The Slovenska Bistrica ultramafic complex (SBUC; Eastern Alps, Slovenia) occupies the south-easternmost part of the Pohorje Mountains, which represent an exhumed piece of continental crust subducted ...during the Cretaceous Eo-Alpine orogeny. The SBUC is composed of serpentinised harzburgites with local occurrences of garnet lherzolite, and is the only known occurrence of ultramafic rocks within the high- to ultrahigh-pressure nappe system apart from a few small dismembered pieces in the near vicinity. The harzburgites are highly depleted following melting within the spinel stability field, as exemplified by high whole-rock MgO contents (41.5–44.3 wt.%), low Al
2O
3 (0.7–1.2 wt.%), low Lu
N (0.1–0.7), and high Cr# of Cr-spinel (ca. 0.5). Fluid-immobile incompatible trace elements (Ti, Sc, V, Zr, HREE, Th) correlate well with MgO, consistent with a melt depletion trend. Other incompatible elements (Ba, Sr, LREE) show little correlation and are probably modified by the serpentinisation process or later metamorphic overprint. However, comparable LREE enrichment of all samples and absence of negative Nb and Th anomalies suggests that this piece of mantle was already metasomatised by melts or fluids before serpentinisation.
Garnet lherzolite in the SBUC recorded an UHP stage (4 GPa, 900 °C) not visible in the harzburgites. Because of the evidence of an earlier lower pressure stage within the spinel stability field, the SBUC represents a piece of subducted mantle. The protolith of the harzburgites is probably oceanic mantle, considering the high degree of melt depletion yet the lack of a subduction-zone signature. It therefore most likely represents a part of previously subducted Meliata oceanic mantle, which was part of a deeper section of the hanging wall along which subduction of the continental crust that is now exposed in Pohorje took place. Alternatively, it may represent mantle depleted and metasomatised in a continental rift zone, which was later incorporated in the hanging wall of the subduction zone and subsequently dragged down to UHP conditions.
The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso- and microscale structures and anisotropy of magnetic ...susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps–Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N–S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE–SW to E–W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE–NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.
We investigate landscape evolution in a region of the Alps that has escaped glacial erosion during periodic glaciations of the last million years. The research is thus suited to investigate ...landscaping processes on a longer time scale at the eastern end of the Alps. Morphometric analysis reveals the presence of incised relict landscapes in several regions. In the Koralpe range topographic analysis is interpreted in terms of the relict landscape being present on both sides of the eastward tilted Koralpe block. This suggests that the relict landscape is younger than the tilting of the range, which is inferred to have taken place between 18 and 16Ma. In the Pohorje region, a relict landscape is developed across the contacts of a 19Ma pluton. We use apatite (U–Th)/He thermochronology to constrain the possible age of the Koralpe and Pohorje relict landscapes. The results indicate that the Pohorje massif had cooled below 70°C by about 15Ma suggesting that the relict landscape must be younger — consistent with the interpretation of the Koralpe range. These results suggest that many relict landscapes of the eastern Alps may have formed after 15Ma in a period of tectonic quiescence and erosion. However, in both ranges channel profile projections show that about 387±105m uplift and incision occurred subsequently. This incision is likely to have occurred during the last 6–5Ma in response to the uplift of the whole region. It testifies to a renewed and ongoing uplift event that is earlier than the glaciation periods but might easily be confused with impact of glacial erosion elsewhere in the eastern Alps.
•Morphologically, Koralpe and Pohorje ranges constitute incised relict landscapes.•Channel profile projection indicates an incision amount of c. 400m.•New apatite (U–Th)/He ages suggest that the relict landscape is <15Ma.
The first evidence for ultrahigh‐pressure (UHP) metamorphism in the Eastern Alps is reported from kyanite eclogites of the Pohorje Mountains in Slovenia. Polycrystalline quartz inclusions surrounded ...by radial fractures in garnet, omphacite, and kyanite are interpreted to be pseudomorphs after coesite. Abundant quartz rods and needles in omphacite indicate an exsolution from a preexisting supersilicic clinopyroxene that contained a Ca‐Eskola component. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + phengite + quartz/or coesite yields peak pressure and temperature conditions of 3.0–3.1 GPa and 760°–825°C, well within the stability field of coesite, thus supporting the microtextural evidence for UHP metamorphism. This records the highest‐pressure conditions of Eo‐Alpine metamorphism during the Cretaceous orogeny in the Alps, implying a very deep subduction of the continental crust to at least 90–100 km depths. The new data are evidence for a regional southeastward increase of peak pressures in the Lower Central Austroalpine, indicating a south‐ to eastward dip of the subduction zone. Subduction was intracontinental; northwestern parts of the Austroalpine (Lower Central Austroalpine) were subducted under southeastern parts (Upper Central Austroalpine). The subduction zone formed in the Early Cretaceous in the northwestern foreland of the Meliata suture after Late Jurassic closure of the Meliata Ocean and the resulting collision, by a forward subduction shift to a Permian rift.
In steady-state orogens, topographic gradients are expected to increase with elevation whereas the European Alps feature a transition from increasing to decreasing slopes. This peculiar pattern has ...been interpreted to reflect either the critical slope stability angle or a premature fluvial landscape but is also consistent with the glacial buzz-saw hypothesis. To disentangle the contributions of each of these principles we split the Alps into contiguous domains of structural units and analyze their slope–elevation distributions emphasizing glaciated and non-glaciated realms. In comparable structural units within the extent of the last glacial maximum (LGM) the transition from increasing to decreasing slopes is located at the equilibrium line altitude (ELA) of the LGM and we interpret this to be evidence for the impact of glacial erosion. Decay rates of glacial landforms towards steady-state slopes depend on lithological properties leading to a landscape characterized by different transient states. Beyond the LGM limits the slope–elevation distributions show local maxima as well, but these are located at varying altitudes implying a tectonic driver. This observation and data from surrounding basins suggests that at least parts of the European Alps experienced a pre-Pleistocene pulse of tectonic uplift. The resulting presence of premature low-gradient terrain above the ELA during the global cooling in Plio–Pleistocene times would have heavily influenced the onset and the extent of an alpine ice cap.
•The transient topography of the Alps is caused by tectonics and glaciation.•Local maxima in slope occur at the LGM ELA: glacial buzz-saw.•Local maxima in slope occur at various altitudes: prematurity.•The persistence of transient landforms depends on lithology: slope stability.•Pre-Pleistocene prematurity may have influenced onset and extent of glaciations.
New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely ...associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.
The investigation of eclogites and a precursor gabbro from the Austroalpine basement domains Koralpe, Saualpe and Pohorje shows that these mafic rocks are similar to oceanic gabbros that were derived ...from a depleted mantle source. The chemical variations of the eclogites are related to differences in the magmatic history of the precursor rocks and to seawater alteration. The trace element composition of the rocks has not changed significantly during the gabbro to eclogite transformation because trace elements are redistributed among the newly formed high-pressure major and accessory minerals. As other recent studies indicate, incompatible trace elements are predominantly hosted in zoisite/clinozoisite (Sr, Pb, U, Th, LREE), apatite (Sr, Pb, REE), phengite (Cs, Rb, Ba), garnet (Y, HREE, Sc), rutile (Ti, Nb, Ta) and zircon (Zr, Hf) at eclogite-facies conditions. Omphacite hosts most of the Li in addition to some Sr and major amounts of Sc and V
. This argues against significant liberation of LILE and LREE during subduction-related dehydration or fluid infiltration of these mafic rocks.
The trace element characteristics of accessory minerals in eclogites help to reconstruct the
P–
T–
t evolution of a subduction complex: U–Pb zircon ages will date the high-pressure event if U–Th characteristics and REE analyses constrain zircon growth to being metamorphic and essentially synchronous with the growth of garnet. Recent studies document that variations in Zr content of rutile grown in the presence of zircon and quartz are mainly attributable to differences in temperature. Zr-in-rutile thermometry of Koralpe, Saualpe and Pohorje eclogites yields temperatures of 700–730 °C (according to Zack et al. Zack, T., Moraes, R., Kronz, A., 2004. Temperature dependence of Zr in rutile: an empirical calibration of a rutile thermometer. Contrib. Mineral. Petrol. 148, 471–488) or between 630–650 °C (according to Watson et al. Watson, E.B., Wark, D.A., Thomas, J.B., 2006. Crystallization thermometers for zircon and rutile. Contrib. Mineral. Petrol. 151, 413–433). No systematic variation in rutile temperatures was observed for matrix rutile and rutile included in garnet, omphacite or kyanite, suggesting that these temperatures represent peak metamorphic conditions and that this part of the Austroalpine basement behaved as a coherent block during subduction.
This study comprises a reassessment of the classical model of lateral extrusion in the Eastern Alps by using recently published geochronological data, sedimentary ages from intramontane basins, ages ...and distribution of magmatic rocks, and information from seismic profiles. Extrusion‐related faulting continuously propagated from the western toward the central eastern part of the Eastern Alps during Oligocene to Middle Miocene times. This is confined by oblique convergence between the Adriatic and European plates. During Middle Miocene times, extrusion became not only lateral in terms of parallel to the trend of the Eastern Alps, but was characterized by a displacement vector at a high angle to the strike of the orogen. This resulted in the exhumation of the Schladming and Pohorje blocks that were exhumed within extensional bridges at the northern and southern terminations of the Pöls‐Lavanttal fault system, respectively. From Middle Miocene to recent times, extrusion was controlled by overall extension between the Dinaric and Carpathian subduction zones. The influence of north directed compression triggered by the northward moving Adriatic plate diminished, and the influence of the retreating Carpathian subduction zone increased. This gave rise to Miocene volcanism that is exclusively found east of the Dinaric subduction zone. We therefore consider that lateral extrusion in the Eastern Alps can be subdivided into distinct tectonic phases, with less pronounced eastward extension‐related displacement between Late Oligocene and Middle Miocene times. As soon as the Eastern Alps passed the Dinaric subduction zone, the entire domain became highly extensive.
Key Points
Reassessment of the classic model of lateral extrusion in the Eastern Alps
Consideration of shortcomings of previously published models
Linkage of lateral extrusion to Mediterranean plate tectonics
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