Mantle rheology inferred from seismic tomography and foreland basin uplift around the Alps-Carpathian arc

Positive P-wave velocity V_p anomalies beneath the Alps resolved with teleseismic tomography (TEL, Paffrath et al. 2021) and local earthqake tomography (LET, Jozi Najafabadi et al. 2022) from AlpArray have geometries that are reminiscent of drips rather than slabs. These slab drips are still attached to the European orogenic lithosphere in the Central Alps, but appear to be partly detached in the Eastern Alps (Handy et al. 2021). The amount of Neogene crustal shortening during north-directed Adriatic indentation of the latter corresponds to the length of the slab drip within the limits of vertical resolution (20-50 km) down to a depth of 300 km (McPhee & Handy, in press). This allows us to establish the ages of the drip (14-21 Ma) and of a horizontal negative V_p-anomaly (≤ 14 Ma) separating this drip from its orogenic lithosphere.

Using these ages, we estimate the sink rate of the hanging part of the European slab drip to be 6-11 mm/yr. A higher sink rate (32 mm/yr) is obtained by using the clockwise migration of depocenter uplift and thrusting in the foreland basin around the Carpathian arc as proxies for the timing of slab detachment from the European orogenic lithosphere (Meulenkamp et al. 1996). Accordingly, the slab drip detached between 19 and 11.5 Ma and is currently entrained in the Mantle Transition Zone, MTZ (Wortel & Spakman 2000). Taken together, these relations indicate that the sink rate of slab drips increased with depth and time. However, we are unable to ascertain whether these drips reached a terminal sink velocity.

The lack of Neogene magmatism in the Alpine orogen suggests that detachment and sinking of the slab drips occurred in the absence of melting of the mantle. Taking the sink rates above at face value, we apply a modified form of the Stoke’s Law equation to obtain a dynamic viscosity of some 10⁸ MPa-s for asthenosphere undergoing solid-state flow around the slab dripping beneath the Eastern Alps. This is several orders of magnitude greater than the generally cited range of asthenospheric viscosities in subduction settings (10¹² – 10¹³ MPa-s) as well as viscosities obtained for polycrystalline dunite undergoing grainsize-insensitive creep (10¹¹-10¹³ MPa-s, grainsize = 1cm), but similar to viscosities for grainsize-sensitive creep (10⁹ MPa-s, grainsize = 50 microns, Handy et al. 1989) in the temperature range (1270-1345 °C) of the aforementioned negative V_p-anomaly (≤ 14 Ma) under the Eastern Alps. While we can only speculate on the temperatures and syntectonic grainsize in the asthenospheric mantle beneath the Alps, our findings indicate that suborogenic asthenophere is anomolously weak, even in the absence of melting, and facilitates the rapid detachment and sinking of slabs at the end of convergence and indentation.