Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and ...involved widespread intracrustal and crust–mantle decoupling during Adria–Europe collision. In particular, the switch from south-directed European subduction to north-directed “wrong-way” Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench–trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps–Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust–mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.
A new kinematic reconstruction that incorporates estimates of post‐20 Ma shortening and extension in the Apennines, Alps, Dinarides, and Sicily Channel Rift Zone (SCRZ) reveals that the Adriatic ...microplate (Adria) rotated counterclockwise as it subducted beneath the European Plate to the west and to the east, while indenting the Alps to the north. Minimum and maximum amounts of rotation are derived by using, respectively, estimates of crustal extension along the SCRZ (minimum of 30 km) combined with crustal shortening in the Eastern Alps (minimum of 115 km) and a maximum amount (140 km) of convergence between Adria and Moesia across the southern Dinarides and Carpatho‐Balkan orogens. When combined with Neogene convergence in the Western Alps, the best fit of available structural data constrains Adria to have moved 113 km to the NW (azimuth 325°) while rotating 5 ± 3° counterclockwise relative to Europe since 20 Ma. Amounts of plate convergence predicted by our new model exceed Neogene shortening estimates of several tens of kilometers in both the Apennines and Dinarides. We attribute this difference to crust‐mantle decoupling (delamination) during rollback in the Apennines and to distributed deformation related to the northward motion of the Dacia Unit between the southern Dinarides and Europe (Moesia). Neogene motion of Adria resulted from a combination of Africa pushing from the south, the Adriatic‐Hellenides slab pulling to the northeast, and crustal wedging in the Western Alps, which acted as a pivot and stopped farther northwestward motion of Adria relative to Europe.
Key Points
Adria has rotated 5 ± 3° counterclockwise and translated 113 km to the NW (azimuth 325°) relative to Europe since 20 Ma
Adria motion was associated with 110 km convergence relative to Moesia, 125 km in Eastern Alps, and 60 km of extension in Sicily Channel
Differences between amounts of shortening and plate convergence suggest crust‐mantle decoupling at active Adria‐Europe boundaries
A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution, from sea-floor spreading through ...subduction to collision in the Alps. Unlike previous models, which relate the fate of Alpine Tethys solely to relative motions of Africa, Iberia and Europe during opening of the Atlantic, our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record. The motions of these microplates (Adria, Iberia, Alcapia, Alkapecia, and Tiszia) relative to both Africa and Europe during Late Cretaceous to Cenozoic time involved the subduction of remnant Tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces: (1) 131–84
Ma intra-oceanic subduction of the Ligurian part of Alpine Tethys attached to Iberia coincided with Eo-alpine orogenesis in the Alcapia microplate, north of Africa. These events were triggered primarily by foundering of the older (170–131
Ma) Neotethyan subduction slab along the NE margin of the composite African–Adriatic plate; subduction was linked by a sinistral transform system to E–W opening of the Valais part of Alpine Tethys; (2) 84–35
Ma subduction of primarily the Piemont and Valais parts of Alpine Tethys which were then attached to the European plate beneath the overriding African and later Adriatic plates. NW translation of Adria with respect to Africa was accommodated primarily by slow widening of the Ionian Sea; (3) 35
Ma–Recent rollback subduction of the Ligurian part of Alpine Tethys coincided with Western Alpine orogenesis and involved the formation of the Gibraltar and Calabrian arcs. Rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable, retreating Adriatic and African slabs during slow convergence of Africa and Europe. The upper European–Iberian plate stretched to accommodate this slab retreat in a very mobile fashion, while the continental core of the Adriatic microplate acted as a rigid indenter within the Alpine collisional zone. The subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the Alps and Apennines, as well as beneath parts of the Pannonian Basin, the Adriatic Sea, the Ligurian Sea, and the Western Mediterranean. The predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10–30%, indicating that parts of slabs may be superposed within the mantle transition zone and/or that some of this subducted lithosphere became seismically transparent.
The Eastern Alps were affected by a profound post‐collisional tectonic reorganisation in Neogene time, featuring indentation by the Adriatic upper plate, rapid uplift and filling of the eastern ...Molasse Basin, exhumation and eastward orogen‐parallel transport of Paleogene metamorphic units in the orogenic core, and a shift from northward thrust propagation in the European plate to southward propagation in the Adriatic plate. We test the idea that these events were triggered by slab detachment by reconstructing the indentation process. This involves sequentially restoring N‐S and E‐W cross‐sections of the orogenic wedge and correcting for out‐of‐section orogen‐parallel transport with a map‐view reconstruction. We propose two phases of indentation: Initially (23 and 14 Ma), the whole Adriatic crust acted as an indenter. Its northward motion was accommodated by upright folding and orogen‐parallel extensional exhumation in the Tauern Window. This phase was followed (14 Ma to Present) by continued orogen‐parallel transport of the orogenic wedge into the Pannonian Basin and deformation of the leading edge of the Adriatic indenter, forming the Southern Alps fold‐thrust belt. The lower crust of the Southern Alps indented the base of the Venediger Nappes in the Tauern Window, forming a high‐velocity (6.8–7.25 km/s) ridge in map view at 30–45 km depth. By correlating the post‐23 Ma orogenic evolution with presently imaged European slab segments in P‐wave teleseismic tomography, we discern two possible Neogene slab removal events: One from 23 to 19 Ma triggering tectonic reorganisation of the Eastern Alps and its foreland basin, and potentially a second event after 14 Ma.
Key Points
The Eastern Alps were first indented by Adriatic lithosphere (23 and 14 Ma), then by Adriatic lower crust as the indenter deformed (14 and 0 Ma)
Shortening of the orogenic wedge since 23 Ma requires 135 km of subduction and 90 km of eastward extrusion of orogenic lithosphere
Slab detachment at 23 and 20 Ma and possibly after 14 Ma is constrained by areal balancing of crust and mantle
We integrate structural, geophysical, and geodetic studies showing that the Dinarides‐Hellenides orogen along the Adria‐Europe plate boundary in the Western Balkan peninsula has experienced clockwise ...oroclinal bending since Eocene‐Oligocene time. Rotation of the Hellenic segment of this orogen has accelerated since the middle Miocene and is associated with a north‐to‐south increase in shortening along the orogenic front. Within the Paleogene nappe pile, bending was accommodated by orogen‐parallel extension, clockwise block rotation, and thrusting in the hanging wall of the Skhoder‐Peja Normal Fault (SPNF). The SPNF and related faults cut the older Skhoder‐Peja Transfer Zone with its pre‐Neogene dextral offset of the West Vardar ophiolite nappe. Rotation of the SPNF hanging wall involved Miocene‐to‐recent, out‐of‐sequence thrusting that was transferred to the Hellenic orogenic front via lateral ramps on dextral transfer zones. Along strike of the Dinarides‐Hellenides and coincident with the southward increase in Neogene shortening, the depth of the Adriatic slab increases from ~160 km north of the SPNF to ~200 km just to the south thereof, to several hundreds of kilometers to the south of the Kefalonia Transfer Zone. The geodynamic driver of tectonics since the early Miocene has been enhanced rollback of the Hellenic segment of the Adriatic slab in the aftermath of Eocene‐Oligocene slab tearing and breakoff beneath the Dinarides, which focused slab pull in the south. The SW‐retreating Hellenic slab segment induced clockwise bending of the southern Dinarides and northern Hellenides, including their Adriatic foreland, about a rotation pole in the vicinity of the Mid‐Adriatic Ridge.
Key Points
The junction of the Dinarides and Hellenides is the site of a N‐to‐S increase in Neogene shortening and upper‐plate extension
This shortening reflects oroclinal bending about a pole near the Mid‐Adriatic Ridge that has been driven by Hellenic rollback subduction
The Shkoder‐Peja Normal Fault accommodated Neogene orogen‐parallel extension and clockwise rotation of shallower levels of the orogen
Continent-derived tectonic units in the Tauern Window of the Alps exhibit stratigraphic and structural traces of extension of continental margins eventually leading to the opening of the Alpine ...Tethys. In this study, we reassess lithostratigraphic data from the central part of the Tauern Window to reconstruct the post-Variscan evolution of this area, particularly the rift-related geometry of the European continental margin. The lithostratigraphy of the Alpine nappes reflects systematic variations of the structure of the European margin. The lowest tectonic units (Venediger nappe system, Eclogite Zone and Trögereck Nappe) are characterized by a thick succession of arkose-rich Bündnerschiefer-type sediments of probably Early Cretaceous age that we interpret as syn-rift sequence and which stratigraphically overlies thinned continental basement and thin pre-rift sediments. In contrast, the highest tectonic unit derived from Europe (Rote Wand Nappe) preserves a thick pre-rift sedimentary sequence overlying thinned continental basement, as well as a thick syn- to post-rift succession characterized by turbiditic Bündnerschiefer-type sediments of probable Cretaceous age. These observations point towards a highly segmented structure of the European rifted margin. We propose that this involved the formation of an outer margin high, partly preserved in the Rote Wand Nappe, that was separated from the main part of the European margin by a rift basin overlying strongly-thinned continental crust. The along-strike discontinuity of the Rote Wand Nappe is proposed to reflect the lateral variation in thickness of the outer margin high that resulted from margin-parallel segmentation of the European continental crust during highly oblique rifting antecedent to the opening of Alpine Tethys.
We present a novel three-dimensional model of compressional wave attenuation (1/
Q
P
) for the Eastern and eastern Southern Alps in Europe that includes the eastern part of the Adriatic indenter, ...termed here the Dolomites Sub-Indenter. Our approach employed waveform data from the SWATH-D network, a dense temporary network operational between 2017 and 2019, as well as selected stations of the larger AlpArray Seismic Network. A spectral inversion method using frequency-independent quality factor
Q
P
, was applied to derive 3578 path-averaged attenuation values (
t*
) from 126 local earthquakes. These were then inverted using the damped least square inversion (local earthquake tomography) for the attenuation structure. The resulting
Q
P
model, which builds on and complements a previously calculated 3-D velocity model (
V
P
and
V
P
/
V
S
), exhibits good resolution down to ~ 20 km depth. Several anomalies can be correlated with the distribution of other physical parameters (
V
P
and
V
P
/
V
S
) and regional tectonic features. Notably, the Friuli-Venetian region exhibits the highest attenuation (lowest
Q
P
) anomaly, coinciding with low
V
P
values and increased
V
P
/
V
S
. This anomaly is likely associated with a high density of faults and fractures, as well as the presence of fluid-filled sediments along the active thrust front in the eastern segment of the Southern Alps. Another intriguing observation is the low attenuation (high
Q
P
) anomaly along the northwestern edge of the Dolomites Sub-Indenter (NWDI), located south of the Periadriatic fault and east of the Giudicarie fault, where seismicity is notably absent. This anomaly coincides with Permian magmatic rocks at the surface and may be a measure of their strength at depth.
Graphical Abstract
We investigate the evolution of the three‐dimensional thermal structure of a palaeo‐subduction channel exposed in the Penninic units of the central Tauern Window (Eastern Alps). Structural and ...petrological observations reveal a sheath fold with an amplitude of some 20 km that formed under high‐P conditions (~2 GPa). The fold is a composite structure that isoclinally folded the thrust of an ophiolitic nappe derived from Alpine Tethys Ocean onto a unit of the distal European continental margin, also affected by the high‐P conditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes deform isograds of the T‐dominated Barrovian metamorphism that itself overprints the high‐P metamorphism partly preserved in the sheath fold. Using Raman spectroscopy on carbonaceous material (RSCM), we are able to distinguish peak‐temperature domains related to the original subduction metamorphism from domains associated with the later temperature‐dominated (Barrovian) metamorphism. The distribution of RSCM temperatures in the Barrovian domain indicates a lateral and vertical decrease of peak temperature with increasing distance from the centres of the thermal domes. This represents a downward increase of palaeo‐temperature, in line with previous studies. However, we observe the opposite palaeo‐temperature trend in the lower limb of the sheath fold, namely an upward increase. We interpret this inverted palaeo‐temperature domain as the relic of a subduction‐related temperature field. Towards the central part of the sheath fold's upper limb, RSCM temperatures increase to a maximum of ~520°C. Further upsection in the hangingwall of the sheath fold, palaeo‐peak temperatures decrease to where they are indistinguishable from the peak temperatures of the overprinting Barrovian metamorphism. Peak‐temperature contours of the subduction‐related metamorphism are oriented roughly parallel to the folded nappe contacts and lithological layering. The contours close towards the northern, western and eastern parts of the fold, resulting in an eye‐shaped, concentric pattern in cross‐section. The temperature contour geometry therefore mimics the fold geometry itself, indicating that these contours were also folded in a sheath‐like manner. We propose that this sheath‐like pattern is the result of a two‐stage process that reflects a change of the mode of nappe formation in the subduction zone from thrusting to fold nappe formation. First, thrusting of a hot oceanic nappe onto a colder continental nappe created an inverted peak‐thermal gradient. Second, sheath folding of this composite nappe structure together with the previously established peak‐temperature pattern during exhumation. This pattern was preserved because temperatures decreased during retrograde exhumation metamorphism and remained less than the subduction‐related peak temperatures during the later Barrovian overprint. The fold ascended with diapir‐like kinematics in the subduction channel.
The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of ...particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearance, is clearly identified along most of the swath. The Variscan lithospheric blocks beneath the Bohemian Massif are imaged with sub-vertical boundaries. Beneath the Eastern Alps, the shape of the Moho is consistent with bi-vergent orogenic thickening, with a steeper and deeper-reaching Adriatic plate plunging northwards beneath the European plate in the north. At the junction of these plates at depth, around the previously proposed Moho gap, the root of the Eastern Alps is a broad trough characterized by a zone of low velocity-gradient that is up to 20 km thick, transitioning between crust and mantle. Our receiver-function results corroborate earlier lithosphere-upper mantle seismic tomography images, and highlight the Adriatic affinity of the Eastern Alpine slab. The zigzag deployment pattern of stations in the EASI experiment also allows distinction of short-wavelength variations perpendicular to the profile, both within the shallow and the deep crust. This underlines the importance of applying 3D imaging in complex geodynamic systems.
Display omitted
•Dense seismological swath maps Eastern Alps and Bohemian Massif structure (13.3°E)•Lithospheric structure points to subducting Adria plate beneath Tauern Window•Eastern Alpine root is a sink, a gradual transition from crustal to mantle velocities•Short-wavelength east-west structural variation within the crust•Significant variation of and sharp contacts between adjacent geological units
We investigate a well‐preserved paleo subduction channel that preserves a coherent part of the European continental margin exposed in the central Tauern Window (Eastern Alps), with the aim of testing ...models of sheath fold nappe formation and exhumation. The subduction zone was active during Paleogene convergence of the European and Adriatic plates, after closure of the Alpine Tethyan ocean. New cross sections and structural data together with new petrological data document a recumbent, tens of kilometers‐scale sheath fold in the center of the Tauern Window that formed during pervasive top‐foreland shear while subducted at high‐pressure (HP) conditions (~2.0 GPa, 500 °C) close to maximum burial depth. The fold comprises an isoclinally folded thrust that transported relicts of the former Alpine Tethys onto a distal part of the former European continental margin. The passive margin stratigraphy is still well preserved in the fold and highlights the special character of this segment of the European continental margin. We argue that this segment formed a promontory to the margin, which was inherited from Mesozoic rifting. In accordance with classical sheath fold theory, this promontory may have acted as an initial structural perturbation to nucleate a fold that was passively amplified to a sheath fold during top‐foreland shear in the subduction zone. The fold was at least partly exhumed and juxtaposed with the surrounding lower pressure units by opposing top‐hinterland and top‐foreland shear zones above and below, respectively, that is, in the sense of a nappe fold formed during channel‐extrusion exhumation.
Key Points
The central Tauern Window preserves a crustal‐scale sheath fold that formed in the Alpine subduction zone in top‐foreland shear
The sheath fold was formed by passive amplification of a rift‐inherited heterogeneity of the European margin
Exhumation of sheath fold nappe from HP conditions was achieved at least partly by channel extrusion
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