Florian Duval’s research while affiliated with French National Centre for Scientific Research and other places

The East African Rift is one of Earth’s largest continental landforms. It is recognized as a critical region for understanding hominin evolution yet has also undergone important transformation through ongoing tectonic and volcanic activity. An understanding of the interplay of rift kinematics, magma genesis and geomorphic evolution requires firm geochronology but this has been lacking for much of the East African Rift. Here we present detailed stratigraphic observations and high-precision ⁴⁰Ar/³⁹Ar ages for major volcanic units in the Central Main Ethiopian Rift. Our new data identify a volumetrically major episode of explosive volcanism between circa 3.85–3.42 million years ago, after aproximately 5-million years-long quiescence. Four other pulses followed but with intensity and magnitude declining over time. We suggest that the observed temporal clustering and the pulsatory volcanic activity may have influenced environmental conditions in the area, with possible implications for hominin evolution.

Using target-matching techniques combining 40Ar/39Ar crystal-mapping with elemental mapping and high-resolution electron microscopy, this study investigates the 40Ar behavior in very-slowly cooled muscovite from the Harney Peak Granite (HPG, South Dakota, USA). Detailed age mapping along (001) in single crystals from different localities of the HPG documents age gradients in excess of ∼ 300-400 m.y., with conspicuous internal 40Ar/39Ar zoning. This suggests (001) layer-parallel 40Ar transport driven by diffusion, consistent with previous 40Ar/39Ar crystal-mapping studies. The age distribution pattern is complex, however, and defines a mosaic of sub-grain domains with more retentive core zones, broadly ∼ 250-300 μm across, separated by zones of high diffusivity varying in shape and extent. The maximum ages preserved in the core domains are independent of their size but vary linearly with the bulk areal extent of the peripheral (or surrounding) high-diffusivity zones. Spatial 40Ar/39Ar relationships inside each grain point to a mechanism of multipath continuum-diffusion interaction between subdomains across the whole crystal, rather than via discrete non-interracting domains such as in K-feldspars. A close spatial correlation exists between younger ages, Na-depleted (K-enriched) zones, and density of microstructural defects. These defects, identified as lenticular voids and basal partings (< 100 nm-long), are developed in response to inward K ↔ Na interdiffusion during late-magmatic stages, in the absence of deformation. Coupled variations in density of microstructural defects and Na-K interchange are inferred to control the bulk diffusion-domain structure of HPG muscovite. Quantitative diffusion modeling of coupled compositional-defect-isotopic variations indicates that 40Ar diffusivity may be enhanced by up to six orders of magnitude in defect-controlled high-diffusivity zones relative to less defective (pristine) domains. On the other hand, empirical diffusivity estimates required to preserve the core ages are commensurate with diffusion estimates independently derived from recent atomistic simulations.

Widespread overprinting of early high‐pressure/low‐temperature (HP/LT) subduction stages due to subsequent collisional or late‐orogenic tectono‐metamorphic events is a common feature affecting the interpretation of geochronologic data from HP/LT orogens. The Betic‐Rif orogen is exemplary in this connection as a great majority of published radiometric ages are found to cluster around 20 Ma. This clustering is commonly interpreted as reflecting a short, yet complex, succession of tectono‐metamorphic events spanning only over a few Myr, including back‐arc extension and overthrusting of the Internal Zones on the External Zones. An alternative explanation consists in the poor preservation of a much earlier HP/LT metamorphic event, presumably Eocene, coeval with subduction and crustal thickening in the Internal Zones, and particularly the Alpujárride Complex. However, this age is vividly debated due to widespread resetting by the Early Miocene HT/LP overprint. In this study, we provide new ⁴⁰Ar/³⁹Ar evidence from white micas selected along an E‐W section of the Internal Betics, from the central to the eastern Alpujárride Complex. Our new data show (a) that exceptionally well‐preserved HP/LT parageneses in this unit retain a well‐defined Eocene age around 38 Ma, and (b) that widespread 20 Ma ages recorded all along the section correspond to a regional stage of exhumation, coeval with a major change in the kinematics of back‐arc extension. Our study provides conclusive evidence that ⁴⁰Ar/³⁹Ar dating of carefully targeted HP/LT associations can overcome the problem of extensive late‐orogenic overprinting, testifying for an Eocene HP event around 38 Ma in the Betic‐Rif orogen.

The relationships between volcanic activity and tectonics at the southernmost termination of the Main Ethiopian Rift (MER), East Africa, still represent a debated problem in the MER evolution. New constraints on the timing, evolution and characteristics of the poorly documented volcanic activity of the Dilo and Mega volcanic fields (VF), near the Kenya-Ethiopia border are here presented and discussed. The new data delineate the occurrence of two distinct groups of volcanic rocks: 1) Pliocene subalkaline basalts, observed only in the Dilo VF, forming a lava basement faulted during a significant rifting phase; 2) Quaternary alkaline basalts, occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift-related faults. ⁴⁰Ar/³⁹Ar dating constrains the emplacement time of the large basal lava plateau to ~3.7 Ma, whereas the youngest volcanic activity characterising the two areas dates back to 134 ka (Dilo VF) to as recent as the Holocene (Mega VF). Volcanic activity developed along tectonic lineaments independent from those of the rift. No direct relations are observed between the Pliocene, roughly N-S-trending major boundary faults of the Ririba rift and the NE-SW-oriented structural trend characteristic of the Quaternary volcanic activity. We speculate that this change in structural trend may be the expression of (1) inherited crustal structures affecting the distribution of the recent volcanic vents, and (2) a local stress field controlled by differences in crustal thickness, following a major episode of reorganization of extensional structures in the region due to rift propagation and abandonment.

New detailed structural data from the Fuegian Andes including new ages and cross-cutting relationships with intrusive rocks, as well as an appraisal of published structural data, support that this orogen evolved as a basement-involved thrust-fold belt after initial formation in an arc-continent collision scenario. New structural data from a deformed 84 Ma intrusive indicate that structures from the collisional event in the Argentine Fuegian Andes are of Campanian age, comprising only the youngest and less intense deformation of the orogenic wedge. In the internal thrust-fold belt, these structures are cut by intrusives with new ages of 74 Ma (Ar/Ar on hornblende). The superposition of thrusts on these early structures indicates a subsequent event in which a thrust-fold belt formed since the Maastrichtian-Danian. Additional new data confirm brittle-ductile thrusting in the central belt, with thrusts joining a common upper detachment in the base of the Lower Cretaceous rocks. These thrusts formed a first-order duplex system that transferred the shortening accommodated in the foreland until the Miocene.