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Based on recent results of AlpArray, we propose a new model of Alpine collision that involves subduction and detachment of thick (180-200 km) European tectosphere. Our approach combines teleseismic P-wave tomography and existing Local Earthquake Tomography (LET) allowing us to image the Alpine slabs and their connections with the overlying orogenic crust at an unprecedented resolution. The images call into question the conventional notion that slabs comprise only seismically fast lithosphere and suggest that the mantle of the downgoing European Plate is heterogeneous, containing both positive and negative Vp anomalies of up to 5-6%. We interpret these as compositional rather than thermal anomalies, inherited from the Variscan and pre-Variscan orogenic cycles. They make up a kinematic entity referred to as tectosphere, which presently dips beneath the Alpine orogenic front. In contrast to the European Plate, the tectosphere of the Adriatic Plate is thinner (100-120 km) and has a lower boundary approximately at the interface between positive and negative Vp anomalies. Horizontal and vertical tomographic slices reveal that beneath the Central and Western Alps, the downgoing European tectospheric slab dips steeply to the S and SE and is only locally still attached to the Alpine crust. However, in the Eastern Alps and Carpathians, the European slab is completely detached from the orogenic crust and dips steeply to the N-NE. This along-strike change in attachment coincides with an abrupt decrease in Moho depth below the Tauern Window, the Moho being underlain by a pronounced negative Vp anomaly that reaches eastward into the Pannonian Basin area. This negative Vp anomaly is interpreted to represent hot upwelling asthenosphere that was instrumental in accommodating Neogene orogen-parallel lateral extrusion of the ALCAPA tectonic unit (upper plate crustal edifice of Alps and Carpathians) to the east. A European origin of the northward-dipping, detached slab segment beneath the Eastern Alps is likely since its imaged down-dip length (300-500 km) matches estimated Tertiary shortening in the Eastern Alps accommodated by south-dipping subduction of European tectosphere. A slab anomaly beneath the Dinarides is of Adriatic origin and dips to the northeast. There is no evidence that this slab dips beneath the Alps. The slab anomaly beneath the northern Apennines, also of Adriatic origin, hangs subvertically and is detached from the Apenninic orogenic crust and foreland. Except for its northernmost segment where it locally overlies the southern end of the European slab of the Alps, this slab is clearly separated from the latter by a broad zone of low Vp velocities located south of the Alpine slab beneath the Po Basin. Considered as a whole, the slabs of the Alpine chain are interpreted as attenuated, largely detached sheets of continental margin and Alpine Tethyan lithosphere that locally reach down to a slab graveyard in the Mantle Transition Zone (MTZ). [ABSTRACT FROM AUTHOR]

Abstract: The Kozara Mountains of northern Bosnia and Hercegovina form part of the internal Dinarides and host two tectonically juxtaposed ophiolitic successions of different age. The southern part of the Kozara Mountains exposes the Western Vardar Ophiolitic Unit, which was obducted onto the Adriatic margin in the Late Jurassic. The northern part exposes a bimodal igneous succession that was thrust onto the Western Vardar Ophiolitic Unit during the latest Cretaceous to Early Paleogene. This bimodal igneous succession comprises isotropic gabbros, doleritic dikes, basaltic pillow lavas and rhyolites. Pelagic limestones, intercalated with pillow lavas, yielded a Campanian globotruncanid association, consistent with concordant U–Pb ages on zircons from dolerites and rhyolites of 81.39±0.11 and 81.6±0.12 Ma, respectively. Chondrite-normalised rare earth element patterns of the bimodal igneous rocks show enrichment of LREE over HREE. Primitive mantle-normalised multi-element diagrams do not reveal significant depletion of HFSE. The εNd(T) and initial 87Sr/86Sr isotopic values range from +4.4 to +6.3 and from 0.70346 to 0.70507 respectively, suggesting an intraoceanic origin. The bimodal igneous succession is unconformably overlain by Maastrichtian to Paleocene siliciclastics that contain abundant ophiolitic detritus, suggesting reworking of the Campanian magmatics. An Eocene turbiditic sandstone succession unconformably covers both the Western Vardar Ophiolitic Unit and the Late Cretaceous bimodal igneous successions. These observations suggest that the Adriatic Plate and the Europe-derived Tisza and Dacia Mega-Units were still separated by a deep basin floored by oceanic lithosphere until the Campanian and that its closure did not occur before the Maastrichtian to earliest Paleogene. This Late Cretaceous oceanic domain probably represented a remnant of the Vardar Ocean, or alternatively, the Alpine Tethys; possibly the traces of both oceanic domains were connected in the area. [Copyright &y& Elsevier]

The Cenozoic foreland fold and thrust belt of the external Dinarides along the eastern shore of the Adriatic Sea shows an overall NW-SE structural strike that changes to E-W in its central part, between Zadar and Mostar. This change is associated with a dextrally transpressive zone, named Split-Karlovac Fault. Segments of this fault straddle the Eo-Oligocene flexural foreland basin system of the external Dinarides. Two balanced cross-sections across the northern and southern segments of this transverse fault zone document highly contrasting styles of deformation and amount of shortening. The northern segment across Velebit Mountain is characterized by a triangle structure with a minimum shortening of 44 km. This triangle structure is formed by a set of thick-skinned top-SW thrust duplexes, topped by a thin-skinned passive top-NE roof backthrust. By contrast, the southern segment, traversing north of Split, shows a thin-skinned top-SW to top-S directed nappe stack, associated with a minimum shortening of 127 km and rooting into a basal detachment in the Paleozoic basement. To better understand parameters controlling these contrasting styles of deformation, we modelled the flexural response of the foreland lithosphere that provided the accommodation space for the Eocene to Oligocene syn-orogenic Promina Beds, a prevailingly carbonate-clastic and conglomerate-bearing sequence making a part of the foreland basin fill. Tectonic load exerted by the hinterland nappe stack was estimated using the two balanced cross-sections. Estimates of tectonic load allowed modelling the amount of flexural response that yielded depth and shape of the basin containing the Promina Beds. The best fitting modelling results provided additional confidence in the validity of the balanced cross-sections. It is shown that shortening in the two structurally contrasting segments of the thick-skinned thrust belt were contemporaneous with dextral strike-slip along the thick-skinned Split-Karlovac Fault and coeval with sedimentation of the Promina Beds. We propose that the Velebit triangle structure acted as a back-stop that prevented the propagation of the deformation front towards the SW. Instead shortening was transferred along the Split-Karlovac Fault into the southern transect, where deformation propagated towards the foreland along a successively shallowing stepped detachment. The contrasting style of Eo-Oligocene deformation probably resulted from the interplay of Permo-Triassic evaporites with the reactivation of inherited Middle Triassic normal faults to both sides of the transpressive zone. • Nappe stack vs. triangle structure in the external Dinarides fold and thrust belt. • Substantial along-strike difference of crustal shortening. • Effects of mechanically weak lithologies on the style of deformation. • Lithosphere deflection modelling in combination with balanced cross-sections. • Impacts of a transverse zone on the flexural foreland basin system geometry. [ABSTRACT FROM AUTHOR]