Constraints on Crustal Structure in the Eastern and eastern Southern Alps

In the course of this study, an extensive seismological dataset from both the temporary SWATH-D network (Heit et al., 2021) and selected stations of the AlpArray Seismic Network (Hetényi et al., 2018) was analyzed. The primary aim of this endeavor was to gain comprehensive insights into the crustal structure of the southern and eastern Alps. The small inter-station spacing (average of ∼15 km within the SWATH-D network) allowed for depicting crustal structure at unprecedented resolution across a key part of the Alps. The methodological approach employed in this study entailed a sequential series of analyses to unveil the underlying features. The preliminary step encompassed the determination of the arrival times of both P and S seismic waves. Subsequently, a Markov chain Monte Carlo inversion technique was deployed to simultaneously calculate robust hypocenters, a 1-D velocity model, and station corrections (Jozi Najafabadi et al., 2021). This data was then utilized for calculation of 3-D VP and VP/VS models (Jozi Najafabadi et al., 2022). In addition, the path-averaged attenuation values were obtained by a spectral inversion of the waveform data of selected earthquakes. The attenuation structure (1/QP model) is then calculated using damped least square inversion of the path-averaged attenuation values (Jozi Najafabadi et al., 2023). These analyses resulted in a multidimensional depiction of the subsurface. The derived models for QP, VP and VP/VS indicate subsurface anomalies that can be attributed to rock’s physical parameters, presence of fluids within rocks and their motion in pores and fractures, temperature, and partial melting. The findings reflect head-on convergence of the Adriatic indenter (the part of the Adriatic Plate that has modified the Alpine orogenic edifice) with the Alpine orogenic crust. Furthermore, a highly heterogeneous crustal structure within the study area was unveiled. The velocity model illuminated decoupling of the lower crust from both its mantle substratum and upper crust. The Moho, taken to be the iso-velocity contour of Vp = 7.25 km/s, provided insights into the southward subduction of the European lithosphere, a phenomenon previously investigated in the Eastern and eastern Southern Alps (e.g., Kummerow et al., 2004 and Diehl et al., 2009). The most pronounced high-attenuation (low QP) anomaly is found to be closely correlated with the high density of faults and fractures in the Friuli-Venetian region, as well as the presence of fluid-filled sediments within the Venetian-Friuli Basin. Furthermore, the northwestern edge of the Dolomites Sub-Indenter (NWDI) corresponds to a low attenuation (high QP) anomaly which is interpreted as a reflection of the NWDI's stronger rocks compared to the surrounding areas

Role of the Giudicarie Belt and eastern Southern Alps in Adriatic Indentation

The Giudicarie Belt (GB) sinistrally offsets the Alpine orogenic edifice by some 70 km, including the front of the Adriatic Indenter as defined at the surface by Periadriatic Fault. The GB is a composite structure, comprising northern and southern segments of the Giudicarie Fault (GF), as well as a ≤50 km wide fold-and-thrust belt that strikes obliquely to ENE-WSW trending thrusts affecting Permo-Mesozoic sediments and basement of the eastern Southern Alps (Fig. 1). Stratigraphic and thermochronological constraints indicate that sinistral transpression within the GB began at 21-22 Ma and ceased no later than latest Miocene time. Minimum shortening across the GB in the range of 12-35 km was accommodated by thrusts and strike-slip faults that are inferred to reach down to 15-20 km and to link with the GF (Verwater et al. 2021). The GB does not offset the Moho and also does not coincide with observed changes in lithospheric mantle structure imaged by teleseismic Vp tomography. It is therefore not the site of a slab gap or tear, but forms part of an intracrustal fault system that is linked to the north with thrusts and strike-slip faults beneath the Tauern Window. In the Southern Alps east of the GB, SE-directed folding and thrusting accommodated shortening of 30-50 km. It initiated at 14 Ma (Langhian-Serravalian flysch beneath the Valsugana thrust) and propagated SE-wards to the active Montello thrust along the orogenic front of the Southern Alps (Fig. 1). Thus, thrusting in the eastern Southern Alps began later than within the GB, though deformation within these domains probably overlapped in mid-late Neogene time. We propose that a 1st phase of Adriatic indentation at 23-14 Ma involving sinistral transpression along the GB was linked to an intracrustal detachment that accommodated rapid exhumation of Penninic units in the Tauern Window and eastward lateral extrusion of orogenic crust in the E. Alps (Fig. 1). A 2nd phase of indentation since 14 Ma involved NNW-SSE-directed shortening that crumpled the leading edge of the Adriatic indenter. Section balancing (McPhee et al., this vol.) indicates that thrusts of this 2nd phase are directly linked to bulging and northward wedging of the Adriatic lower crust, as also indicated by local earthquake tomography obtained from Swath D (Fig. 2, Jozi Najafabadi et al. 2022). We note that the model above differs from our original interpretation of broadly coeval activity of the GB and the eastern Southern Alps during late Paleogene-Neogene Adria-Europe convergence (Verwater et al., 2021). In our present view, the Trento-Cles strike-slip fault accommodated differential shortening only within the GB and was not linked to the Schio-Vicenza fault system. The latter is marked by only minor (≤ 4 km) sinistral offset and was reactivated as a Mio-Pliocene normal fault in the foreland of the Apennines (Verwater et al. 2021)

Constraints on Crustal Structure in the Vicinity of the Adriatic Indenter (European Alps) From Vp and Vp/Vs Local Earthquake Tomography

In this study, 3-D models of P-wave velocity (Vp) and P-wave and S-wave ratio (Vp/Vs) of the crust and upper mantle in the Eastern and eastern Southern Alps (northern Italy and southern Austria) were calculated using local earthquake tomography (LET). The data set includes high-quality arrival times from well-constrained hypocenters observed by the dense, temporary seismic networks of the AlpArray AASN and SWATH-D. The resolution of the LET was checked by synthetic tests and analysis of the model resolution matrix. The small inter-station spacing (average of ∼15 km within the SWATH-D network) allowed us to image crustal structure at unprecedented resolution across a key part of the Alps. The derived P velocity model revealed a highly heterogeneous crustal structure in the target area. One of the main findings is that the lower crust is thickened, forming a bulge at 30–50 km depth just south of and beneath the Periadriatic Fault and the Tauern Window. This indicates that the lower crust decoupled both from its mantle substratum as well as from its upper crust. The Moho, taken to be the iso-velocity contour of Vp = 7.25 km/s, agrees with the Moho depth from previous studies in the European and Adriatic forelands. It is shallower on the Adriatic side than on the European side. This is interpreted to indicate that the European Plate is subducted beneath the Adriatic Plate in the Eastern and eastern Southern Alps

Seismic wave attenuation (1/Qp) in the crust underneath the Eastern and eastern Southern Alps (Europe): imaging effects of faults, fractures, and fluids

Abstract 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 Abstrac

Neogene kinematics of the Giudicarie Belt and eastern Southern Alpine orogenic front (northern Italy)

Neogene indentation of the Adriatic plate into Europe led to major modifications of the Alpine orogenic structures and style of deformation in the Eastern and Southern Alps. The Giudicarie Belt is a prime example of this, as it offsets the entire Alpine orogenic edifice; its activity has been kinematically linked to strike-slip faulting and lateral extrusion of the Eastern Alps. Remaining questions on the exact role of this fold-and-thrust belt in the structure of the Alpine orogen at depth necessitate a quantitative analysis of the shortening, kinematics, and depth of decoupling beneath the Giudicarie Belt and adjacent parts of the Southern Alps. Tectonic balancing of a network of seven cross sections through the Giudicarie Belt parallel to the local NNW–SSE shortening direction reveals that this belt comprises two kinematic domains that accommodated different amounts of shortening during overlapping times. These two domains are separated by the NW–SE-oriented strike-slip Trento-Cles–Schio-Vicenza fault system, which offsets the Southern Alpine orogenic front in the south and merges with the Northern Giudicarie Fault in the north. The SW kinematic domain (Val Trompia sector) accommodated at least ∼ 18 km of Late Oligocene to Early Miocene shortening. Since the Middle Miocene, this domain experienced at least ∼ 12–22 km shortening, whereas the NE kinematic domain accommodated at least ∼ 25–35 km shortening. Together, these domains contributed an estimated minimum of ∼ 40–47 km of sinistral strike-slip motion along the Northern Giudicarie Fault, implying that most offset of the Periadriatic Fault is due to Late Oligocene to Neogene indentation of the Adriatic plate into the Eastern Alps. Moreover, the faults linking the Giudicarie Belt with the Northern Giudicarie Fault reach ∼ 15–20 km depth, indicating a thick-skinned tectonic style of deformation. These fault detachments may also connect at depth with a lower crustal Adriatic wedge that protruded north of the Periadriatic Fault and are responsible for N–S shortening and eastward, orogen-parallel escape of deeply exhumed units in the Tauern Window. Finally, the E–W lateral variation of shortening across the Giudicarie Belt indicates internal deformation and lateral variation in strength of the Adriatic indenter related to Permian–Mesozoic tectonic structures and paleogeographic zones

Relocation of earthquakes in the southern and eastern Alps (Austria, Italy) recorded by the dense, temporary SWATH-D network using a Markov chain Monte Carlo inversion

In this study, we analyzed a large seismological dataset from temporary and permanent networks in the southern and eastern Alps to establish high-precision hypocenters and 1-D VP and VP/VS models. The waveform data of a subset of local earthquakes with magnitudes in the range of 1–4.2 ML were recorded by the dense, temporary SWATH-D network and selected stations of the AlpArray network between September 2017 and the end of 2018. The first arrival times of P and S waves of earthquakes are determined by a semi-automatic procedure. We applied a Markov chain Monte Carlo inversion method to simultaneously calculate robust hypocenters, a 1-D velocity model, and station corrections without prior assumptions, such as initial velocity models or earthquake locations. A further advantage of this method is the derivation of the model parameter uncertainties and noise levels of the data. The precision estimates of the localization procedure is checked by inverting a synthetic travel time dataset from a complex 3-D velocity model and by using the real stations and earthquakes geometry. The location accuracy is further investigated by a quarry blast test. The average uncertainties of the locations of the earthquakes are below 500 m in their epicenter and ∼ 1.7 km in depth. The earthquake distribution reveals seismicity in the upper crust (0–20 km), which is characterized by pronounced clusters along the Alpine frontal thrust, e.g., the Friuli-Venetia (FV) region, the Giudicarie–Lessini (GL) and Schio-Vicenza domains, the Austroalpine nappes, and the Inntal area. Some seismicity also occurs along the Periadriatic Fault. The general pattern of seismicity reflects head-on convergence of the Adriatic indenter with the Alpine orogenic crust. The seismicity in the FV and GL regions is deeper than the modeled frontal thrusts, which we interpret as indication for southward propagation of the southern Alpine deformation front (blind thrusts)