A fine structural section through the crust and lower lithosphere along the axial region of the Alps

Abstract

SUMMARY A unified two‐dimensional lithospheric model has been derived for the P‐wave velocity structure beneath the Alpine Longitudinal Profile (ALP'75) main line by making new phase correlations and using ray‐tracing and ray‐theoretical seismogram calculations in laterally inhomogeneous media. Due essentially to an increase in thickness of the lower crust, the total crustal thickness increases from 30–40 km in the peripheral regions to about 50 km in the central region of the Alps. The thickness change is more abrupt at the eastern end than at the western end of the line. The boundary between the upper and lower crust occurs at 18–20 km depth except in the peripheral region at the eastern end of the line where it occurs at 14–16 km depth. Along the whole line the seismic basement has velocities averaged over some tens of kilometres of 5.85‐6.15 km s‐1 with individual values having uncertainties of ±0.05‐0.1 km s‐1 at least east of shot‐point B. The lower part of the upper crust has been modelled as a low velocity layer, which is most pronounced beneath the peripheral regions, and which is generally underlain by a transition zone at the base of which the top of the lower crust occurs. The top of the lower crust has been modelled with velocities ranging from 6.25‐6.5 km s‐1 beneath the peripheral regions to 6.5‐6.65 km s‐1 beneath the central Alps. Velocity uncertainties at the top of the lower crust are estimated to be around ±0.2 km s‐1. Beneath the central Alps a low velocity layer has also been modelled in the lower crust. The top 6 km of the mantle has been modelled with velocities of 8.0–8.2 km s‐1 with individual estimates having uncertainties of ±0.15 km s‐1. Below this, the upper mantle down to almost 100 km depth has been modelled as a series of high velocity layers embedded in material with lower velocities of 8.0–8.2 km s‐1. The combined effects of tectonic overthrusting and uplift and erosion have probably led to the lower crust being apparently thicker than the upper crust in the central Alps at the present day.

Active strike-slip faulting in the Chablais area (NW Alps) from earthquake focal mechanisms and relative locations

“…The rectangle delimits the area shown in Figure 1 and the straight lines correspond to the ray-trace profiles analysed in this study (see Fig.3 for the diagram of the N-37°profile). Waldhauser et al (1998) and on P-wave velocities obtained from various crustal refraction studies (Yan & Mechie 1989;Maurer & Ansorge 1992) and earthquake studies (Deichmann & Rybach 1989;Pfister 1990;Maurer 1993). The ray-trace algorithm used was modified from Gebrande (1976).…”

A New Seismic Data Set on the Depth of the Moho in the Alps

“…The model is based on the refraction profile ALP75 that runs along the crest of the entire Alpine arc. We extract layer thickness and V p velocities of the 2D ALP75 model (YAN and MECHIE 1989), where the profile is close to station A313. The model features an intracrustal interface (velocity increase) at 16 km depth, a crustal thickness of 30 km, and an interface in the upper mantle at 70 km depth.…”

“…The presence of reverberated phases within the sedimentary basin and hence destructive interference may inhibit the detection of this feature. YAN and MECHIE (1989) show an equivalent intracrustal interface at similar depths, and MOLINARI et al (2012) model a layered crust, where the discontinuity between the upper and lower crust is a smooth surface located between 18 and 24 km depth. BRÜ CKL et al (2007) detect a reflector at about 20 km depth, which they interpret as a thin zone of magmatic intrusion below the Bohemian massif and Molasse zone.…”

“…Information on crustal thickness in the Eastern Alps is derived mainly from vintage to recent active source 2D and 3D experiments (ALPINE EXPLOSION SEISMOLOGY GROUP 1976;YAN and MECHIE 1989;BRÜ CKL et al 2003LUESCHEN et al 2004;BLEIBINHAUS and GEBRANDE 2006;BEHM et al 2007;GRAD et al 2009a;BEHM 2009). Teleseismic studies in the Western and Eastern Alps (LIPPITSCH et al 2003;DANDO et al 2011;MITTERBAUER et al 2011) targeted the deep lithosphere and contribute to an ongoing debate on slab geometry and subsequent geodynamic implications (e.g., KISSLING et al 2006;BRÜ CKL 2011).…”

Lithospheric Structure and Tectonics of the Eastern Alps – Evidence from New Seismic Data

“…The Alpine longitudinal profile (ALP'75) extended along the axis of the Western and Eastern Alps to the Pannonian basin (e.g., Yan and Mechie, 1989;Scarascia and Cassinis, 1997). WAR/R as well as steep angle reflection profiles were also measured in the Bohemian Massif, the Pannonian Basin, and the Dinarides (e.g., Beránek and Zátopek, 1981;Posgay et al, 1981;Joksović and Andrić, 1983).…”