Density distribution across the Alpine lithosphere constrained by 3D gravity modelling and relation to seismicity and deformation

Abstract

Abstract. The Alpine Orogen formed as a result of the collision between the Adriatic and European plates. Significant crustal heterogeneity exists within the region due to the long history of interplay between these plates, other continental and oceanic blocks in the region, and inherited crustal features from earlier orogenys. Deformation relating to the collision continues to the present day. Here, a seismically constrained, 3D, structural and density model of the lithosphere of the Alps and their respective forelands, derived from integrating numerous geoscientific datasets, was adjusted to match the observed gravity field. It is shown that the distribution of seismicity and deformation within the region correlates strongly to thickness and density changes within the crust, and that the present day Adriatic crust is both thinner and denser (22.5 km, 2800 kg/m3) than the European crust (27.5 km, 2750 kg/m3). Alpine crust derived from each respective plate is found to show the same trend with zones of Adriatic provenance (Austro-Alpine and Southern Alps) found to be denser and those of European provenance (Helvetic Zone and Tauern Window) to be less dense suggesting the respective plates and related terrains had similar crustal properties to the present day prior to orogenesis. The model generated here is available for open access use to further discussions about the crust within the region.

Gravity modeling of the Alpine lithosphere affected by magmatism based on seismic tomography

“…). This density relation has been also noted by (Spooner et al, 2019), indicating that this physical parameter could be used as a marker for the different tectonic domains.…”

“…We propose an innovative approach which includes several numerical algorithms and that provides a Moho definition, a velocity-density conversion, gravimetrical forward modelling and finally a linear inversion based on the gravimetric residual map. The results for the Alpine structure highlight that all terrains of Adriatic provenance (Austro-Alpine and South-Alpine) are denser (2850 kg m -3 ) than the 445European terrains (2750 kg m -3 ), suggesting that this physical parameter could be used as a marker to characterize the different tectonic domains involved in the Alpine regions (Spooner et al, 2019). We also considered the alternative source models for this gravity anomaly, in which different authors provide a stretched and thinned Adriatic crust located between the Giudicarie thrust front and the Schio-Vicenza fault, but today several Moho models of the Alps exist, that refute this model.…”

“…Several geophysical models have been developed for the Alpine lithosphere in the past, especially using seismic (Gebrande et al, 2006;Kästle et al, 2018) and gravity data as modelling constraints (Ebbing et al, 2006;Spooner et al, 2019). In this work for the first time, a high resolution three-dimensional seismic tomography model has been adopted as starting model for the computation of a 3D density model using the algorithm discussed before.…”

“…Infact, the Bouguer values are -60 mGal over the Alpine arc, whereas the regional Bouguer values are classically below -180 mGal e.g. (Braitenberg et al, 2013;Spooner et al, 2019;Zanolla et al, 2006). Another area with clear negative values is close to the front of the Tuscan-Emilian Appennines, presumably due to the combined effect of sediments of the Po plain and the crustal thickening due to 170 stacking of sediment layers of the Appennines.…”

“…The Alpine gravity field has been modelled before (Ebbing et al, 2006;40 Spooner et al, 2019), novel in our study is the use of the Bouguer field corrected for the global topography, which is more realistic than the 167 km classical Hayford radius reduction. A recent study (Spooner et al, 2019) used an approach in which density is constant for lateral crustal domains of typical extension of 80 km to 150 km, and the lithosphere is defined in layers of constant density. The layers have been defined with one layer for unconsolidated and consolidated sediments each, two layers for crystalline crust, one layer for subcrustal lithosphere, and one constant layer for the asthenosphere.…”