Amit Segev

Effects of Cretaceous plume and convergence, and Early Tertiary tectonomagmatic quiescence on the central and southern Levant continental margin

Abstract: This study synthesizes geological and geophysical evidence concerning the structure and character of the central and southern Jurassic Levant continental margin during Cretaceous–Tertiary time. From the beginning of the Cretaceous and until Cenomanian time, the Levant margin was strongly affected by extensional tectonics, cyclical igneous activity and rifting coupled with thermal and vertical fluctuations. It is suggested here that during the Senonian–Maastrichtian convergence of Afro-Arabia and the Mesotethys, and the Tauride part of Eurasia, the Herodotus basin oceanic crust subducted along the Eratosthenes Arc, below the short-lived abandoned Levant back-arc basin. Such a plate configuration assumes regional shear zones, as follows: (1) between the Eratosthenes Arc from the south and the Kyrenia Arc from the north: the NW–SE Carmel–Azraq–Sirhan fault system; (2) between the Sinai and the African plates: the Suez fault system; (3) between the Mesotethys and the African plates: the northern Egypt–Sinai–Negev west–east transversal fault system. Distinct tectonomagmatic quiescence between Late Maastrichtian and Late Eocene time allowed thermal relaxation and subsidence of the Levant margin until the apparent achievement of local isostatic compensation and the consequent development of the longest transgression over the Afro-Arabian ramp. Supplementary material: Details on the construction of the Bouguer gravity map are available at http://www.geolsoc.org.uk/SUP18404.

Melt instabilities in an intraplate lithosphere and implications for volcanism in the Harrat Ash-Shaam volcanic field (NW Arabia)

We investigate melt generation in a slowly extending lithosphere with the aim of understanding the spatial and temporal relationships between magmatism and preexisting rift systems. We present numerical models that consider feedback between melt generation and lithospheric deformation, and we incorporate three different damage mechanisms: brittle damage, creep damage, and melt damage. Melt conditions are calculated with a Helmholtz free energy minimization method, and the energy equation is solved self-consistently for latent heat and shear heating effects. Using a case of a slowly extending (1-1.5mm/yr) continental lithosphere with a relatively low surface heat flow (similar to 50mW/m(2)), we show that melt-rich shear bands are nucleated at the bottom of the lithosphere as a result of shear heating and damage mechanisms. Upon further deformation, melt zones intersect creep damage zones, thus forming channels that may be used for the melt to migrate upward. If a preexisting structure resides only in the brittle crust, it does not control the path of melt migration to the surface, and melt-filled channels propagate from the bottom upwards, independently of upper crustal structures. In contrast, a preexisting weak structure that reaches a critical depth of 20km allows fast (similar to 2Ma) propagation of melt-filled channels that link melt damage from the bottom of the lithosphere to near-surface structures. Our model results may explain the short time scale, volume, and magma extraction from the asthenosphere through a low surface heat flow lithosphere, such as observed, for example, in the Harrat Ash-Shaam volcanic field (northwestern Arabia), which developed in the Arabian Plate and is spatially linked to the Azraq-Sirhan Graben.