Abstract
<p>The direction and location of subducting slab segments in the Alpine area is highly debated.  Here, we use seismic crustal depth estimates and different upper mantle tomographies to define hypotheses for the geometry of the subducting slab segments. Based on a new surface wave tomography of the upper mantle in the Alpine region, we also include a new hypothesis with a long Eurasian slab in the central Alps, a short slab segment in the western Alps, and bivergent subduction in the eastern Alps. In addition, we consider the south-dipping slab segment beneath the northern Apennines.  </p><p>Next, we study the possible slab related effects of the various considered slab geometries on the gravity field. Specifically, we calculate the gravity effects at the surface and at satellite altitude. In addition to the vertical gravity effect we also show gravity gradients. Two approaches are compared.  First, we convert seismic velocities directly to density using accepted conversion factors. Such direct conversion results in relatively scattered gravity anomalies. In the second approach, we assign density contrasts to predefined slab geometries. Starting from simple models with a constant slab density, we increase the complexity by considering temperature and pressure related density changes according to mantle composition. For such models, the density contrast of the slabs to the ambient mantle diminishes with depth. These models based on predefined slab geometries allow to analyse contributions by the different slab segments independently in greater detail. Combining the slab models with recent 3D crustal models of the Alps is needed in order to establish realistic density models of the Alpine realm for geodynamic applications.</p>
Highlights
The upper limit of 70 km is introduced because (i) we focus on the contribution of the slab segments removing crustal information from the model; (ii) the MeRE2020 tomography model is not sensitive to shallow structures – as a result, the slabs are not well recovered in depths shallower than 70 km; (iii) we want to ensure a uniform upper boundary
The highest magnitude of the forward-calculated gravity signal is on the order of 110 mGal and is observed for a slab model with a density contrast of 80 kg m−3 and a constant slab thickness of 100 km, while the lowest signal is produced by a combination of 20 km m−3 density contrast and a slab thickness of 60 km
The signal pattern is influenced by the predefined slab geometry, while the magnitude of the gravity signal depends on the density contrast and thickness (Fig. 7)
Summary
Interpretation of gravity anomalies can reveal information on the architecture and tectonic setting of the lithosphere (e.g. Zeyen and Fernàndez, 1994; McKenzie and Fairhead, 1997; Holzrichter and Ebbing, 2006; Braitenberg, 2015; Spooner et al, 2019). The Alpine mountain belt (Fig. 1a) is chosen for this sensitivity study because firstly a large range of recent seismic tomography studies imaged subducting slab segments in the Alpine region (e.g. Babuška et al, 1990; Lippitsch et al, 2003; Spakman and Wortel, 2004; Mitterbauer et al, 2011; Karousová et al, 2013; Zhao et al, 2016; Kästle et al, 2018; El-Sharkawy et al, 2020). Those different studies suggest different configurations of slab segments Those different studies suggest different configurations of slab segments (see Sect. 1.1), al-
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