F. Innocenti

The Yerer-Tullu Wellel volcanotectonic lineament: a transtensional structure in central Ethiopia and the associated magmatic activity

Abstract An east-west trending structure in central Ethiopia, the Yerer Tullu-Wellel volcanotectonic lineament, intersects the Main Ethiopian Rift and lies between latitudes 8°20′N and 9°05′N. Interpretation of Landsat images, calibrated by field study, indicates the relative chronology of the observed fracture systems (northeast-southwest, east-west, north-south and northwest-southeast). Along this structure there are several central volcanoes concentrated along the latitude of 8°45′N. Most of these volcanoes have basic lava flows at their base with acid dome, plug, and/or pyroclastic deposits as their evolved end members. The chemistry of these volcanic rocks shows a progressive decrease in alkalinity and silica undersaturation from western areas towards the rift margin. Radiometric ages of these volcanoes range from 12 Ma to Recent and indicate that the volcanic activity shifted eastward in time. The central volcanoes are usually located at the intersections of the above described fracture systems. The role of the transtensional Yerer Tullu-Wellel volcano-tectonic lineament since Late Miocene times is discussed within the geodynamic evolution of the Main Ethiopian Rift.

Slab window-related magmatism from southernmost South America: the Late Miocene mafic volcanics from the Estancia Glencross Area (∼52°S, Argentina–Chile)

Abstract The Estancia Glencross Area (EGA) volcanic rocks form a series of five isolated buttes located at the southern end (∼52°S) of the discontinuous belt of Cenozoic basaltic lava formations occurring in the extra-Andean Patagonia. EGA volcanics are subalkaline basalts and basaltic andesites erupted at 8.0–8.5 Ma in a region closely behind the Andean Cordillera. EGA volcanism predated by about 4–5 my the onset of the volcanism in the nearby Pali Aike Volcanic Field, which produced highly primitive, alkaline lavas. Incompatible trace-element distributions and Sr–Nd isotope compositions of EGA rocks are those typical of within-plate OIB-type basalts and are indicative of minimal interaction of sub-lithospheric magmas with enriched reservoirs. The geochemical characteristics of EGA volcanics, as well as their age and location are consistent with a model of slab window opening beneath this region. The high silica content and the garnet signature of the estimated EGA primary magma are explained by a two-stage process involving the initial production of melts from a garnet lherzolite source followed by the reaction of these melts with harzburgite country rocks during their ascent through the mantle lithosphere. The melt/harzburgite reaction, favoured by a slow melt ascent rate, as well as the low magma production at EGA, are likely related to the dominantly compressive stress regime operating in this area during Late Miocene.

Geodynamic evolution of the Aegean: constraints from the Plio-Pleistocene volcanism of the Volos–Evia area

Abstract: The Plio-Pleistocene lava flows and domes of the Volos–Evia area were erupted between 3.4 and 0.5 Ma ago on the western continuation of the North Anatolian Fault, in a back-arc position with respect to the active arc. They are mainly high-K calc-alkaline trachyandesites. Based on their Sr–Nd–Pb isotopic compositions, the mantle source of the Volos–Evia area lavas is similar to that of a large volcanic belt that developed north of the Pelagonian–Attic–Cycladic–Menderes massifs, encompassing a 35 Ma timespan and widespread over a large area from NW Greece–Macedonia to the Aegean–western Anatolia. In contrast, southern Aegean arc rocks have a similar subduction fingerprint but distinctly lower Sr and higher Nd isotopic compositions. The geochemical and isotopic differences between southern and northern Aegean rocks may be ascribed to the different nature of the mantle wedge: depleted asthenosphere under the the southern Aegean, and lithosphere northward. The lack of an asthenospheric mantle wedge below the northern Aegean fits with the hypothesis of an almost horizontal subduction of the African slab. In the mantle reference frame the African slab is moving out of the mantle, and a slab-driven suction flow of the underlying mantle may be responsible for the recent development of a thin asthenospheric layer in the southern Aegean mantle wedge.