Erin L. Matchan

The Turkana Basin in NW Kenya and SW Ethiopia hosts remarkable fossil-rich sediments that are central to our understanding of early hominin evolution, with interbedded volcanic tuffs providing critical time markers. However, the resolution of existing Early Pleistocene–Pliocene ages is limited to c. 20–60 kyr, inhibiting the evaluation of climatic and environmental drivers of evolution. We present high-precision, single-feldspar ⁴⁰ Ar/ ³⁹ Ar age and elemental data for four stratigraphically significant tuffs. These samples exhibit variably dispersed age distributions correlated with feldspar compositional trends, interpreted to indicate the partial retention of inherited ⁴⁰ Ar related to crustal ‘cold storage’ and rapid melt infiltration preceding eruption. We evaluated various statistical methods and calculated astronomically calibrated Bayesian age estimates of 1879.1 ± 0.6 ka (±2.4 ka including external errors) for the Kay Behrensmeyer Site (KBS)/H2 Tuff, 1837.4 ± 0.9 ka (±2.4 ka) for the Malbe/H4 Tuff, 1357.5 ± 1.8 ka (±2.5 ka) for the Chari/L Tuff and 1315.4 ± 1.9 ka (±2.5 ka) for the Gele Tuff. Our results permit refined age constraints for important early Homo fossils, including the cranium KNM-ER1813 ( Homo habilis ) and various Homo erectus fossils. The KBS Tuff age also provides an important calibration locus for orbital tuning of palaeoclimate proxy records, revealing the complex interplays between palaeoclimate and geological drivers of sedimentation. Supplementary material : The supplementary tables and figures include previously published ages (Table S1), sample and irradiation information (Table S2), electron probe microanalytical results for tuff glasses (Table S3, Figs S2 and S3) and feldspars (Table S4), ⁴⁰ Ar/ ³⁹ Ar analytical data (Tables S5 and S6, Fig. S4), orbital tuning model parameters (Table S7), a summary of previous and revised hominin ages (Table S8) and a worked Bayesian estimation example (Fig. S1) and are available at https://doi.org/10.6084/m9.figshare.c.6602994

Field observations and unmanned aerial vehicle surveys from Caldera Taburiente (La Palma, Canary Islands, Spain) show that pre‐existing dykes can capture and re‐direct younger ones to form multiple dyke composites. Chill margins suggest that the older dykes were solidified and cooled when this occurred. In one multiple dyke example, an ⁴⁰Ar/³⁹Ar age difference of 200 kyr was determined between co‐located dykes. Petrography and geomechanical measurements (ultrasonic pulse and Brazilian disc tests) show that a microscopic preferred alignment of plagioclase laths and sheet‐like structures formed by non‐randomly distributed vesicles give the solidified dykes anisotropic elastic moduli and fracture toughness. We hypothesize that this anisotropy led to the development of margin‐parallel joints within the dykes, during subsequent volcanic loading. Finite element models also suggest that the elastic contrast between solidified dykes and their host rock elevated and re‐oriented the stresses that governed subsequent dyke propagation. Thus, the margin‐parallel joints, combined with local concentration and rotation of stresses, favored the deflection of subsequent magma‐filled fractures by up to 60° to form the multiple dykes. At the edifice scale, the capture and deflection of active intrusions by older ones could change the organization of volcanic magma plumbing systems and cause unexpected propagation paths relative to the regional stress. We suggest that reactivation of older dykes by this mechanism gives the volcanic edifice a structural memory of past stress states, potentially encouraging the re‐use of older vents and deflecting intrusions along volcanic rift zones or toward shallow magma reservoirs.