The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world's oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present (40)Ar-(39)Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale.

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Six tuffaceous beds within the Omo Group of the Omo–Turkana Basin have been dated using the 40 Ar/ 39 Ar single crystal total fusion method on anorthoclase, yielding eruption ages. The Omo Group constitutes up to 800 m of subaerially exposed sediments surrounding Lake Turkana within the East African Rift system in northern Kenya and southern Ethiopia. Rhyolitic explosive eruptions produced tuffs and pumice clasts that are considered to have been deposited shortly after eruption. The new age data on feldspars from the pumice clasts range from 4.02 ± 0.04 Ma for the Naibar Tuff of the Koobi Fora Formation to 1.53 ± 0.02 Ma for Tuff K of the Shungura Formation. The Orange Tuff in the KBS Member of the Koobi Fora Formation was dated at 1.76 ± 0.03 Ma, providing good control in this part of the sequence where formerly there was a >200 ka gap. Data are consistent with earlier measurements and significantly improve age resolution within the Omo Group, which has yielded many vertebrate fossils, including hominin fossils comprising a number of species. We suggest new age estimates for a limited number of hominin specimens. Supplementary material: Eleven tables and nine figures are available at www.geolsoc.org.uk/SUP18506 .

The metamorphic history of martian meteorite NWA 7034 suggests that the martian crustal dichotomy may have formed within 100 million years of planetary formation.