 300 ka, thus refuting the previously used stratigraphic marker for terrestrial end-Permian extinction., The end-Permian is associated with major changes in both marine and terrestrial biodiversity. Here, Gastaldo et al. present high resolution dating of the Lystrosaurus Assemblage Zone in the Karoo Basin, South Africa, demonstrating that the marine crisis did not mirror a coeval event on land.

The initiation of intraslab earthquakes and their location depend on multiple factors such as slab heterogeneity, pre-subduction anisotropy, thermal parameters and the metamorphic history of the lithosphere slab. As a result, these failures occur over a range of temperature-pressure conditions through different mechanisms. Therefore, to unravel the intricacy of intraslab seismicity, it is crucial to investigate the details of failure mechanisms using multiple datasets, at different scales. Details on rupture mechanisms are now accessible through advanced, high-resolution seismologic approaches. However, seismologic archives are limited to events of the past few decades. Further, quasi-synchronous multiple-source mechanisms can be challenging to distinguish from seismic data only. Alternatively, the geologic records of intraslab seismicity, preserved in pseudotachylytes, provide a higher spatiotemporal resolution than seismic data, resulting in direct insight into the rupture triggering mechanisms.. These fault rocks also inform, over the geologic timescale, on the integrated deformation history of the slab. Despite this fact, only a handful of natural exposures have been investigated, and these include localities in Corsica, Italy, Zambia, and Japan. In addition, despite some strain rate limitations, deformation experiments, conducted using a wide range of starting materials and over broad pressure-temperature conditions, may further constrain the causes of intraslab rupture. These experiments, being intrinsically performed on relatively small rock volumes cannot capture large scale aspects of intraslab deformation. Finally, numerical modelling may overcome some of the above limitations as long as accurate and realistic parameters are chosen. Ultimately, our review shows that to fill the gap in our knowledge of intraslab seismicity, we need a multiscale-multimethod approach that integrates seismologic, geologic, experimental and numerical perspectives.

International audience; Fault pseudotachylytes form by frictional melting during seismic slip and therefore are widely interpreted as “earthquake fossils.” Rapid movement along a rupture surface typically forms a pseudotachylyte generation vein, the thickness of which increases with earthquake magnitude. The direction and sense of seismic slip cannot always be determined due to the generally complex geometry of pseudotachylyte veins. Here we show, for the first time, that the orientation of the magnetic fabric of fault pseudotachylytes indicates both direction and sense of seismic slip. The magnetic fabric, acquired in a manner similar to that of other magmas, arises in this case from the asymmetric preferred orientation of paramagnetic grains during viscous shear of the friction melt. This kinematic information, coupled with fault plane orientation and generation vein thickness, provides new and critical insight for the earthquake focal mechanism. The magnetic fabric of pseudotachylytes therefore not only constitutes a valuable kinematic criterion for these fault rocks, but also could expand our knowledge of prehistoric seismic events.

Fault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting. Here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine-grained magnetite. This confirms previous studies on friction melt experiments. Stoichiometric magnetite, produced during earthquakes by the breakdown of ferromagnesian silicates, records the ambient magnetic field during seismic slip. We find that most fault pseudotachylytes exposed in the Santa Rosa Mountains, southern California, a classic pseudotachylyte locality, acquired their natural remanent magnetization (NRM) upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization. NRM intensities typical of most pseudotachylyte veins range from 1 to 60·10−4 Am2/kg. A few specimens, however, contain magnetizations significantly higher than that caused by the Earth's field as well as magnetization directions that are highly variable over short distances. Other magnetization processes, possibly related to coseismic electric currents, may be involved during the seismogenic process to control NRM acquisition.

We report the ^(238)U-^(230)Th age derived from samples from three basalt flows from the Albuquerque Volcanoes Field, central New Mexico (USA), interpreted to have erupted during one of the Bruhnes Chron geomagnetic field polarity events. Whole-rock samples and magnetite separates from two of the samples define an isochron with an age of 156±29 ka (2σ). It is now possible to determine the ages of geomagnetic excursions with reasonable precision, by the ^(238)U-^(230)Th method, using basalts erupted between possibly 25 and up to 250 ka. Although a strict correlation of the polarity event recorded by the Albuquerque lavas with other high amplitude field phenomena in the latest Quaternary is not yet possible, the Blake and the less well-documented Old Crow and Biwa I/Jamaica events are likely candidates.