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The Timing of Rifting Events in the Levant Margins: U-Pb Dating of Zircons From Volcanic Rocks in the Eastern Mediterranean


The Levant basin, at the easternmost Mediterranean, formed during the opening of the NeoTethys. Whereas other Tethyan basins were later consumed during Alpine orogenesis, the Levant basin remained out of reach of orogenic deformation and has preserved its passive margins and structural continuity with the bordering land. The Levant basin and its passive margins were shaped by several rifting phases from Early Permian to Middle Jurassic times, as recorded by subsidence and magmatism. While the subsurface extensional structure along its SE passive margins (coastal Israel) was reconstructed in detail, the spatial and temporal distribution of igneous activity is not well known. The most important evidence of rifting-related magmatism in the Levant is a 2.5 km thick mostly alkaline basalt sequence, titled Asher volcanics, which is preserved in a roughly coast-parallel deeply buried graben in N Israel. Previous K-Ar and Ar-Ar dating assisted by biostratigraphic considerations constrained the age of Asher Volcanics to early to middle Jurassic times. However, these are suspected of being partially reset by low grade metamorphism. Here, rare zircon grains, laboriously separated from rock cuttings of volcanic intervals of four deep boreholes, determine the timing and duration of the Asher volcanics magmatism (Atlit – 1A, Elijah-3, Meged – 2, and Deborah – 2A). U-Pb zircon ages of rocks from the base and the top of Asher volcanics in Atlit-1 and Elijah-3 boreholes, respectively, show that the 2.5 km thick section erupted in a rather short time interval in the latest Triassic (Rhaetian), from 207 to 201 Ma. Basaltic horizons interlayered with Norian reefal limestones beneath the Asher volcanics in Atlit – 1A borehole herald the flood-volcanism to come. Rare zircon xenocrysts from the Elijah – 3 borehole yielded an Early Permian age (289 ± 4 Ma) concomitant with faulting and sedimentation in the intra-continental Palmyra trough, which marks the onset of Neotethyan rifting. Ti-in-zircon thermometry and trace element characteristics indicate that unlike their Triassic counterparts, the Permian zircons crystallized in felsic magma. This is the first evidence of early Permian magmatism in the Levant. The mafic volcanic and sub-volcanic sequence in the Deborah – 2A borehole named Devora volcanics was stratigraphically correlated to the Mid-Late Jurassic Bhannes magmatism in Mt. Lebanon. Nonetheless the new U-Pb zircon age, 137 Ma, shows that the Devora volcanics are actually coeval with the Early Cretaceous Tayasir volcanics, which are unrelated to extension, but represent the activity of a mantle plume. A compilation of all the data show that a single major alkaline volcanism in the Levant margins occurred ~10 m.y. earlier than thought before, at Upper Triassic rather than Lower Jurassic times. This is also the timing of major tectonic subsidence at the shelf edge and significant vertical movements inland. The Rhaetian voluminous magmatism marks the culmination of lithospheric thinning in the Levant margins, which had begun by crustal – extension and melting ~85 Myr earlier and continued intermittently through Permo-Triassic times. Further subsidence in the Jurassic may be a residual thermal effect. Other rifting-related volcanic sequences in the Eastern Mediterranean, the Mamonia complex of SW Cyprus and the Gödene zone of the Antalya complex, S Turkey, erupted 10-20 Myr earlier than the Asher volcanics in a more distal, basinward location and are associated with reworked reefal limestones derived from a nearby shelf. The latter may be correlated with the Norian atoll-like limestone build-ups in the Levant margins, and thus may represent the conjugate NW margins of the Eastern Mediterranean basin.