THE VOLCANO-TECTONIC EVOLUTION OF THE NORTHERN PART OF THE

 ARABIAN PLATE (SYRIA): GEODYNAMIC FRAMEWORK, CHRONOLOGY K-AR,

 GEOCHEMICAL CHARACTERS, MAPPING

 (REMOTE SENSING AND GIS)

ABSTRACT

 The Cainozoic volcanic activity in the Arabian plate offers an excellent opportunity to study the intra-plate volcanism related to a complex tectonic setting. After the emplacement of the Yemeni-Ethiopian continental flood basalt plateau, ~ 31 Ma, since the Late Oligocene, widespread volcanic activity has erupted, accompanying the separation of the Arabian-Nubian Shield (development of Red Sea rifting) and the convergence between the Arabian and Eurasian plates (building of the Bitlis-Zagros thrust belts). In the northern part of the Arabian platform, the Syrian volcanism has taken place in a general compressional context, surrounding the Palmyride fold belt and adjacent to other deformation zones (e.g. the Euphrates graben and Dead Sea fault system).

 This thesis focuses on the volcano-tectonic evolution of the northern part of the Arabia plate, particularly in Syria, and essentially combines geochronological, geochemical, and morpho-structural studies, in addition to supplementary geophysical models.

Our morpho-structural analyses of the Harrat Ash Shaam volcanic province (HASV) to the south of Palmyride, digitally characterise more than 800 monogenic volcanic cones placed in Syria, Jordan, and Saudi Arabia. These new data, together with the availability of sediment thickness data, give rise to a new volcano-tectonic approach. This study shows that the consistent negative correlation between the intensity of volcanism and basement depth is influenced by the tectonic setting. The normative analysis of the distribution of volcanic cones in relation to sediment thicknesses is critical when comparing the extension of tectonics in different zones.

Remote sensing imagery, field work and our > 40 new K-Ar ages dataset ranging from ~0.05 million years (Ma) to ~18 Ma allow us to precise the Syria volcano-tectonic evolution through time. Regarding the youngest lava flows of HASV, the integration of the results makes it possible to suggest a chronological model for the alteration processes in relation to Quaternary palaeoclimatic changes. We reconstruct the volcano-tectonic evolution in Syria during the Cainozoic, and suggest different extension styles to explain the volcanism. It started during the Late Oligocene and the Early Miocene,  between ~26 Ma and ~16 Ma to the South of Palmyride at HASV in an extensional tectonic context. From the Miocene to the Quaternary, between ~19 Ma and ~0.08 Ma, the volcanism developed to the North under second order extension tectonic conditions. Since the Mid-Miocene, the compression has increased and the magma erupted in relation with a possible counter-clockwise rotation tectonic relative motion. South of Palmyride it corresponds to the widespread eruptive phase during the last 13 Ma. To the North, this phase, linked to rotational tectonics appears concentrated in superficies and time; it corresponds to the Homs plateau, NW Palmyride, between 6.3 and 4.3 Ma.

We suggest a new volcano-tectonic evolution model for the HASV. It highlights the essential role of lithosphere heterogeneity beneath Lebanon, in particular the anti Lebanon Mountains and Palmyride thrust belts, in triggering the Mid-Miocene volcanism.

Our geophysical models estimate mean lithosphere – asthenosphere boundaries at about 150 km depth.  According to geochemical data, the zone of shallowest depth ~110 km, W of HASV, could be the result of a thermal anomaly, instead of an asthenospheric  upwelling.

Geochemically, the Cainozoic Syrian lavas are alkaline and subalkaline rocks, typical of magma emitted in continental intraplate contexts. They are basanites and tephrites, basalts, basaltic andesites, basaltic trachyandesites, and trachybasalts. Thirty samples from different Syrian volcanic provinces show significant variation in terms of incompatible trace element signatures. Crustal contamination plays a negligible role in the process of magma genesis, as does crystal fractionation, essentially restricted to olivine and clinopyroxene. Our results show that the Syrian lava has been generated by variable rates of partial melting from different levels of a locally heterogeneous lithospheric mantle. The LREE/MREE ratio not only illustrates how the degree of partial melting was changed spatially and temporally during the last ~18 Ma, but it also illustrates how the degree and style of extension tectonics changed through time.

One of the consequences of this tectonic setting could be a prospective of westward hydrocarbon migration which was probably triggered by a Plio-Quaternary extension in the crust at the Euphrates Graben in the East, and probably guided towards a previously stretched crust at the NW Syria.

We conclude that the Cainozoic Syrian volcanism is a consequence of extensional tectonic, under periodical influence of the north and east-ward convergence at the Arabia-Eurasia margin, which induces rotational tectonic styles ; this controls the partial melting at various depths in the mantle. The volcanism of northern Arabia develops in the framework of the Red Sea rifting and initiated at the same time as the southern Red Sea volcanism. It extends up to historical time, progressively smoothed to the North in a contradictory relation with the compression tensional setting of Arabia-Eurasia margin.

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Al Kwatli D.M.A

Ph.D. Earth Sciences, 

Université Paris-Saclay

Mobile: 00 33 (0)6 95 80 05 14

E-mail : dr.alkwatli@gmail.com

(Linkedin) http://fr.linkedin.com/in/dralkwatli