Conference Agenda

Despite the amount of research focused on the Alpine orogen, different hypotheses still exist regarding varying seismicity distribution patterns throughout the numerous crustal blocks of different physical properties that comprise the region. The thermal field, also varying as a consequence of heterogeneous lithospheric properties, is a key controlling factor for rock strength via thermally activated creep and it exerts a first order influence on the depth of the brittle-ductile transition zone, the lower bound to the seismogenic zone and the spatial distribution of seismicity. Previous measurement constrained works across the orogen and its forelands that have defined the 3D lithospheric density distribution and thermal field facilitate the generation of an observation based rheological model of the region. Here we present rheological modelling results from across the Alps and their forelands and compare calculated strengths to observed seismicity patterns, in order to explain the localisation of deformation. The depth of peak seismic energy release in almost all regions was found to correlate to the calculated brittle-ductile transition, adding validity to the results achieved. We find a strong correlation between the lateral distribution of seismicity and the total strength of the lithosphere, occurring mainly in weaker zones, with crustal thickness and LAB depth highly influential.

The modern-day coverage and availability of broad-band stations in the greater Alpine area offered by AlpArray, Swath-D and the European seismological networks allows for imaging seismic wave-fields at yet unprecedented resolution. In the AlpArray area and in Italy, the distance of any point to the nearest station is less than 30km, resulting in an average inter-station distance of about 45km. With a much denser deployment in a smaller region of the Alps (320km in length and 140km wide), the Swath-D network possesses an average inter-station distance of about 15km.

We show single event seismogram sections, time slices of teleseismic and regional wave-fields, and wave-field animations to reveal both the resolution capabilities of this dense station distribution as well as the enormous spatio-temporal complexity of seismic wave propagation. The time slices and wave-field animations demonstrate the need for dense regional arrays of broad-band stations, such as provided by AlpArray and neighboring networks, to resolve properties of teleseismic wave-fields. Here we present the images of coherent arrivals of direct body and surface waves, multiple body wave reflections, and multi-orbit phases for teleseismic events.

Spatial observations of the wave-fields illustrate e.g. the decrease in horizontal wavelength from P to S to surface waves and the way in which they considerably deviate from plane waves, due to heterogeneous earth structures along the path from the source to the array and beneath the regional array itself. Tomographic imaging techniques for the deep structure beneath the regional array have to take this spatio-temporal variability into account and correct for it.

The lateral resolution of the regional broad-band array is however dependent on station density, in this case limited to about 100km. Only even denser station distributions like those provided by Swath-D suffice to recover wave-fields of short period body and surface waves.

The present-day tectonics of the Alps are driven by highly fragmented subducted slabs. Detailed imaging of the Alpine lithosphere-asthenosphere system is prerequisite for quantitative geodynamic modelling of the plate deformation, however this is a challenging task due to the Alpine highly-curved geometry and relatively small-sized subducted slab segments. New high-resolution isotropic as well as azimuthally anisotropic Rayleigh-wave phase-velocity maps are calculated from automated inter-station phase-velocity measurements in a very broad period range (8 - 350 s). The maps are inverted, point-wise, for a 3-D, shear-wave velocity model (MeRE2020) using a newly elaborated stochastic, particle-swarm-optimization algorithm (PSO). In central Alps and partly beneath the northern foreland, MeRE2020 shows a nearly-vertical southward subduction of the Eurasian lithosphere down to ~250 km depth. The western Alpine slab segment is, in contrast, imaged down to approximately 100 km depth. The downward-imaged low-velocities support the suggested slab break-off. On the other hand, MeRE2020 shows the high-velocity anomaly beneath the Dinarides is indenting the eastern Alps and hints at the presence of a shallow Adriatic slab beneath the southern part of the eastern Alps down to about 150 km depth. In addition, another high-velocity anomaly is found beneath the northern flank of the eastern Alps and the Molasse Basin. We interpret this anomaly as southward subducting Eurasian mantle lithosphere. Therefore, we propose that subducting mantle lithosphere of both Eurasian and Adriatic origin may be present in the eastern Alps. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Dinarides down to about 150 km depth. Beneath the northern Apennines, MeRE2020 shows an attached Adriatic slab, whereas a slab window in the central Apennines is present.

In the multinational AlpArray project over 600 seismic broadband stations have been installed and operated in the broader Alpine region over the last years. Supplemented by the existing permanent stations in the area it is one of the most densely spaced seismic networks worldwide. The huge amount of stations offers an excellent opportunity to study crustal seismicity and to calculate focal mechanims even for small magnitude earthquakes with high accuracy.
In our study we focus on small to intermediate earthquakes that occurred in the area of the Northern Alpine chain. The events are roughly clustered in four distinct sub-regions. From West to East these are (1) the Lake Constance area (2) the Arlberg region (3) the area of Garmisch-Partenkirchen and (4) the broader region of Innsbruck. We calculated the focal mechanisms using the FOCMEC program (Snoke, 2003) and used polarities of P waves as well as amplitude ratios of SH to P as input parameters in the inversions.
Altogether, we calculated focal mechanisms for 30 earthquakes in the magnitude range between 2.5 and 3.5 from the time period 2016 to 2020. The focal mechanisms preferably show reverse or strike-slip faulting, normal faulting events are rather the exception. We analyse the mechanisms with respect to lateral changes along the Northern Alpine chain and compare them to moment tensors of events of slightly larger magnitudes (Petersen et al., 2019). Furthermore, we compare our mechanisms to the orientations of faults and use the focal mechanisms as input to invert for the stress field.

Breaking Back - Parameters controlling the Presence of Backthrust Structures at the Tip of FTB - Insights from the Alpine-Carpathian Belt

Backthrusts at the deformation front of orogens, so-called triangle zones, are known from various fold-thrust belts in the world. However, our understanding of kinematic and mechanic requirements for their formation is limited. Herein, we present a compilation of published data from the Alpine-Carpathian Belt, an orogen with well-known structural inventory, mechanical stratigraphy and along-strike changes. This offers the opportunity to compare different parameters conjectured responsible for the formation of triangle zones at the front of one single orogenic system. We analysed fault geometries and mechanical stratigraphy from 156 published geologic cross-sections, which enabled us to map areas, where triangle zones occur. Additionally, we compiled structural data of earlier (pre-Alpine) tectonics in the underlying substratum. Our results show that, despite the large structural variability along-strike, the frontal structures can be subdivided into three classes:

(1) Where no weak detachment horizons are present, backthrusts are absent. (2) Where weak detachment horizons (dominantly shale and salt) are present, triangle zones form.

(3) Where the elastic thickness of the lower plate is low or reduced by inherited structures, a triangular shape of the foreland basin is formed by flexural unconformities.

This finding allows us to compare the evolution of seemingly unrelated structures across the entire orogenic system and provides insights to mechanical parameters, which control the structure of deformation fronts, up to lithospheric scales.

At the southern margin of the eastern Southern Alps, the style of active faulting changes from thrusting on ~E-W trending faults at the Alpine front to right-lateral strike-slip faulting on NW-SE trending structures further south. The strike-slip fault system is ~100 km long and consists of a series of parallel subvertical faults. While the transition zone in the north is relatively well investigated, the southern termination of the strike-slip faults in the Slovenia-Croatia border region is less well understood. This is due to low seismicity, low deformation rates, a coarse geodetic network, and a karst landscape unfavourable of preserving geomorphological markers of fault activity. However, this area is interesting as it marks the northern edge of a proposed slab gap under the Dinarides. Studies on active faulting there help to better understand the surface expression of deep-seated processes. We present new data from the Selce Fault in SW Slovenia. The fault is mapped as 15 km long and a recent study showed swarm activity at depths of 5-18 km. Using tectonic geomorphology, paleoseismological trenching, and chronology obtained by OSL and ¹⁴C dating, we show that this strike-slip fault has probably been active in the Holocene. A surface-rupturing earthquake, creep, or a combination thereof affected the youngest geological units. We hypothesize that the slab gap controls the style of faulting. Instead of long and continuous faults as seen further north, smaller structures prevail. However, even short faults such as the Selce Fault might significantly contribute to the regional moment release.

The Dinarides fold and thrust belt, situated on the Balkan Peninsula, results from the convergences between the Adriatic and Eurasian plates since Mid-Jurassic times. The Late Jurassic obduction of ophiolites, the Early Cretaceous composite nappe stacking and the continent-continent collision in latest Cretaceous times led to folding and thrusting in the most external part of the Dinarides. This extensive last phase of substantial crustal shortening and thickening is associated with the syn-tectonic deposition of flexural foreland deposits of Middle Eocene to Lower Oligocene age.

Within these syn-tectonic sediments and in older Mesozoic carbonate platform rocks we document a set of uplifted flat marine terraces, stretching along the entire Dinaric coast and ranging up to 600 m in elevation. The location of these terraces correlates well with the position of a reported positive P-wave tomography anomaly beneath the Dinarides. This “shallow”, up to 180 km deep anomaly correlates also with the thinnest part of the Adriatic lithosphere and the drainage divide, which separates the Adriatic and Black sea catchments. River incision profiles on both sides of the drainage divide reveal a symmetric river incision pattern corresponding to the mean elevation of the documented marine terraces. This suggests an orogen-wide surface uplift.

This uplift event can be relatively dated to Oligocene-Miocene (28-17.1 Ma) and is contemporaneous with the emplacement of igneous rocks with mantle affinity (33-22 Ma) in the internal Dinarides. Geomorphic, geophysical, and petrological data reveal that the post-collisional reorganization of the Dinarides is associated with an Oligoene-Miocene mantle delamination resulting in an orogen-wide uplift event.

Indentation is used to define the collisional mechanics of small plates flanked by large subduction systems creating significant thickening and lateral escape of continental units. The close spatial proximity creates a situation where deformation is actively transferred between different subduction systems during their different stages of orogenic evolution. We study this transfer by observation and modelling of indentation and orogenic evolution in the Central Mediterranean system. The evolution of the greater Adriatic microplate has been described by studies in the Central and Eastern Alps, Apennines, their connection with the Dinarides and the lateral escape into the Pannonian Basin. We know that this lateral escape was partly accommodated by the Pannonian extension that ceased once the Carpathians subduction system was locked at ~ 8 Ma. What do the Dinarides really tell us? This orogenic system shows a generalized phase of Miocene, which was followed by inversion after 9 Ma by creating a coherent regional system of large offset dextral strike-slip faults, which transfer gradually their offsets to thrusts and high-angle reverse faultsNumerical modelling in the Dinarides and Carpathians demonstrates a mechanism of rapid slab roll-back followed by detachment and out-of-sequence crustal thickening during indentation and slab migration. These observations and models lead to the conclusion that the Adriatic indentation in the Alps and the shortening in the Apennines was accommodated at two different subduction systems (or plate margins) with an overlapping time at ~9-8 Ma. As long as the Miocene Carpathians subduction system was active until 8 Ma, most indentation was accommodated by the lateral escape into the Carpathians movement, while Dinarides recorded little to no deformation at its plate margins. After 9 Ma, the Adriatic indentation partitioned deformation mostly to oblique transpression in the Dinarides. This deformation accommodated the differential N- to NE-wards motion of Adria in respect to the rapid S- to SW- ward movement of a Hellenides area situated SE of the Kefalonia Fault, driven by the Aegean slab-roll back, facilitated by the thinned continental to oceanic nature of the Ionian lithosphere involved in the subduction system. The post- 9 Ma system of strike-slip, reverse and thrust faults mapped in the Dinarides must be nothing else but a large-scale crustal horizontal drag zone accommodating the differential motion between the Adriatic indentation and Aegean slab roll-back, connecting the present-day deformation observed in the Alps and Hellenides.