Genesis of the Eastern Adamello Plutons (Northern Italy): Inferences for the Alpine Geodynamics

The Corno Alto–Monte Ospedale magmatic complex crops out at the eastern border of the Adamello batholith, west of the South Giudicarie Fault (NE Italy). This complex includes tonalites, trondhjemites, granodiorites, granites and diorites exhibiting an unfoliated structure suggesting passive intrusion under extensional-to-transtensional conditions. Major, minor elements, REE and isotopic analyses and geochemical and thermodynamic modelling have been performed to reconstruct the genesis of this complex. Geochemical analyses unravel a marked heterogeneity with a lack of intermediate terms. Samples from different crust sections were considered as possible contaminants of a parental melt, with the European crust of the Serre basement delivering the best fit. The results of the thermodynamic modelling show that crustal melts were produced in the lower crust. Results of the geochemical modelling display how Corno Alto felsic rocks are not reproduced by fractional crystallization nor by partial melting alone: their compositions are intermediate between anatectic melts and melts produced by fractional crystallization. The tectonic scenario which favored the intrusion of this complex was characterized by extensional faults, active in the Southalpine domain during Eocene. This extensional scenario is related to the subduction of the Alpine Tethys in the Eastern Alps starting at Late Cretaceous time.

Comparison of extensometric results measured in the Vyhne Tidal Station (Slovakia) and in the Sopronbánfalva Geodynamic Observatory (Hungary)

Two extensometer stations have been set up at the margin of the Pannonian Basin to monitor tectonic movements as well as Earth tides and related phenomena. Because the Sopronbánfalva Geodynamic Observatory (SGO) in Hungary and the Vyhne Tidal Station (VTS) in Slovakia are located in different geological, topographic, and tectonic environments, the analysis and comparison of the extensometer data measured here provides a useful opportunity to interpret the observed data. The tectonic deformation at the SGO shows an average contraction of: −2.94 μstr y−1 (1 μstr is 10−6 relative deformation) which can be explained by the uplift of the Alps and the anticlockwise motion of the Adria microplate, causing compression in the Eastern Alps. At the VTS an average compression of −14.8 nstr y−1 (1 nstr is 10−9 relative deformation) was measured which can be explained by the NW compression direction in this area. The measured deformations in both observatories show a good agreement with the results of GPS measurements. The deformation at the VTS is characterized by small dilatation anomalies caused by the different topographic, tectonic environment and probably by the high heat flow in the area of the station. At this station the calculated amplitude factors for O1, P1, K1, M2 are 1.01482, 1.21691, 0.83173, 1.09392 and the ocean load corrected values are 1.10817, 1.35717, 0.92809, 1.28812, respectively. At the SGO the calculated amplitude factors for the same tidal components are 0.58776, 0.38967, 0.41548, 1.00564 and the ocean load corrected values are 0.98893, 1.89117, 1.00430, 1.04962, respectively. These results show that the effect of the ocean tide loading is greater at Sopronbánfalva, than at Vyhne. Based on the comparison, we can say that the result of the local strain measurement can be considered realistic.

Reuniting the Ötztal Nappe: the tectonic evolution of the Schneeberg Complex

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.

Orogenic lithosphere and slabs in the greater Alpine area – interpretations based on teleseismic P-wave tomography

Based on recent results of AlpArray, we propose a new model of Alpine collision that involves subduction and detachment of thick (∼ 180 km) European lithosphere. 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 lithosphere at an unprecedented resolution. The images call into question the conventional notion that downward-moving lithosphere and slabs comprise only seismically fast lithosphere. We propose that the European lithosphere is heterogeneous, locally containing layered positive and negative Vp anomalies of up to 5 %–6 %. We attribute this layered heterogeneity to seismic anisotropy and/or compositional differences inherited from the Variscan and pre-Variscan orogenic cycles rather than to thermal anomalies. The lithosphere–asthenosphere boundary (LAB) of the European Plate therefore lies below the conventionally defined seismological LAB. In contrast, the lithosphere of the Adriatic Plate is thinner and has a lower boundary approximately at the base of strong positive Vp anomalies at 100–120 km. Horizontal and vertical tomographic slices reveal that beneath the central and western Alps, the European slab dips steeply to the south and southeast and is only locally still attached to the Alpine lithosphere. However, in the eastern Alps and Carpathians, this slab is completely detached from the orogenic crust and dips steeply to the north to northeast. 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 as representing hot upwelling asthenosphere that heated the overlying crust, allowing it to accommodate Neogene orogen-parallel lateral extrusion and thinning 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 down-dip length matches estimated Tertiary shortening in the eastern Alps accommodated by originally south-dipping subduction of European lithosphere. 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 highly attenuated, largely detached sheets of continental margin and Alpine Tethyan oceanic lithosphere that locally reach down to a slab graveyard in the mantle transition zone (MTZ).

Thrust tectonics in the Wetterstein and Mieming mountains, and a new tectonic subdivision of the Northern Calcareous Alps of Western Austria and Southern Germany

We investigate the tectonic evolution of the Wetterstein and Mieming mountains in the western Northern Calcareous Alps (NCA) of the European Eastern Alps. In-sequence NW-directed stacking of thrust sheets in this thin-skinned foreland thrust belt lasted from the Hauterivian to the Cenomanian. In the more internal NCA major E-striking intracontinental transform faults dissected the thrust belt at the Albian–Cenomanian boundary that facilitated ascent of mantle melts feeding basanitic dykes and sills. Afterwards, the NCA basement was subducted, and the NCA were transported piggy-back across the tectonically deeper Penninic units. This process was accompanied by renewed Late Cretaceous NW-directed thrusting, and folding of thrusts. During Paleogene collision, N(NE)-directed out-of-sequence thrusts developed that offset the in-sequence thrust. We use this latter observation to revise the existing tectonic subdivision of the western NCA, in which these out-of-sequence thrusts had been used to delimit nappes, locally with young-on-old contacts at the base. We define new units that represent thrust sheets having exclusively old-on-young contacts at their base. Two large thrust sheets build the western NCA: (1) the tectonically deeper Tannheim thrust sheet and (2) the tectonically higher Karwendel thrust sheet. West of the Wetterstein and Mieming mountains, the Imst part of the Karwendel thrust sheet is stacked by an out-of-sequence thrust onto the main body of the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.

Deep phylogeographic splits but no taxonomic structure in the disjointly distributed Draba pacheri (Brassicaceae), a subendemic of the Eastern Alps

Draba (Brassicaeae), a model group for diversification and evolution in Arctic and mountain habitats, is taxonomically challenging and many of its species are insufficiently investigated. One such species is D. pacheri, an endemic of the eastern European Alps and the western Carpathians (here presumably extinct). Several hypotheses exist with respect to the phylogenetic position and the taxonomy of this species, but none of these has ever been tested using molecular data. In this article we examine (i) DNA sequence data to assess the phylogenetic position of D. pacheri within the genus and (ii) AFLP fingerprint data as well as morphometric data to address whether this species can be divided taxonomically into species or subspecies. DNA sequence data firmly place D. pacheri within the Core Draba Group III, whose internal relationships are, however, insufficiently resolved to precisely identify the closest relative of D. pacheri. AFLP data identify several genetically divergent lineages corresponding to geographically distinct regions. Although these lineages are congruent with hypotheses distinguishing either two species (D. pacheri s. str., D. norica) or one species with several subspecies, the lack of clear morphological separation, both with respect to the entire set of traits and single presumably diagnostic characters such as trichome morphology, renders recognition of a single species D. pacheri, as suggested previously, the best taxonomic solution. The deep and geographically strongly structured splits of D. pacheri likely are the result of isolation in several Pleistocene refugia and warrant that conservation efforts should involve populations from each of the main geographic subgroups.