Post-Collisional Reorganisation of the Eastern Alps in 4D – Crust and Mantle Structure

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

Rift-related paleogeography of the European margin in the Eastern Alps (Central Tauern Window)

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

Evolution of a Fossil Subduction Zone: Insights from the Tauern Window, Eastern Alps

Subduction zones play a crucial role in the evolution of Earth's lithosphere. In many orogens, deeply subducted coherent high-pressure (HP) nappes were exhumed from deep to shallow parts of subduction channels. This process significantly affects the deformation pattern and internal structure of the orogen. Exhumation seems to occur preferentially during the transition from subduction to collision, when dense oceanic lithosphere has been consumed entirely and more buoyant continental lithosphere from a passive continental margin enters the subduction zone. Here, we present a detailed study on the structural, kinematic, and metamorphic evolution of a well-preserved paleo-subduction channel within the Tauern Window (Alps). First, we reevaluated the metamorphic history and regional tectono-stratigraphy of the tectonic units in the central Tauern Window. These units originate from the Alpine Tethys oceanic domain and the adjacent European passive continental margin. They experienced HP conditions during Alpine subduction, which was followed by exhumation to their current position in the Alpine nappe stack. By integrating new structural data and the well-preserved stratigraphy of the ocean-continent transition, we reconstructed the structure and kinematics of the nappes in great detail. Notably, we document a recumbent, tens-of-kilometers-scale sheath fold formed during pervasive top-to-the-foreland shear. This sheath fold comprises an isoclinally folded thrust that transported ophiolite relicts from the former Alpine Tethys onto a distal part of the European continental margin during early stages of subduction. It formed under HP conditions, immediately after the Europe-derived rocks in its core reached their maximum burial depth. The non-cylindrical shape of the sheath fold suggests its nucleation at a promontory of the former margin, inherited from Mesozoic rifting and subsequently amplified to a sheath geometry during top-to-the-foreland shear in the subduction zone. To gain insight into the temperature (T) structure of the sheath fold, we employed Raman spectroscopy on carbonaceous material (RSCM) thermometry on a large number of samples with high spatial resolution. The systematic spatial temperature trends reveal distinct domains related to the original subduction metamorphism and later T-dominated (Barrovian) metamorphic overprint. Integrating the peak-temperature pattern with the fold geometry unveils a two-stage process of nappe formation and sheath folding during exhumation. Our results highlight the existence of considerable along-strike heterogeneity within the deep portion of a fossil subduction zone, likely influenced by inherited rift structures and exhumation processes. Understanding such heterogeneities is crucial for interpreting seismic sections and numerical simulations of subduction zones, emphasizing the need to consider three-dimensional complexities beyond the idealized cylindrical models often used. By unraveling the structural and metamorphic evolution of exhumed HP nappes in the Tauern Window, this study contributes to a better understanding of the dynamic processes operating within subduction zones and their implications for mountain building

Investigating the post-collisional reorganisation of the Eastern Alps using a 4D reconstruction

In Neogene time, the Eastern Alps underwent a profound post-collisional tectonic reorganisation. This featured indentation of the Alpine orogenic wedge by the Adriatic upper plate, eastward lateral extrusion between conjugate strike-slip faults, and a shift from thrust propagation on the European lower plate to the Adriatic upper plate, accreting the eastern South Alps fold-thrust belt. The triggers and driving forces of this tectonic reorganisation remain hotly debated. We present new sequentially restored orogen-scale cross sections along the TRANSALP (12°E) and EASI (13.3°E) transects, plus an E-W orogen-parallel section (46.5°E) to investigate the kinematic evolution of the Neogene tectonic reorganisation in 4D. These transects were affected by eastward lateral extrusion, and so we used a map-view reconstruction to restore out-of-section transport of rock at the onset of rapid extrusion (23 Ma), and the onset of thick-skinned thrusting in the eastern South Alps fold-thrust belt (14 Ma). We then compared our results with Vp LET and teleseismic models of the crust and upper mantle. The geologic record reveals two phases of indentation in the Tauern Window: (Phase 1, 23-14 Ma) The Adriatic crust acted as a coherent indenter, with northward motion relative to Europe accommodated by shortening within the Eastern Alps orogenic wedge as well as sinistral motion along the Giudicarie Fault. Initially, upright folding of Penninic units, including the Venediger nappes, in the Tauern Window accommodated most shortening, but by middle Miocene time, eastward lateral extrusion of the entire metamorphic edifice and NCA was the primary mechanism accommodating N-S shortening. This shortening required ongoing subduction of the European lithosphere, ruling out previous models involving north-dipping Adriatic subduction. A purported detachment below the Venediger Duplex is inferred to have served as the base of the laterally extruding wedge, which comprised the previously subducted and exhumed European crust. (Phase 2, 14 Ma-Present): Since the middle Miocene, the leading edge of the Adriatic indenter has been deforming, forming the thick-skinned South Alps fold-thrust belt. The onset of S-directed shortening is recorded by Langhian-Serravallian rocks beneath the Valsugana Thrust. In contrast, the Adriatic lower crust of the fold-thrust belt was decoupled and transported northwards into the orogenic wedge. In the TRANSALP section, the European lithospheric mantle currently extends beneath the orogenic wedge, whereas in the EASI section the subducted European lithosphere has detached. The Adriatic lower crust indented the deeply buried equivalents of the European Venediger rocks exposed in the Tauern Window. A high-velocity (6.8-7.25 km/s) bulge in LET models of the TRANSALP section images this indenter, and possibly includes accreted European lower crust. We find that when the European slab detached beneath the Eastern Alps, shortening, exhumation, and lateral extrusion of the Eastern Alps orogenic wedge became increasingly important in accommodating Adria-Europe convergence. This culminated in the accretion of the South Alps which now forms the southern part of the orogenic wedge and primarily accommodates ongoing convergence. We note that in the E-W orogen-parallel section, a vertical gap within the slab anomaly, interpreted as a horizontal slab detachment, occurs east of the western boundary of the Tauern Window and the north projection of the Giudicarie Fault. Slab detachment (Handy et al., this volume) is an appealing explanation for the Neogene evolution by eliminating slab pull and redirecting the shortening into the south part of the orogenic wedge

A new 4D model of Alpine orogenesis based on AlpArray

Wholesale slab breakoff or detachment in the Alps in late Paleogene time has been invoked to explain Periadriatic calc-alkaline magmatism (43-29 Ma), rapid exhumation of HP metamorphics, as well as clastic infill of proximal parts of the Alpine Molasse basin (30-28 Ma). However, the 14 My timespan of these events exceeds the duration of slab detachment estimated from thermomechanical modelling (2-8 My) and from foreland depocenter migration (~5 My) along equivalent lengths of neighboring Alpine orogens with torn slabs (Carpathians, Apennines). Moreover, wholesale slab detachment does not explain major E-W differences in Neogene crustal structure, basin evolution, erosion and indentation in the Alps. Teleseismic Vp tomography from AlpArray suggests that the slab segment beneath the Central Alps comprises European lithosphere, is attached to its orogenic lithosphere and extends down to ~250 km depth, in parts possibly even to the Mantle Transition Zone (Fig. 1). This marks a first phase of partial slab detachment, probably in late Paleogene time based on comparing slab length with shortening in the C. Alps and of Adria-Europe convergence since 35 Ma. In contrast, the slab segment beneath the Eastern Alps is detached between 80-150 km depth. The age of this second phase of slab detachment is bracketed at 23-19 Ma by criteria below and by comparing vertical detachment distance with global slab sink rates. We propose a new model of Alpine mountain-building that features the northward motion of subduction singularities above delaminating and detaching Alpine slab segments, respectively in the C. and E. Alps (Fig. 2), to explain the aforementioned E-W differences in Oligo-Miocene structure, magmatism, and foreland sedimentation. Mountain-building began at ~35 Ma with a decrease in Adria-Europe convergence to <1cm/yr collision, causing the European slab to steepen and detach beneath both the Central and Eastern Alps. Periadriatic magmatism may have initiated prior to slab detachment due to fluxing of the cold mantle wedge by fluids from devolatilizing crust along the steepened Alpine slab. Thereafter, the Central and Eastern Alps evolved separately (Fig. 2). Northward motion of the singularity during slab delamination in the Central Alps increased both horizontal shortening and the taper angle of the orogenic wedge, with rapid exhumation and denudation in the retro-wedge. Slab steepening and delamination are inferred to have been more pronounced in the Eastern Alps, possibly due to the greater negative buoyancy of the slab in the absence of Brianconnais continental lithosphere in the eastern part of Alpine Tethys. The delaminating slab in the east drove subsidence and continued marine sedimentation in the E. Molasse basin from 29-19 Ma, while the western part of the basin in the C. Alps filled with terrigeneous sediments. Slab detachment beneath the E. Alps at ~20 Ma coincided broadly with several dramatic events within only 5 Ma (23-17 Ma): (1) a switch from advance of the northern thrust front to indentation of the E. Alps by the eastern S. Alps along the sinistral Giudicarie Fault; (2) rapid exhumation of Penninic nappes in the core of the orogen (Tauern Window) and orogen-parallel escape of orogenic crust toward the Pannonian Basin; (3) rapid filling of the E. Molasse basin. These events are attributed to a northward and upward shift of the singularity to within the orogenic crust during Adriatic indentation (Fig. 2). The eastward propagation of the uplifting depocenter in the E. Molasse basin is interpreted to reflect propagation of a subhorizontal slab tear beneath the E. Alps which is imaged by Vp teleseismic tomography. This slab tearing ultimately accompanied Miocene rollback subduction in the Carpathians, as inferred from the migrating depocenter around the orogenic foredeep

Imaging structure and geometry of slabs in the greater Alpine area – a P-wave travel-time tomography using AlpArray Seismic Network data

We perform a teleseismic P-wave travel-time tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po Plain to the river Main. Teleseismic travel times and travel-time residuals of direct teleseismic P waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162 000 highly accurate absolute P-wave travel times and travel-time residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Predictions of travel times are computed in a hybrid way applying a fast TauP method outside the model domain and continuing the wave fronts into the model domain using a fast marching method. We iteratively invert demeaned travel-time residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from a 3D a priori model of the crust and uppermost mantle and a 1D standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are a partly overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab whose easternmost end is located at about 100 km depth beneath the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex internal structure generally dipping to the SE down to about 400 km suggests a slab of European origin limited to the east by the Giudicarie fault in the upper 200 km but extending beyond this fault at greater depths. In its eastern part it is detached from overlying lithosphere. Further to the east, well-separated in the upper 200 km from the slab beneath the central Alps but merging with it below, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the eastern Alps of presumably the same origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent thick subducting lithosphere in this region. Alternatively, these regions may signify the presence of seismic anisotropy with a horizontal fast axis parallel to the Alpine belt due to asthenospheric flow around the Alpine slabs. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to their position above the European slab

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)

Evolving temperature field in a fossil subduction channel during the transition from subduction to collision (Tauern Window, Eastern Alps)

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-Pconditions (similar to 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-Pconditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes deform isograds of theT-dominated Barrovian metamorphism that itself overprints the high-Pmetamorphism 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 similar to 520 degrees 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

U.S. Fresh Fruit and Vegetable Marketing: Emerging Trade Practices, Trends, and Issues

In the past year, trade practices between fresh produce shippers and food retailers gained national attention. Shippers are concerned that recent retail consolidation has led to market power and the growing incidence of fees and services. Retailers argue that these new trade practices reflect their costs of doing business and the demands of consumers. Trade practices include fees such as volume discounts and slotting fees, as well as services like automatic inventory replenishment, special packaging, and requirements for third-party food safety certification. Trade practices also refer to the overall structure of a transaction-for example, long-term relationships or contracts versus daily sales with no continuing commitment. This study compares trade practices in 1999 with those prevalent in 1994, placing them in the broader context of the evolving shipper/retailer relationship. Most shippers and retailers reported that the incidence and magnitude of fees and services associated with transactions has increased over the last 5 years. Fees paid to retailers are usually around 1-2 percent of sales for most of the commodities we examined, but 1-8 percent for bagged salads. Information on the incidence and magnitude of these new practices is scarce. To augment information that is publicly available, we interviewed a limited number of shippers, retailers, and wholesalers about their firms and trade practices. We received a high level of voluntary cooperation from the interviewed firms.produce, fresh fruit and vegetables, fresh-cut produce, trade practices, fees and services, slotting fees, retail consolidation, produce shipper consolidation, Crop Production/Industries, Marketing,