Geometry of the magma chamber beneath Damavand volcano (N Iran) from a joint study of gravity and magnetic data

Imaging the intra-sediment magma chamber in the Damavand region, northern Iran, is beyond the resolution of the local seismic observations. Gravity anomalies can precisely image the lateral extension of magma reservoir. In order to provide vertical extension of the magma chamber, we apply the inversion of magnetic data with a higher sensitivity to shallow structure in comparison to the gravity data. More importantly, the knowledge of the magma chamber’s density allows the prediction of its mechanical behaviour, including the potential of eruption. As Damavand is estimated to be an active volcano, it is important to revisit the physical properties of the magma chamber to be able to evaluate the potential of eruption. Here, we apply the sparse norm inversion of the Bouguer gravity anomaly and magnetic data to model the uppermost crust beneath the Damavand volcano. The qualitative analysis of the Bouguer anomaly shows that the power of the spectrum remains almost unchanged by upward continuation using heights greater than 4 km. Thus, we conclude that the 4-km upward continued Bouguer anomaly represents the regional gravitational effects free from very shallow effects. The inversion of the magnetic anomaly, interestingly, shows a susceptibility structure, with susceptibility contrast of up to ~ +0.025 SI, in the same place as the density anomaly. This study proposes a 10- km wide magma chamber beneath Damavand from depth ~3 km to depth ~12 km. The resulting density structure is comparable with the obtained values from derived densities (using the thermodynamic mineral phase equilibrium) based on geochemical data and those from the conversion of seismic velocity to density. According to the geochemical data analysis, the lava is andesitic which is categorized among dense crustal rocks (2.8 g/cm³). But our modelling results shows a density contrast of maximum + 0.25 g/cm³ between the magma chamber and the surrounding sedimentary rocks (with a density of 2.45 g/cm³) above 5 km. Therefore, we can conclude that the shallow magma chamber, composed of dense andesite, is relatively warm and probably not completely consolidated. The high temperature of the magma chamber appears to be neutralized by the impact of the high density of andesite (naturally dense rocks) to result in a moderate negative anomaly in tomography (i.e., ∆Vs=~ -2 %). The magma chamber’s temperature might exceed 750-800 ºC which is still beyond the solidus-liquid transition temperature of 1100 ºC. Therefore, we can conclude that the magma is no liquid and is partially consolidated.