Density and magnetic architecture of the Gurghiu Mountains volcanoes as inferred from geophysical data

Abstract

The paper is mainly aimed at presenting some results of the geophysical investigations focused on the Gurghiu Mountains volcanism.The Gurghiu Mountains are located in the central inner (western) part of Eastern Carpathians, Romania. They represents the middle segment of the approx. 160 km long Neogene to Quaternary volcanic chain Călimani-Gurghiu-Harghita (CGH), the southeastern end of the magmatic arc adjoining the Carpathians from Slovakia to Romania. CGH is a typical andesite-dominated calc-alkaline volcanic range. As part of it, Gurghiu Mountains exhibits (with minor exceptions) monotonous volcanic rocks, clearly dominated by andesites and pyroxene andesites.Several years ago, CGH volcanism was subject to research within a specific project funded by the Romanian National Agency for Scientific Research. During the project, gravity and geomagnetic investigations were conducted in the Gurghiu Mountains areal to help unveiling the composition and structure of the volcanic edifices. Thus, consistent gravity and geomagnetic data sets over the studied area were obtained. Furthermore, various data mining techniques (e.g., Bouguer anomaly for various reference densities, geomagnetic and reduced-to-the-pole geomagnetic anomaly, regional-residual separation through upward/downward continuation and/or polynomial regression, high-order derivatives) were applied in order to create more intuitive images helping in the qualitative interpretation of the geophysics results.In a second stage, quantitative approaches were employed for unveiling the hidden structure of the shallow part of the crust. Consequently, 2D and 3D models of the density and magnetic structure of the main volcanic forms in the area (e.g., Fâncel-Lăpusna caldera, Seaca-Tătarca, Sumuleu and Ciumani-Fierăstraie crater areas) were inferred from joint inversion of gravity and geomagnetic data.Finally, based on the inversion results, attempts to construct 3D models of the shallow crust architecture were made by employing the forward modelling approach under constraints provided by rock physics studies and exploration wells. Key words:  gravity, geomagnetism, density, magnetic susceptibility, inversion, forward modelling, volcanism, Gurghiu Mountains, Eastern Carpathians