The magmatic history of the Pelagonian Zone, in northern Greece, is constrained with secondary ion mass spectrometer (SIMS) U-Pb dating on zircons of various granitoids whose structural positions were defined with respect to the regional main foliation. Ages pertain to four groups: (i) Mesoproterozoic (circa 1430 Ma) crystallization of granites inferred from inherited magmatic zircon cores that have been partially molten during the (ii) Neoproterozoic at circa 685 Ma (metamorphic zircon rims) and subsequently intruded by a Neoproterozoic leucogranite (circa 600 Ma). (iii) Late-or post-Variscan calc-alkaline granitoids (315-301 Ma) were in turn intruded by a subvolcanic dike at about 280 Ma. In the Early Permian the εNd(t) in magmas decreased from À7.3 to À1.3, hinting to mantle-derived melts produced during extension. Rifting is further heralded by two acidic and one mafic dike containing Lower-Middle Triassic zircons (246-242 Ma). (iv) Early Cretaceous anatectic melts at 117 ± 8 Ma formed during regional metamorphism. This age is the first report of in situ anatexis in the Pelagonian Zone. Cretaceous anatexis developed during the Mesozoic collision of Pelagonia with the Eurasian margin. Major-and trace-element geochemistry of amphibolites further attests for the complex pre-Alpine tectonic history with Neoproterozoic calc-alkaline and back-arc geochemical signature and Triassic alkali-magmatism.

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins.

Abstract. The Pelagonian zone, situated between the External Hellenides/Cyclades to the west and the Axios/Vardar/Almopias zone (AVAZ) and the Rhodope to the east, was involved in late Early Cretaceous and in Late Cretaceous-Eocene orogenic events whose duration and extent are still controversial. This paper constrains their late thermal imprints. New and previously published zircon (ZFT) and apatite (AFT) fission-track ages show cooling below 240 • C of the metamorphic western AVAZ imbricates between 102 and 93-90 Ma, of northern Pelagonia between 86 and 68 Ma, of the eastern AVAZ at 80 Ma and of the western Rhodope at 72 Ma. At the regional scale, this heterogeneous cooling is coeval with subsidence of Late Cretaceous marine basin(s) that unconformably covered the Early Cretaceous (130-110 Ma) thrust system from 100 Ma. Thrusting resumed at 70 Ma in the AVAZ and migrated across Pelagonia to reach the External Hellenides at 40-38 Ma. Renewed thrusting in Pelagonia is attested at 68 Ma by abrupt and rapid cooling below 240 • C and erosion of the gneissic rocks. ZFT and AFT in western and eastern Pelagonia, respectively, testify at ∼ 40 Ma to the latest thermal imprint related to thrusting. Central-eastern Pelagonia cooled rapidly and uniformly from 240 to 80 • C between 24 and 16 Ma in the footwall of a major extensional fault. Extension started even earlier, at ∼ 33 Ma in the western AVAZ. Post-7 Ma rapid cooling is inferred from inverse modeling of AFT lengths. It occurred while E-W normal faults were cutting Pliocene-torecent sediment.