Vahid Ebrahim Zadeh Ardestani - Institute of Geophysics, University of Tehran, Tehran, Iran.
Naeim Mousavi - International Institute of Earthquake Engineering and Seismology, Tehran, Iran.
Mohammad Tatar - International Institute of Earthquake Engineering and Seismology, Tehran, Iran.

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 magma chamber, we apply inversion of magnetic data with a higher sensitivity to shallow structure in comparison to gravity data. More importantly, knowledge of magma chamber’s density allows 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 Bouguer gravity anomaly and magnetic data to model the uppermost crust beneath Damavand volcano. Qualitative analysis of the Bouguer anomaly shows that the power of the spectrum remains almost unchanged by upward continuation using heights greater than ۴ km. Thus, we conclude that the ۴-km upward continued Bouguer anomaly represents the regional gravitational effects free from very shallow effects. Inversion of magnetic anomaly, interestingly, shows a susceptibility structure, with susceptibility contrast of up to ~ +۰.۰۲۵ SI, in the same place as density anomaly. This study proposes a ۱۰-km wide magma chamber beneath Damavand from depth ~۳ km to depth ~۱۲ km. The resulted density structure is comparable with the obtained values from derived densities (using thermodynamic mineral phase equilibrium) based on geochemical data and those from conversion of seismic velocity to density. According to the geochemical data analysis, the lava is andesitic which is categorized among dense crustal rocks (۲.۸ g/cm۳). But, our modeling results shows a density contrast of maximum + ۰.۲۵ g/cm۳ between the magma chamber and the surrounding sedimentary rocks (with density of ۲.۴۵ g/cm۳) above ۵ 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 magma chamber appears to be neutralized by the impact of high density of andesite (naturally dense rocks) to result in moderate negative anomaly in tomography (i.e., ∆Vs=~ -۲ %). Magma chamber’s temperature might exceed ۷۵۰-۸۰۰ ºC which is still beyond the solidus-liquid transition temperature of ۱۱۰۰ ºC. Therefore, we can conclude that the magma is no liquid and is partially consolidated.

Damavand Volcano, Magma chamber, Bouguer anomalies, Magnetic data, Sparse norm inversion